Computationally designed tandem direct selective oxidation ... · PDF fileComputationally designed tandem direct selective oxidation using molecular oxygen ... production processes

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PAPER

Computationally

Department of Chemical & Materials Engine

Cruces, NM 88003-8001, USA. E-mail: tman

† Electronic supplementary information (tables including: triplet–quintet crossingoxo complex; singlet–triplet crossing curthe RuTDCPP bare complex; singlet–tRuTDCPP(PO)S from RuTDCPP(PO*)T; ta(ESCF, EZP, H, and G) for the DMZB catalvarious oxygen groups and/or adsorbeactivation barriers and net reaction enreaction steps in Fig. 12, Fig. 13, Fig. 15,DMZB catalyst; tables of assigned spin m

Cite this: RSC Adv., 2016, 6, 88189

Received 11th July 2016Accepted 31st August 2016

DOI: 10.1039/c6ra17731j

www.rsc.org/advances

This journal is © The Royal Society of C

designed tandem direct selectiveoxidation using molecular oxygen as oxidantwithout coreductant†

Bo Yang and Thomas A. Manz*

Developing greener technologies to produce chemicals has attractedmuch recent attention. For the selective

oxidation of organic compounds, direct selective oxidation in whichmolecular O2 is utilized as oxidant without

using a co-reductant or co-oxidant is desirable to avoid forming waste co-products; however, existing

catalysts are limited in the types of substrates that can be used to achieve this. To address this challenge,

we introduce a tandem direct selective oxidation process that separates the molecular oxygen activation

step from the substrate oxidation step. Specifically, two reactions occur in separate reactors over two

different catalysts: (1) molecular oxygen activation via reaction with an oxygen acceptor molecule to

produce an oxygen transfer intermediate, and (2) substrate oxidation via reaction with the oxygen transfer

intermediate to produce substrate oxide and regenerate the oxygen acceptor molecule. The oxygen

acceptor molecule should be recycled back to the first reactor to achieve a net reaction of 2 substrate +

O2 / 2 substrate oxide. This separation of molecular oxygen activation and substrate oxidation steps

reduces by-product formation by avoiding some side reactions. We use density functional theory (DFT) to

study propene epoxidation as an important example. This reaction is of interest because: (a) propylene

oxide (PO) is one of the leading commodity chemicals worldwide, (b) all current commercial PO

production processes produce co-products, and (c) propene epoxidation illustrates the class of difficult

terminal alkene epoxidations for substrates containing allylic hydrogen atoms. Using DFT calculations, we

identify plausible candidates for the oxygen transfer intermediate and catalysts for the molecular oxygen

activation and substrate oxidation reactions. The Zr(C6H4-1,2-(N(C6H3-20,60-(CH3)2)O)2)2 [DMZB] and the

Ru(meso-tetrakis(2,6-dichlorophenyl)porphyrin) [RuTDCPP] catalysts were chosen for reactions (1) and (2),

respectively. Several pyridine based N-oxides were tested as oxygen transfer intermediates. Our DFT

computations indicate 2,6-dimethylpyridine N-oxide should perform well. The RuTDCPP catalyst was prior

experimentally demonstrated to oxidize organic substrates (e.g., 1-octene) using aromatic N-oxides as

oxidants with above 90% selectivity towards the desired product under mild conditions. For molecular O2

activation (reaction (1)) and propene epoxidation (reaction (2)), our computed enthalpic energetic spans are

33.3 and 31.6 kcal mol�1, respectively, predicting decent activities for both catalysts.

1. Introduction

Catalytic selective oxidation is of great importance forproducing oxidized organic compounds such as alcohols,

ering, New Mexico State University, Las

[email protected]

ESI) available: Additional gures andcurves for O2 addition to the DMZBves for Me2PyO donating O atom toriplet crossing curves for formingbles of computed relative energiesyst and the RuTDCPP catalysts withd molecules; tables of computedergies (ESCF, EZP, H, and G) for alland direct ethylene epoxidation overagnetic moments (ASMs) for triplet

hemistry 2016

aldehydes, ketones, acids, and epoxides, ranging from nechemicals to large-scale commodities.1–4 Due to both economicand environmental reasons, catalytic selective oxidationprocesses in which molecular O2 is utilized as oxidant without

complexes; the energy prole diagram of reaction cycle 2 for ethyleneepoxidation over the DMZB catalyst; catalytic cycles and associated energyproles for 2,6-dimethylpyridine, 2,6-dichloropyridine, pyridine,4-nitroquinoline, and 2-chloropyridine oxidation using the DIZB catalyst. Allof the DFT-optimized geometries and associated energies are presented withimaginary frequencies provided for transition states. A zip format archivecontaining .xyz les (which can be read using any text editor or the free Jmolvisualization program) containing net atomic charges, bond orders, and ASMs(for spin-polarized systems) for all of the DFT-optimized geometries. See DOI:10.1039/c6ra17731j

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http://crossmark.crossref.org/dialog/?doi=10.1039/c6ra17731j&domain=pdf&date_stamp=2016-09-14

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using any co-reductant or co-oxidant is preferable.3–5 First, usingmolecular O2 without any co-reductant or co-oxidant canpotentially avoid co-product formation. Second, molecular O2 ischeap due to its abundance in air. Third, using O2 as oxidantdoes not lead to unwanted salts such as those produced whenusing sodium hypochlorite (i.e., bleach) as oxidant. The keychallenge is to develop catalysts that can effectively activateboth oxygen atoms in molecular O2 under mild conditions andselectively transfer them to substrate molecules. This turns outto be an extremely difficult task.5 Great efforts have been madefor nding such catalysts with some successes, but much workremains.3,4,6–17 Of particular interest, Wang et al. achieved 75%selectivity at 63% conversion for aerobic 1-octene epoxidationover a zeolite-encapsulated Mn Schiff-base complex.129

There is intense motivation to improve the efficiency oflarge-scale catalytic selective oxidation processes.2,18 Ethyleneoxide (EO), propylene oxide (PO), and phenol ranked among thetop 18 most produced chemicals worldwide by mass in 2010and have been identied by the International Energy Agency asneeding energy and greenhouse gas reductions.19 PO is a keyintermediate in the production of many chemical productsincluding polyether polyols (used to make polyethers andpolyurethanes), propylene glycols (used to make unsaturatedpolyester resins and industrial uids), and propylene glycolethers (used to make paints).20 Despite the fact that PO hasa long history in the chemical industry and is produced in hugeamount worldwide, no direct propene epoxidation process iscommercially available.20 In contrast, EO is already commer-cially produced by the direct epoxidation of ethylene usingmolecular oxygen over silver-based catalysts.21–23 Phenol iswidely used to produce plastics and other chemical products.Today, more than 90% of phenol is produced by the partialoxidation of cumene with acetone produced as a co-product.24

Fig. 1 Currently existing manufacturing processes for large-scale propyused in place of NaOH. In the coproduct route, isobutane can be used

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The economic value of this process is highly inuenced by themarket price of acetone.24 The direct hydroxylation of benzeneusing molecular oxygen as oxidant without co-reductant isdesirable to avoid co-product formation, but efficient catalystsfor this reaction have not been developed yet.25,26

In this article, we will focus on PO production. As shown inFig. 1, PO is currently manufactured using a variety ofprocesses.20,23 In chlorohydrin processes, propene is rst reac-ted with chlorine gas and water to produce chloro-propanol.The chloro-propanol is then reacted with NaOH (or Ca(OH)2)to produce PO plus 2NaCl (or CaCl2).20,23 Although the yield toPO is good, this process generates�40 pounds of brine for everypound of PO produced.20,23 The co-product processes utilizeorganic hydroperoxides as oxidants and produce co-productssuch as styrene or methyl tert-butyl ether (MTBE).20 Sinceequal molar quantities of PO and co-product are produced inthese processes, the economic value is strongly inuenced bythe co-product's market value. A variation has been developedthat avoids co-products.23 In this variation, cumene is convertedto its hydroperoxide which oxidizes propene to PO and forms analcohol that is passed over a catalyst to regenerate cumene.23

This process gives high yield and selectivity to PO,23 butconsumes one mole of H2 (to regenerate cumene) for each moleof PO produced. The most recently developed process useshydrogen peroxide (H2O2) as an intermediate oxidant.20,23,27,28

Transporting H2O2 for this process is uneconomical, so it mustbe generated on site.29 Theoretically, this process would onlyproduce water as a co-product.29 The required H2O2 is generatedby the reaction H2 + O2 / H2O2 over a catalyst or via theanthraquinone process.29 Although high conversion and selec-tivity to PO can be obtained, this process has the disadvantageof consuming at least one mole of H2 for every mole of POproduced.

lene oxide (PO) production. In the chlorohydrin route, Ca(OH)2 can beas co-reactant in place of ethylbenzene.

This journal is © The Royal Society of Chemistry 2016

Table 1 Summary of some basic economic and environmental facts regarding existing propylene oxide and ethylene oxide manufacturingprocesses. GHG stands for greenhouse gas

ProductRanka

(mass based)Production volumea

(kg per year)Price($ kg�1)

Energy consumptiona

(kJ kg�1)GHG emissiona

(kgCO2-eq. per kg)Existing manufacturingprocessa

Propylene oxide 16 7.06 � 109 2.09b 22.53 � 103 1.61 Indirect epoxidationprocesses

Ethylene oxide 12 2.12 � 1010 1.32c 10.18 � 103 1.71 Direct epoxidation usingAg catalysts

a From ref. 19. b From ref. 32. c From ref. 33.

Paper RSC Advances

Even though the cumene and hydrogen peroxide processes onlyproduce water as co-product under idealized conditions, theenergy consumption of these processes is enormous and haspotentially reduced the prot for PO production. As summarizedin Table 1, the energy consumption of making 1 kg of PO isroughly twice the energy consumption ofmaking 1 kg of EO, whichcorresponds to �24 � 109 kW h of extra energy consumed eachyear. There is continuing interest to improve the economic andenvironmental aspects of propylene oxide manufacture.2,18,30,31

A direct propene epoxidation process in which propenedirectly reacts with O2 over a catalyst to produce PO withoutusing any co-reductant is most desired.20 The principle chal-lenge is to nd a suitable catalyst for such reaction. This catalystshould simultaneously give a high conversion and high selec-tivity to PO with high catalyst stability. A large number ofcatalysts, including silver-based catalysts used for large-scaledirect ethylene epoxidation,20 Zr/Hf organometalliccompounds with two bidentate ligands,34–36 and other transitionmetal catalyts37–42 have being thoroughly tested for direct pro-pene epoxidation either through experimental tests or compu-tational simulations, but none was commercialized yet. Acatalyst has not yet been able to achieve high enough conver-sion, selectivity, and catalyst stability to make direct propeneepoxidation commercializable.20

Fig. 2 Basic steps in the Manz–Yang selective oxidation catalyticcycle.34–36 Notation: A ¼ oxygen acceptor molecule, AO ¼ oxidizedoxygen acceptor molecule, L¼ chelating ligand. Additional adsorbatesmay optionally be bound to the metal atom during this catalytic cycle.Besides Zr, the central metal atom could be another transition metalsuch as Hf.


In previous computational studies, we introduced a newcatalytic route for molecular oxygen activation and transfer ofboth oxygen atoms to substrate without using a co-reductant orco-oxidant.34–36 As shown in Fig. 2, four elementary steps areinvolved in the catalytic cycle: (1) an O2 molecule reacts with anoxo group to generate an h2-ozone group, (2) the h2-ozone grouprearranges to form an h3-ozone group, (3) the h3-ozone groupreleases an O atom to substrate to form substrate oxide anda peroxo group, and (4) the peroxo group releases another Oatom to substrate to form another substrate oxide and regen-erate the oxo group. Using this catalytic cycle, we demonstratedthat the diisopropyl-substituted Zr_benzol (DIZB) organome-tallic catalyst effectively activates molecular O2 with a computedactivation barrier of only 16.9 kcal mol�1.36 We computed anoverall enthalpic energetic span of 27.1 kcal mol�1 for directethylene epoxidation.36

However, this catalytic cycle does not work for direct propeneepoxidation.35,36 As shown in Fig. 3, a common bonding motif isshared between the ozone complexes and the unwanted allylicH transfer side-product. The ozone complexes are key structuresthat ensure both O atoms in molecular O2 are activated and canreact with two substrate molecules sequentially withoutrequiring too much energy. In an h2-ozone complex, as shownin Fig. 3(a), two outer O atoms of the ozone group bond to the Zratom through two single bonds. As shown in Fig. 3(b), a similarbonding structure occurs in the allylic H transfer product. Thisunwanted allylic H transfer by-product is a lower energy statethat is more thermodynamically favored than the desired POformation reaction.35,36 Attempting to impose kinetic limita-tions by increasing the allylic H transfer reaction barrier isfraught with difficulty, because the common bonding motifmakes it almost impossible to destabilize this side productwithout also destabilizing the ozone group and raising theenergy required for O2 activation. A completely differentapproach is required.

