C−H Bond Activation by a Palladium(II) Thioether Complex:Formation of the Bis(nitromethanate) Complex [Pd(9S3)(CH2NO2)2]John P. Lee,*,† C. Luke Keller,† Ashley A. Werlein,† Daron E. Janzen,‡ Donald G. VanDerveer,§

and Gregory J. Grant*,†

†Department of Chemistry, The University of Tennessee at Chattanooga, 615 McCallie Avenue, Chattanooga, Tennessee 37403,United States‡Department of Chemistry, St. Catherine University, 2004 Randolph Avenue, St. Paul, Minnesota 55105, United States§Department of Chemistry, Clemson University, Clemson, South Carolina 29634, United States

*S Supporting Information

ABSTRACT: Palladium(II) acetate reacts with 1 equiv of 1,4,7-trithiacyclononane (9S3) at room temperature to produce theneutral complex [Pd(9S3)(OAc)2] (1; OAc

− = CH3COO−) as an analytically pure yellow solid, which has been characterized

using 1H and 13C NMR spectroscopy and single-crystal X-ray diffraction. The crystal structure of 1 shows the first example of anexodentate third sulfur of the 9S3 ligand in a Pd(II) complex. Complex 1 reacts with nitromethane at room temperature inmethanol to produce [Pd(9S3)(CH2NO2)2] (2) and acetic acid, as confirmed by 1H NMR and 13C NMR spectroscopy.Moreover, complex 2 has been characterized by single-crystal X-ray diffraction. Its crystal structure is the first example of anytransition-metal complex containing two nitromethanate (anionic nitromethane) ligands. The complex shows the more typicalelongated-square-pyramidal structure and [S2C2 + S1] coordination with one long Pd−S axial interaction at 2.823(2) Å and twoC-bound nitromethanate ligands. Interestingly, each nitromethanate ligand is in a different coordination environment and variesin their trans-directing abilities. Reactivity studies suggest that the complexation behavior of the 9S3 ligand and the Pd(II) centeras well as the σ-donor ability of the leaving group play key roles in the C−H activation of the nitromethane. Two related metalcomplexes, [Pd(dppe)(OAc)2] (dppe = 1,2-bis(diphenylphosphino)ethane) and [Pd(9S3)(CF3COO)2], were synthesized, butneither of these react in a similar fashion to form a nitromethanate complex. Also, the reaction of 1 with nitrobenzene andnitrocyclopentane was studied, but these nitro-organics do not undergo C−H bond activation like nitromethane.

■ INTRODUCTION

Over the past several decades there has been a large amount ofwork centered on the development of robust and activehomogeneous transition-metal complexes for the selectiveactivation of C−H bonds.1,2 Though much effort in thisarena has been focused on methane, benzene, and other relatedhydrocarbons, the ability to functionalize additional C−Hbonds of organic compounds that contain heteroatoms is ofimportance as well.3 One particular area of interest is the metal-mediated activation of nitroalkanes (e.g., nitromethane) andreaction with aldehydes in a nitro-aldol reaction. For example,the use of chiral Cu catalysts can lead to asymmetric nitro-aldolreactions (Henry reaction).4−9 In the reaction of nitromethanewith an aldehyde in the presence of a transition-metal catalyst,the deprotonation of nitromethane to form a nitromethanateanion ([CH2NO2]

−) is believed to be a key reaction step orintermediate.6,7 To date, no direct observation of this

intermediate in the nitro-aldol reaction has been observed.Indeed, the direct observation of C−H activation of nitro-methane is relatively rare. Goldman has reported a well-definedPCP pincer ligand supported Ir(I) complex that can oxidativelyadd the C−H bond of nitromethane to form an Ir(III) complexwith nitromethanate and hydride ligands.10 In the Goldman Irsystem, all three potential linkage isomers of the nitro-methanate ligand were isolated and characterized, whichinclude κ2-O,O-CH2NO2, κ

1-O-CH2NO2, and η1-CH2NO2. Ina second example, Gunnoe has reported a tris(pyrazolyl)-borate−Ru(II) methyl complex that reacts with nitromethaneto produce a putative [Ru(Tp)(PMe3)(CH2NO2] intermediatevia C−H activation of nitromethane and loss of methane thatfurther reacts with additional nitromethane to produce

Received: July 9, 2012

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[Ru(Tp)(PMe3)(κ2-O,N-N(O)C(H)(NO2))].

11 Most germaneto the work presented here, there have been a few reports of Pdcompounds that react with nitromethane to form Pd complexesthat bear a single nitromethanate ligand. However, each ofthese examples requires vigorous reaction conditionstheaddition of an external strong base such as NaOH,12 an innercoordination sphere hydroxide ligand,13,14 or a Pd(I)−Pd(I)metal−metal σ-bond.15

The complexation properties of the tridentate ligand 1,4,7-trithiacyclononane (9S3) and related thia crowns have beenextensively explored during the past 30 years.16−18 A keyfeature that results in such an extensive coordination chemistryis that the 9S3 ligand adopts an exclusive endodentateconformation as its lowest energy form which enables facilefacial coordination to metal centers.19 In its complexesinvolving d8 metals, such as Pd(II) and Pt(II), the 9S3 ligandcoordinates in a fashion that can best be considered as an S2 +S1 chelate in which two sulfur donors coordinate in the squareplane of the complex (thioether sulfur bond lengths are 2.2−2.4Å in Pd(II) and Pt(II) complexes), but the third sulfur displaysa metal−sulfur axial interaction at a much greater distance. Thelong-distance interaction arises from an “orbital mismatch”between the electronic preferences of the d8 metal ion to form asquare-planar complex and conformational preferences of thetrithiacrown to bind facially.20 Heteroleptic Pd(II) and Pt(II)complexes will have the general formula [M(9S3)(L2)]

n+ anddisplay elongated-square-pyramidal geometries.21 The length ofthe M−S axial interaction is highly dependent on the σ-donorability of the ancillary ligand L and can vary by as much as 0.7 Å(2.51−3.21 Å).22 Note, however, that the longest of thesemetal−sulfur distances is still less than the sum of the van derWaals radii for either one (Pd−S = 3.4 Å, Pt−S = 3.5 Å).23 Asecond important general feature in d8 metal 9S3 coordinationchemistry is the fluxionality of the trithia crown.24,25 In its 13CNMR spectrum with these metal ions, the six methylenecarbons of 9S3 virtually always appear as a single resonance.The fluxionality of the trithia crown arises from a 1,4-metallotropic shift that rapidly exchanges the three sulfurdonors.26

