Revisiting the Aufbau Reaction with Acetylene: Further Insights from Experiment and Theory

Published: February 23, 2011

r 2011 American Chemical Society 1569 dx.doi.org/10.1021/om101114c |Organometallics 2011, 30, 1569–1576

ARTICLE

pubs.acs.org/Organometallics

Revisiting the Aufbau Reaction with Acetylene: Further Insightsfrom Experiment and TheorySamuel S. Karpiniec,† David S. McGuinness,*,† Michael G. Gardiner,† Brian F. Yates,† and Jim Patel‡

†School of Chemistry, University of Tasmania, Private Bag 75, Hobart 7001, Australia‡CSIRO Earth Science and Resource Engineering, Bayview Avenue, Clayton, Melbourne 3168, Australia

bS Supporting Information

1. INTRODUCTION

We have recently been exploring the metal-catalyzed oligo-merization of acetylene to liquid products.1,2 Acetylene/hydro-gen mixtures can be produced by natural gas pyrolysis; hence thisstep coupled with acetylene oligomerization to fuel-range pro-ducts represents a potential alternative gas-to-liquid (GTL)methodology.3-5 While trialing a range of potential catalystsbased upon transition metal and lanthanide complexes, we weresurprised to discover that the majority of chain growth was in factoccurring at the cocatalyst, triethylaluminum.2

The stepwise growth of aluminum alkyls through ethyleneinsertion, termed the Aufbau reaction, was reported by Ziegler inthe early 1950s (reaction 1).6 This reaction has formed the basisof large-scale industrial production of ethylene oligomers for overhalf a century, with the advantage that a relatively narrow andcontrollable Poisson distribution of chain lengths results.7,8 Theanalogous Aufbau reaction with acetylene (reaction 2) wasexplored by Ziegler and Wilke, but met with limited success.9

The reaction was found to cease after one insertion, leading toEt2Al(CHdCHEt). It is somewhat surprising to find that thisreaction has apparently not been the subject of further investiga-tion, given the historical importance of the Aufbau reaction. Ourfindings that, under modified conditions, unsaturated oligomerscan be formed by progressive acetylene insertions are thus offundamental interest.

While we have investigated the reaction between AlEt3 andacetylene in some depth,1,2 a number of observations remain tobe accounted for. The first insertion of acetylene into the Al-Etbond is reasonably facile, yet the subsequent insertion is moredifficult. Moreover, while this second insertion into the Al-butenyl group is apparently more difficult, it is still more facilethan insertion into a second Al-Et bond. As such, oligomergrowth occurs predominately at a single chain of each Al, with theremaining two ethyl groups remaining largely unreacted until thelatter stages of growth. A further intriguing aspect of this reactionis the introduction of branching in longer chain oligomers by ahitherto unknown mechanism, although this has not beeninvestigated in the present study. The ability to limit chain lengththrough σ-bond metathesis with hydrogen (chain transfer) wasalso investigated (reaction 3). While this technique could inprinciple be used to make the process catalytic (through sub-sequent insertion into the Al-H bond), it was of limited success.

Herein we have studied the individual reaction steps of theseprocesses, and the nature of the products, through a combinationof experiment and theory. Sequential migratory insertion ofacetylene beginning with AlEt3 has been studied, as has σ-bondmetathesis with hydrogen. The results provide insight into theearly stages of acetylene chain growth at AlEt3 and answer someof the remaining questions surrounding this system.

Received: November 29, 2010

ABSTRACT: The first steps of acetylene chain growth at AlEt3, via migratoryinsertion, have been investigated both experimentally and theoretically. The firstinsertion into the Al-Et bond occurs readily, leading to the stable alkenyl-bridgeddimer [{Et2Al(μ-CHdCHEt)}2] (1). The alkenyl bridging mode has been ob-served through isolation and structural analysis of Al2Et2(OC6H3Ph2)2(μ-CHdCHEt)(μ-OC6H3Ph2) (2), synthesized by way of controlled reaction of 1with 2,6-Ph2C6H3OH. This stable binding mode increases the barrier to a second insertion of acetylene, as insertion proceedsthrough monomeric Al-acetylene adducts. The energetics of further chain growth, dimer formation, and chain termination viahydrogenolysis were investigated theoretically. The results provide further insight and explanation for previous experimentalfindings in relation to the Aufbau reaction with acetylene.