To avoid this problem, we propose separating molecularoxygen activation and propene epoxidation into two separatereactions occurring in two separate reactors over two differentcatalysts. As shown in Fig. 4, the rst reaction oxidizes anoxygen acceptor molecule (e.g., 2,6-dimethylpyridine) usingmolecular oxygen as the oxidant to form an oxygen transferintermediate (e.g., 2,6-dimethylpyridine N-oxide). The secondreaction oxidizes propene to propylene oxide using the oxygentransfer intermediate as the oxidant and regenerates the oxygenacceptor molecule. The net reaction generates propylene oxide

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Fig. 3 Inherent difficulty for direct propene epoxidation usingmolecular O2 over the Zr organometallic catalysts. A similar bondingstructure (highlighted in red) exists within (a) h2-ozone complex and(b) allylic H transfer side product. L is an abbreviation for an organicchelating ligand. Fig. 5 Simplified process flow diagram for the tandem direct selective

oxidation process. The oxygen acceptor molecule is indicated by thesymbol A. The oxygen transfer intermediate is indicated by the symbolAO. The molecular oxygen activation step occurs in reactor 1 overcatalyst 1, where A is converted to AO. AO is then transferred to reactor2 where substrate oxidation occurs over catalyst 2 to reduce AO to A.The exit stream of reactor 2 is sent to a separation unit where A isseparated from substrate oxide (and possibly other chemical species).A is recycled back to reactor 1. The intermediate streams will likely alsocontain solvent molecules as well as some unreacted reactants andpossibly other chemical species. For simplicity, these are not explicitlylabeled on the diagram. The solvent(s) and unreacted reactants couldbe separated from one or more intermediate streams and recycledback to the appropriate reactor 1 and/or reactor 2. The gas streamfrom reactor 1 would likely be passed through a cold trap to condensesolvent, A, and AO molecules that are returned to reactor 1.

RSC Advances Paper

using only propene and molecular oxygen as reactants. We callthis two-step process “tandem direct selective oxidation”. Itcould potentially be used to selectively oxidize a wide range ofchallenging organic compounds using molecular oxygen as theoxidant without co-reductant or co-oxidant.

Fig. 5 illustrates how tandem direct selective oxidation couldbe implemented as a chemical ow process. (This could beimplemented as either a continuous or a batch process.) Oxygenacceptor molecule (A) and O2 (pure or as air or diluted gasstream) would be fed to the rst reactor where they would reactover catalyst 1 to form the oxygen transfer intermediate (AO). Agas stream exiting the rst reactor would be passed througha cold trap. The condensables (e.g., A, AO, solvent) would berecycled back to the rst reactor while the non-condensables(e.g., O2, inert gas components) would be sent to exhaust. Aliquid stream containing AO drawn off the rst reactor would befed into the second reactor. The organic substrate would be fedinto reactor 2 where it would react with AO over catalyst 2 toform the desired product (i.e., substrate oxide) and regenerateA. The effluent from the second reactor would be passed intoseparation units to separate substrate oxide from the oxygenacceptor molecule (A). A recycle stream containing A would befed back into the rst reactor.

Fig. 4 Illustration of proposed two-step selective oxidation processusing propene epoxidation reaction as an example. Reaction (1) is thesubstituted pyridine oxidation. 2,6-Dimethylpyridine (Me2Py) is shownas an example for oxygen acceptor molecule. Reaction (2) is propeneepoxidation. The overall reaction is substrate oxidation using molec-ular oxygen without co-reductant.

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In this article, we study the catalysts for reactions (1) and (2)shown in Fig. 6. For reaction (1), the key character we arelooking for is that the chosen catalyst should be able to effec-tively activate molecular O2 and should behave like an O atom“sponge” that freely accepts O atoms and transfers them to thechosen oxygen acceptor molecule with little change in the Oatom chemical potential. The dimethyl-substituted Zr_benzol(DMZB) organometallic complex is our catalyst choice forreaction (1), yet other catalysts that fulll the same requirementcould also be applied. In reaction (2), by using a Ru basedporphyrin catalyst, Me2PyO is utilized as the oxidant for pro-pene epoxidation. Experimental studies show that Ru basedporphyrin complexes can exhibit high selectivity and high yieldfor alkene epoxidations using aromatic N-oxides as oxidantunder mild conditions.43–45 Because other transition metal (e.g.,Fe, Mn, Co, etc.) based porphyrin complexes require strongeroxidants such as PhIO and H2O2,46–48 they are not suitable forour study. Here, we choose the MCM-41 tethered Ru(meso-tetrakis(2,6-dichlorophenyl)porphyrin) complex43 as the catalystfor our second reaction. Using this catalyst, Liu et al. obtaineda selectivity of 92% at 80% conversion for 1-octene epoxidationusing 2,6-dichloropyridine N-oxide as the oxidant for 24 hoursof reaction at 40 �C.43 The MCM-41 tethering makes thisorganometallic complex heterogeneous for easier catalystseparation and reuse. In the experiment, a NH2(CH2)3Si linkagewas used to immobilize this complex. It attaches to the Ru atomon the N end and tethers onto MCM-41 on the Si end throughthree Si–O–Si bonds. Some other tethering methods could alsobe applied but are not tested in our study.45,49–55 In our study,a Si–O–H bonding model was used to replace the Si–O–Si bondsand the tethered MCM-41 structure. Fig. 6 illustrates theexperimental tethering linkage and our computational model.


Fig. 6 Catalysts studied in this work. On the left is shown the DMZB catalyst for oxygen acceptormolecule oxidation reaction usingO2 as oxidant(reaction (1)). In the middle is shown the tethered RuTDCPP catalyst for propene epoxidation reaction using oxygen transfer intermediate asoxidant (reaction (2)). On the right is shown the tethering linkage used in the experiments and our computational model.

Paper RSC Advances

Selected examples of prior computational studies of Ruporphyrin catalysts include: (a) the mechanism of methanehydroxylation by a Ru porphyrin hydrogen sulde complex,130,131

(b) the mechanism of O2 activation via the formation of [Ruporphyrin]–O2–[Ru porphyrin] complexes,132 (c) the mechanismof photocatalyzed epoxidation of alkenes in water by a Ruporphyrin carbon monoxide complex,133 and (d) the mechanismof Ru porphyrin catalyzed intramolecular amidation of a sulfa-mate ester.134

The remainder of this article is organized as following.Section 2 summarizes the computational methods applied forcomputing molecular geometries and energies. Section 3.1summarizes different types of selective oxidation reactions thatcan be achieved using Ru based porphyrin catalysts witharomatic N-oxides as oxidants. Section 3.2 presents anddiscusses chemical potential diagrams in which the relativeenergies of different catalyst forms are plotted as a function ofthe oxygen chemical potential. These diagrams give insightsinto the preferred catalyst forms under reaction conditions.Section 3.3 presents and discusses computed catalytic cycles,reaction energy proles, adsorption energies, and charge andbonding analysis for reaction (1). Section 3.4 presents anddiscusses computed catalytic cycles, reaction energy proles,adsorption energies, and charge and bonding analysis forreaction (2). Section 3.5 compares computed energetic spansusing different basis sets with and without implicit solvation.Section 4 presents and discusses reaction cycles and energeticspans for direct ethylene epoxidation over the DMZB catalyst.Themain conclusions of this work are summarized in Section 5.

2. Computational methods

Geometries and energies of ground states, transition states, andother reactive intermediates were computed to study reactionmechanisms and activation barriers. All DFT-based calculationswere conducted using GAUSSIAN 09 soware.56 Becke's three-parameter hybrid method involving the Lee–Yang–Parr corre-lation functional57,58 (B3LYP) and LANL2DZ basis sets56 werechosen to achieve a good balance between high geometryaccuracy and low computational cost. Geometries were


optimized to a maximum step size of 0.01 a.u. for atomdisplacements and a maximum force of 0.0025 a.u. Repeatingall the calculations with a substantially larger basis set or tighterconvergence criteria would be extremely computationallyexpensive. For one of the catalysts, we repeated calculations forrate-determining intermediates using larger basis sets, tighterconvergence criteria, and an implicit solvent model (Section3.5). Except where otherwise indicated, catalyst forms werestudied in vacuum.

The DFT calculation for each ground and transition state wasconducted as described previously.34–36 For ground state calcu-lations, various initial geometries were considered and fullgeometry optimizations were carried out to determine thelowest energy conformation. For each transition state, con-strained optimizations were initially performed to generatea geometry estimate that was subsequently optimized using thequadratic synchronous transit or eigenmode followingmethods. Frequency analysis was performed on each groundstate and transition state. We veried that all frequencies arepositive for each ground state and only one imaginary frequencyexists (within a computational tolerance of 30 cm�1) for eachtransition state. For each transition state, the imaginaryfrequency was animated in GaussView to verify its vibration wasalong the desired reaction coordinate. Thermochemical anal-ysis was performed under standard condition (1 atm. pressureand 298.15 K temperature) using the harmonic approximation.The electronic energy without vibrational correction (ESCF), theelectronic energy including zero-point vibrational correction(EZP), the enthalpy (H), and the Gibbs free energy (G) for eachoptimized geometry is reported in the ESI.†

As explained in prior literature, for reactions in which thereactants and products have different spin states (aka ‘two-statereactivity’59), the potential energy surfaces dened for thesedifferent spin states will cross along the reaction pathway. Ifthis crossing occurs at a relatively low energy along the reactionpathway, the reaction barrier will be characterized by a regulartransition state for one of the spin states.59,60 On the other hand,if this crossing occurs at a relatively high energy along thereaction pathway, the reaction barrier will be characterized bya minimum energy crossing point between the two spin

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states.59,60 Results for all of the crossing point calculations inthis study are given in Section 1.1 the ESI.†

3. Computational study of the 2-steppropene epoxidation reaction using2,6-dimethylpyridine (Me2Py) as theoxygen transfer intermediate3.1 Background information on ruthenium porphyrinsystems

Cytochrome P450s are one type of the terminal oxidase enzymeswhich hold the capability of selectively oxidizing organicsubstrates in vivo.61 Ru based porphyrin complexes, with theircore structure resembling that of the Fe porphyrin core ofP450s, have been extensively studied as selective oxidationcatalysts for organic synthesis.45,62–68 Compared to other metal-loporphyrins, Ru based systems are of particular interest to ourstudy since they have demonstrated capabilities to utilizearomatic N-oxides as oxidants, while Fe, Mn, Co, Mo, and Osporphyrins do not perform well with these oxidants.46,47

Table 2 summarizes styrene epoxidation over the Ru(TMP)catalyst shown in Fig. 7 using different types of aromatic N-oxides as oxidants. The listed reaction time gives an approxi-mate measure of catalyst reactivity, with the more activeoxidants requiring shorter reaction times. Using 2,6-dichlor-opyridine N-oxide as the oxidant resulted in the shortest reac-tion time and 100% yield. Yields from 94 to 99% were obtainedwith 2-chloropyridine N-oxide, 2,6-dimethylpyridine N-oxide,2,6-dibromopyridine N-oxide, and 4-nitroquinoline N-oxide.Poor yields were obtained with 2-methylpyridine N-oxide and 2-methylquinoline N-oxide. Almost no reactivity was observed forpyridine N-oxide. These results clearly show the aromatic N-oxide substituent groups have a large effect on reactivity.

Table 2 Summary of styrene oxidation reactions over the Ru(TMP) catalwere conducted under argon at room temperature in benzene with styr

Entry Oxidant Time (h) Epoxide yield Ref.

1 2 100% 46

2 16 to 20 94% 46

3 16 to 20 95% 46

4 16 to 20 26% 46

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Table 3 briey summarizes different types of selectiveoxidation reactions that can be carried out using Ru basedporphyrin catalysts (see Fig. 6 and 7) with aromatic N-oxides asoxidant. Since our purpose was to illustrate the differentreaction types that can be catalyzed, for each reaction type welisted one example from the literature reporting a high reac-tion yield. Additional examples can be found in the citedreferences. Epoxidation of linear terminal alkenes containingallylic H atoms is especially challenging due to the highreactivity of the allylic H atoms. A linear terminal epoxide, 1,2-epoxyoctane, was obtained in 91–99% selectivities from theepoxidation of 1-octene with 2,6-dichloropyridine N-oxide (2,6-Cl2PyO) catalyzed by several Ru porphyrins includingRuTDCPP(MCM41),43 Ru(F20-TPP)(PEG),54 Ru(F20-TPP),75 Ru(4-Cl-TPP)(PEG),76 and Ru(4-Cl-TPP)(MPR).77 For theRuTDCPP(MCM41) system, a turnover frequency of 158 perhour was reported.43 Other systems such as the chiral Ru(tetra-anthracene-porphyrin) reported by Berkessel, et al.44 were alsoapplied for such epoxidation reaction, but less than 20% yieldswere achieved. Aromatic terminal alkenes (e.g., styrene) areone of the most tested olen families for the epoxidationreaction using the Ru porphyrin catalyticsystems.44,46,48,69,74,76,78–80 Greater than 97% yields were achievedfor styrene epoxidation by different research groups using 2,6-Cl2PyO as oxidant at room temperature.44,76 Higuchi, et al.46

reported a styrene oxide yield of 100% using the Ru(TMP)catalyst. Other non-terminal alkenes were also epoxidizedusing pyridine based oxidants and Ru porphyrin cata-lysts.44,46,69,75,81 Berkessel, et al.44 conducted dihydronaph-thalene epoxidation with 2,6-Cl2PyO using the Ru(tetra-anthracene-porphyrin) system and a selectivity of 89% wasachieved for the epoxide. Zhang, et al.69 performed a series ofnon-terminal alkene epoxidations using a dendritic ruthe-nium porphyrin plus 2,6-Cl2PyO catalytic system. Epoxideselectivities greater than 99% were achieved for cis-stilbene,

yst using different types of aromatic N-oxides as oxidants. All reactionsene/oxidant molar ratio ¼ 100/110

Entry Oxidant Time (h) Epoxide yield Ref.

