Theoretical Structure–Reactivity Study of Ethylene Insertion into Nickel

Article pubs.acs.org/Organometallics

Theoretical Structure−Reactivity Study of Ethylene Insertion into Nickel−Alkyl Bonds. A Kinetically Significant and Unanticipated Role of trans Influence in Determining Agostic Bond Strengths Faraj Hasanayn,*,† Patrick Achord,‡ Pierre Braunstein,§ Hamza Javar Magnier,† Karsten Krogh-Jespersen,*,‡ and Alan S. Goldman*,‡ †

Department of Chemistry, The American University of Beirut, Beirut, Lebanon Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States § Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, F-67081 Strasbourg Cédex, France ‡

S Supporting Information *

ABSTRACT: Theoretical methods (B3LYP, M06, and CCSD(T)) have been used to study the kinetics and thermodynamics of ethyl migratory insertion in a series of square-planar [(X∧Y)Ni(ethyl)(ethylene)] complexes (X∧Y = anionic bidentate ligand). The results are discussed qualitatively using general trans-influence arguments. When X ≠ Y, the reactions of the two possible isomers have been compared. The results reveal that when one of the coordinating groups exerts a strong trans influence (STI) and the other a weak trans influence (WTI), as in a STI∧WTI chelating ligand such as a phosphinoenolate (P∧O), one of the two isomers has an activation energy for ethylene insertion (i.e., ethyl migration) that is much less than that calculated for symmetrical bidentate ligands of either the WTI∧WTI or STI∧STI types. Specifically, a low activation energy is found when an ethyl group, coordinated trans to the STI group, migrates to the ethylene coordinated trans to the WTI group. The converse pathway in the STI∧WTI system, wherein ethyl migrates from a position trans to a WTI group, encounters a very high barrier. However, the kinetic barrier to isomerization (prior to migration) is sufficiently low to allow repeated insertions to proceed via the low-barrier pathway, in which an alkyl group in effect migrates from the position trans to the STI group to the position trans to the WTI group. The overall barrier (isomerization plus insertion) for an [(STI∧WTI)Ni(ethyl)(ethylene)] complex is less than that calculated for insertion in a WTI∧WTI analogue. Ethylene dissociation from [(X∧Y)Ni(ethyl)(ethylene)] leads to an intermediate exhibiting a Ni−ethyl β-agostic bond. Unexpectedly, the data reveal that increased trans influence exerted by the ligand trans to the ethyl α-carbon results in a strengthening of the β-agostic interactions. The [(STI∧STI)Ni(ethyl)] species, therefore, have a surprisingly low energy agostic resting state. As a result, ethylene binding to [(STI∧STI)Ni(ethyl)] is predicted to be endoergic; this contributes to an overall barrier to catalytic ethylene insertion which is greater than that calculated for (STI∧WTI)Ni-based species. These results may explain, at least in part, the favorable role of STI∧WTI chelating ligands in nickel-catalyzed olefin oligomerization. They likely also have bearing on factors influencing the activity of late-transition-metal catalysts for olefin oligomerization and polymerization more generally.

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(P∧O).10 This system evolved into the industrial Shell Higher Olefin Process (SHOP) for the selective production of linear αolefins in the C6−C20 range, a process that has become one of the most important industrial applications of homogeneous catalysis.11 In 1978 Keim reported the synthesis of a Ni complex having a phosphino-enolate ligand of the general formula [(P∧O)Ni(Ph)(PPh3)] that is related to the catalysts involved in the SHOP process.12,13 A generally accepted mechanism of activity by this system is outlined in Scheme 1.14 Since the initial reports on square-planar (P∧O)Ni-type catalysts, there have been extensive research efforts in this field