1570 dx.doi.org/10.1021/om101114c |Organometallics 2011, 30, 1569–1576

Organometallics ARTICLE

2. RESULTS AND DISCUSSION

2.1. First Insertion of Acetylene. The reaction of acetylenewith triethylaluminumwas reported byWilke9 in 1960, followingfrom Ziegler’s studies of the Aufbau reaction.6 Wilke documen-ted the single insertion of acetylene to form Et2Al-CHdCHEt.The growth of higher products due to further insertion wasthought not to occur, but the formation of higher branchedspecies was reported to occur due to condensation of unsaturatedorganoaluminum species. We have recently shown that furtherinsertion, leading to unsaturated oligomeric chains, is in factpossible at elevated temperatures and under more dilute condi-tions (which disfavors intermolecular condensation reactions).1,2

Nonetheless, our results concur with those ofWilke, insomuch asthe first insertion appears facile, while subsequent insertions aremuch more difficult. As such, this first fundamental step has beenexperimentally re-examined and modeled with the aid of theore-tical methods.When a toluene solution of AlEt3 was heated to 50 �C in the

presence of 1 bar gauge acetylene, complete conversion (byNMR) to Et2Al-CHdCHEt occurred within 1 h. The productwas isolated as a pyrophoric liquid after removal of toluene undervacuum and characterized by NMR spectroscopy (1H, 13C,COSY, HSQC; see Experimental Section and Supporting In-formation). Wilke postulated that a butenyl-bridged dimericstructure was likely, and our characterization of this compoundleads to the same conclusion. In particular, a very large downfieldchemical shift of the alkene β-hydrogen resonance (7.45 ppm)indicates significant polarization of the double bond towardaluminum, as illustrated in structure 1. The corresponding 13CNMR resonance occurs at 187.7 ppm.

In the interest of structurally authenticating this bridgingmode, we investigated methods for conversion to a compoundmore amenable to crystallization. It was reasoned that controlledprotonolysis with phenol may selectively replace the terminal

ethyl groups. Unfortunately, treatment of the dimer with fourequivalents of PhOH led to the isolation of Et4Al4(μ-OPh)6-(OPh)2, in which the putative bridging butenyl goups have beenreplaced by bridging phenoxides. This compound has beenstructurally characterized and is presented in the SupportingInformation. It was thought that greater steric bulk on the phenolmay hinder formation of two aryloxide bridges, in particular thatphenyl groups flanking the oxygen may make it difficult toaccommodate a second phenol unit in a bridging position. Assuch, two equivalents of 2,6-diphenylphenol were reacted withthe dimer and a colorless crystalline solid was isolated afterconcentration and cooling. Crystal structure analysis revealed themonobutenyl-bridged dimer Et2Al2(OAr)2(μ-CHdCHEt)(μ-OAr) (2), shown in Figure 1. Evidently, a number of productsare formed, as judged by the complexity of the NMR spectrum ofthe bulk material. Nonetheless, the crystal structure obtainedserves to illustrate the presence of butenyl bridging between twoaluminum centers. The hybrid stick/ORTEP representation(Figure 1a) shows the basic structure, featuring the bridgingbutenyl group. The double bond of this group shows the protonsto be arranged in a cis fashion, which is consistent with thecomputationally predicted geometry resulting from insertion viaa four-center transition state (see below). The bulky 2,6-diphe-nylphenol groups clearly block access to the bridging alkene,preventing further hydrolysis and preserving this structuralfeature. This steric obstruction is also depicted in the partialspace-filling model in Figure 1b. The distances from the bridgingoxygen of the phenoxide to the aluminum atoms are roughlyequivalent at 1.868(2) and 1.874(2) Å and, as expected, arelonger than the respective Al-Oterminal bonds of 1.732(2) and1.734(2) Å. The two Al-Cbridging distances differ by 0.021 Å[2.098(3) and 2.077(4) Å], perhaps suggesting a slight asymme-trical distribution of electron density around this three-centerAl-C-Al bond.In order to investigate this reaction theoretically (theoretical

