Revisiting the Aufbau Reaction with Acetylene: Further Insights from

Feb 23, 2011 - The first steps of acetylene chain growth at AlEt3, via migratory insertion, have been investigated both experimentally and theoretical...
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Revisiting the Aufbau Reaction with Acetylene: Further Insights from Experiment and Theory Samuel 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 ABSTRACT: The first steps of acetylene chain growth at AlEt3, via migratory insertion, have been investigated both experimentally and theoretically. The first insertion into the Al-Et bond occurs readily, leading to the stable alkenyl-bridged dimer [{Et2Al(μ-CHdCHEt)}2] (1). The alkenyl bridging mode has been observed through isolation and structural analysis of Al2Et2(OC6H3Ph2)2(μCHdCHEt)(μ-OC6H3Ph2) (2), synthesized by way of controlled reaction of 1 with 2,6-Ph2C6H3OH. This stable binding mode increases the barrier to a second insertion of acetylene, as insertion proceeds through monomeric Al-acetylene adducts. The energetics of further chain growth, dimer formation, and chain termination via hydrogenolysis were investigated theoretically. The results provide further insight and explanation for previous experimental findings in relation to the Aufbau reaction with acetylene.

1. INTRODUCTION We have recently been exploring the metal-catalyzed oligomerization of acetylene to liquid products.1,2 Acetylene/hydrogen mixtures can be produced by natural gas pyrolysis; hence this step coupled with acetylene oligomerization to fuel-range products represents a potential alternative gas-to-liquid (GTL) methodology.3-5 While trialing a range of potential catalysts based upon transition metal and lanthanide complexes, we were surprised to discover that the majority of chain growth was in fact occurring at the cocatalyst, triethylaluminum.2 The stepwise growth of aluminum alkyls through ethylene insertion, termed the Aufbau reaction, was reported by Ziegler in the early 1950s (reaction 1).6 This reaction has formed the basis of large-scale industrial production of ethylene oligomers for over half a century, with the advantage that a relatively narrow and controllable Poisson distribution of chain lengths results.7,8 The analogous Aufbau reaction with acetylene (reaction 2) was explored by Ziegler and Wilke, but met with limited success.9 The reaction was found to cease after one insertion, leading to Et2Al(CHdCHEt). It is somewhat surprising to find that this reaction has apparently not been the subject of further investigation, given the historical importance of the Aufbau reaction. Our findings that, under modified conditions, unsaturated oligomers can be formed by progressive acetylene insertions are thus of fundamental interest.

While we have investigated the reaction between AlEt3 and acetylene in some depth,1,2 a number of observations remain to be accounted for. The first insertion of acetylene into the Al-Et bond is reasonably facile, yet the subsequent insertion is more difficult. Moreover, while this second insertion into the Albutenyl group is apparently more difficult, it is still more facile than insertion into a second Al-Et bond. As such, oligomer growth occurs predominately at a single chain of each Al, with the remaining two ethyl groups remaining largely unreacted until the latter stages of growth. A further intriguing aspect of this reaction is the introduction of branching in longer chain oligomers by a hitherto unknown mechanism, although this has not been investigated in the present study. The ability to limit chain length through σ-bond metathesis with hydrogen (chain transfer) was also investigated (reaction 3). While this technique could in principle be used to make the process catalytic (through subsequent insertion into the Al-H bond), it was of limited success.

Herein we have studied the individual reaction steps of these processes, and the nature of the products, through a combination of experiment and theory. Sequential migratory insertion of acetylene beginning with AlEt3 has been studied, as has σ-bond metathesis with hydrogen. The results provide insight into the early stages of acetylene chain growth at AlEt3 and answer some of the remaining questions surrounding this system.

Received: November 29, 2010 Published: February 23, 2011 r 2011 American Chemical Society

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Figure 1. Molecular structure of Al2Et2(OC6H3Ph2)2(μ-C4H7)(μ-OC6H3Ph2) (2). (a) Hybrid stick/ORTEP view. Selective bond distances (Å) and angles (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-Al2 83.95(12). (b) Partial space-filling model.