In this paper, we report the room-temperature C−H bondactivation reaction of nitromethane by a Pd(II) 9S3 complex,[Pd(9S3)(OAc)2], to yield a bis(nitromethanate) complex,[Pd(9S3)(CH2NO2)2]. A reaction mechanism is proposed onthe basis of several reactivity studies with related complexes. Inaddition to the reactivity of [Pd(9S3)(OAc)2], both [Pd(9S3)-(OAc)2] and [Pd(9S3)(CH2NO2)2] have been structurallycharacterized by X-ray crystallography.

■ RESULTS AND DISCUSSION

Formation of [Pd(9S3)(OAc)2] (1). The reaction of[Pd(OAc)2] (OAc− = acetate, commercially available as acyclic trimer) with 1 equiv of 9S3 quantitatively produces[Pd(9S3)(OAc)2] (1) as a yellow solid (Scheme 1). Complex 1has been characterized by its 1H and 13C NMR spectra, whichshow the expected number of peaks, intensities, and splitting

patterns. In the 1H NMR spectrum, a singlet is observed for thechemically equivalent cis acetate ligands, while the coordinated9S3 shows its typical AA′BB′ pattern. Due to the expectedfluxionality of the 9S3 ligand, the 13C NMR spectrum showsonly a singlet for the coordinated 9S3 in solution.25,26

Complex 1 has also been characterized by single-crystal X-raycrystallography, and the structure is shown in Figure 1.

Crystallographic data for complex 1 as well as for the othercomplexes described in this report appear in Tables 1 and 2.The crystal structure of complex 1 is unusual in two aspects.First, the axial sulfur of the 9S3 ligand lies in an exodentatefashion relative to the Pd(II) center. There are over 35

Scheme 1. Synthesis of [Pd(9S3)(OAc)2] (1)

Figure 1. ORTEP drawing of [Pd(9S3)(OAc)2] (1). Hydrogen atomshave been omitted for clarity. Ellipsoids are shown at the 50%probability level.

Table 1. Crystallographic Data Summary for[Pd(9S3)(OAc)2] (1) and [Pd(9S3)(CH2NO2)2] (2).

1 2

formula C10H18O4PdS3 C8H16N2O4PdS3habit, color yellow, block red, blocklattice type monoclinic monoclinicspace group P21/c P21/na, Å 7.9701(16) 7.8595(16)b, Å 13.576(3) 9.904(2)c, Å 13.316(3) 18.205(4)α, deg 90.00 90β, deg 96.490(8) 95.96(3)γ, deg 90.00 90V, Å3 1431.6(5) 1409.4(5)Z 4 4temp, K 173(2) 183(2)formula wt 404.85 406.81Dc, Mg m−3 1.878 1.917μ, mm−1 1.736 1.767no. of rflns collected 2531 2487no of unique rflns 1718 (R(int) =

0.0913)2266 (R(int) =0.0513)

no. of data, restraints,params

2531/0/165 2487/0/163

R1, wR2 (I > 2σ(I)) 0.0853, 0.1990 0.0405, 0.0779R1, wR2 (all data) 0.1316, 0.2519 0.0478, 0.0804goodness of fit (F2) 1.056 1.167largest diff peak/hole, e Å−3 2.541/−1.194 0.636/−0.663

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examples of crystal structures involving Pd(II) 9S3 complexesin the Cambridge Structural Database, but all prior examplesdisplay an endodentate sulfur.27,28 The distance between the Pdcenter and the exodentate sulfur atom is 4.093(4) Å, wellbeyond than the sum of the van der Waals radii. Thus, thesolid-state structure shows no long-distance Pd−S axialinteraction. The Pd(II) center is surrounded in a distorted-square-planar fashion by the remaining two sulfur donors fromthe 9S3 as well as two oxygen donors from the two acetategroups. This is the second unusual aspect of the complex’sstructurethe presence of these oxygen donors. In over 100crystal structures of Pt(II) and Pd(II) 9S3 complexes, there areno prior examples of any oxygen donor ligand.27 The length ofthe Pd−Sax bond is highly dependent upon the σ-donorproperties of the ancillary ligands.22 We believe that the strongσ-donating ability of the anionic acetate ligand produces a moreelectron-rich palladium center and, thereby, the rare exodentatecoordination of the 9S3. Furthermore, the 13C NMR chemicalshift for the fluxional 9S3 ligand, in solution, is very high (40.8ppm in CDCl3), the most downfield chemical shift observed forthis ligand among all of its Pd(II) complexes.22 An extendedlattice view of 1 reveals that the complex exists in the solid stateas dimers with weak Pd−Pd interactions (see the SupportingInformation). This internuclear Pd−Pd distance at 3.41 Å isslightly longer than the sum of the van der Waals radii (3.3 Å).This weak intermolecular interaction may be a contributingfactor to the exodentate conformation of the coordinated 9S3in the solid state.Formation of [Pd(9S3)(CH2NO2)2] (2). Complex 1 reacts

with excess nitromethane at room temperature in methanolover the course of 16 h to produce [Pd(9S3)(CH2NO2)2] (2)and concomitant formation of acetic acid (HOAc), asconfirmed by both 1H and 13C NMR spectroscopy. We alsonote a significant color change from yellow to red as thereaction proceeds, and the reaction is nearly quantitativewithout the formation of side products. The reaction is shownin Scheme 2, and these experimental conditions were found tobe the best ones to facilitate formation of 2. We note that thereaction is unusual in that the stronger acid, acetic acid (Ka of