methods are discussed in Section 4.7), the likely reactionintermediates considered were a coordination complex oftriethylaluminum and acetylene and a transition state for theinsertion of acetylene into an Al-Et bond. Also considered wasthe monomer-dimer equilibrium of triethylaluminum. Thedimeric species is known to be more energetically favorable thanthe monomer; hence the energy required to break the dimer

Figure 1. Molecular structure of Al2Et2(OC6H3Ph2)2(μ-C4H7)(μ-OC6H3Ph2) (2). (a) Hybrid stick/ORTEP view. Selective bond distances (Å) andangles (deg): Al1-O1 1.732(2); Al1-C1 1.944(3); Al1-O2 1.868(2); Al1-C5 2.098(3); C5-C6 1.352(5); Al2-O2 1.874(2); Al2-C5 2.077(4).O1-Al1-C1 115.23(13); O2-Al1-C5 86.04(11); C1-Al1-C5 113.03(15); O1-Al1-O2 112.49(11); Al1-O2-Al2 96.52(9); Al1-C5-Al283.95(12). (b) Partial space-filling model.



into the monomeric form is relevant to the overall process.10,11

The calculations were performed on the basis that coordinationand insertion do not occur while aluminum is in its dimeric form;attempts to model the coordination of acetylene to the Al2Et6dimer did not yield any viable intermediates. The relative energysurface for this reaction is shown in Figure 2, with all energiesrelative to Al2Et6 and free acetylene (on a per-Al basis, i.e., 1/2Al2Et6 þ HCtCH). All energies shown are ΔG values, whichhave been scaled to better represent solution values, as isdiscussed in Section 4.7. The barrier to monomer formationthus calculated is 12.4 kJ 3mol-1, which is in reasonable agree-ment with experimental values (8.212 and 12.113 kJ 3mol-1) .Geometry optimization led to a coordination complex of AlEt3and C2H2 with acetylene bound at around 2.7 Å (Al-C) fromplanar triethylaluminum. However, on the free energy surfacethis complex is marginally higher in energy than the separatedfragments, suggesting that a discrete AlEt3-C2H2 adduct doesnot form prior to insertion. The same observation has beenmadefor ethylene insertion into AlEt3.

8 The transition state involvesthe expected four-center structure and is similar to that in aprevious computational study, where direct insertion of acetyleneinto the Al-C bond of H2Al-CH3 was considered.14,15 Theeffective activation energy from the dimer is 83.4 kJ 3mol-1.Optimization beyond this point leads to Et2Al-CHdCHEt asthe primary product, which is 127.1 kJ 3mol-1 more stable thanthe reactants; however the final product is predicted to be thebutenyl-bridged dimer, which lies 171 kJ 3mol

-1 below the reac-tants. This strong stabilization brought about through dimerizationis relevant to the subsequent insertion steps; see below. Theseresults seem to support the experimental observations, which seesome of this insertion product being formed at room temperature,

but much more once the temperature is increased. It should benoted that the end product arrived at by geometry optimizationfeatures a cis-butenyl moiety at aluminum, as observed experimen-tally. The trans structure was also modeled for comparison, but wasfound to be only 3.4 kJ 3mol