INTRODUCTION Linear α-olefins of varying chain lengths are in great demand by the chemical industry, due to their wide range of applications as comonomers for the production of high-density polyethylene and for the production of linear low-density polyethylene, plasticizers, lubricants, and biodegradable detergents.1−3 The major route for the large-scale production of these olefins proceeds via the oligomerization of ethylene. Homogeneous catalysts based on transition metals play a very significant role in the field of ethylene oligomerization, and several first-row metals, including Ti,4,5 Fe,6 Cr,7 Ni,8 and Co,9 have proven valuable for this purpose. A particularly prominent class of catalysts, discovered at Shell Laboratories in the 1970s, resulted from the combination of a nickel compound with a phosphino-carboxylate or phosphino-enolate chelating ligand © 2012 American Chemical Society

Received: January 1, 2012 Published: June 25, 2012 4680

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calculated effects are discussed using qualitative trans-influence arguments.

Scheme 1. Typical Ethylene Oligomerization by Keim-Type Precatalysts

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COMPUTATIONAL DETAILS

The majority of prior theoretical studies relevant to the present work have been conducted at the DFT level.19−22,24 Using an ECP on Ni and a relatively small basis set, Sakaki et al.24 found excellent agreement between energies derived from DFT calculations using the B3LYP25 exchange-correlation functionals and ab initio CCSD(T)26 calculations for olefin coordination and insertion in [Ni(CH3)(HNCHCHNH)]+. In recent years, some method validation studies have recommended the use of functionals from the “Minnesota family” for the study of organometallic reactions.27,28 These contemporary, elaborate functionals include, inter alia, improved treatments of noncovalent interactions. Thus, in the present study we initially used both the B3LYP and M0629 functionals with 6-311G(2d,2p)30 basis sets. Calculations applying the M06 functionals were found to provide ethylene coordination energies systematically 6−8 kcal/mol stronger (i.e., more negative ΔEcoord) than B3LYP; on the other hand, the migratory insertion barriers (ΔE⧧) were smaller by 2−4 kcal/mol using M06. The geometries and the trends in ΔEcoord and ΔE⧧ obtained from use of the two sets of functionals were essentially identical. However, when diffuse functions were included in the basis set for all atoms in either the B3LYP or the M06 calculations, ΔEcoord became weaker by 6−8 kcal/mol, whereas ΔE⧧ values did not change significantly (comparisons based on full geometry optimizations at both computational levels). Single-point calculations of the [(P∧O)Ni(ethyl)(ethylene)] system at the CCSD(T)/6-311++G(2d,2p) level and optimized M06 geometries produced ΔEcoord values that were similar to the M06 values and activation energy barriers that agreed satisfactorily with both the B3LYP and M06 results. Except for the relative energy of the barrier to cis−trans isomerization of the squareplanar ethyl ethylene P∧O complex, the B3LYP/6-311G(2d,2p) and M06/6-311++G(2d,2p) levels afford comparable energies; importantly, they lead to identical conclusions concerning the primary questions under investigation here. For convenience and to keep the discussion focused, we present and discuss in the text primarily B3LYP/6-311G(2d,2p) geometries and energies; we refer to the M06/ 6-311++G(2d,2p) and CCSD(T)/6-311++G(2d,2p) results only when relevant. Validation of the B3LYP data against the M06 and CCSD(T) levels of theory for the P∧O system is provided in Table 1. We also examined the structural and energetic effects of solvation using a polarizable continuum model (PCM),31 portraying toluene and 1,2-ethanediol as model solvents. Solvation effects were overall found to be minor, and their inclusion did not alter any trends. The complete set of M06 results, including the effects of the model solvents, are available in the Supporting Information. Discussions in the text related to energies are based mostly on Gibbs free energy quantities (G°) calculated at P = 1 atm and T = 298.15 K.32 All calculations made use of Gaussian09.33