5 16 to 20 Trace 46

6 6 98% 46

7 24 99% 74

8 24 46% 74


Fig. 7 Schematic structures of the Ru(F20-TPP)-PEG,54 the Ru(F20-TPP),71 the dendritic Ru(TPP),69 the Ru(TPP),46 and the Ru(TMP)70 catalysts.

Paper RSC Advances

cyclohexene, and cyclooctene. Ru based porphyrin catalystshave also been applied to oxidize other organic substratesusing aromatic N-oxide based oxidants. These substratesincluded different alkanes,46,48,54,55,75,79,82–84 alco-hols,46,48,55,74,79,84 arenes,75,85 amides,70 and ethers.71

As shown in Table 3, some chemicals steps that can becatalyzed by Ru porphyrin catalysts include: (a) O atom addi-tion, (b) C–H bond separation, (c) isomerization, (d) hydrolysisand hydration, and (d) N–C bond cleavage. Relative rates ofthese different chemical steps is strongly inuenced by reactionmedium composition. As shown in Table 3, addition of HBr orHCl facilitates C–H bond separation to form alcohols andoxidation of alcohols or ethers to ketones, while the addition offormic acid followed by the addition of K2CO3 facilitates alkenedihydroxylation.

Stereo-specic and site-specic epoxidation reactions can becatalyzed by Ru porphyrin catalysts.46,49,69,86–91 In 1996, Grosset al. reported for the rst time the enantioselective styreneepoxidation using a homochiral Ru porphyrin catalyst.86 Enan-tioselectivity of 57% was achieved using 2,6-Cl2PyO as theoxidant with benzene as the solvent.86 Berkessel and Frauenk-ron studied the asymmetric epoxidation of various unfunc-tionalized olens.88 Enantioselectivity of 77% with yield of 90%was achieved for the 1,2-dihydronaphtalene epoxidation using2,6-Cl2PyO as the oxidant.88 Higuchi and Hirobe46 and Ohtakeet al.90 studied the epoxidation of different conformations ofterpene in the acetate form. RuTMP(O)2 was utilized as the


catalyst in benzene solution with 2,6-Me2PyO as the oxidant.46,90

6,7-Double bonds were epoxidized selectively with site-selectivities in the range of 78–98%.46,90

Aerobic olen epoxidation was rst reported by Groves andQuinn, but the total number of turnovers achieved was low (i.e.,<50).92 Lai et al. studied enantioselective epoxidation of olensusing O2 as oxidant without any co-reductant, but the totalnumber of turnovers achieved was also low.89 Enantioselectivityof 73% was achieved for chiral alkene epoxidation under oxygenpressure of 8 atm.89 Both Groves and Quinn and Lai et al. usedruthenium porphyrin catalysts.89,92 A recent study provideda breakthrough in which the addition of NaHCO3 solutionfacilitated the tandem epoxidation–isomerization reactioncatalyzed by ruthenium porphyrin complexes to produce alde-hydes from olens using molecular oxygen as the terminaloxidant with approximately 1000 catalyst turnovers achieved.93

Asymmetric aerobic epoxidation of olens over a rutheniumsalen complex in the presence of light has been reported byTanaka et al.94

We now briey discuss some unwanted side reactions thatmay occur during Ru porphyrin catalyzed selective oxidationreactions. In the presence of an oxidant, alkene substrates mayirreversibly react with the porphyrin ring to deactivate thecatalyst via the formation of a metal–O–C–C–N–(metal)ring.135–137 To minimize oxidative attack of the porphyrin ring, itis preferable to use as mild of oxidant as feasible to perform thedesired selective oxidation reaction. For example, strong

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Table 3 Summary of different types of selective oxidation reactions which utilize 2,6-dichloropyridine oxide as the oxidant and Ru-basedporphyrin catalysts

Entry Reaction type Substrate Oxidant Catalysta Product Selectivity Ref.

1 Alkene epoxidation

1.1 eq.Ru(F20-TPP)-PEG 99% 54RuTDCPP(MCM41) 92% 43

1.1 eq.

Ru(TMP) 100% 46RuTDCPP(MCM41) 91% 43

1.1 eq.

Dendritic Ru(TPP) >99% 69RuTDCPP(MCM41) 91% 43

2 Alkane hydroxylation 2.0 eq. (plus HBr) Ru(TPP) 94% 46

3 Alkane oxidation 2.2 eq. (plus HCl) Ru(F20-TPP)-PEG 99% 54

4 Alcohol oxidation 1.1 eq. (plus HCl) Ru(TMP) 88% 46

5 Arene oxidation 2.0 eq. (plus HCl) Ru(TPP) 97% 46

6 Amide oxidation 2.5 eq. Ru(TMP) 89% 70

7 Ether oxidation 2.5 eq. Ru(F20-TPP) 90% 71

8 Alkene to ketone 1.03 eq. Ru(TDCPP)Cl2 87% 72

9Alkenedihydroxylation

1.5 eq.(plus HCOOH, K2CO3)

Ru(TDCPP)Cl2 98% 73

a The structure of the RuTDCPP(MCM41) catalyst is given in Fig. 6. Ru(TDCPP)Cl2 has a similar structure except without a tether and with axiallybound Cl atoms. The other catalyst structures are illustrated in Fig. 7.

RSC Advances Paper

oxidants like molecular O2 have been observed to produce lowtotal turnovers.89,92 For catalysts tethered via a Lewis adduct(such as the one studied in this work), catalyst leaching mightbe an appreciable problem. For the RuTDCPP(MCM41) catalystof Liu et al.43 shown in Fig. 6 and computationally modeled inthis work, 67% of the catalyst activity was retained aer 11 691turnovers (three runs).43 This loss in catalyst activity wasattributed to either catalyst leaching or other forms of deacti-vation.43 To avoid catalyst leaching, the catalyst should betethered via a covalent bond that cannot be easily broken. Yu

88196 | RSC Adv., 2016, 6, 88189–88215

et al. prepared Ru porphyrin catalysts covalently bound topolymer supports and observed no catalyst leaching for severalreuses of the catalyst achieving up to 3 � 104 turnovers.77 Forthe reaction type studied in this work (i.e., linear alkene epox-idation), the formation of unwanted by-products over the Ruporphyrin catalysts have not been described in detail in priorreports. One could suspect the formation of trace amounts ofaldehydes or ketones via combined oxidation–isomerization, orof glycols (in aqueous solutions) via combined oxidation–hydration.


Paper RSC Advances

A key consideration is that the catalyst price per mass ofchemical product produced should be low compared to themarket value per mass of chemical product produced. There-fore, it is important to maximize the total number of turnoversthat can be produced by the catalyst as well as to minimize thecatalyst replacement cost. Because ne chemicals have higherprice per mass than large-scale commodity chemicals, it shouldbe easier to make the catalyst price economically affordableproducing a ne chemical.

3.2 Computed chemical potential diagrams

Several key points must be fullled by the catalysts and theoxygen acceptor molecule in order to perform two-step propeneepoxidation by using only O2 as net co-reactant. For the O2

activation reaction (reaction (1)), the selected catalyst should beable to react with O2 molecules to form oxygenated complexes.This should produce a sufficiently weakened bond between twoadjacent O atoms so an oxygen acceptor molecule could easilyextract one of the oxygen atoms from the complex withoutpaying a large energy penalty for breaking the O–O bond. Aergiving out the rst oxygen atom, the catalyst must also be able totransfer the second oxygen atom (since there are two oxygenatoms in an O2 molecule) to another oxygen acceptor moleculewith a low energy barrier to produce another oxygen transferintermediate. For the propene epoxidation reaction (reaction(2)), the selected catalyst must be able to utilize the oxygentransfer intermediate from reaction (1) as oxidant to selectivelyoxidize a propene molecule with a relatively low activationbarrier. The selected oxygen transfer intermediate, therefore,should have a lower oxygen atom chemical potential than the O2

molecule so that it can be easily oxidized by O2. It should alsohave higher oxygen atom chemical potential than propyleneoxide in order to be used as an oxidant for propene epoxidation.

Chemical potential diagrams (Fig. 8–10) were computed tostudy the relative energies of different catalyst forms as a func-tion of oxygen atom chemical potential. In these gures, the x-axis represents the oxygen atom chemical potential. The oxygen

Fig. 8 Computed chemical potential diagram for DMZB catalyst. SingletThe right panel is a partial copy of the left panel for easy comparison. Theplanar triplet bisperoxo intermediate does not exist.


atom chemical potential of an O2 molecule is taken to be thereference state of 0 kcal mol�1 (referred to as O2 side). On the O2

side, complexes with higher relative energies tend to formcomplexes with lower relative energies by releasing or reactingwith O2 molecules. Accordingly, the computed oxygen atomchemical potential for Me2PyO is �7.24 kcal mol�1 (referred toas Me2PyO side) based on the reaction Me2Py + 1/2O2 /

Me2PyO. The computed oxygen atom chemical potential for POis �15.94 kcal mol�1 (referred to as PO side) based on thereaction propene + 1/2O2 / PO. On the Me2PyO or PO side,complexes with higher relative energies may react with Me2Py,propene, Me2PyO, or PO to produce complexes with lowerrelative energies. The y-axis represents the relative energy foreach intermediate. On the O2 side, the relative energy for eachstructure (A) was calculated according to the formula:

EA þ N

2EO2

� Ereference complex � Eadsorbed molecules:

On the Me2PyO side, the relative energy for each structure (A)was calculated according to the formula:

EA + N(EMe2PyO� EMe2Py

) � Ereference complex � Eadsorbed molecules.

On the PO side, the relative energy for each structure (A) wascalculated according to the formula:

EA + N(EPO � EP) � Ereference complex � Eadsorbed molecules

Here, N represents the difference of oxygen atom numberbetween structure (A) and the reference complex. The term“adsorbed molecules” refers to the isolated forms of Me2Py,Me2PyO, PO, or EO that are adsorbed to the catalyst in structureA. For the DMZB catalyst, the reference state is the triplet spirobisperoxo form of the catalyst (M$(O2)2

T). For the RuTDCPPcatalyst, the reference state is the triplet oxo form of the catalyst(RuTDCPP(O)T). E is the DFT based energy calculated for thecorresponding intermediate. Fig. 8–10 display results based on

and triplet forms are displayed in the left and center panel, respectively.spiro triplet bisperoxo intermediate (M$(O2)2

T) is the reference state. A

RSC Adv., 2016, 6, 88189–88215 | 88197

Fig. 9 Computed chemical potential diagram for DMZB catalyst with adsorbed 2,6-dimethylpyridine, 2,6-dimethylpyridineN-oxide, or ethyleneoxide molecule. Singlet and triplet forms are displayed in the left and center panel, respectively. The right panel is a partial copy of the left panelfor easy comparison. The spiro triplet bisperoxo intermediate (M$(O2)2

T) is the reference state.

RSC Advances Paper

ESCF. Tables S1–S3 (see ESI†) summarize the correspondingrelative energies based on ESCF, EZP, H, and G.

For convenience, we introduce a few notations. We use “M” torepresent the Zr(C6H4-1,6-(N(C6H3-20,60-(CH3)2)O)2)2 (aka DMZB)bare structure, “M0” to represent the Zr(C6H4-1,6-(N(C6H3-20,60-(CH(CH3)2)2)O)2)2 (aka DIZB) bare structure, “O” to represent oneoxo group, “(O2)” to represent one peroxo or weakly adsorbed O2

group, “(h2-O3)” and “(h3-O3)” to represent one h2- and one h3-ozone group, respectively. Subscripted numbers indicate thequantity of the corresponding groups, and the symbol “$” indi-cates a weakly adsorbed group. For example, the notation“M$(O2)2” represents a DMZB intermediate in which the Zr atomweakly adsorbs two O2 groups. Spin states are represented bya superscript: S (singlet), T (triplet), Q (quintet).