-1 more stable than the cis isomer. Wedid not investigate the barrier for such an isomerization orinvestigate its direct formation via a different mechanism.To the best of our knowledge, the barrier to acetylene

insertion into AlEt3 has not previously been calculated. Theclosest comparisons that can be drawn are for insertion intoH2Al-R (R = Me, Et). The barriers calculated in these cases arerelative to the monomeric reactants (dimers were not consider-ed) and range from 67 kJ 3mol-1 {MP4(SDTQ)/6-31G(*)//HF/6-31G(*)}14 to ca. 80 kJ 3mol-1 {MP2/6-311þG(**)//HF/3-21G(*)}.15 Our barrier from the separated and monomer-ic reactants to the TS (71 kJ 3mol-1) is comparable.2.2. Second Insertion of Acetylene. Several permutations of

the reaction with a second equivalent of acetylene were con-sidered. We previously experimentally found that subsequentinsertion of acetylene occurs preferentially into the Al-butenylbond, rather than into a second Al-Et bond.2 As such, bothpossibilities have been studied, namely, insertion into the Al-butenyl group and insertion into a second Al-Et group. Wereasoned that the strong butenyl-bridging formed after oneacetylene insertion may explain why a second insertion is moredifficult. Indeed, as shown in Figure 3, the barrier to insertioninto the Al-butenyl group (103.3 kJ 3mol-1) is significantlyincreased relative to the first insertion into Al-Et. This increaseof ca. 20 kJ 3mol-1 in activation energy derives from the increasein energy required for dissociation of the butenyl-bridged dimer.In fact, the barrier from the acetylene-coordinated monomer for

Figure 2. Relative energy surface for the first insertion of acetylene into AlEt3 (kJ 3mol-1).



the second insertion (60.4 kJ 3mol-1) is somewhat lower thanthat for the first insertion (70.4 kJ 3mol-1). As such, thereluctance of the first insertion product to react further withacetylene simply relates to the stabilizing influence of butenylbridging (see below).The alternative of insertion into a second ethyl group follows a

similar pathway, except that the barrier to the transition state is14.6 kJ 3mol

-1 higher (117.9 kJ 3mol-1) than that for insertioninto the butenyl group. This agrees with the experimental resultsthat suggest some insertion occurs beyond the first ethyl group,especially in longer experiments; however the major kineticproduct would come from repeated insertion at Al-alkenyl.Herein we have only illustrated a second acetylene insertion

into Et2Al-CHdCHEt with a cis double-bond arrangement(the kinetic product of the first insertion). Experimentally, we seeno evidence for isomerization to the trans isomer at roomtemperature with the isolated product; however it may bepossible at the elevated temperatures of oligomerization experi-ments. As such, the reaction pathway for the trans isomer has alsobeen computed and is shown in the Supporting Information. Thesituation is very similar to that shown herein for the cis isomer,with the energies of the individual steps differing by only a fewkJ 3mol

-1.2.3. Third Insertion of Acetylene. The third insertion of

acetylene, into the Al-hexadienyl bond, was modeled beginningfrom the cis,cis-AlEt2(hexadienyl) dimer and proceeding to theprimary product cis,cis,cis-AlEt2(octatrienyl). The dimerized pro-duct was not considered, as the stabilization expected for this isvery similar to that for the reactant (see below, Section 2.4). Therelative energy surface for this reaction is shown in Figure 4. Themost notable feature is a significantly reduced overall barrier for

this third acetylene insertion (84.2 kJ 3mol-1) relative to thesecond insertion into an Al-butenyl bond (in fact the thirdinsertion is comparable in barrier to the first insertion intoAlEt3). The reason for this change is a lower barrier from themonomeric acetylene coordination complex to the transitionstructure (45.1 kJ 3mol-1), while the energy required to breakthe alkenyl-bridged dimer remains much the same. This differ-ence may be attributable to the effect of double-bond conjuga-tion in the case of the Al-hexadienyl structure, which apparentlyleads to a more facile acetylene insertion in this case. This effectwould be expected to persist in subsequent insertion steps,although we did not explore this beyond a third insertion. Itmay be concluded that, were it not for the formation of stablealkenyl-bridged dimers, acetylene chain growth at Al wouldbecome reasonably facile following the first two insertions. Thevarious modes of dimer formation that are possible are thereforeexplored below.2.4. Energetics of Dimerization. In calculating the energy