2. RESULTS AND DISCUSSION 2.1. First Insertion of Acetylene. The reaction of acetylene with triethylaluminum was reported by Wilke9 in 1960, following from Ziegler’s studies of the Aufbau reaction.6 Wilke documented the single insertion of acetylene to form Et2Al-CHdCHEt. The growth of higher products due to further insertion was thought not to occur, but the formation of higher branched species was reported to occur due to condensation of unsaturated organoaluminum species. We have recently shown that further insertion, leading to unsaturated oligomeric chains, is in fact possible at elevated temperatures and under more dilute conditions (which disfavors intermolecular condensation reactions).1,2 Nonetheless, our results concur with those of Wilke, insomuch as the first insertion appears facile, while subsequent insertions are much more difficult. As such, this first fundamental step has been experimentally re-examined and modeled with the aid of theoretical methods. When a toluene solution of AlEt3 was heated to 50 °C in the presence of 1 bar gauge acetylene, complete conversion (by NMR) to Et2Al-CHdCHEt occurred within 1 h. The product was isolated as a pyrophoric liquid after removal of toluene under vacuum and characterized by NMR spectroscopy (1H, 13C, COSY, HSQC; see Experimental Section and Supporting Information). Wilke postulated that a butenyl-bridged dimeric structure was likely, and our characterization of this compound leads to the same conclusion. In particular, a very large downfield chemical shift of the alkene β-hydrogen resonance (7.45 ppm) indicates significant polarization of the double bond toward aluminum, as illustrated in structure 1. The corresponding 13C NMR resonance occurs at 187.7 ppm.

In the interest of structurally authenticating this bridging mode, we investigated methods for conversion to a compound more amenable to crystallization. It was reasoned that controlled protonolysis with phenol may selectively replace the terminal

ethyl groups. Unfortunately, treatment of the dimer with four equivalents of PhOH led to the isolation of Et4Al4(μ-OPh)6(OPh)2, in which the putative bridging butenyl goups have been replaced by bridging phenoxides. This compound has been structurally characterized and is presented in the Supporting Information. It was thought that greater steric bulk on the phenol may hinder formation of two aryloxide bridges, in particular that phenyl groups flanking the oxygen may make it difficult to accommodate a second phenol unit in a bridging position. As such, two equivalents of 2,6-diphenylphenol were reacted with the dimer and a colorless crystalline solid was isolated after concentration and cooling. Crystal structure analysis revealed the monobutenyl-bridged dimer Et2Al2(OAr)2(μ-CHdCHEt)(μOAr) (2), shown in Figure 1. Evidently, a number of products are formed, as judged by the complexity of the NMR spectrum of the bulk material. Nonetheless, the crystal structure obtained serves to illustrate the presence of butenyl bridging between two aluminum centers. The hybrid stick/ORTEP representation (Figure 1a) shows the basic structure, featuring the bridging butenyl group. The double bond of this group shows the protons to be arranged in a cis fashion, which is consistent with the computationally predicted geometry resulting from insertion via a four-center transition state (see below). The bulky 2,6-diphenylphenol groups clearly block access to the bridging alkene, preventing further hydrolysis and preserving this structural feature. This steric obstruction is also depicted in the partial space-filling model in Figure 1b. The distances from the bridging oxygen of the phenoxide to the aluminum atoms are roughly equivalent at 1.868(2) and 1.874(2) Å and, as expected, are longer than the respective Al-Oterminal bonds of 1.732(2) and 1.734(2) Å. The two Al-Cbridging distances differ by 0.021 Å [2.098(3) and 2.077(4) Å], perhaps suggesting a slight asymmetrical distribution of electron density around this three-center Al-C-Al bond. In order to investigate this reaction theoretically (theoretical methods are discussed in Section 4.7), the likely reaction intermediates considered were a coordination complex of triethylaluminum and acetylene and a transition state for the insertion of acetylene into an Al-Et bond. Also considered was the monomer-dimer equilibrium of triethylaluminum. The dimeric species is known to be more energetically favorable than the monomer; hence the energy required to break the dimer 1570