HCH2NO2 = 1.0 × 10−10 vs Ka of HOAc = 1.8 × 10−5), is theproduct formed. The requirement of excess nitromethane tofacilitate formation of the nitromethanate complex suggests anequilibrium between complexes 1 and 2. The presence of anequilibrium is further supported by H/D exchange in CD3NO2,which forms a coordinated CH2NO2 ligand, but only in thepresence of 1. Also, we note that the red solid of complex 2 isonly stable in solution in the presence of excess nitromethane(the color changes from red to green upon attempting toisolate). In methanol-d4, the nitromethanate resonances areobserved at 4.83 and 4.78 ppm (two protons for eachresonance) in the 1H NMR spectrum, which match chemicalshifts in other reported Pd(II) nitromethanates.12,14 Weobserve no coalescence of these two peaks at temperaturesup to 55 °C and believe that the appearance of two differentproton resonances arises from each of the nitromethanateligands being in a different structural environment (seediscussion below). There is only one prior report ofbis(nitromethanate) complexes with any transition metal (byMilani and co-workers), but all of those complexes showedequivalent CH2NO2 ligands at room temperature.12 Also, theirnitromethanate complexes were formed by deprotonation ofthe nitromethane in strong base (excess 0.2 M NaOH),followed by ligand substitution and not C−H bond activationas occurs here. Similarly, the 13C NMR chemical shifts of thenitromethanates are in agreement with two previous reports ofC-bound nitromethanate complexes by Goldman and Ikariya,respectively.10,29 Of note, the proton NMR resonances for thenitromethanate ligands are shifted 0.49 ppm downfield ofnitromethane solvent, while in the 13C NMR spectrum, thenitromethanate resonance is at 32.8 ppm, 29 ppm upfield ofnitromethane solvent. We were unable to resolve thechemically different −CH2NO2 resonances (as shown in the1H NMR) due to overlap with the coordinated 9S3. Indeed,comparison of peak widths at half-height (LW1/2) shows thatone resonance is considerably broader than the other peaks(see the Experimental Section). As expected, two N−Ostretches for the nitro groups are observed at 1486 and 1338cm−1.

Table 2. Selected Bond Distances (Å) and Angles (deg) for [Pd(9S3)(OAc)2] (1) and [Pd(9S3)(CH2NO2)2] (2).

[Pd(9S3)(OAc)2] (1) [Pd(9S3)(CH2NO2)2] (2)

distance angle distance angle

Pd−S1 4.093(4) S4−Pd−S7 89.86(12) Pd−S1 2.823(1) S4−Pd−S7 88.52(5)Pd−S4 2.253(3) O1−Pd−O3 86.1(4) Pd−S4 2.338(1) C11−Pd−C10 84.0(2)Pd−S7 2.264(4) S7−Pd−O3 178.2(3) Pd−S7 2.328(1) S7−Pd−C10 172.75(15)Pd−O1 2.049(9) S4−Pd−O1 171.9(3) Pd−C11 2.075(5) S4−Pd−C11 173.44(15)Pd−O3 2.049(9) O1−C11−O2 123.9(13) Pd−C10 2.092(5) C10−N1−O1 119.3(5)C11−O1 1.273(18) O3−C13−O4 123.1(13) C10−N1 1.446(7) C10−N1−O2 119.0(6)C11−O2 1.226(18) C11−N2 1.425(7) C11−N2−O3 117.0(7)C13−O3 1.295(17) N1−O1 1.231(7) C11−N2−O4 120.2(6)C13−O4 1.252(18) N1−O2 1.232(7) O1−N1−O2 121.7(5)

N2−O3 1.238(7) O3−N2−O4 122.8(7)N2−O4 1.218(8)

Scheme 2. Synthesis of [Pd(9S3)(CH2NO2)2] (2)

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The precise role of the polar protic solvent, methanol, isunclear. However, analysis of the reaction product mixtures of 1and nitromethane in methanol-d4 by 2H NMR shows nodeuterium incorporation into 2, which discounts the solventacting as a proton source. An identical reaction in neatnitromethane results in multiple products (bis(nitromethanate)complex + starting complex), as observed by NMR analysis ofthe expected nitromethanate region. Complex 1 is insoluble inmost common organic solvents, but it is partially soluble inmethanol. We believe that this partial solubility in methanolenables complex 1 to react with nitromethane. The reaction of1 in CD3NO2 with 20 equiv of CH3OH produces complex 2.However, there is no C−D coupling observed in thenitromethanate region by 13C NMR spectroscopy. Similarly,the reaction of 1 in CD3OD with 20 equiv of CD3NO2 showsno C−D coupling in the nitromethanate region by 13C NMRspectroscopy. Water present in the CD3NO2 NMR solventappears to be the proton source for the H/D exchange, as thereaction in neat CD3NO2 also shows no C−D coupling. Theseresults are in agreement with the formation of complex 2/aceticacid being in equilibrium with 1/nitromethane (see Mecha-nistic Discussion below).Due to the chemical equilibrium between 1/nitromethane

and 2/acetic acid, we were unable to isolate it as an analyticallypure solid. However, complex 2 was successfully crystallizedfrom nitromethane and characterized by single-crystal X-raycrystallography. A thermal ellipsoid perspective of the structureis shown in Figure 2a, and crystallographic data are presented inTables 1 and 2. To our knowledge, 2 represents the firstexample of a crystallographically characterized complex of anytransition-metal ion that contains two coordinated nitro-methanate ligands.27 Due to the unusual nature of the structure,we obtained a second confirmatory crystallographic data setusing a different diffractometer and a second crystal grown froma different solvent, methanol (see the Supporting Informationfor details). The unit cell parameters and space group areidentical for both structures. In contrast to complex 1, 2 showsa Pd(II) center surrounded by an elongated-square-pyramidalgeometry defined by the endodentate 9S3 ligand and the twocarbon-bound nitromethanates. The coordination geometryaround the Pd is best described as [S2C2 + S1], and the axialPd−S distance is 2.823(2) Å. The Pd(II) ion is displaced 0.086Å out of the mean least-squares square plane ([S2C2]) andtoward the axial sulfur.Surprisingly, the two nitromethanate ligands are found to be