involving both symmetrical and asymmetrical, anionic and neutral bidentate ligands. These efforts have yielded a large number of (pre)catalysts with demonstrated high selectivities toward ethylene dimerization, short-chain oligomerization, or high-molecular-weight polymerization. Interestingly, a majority of the more effective Ni(II) catalysts continue to be based on bidentate ligands of the heteroatom coordination type akin to Keim’s seminal P∧O system.8,15,16 A notable exception is a class of catalysts developed by Brookhart, which is based on symmetrical, neutral diimine bidentate ligands.17 Olefin oligomerization has also been effected using Pd(II)-based square-planar complexes.18 Theoretical investigations, most notably by the Ziegler19 and Morokuma20 groups, have afforded valuable insights toward understanding many aspects of the elementary steps for olefin oligomerization by square-planar Ni(II) and Pd(II) complexes.21 Recently, Jensen and co-workers used DFT-based methods to support the accessibility of a low-energy chain termination mechanism different from the conventional βhydride elimination illustrated in Scheme 1.22 An important finding of the previous theoretical studies19−22 establishes that when the chelating ligand in ethyl ethylene square-planar complexes is asymmetrical (as in P∧O), then the higher energy isomer tends to have a markedly smaller barrier for ethyl insertion than the thermodynamically more stable isomer. This phenomenon has been also noted in acrylate insertion reactions.23 However, to our knowledge there has been no systematic investigation concerning whether this result, characteristic of asymmetric ligands, affords an intrinsic overall advantage to the olefin insertion kinetics. In the present work, we use electronic structure calculations to study the energetics of ethylene insertion and dissociation in a series of [(X∧Y)Ni(ethyl)(ethylene)] complexes (X∧Y = anionic bidentate ligand). The study focuses on the principal electronic effects of the donor atoms provided by X∧Y and does not specifically address steric effects. Although the series of ligands considered is not exhaustive, the data suggest that the P∧O ligands, which launched the chemistry under study four decades ago, may intrinsically contain nearly optimal electronic properties for the olefin insertion step in a neutral Ni(II) system. To facilitate the presentation of the results, we first discuss the P∧O system in detail and then consider how the barriers would change when the P and O donor atoms are substituted by other atoms. The

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RESULTS AND DISCUSSION trans-Influence and Agostic Bonding Considerations. We begin the study by inspecting the structures of the threecoordinate [(P∧O)Ni(methyl)] and [(P∧O)Ni(ethyl)] fragments and their square-planar ethylene adducts (P∧O = 2phosphinoethenolate; Figures 1 and 2). The closed-shell singlet state of [(P∧O)Ni(Me)] has a planar T-shaped geometry (Cs point group) that allows for two isomers (1a-T and 1b-T, Figure 1). In both isomers, the methyl group adopts a conformation in which an in-plane hydrogen is significantly bent toward Ni, yielding Ni−C−H angles of ca. 100°, thus suggesting the presence of an α-C−H agostic interaction. However, in both isomers the CαH−Ni bond distance is relatively long (∼2.3 Å), and the barrier for methyl group rotation is small (∼1 kcal/mol). Thus, the equilibrium αagostic geometries do not seem to offer any significant 4681

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stabilization to the [(P∧O)Ni(Me)] fragment. Similar conclusions regarding weak α-agostic interactions engendered by significantly bent M−Cα−H bonds have been noted in other systems.34 The [(P∧O)Ni(Me)] isomer having the methyl trans to the oxygen atom, 1a-T, is calculated to be 13.4 kcal/mol lower in free energy than 1b-T with a trans Me−Ni−P configuration (Figure 1). This substantial energy of isomerization implies that