surfaces above, the dimers considered are those resulting fromhomodimerization of identical monomeric Al compounds. Inreality, once acetylene insertion has commenced, a range ofcompounds are present, and as such, a range of oligomerizedbridging ligands are possible. In order to evaluate the effect thismay have on the reaction, we have compared the stabilizationenergy realized through various bridging modes as shown inChart 1. A variety of combinations of AlEt3 and AlEt2(butenyl)were compared, in which the alkenyl group is cis. The transstructures were also investigated, and all displayed relativeenergies within ca. 3 kJ 3mol-1 of the cis structures. The dimerscan be comprised of one of each monomer (4 and 7) or twoof the same (3 for AlEt3, 5, 6, and 1 for AlEt2(butenyl)).

Figure 3. Relative energy surface for the two possible modes of acetylene insertion into cis-AlEt2(butenyl) (kJ 3mol-1).



In considering the relative energies, there seem to be three appro-ximate energy levels corresponding to the type of bridging ligand.Two bridging ethyl groups lead to a stabilization of around12 kJ 3mol

-1, relative to the monomers. With one butenyl bridge,the stabilization energy is increased to 27 kJ 3mol

-1, while twobutenyl bridges lead to even further stabilization as discoveredabove (ca. 44 kJ 3mol

-1). The hexadienyl-bridged structure 8 wasalso considered and has a stabilization energy close to the butenyl-bridged dimer 1. As such, conjugation leads to no effect on bridgingstrength, and this stabilization is expected to be similar for higherconjugated oligomers as well.These results are in accord with the known preference for

phenyl bridging in [Et4Al2(μ-Ph)2].16-18 Like the butenyl group,

the sp2-hybridized phenyl ligand hasπ-orbital electron density tocontribute to the three-centered bond. This is not available withbridging alkyl groups, resulting in an electron-deficient (andweaker) three-centered, two-electron bond. We note a similarpolarization to that shown in structure 1 has been invoked for[Et4Al2(μ-Ph)2].

19 We expect structures with two bridgingalkenyl groups to be strongly favored, which validates the aboveanalysis of the relative energy surfaces.2.5. Chain Transfer with Dihydrogen. As discussed in the

Introduction, our previous attempts to promote chain transferwith dihydrogen, in order to make the process catalytic in Al,were met with limited success. In particular, hydrogenolysis ofthe Al-C bond is hard to achieve, and when it does occur, itseems to lead to deactivation toward further insertion.2 As such,this reaction has been modeled by consideration of the reactionbetween Et2Al(butenyl) and H2 (Figure 5). The high barrier toσ-bond metathesis (160.7 kJ 3mol-1) displayed for this reactionis consistent with the experimental findings. This barrier is much

higher than further acetylene insertion, and thus explains whyhigh proportions of dihydrogen (H2:HCtCH ≈ 5:1) arerequired before significant chain transfer is observed. We wereunable to locate a discrete H2 coordination complex prior to thetransition structure for this reaction.The product hydride, Et2AlH, is known to exist as a trimer,10

and indeed our modeling predicts a very high stabilization energy

Figure 4. Relative energy surface for the insertion of acetylene into cis,cis-AlEt2(hexadienyl) (kJ 3mol-1).

Chart 1a

aRelative energies (kJ 3mol-1) of alkyl/alkenyl-bridged dimers. Energiesshown are for the reaction Et2Al-R f 1/2 Al2Et4(μ-R)2.



for this structure relative to the monomer (-57.4 kJ 3mol-1 per

Al). The hydride-bridged dimer was also modeled and lies 46.5kJ 3mol

-1 below the monomer. We previously speculated thatsuch hydride-bridged structures might be very stable and sug-gested that this may explain the lack of reactivity of Et2AlHtoward further insertion. The theoretical results presented hereinsupport this notion.