For both DMZB and RuTDCPP catalytic systems, energies forsinglet geometries are shown in the le panel of each chemicalpotential diagram, energies for triplet geometries are shown inthe middle panel, and the right panel is a partial copy of the lepanel to enable direct comparison of different spin state

Fig. 10 Computed chemical potential diagram for the RuTDCPP catalyrespectively. The right panel is a partial copy of the left panel for easy co

88198 | RSC Adv., 2016, 6, 88189–88215

energies. Fig. 8 and 9 present the energies for DMZB complexeswithout and with adsorbed molecules, respectively. For theRuTDCPP catalytic system, quintet states of the complexes werealso studied. The computed energies for quintet geometries(summarized in Table S3 of the ESI†) are higher than that oftheir corresponding singlet and triplet geometries. Therefore,all quintet conformations of the RuTDCPP catalyst are ther-modynamically unstable within the oxidation system and wouldtransform to either singlet or triplet conformations readily.

For the DMZB catalytic system with Me2Py and O2 as reac-tants, as shown in Fig. 8 and 9, the triplet oxo complex withadsorbed Me2Py molecule (M(O)$(Me2PyO)

T) is the preferredstructure among all catalyst forms across the majority of oxygenchemical potential range. M(O)$(Me2PyO)

T holds a lower rela-tive energy (�3.1 kcal mol�1 on O2 side and�25.3 kcal mol�1 onMe2PyO side) than its singlet conformation (8.8 kcal mol�1 onO2 side and �13.4 kcal mol�1 on Me2PyO side). On the singletpanel, the h3-ozone complex with adsorbed Me2Py molecule(M(h3-O3)$(Me2Py)

S) holds the highest relative energy (39.0 kcal

st. Singlet and triplet forms are displayed in the left and center panel,mparison. The oxo intermediate (RuTDCPP(O)T) is the reference state.


Paper RSC Advances

mol�1) on O2 side which indicates M(h3-O3)$(Me2Py)S will not be

stable within the catalytic system. It can generate lower energyM(h3-O3)

S by ejecting the adsorbed Me2Py molecule. On thetriplet panel, the bare complex (MT) possesses the highestrelative energy on the both O2 and Me2PyO sides. By reactingwith O2 molecule(s), MT could easily form oxygenatedcomplexes. On both singlet and triplet panels, major interme-diates of the catalyst that range from one adsorbed oxygen atomto six adsorbed oxygen atoms settle within a �40 kcal mol�1

window on the O2 side. This suggests the DMZB complex shouldwork efficiently for activating O2 molecules.

As shown in Fig. 10, the singlet RuTDCPP complex withadsorbed PO molecule (RuTDCPP(PO)S) is the preferred struc-ture among all intermediates across the majority of oxygenchemical potential range. On the triplet panel, the oxo form ofthe RuTDCPP complex (RuTDCPP(O)T) holds the lowest energyon both Me2PyO and PO sides. On the singlet panel, RuTDCP-P(O)S holds the highest relative energies on both Me2PyO andPO sides. On Me2PyO (PO) side, RuTDCPP(O)S can react withMe2Py (propene) molecule to generate the low-lyingRuTDCPP(Me2PyO)

S (RuTDCPP(PO)S) structure. On the tripletpanel, the bare RuTDCPP complex holds the highest relativeenergy on both the Me2PyO and PO sides and would adsorbeither a Me2PyO or PO molecule to form a corresponding low-lying conformation.

A few words are in order to clarify how the nature of the axialtether and adsorbed species effect the spin states that can beformed in Ru porphyrin catalysts. Systems having an oddnumber of electrons can form spin states having an oddnumber of unpaired electrons (i.e., doublet, quartet, etc.), whilesystems having an even number of electrons can form spinstates having an even number of unpaired electrons (i.e.,singlet, triplet, quintet, etc.). An uncharged system consisting ofa ruthenium atom in a porphyrin ring with no adsorbates ortether has an even number of electrons. The Ru porphyrincatalyst studied in this work contains a tether in which the lonepair of the tethering amine's N atom is bound to the Ru metalatom to form a Lewis adduct, and this tether contains an evennumber of electrons. Consequently, the bare form of this teth-ered catalyst as well as complexes with adsorbates containingan even number of electrons can form spin states having aneven number of unpaired electrons (i.e., singlet, triplet, quintet,etc.). In this work, the adsorbates of interest (e.g., oxo, Me2Py,Me2PyO, P, PO, etc.) contain an even number of electrons.

Spin magnetic moments of the triplet states of the DMZBcatalyst are listed in Tables S7 and S8 (ESI†) to quantify thedistribution of spin magnetization for the following groups ofatoms: (a) Zr metal atom, (b) strongly adsorbed O groups, (c)weakly adsorbed O groups, (d) N atoms in ligand 1, (e) N atomsin ligand 2, and (f) the remainder of the structure. For each ofthese atom groups, the spin magnetic moment was computedby summing the atomic spin moments (ASMs) for all atoms inthe group. ASMs were computed using the Density-DerivedElectrostatic and Chemical (DDEC6) method.95–100 In thetables, ligand 1 is arbitrarily designated as the ligand havinglarger ASMs for nitrogen atoms. The combined ASMs for allparts sum to 2.00, representing the two unpaired electrons. As


shown in Tables S7 and S8 (ESI†), except for the bare complex(MT), the Zr atoms in other structures have ASM magnitudes#0.05. In structures containing them, a large portion of the spindensity resides on weakly adsorbed peroxo or ozone groups.Ligand nitrogen atoms also hold a substantial fraction of thespin.

Spin magnetic moments of the triplet states of the RuTDCPPcatalyst are listed in Table S9 (ESI†) to quantify the distributionof spin magnetization for the following groups of atoms: (a) Rumetal atom, (b) N and O atoms in adsorbed group, (c) N atom inthe tethering group, (d) N atoms in the porphyrin ring, (e) theremainder of the structure. As shown in Table S9 (ESI†), exceptfor the bare complex (RuTDCPPT), the Ru atom in other struc-tures has an ASM magnitude #0.9. The oxo group in(RuTDCPP(O)T) has an ASM of �1. The tethering group andadsorbed Me2Py, Me2PyO, P, and PO molecules hold very littlespin. A substantial amount of spin resides in the porphyrin ringfor all of the complexes except the RuTDCPPT bare complex.

3.3 Reaction (1): 2,6-dimethylpyridine oxidation over DMZBcatalyst

3.3.1 Computed reaction cycles, energy proles, andcharge and bonding analyses. We rst investigated 2,6-dime-thylpyridine oxidation over the DIZB catalyst. The DIZB catalystwas rst introduced by us in a prior publication that focused onethylene epoxidation using molecular oxygen as oxidantwithout coreductant.36 Our computations predicted the DIZBcatalyst to be effective for direct ethylene epoxidation.36 Incontrast to ethylene, our DFT computations show the DIZBcatalyst has too much steric congestion near the metal center toaccommodate the larger size of the 2,6-dimethylpyridinemolecule. As shown in Fig. 11, an initial geometry guess con-sisting of 2,6-dimethylpyridine N-oxide adsorbed on the DIZBoxo peroxo complex leads to a nal geometry in which one of thebenzol ligands has been ejected from the catalyst. This indicatesthat the steric congestion near the metal center is too great toaccommodate both benzol ligands, the peroxo group, the oxogroup, and an adsorbed 2,6-dimethylpyridine N-oxide molecule.Based on these results, we decided to try the less stericallycongested DMZB catalyst that has methyl instead of isopropylsubstituent groups on the benzol ligands. As shown in Fig. 11,the DMZB catalyst can accommodate both benzol ligands,a peroxo group, an oxo group, and an adsorbed 2,6-dime-thylpyridine N-oxide molecule. This is important, becausecatalytic cycles passing through a peroxo h3-ozone complexinvolve a transition state in which a 2,6-dimethylpyridinemolecule extracts an oxygen atom from a bisperoxo complex toform 2,6-dimethylpyridine N-oxide plus the oxo peroxocomplex. For both the DIZB and DMZB catalysts, we computedreaction cycles passing through the triplet peroxo h3-ozonecomplex (1st cycle) and triplet h3-ozone complex (2nd cycle). Thecomputed ESCF cycle activation barriers (in kcal mol�1) were39.9 (DIZB, 1st cycle, Fig. S5 of ESI†), 40.6 (DIZB, 2nd cycle,Fig. S6 of ESI†), 32.0 (DMZB, 1st cycle, Fig. 12), and 33.9 (DMZB,2nd cycle, Fig. 13). These lower cycle activation barriers show theDMZB catalyst is better for this reaction. Consequently, all of

RSC Adv., 2016, 6, 88189–88215 | 88199

Fig. 11 Computational tests to determine the stability of benzol ligand desorption in the presence of 2,6-dimethylpyridine N-oxide adsorbed tothe triplet oxo peroxo complex. For the DMZB catalyst (top), when the input geometry guess contained desorbed benzol ligand, the calculationconverged to an optimized geometry with adsorbed benzol ligand. This shows benzol ligand desorption is unfavorable for the DMZB catalyst. Forthe DIZB catalyst (bottom), when the input geometry guess contained adsorbed benzol ligand, the calculation converged to an optimizedgeometry with desorbed benzol ligand. This shows benzol ligand desorption is favorable for the DIZB catalyst in the presence of 2,6-dime-thylpyridine N-oxide adsorbed to the triplet oxo peroxo complex.

RSC Advances Paper

the remaining results in this section focus on the DMZBcatalyst.

We now describe in detail the 1st and 2nd catalytic cyclespassing through M(h2-O3)$(O2)

T and M(h2-O3)T intermediates,

respectively. In Fig. 12 (1st cycle) and Fig. 13 (2nd cycle), thecomputed catalytic cycle is shown on the top panel and thecorresponding energy prole is displayed on the bottom panel.For both reaction cycle diagrams, the SCF energies for eachreaction step are shown in kcal mol�1. Complete geometries forall calculated intermediates and transition states are providedin ESI.† “CP” refers to a spin state crossing point. (We used thetriplet–quintet crossing point to estimate the activation barrierfor O2 adsorption to M(O)T to form M(h2-O3)

T, because there isno regular transition state for this reaction. See Fig. S1 of ESI†for details.)

The overall catalytic cycle (aka ‘master cycle’) may becomprised of several alternate catalytic cycles (aka ‘juniorcycles’). For each junior cycle, the cycle activation barrier isdened as the largest multistep energy barrier along the

88200 | RSC Adv., 2016, 6, 88189–88215

forward direction of the catalytic cycle including the catalystresting state. The energetic span is then dened as theminimum of the cycle activation barriers among those juniorcycles producing the target net reaction. As reviewed by Kozuchand Shaik,101 the energetic span is a kinetic assessment ofcomputed catalytic cycles. The energetic span equals the energydifference between the turnover-determining intermediate(TDI) and the turnover-determining transition state (TDTS)along the forward direction of a catalytic cycle. The TDI andTDTS were determined independently for the SCF energy (ESCF),zero-point energy (EZP), enthalpy (H), and Gibbs free energy (G).In this section, we discuss the ESCF results. The EZP, H, and Genergetic spans are discussed in Section 3.4.

As shown in Fig. 12, the rst reaction cycle for Me2Pyoxidation by DMZB catalyst involves MO$(O2)

T, M(h2-O3)$(O2)T,

M(h3-O3)$(O2)T, and M$(O2)2

T catalyst forms. The cycle startswith MO$(O2)

T. By adsorbing one O2 molecule, MO$(O2)T turns

into M(h2-O3)$(O2)T. The activation barrier associated with this

step is 15.8 kcal mol�1 and the computed net reaction energy is


Fig. 12 (upper panel) Triplet peroxo h3-ozone intermediate (M(h3-O3)$(O2)T) involved catalytic cycle (the 1st cycle) for 2,6-dimethypyridine

oxidation over the DZMB catalyst. For each step, the energies are labeled in kcal mol�1. (lower panel) SCF energy profile for this catalytic cycle.M$(O2)2

T is the reference state. Activation barriers for major reaction steps and the whole catalytic cycle are presented in kcal mol�1.

This journal is © The Royal Society of Chemistry 2016 RSC Adv., 2016, 6, 88189–88215 | 88201

Paper RSC Advances

Fig. 13 (upper panel) Triplet h3-ozone intermediate (M(h3-O3)T) involved catalytic cycle (the 2nd cycle) for 2,6-dimethylpyridine oxidation over

the DMZB catalyst. For each step, the energies are labeled in kcal mol�1. (lower panel) SCF energy profile for this catalytic cycle. M$(O2)2T is the

reference state. Activation barriers for major reaction steps and the whole catalytic cycle are presented in kcal mol�1.