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

into the monomeric form is relevant to the overall process.10,11 The calculations were performed on the basis that coordination and insertion do not occur while aluminum is in its dimeric form; attempts to model the coordination of acetylene to the Al2Et6 dimer did not yield any viable intermediates. The relative energy surface for this reaction is shown in Figure 2, with all energies relative to Al2Et6 and free acetylene (on a per-Al basis, i.e., 1/2 Al2Et6 þ HCtCH). All energies shown are ΔG values, which have been scaled to better represent solution values, as is discussed in Section 4.7. The barrier to monomer formation thus calculated is 12.4 kJ 3 mol-1, which is in reasonable agreement with experimental values (8.212 and 12.113 kJ 3 mol-1) . Geometry optimization led to a coordination complex of AlEt3 and C2H2 with acetylene bound at around 2.7 Å (Al-C) from planar triethylaluminum. However, on the free energy surface this complex is marginally higher in energy than the separated fragments, suggesting that a discrete AlEt3-C2H2 adduct does not form prior to insertion. The same observation has been made for ethylene insertion into AlEt3.8 The transition state involves the expected four-center structure and is similar to that in a previous computational study, where direct insertion of acetylene into the Al-C bond of H2Al-CH3 was considered.14,15 The effective activation energy from the dimer is 83.4 kJ 3 mol-1. Optimization beyond this point leads to Et2Al-CHdCHEt as the primary product, which is 127.1 kJ 3 mol-1 more stable than the reactants; however the final product is predicted to be the butenyl-bridged dimer, which lies 171 kJ 3 mol-1 below the reactants. This strong stabilization brought about through dimerization is relevant to the subsequent insertion steps; see below. These results seem to support the experimental observations, which see some of this insertion product being formed at room temperature,

but much more once the temperature is increased. It should be noted that the end product arrived at by geometry optimization features a cis-butenyl moiety at aluminum, as observed experimentally. The trans structure was also modeled for comparison, but was found to be only 3.4 kJ 3 mol-1 more stable than the cis isomer. We did not investigate the barrier for such an isomerization or investigate 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. The closest comparisons that can be drawn are for insertion into H2Al-R (R = Me, Et). The barriers calculated in these cases are relative to the monomeric reactants (dimers were not considered) and range from 67 kJ 3 mol-1 {MP4(SDTQ)/6-31G(*)// HF/6-31G(*)}14 to ca. 80 kJ 3 mol-1 {MP2/6-311þG(**)// HF/3-21G(*)}.15 Our barrier from the separated and monomeric reactants to the TS (71 kJ 3 mol-1) is comparable. 2.2. Second Insertion of Acetylene. Several permutations of the reaction with a second equivalent of acetylene were considered. We previously experimentally found that subsequent insertion of acetylene occurs preferentially into the Al-butenyl bond, rather than into a second Al-Et bond.2 As such, both possibilities have been studied, namely, insertion into the Albutenyl group and insertion into a second Al-Et group. We reasoned that the strong butenyl-bridging formed after one acetylene insertion may explain why a second insertion is more difficult. Indeed, as shown in Figure 3, the barrier to insertion into the Al-butenyl group (103.3 kJ 3 mol-1) is significantly increased relative to the first insertion into Al-Et. This increase of ca. 20 kJ 3 mol-1 in activation energy derives from the increase in energy required for dissociation of the butenyl-bridged dimer. In fact, the barrier from the acetylene-coordinated monomer for 1571

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

the second insertion (60.4 kJ 3 mol-1) is somewhat lower than that for the first insertion (70.4 kJ 3 mol-1). As such, the reluctance of the first insertion product to react further with acetylene simply relates to the stabilizing influence of butenyl bridging (see below). The alternative of insertion into a second ethyl group follows a similar pathway, except that the barrier to the transition state is 14.6 kJ 3 mol-1 higher (117.9 kJ 3 mol-1) than that for insertion into the butenyl group. This agrees with the experimental results that suggest some insertion occurs beyond the first ethyl group, especially in longer experiments; however the major kinetic product 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 see no evidence for isomerization to the trans isomer at room temperature with the isolated product; however it may be possible at the elevated temperatures of oligomerization experiments. As such, the reaction pathway for the trans isomer has also been computed and is shown in the Supporting Information. The situation is very similar to that shown herein for the cis isomer, with the energies of the individual steps differing by only a few kJ 3 mol-1. 2.3. Third Insertion of Acetylene. The third insertion of acetylene, into the Al-hexadienyl bond, was modeled beginning from the cis,cis-AlEt2(hexadienyl) dimer and proceeding to the primary product cis,cis,cis-AlEt2(octatrienyl). The dimerized product was not considered, as the stabilization expected for this is very similar to that for the reactant (see below, Section 2.4). The relative energy surface for this reaction is shown in Figure 4. The most notable feature is a significantly reduced overall barrier for