3. SUMMARY AND CONCLUSIONS

We have investigated the first steps of acetylene chain growthat AlEt3. The first insertion into the Al-Et bond occurs readily,leading to a stable alkenyl-bridged dimer, which has beencharacterized both experimentally and theoretically. Subsequentinsertion is hampered by this strong bridging mode. The resultspredict that growth at Al would become reasonably facile were itnot for the formation of these stable dimers and highlight theimportance of considering the energetic cost of the dimer-monomer transformation in these studies. This is expected to beparticularly the case in experimentally relevant reactions in thecondensed phase. The effect of dimerization has not beenconsidered in past studies of alkene and alkyne insertion intoalkylaluminum,14,15,20,21 although we note that this will be lessimportant for alkene insertion reactions, where strong alkenylbridging is not a factor.

It is interesting to compare the conventional Aufbau reactionwith ethylene to the process studied herein. The barrier toethylene insertion into AlEt3 has been evaluated experimentally,and the free energy of activation at 500 K is reported as 133kJ 3mol

-1 relative to the monomeric reactants in a gas phaseprocess.22 As such, insertion of acetylene into AlEt3 probably hasa significantly lower initial barrier (70 kJ 3mol-1 from themonomer) and most likely still has a lower barrier in subsequentsteps, when strong alkenyl bridging becomes relevant. It mighttherefore be expected that an Aufbau process with acetylene

should be trivial, given the success of the reaction with ethylene.The main explanation for why this is not the case relates toreaction conditions, insomuch as the growth reaction withethylene is conducted at pressures of 50-100 bar.8 Suchpressures are impractical for acetylene, which is at risk ofspontaneous explosive decomposition at pressures greater thanseveral atmospheres.23 As such, our experimental studies wererestricted to acetylene pressures of 1 bar gauge (2 barabsolute).1,2 Even under these conditions, with long reactiontimes we did witness the formation of highly insoluble poly-acetylene, illustrating that growth does advance. This highlightsanother difference between the ethylene and acetylene growthreactions. With ethylene, chain transfer via β-H elimination iscompetitive, which results in a practical upper limit on oligomerchain length of ca. 100 insertions.8 Hence, insoluble polyethyleneis not formed. No such chain transfer reaction occurs foracetylene growth. This can be partially overcome by introducinghydrogen, although it has been shown herein that σ-bondmetathesis with dihydrogen has a high barrier. As such, it hasproven very difficult to effectively control the oligomer chainlength distribution or to prevent the formation of polyacetylenein longer runs. Ultimately, it may prove more effective to partiallyhydrogenate acetylene to ethylene if it is to be transformed tofuel-range oligomers.4 Finally, we note that the present work hasbeen restricted to the early steps of acetylene chain growth at Al.The peculiar formation of branched higher oligomers1,2 remainsunresolved, and in this regard further work is required.

4. EXPERIMENTAL AND THEORETICAL METHODS

4.1. General Procedures. All manipulations were performedunder an atmosphere of UHP argon (BOC gases) using standardSchlenk techniques or in an MBraun nitrogen glovebox. Solvents werepurified by passage through an Innovative Technologies solvent pur-ification system and, where appropriate, stored over a sodium mirror.

Figure 5. Relative energy surface for σ-bond metathesis between Et2Al-CHdCHEt and H2 (kJ 3mol-1).



Acetylene (BOC Gases instrumental grade) was purified by passagethrough a column of activatedmolecular sieves (3 Å) and alumina. NMRspectra were recorded on a Varian Mercury Plus NMR spectrometeroperating at 300 MHz (1H) or 75 MHz (13C).4.2. Preparation of [Et2Al(CHdCHEt)]2 (1). A toluene solution