structurally different in the solid state. We have designated thetwo nitromethanate ligands as “up”, which corresponds to theC−N bond of the ligand pointing up and away from the Pdcenter (perpendicular to basal square plane), while “out” hasthe C−N bond in the ligand pointing out and away from themetal center (parallel to the basal square plane). The dihedralangles between the [PdS2C2] basal plane and each of the PdCNplanes differentiate the two. For the up nitromethanate ligand,the dihedral angle is 82.1°, showing that the ligand lies nearlyperpendicular to the basal plane. However, this angle in the outligand is 2.1°, demonstrating that the two planes are nearlycoplanar. The two nitromethanates not only have differentorientations within the complex but they also have differentinternal bond lengths. Interestingly, only the up nitromethanateligand shows N−O bond resonance with both bonds of equallength at 1.232(7) and 1.231(7) Å. We have identified 14examples in the CSD which contain one nitromethanate ligandcoordinated to a transition-metal ion (see the Supporting

Information for a listing of these 14 structures; as noted, thereare no CSD examples with two ligands).27 These cases includeIr organometallic complexes,10,29 Pt terpy complexes,13,30 Cocobalamins,31−33 A-frame Au and Pd complexes,15,34 Cd andCo polynuclear complexes,35,36 and mononuclear Pd(II)complexes.12,14 Approximately half of these structures containnitromethanates with resonance in their N−O bonds. Incontrast to the resonance observed in the up ligand, the outnitromethanates display slightly differing N−-O bond lengths at1.238(8) and 1.218(9) Å, with the shorter N−O bond (N2−O4) directed toward the axial sulfur and away from the squareplane. The C−N bond lengths are different as well, with the upligand at 1.448(8) Å and the out ligand at 1.424(8) Å. Bothlengths are shorter than a typical C−N single bond (1.52 Å),possibly suggesting some double-bond character in the outligand.23 The two Pd−C−N bond angles are also quite different(up, 108.2°; out, 115.1°) with up closer to an ideal tetrahedralvalue. In addition, the two nitromethanates have different trans

Figure 2. (a) ORTEP drawing of [Pd(9S3)(CH2NO2)2] (2). Onlyhydrogen atoms on the nitromethanate ligands are shown. Ellipsoidsare shown at the 50% probability level. (b) Space-filling model of[Pd(9S3)(CH2NO2)2] (2). The perspective is looking toward the twonitromethanate ligands.

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donor effects, with the up being the better trans donor of thetwo. The equatorial Pd−S bond trans to the up nitromethanateis shorter at 2.328(1) Å, consistent with it being a slightly bettertrans donor ligand than the out, which has a bond length of2.338(1) Å. A space-filling model of the structure is shown inFigure 2b, and the steric interaction between the twonitromethanates can clearly be seen. The different up and outorientations of the nitromethanates are a consequence of thissteric interaction and we also believe are the source for thedifference in chemical shifts of the methylene protons on eachligand. In summary, there are two unusual features presentthe first example of two nitromethanates coordinated to anytransition-metal ion as well as significant electronic andstructural differences between each ligand.Reactivity Studies of Nitromethane with Related

Pd(II) Complexes. The reaction of 1 with nitromethane is arare example of C−H activation of nitromethane at roomtemperature.10,11 Furthermore, the reaction proceeds readilywithout the presence of a strong base. We note that the originalinorganic reagent, [Pd(OAc)2], does not react with nitro-methane under the same conditions. In addition, the knowncomp l e x [Pd (dppe ) (OAc) 2 ] ( dppe = 1 , 2 - b i s -(diphenylphosphino)ethane) was synthesized to mimic thebehavior of complex 1.37 The chelated phosphine complex issimilar to 1 in that two acetate ligands are also oriented in a cisarrangement at the Pd(II) center. The complex [Pd(dppe)-(OAc)2] does not react with nitromethane in methanol at roomtemperature, nor did it show any reactivity when heated to 50°C for 24 h. Given the failure of the bidentate dppe ligand tofacilitate the reaction, we believe that the 9S3 ligand plays a keyrole in the formation of 2 through its axial Pd−S interactionand fluxionality.Having established the critical role that 9S3 plays, we wished

to probe the effects of the leaving acetate group. The novelcomplex [Pd(9S3)(O2CCF3)2] (3) was prepared by thereaction of 9S3 and the commercially available palladium(II)trifluoroacetate (note that this exists as a monomer and not atrimer like the acetate analogue) using reaction conditionsidentical with those for the formation of 1. It was isolated as abrown solid and has been fully characterized by severalmethods. The 1H NMR spectrum only shows the typicalAA′BB′ pattern for 9S3, while the 19F NMR spectrum showsone peak at −75.1 ppm indicative of a single complex. In the13C NMR spectrum both the trifluoromethyl carbon and thecarbonyl carbon of the trifluoroacetate resonate as quartets(1JCF = 291 Hz and 2JCF = 37 Hz), and 9S3 shows itscharacteristic singlet at 35.5 ppm. As expected, the infraredspectrum of the complex shows the two CO stretchingfrequencies, shifting to higher wavenumbers by approximately90 cm−1 in comparison to the acetate complex. We alsoobtained a partial crystal structure for 3, but disorder in the−CF3 substituents and 9S3 ring resulted in a moderately largeR value of 7.7 and GOF = 1.053 (see the SupportingInformation for crystallographic details). However, thestructure does establish the general coordination sphere aroundthe Pd(II); therefore, our discussion will be limited to justthose aspects. In contrast to 1, the better electron-withdrawingtrifluoroacetates result in a more electropositive Pd center,which switches 9S3 to an endodentate coordination mode.Accordingly, the complex shows an elongated-square-pyramidalshape with [S2O2 + S1] coordination and an axial Pd−Sdistance of 2.853(4) Å. The Pd−O bonds in the trifluoroacetateare generally longer and weaker (2.068(4), 2.051(5) Å) than in

complex 1, in which they average 2.049(9) Å. Importantly,complex 3 showed no reactivity with nitromethane at roomtemperature after 16 h. After several days (48 h) a smallamount of a new complex is observed by 1H and 19F NMRspectroscopy; however, resonances for neither a nitromethanateligand (1H NMR) nor trifluoroacetic acid (19F NMR) areobserved. We also tested the previously prepared complex[Pd(9S3)Cl2], but it also showed no reactivity with nitro-methane in methanol at room temperature.38 The complexwould require the formation of the strong acid HCl if C−Hbond activation were to occur. However, the complex has verylimited solubility in both nitromethane and methanol, and thelow solubility cannot be fully discounted for its lack ofreactivity.