RSC Advances Paper

15.8 � 7.7 ¼ 8.1 kcal mol�1. Subsequently, M(h2-O3)$(O2)T

transforms into M(h3-O3)$(O2)T with an activation barrier of 4.4

kcal mol�1 and a net reaction energy of 4.4 � 1.5 ¼ 2.9 kcalmol�1. In the third step, M(h3-O3)$(O2)

T reacts with a Me2Py

88202 | RSC Adv., 2016, 6, 88189–88215

molecule to form Me2PyO and M$(O2)2T. In the nal step,

M$(O2)2T reacts with a Me2Py molecule to form Me2PyO and

regenerate MO$(O2)T to nish one whole catalytic cycle. The

activation barriers for the third and nal reaction steps are 12.8


Paper RSC Advances

and 25.4 kcal mol�1, respectively. Here, we regard MO$Me2PyOT

(the resting state within our Me2Py oxidation system as dis-cussed in the previous section) as a reversible state of thecatalyst that goes through MOT intermediate and forms anequilibrium with MO$(O2)

T by adsorbing/desorbing Me2PyOand O2 molecules. Specically, as shown in Fig. 12, by over-coming an energy barrier of 8.4 kcal mol�1, Me2PyO desorbsfrom MO$Me2PyO

T and generates MOT. Then, MOT adsorbs anO2 molecule to form MO$(O2)

T by overcoming an activationbarrier of 7.7 kcal mol�1. Moreover, MO$(O2)

T can desorb an O2

and adsorb a Me2PyO molecule to produce MO$Me2PyOT. The

activation barrier associated with O2 desorption is 8.0 kcalmol�1. The M(h2-O3)(h

3-O3)T intermediate is also included

through a detour. With an activation barrier of 13.8 kcal mol�1,M(h2-O3)(h

3-O3)T donates an O atom to the Me2Py molecule to

produce M(h2-O3)$(O2)T and join the cycle.

Here we describe how the SCF energetic span was calculatedfor this cycle. All other energetic spans (i.e., EZP,H, and G based)were computed in a similar manner with TDI and TDTS

Fig. 14 Charge transfer and bonding analysis for the 1st cycle for 2,6-dimare labeled with DDEC6 computed NACs and bond orders, respectivelparentheses. Benzol ligands and H atoms in 2,6-dimethylpyridine molecuhere. NACs are presented in black with bond orders in red and bond len


determined independently for each energy type. As shown in thechemical potential diagram in Fig. 8 and 9, MO$Me2PyO

T holdsthe lowest relative energy among all singlet and triplet inter-mediates across the majority of oxygen atom chemical potentialrange. It is expected that the highest forward energy barrier willbe achieved with this complex included in the complete forwardreaction cycle. The forward barrier for this cycle is maximized at8.4 + 7.7 � 8.0 + 15.8 � 7.7 + 4.4 � 1.5 + 12.8 ¼ 32.0 kcal mol�1

with MO$Me2PyOT as TDI and TS3 as TDTS. Since the other

cycles in the DMZB system exhibit higher cycle activationbarriers (detailed description will be given in the followingparagraphs), 32.0 kcal mol�1 is also the overall SCF energeticspan for the DMZB catalyzed Me2Py oxidation process.

As shown in Fig. 13, the second cycle involves four keyintermediates which are MOT, M(h2-O3)

T, M(h3-O3)T, and

M(O2)T catalyst forms. The cycle starts with MOT. One O2

molecule adds to MOT to formM(h2-O3)T. The activation barrier

associated with this reaction step, as determined by the triplet–quintet constrained optimization crossing-point, is 24.1 kcal

ethylpyridine oxidation. Atoms and chemical bonds in all intermediatesy. Bond lengths (A) from the DFT-optimized geometries are given inle were included in all calculations but for conciseness are not showngths in blue.

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RSC Advances Paper

mol�1. In the second reaction step, M(h2-O3)T transforms into

M(h3-O3)T with an activation barrier of 3.9 kcal mol�1. Then,

M(h3-O3)T reacts with Me2Py to formMe2PyO and M(O2)

T. In thenal step, M(O2)

T reacts with a Me2Py molecule to formMe2PyOand regenerate MOT to nish one whole catalytic cycle. Theactivation barriers for the third and nal reaction steps are 13.8and 19.8 kcal mol�1, respectively. The resting state of thecatalyst, MO$Me2PyO

T, is also included in this cycle as anequilibrium state. The forward barrier for this catalytic cycle ismaximized at 33.9 kcal mol�1 with MO$Me2PyO

T as TDI and TS8as TDTS.

Two other potential catalytic cycles were also considered butshown to be unimportant by our calculations. These two cyclesgo through two different h3-ozone intermediates: (a) triplet oxoh3-ozone intermediate (MO(h3-O3)

T), and (b) triplet h3-ozoneintermediate with adsorbed Me2PyO molecule (M(h3-O3)$Me2-PyOT). For the cycle that goes through MO(h3-O3)

T, four reactionsteps are involved: (1) MO(h3-O3)

T + Me2Py / MO$(O2)T +

Me2PyO, (2) MO$(O2)T + Me2Py/M(O)2

T + Me2PyO, (3) M(O)2T +

O2 /MO(h2-O3)T, and (4) MO(h2-O3)

T /MO(h3-O3)T. Based on

our calculations, the activation barrier for reaction step 2 is 36.7kcal mol�1, and therefore the energetic span of the whole cyclewould never be lower than 36.7 kcal mol�1. Since the computedcycle energetic spans of the rst and second cycles (32.0 and 33.9kcal mol�1, respectively) are lower than 36.7 kcalmol�1, the cyclethat goes through MO(h3-O3)

T would be energetically unfavor-able. For a reaction cycle going through M(h3-O3)$Me2PyO

T, thefour hypothetical reaction steps would be: (1) M(h3-O3)$Me2PyO

T

+ Me2Py / M(O2)$Me2PyOT + Me2PyO, (2) M(O2)$Me2PyO

T +Me2Py / MO$Me2PyO

T + Me2PyO, (3) MO$Me2PyOT + O2 /

M(h2-O3)$Me2PyOT, and (4) M(h2-O3)$Me2PyO

T / M(h3-O3)$Me2PyO

T. Based on our geometry optimization, the h3-ozonegroup in the M(h3-O3)$Me2PyO

T intermediate is stericallyhindered and unable to react with Me2Py. The optimizedgeometry of M(h3-O3)$Me2PyO

T is provided in the ESI.† Withinthe M(h3-O3)$Me2PyO

T complex, the h3-ozone group and theMe2PyO molecule are adsorbed on the opposite sides of the Zrmetal. Since the adsorbedMe2PyOmolecule requiresmore space

Table 4 DDEC6 results for the Zr sum of bond orders (Zr SBO), the toorganic ligands (total Zr–O(benzol ligands) BOs), and the total charge resifor each intermediate in the rate-determining cycle (1st cycle)

Intermediates Zr SBOa Total Zr–O

M(h3-O3)$(O2)T 2.6 1.2

M$(O2)2T 2.7 1.7

MO$(O2)T 2.8 1.2

M(h2-O3)$(O2)T 2.6 1.2

MOT 2.9 1.7MO$Me2PyO

T 2.8 1.4TS1 2.6 1.2TS2 2.6 1.1TS3 2.7 1.0TS4 2.8 1.1TS5 2.8 1.2

a The sum of bond orders between Zr and all other atoms in the system. b

ligands. c The total net charge of the benzol ligands; this is the total for b

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than the ozone group, the benzol ligands of the DMZB complexare forced to tilt towards the h3-ozone group and block thepathway through which the oxygen acceptor comes in to reactwith the ozone group. Consequently, the coming Me2Py mole-cule cannot react with the ozone group unless the weaklyadsorbedMe2PyOmolecule desorbs from the catalyst to generateM(h3-O3). The M(h3-O3) intermediate will then donate one Oatom to the Me2Py molecule to produce Me2PyO and the M(O2)intermediate following the 2nd cycle (Fig. 13).

To better understand the charge transfer and bondingproperties of the DMZB catalyst during selective oxidation, netatomic charges (NACs) and bond orders were computed usingthe DDEC6 method.98–100 The computed NACs, ASMs, and bondorders for every atom in various ground and transition statestructures are given in the ESI.† The Density Derived Electro-static and Chemical (DDEC) method represents NACs, ASMs,and bond orders as functionals of the electron and spin densitydistributions with no explicit basis set dependence.95,96,98,99 TheDDEC NACs are simultaneously optimized to reproduce thechemical states of atoms in a material and the electrostaticpotential surrounding the material with excellent conforma-tional transferability.95,96,98,99 This makes DDEC NACs, ASMs,and bond orders well-suited for studying charge transfer, spintransfer, and bond order changes during chemical reactions.

Fig. 14 and Table 4 give key results. Fig. 14 lists NACs, bondorders, and bond lengths for intermediates in the rate-determining cycle (i.e., 1st cycle). For the situation whereatoms or bonds are in a same (or very similar) chemical envi-ronment, only one of these atoms or bonds was labeled to avoidredundant information. For example, for a non-reacting peroxogroup, only one O atom was labeled with charge and only oneZr–O bond along with the O–O bond were labeled with bondorders and bond lengths. Under such circumstance, all labeledvalues are the average values between the chemically similaratoms or bonds. Table 4 lists the sum of bond orders (SBO) forthe Zr atom in each intermediate, the summed bond orderbetween the Zr atom and the four O atoms of the benzol ligands,and the total net charge of the benzol ligands.

tal bond orders between Zr and four oxygen atoms of two bidentateding on the two bidentate organic ligands (total benzol ligands charges)

(benzol ligands) BOsb Total benzol ligands chargesc

�0.4�1.1�0.5�0.4�1.2�1.3�0.4�0.5�0.5�0.5�0.5

The sum of bond orders between Zr and the four O atoms of the benzoloth ligands not per ligand.


Paper RSC Advances

We now summarize key results. (1) The nearly constant NACfor Zr (2.1 to 2.2) across ground and transition states indicatesthe Zr atom serves as an electron transfer bridge between theoxygenated group(s) and the bidentate redox ligands. (2) The ZrSBO of �2.7 rather than 4 (there are four valence electrons ina Zr atom) indicates polarized covalent bonding in which someelectrons have transferred from the Zr atom to the ligands andadsorbate groups. This agrees with the positive NACs for Zr. (3)Zr–O bond orders (bond lengths) are higher (shorter) forstrongly adsorbed O atoms than for weakly adsorbed O atoms.(4) The NACs for strongly adsorbed O atoms were more negativethan those for weakly adsorbed O atoms. A total of 0.6 to 1.3electrons were transferred to each oxo, peroxo, or ozone group.(5) Due to the low-lying unoccupied p* (anti-bonding) orbitalsof O2 and O3, the transfer of electrons into the O2 and O3 groupsweakens their O–O bonds. Accordingly, O–O bonds with morenegative O atoms exhibit longer bond lengths and lower bondorders. This weakening of the O–O bonds makes it easier for theMe2Py molecule to remove one of the O atoms. (6) The inter-action between Zr and the m-O is weak even in an h3-ozonegroup with the computed bond order of�0.2. (7) The peroxo h2-ozone to peroxo h3-ozone reaction is mainly a geometric rear-rangement. No obvious charge transfer is observed for theozone transformation transition state (TS2). (8) In both Me2PyOformation transition states (i.e., TS3 and TS4), a weak bond (withbond order of 0.3 to 0.4) is formed between the N atom in theMe2Py molecule and the reactive O atom in the peroxo or ozonegroup. At the same time, the O–O bond of the reactive O atom inthe oxygenated group is weakened with increased bond lengthand decreased bond order compared to the reactant state. Thesecomputed charges and bond orders for Me2Py oxidation overthe DMZB catalyst followed similar trends to those for ethylene

Table 5 Summary of computed adsorption energies of Me2PyO and Me2Relative energies with respect MOT using molecular O2 as the oxygen re

Adsorbed state AdsorbentAdsorbate(resided molecule)

Adsorpt

ESCF

MO$Me2PyOS MOS Me2PyO �8.4

MO$Me2PyOT MOT Me2PyO �8.4

M(O2)$Me2PyOS M(O2)

S Me2PyO �6.2M(O2)$Me2PyO

T M(O2)T Me2PyO �6.5

M(h2-O3)$Me2PyOS M(h2-O3)

S Me2PyO �3.8M(h2-O3)$Me2PyO

T M(h2-O3)T Me2PyO �2.6

M(h3-O3)$Me2PyOS M(h3-O3)

S Me2PyO �0.8M(h3-O3)$Me2PyO

T M(h3-O3)T Me2PyO �3.8

MO$Me2PyS MOS Me2Py �2.1

MO$Me2PyT MOT Me2Py

a

M(O2)$Me2PyS M(O2)

S Me2Py 5.7M(O2)$Me2Py

T M(O2)T Me2Py 4.0

M(h2-O3)$Me2PyS M(h2-O3)

S Me2Py 7.1M(h2-O3)$Me2Py

T M(h2-O3)T Me2Py 5.3

M(h3-O3)$Me2PyS M(h3-O3)

S Me2Py 11.7M(h3-O3)$Me2Py

T M(h3-O3)T Me2Py 4.7

a Me2Py does not adsorb onto MOT; the Me2Py molecule was spontaneousllike initial guess.


epoxidation over the DIZB catalyst we reported in a previouspaper.36

3.3.2 Adsorption energies. As shown in the second reactioncycle (Fig. 13), several intermediates within the DMZB catalyticsystem have only one oxygenated group on one side of the Zratom and leave the other side open. Small molecules in thesystem including Me2PyO and Me2Py could potentially adsorbonto these catalyst conformations and shi their energy level.To understand the adsorption effects in our system, wecomputed adsorption energies of Me2PyO and Me2Py onto oxo,peroxo, and ozone forms of the DMZB catalyst, as shown inTable 5. The computed enthalpies of Me2PyO adsorption ontothese catalyst forms ranged from �6.4 kcal mol�1 (mildlyexothermic) for the singlet and triplet oxo complexes to 0.1 kcalmol�1 (slightly endothermic) for the singlet h3-ozone complex.The computed enthalpies of Me2Py adsorption onto thesecatalyst forms ranged from 0.2 kcal mol�1 (slightly endo-thermic) for the singlet oxo complex to 13.1 kcal mol�1 (endo-thermic) for the singlet h3-ozone complex. For Me2Pyadsorption onto the triplet oxo complex (MOT), a stable adsor-bed geometry could not be achieved with several trial calcula-tions. Input geometries were constructed with a Me2Pymolecule adsorbed on one side of the Zr metal and the oxogroup on the other side, but the Me2Py molecule was sponta-neously pushed out during geometry optimization.