of AlEt3 (20 mL, 1.9 M) was heated in an oil bath to 50 �C with stirring,before the flask was exposed to 1 bar gauge of acetylene and flushed 4 or5 times. Stirring was continued for 1 h under a continuous supply ofacetylene; then the acetylene was purged with argon and the flask left tocool. GC analysis of a quenched sample at this stage showed the solutionto contain primarily ethane and 1-butene and no higher oligomers. Thesolvent was gently removed under vacuum over around 6 h, reducing thevolume by ∼40% and yielding a viscous, yellow, and pyrophoric liquid.NMR showed the conversion to be quantitative. 1H and 13C NMRsignals were assigned with the aid of HSQC. 1H NMR in C6D6: δ 7.45(m, 1H, Al-CHdCH-Et), 5.54 (d, J = 15 Hz, 1H, Al-CHdCH-Et),2.15 (m, 2H, Al-CHdCH-CH2-CH3), 1.17 (t, J = 8 Hz, 6H, Al-CH2-CH3), 0.84 (t, 3H, Al-CHdCH-CH2-CH3), 0.16 (q, J = 8 Hz, 4H,Al-CH2-CH3).

13CNMR inC6D6:δ 187.7 (Al-CHdCH-Et), 122.6 (Al-CHdCH-Et), 32.4 (Al-CHdCH-CH2-CH3), 12.3 (Al-CHdCH-CH2-CH3), 8.9 (Al-CH2-CH3), 1.9 (Al-CH2-CH3).4.3. Preparation of Al2Et2(OC6H3Ph2)2(μ-C4H7)(μ-OC6-

H3Ph2) (2). To a Schlenk flask under argon was added [{AlEt2-(C4H7)}2] (1, 431 mg, 1.54 mmol). In a separate flask, 2,6-diphenyl-phenol (765 mg, 3.11 mmol) was dissolved in toluene (4 mL). The flaskcontaining 1 was submerged in an ice bath at -12 �C, and the alcoholadded dropwise over 15 min with stirring. The yellow solution waswarmed to room temperature, concentrated under vacuum to an oilyconsistency, and placed in a freezer at-20 �C. Colorless crystals slowlyformed, which were suitable for X-ray diffraction (see below andSupporting Information). GC analysis of a quenched sample of the bulksolid showed an ethane:butene ratio of 3:1. The apparent disproportio-nation that occurs in this reaction meant that clean NMR and micro-analysis could not be obtained for this product, as it evidently contains amixture of compounds.4.4. Collection and Treatment of X-ray Crystallographic

Data. Data were collected at 100(2) K for crystals of 2 and Al4Et4-(OPh)8 mounted on Hampton Scientific cryoloops at the MX1 beam-line of the Australian Synchrotron. Data collection used BluIce software,and XDS was used for data reduction.24 The structures were solved bydirect methods with SHELXS-97, refined using full-matrix least-squaresroutines against F2 with SHELXL-97,25 and visualized using X-SEED.26

Details of the refinements appear in the cif files, including modelingdetails for a disordered phenoxide ligand in Al4Et4(OPh)8, but standardprocedures involved all non-hydrogen atoms being refined anisotropi-cally and hydrogen atoms being placed in calculated positions andrefined using a ridingmodel with fixed C-Hdistances of 0.95 Å (sp2C-H) and 0.98 Å (CH3), and Uiso(H) = 1.2Ueq(C) (sp

2) and 1.5Ueq(C)(sp3). For 2, all hydrogen atoms on the ethyl and butenyl ligands werelocated and positionally refined to offer additional identification of theseparticipative ligands in the reaction (some were later restrained; see ciffile for details). A summary of crystallographic data of the structures isgiven below, with full CIF files provided in the Supporting Information.4.5. Crystal data for 2:C62H56Al2O3,M = 903.03, colorless prism,

0.06� 0.03� 0.03 mm3, triclinic, space group P1 (No. 2), a = 9.961(3)Å, b = 11.907(3) Å, c = 20.919(5) Å, R = 78.717(3)�, β = 88.993(11)�,γ = 81.797(16)�, V = 2408.1(12) Å3, Z = 2, Dc = 1.245 g/cm3, F000 =956, μ = 0.108 mm-1, 3-BM1 Australian Synchrotron, Synchrotronradiation, λ = 0.77500 Å,T = 100(2) K, 2θmax = 52.9�, 26 394 reflectionscollected, 6908 unique (Rint = 0.0910). Final GooF = 1.031, R1 = 0.0677,wR2 = 0.1738, R indices based on 5509 reflections with I >2σ(I)(refinement on F2), 656 parameters, 6 restraints.4.6. Crystal data for Al4Et4(OPh)8: C56H60Al4O8, M = 968.96,

colorless prism, 0.08� 0.08� 0.05 mm3, monoclinic, space group P21/

n (No. 14), a = 12.819(3) Å, b = 15.4020(9) Å, c = 13.3430(9) Å, β =92.923(2)�, V = 2631.0(6) Å3, Z = 2,Dc = 1.223 g/cm