Reaction of 1 with Other Nitro-Organics and Solvents.The reaction of 1 in CD3OD with related nitro-containingcompounds was probed. No reaction of 1 with nitrobenzene inmethanol was observed at room temperature by 1H NMRspectroscopy. Upon heating to 50 °C, the presence of free 9S3ligand confirmed the decomposition of 1. The reaction of 1with nitrocyclopentane at room temperature resulted in thepartial formation of a new uncharacterized complex, asevidenced by the 9S3 resonance in its 1H NMR spectrum.However, the new complex did not involve incorporation ofnitrocyclopentane as a coordinating ligand. This observation islikely due to the sterically demanding cyclopentyl group beingunable to coordinate to the Pd center in a vacant site. Complex1 was also reacted with acetonitrile (pKa = 25) and acetone(pKa = 19) to compare their behavior with nitromethane (pKa= 10). For acetonitrile, no reaction was observed at roomtemperature, and upon heating to 60 °C complex 1decomposes to multiple uncharacterized products. For acetone,a prolonged reaction time at room temperature resulted in theformation of a new uncharacterized 9S3 complex.

Mechanistic Discussion. Our data suggest that this is ametal-mediated pathway, although they do not distinguishamong several mechanisms. Potential mechanisms of thereaction of 1 with nitromethane would include an oxidativeaddition/reductive elimination pathway, C−H bond homolysisthrough a Pd(III) intermediate, a proton transfer through anion pair intermediate (H+CH2NO2

−) or a σ-bond metathesis.39

The oxidative addition of a C−H bond in nitromethane wouldinvolve the formation of a Pd(IV) intermediate (followed byreductive elimination of acetic acid), and the oxidativeaddition/reductive elimination pathway is unlikely for tworeasons. First, the π-acid characteristics of the 9S3 ligand willhinder the formation of the high-oxidation-state, electron-deficient Pd(IV) intermediate. The 9S3 ligand stabilizesunusual lower oxidation states of metal ions such as Au(II)and Rh(II), not higher ones.40 Second, if oxidation of the Pdcenter were to occur, the axial sulfur would be coordinated tothe metal, blocking access to that coordination site for thenitromethanate ligand.41 Although Pd(III) chemistry is well-known for 9S3 complexes, free radicals of nitromethanes havenot been observed in other C−H bond activation reactions ofthe compound. This makes C−H bond homolysis through aPd(III) intermediate unlikely.39 Regarding the third possiblemechanism, proton transfer via an ion pair intermediate, wenote that the related complex [Pd(9S3)(en)](PF6)2 (en =ethylenediamine) was prepared by Reber and co-workers.42

Even though ethylenediamine is a considerably stronger basethan acetate (Kb of en = 8.0 × 10−5 vs Kb of OAc

− = 5.6 ×10−10), there is no indication of C−H bond activation by this

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particular Pd(II) 9S3 complex. We feel that the most likelymechanism is that through C−H σ bond metathesis, which wasproposed by Ikariya et al. in an Ir(III) Cp* complex.29

However, we note that the principal focus of this work is thepreparation and characterization of the novel bis-(nitromethanate) complex. Current investigations are under-way to elucidate the mechanism in greater detail.A proposed mechanism is shown in Scheme 3, where the

fluxionality of 9S3 (see above) is used to account for both the

loss of two acetate ligands as HOAc and the production of thebis(nitromethanate) complex. The first step involves a ligandassociation of nitromethane. The nitromethane association islikely facilitated by the third sulfur interaction, which can supplyextra electon density by adjusting its Pd−S axial distance whilein solution. We have observed this previously in hetero-bimetallic systems involving [Pt(9S3)(ppy)]+ (ppy = 2-phenylpyridine) moieties of the trinuclear complex [{Pt(phpy)-(9S3)}2Ag(CH3CN)2]

3+, in which Pt(II)→Ag(I) dative bondsare formed by a dramatic shortening (2.952(2) to 2.687(2) Å)of the axial Pt−S distance.43 In our system, we believe that theaxial sulfur similarly stabilizes the Pd interaction with a weaknitromethane ligand. The transition state likely involves a six-membered ring with simultaneous bond making (Pd−C andH−OAc) and bond breaking (Pd−OAc and C−H), and it islikely at this step that intermolecular proton exchange withwater occurs. We feel the most likely mechanism is a concertedσ-bond metathesis, which was proposed by Ikariya et al. for C−H bond activation in nitromethane by an Ir(III) Cp* complexcontaining a basic amido ligand.29 These same steps are thenrepeated for the addition of the second nitromethane. Thefluxional nature of the 9S3 ligand plays a critical role inscrambling the sulfur donors and enabling the formation of thebis(nitromethanate) complex 2. Neither the mono-

(nitromethanate) complex nor any other linkage isomer ofnitromethanate is observed. We note that all of the componentsof the [Pd(9S3)(OAc)2] appear to play a role in the C−Hbond activation process, but the key player is the 9S3 throughits axial Pd−S interaction and fluxionality.