Because adsorption lowers the molecular entropy, thecomputed adsorption was endergonic. The computed adsorp-tion free energies ranged from 5.8 to 16.9 kcal mol�1 for Me2PyOadsorption and 13.6 to 29.0 kcal mol�1 for Me2Py adsorption.

Relative energies with respect to the MOT complex werecomputed to determine the low energy state for each energy type(i.e., ESCF, EZP, H, and G). The MOT complex is the most stable

Py adsorbed onto oxo, peroxo, and ozone forms of the DMZB catalyst.servoir are given for comparison

ion E (kcal mol�1)Relative E compare to MOT

(kcal mol�1)

EZP H G ESCF EZP H G

�7.0 �6.4 5.8 3.5 5.2 5.7 19.0�7.1 �6.4 6.1 �8.4 �7.1 �6.4 6.1�4.8 �4.7 9.5 5.7 8.4 8.4 28.6�5.0 �4.4 8.4 �4.9 �2.5 �2.3 17.0�1.6 �1.9 14.2 13.8 18.3 17.6 45.7�1.1 �1.2 13.5 5.4 9.0 8.6 34.30.6 0.1 16.9 21.2 24.6 23.8 52.0�2.1 �2.4 13.9 7.9 11.2 10.8 37.0�0.3 0.2 13.6 9.8 11.9 12.3 26.7a a a a a a a

7.8 7.5 23.9 17.5 20.9 20.6 43.16.1 6.5 20.8 5.6 8.6 8.5 29.58.7 8.3 24.8 24.8 28.7 27.8 56.36.9 6.9 22.2 13.3 17.0 16.7 43.013.2 13.1 29.0 33.6 37.2 36.8 64.26.7 6.5 22.9 16.5 20.0 19.7 46.0

y ejected during geometry optimization when started from a MO$Me2PyT

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RSC Advances Paper

structure within the DMZB catalytic system without any adsor-bate molecule. When Me2PyO and Me2Py adsorbate moleculesare considered, MO$Me2PyO

T holds the lowest ESCF, EZP, and Hwhile MOT holds the lowest G.

3.3.3 Other oxygen acceptor molecules. Different oxygenacceptor molecules were tested to determine the most suitablecandidate for our two-step process. Energetic spans foroxidizing these oxygen acceptor molecules over the DIZB cata-lyst were computed. The tested molecules included: 2,6-dichloropyridine (2,6-Cl2Py), 2-chrolopyridine (2-ClPy), pyridine(Py), 4-nitroquinoline (4-NQ), and 2,6-dimethylpyridine (2,6-Me2Py). (Another name for 2,6-dimethylpyridine is 2,6-lutidine.)Table 6 lists the computed energetic spans for oxidizing thesemolecules. The corresponding SCF reaction cycles and energyproles are given in Section 1.5 of the ESI.†

The last column in Table 6 lists the B3LYP/LANL2DZcomputed SCF reaction energy per oxygen acceptor molecule.This is the SCF oxygen chemical potential of the substitutedpyridine-based intermediate oxidant. In order for both reaction(1) (forming intermediate oxidant) and reaction (2) (propeneoxidation) to be thermodynamically favored, the oxygen chem-ical potential of the intermediate oxidant should lay betweenthat of the O2 molecule (0.0 kcal mol�1) and that of thepropylene oxide molecule. The oxygen chemical potential of POis �15.9 (B3LYP/LANL2DZ SCF energy34) and �27.5 (experi-mental enthalpy102,103) kcal mol�1. The oxygen chemical poten-tial of PyO is �4.0 (B3LYP/LANL2DZ SCF energy) and �12.6(experimental enthalpy104,105) kcal mol�1. Although 2,6-Cl2PyO is

Table 6 Summary of computed energetic spans (kcal mol�1) for the oxidover the DIZB catalyst. The TDIs and TDTSs are also listed. Computed encomparison. The last column (DESCF) gives the B3LYP/LANL2DZ computethe SCF oxygen atom chemical potential of the substituted pyridine-bas

Reactant Catalyst ESCF EZP

Energetic span2,6-Cl2Py DIZB 53.1 54.92-ClPy DIZB 39.0 42.2Py DIZB 34.1 35.34-NQ DIZB 22.7 25.2Me2Py DIZB 39.9 41.9Me2Py DMZB 32.0 34.5

Turn-over determining intermediate (TDI)2,6-Cl2Py DIZB M0O$Cl2PyO

T M0O$2-ClPy DIZB M0O$ClPyOT M0O$Py DIZB M0O$PyOT M0O$4-NQ DIZB M0O$(4-NQO)T M0O$Me2Py DIZB M0O$Me2PyO

T M0O$Me2Py DMZB MO$Me2PyO

T MO$M

Turn-over determining transition state (TDTS)2,6-Cl2Py DIZB TS29 TS312-ClPy DIZB TS40 TS40Py DIZB TS34 TS344-NQ DIZB TS36 TS36Me2Py DIZB TS22 TS26Me2Py DMZB TS3 TS3

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a good oxidant candidate for the RuTDCPP catalytic system, itholds a B3LYP/LANL2DZ SCF computed oxygen chemicalpotential 10.6 kcal mol�1 higher than that of the O2 molecule.Accordingly, oxidizing 2,6-Cl2Py using O2 molecule is thermo-dynamically unfavorable. Me2PyO exhibits a desired oxygenchemical potential of �7.4 kcal mol�1 (B3LYP/LANL2DZ SCF)that sits between that of O2 and PO. Accordingly, Me2Py isa good candidate for the oxygen acceptor molecule.

Among all of the aromatic N-oxides we tested, 2,6-Cl2PyO isthe most commonly used oxidant in Ru porphyrin oxidationsystems as demonstrated in Table 3. Compared to an unsub-stituted pyridine molecule, the two Cl atoms in 2,6-Cl2Pyprovide an important steric effect that prevents 2,6-Cl2Py fromadsorbing too strongly onto the RuTDCPP bare complex. A moredetailed discussion regarding the RuTDCPP adsorption ener-gies will be given in the next section. As shown in Table 6, a SCFenergetic span of 53.1 kcal mol�1 is achieved for 2,6-Cl2Pyoxidation using O2 and the DIZB catalyst. Because this energeticspan is large, 2,6-Cl2Py is not a suitable oxygen acceptor mole-cule for the DIZB oxidation system. The 2-ClPy moleculeexhibits the similar problem. 2-ClPy has a B3LYP/LANLDZ SCFcomputed oxygen chemical potential 3.7 kcal mol�1 higher thanO2, and its computed SCF energetic span for oxidation bymolecular O2 over DIZB is 39.0 kcal mol�1.

The two methyl groups in Me2Py offer a similar steric effectas the Cl atoms in 2,6-Cl2Py. When Me2PyO was applied asoxidant for styrene epoxidation over the Ru(TMP) catalyst,a yield of 95% was achieved compared to 100% yield when 2,6-

ation of different oxygen acceptors using molecular O2 as the oxidantergetic spans for Me2Py oxidation over the DMZB catalyst is given ford energy of reaction (kcal mol�1) per oxygen acceptor molecule; this ised intermediate oxidant

H G DESCF

53.5 79.0 10.640.6 56.9 3.734.0 51.0 �4.024.6 52.7 �7.240.8 59.2 �7.433.3 54.2 �7.4

Cl2PyOT M0O$Cl2PyO

T M0OT

ClPyOT M0O$ClPyOT M0OT

PyOT M0O$PyOT M0OT

(4-NQO)T M0OT M0OT

Me2PyOT M0O$Me2PyO

T M0OT

e2PyOT MO$Me2PyO

T MOT

TS31 TS31TS40 TS42TS34 TS34TS36 TS38TS26 TS26TS3 TS8


Paper RSC Advances

Cl2PyO was applied as the oxidant.46 We computed a SCFenergetic span of 39.9 kcal mol�1 for Me2Py oxidation over theDIZB catalyst. This value is higher than our computed SCFenergetic span of 32.0 kcal mol�1 for Me2Py oxidation over theDMZB catalyst. As discussed in Section 3.3.1 and Fig. 11 above,high steric congestion in the DIZB catalyst makes it difficult forMe2Py to access the active site.

We also considered 4-nitroquinoline N-oxide (4-NQO).Higuchi et al.46 applied 4-NQO for styrene epoxidation using theRu(TMP) catalyst and achieved a yield of 99% towards styreneoxide. We computed a SCF energetic span of only 22.7 kcalmol�1 for 4-NQ oxidation over the DIZB catalyst. However, 4-NQO is a highly carcinogenic compound that was oen appliedfor inducing cancer in animal models for drug develop-ment.106,107 Even though 4-NQ works great as an oxygen acceptor

Fig. 15 (upper panel) Catalytic cycle for the propene epoxidationusing Me2PyO as the oxidant over the RuTDCPP catalyst. For eachstep, the energies are labeled in kcal mol�1 (lower panel) SCF energyprofile for this catalytic cycle. RuTDCPPS is the reference state. Acti-vation barriers for major reaction steps and the whole catalytic cycleare presented in kcal mol�1.


molecule, its high toxicity prevents its potential commercialapplications.

3.4 Reaction (2): propene epoxidation over the RuTDCPPcatalyst

Fig. 15 shows our reaction cycle for propene epoxidation overthe RuTDCPP catalyst. The energies reported in Fig. 15 are ESCFvalues; the corresponding EZP, H, and G values are listed inTable S5 of the ESI.† As shown in the chemical potentialdiagram (Fig. 10), RuTDCPP(PO)S is the resting state of theRuTDCPP catalyst. The cycle starts with RuTDCPPS. OneMe2PyO molecule adds to RuTDCPPS to form RuTDCPP(Me2-PyO)S. In the second reaction step, RuTDCPP(Me2PyO)

S ejectsone Me2Py molecule to form RuTDCPP(O)T. The computed SCFactivation barrier for this step is 28.7 kcal mol�1 based on theestimated energy of the singlet–triplet crossing-point CP2. Inthe third step, a RuTDCPP(O)T complex reacts with one propenemolecule and generates RuTDCPP(PO*)T. The computed acti-vation barrier is 17.9 kcal mol�1. The structure ofRuTDCPP(PO*)T is marked with a *, because the oxygen atom inthe adsorbed PO* is displaced such that it is bonded betweenthe Ru metal and the center carbon. Our computations revealRuTDCPP(PO*)T is a radical intermediate with DDEC6 ASM of0.92 (i.e., approximately one unpaired electron) on the CH3–

C(O)H–CH2 carbon atom, 0.38 on the CH3–C(O)H–CH2 oxygenatom, 0.65 on the Ru atom, and negligible ASMs on all otheratoms. In the next step, the oxygen atom shis to bond betweenthe two carbon atoms. Specically, RuTDCPP(PO*)T transformsthrough a triplet–singlet crossing-point (CP3) to generate theRuTDCPP(PO)S intermediate. In the nal step, RuTDCPP(PO)S

ejects PO to regenerate RuTDCPPS to nish one whole catalyticcycle. The forward barrier is maximized at 20.4 � 15.7 + 28.7 ¼33.4 kcal mol�1 with RuTDCPP(PO)S as TDI and CP2 as TDTS.

Table 7 summarizes the energetic spans for our two-steppropene epoxidation process. Energetic spans given here arecomputed based on the electronic energy (ESCF), the zero-pointenergy (EZP), the enthalpy (H), and the Gibbs free energy (G).Among all four energetic spans (ESCF, EZP, H and G), theenthalpic energetic span (H) provides an estimate of theapparent activation energy for the entire catalytic cycle. Foroxidation of Me2Py to Me2PyO over the DMZB catalyst, anoverall enthalpic energetic span of 33.3 kcal mol�1 is achieved.For propene epoxidation using the RuTDCPP catalyst andMe2PyO as the oxidant, an overall enthalpic energetic span of31.6 kcal mol�1 is achieved. Although an enthalpic energeticspan over 30 kcal mol�1 is not ideal, the epoxidation reactionwith this energy barrier could still achieve a decent reaction rateby using higher than ambient temperature in the reactor.