trans influence (presumably the weakest possible). Thus, the much greater stability of 1a-T over 1b-T is easily explained as a result of each of the STI ligands (the phosphine and the methyl) being positioned trans to a WTI ligand (the O donor or the vacant site) in 1a-T, as opposed to the ligands being mutually trans in 1b-T. In accord with the above considerations based on trans influence, the Ni−P bond distances are markedly different in 1a-T and 1b-T: 2.07 and 2.28 Å, respectively. Interestingly, however, the Ni−Me bond distances are calculated to be essentially equal in the two isomers (1.87 and 1.88 Å). Similarly, hydride is an STI ligand; however, in the isomers of [(P∧O)NiH] the Ni−H bond lengths are calculated to be very similar (1.46 and 1.47 Å), whereas the Ni−P bond lengths are quite different (2.05 and 2.28 Å). Coordination of ethylene to the site trans to the phosphine in 1a-T is exoergic by 13.5 kcal/mol and yields the square-planar 1a-C2H4 (Figure 1). Ethylene coordination trans to the O donor in 1b-T is more exoergic (20.2 kcal/mol) and yields 1bC2H4, which features shorter bond distances between the metal and the ethylene carbons than those in 1a-C2H4 (2.05 Å vs 2.14 Å). (The corresponding enthalpies (ΔH°) of ethylene coordination to 1a-T and 1b-T are −25.0 and −32.2 kcal/ mol, respectively.) The much greater exothermicity of ethylene binding to 1b-T demonstrates the greater preference of the phosphine (relative to oxygen) to remain trans to a vacant site. The 1b-C2H4 isomer with methyl and phosphine groups in a mutually trans arrangement is 6.7 kcal/mol above 1a-C2H4, in accord with a weaker trans influence of ethylene vs methyl. This value is significantly less than the 13.4 kcal/mol difference found for the three-coordinate species 1a-T and 1b-T, in accord with a stronger trans influence exerted by the ethylene ligand relative to the vacant coordination site. As found in 1a-T and 1b-T, the Ni−P bond distance is pronouncedly longer in the higher energy 1b-C2H4 species than in 1a-C2H4 (2.34 vs 2.18 Å), although the difference of 0.16 Å is not as large as that computed between the T-shaped isomers (0.19 Å). The Ni− Me bond distances are again nearly equal in the two Ni− ethylene isomers (1.93 and 1.94 Å), even though the Me groups are situated trans to very different ligands. Figure 2 gives geometrical parameters and free energies pertaining to the [(P∧O)Ni(ethyl)] system. As found in related computational studies,19,20 the lowest energy structure of the three-coordinate [(P∧O)Ni(ethyl)] is characterized by a significantly bent Ni−CH2−CH3 angle (ca. 76°) and the presence of an agostic bond between the metal and a βhydrogen of the ethyl group (2a-agos and 2b-agos, Figure 2).38 Experimental evidence for this agostic bonding mode in threecoordinate late-transition-metal complexes was established by Spencer and co-workers for a series of isolable cationic Pt− diphosphine alkyl systems.39 Strassner and co-workers synthesized diimine Ni−alkyl complexes and provided crystallographic, solution NMR, and computational evidence in support of β-agostic bonding.40 More recently, Scherer and co-workers isolated agostic cationic Ni Spencer-type complexes and analyzed their charge densities using experimental and computational techniques.41 The structural parameters associated with the β-agostic moieties in Figure 2 are supportive of a much greater Cβ−H− Ni interaction in 2b-agos, where the Cβ−H bond is roughly trans to O, than in 2a-agos, where the Cβ−H bond is trans to P. This is indicated by a significantly longer Cβ−H bond (1.21 vs 1.16 Å) and a shorter CβH−Ni distance (1.59 vs 1.70 Å) in 2b-

Figure 1. B3LYP/6-311G(2d,2p) geometries (in Å and deg) and relative energies (gas phase, G° (kcal/mol) at T = 298 K and P = 1 atm) of the two [(P∧O)NiMe] isomers and their ethylene adducts.

major differences in bonding interactions exist between the two isomers; we analyze these differences qualitatively using general trans-influence arguments. The trans influence pertains to the effects exerted by a given ligand on the strength of the bond trans to it in a given complex.35,36 Ligands are classified as exerting strong or weak trans influence empirically on the basis of structural or spectroscopic data in a set of related complexes. It is generally unfavorable for two strong trans-influence (STI) ligands to be positioned mutually trans, presumably because they would compete for donation into the same metal-based orbitals.37 Phosphines and alkyl groups are prototypical strong transinfluence ligands, whereas oxygen donor ligands are positioned toward the weaker end of the trans-influence scale. The Tshaped geometry of [(P∧O)Ni(Me)] can be viewed as derived from the square-planar motif with one vacant coordination site. The vacant site is then equivalent to a ligand with a very weak 4682