■ CONCLUSIONA Pd(II) complex incorporating a 9S3 ligand was found to reactwith excess nitromethane at room temperature to form abis(nitromethanate) complex. This is the first example of astructurally characterized metal complex containing twonitromethanate ligands and is likely attributable to the abilityof 9S3 to adjust its Pd−S axial distance as well as its fluxionality.The solid-state structure of [Pd(9S3)(CH2NO2)2] (2) showsthe typical S2 + S1 coordination of the 9S3 ligand as well as twostructurally distinguished C-coordinated nitromethanate li-gands. Due to steric interactions, the nitromethanate ligandsare distinct in terms of their internal bond lengths and angles aswell as their trans-directing abilities.The use of the thia crown 9S3 represents a new and

unexplored ancillary ligand for a transition-metal-catalyzed C−H activation reaction. The 9S3 ligand is more electron-deficient(π-acidic) than the ligands previously used in C−H bond-breaking reactions. To date, the primary ancillary ligandsemployed in transition-metal-catalyzed C−H activation areelectron rich (σ-donating) and include, but are not limited to,aza crown ethers, cyclopentadienyl (Cp) and related deriva-tives, tris(pyrazolyl)borate (Tp) and related derivatives, andtridentate pincer ligands (PCP, PNP, etc.). However, it is likelythe intramolecular fluxionality on the metal center and not theπ-acidity of the 9S3 ligand that facilitates the C−H activation ofnitromethane. The work presented herein shows that the 9S3ligand plays a key role in breaking the C−H bond of twonitromethane molecules to produce a bis(nitromethanate)compound. Though the C−H bond of nitromethane is notinert (pKa = 10) in comparison to hydrocarbons such asmethane and benzene, future studies with other thia-crown-containing complexes could be explored for these molecules.

■ EXPERIMENTAL SECTIONGeneral Methods. Unless otherwise noted, all reactions and

procedures were performed under standard laboratory conditions. Allsolvents and reagents were purchased from commercial sources andused as received. 1H, 2H, 13C, and 19F NMR spectra were obtained ona JEOL ECX 400 MHz spectrometer (operating frequencies for 2H,13C, and 19F NMR spectra are 61, 100, and 376 MHz, respectively)and referenced against tetramethylsilane using residual solvent protonsignals (1H NMR), the deutarated solvent (2H NMR), or the 13Cresonances of the deuterated solvent (13C NMR). 13C DEPTspectroscopy was used to confirm proton connectivity. 19F NMRspectra were referenced against an external standard of hexafluor-obenzene (δ −164.9). Unless otherwise noted, NMR spectra wereacquired at room temperature. IR spectra were obtained on a BrukerAlpha FT-IR instrument using solid samples or as thin films on a ZnSeATR crystal. The compound [Pd(dppe)(OAc)2] was prepared aspreviously reported.37

[Pd(9S3)(OAc)2] (1). A mass of {Pd(OAc)2}3 (0.1530 g, 0.6814mmol) was added to a colorless solution of 9S3 (0.1260 g, 0.6979mmol) in acetone (20 mL), and the mixture was stirred at roomtemperature for 1 h. The resulting yellow precipitate was collected byvacuum filtration, washed with acetone, and dried in vacuo (0.269 g,98.0% yield). 1H NMR (CD3OD, δ): 3.44−3.15 (12 H, AA′BB′, 9S3,−CH2) and 1.96 (6 H, s, OAc−, −CH3).

13C NMR (CD3OD, δ):176.8 (s, OAc−, −CO), 37.5 (br s, 9S3, −CH2), 21.9 (s, OAc

−, −CH3).13C DEPT NMR (CDCl3, δ): 40.9 (9S3, −CH2), 22.4 (OAc

−, −CH3).

Scheme 3. Proposed Mechanism of Formation of[Pd(9S3)(CH2NO2)2] (2) from [Pd(9S3)(OAc)2] (1) andNitromethanea.

aThe dashed lines represent a long-distance interaction, dotted linesrepresent bond making/breaking, and the curved arrows represent the9S3 intramolecular fluxtionality. Note that the labeled sulfur atomsmatch those in Figure 1.

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13C DEPT NMR (CD3NO2, δ): 38.5 (9S3, −CH2), 22.4 (OAc−,−CH3). Anal. Calcd for C10H18O4PdS3: C, 29.67; H, 4.48; S, 23.76.Found: C, 29.65; H, 4.39; S, 23.49. IR (solid, cm−1): 2990 (C−H),2956 (C−H), 2914 (C−H), 2895 (C−H), 1612, 1591 (s, CO, νa,OAc−), 1366, 1313 (s, CO, νs, OAc

−), 1006, 925, 876, 676 (C−S),613. The electronic absorption spectrum (800−200 nm) measured inmethanol showed one λmax at 283 nm (ε = 8 × 103 L mol−1 cm−1) withshoulder peaks at 420 and 340 nm. Due to the limited solubility of 1 inmethanol, an accurate calculation of the molar extinction coefficientswas not possible. A crystal of 1 suitable for X-ray diffraction was grownby slow evaporation from a chloroform-d solution.[Pd(9S3)(CH2NO2)2] (2). A mass of [Pd(9S3)(OAc)2] (1; 0.0191

g, 0.0472 mmol) was partially dissolved in methanol-d4 (0.75 mL) andtransferred to an NMR tube. Nitromethane (0.050 mL, 0.93 mmol)was added to the slightly heterogeneous yellow solution, whichimmediately turned homogeneous and red. The reaction wasmonitored periodically by 1H NMR spectroscopy over the course of16 h at room temperature, at which time complete conversion to 2 wasaccomplished. 1H NMR (CD3OD, δ): 4.83 (2 H, s, −CH2NO2), 4.78(2 H, s, −CH2NO2), 3.25 − 2.95 (12 H, m, AA′BB′, 9S3, −CH2), 1.91(6 H, s, HOAc, −CH3).