Table 7 Computed energetic spans (kcal mol�1) for the Me2Pyoxidation reaction and the propene epoxidation reaction

Reactants Products Catalyst ESCF EZP H G

Me2Py, O2 Me2PyO DMZB 32.0 34.5 33.3 54.2P, Me2PyO PO, Me2Py RuTDCPP 33.4 31.3 31.6 32.0

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Table 8 Summary of computed adsorption energies for Me2Py, Me2PyO, Py, PyO, P, or PO molecule adsorbed onto the RuTDCPP catalyst

Adsorbed state Adsorbent Adsorbate

Adsorption E (kcal mol�1)

ESCF EZP H G

RuTDCPP(Me2Py)T RuTDCPPT Me2Py �2.0 0.1 �0.5 17.1

RuTDCPP(Me2Py)S RuTDCPPS Me2Py �1.5 0.2 0.1 15.6

RuTDCPP(Me2PyO)T RuTDCPPT Me2PyO �19.9 �18.4 �18.5 �3.1

RuTDCPP(Me2PyO)S RuTDCPPS Me2PyO �15.7 �15.0 �14.5 �1.1

RuTDCPP(Py)T RuTDCPPT Py �30.5 �29.0 �29.1 �15.0RuTDCPP(Py)S RuTDCPPS Py �31.5 �30.1 �30.0 �16.5RuTDCPP(PyO)T RuTDCPPT PyO �13.4 �12.7 �12.5 1.2RuTDCPP(PyO)S RuTDCPPS PyO �21.6 �21.0 �20.5 �9.1RuTDCPP(P)T RuTDCPPT P �9.2 �7.2 �7.7 6.7RuTDCPP(P)S RuTDCPPS P �11.8 �10.2 �10.4 3.2RuTDCPP(PO)T RuTDCPPT PO �9.7 �8.7 �8.6 3.9RuTDCPP(PO)S RuTDCPPS PO �20.4 �19.2 �19.1 �6.4

RSC Advances Paper

Table 8 summarizes computed adsorption energies forMe2Py, Me2PyO, Py, PyO, P, and PO molecules adsorbed ontothe singlet and triplet bare RuTDCPP complexes. BecauseRuTDCPPT has a much higher relative energy than RuTDCPPS

(see Fig. 10), adsorption energies onto RuTDCPPS are moreimportant to the reaction kinetics. Notably, Py and PyO adsorbstronger than PO and P onto RuTDCPPS, while Me2PyO andMe2Py adsorb weaker than PO onto RuTDCPPS. This is of keyimportance, because RuTDCPP(PO)S is the TDI. Stronger thanPO adsorbates in the system would therefore create a lowerenergy TDI that would lead to an increase in the energetic span

Fig. 16 Charge transfer and bonding analysis for the reaction cycle forbonds in all intermediates are labeled with DDEC6 computed NACs andgeometries are given in parentheses. The porphyrin ring, tether linkage, acalculations but for conciseness are not shown here. NACs are presenteddual spin states at crossing points, NACs and bond orders were not com

88208 | RSC Adv., 2016, 6, 88189–88215

and hence lower the reaction rate. Consequently, PyO is nota suitable oxidant for propene epoxidation over this catalyst.

Our computational results agree with experimental resultsin several aspects. The adsorption energy of substrate oxideonto RuTDCPPS, which forms the TDI for PO production (i.e.,RuTDCPP(PO)S), determines the lower bound of the overallenergetic span. Stronger than PO adsorbates (e.g., Py asshown in Table 8), would therefore produce a higher ener-getic span. This explains why PyO is unsuitable as an oxygentransfer intermediate for this type of catalyst. The displace-ment of adsorbed PO by Me2PyO is endothermic by

propene epoxidation over the RuTDCPP catalyst. Atoms and chemicalbond orders, respectively. Bond lengths (A) from the DFT-optimized

nd all H atoms in propene and 2,6-dimethypyridine were included in allin black with bond orders in red and bond lengths in blue. Due to theputed for CP2 and CP3.


Table 9 Summary of DDEC6 results for ground and transition states in the main catalytic cycle for propene epoxidation over the RuTDCPPcatalyst

Intermediates Ru SBOa Ru–N(Por) BOsb Porphyrin chargec Tether charged

RuTDCPPS 3.7 2.8 �1.0 0.3RuTDCPP(Me2PyO)

S 3.8 2.6 �1.1 0.3RuTDCPP(O)T 4.0 2.4 �0.7 0.2TS10 3.9 2.4 �0.9 0.2RuTDCPP(PO*)T 3.8 2.4 �0.8 0.2RuTDCPP(PO)S 3.9 2.6 �1.1 0.3

a Sum of DDEC6 bond orders between Ru atom and all other atoms in the system. b Sum of DDEC6 bond orders between Ru atom and the nitrogenatoms in the porphyrin ring. c Sum of DDEC6 NACs for all atoms in the TDCPP ring. d Sum of DDEC6 NACs for all atoms in the tether.

Paper RSC Advances

approximately 4.6 kcal mol�1. For substrate oxides thatadsorb weaker than the intermediate oxidant, the adsorbedintermediate oxide (e.g., RuTDCPP(Me2PyO)

S) rather than theadsorbed substrate oxide would be the TDI. Under suchcircumstances, epoxidizing different olens (having weaklyadsorbing epoxides) using the same oxidant would yieldsimilar reaction rates. This agrees with the experimentalresults reported by Liu, et al.43 where turn-over-frequencies of�180 per hour were achieved for different olen epoxidationsusing Cl2PyO as the oxidant over the RuTDCPP(MCM41)catalyst. Further experimental support for our reactionmechanism comes from substrate competition studies.Specically, Groves et al. reported that adamantane oxidizedmore rapidly than 1-octene when each substrate was pre-sented alone to the Ru(F20-TPP) catalyst, but that 1-octeneoxidized more rapidly than adamantane when an equimolarmixture of the two substrates was presented to the catalyst.75

We propose this suggests a TDI in which some derivative ofthe substrate (e.g., substrate oxide) is complexed to thecatalyst. According to our proposed reaction mechanism, thelower oxidation rate of 1-octene alone compared to ada-mantane alone could be interpreted as a stronger catalystbinding affinity for 1-octene oxide than for adamantanederivative, which would also make 1-octene more competitivethan adamantane for selective oxidation in the 1-octene/adamantane substrate mixture.

As summarized in Fig. 16 and Table 9, NACs, bond orders,and bond lengths for intermediates of the propene epoxidationcycle were also computed to understand the bonding and

Table 10 Summary of computed cycle activation barriers (kcal mol�1)catalyst at different levels of theory. Overall energetic spans are highlighte(G) cycle activation barriers, but it was only computed at the B3LYP/LANare not available for the other levels of theory

Level of theory

First cycle

ESCF EZP H

B3LYP/LANL2DZ 32.0 34.5 33.3B3LYP/LANL2DZ w/tight 32.0 34.5 33.3B3LYP/LANL2DZ w/PCM 29.0 31.5 30.3B3LYP/custom basis set 37.1 40.0 38.6B3LYP/custom basis set w/PCM 34.1 37.2 35.7


charge transfer properties of the RuTDCPP catalyst. First, theNACs and SBOs for the Ru metal center do not change muchduring the reaction cycle. The NACs of Ru ranged from +0.7 to+1.0, and the Ru SBO ranged from 3.7 to 4.0. The summed bondorder between the Ru atom and the nitrogen atoms of theporphyrin ring ranged from 2.4 to 2.8. The bond order betweenthe Ru atom and the tethering ligand was about 0.5 with a bondlength of �2.15 A. These quantities show the Ru atom main-tains a similar net charge and sum of bonds throughout thecatalytic cycle. When the Ru atom forms a bond to an oxo groupor an adsorbate molecule, there is a slight decrease in the Ru-tether and Ru-porphyrin bond orders to offset this in order tomaintain a nearly constant SBO. The computed bond lengthsand bond orders clearly show the oxygen atom inRuTDCPP(PO*)T has single bonds to the Ru atom and thecentral C atom. Also of interest, the Ru–O bond order inRuTDCPP(O)T is approximately 1.1.

3.5 Level of theory justication

Different levels of theory were tested to justify the appropri-ateness of our choice of computational model. Due to the largecomputational cost of calculations involving large basis sets, weonly re-optimized some intermediates that directly relate to theoverall energetic span of the DMZB catalytic cycle. Specically,the MO$Me2PyO

T ground state and the TS3 and TS8 transitionstate geometries were re-optimized for each level of theorytested. The overall energetic spans for Me2Py oxidation were re-computed accordingly. For the RuTDCPP catalytic system,

for the 1st and 2nd catalytic cycles of Me2Py oxidation over the DMZBd in bold. MOT is the rate determining intermediate for the free energyL2DZ level of theory. Thus, the free energy (G) cycle activation barriers

Second cycle

G ESCF EZP H G

62.4 33.9 35.3 34.2 54.233.9 35.3 34.231.5 32.5 31.638.7 40.2 39.037.8 39.4 38.2

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crossing-point (CP) geometries are directly related to the ener-getic span. Since every CP geometry requires at least 10 opti-mized constrained geometries and consumes a huge amount ofcomputational hours, we did not re-compute the energeticspans for the RuTDCPP catalytic system.

Levels of theory we evaluated included: (a) B3LYP/LANL2DZusing the “opt ¼ loose” optimization convergence criteria(maximum step size of 0.01 a.u. for atom displacements anda maximum force of 0.0025 a.u.) and without any solvent, (b)B3LYP/LANL2DZ using the “opt ¼ tight” optimization conver-gence criteria (maximum step size of 0.0018 a.u. for atomdisplacements and a maximum force of 0.00045 a.u.) andwithout any solvent, (c) B3LYP/LANL2DZ using the “opt ¼loose” optimization convergence criteria combined withimplicit toluene solvent using the Polarizable ContinuumModel (PCM),108–112 (d) B3LYP paired with a custom basis set(LANL2DZ basis set for all H and C atoms, LANL2DZdp basis setfor all N and O atoms, and LANL2TZ(f) basis set for the Zr atom(which uses the same effective core potential as LANL2DZ))using the “opt ¼ loose” optimization convergence criteriawithout any solvent, (e) B3LYP paired with the custom basis setusing the “opt ¼ loose” optimization convergence criteriacombined with implicit toluene solvent (PCM). The N and OLANL2DZdp basis sets add a diffuse p function and a polarizingd function to the LANL2DZ basis set.113 The Zr LANL2TZ(f) basisset includes a polarizing f function, a triple zeta valence space,a diffuse s function, and a diffuse p function.113 The PCMcalculations included the self-consistent reaction eld (SCRF)energies that describe the electronic polarization due to solventdielectric effects, but do not include the dispersion or cavitationenergies. (The dispersion and cavitation energies were omittedbecause they are not computed self-consistently.)

Table 10 summarizes the computed cycle activation barriersfor the 1st and 2nd catalytic cycles of Me2Py oxidation over theDMZB catalyst at different levels of theory. Overall energeticspans are highlighted in bold. SCF energies, zero-point ener-gies, enthalpies, and B3LYP/LANL2DZ Gibbs free energies arereported. The cycle activation barriers for these two cycles aresimilar and nomajor differences were observed for the differentlevels of theory tested. For the enthalpic energetic span, thecomputed values ranged from 30.3 to 38.6 kcal mol�1 with anaverage value of 34.2 kcal mol�1 and a standard deviation of 3.1

Table 11 Summary of computed EO adsorption energies onto oxo, perespect to MOT using molecular O2 as the oxygen reservoir are given fo

Adsorbed state Adsorbent Adsorbate

Adsorption E (kca

ESCF EZP

MO$EOS MOS EO 0.0 2.6MO$EOT MOT EO �11.4 �9.6M(O2)$EO

S M(O2)S EO �5.1 �3.1

M(O2)$EOT M(O2)

T EO �5.6 �3.7M(h3-O3)$EO

S M(h3-O3)S EO �2.3 �0.3

M(h3-O3)$EOT M(h3-O3)

T EO �3.8 �1.8M(h2-O3)$EO

S M(h2-O3)S EO �4.5 �2.4

M(h2-O3)$EOT M(h2-O3)

T EO �3.7 �2.2

88210 | RSC Adv., 2016, 6, 88189–88215

kcal mol�1. This demonstrates that the B3LYP/LANL2DZ level oftheory is good enough for determining key trends in the cata-lytic performance for 2-step propene epoxidation.