this third acetylene insertion (84.2 kJ 3 mol-1) relative to the second insertion into an Al-butenyl bond (in fact the third insertion is comparable in barrier to the first insertion into AlEt3). The reason for this change is a lower barrier from the monomeric acetylene coordination complex to the transition structure (45.1 kJ 3 mol-1), while the energy required to break the alkenyl-bridged dimer remains much the same. This difference may be attributable to the effect of double-bond conjugation in the case of the Al-hexadienyl structure, which apparently leads to a more facile acetylene insertion in this case. This effect would be expected to persist in subsequent insertion steps, although we did not explore this beyond a third insertion. It may be concluded that, were it not for the formation of stable alkenyl-bridged dimers, acetylene chain growth at Al would become reasonably facile following the first two insertions. The various modes of dimer formation that are possible are therefore explored below. 2.4. Energetics of Dimerization. In calculating the energy surfaces above, the dimers considered are those resulting from homodimerization of identical monomeric Al compounds. In reality, once acetylene insertion has commenced, a range of compounds are present, and as such, a range of oligomerized bridging ligands are possible. In order to evaluate the effect this may have on the reaction, we have compared the stabilization energy realized through various bridging modes as shown in Chart 1. A variety of combinations of AlEt3 and AlEt2(butenyl) were compared, in which the alkenyl group is cis. The trans structures were also investigated, and all displayed relative energies within ca. 3 kJ 3 mol-1 of the cis structures. The dimers can be comprised of one of each monomer (4 and 7) or two of the same (3 for AlEt3, 5, 6, and 1 for AlEt2(butenyl)). 1572

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

In considering the relative energies, there seem to be three approximate energy levels corresponding to the type of bridging ligand. Two bridging ethyl groups lead to a stabilization of around 12 kJ 3 mol-1, relative to the monomers. With one butenyl bridge, the stabilization energy is increased to 27 kJ 3 mol-1, while two butenyl bridges lead to even further stabilization as discovered above (ca. 44 kJ 3 mol-1). The hexadienyl-bridged structure 8 was also considered and has a stabilization energy close to the butenylbridged dimer 1. As such, conjugation leads to no effect on bridging strength, and this stabilization is expected to be similar for higher conjugated 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 has π-orbital electron density to contribute to the three-centered bond. This is not available with bridging alkyl groups, resulting in an electron-deficient (and weaker) three-centered, two-electron bond. We note a similar polarization to that shown in structure 1 has been invoked for [Et4Al2(μ-Ph)2].19 We expect structures with two bridging alkenyl groups to be strongly favored, which validates the above analysis of the relative energy surfaces. 2.5. Chain Transfer with Dihydrogen. As discussed in the Introduction, our previous attempts to promote chain transfer with dihydrogen, in order to make the process catalytic in Al, were met with limited success. In particular, hydrogenolysis of the Al-C bond is hard to achieve, and when it does occur, it seems to lead to deactivation toward further insertion.2 As such, this reaction has been modeled by consideration of the reaction between Et2Al(butenyl) and H2 (Figure 5). The high barrier to σ-bond metathesis (160.7 kJ 3 mol-1) displayed for this reaction is consistent with the experimental findings. This barrier is much

Chart 1a

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

higher than further acetylene insertion, and thus explains why high proportions of dihydrogen (H2:HCtCH ≈ 5:1) are required before significant chain transfer is observed. We were unable to locate a discrete H2 coordination complex prior to the transition 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 1573

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

for this structure relative to the monomer (-57.4 kJ 3 mol-1 per Al). The hydride-bridged dimer was also modeled and lies 46.5 kJ 3 mol-1 below the monomer. We previously speculated that such hydride-bridged structures might be very stable and suggested that this may explain the lack of reactivity of Et2AlH toward further insertion. The theoretical results presented herein support this notion.