13C NMR (CD3OD, δ): 174.4 (s, acetic acidCO), 34.0 (br s, 9S3 and −CH2NO2, -CH2, LW1/2 = 25 Hz), 32.8 (s,−CH2NO2, LW1/2 = 17 Hz), 19.5 (s, acetic acid CH3, LW1/2 = 15 Hz).13C NMR (CD3NO2, δ): 174.4 (s, HOAc, −CO), 36.2 (br s, 9S3,−CH2), 32.8 (br s, −CH2NO2), 19.5 (s, HOAc, −CH3). IR (thin film,cm−1): 1486 (s, −NO2, νa), 1338 (s, −NO2, νs). The electronicabsorption spectrum (800−200 nm) measured in methanol showedone λmax at 270 nm (ε = 1.01 × 103 L mol−1 cm−1) with a weakershoulder peak at 450 nm, which is likely a d−d transition and accountsfor the red color of 2. A crystal of 2 suitable for X-ray diffraction wasgrown by diffusion of diethyl ether into a nitromethane solution of 2.Due to the unusual structure of 2, a second and confirmatory crystalstructure was determined using a different diffractometer (see theSupporting Information), and the crystal was grown from the slowevaporation of a methanol-d4 solution.[Pd(9S3)(O2CCF3)2] (3). A mass of [Pd(O2CCF3)2] (0.1500 g,

0.4512 mmol) was added to a colorless solution of 9S3 (0.0814 g,0.451 mmol) in acetone (10 mL), and the mixture was stirred at roomtemperature for 1 h. The resulting brown-red precipitate was collectedby vacuum filtration, washed with acetone, and dried in vacuo (0.184g, 79.6% yield). 1H NMR (CDCl3, δ): 3.48−3.08 (12 H, AA′BB′, 9S3,−CH2).

13C NMR (CDCl3, δ): 161.3 (q, 2JCF = 37 Hz, −O2CCF3),110.5 (q, 1JCF = 291 Hz, −O2CCF3), 35.5 (br s, 9S3, −CH2).

19F NMR(CDCl3, δ): −75.1 (s, −CF3). IR (solid, cm−1): 2980 (C−H), 2928(C−H), 1676 (s, CO, νa), 1450, 1409 (s, CO, νs), 1220, 1194,1137, 1104 (s, C−F), 922, 888, 815, 795, 715, 677 (C−S), 615, 550.Anal. Calcd for C10H12F6O4PdS3: C, 23.42; H, 2.36; S, 18.76. Found:C, 23.88; H, 2.41; S, 19.30.44 A partial structure was obtained on acrystal grown by diffusion of diethyl ether into a nitromethane solutionof 3 (see the Supporting Information).Attempt To Isolate [Pd(9S3)(CH2NO2)2] (2). A yellow

heterogeneous solution of 1 (0.1223 g, 0.3021 mmol) in degassedCH3OH (10 mL) was transferred to a round-bottom flask. The flaskwas purged with N2 for 20 min. Nitromethane (0.32 mL, 4.61 mmol)was added by syringe, and the mixture was stirred at room temperatureunder N2 for 24 h, at which time the solution had turnedhomogeneous and red. The solvent was reduced in vacuo on a rotaryevaporator, and diethyl ether was added to precipitate the product. Asmall amount of red solid was collected by vacuum filtration andwashed with degassed diethyl ether. 1H NMR in CD3OD showed anapproximately 6:1 ratio of complex 2 to complex 1. However, while insolution the color changed from red to green, indicating decom-position (the likely product is the green [Pd(9S3)2]

2+ complex) in theabsence of excess nitromethane.Reaction with Related Complexes. Reaction of [Pd(dppe)-

(OAc)2] with Nitromethane. A yellow homogeneous solution of[Pd(dppe)(OAc)2] (0.0050 g, 0.0093 mmol) in CD3OD (0.75 mL)was transferred to an NMR tube. Nitromethane (10 μL, 0.186 mmol)was added, and the reaction was followed by 1H and 31P NMR

spectroscopy. No reaction was observed, even upon heating to 50 °Cfor 16 h.

Reaction of [Pd(9S3)(O2CCF3)2] (3) with Nitromethane. A yellowheterogeneous solution of 1 (0.0110 g, 0.0215 mmol) in CD3OD(0.75 mL) was transferred to an NMR tube. Nitromethane (23 μL,0.428 mmol) was added. No reaction was observed after 16 h at roomtemperature by 1H and 19F NMR spectroscopy. After 48 h, a smallamount of a new 9S3-containing compound was observed by 1H and19F NMR spectroscopy; however, this product did not involvenitromethane and trifluoroacetic acid was not observed.

Reaction of [Pd(9S3)(Cl)2] with Nitromethane. An orangeheterogeneous solution of [Pd(9S3)(Cl)2] (0.0125 g, 0.0345 mmol)in CD3OD (0.75 mL) was transferred to an NMR tube. Nitromethane(38 μL, 0.699 mmol) was added, and the reaction was followed by 1HNMR spectroscopy. No reaction was observed even after 16 h at roomtemperature.

Reaction of [Pd(9S3)(OAc)2] (1) with Nitro-Organics andSolvents. Nitrobenzene. A yellow heterogeneous solution of 1(0.0100 g, 0.0247 mmol) in CD3OD (0.75 mL) was transferred to anNMR tube. Nitrobenzene (40 μL, 0.389 mmol) was added, and thereaction was followed by 1H NMR spectroscopy. No reaction wasobserved at room temperature, and upon heating to 50 °C there wasdecomposition to multiple uncharacterized products that did notinvolve nitrobenzene.

Nitrocyclopentane. A yellow heterogeneous solution of 1 (0.0119g, 0.0294 mmol) in CD3OD (0.75 mL) was transferred to an NMRtube. Nitrocyclopentane (15 μL, 0.142 mmol) was added. After 24 h,approximately 75% conversion to a single, new, uncharacterized 9S3-containing product was observed by 1H NMR spectroscopy; however,this product did not involve the nitrocyclopentane.

Acetone. A yellow heterogeneous solution of 1 (0.0102 g, 0.0025mmol) in CD3OD (0.75 mL) was transferred to an NMR tube.Acetone (37 μL, 0.504 mmol) was added. No reaction was observedinitially by 1H NMR spectroscopy; however, after 48 h a new unknown9S3-containing complex was observed that did not involve acetone.

Acetonitrile. A yellow heterogeneous solution of 1 (0.0095 g,0.0024 mmol) in CD3OD (0.75 mL) was transferred to an NMR tube.Acetonitrile (25 μL, 0.469 mmol) was added. No reaction wasobserved by 1H NMR spectroscopy, and upon heating to 60 °Cdecomposition to multiple uncharacterized products was observed.