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Figure 2. B3LYP/6-311G(2d,2p) geometries (in Å and deg) and relative energies (gas phase, G° (kcal/mol) at T = 298 K and P = 1 atm) of the two [(P∧O)Ni(ethyl)] isomers and their ethylene adducts.

attributable to agostic C−C interactions, which in recent years have been proposed for several systems.45,46 The rotation of the ethyl groups in 2a-agos and 2b-agos into conformations that remove Ni−Me interactions requires 10.3 and 16.7 kcal/mol, respectively. The resulting species (2a-T and 2b-T, Figure 2) are true minima, but they too exhibit a significant degree of Ni−Cα−H angle bending (ca. 93°). Further ethyl group rotation from 2a-T and 2b-T into the 2aT-rot and 2b-T-rot species described above (Figure 2) has ΔG° values of only 1.7 and 1.2 kcal/mol (the corresponding ΔE values are only ca. 0.5 kcal/mol). As found in the methyl system and in spite of the larger degree of Ni−Cα−H bending, the α-agostic interactions reflected in the geometries of 2a-T and 2b-T do not appear to be energetically important. Therefore, the 10.3 and 16.7 kcal/mol rotational barriers from 2a-agos and 2b-agos to 2a-T and 2b-T may represent a reasonable estimate of the strength of the total β-agostic interactions in the P∧O system. This indicates that the agostic stabilization in 2b-agos in the gas phase is greater by 6.4 kcal/ mol than that in 2a-agos. Such a result could be expected on the basis of the different trans influences of the P and O donors trans to the Cβ−H bond in the two isomers since, in general, any bond will be stronger when it is formed trans to a weaker trans-influence ligand (in this case trans to O in 2b-agos). However, inspection of the geometries in Figure 2 indicates that the greater agostic stabilization in 2b-agos (as evaluated for the transformation from 2-T-rot to 2-agos) is attributable to

agos. (For comparison, the lengths of the fully formed Ni−H bonds in the two isomers of [(P∧O)NiH] are 1.46 and 1.47 Å). The stronger Cβ−H−Ni interaction in 2b-agos is also associated with a shorter Cα−Cβ bond of the ethyl group in comparison to that in 2a-agos: 1.47 vs 1.50 Å. Consistent with the structural indications of a stronger Cβ− H−Ni interaction in 2b-agos, the adiabatic barrier for rotation of the methyl group in the ethyl ligand (via the 2a-agos-rot and 2b-agos-rot transition states in Figure 2) is much larger in 2bagos than in 2a-agos (12.1 vs 6.2 kcal/mol). Interestingly, in the TS's for rotation the Ni−CH2−CH3 angle continues to be bent to near 80° in both isomers. However, the distances between Ni and the equivalent methyl hydrogens in 2a-agosrot and 2b-agos-rot are 2.28 and 2.20 Å, respectively, which appear to be too long to implicate CβH−Ni bonding as the primary driving force for the bent Ni−CH2−CH3 angles. Nonetheless, the energy needed to open the Ni−CH2−CH3 angle further to 110° in either isomer is ca. 6 kcal/mol (on the basis of geometry optimization in the Cs point group of 2a-Trot and 2b-T-rot (Figure 2) with the Ni−CH2−CH3 angle kept fixed at 110°). Theoretical studies by Eisenstein,42 Scherer,43 and Etienne44 have noted that M−CH2−CH3 bending in complexes that have agostic geometries but lack significant CβH−M bonding components can follow from electronic reorganizations that are favorable for the M−Cα bond. It is also likely that angle bending in such cases has a component 4683

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Figure 3. Gas-phase B3LYP/6-311G(2d,2p) free energy profiles for ethylene insertion in 2a-C2H4 and 2b-C2H4 and key geometrical parameters (in Å and deg). Values in parentheses are free energies (G° at T = 298 K and P = 1 atm; in kcal/mol) defined relative to 2a-C2H4.