3. SUMMARY AND CONCLUSIONS We have investigated the first steps of acetylene chain growth at AlEt3. The first insertion into the Al-Et bond occurs readily, leading to a stable alkenyl-bridged dimer, which has been characterized both experimentally and theoretically. Subsequent insertion is hampered by this strong bridging mode. The results predict that growth at Al would become reasonably facile were it not for the formation of these stable dimers and highlight the importance of considering the energetic cost of the dimermonomer transformation in these studies. This is expected to be particularly the case in experimentally relevant reactions in the condensed phase. The effect of dimerization has not been considered in past studies of alkene and alkyne insertion into alkylaluminum,14,15,20,21 although we note that this will be less important for alkene insertion reactions, where strong alkenyl bridging is not a factor. It is interesting to compare the conventional Aufbau reaction with ethylene to the process studied herein. The barrier to ethylene insertion into AlEt3 has been evaluated experimentally, and the free energy of activation at 500 K is reported as 133 kJ 3 mol-1 relative to the monomeric reactants in a gas phase process.22 As such, insertion of acetylene into AlEt3 probably has a significantly lower initial barrier (70 kJ 3 mol-1 from the monomer) and most likely still has a lower barrier in subsequent steps, when strong alkenyl bridging becomes relevant. It might therefore 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 to reaction conditions, insomuch as the growth reaction with ethylene is conducted at pressures of 50-100 bar.8 Such pressures are impractical for acetylene, which is at risk of spontaneous explosive decomposition at pressures greater than several atmospheres.23 As such, our experimental studies were restricted to acetylene pressures of 1 bar gauge (2 bar absolute).1,2 Even under these conditions, with long reaction times we did witness the formation of highly insoluble polyacetylene, illustrating that growth does advance. This highlights another difference between the ethylene and acetylene growth reactions. With ethylene, chain transfer via β-H elimination is competitive, which results in a practical upper limit on oligomer chain length of ca. 100 insertions.8 Hence, insoluble polyethylene is not formed. No such chain transfer reaction occurs for acetylene growth. This can be partially overcome by introducing hydrogen, although it has been shown herein that σ-bond metathesis with dihydrogen has a high barrier. As such, it has proven very difficult to effectively control the oligomer chain length distribution or to prevent the formation of polyacetylene in longer runs. Ultimately, it may prove more effective to partially hydrogenate acetylene to ethylene if it is to be transformed to fuel-range oligomers.4 Finally, we note that the present work has been restricted to the early steps of acetylene chain growth at Al. The peculiar formation of branched higher oligomers1,2 remains unresolved, and in this regard further work is required.

4. EXPERIMENTAL AND THEORETICAL METHODS 4.1. General Procedures. All manipulations were performed under an atmosphere of UHP argon (BOC gases) using standard Schlenk techniques or in an MBraun nitrogen glovebox. Solvents were purified by passage through an Innovative Technologies solvent purification system and, where appropriate, stored over a sodium mirror. 1574

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Organometallics Acetylene (BOC Gases instrumental grade) was purified by passage through a column of activated molecular sieves (3 Å) and alumina. NMR spectra were recorded on a Varian Mercury Plus NMR spectrometer operating 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 or 5 times. Stirring was continued for 1 h under a continuous supply of acetylene; then the acetylene was purged with argon and the flask left to cool. GC analysis of a quenched sample at this stage showed the solution to contain primarily ethane and 1-butene and no higher oligomers. The solvent was gently removed under vacuum over around 6 h, reducing the volume by ∼40% and yielding a viscous, yellow, and pyrophoric liquid. NMR showed the conversion to be quantitative. 1H and 13C NMR signals 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-CH2CH3), 0.84 (t, 3H, Al-CHdCH-CH2-CH3), 0.16 (q, J = 8 Hz, 4H, Al-CH2-CH3). 13C NMR in C6D6: δ 187.7 (Al-CHdCH-Et), 122.6 (AlCHdCH-Et), 32.4 (Al-CHdCH-CH2-CH3), 12.3 (Al-CHdCH-CH2CH3), 8.9 (Al-CH2-CH3), 1.9 (Al-CH2-CH3).