H/D Reaction Studies with [Pd(9S3)(OAc)2] (1). CH3NO2/CD3OD with 1. A yellow heterogeneous solution of 1 (0.0204 g,0.0504 mmol) in CD3OD (0.75 mL) was transferred to an NMR tube.Nitromethane (14 μL, 0.0261 mmol) was added, and the reaction wasfollowed by 1H NMR spectroscopy until it had reached completion bymonitoring the nitromethanate and 9S3 region. At that time a 2HNMR spectrum was taken, which showed no deuterium incorporationin the nitromethanate region.

CD3NO2/CH3OH with 1. A yellow heterogeneous solution of 1(0.0197 g, 0.0487 mmol) in CD3NO2 (0.75 mL) was transferred to anNMR tube. Methanol (39 μL, 0.964 mmol) was added, and thereaction was followed by 1H NMR spectroscopy until it had reachedcompletion by monitoring the nitromethanate and 9S3 region. At thattime a 13C NMR spectrum was taken, which showed no deuteriumcoupling in the nitromethanate region, and the nitromethanateresonance was observed in the 1H NMR.

CD3NO2/CD3OD with 1. A yellow heterogeneous solution of 1(0.0101 g, 0.0249 mmol) in CD3NO2 (0.75 mL) was transferred to anNMR tube. Methanol-d4 (20 μL, 0.497 mmol) was added, and thereaction was followed by 1H NMR spectroscopy until it had reachedcompletion by monitoring the nitromethanate and 9S3 region. At thattime a 13C NMR spectrum was taken, which showed no deuteriumcoupling in the nitromethanate region, and the nitromethanateresonance was observed in the 1H NMR spectrum.

CD3NO2/CH3OH without 1. A 0.1 mL portion of methanol wasadded to 0.7 mL of nitromethane-d4 in an NMR tube. After 24 h atroom temperature, no deuterium incorporation into the methanolfrom the nitromethane-d4 was observed by 13C NMR.

CD3NO2 with 1. A yellow heterogeneous solution of 1 (0.0106 g,0.0487 mmol) in CD3NO2 (0.75 mL) was transferred to an NMR

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tube. The reaction was followed by 1H NMR spectroscopy until it hadreached completion by monitoring the nitromethanate and 9S3 region.At that time a 13C NMR spectrum was taken, which showed nodeuterium coupling in the nitromethanate region, and the nitro-methanate resonance was observed in the 1H NMR.Crystallographic Information. Complex 1. A crystal (approx-

imate dimensions 0.17 × 0.17 × 0.04 mm3), grown from a methanolsolution, was placed onto the tip of a 0.1 mm diameter glass capillaryand mounted on a Rigaku AFC8S Mercury CCD diffractometer fordata collection at 173(2) K. A preliminary set of cell constants wascalculated from reflections harvested from several frames. The datacollection was carried out using Mo Kα radiation (λ = 0.710 73 Å,graphite monochromator). The intensity data were corrected forabsorption and decay (REQAB).45 Final cell constants were calculatedfrom the xyz centroids of 12 102 strong reflections from the actual datacollection after integration (CrystalClear).46 Please refer to Table 1 foradditional crystal data and refinement information. The structure wassolved using SHELXS-97 and refined using SHELXL-97.47 The spacegroup P21/c was determined on the basis of systematic absences andintensity statistics. A direct-methods solution was calculated, whichprovided most non-hydrogen atoms from the E map. Full-matrix least-squares/difference Fourier cycles were performed, which located theremaining non-hydrogen atoms. All non-hydrogen atoms were refinedwith anisotropic displacement parameters. All hydrogen atoms wereplaced in ideal positions and refined as riding atoms with relativeisotropic displacement parameters. The final full-matrix least-squaresrefinement converged to R1 = 0.1316 and wR2 = 0.2519 (F2, all data).Complex 2. A crystal (approximate dimensions 0.48 × 0.07 × 0.07

mm3), grown from diethyl ether diffusion into a nitromethanesolution, was placed onto the tip of a 0.1 mm diameter glass capillaryand mounted on a Rigaku AFC8S Mercury CCD diffractometer fordata collection at 183(2) K. A preliminary set of cell constants wascalculated from reflections harvested from several frames. The datacollection was carried out using Mo Kα radiation (λ = 0.710 73 Å,graphite monochromator). The intensity data were corrected forabsorption and decay (REQAB).45 Final cell constants were calculatedfrom the xyz centroids of 11 480 strong reflections from the actual datacollection after integration (CrystalClear).46 Please refer to Table 1 foradditional crystal data and refinement information. The structure wassolved using SHELXS-97 and refined using SHELXL-97.47 The spacegroup P21/c was determined on the basis of systematic absences andintensity statistics. A direct-methods solution was calculated whichprovided most non-hydrogen atoms from the E map. Full-matrix least-squares/difference Fourier cycles were performed which located theremaining non-hydrogen atoms. All non-hydrogen atoms were refinedwith anisotropic displacement parameters. All hydrogen atoms wereplaced in ideal positions and refined as riding atoms with relativeisotropic displacement parameters. The final full-matrix least-squaresrefinement converged to R1 = 0.0478 and wR2 = 0.0804 (F2, all data).Complex 2. A second crystal (approximate dimensions 0.40 × 0.20

× 0.10 mm3, grown from a methanol solution) was placed onto aBruker SPINE-pin and mounted on a Bruker SMART X2Sdiffractometer. A data set was collected. The unit cell parametersand space group were identical with those for the other complex 2study. Crystallographic data for this confirmatory structure arepresented in the Supporting Information.

■ ASSOCIATED CONTENT*S Supporting InformationCIF files giving crystallographic data for complexes 1 and 2 andtables, text, and figures giving additional experimental data. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: John-Lee@utc.edu (J.P.L.); Greg-Grant@utc.edu(G.J.G.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSAcknowledgements are made to the following for theirgenerous support of this research: the National ScienceFoundation RUI Program (CHE-0841659 and CHE-0303958), the National Science Foundation MRI Program(CHE-0951711) for purchase of the Bruker SMART X2Ssingle-crystal diffractometer at the University of Tennessee atChattanooga, and the Grote Chemistry Fund at UTC.

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