3, F000 = 1024, μ =0.141mm-1, 3-BM1Australian Synchrotron, Synchrotron radiation, λ =0.77500 Å, T = 100(2) K, 2θmax = 53.2�, 27 961 reflections collected,4075 unique (Rint = 0.0937). Final GooF = 1.050, R1 = 0.0733, wR2 =0.2063, R indices based on 3500 reflections with I >2σ(I) (refinementon F2), 338 parameters, 0 restraints.4.7. Theoretical Methods. All calculations were performed using

Gaussian0327 or Gaussian09,28 utilizing hardware from the AustralianPartnership for Advanced Computing Program (APAC), or NationalComputational Infrastructure. Geometry optimizations were performedusing the B3LYP29-32 functional, using the 6-31G(d) basis set.33,34

Single-point energies were calculated using 6-311þG(2d,p).35,36 It hasbeen noted that in the computational modeling of certain systems, forexample olefin polymerization, density functionals often do not accu-rately describe a number of mid-long-range interactions. This effectwas considered relevant to the system being studied herein. There areseveral approaches that are used to address this shortcoming, and anumber have been compared recently for the description ofhydrocarbons.37 Thus, a dispersion correction described by Grimmewas applied to the B3LYP single-point energies, with a scaling factor of1.05, to yield B3LYP-D values.38 Grimme’s method has been found tomore accurately describe long-range van der Waals forces in manysystems.

Gibbs free energy corrections in which the entropy contribution hasbeen scaled were applied throughout. Gas phase calculations provide apoor estimate of the true free energy changes in solution, and this isaccentuated when the number of molecules changes, as is the case in thefirst three steps of the reaction pathway. For instance, applying full freeenergy corrections predicted that monomeric AlEt3 is more stable thandimeric Al2Et6, which is experimentally not the case (triethylaluminumexists as the dimer in hydrocarbon solutions).12 A reviewer suggestedscaling the entropy contribution by a factor of 0.67 for benzene. As ourcatalysis was conducted in toluene solution, this seemed a suitableapproach and has been applied throughout. The free energy correctionthus applied to single-point energies wasGcorr =Hcorr- (TScorr� 0.67).This resulted in a calculated free energy of monomer formation of 12.4kJ 3mol-1, in reasonable agreement with experimental values (8.212 and12.113 kJ 3mol-1). It is worth noting that applying this correction doesnot significantly change the predicted activation free energies for thereactions. For instance, the barrier to the first insertion (83.4 kJ 3mol-1)with entropy contributions scaled is only modestly lower than thecalculated barrier with the full free energy correction applied (88.5kJ 3mol-1). The corresponding enthalpy of activation for this processwas calculated to be 73.1 kJ 3mol-1.

’ASSOCIATED CONTENT

bS Supporting Information. X-ray crystallographic files(CIF) for 2 and Al4Et4(OPh)8, structure of Al4Et4(OPh)8,additional NMR data for 1, and optimized geometries withabsolute energies of all stationary points (PDF). This materialis available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

D.S.M. and S.S.K. thank the CSIRO for funding, particularlythe late Prof. David Trimm, with whom this collaboration wasinitiated. This work was also funded by the Australian Research



Council through Discovery Project DP0665058. We thank theNational Computational Infrastructure (NCI) and the AustralianPartnership for Advanced Computing (APAC) for provision ofcomputing resources. X-ray data were obtained on MX1 at theAustralian Synchrotron, Victoria, Australia.24

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