4.3. Preparation of Al2Et2(OC6H3Ph2)2(μ-C4H7)(μ-OC6H3Ph2) (2). To a Schlenk flask under argon was added [{AlEt2(C4H7)}2] (1, 431 mg, 1.54 mmol). In a separate flask, 2,6-diphenylphenol (765 mg, 3.11 mmol) was dissolved in toluene (4 mL). The flask containing 1 was submerged in an ice bath at -12 °C, and the alcohol added dropwise over 15 min with stirring. The yellow solution was warmed to room temperature, concentrated under vacuum to an oily consistency, and placed in a freezer at -20 °C. Colorless crystals slowly formed, which were suitable for X-ray diffraction (see below and Supporting Information). GC analysis of a quenched sample of the bulk solid showed an ethane:butene ratio of 3:1. The apparent disproportionation that occurs in this reaction meant that clean NMR and microanalysis could not be obtained for this product, as it evidently contains a mixture 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 beamline of the Australian Synchrotron. Data collection used BluIce software, and XDS was used for data reduction.24 The structures were solved by direct methods with SHELXS-97, refined using full-matrix least-squares routines against F2 with SHELXL-97,25 and visualized using X-SEED.26 Details of the refinements appear in the cif files, including modeling details for a disordered phenoxide ligand in Al4Et4(OPh)8, but standard procedures involved all non-hydrogen atoms being refined anisotropically and hydrogen atoms being placed in calculated positions and refined using a riding model with fixed C-H distances of 0.95 Å (sp2CH) and 0.98 Å (CH3), and Uiso(H) = 1.2Ueq(C) (sp2) and 1.5Ueq(C) (sp3). For 2, all hydrogen atoms on the ethyl and butenyl ligands were located and positionally refined to offer additional identification of these participative ligands in the reaction (some were later restrained; see cif file for details). A summary of crystallographic data of the structures is given 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, Synchrotron radiation, λ = 0.77500 Å, T = 100(2) K, 2θmax = 52.9°, 26 394 reflections collected, 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/

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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/cm3, F000 = 1024, μ = 0.141 mm-1, 3-BM1 Australian 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) (refinement on F2), 338 parameters, 0 restraints. 4.7. Theoretical Methods. All calculations were performed using Gaussian0327 or Gaussian09,28 utilizing hardware from the Australian Partnership for Advanced Computing Program (APAC), or National Computational Infrastructure. Geometry optimizations were performed using 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 has been noted that in the computational modeling of certain systems, for example olefin polymerization, density functionals often do not accurately describe a number of mid-long-range interactions. This effect was considered relevant to the system being studied herein. There are several approaches that are used to address this shortcoming, and a number have been compared recently for the description of hydrocarbons.37 Thus, a dispersion correction described by Grimme was applied to the B3LYP single-point energies, with a scaling factor of 1.05, to yield B3LYP-D values.38 Grimme’s method has been found to more accurately describe long-range van der Waals forces in many systems. Gibbs free energy corrections in which the entropy contribution has been scaled were applied throughout. Gas phase calculations provide a poor estimate of the true free energy changes in solution, and this is accentuated when the number of molecules changes, as is the case in the first three steps of the reaction pathway. For instance, applying full free energy corrections predicted that monomeric AlEt3 is more stable than dimeric Al2Et6, which is experimentally not the case (triethylaluminum exists as the dimer in hydrocarbon solutions).12 A reviewer suggested scaling the entropy contribution by a factor of 0.67 for benzene. As our catalysis was conducted in toluene solution, this seemed a suitable approach and has been applied throughout. The free energy correction thus applied to single-point energies was Gcorr = Hcorr - (TScorr  0.67). This resulted in a calculated free energy of monomer formation of 12.4 kJ 3 mol-1, in reasonable agreement with experimental values (8.212 and 12.113 kJ 3 mol-1). It is worth noting that applying this correction does not significantly change the predicted activation free energies for the reactions. For instance, the barrier to the first insertion (83.4 kJ 3 mol-1) with entropy contributions scaled is only modestly lower than the calculated barrier with the full free energy correction applied (88.5 kJ 3 mol-1). The corresponding enthalpy of activation for this process was calculated to be 73.1 kJ 3 mol-1.

’ ASSOCIATED CONTENT 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 with absolute energies of all stationary points (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

bS

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT D.S.M. and S.S.K. thank the CSIRO for funding, particularly the late Prof. David Trimm, with whom this collaboration was initiated. This work was also funded by the Australian Research 1575

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Organometallics Council through Discovery Project DP0665058. We thank the National Computational Infrastructure (NCI) and the Australian Partnership for Advanced Computing (APAC) for provision of computing resources. X-ray data were obtained on MX1 at the Australian Synchrotron, Victoria, Australia.24

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