In addition, we also conducted an ab initio moleculardynamics calculation using the VASP114–117 program startingwith the triplet peroxo h3-ozone form of the DMZB catalyst(M(h3-O3)$(O2)

T). The PBE118 exchange–correlation functionalwas applied in combination with a planewave cutoff energy of400 eV and the projector augmented wave (PAW119,120) method. Atotal of 184 geometry steps were nished with 1 femtosecondper geometry step. A Nose thermostat was used to simulatea canonical ensemble at 350 K temperature.121 During thecalculation, the M(h3-O3)$(O2)

T complex naturally transformedinto the M(h2-O3)$(O2)

T complex. This agrees with our otherDFT results that M(h3-O3)$(O2)

T has a slightly higher relativeenergy than M(h2-O3)$(O2)

T and the activation barrier forinterconverting between these two forms is small.

4. Computational study of the directethylene epoxidation reaction

Since ethylene has no allylic H atoms, it cannot participate inthe unwanted allylic H transfer side reactions that are so chal-lenging during propene epoxidation. This makes it much easierto develop a good catalyst for direct ethylene epoxidation thanfor direct propene epoxidation. In our previous work, westudied direct ethylene epoxidation over the DIZB catalyst usingmolecular O2 as oxidant without co-reductant.36 We now givea brief summary of the results for direct ethylene epoxidationusing the DMZB catalyst.

Table 11 summarizes energies for EO adsorption onto theoxo, peroxo, and ozone forms of the DMZB catalyst. Except foradsorption of EO onto MOS, the computed adsorption energieswere exothermic. However, the loss in entropy upon adsorptionled to endergonic adsorption in all cases. Because MO$EOT hasthe lowest relative SCF energy among all possible intermediatesas shown in the chemical potential diagram (Fig. 9 and Table11), it was included in determining each cycle activation barrier

Four junior cycles that go through four different h3-ozoneintermediates were investigated. The four h3-ozone intermedi-ates involved are: M(h3-O3)$EO

T, M(h3-O3)$(O2)T, M(h3-O3)

T,and M(O)(h3-O3)

T. Our computed energetic spans for direct

roxo, and ozone forms of the DMZB catalyst. Relative energies withr comparison

l mol�1) Relative E compare to MOT (kcal mol�1)

H G ESCF EZP H G

2.7 15.2 11.9 14.8 14.8 28.3�9.2 2.3 �11.4 �9.6 �9.2 2.3�3.4 9.8 6.8 10.0 9.7 28.9�3.4 8.1 �4.1 �1.2 �1.3 16.7�0.7 13.7 19.7 23.7 23.0 48.9�2.2 10.6 8.0 11.4 11.1 33.7�2.7 11.0 13.1 17.6 16.8 42.5�2.8 11.1 4.3 7.9 7.0 31.9


Fig. 17 (top panel) Triplet h3-ozone intermediate with adsorbed EOmolecule (M(h3-O3)$EO

T) involved catalytic cycle for direct ethyleneepoxidation using the DMZB catalyst. For each step, the energies arelabeled in kcal mol�1. (bottom panel) SCF energy profile for thiscatalytic cycle. M$(O2)2

T is the reference state. Activation barriers formajor reaction steps and the whole catalytic cycle are presented inkcal mol�1.

Paper RSC Advances

ethylene epoxidation over the DMZB catalyst (ESCF ¼ 34.0, EZP ¼36.2, H ¼ 34.9, G ¼ 58.4 kcal mol�1) are higher than those wepreviously reported36 for the DIZB catalyst (ESCF ¼ 24.4, EZP ¼28.3, H ¼ 27.1, G ¼ 52.9 kcal mol�1).

First cycle: Fig. 17 gives the catalytic cycle and energy prolepassing through the M(h3-O3)$EO

T intermediate. The reaction


steps involved are: (1) MO$EOT adsorbs an O2 molecule to formM(h2-O3)$EO

T, (2) M(h2-O3)$EOT transforms to M(h3-O3)$EO

T,(3) M(h3-O3)$EO

T donates one O atom to ethylene to produce EOand M(O2)$EO

T, and (4) M(O2)$EOT donates one O atom to

ethylene to produce EO and regenerate MO$EOT. The SCF cycleactivation barrier is 34.1 kcal mol�1 with MO$EOT as the TDIand TS13 as TDTS.

Second cycle: Fig. S4 of the ESI† gives the catalytic cycle andenergy prole passing through the M(h3-O3)$(O2)

T interme-diate. The reaction steps involved are: (1) MO$EOT desorbs EOto form MOT, (2) MOT adsorbs an O2 molecule to formMO$(O2)

T, (3) MO$(O2)T adsorbs an O2 molecule to form M(h2-

O3)$(O2)T, (4) M(h2-O3)$(O2)

T rearranges to formM(h3-O3)$(O2)T,

(5) M(h3-O3)$(O2)T donates one O atom to ethylene to produce

EO and M$(O2)2T, and (6) M$(O2)2

T donates one O atom toethylene to produce EO and regenerate MO$(O2)

T. The SCF cycleactivation barrier is 34.0 kcal mol�1 with MO$EOT as the TDIand TS15 as TDTS.

Third cycle: for the cycle passing through the M(h3-O3)T

complex, three reaction steps were computed: (1) M(O)T adsorbsan O2 molecule to form M(h2-O3)

T, (2) M(h2-O3)T transforms

into M(h3-O3)T, and (3) M(h3-O3)

T reacts with ethylene toproduce EO and M(O2)

T. The computed SCF activation barriers(net reaction energies) for steps (1), (2), and (3) are 23.3 (8.0), 3.9(3.7), and 13.1 (�23.9) kcal mol�1, respectively. The energydifference between the M(O)$(EO)T catalyst resting state and thetransition state involved in the reaction step (3) is: 11.4 + 23.3 �15.3 + 3.9 � 0.2 + 13.1 ¼ 36.3 kcal mol�1. Since this energydifference is already larger than the cycle activation barriers ofthe two previously described cycles, this reaction cycle is ener-getically unfavorable within the system and no further compu-tation was carried out.

Fourth cycle: in the cycle passing through the M(O)$(h3-O3)T

complex, M(O)$(O2)T reacts with ethylene to generate EO and

M(O)2T. A SCF activation barrier of 27.4 kcal mol�1 was achieved

for this step. The energy difference between the M(O)$(EO)T

catalyst resting state and the transition state involved in thisstep is 38.5 kcal mol�1 which is also higher than the cycleactivation barriers of the two previously described cycles. (Thereactions involved between M(O)$(EO)T and the transition stateare: EO desorbs from M(O)$(EO)T to generate MOT, MOT

adsorbs an O2 to produce M(O)$(O2)T, and M(O)$(O2)

T reactswith an ethylene molecule.) Therefore, this reaction cycle is alsoenergetically unfavorable within the system and no furthercomputation was carried out.

5. Conclusions

In this article, we introduced a new two-step process that can beutilized for selectively oxidizing difficult organic substrates (e.g.,propene) using molecular O2 as the terminal oxidant. Tworeactions occur in separate reactors over two different catalysts:(1) molecular oxygen activation via reaction with an oxygenacceptor molecule to produce an oxygen transfer intermediate,and (2) substrate oxidation via reaction with the oxygen transferintermediate to produce substrate oxide and regenerate theoxygen acceptor molecule. The oxygen acceptor molecule

RSC Adv., 2016, 6, 88189–88215 | 88211

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should be recycled back to the rst reactor to achieve a netreaction of 2 substrate + O2 / 2 substrate oxide. We call thisprocess “tandem direct selective oxidation”. This separation ofmolecular oxygen activation and substrate oxidation stepsreduces by-product formation by avoiding some side reactions.Because this process does not require a sacricial co-reductantor co-oxidant that would produce an unwanted co-product, itshould nd applications to reduce chemical waste generation inthe selective oxidation of difficult organic substrates.

Using separate reactors for O2 activation and substrateoxidation is important for the epoxidation of terminal alkenescontaining allylic H atoms. The key idea is to prevent theterminal alkene molecule from reacting with two activated Oatoms at the same time. This avoids a side reaction in which anallylic H atom transfers from the terminal alkene to a secondactivated O atom, such as is present in peroxo or ozone inter-mediates. Specically, the catalyst in step (1) needs to activatetwo oxygen atoms simultaneously from molecular O2, while thecatalyst in step (2) should activate only one oxygen atom ata time in order to avoid the allylic H transfer reaction. In therst reactor, both O atoms in one O2 molecule are activatedthrough two oxygen acceptor molecules to produce two oxygentransfer intermediate molecules. Since the oxygen acceptormolecule does not contain any allylic H atoms, the allylic Htransfer side reaction will not occur. In the second reactor,a catalyst is specically chosen so that it only utilizes one oxygentransfer intermediate molecule as oxidant at one time. Underthis circumstance, only one activated O atom is provided to theterminal alkene molecule and it will react with the C–C doublebond. Since there is no extra activated O atom available, theallylic H transfer side reaction will also be avoided during theselective oxidation of substrate in step 2.

Using computational screening, our choice of catalysts forreactions (1) and (2) are dimethyl-substituted Zr_benzol (DMZB)catalyst and tethered Ru(meso-tetrakis(2,6-dichlorophenyl)porphyrin) (RuTDCPP) catalyst, respectively. 2,6-Dimethylpyr-idine (Me2Py) was selected as the oxygen acceptor molecule forpropene epoxidation. The DMZB catalyst was chosen for itscomputationally predicted capability to activate molecular O2

efficiently under mild conditions. Tethered RuTDCPP complexwas previously experimentally demonstrated to be a suitablecatalyst for selectively oxidizing terminal alkenes usingaromatic N-oxides as oxidants.43 Our selection of Me2Py as theoxygen acceptor molecule is based on three factors: (a) thechemical potential energy of Me2PyO sits between that of O2 andpropylene oxide to make both reactions in our two-step processthermodynamically favorable, (b) our DFT-computed catalyticcycles show Me2Py leads to acceptable energetic spans over thecatalysts chosen for reactions (1) and (2), and (c) the toxicity ofMe2Py and Me2PyO appear to be manageable through appro-priate safety precautions (as contrasted with 4-NQ and 4-NQO).

We used DFT computations to study reaction mechanisms,energy proles, and possible side reactions for the DMZB andthe RuTDCPP catalytic systems. For both systems, no energeti-cally important deactivation products or side reactions wereidentied. The computed enthalpic energetic spans are 33.3and 31.6 kcal mol�1 for O2 activation and propene epoxidation,

88212 | RSC Adv., 2016, 6, 88189–88215

respectively. Accordingly, both catalysts should work efficientlyunder mild conditions.

NACs and bond orders were computed to gain chemicalinsights into the reaction chemistry. For the DMZB catalyst,the redox active bidentate ligands function as an electronreservoir and the Zr atom acts as an electron transfer bridgebetween the bidentate ligands and the adsorbed oxygenatedgroups and substrate molecules. The Zr NAC (+2.1 to +2.2) andSBO (2.6 to 2.9) remained approximately constant during themain catalytic cycle. The main electron transfer effects occurbetween the redox bidentate ligands, the adsorbed oxygenatedgroups, and the reacting oxygen acceptor molecules. For theRuTDCPP catalyst, the Ru NAC (+0.7 to +1.0) and SBO (3.7 to4.0) also remained approximately constant during the maincatalytic cycle. Bonds to an oxo group or adsorbed moleculeare partially offset by slightly weakened Ru-tether and Ru-porphyrin bond orders.

Finally, our selective oxidation route differs from theSharpless and Jacobsen epoxidation routes in key aspects.Sharpless epoxidation uses tBuOOH as the oxidant overTi(OiPr)4 catalyst to stereoselectively produce chiral 2,3-epox-yalcohols from allylic alcohols.122 Jacobsen epoxidation usesNaOCl as the oxidant over manganese organometallic catalystscontaining a salen-like ligand to stereoselectively produce chiralepoxides from unfunctionalized olens.123 Our tandem directselective oxidation process uses molecular oxygen as theterminal oxidant without co-reductant to produce epoxides orother desired selective oxidation products. A key advantage ofour approach is the great variety of different functional groups(e.g., epoxides, alcohols, ketones, aldehydes as shown in Table3) that it can potentially enable to produce via tandem directselective oxidation.

Future work needs to address the experimental synthesisand reaction testing of the DMZB catalyst. Other interme-diate oxidant and catalyst choices could potentially be madeand may be worthy of study in future work. Future work alsoneeds to address the separation of reaction products from thecatalysts and the recycle of oxygen acceptor molecule fromreactor 2 to reactor 1. Using a catalyst tether or a uorousbiphasic mixture are two strategies to facilitate catalystseparation.124–127

Acknowledgements

This project was funded in part by National Science Foundation(NSF) grant IIP-1640621. Supercomputing resources wereprovided by the Extreme Science and Engineering DiscoveryEnvironment (XSEDE).128 XSEDE is funded by NSF grant ACI-1053575. XSEDE project grant TG-CTS100027 provided alloca-tions on the Stampede cluster at the Texas AdvancedComputing Center (TACC) and the Comet clusters at the SanDiego Supercomputing Center (SDSC). The authors sincerelythank the technical support staff of XSEDE, TACC, and SDSC.The authors and NMSU's Office of Intellectual Property(Arrowhead Center, Inc.) have applied for a patent on some ofthe results described in this paper.


Paper RSC Advances

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