additional effects involving the α-carbon (ethyl)−Ni bond. The Ni−P bond distance is lengthened from 2.08 to 2.13 Å as the Cβ-H agostic bond is introduced trans to the phosphine by the transformation of 2a-T-rot into 2a-agos. Likewise, conversion of 2b-T-rot to 2b-agos, which introduces an agostic interaction trans to O, increases the Ni−O bond length from 1.83 to 1.87 Å. Surprisingly, however, in the latter transformation the Ni−P distance contracts from 2.29 to 2.23 Å, implying that formation of the ethyl Cβ−H agostic bond trans to O is accompanied by a significant weakening in the trans influence exerted by the αcarbon of the ethyl ligand. As discussed above, prior studies have revealed major electronic effects in the M−alkyl bond when the M−Cα−Cβ angle is bent43,44 to form either a β-H agostic bond or what appears to be a β-C agostic bond. In the case of 2b-agos, the relatively long Cβ−H and short CβH−Ni and Cα−Cβ bond distances are presumably associated with Cα−Cβ double-bond character (Cα−Cβ = 1.47 Å in 2b-agos compared with 1.52 Å in 2b-T-rot), as well as hydridic character of the agostic Cβ-bound H (consistent also with the elongated Ni−O bond). In other words, the agostic ethyl group has ethylene/hydride character, in accord with the common perception of it as an intermediate on the pathway toward β-hydrogen elimination. These models anticipate that stronger ethyl β-agostic bond formation should be associated with increased trans-influence strength of the Cβ− H bond (due to more charge transfer to Cβ−H) and a weakened trans influence of the α-carbon (due to less negative charge on Cα). Regardless of how the underlying electronic factors are best characterized, an important corollary of the weakened trans influence of the ethyl α-carbon, as inferred from the structural data, is that the β-agostic interaction should be stabilized by increased trans inf luence of the coordinating group trans to the alkyl α-carbon. In later sections, we will return to this point to provide more evidence in support of this conclusion and to

discuss its potential significance to the reactivity of primary interest in this study. The presence of the β-agostic bond in the ethyl-containing nickel complexes should naturally reduce their driving force to bind ethylene as compared to methyl. In fact, the B3LYP/6311G(2d,2p) free energies of ethylene coordination to 2a-agos and 2b-agos (yielding 2a-C2H4 and 2b-C2H4, respectively; Figure 2) are calculated to be only slightly favorable: ΔG°coord = −1.1 and −1.5 kcal/mol,47 respectively, as compared with −13.5 and −20.2 kcal/mol for the methyl analogues 1a-T and 1b-T. In addition to relatively weak ethylene coordination, the minor dif ference of ΔΔG°coord = 0.4 kcal/mol between the binding energies in the two isomers is noteworthy, as contrasted with the large difference of ΔΔG°coord = 6.7 kcal/ mol for the methyl analogues. In conclusion, the isomeric pairs generated by the P∧O ligand in the three- and four-coordinate complexes presented in Figures 1 and 2 are calculated to have substantially different energies. Importantly, in each pair of isomers, the relative strength of the Ni−P bond (as indicated by the Ni−P bond distance) correlates satisfactorily with the relative energy of the isomer. Accordingly, we will monitor the Ni−P bond distance closely in the context of activation and relative energies of the isomeric transition states discussed below. Migratory Insertion in the P ∧ O Ethyl System. Comparative Gibbs free energy profiles for ethylene insertion and dissociation in 2a-C2H4 and 2b-C2H4 are presented in Figure 3. The values in parentheses are energies of the corresponding species defined relative to the 2a-C2H4 reactant, which represents the lowest energy point on the given free energy surface. The figure includes selected geometrical parameters relevant to the discussion of the activation energies. Ethylene insertion in 2a-C2H4 affords the Ni−butyl complex 2a-prd as the product. In 2a-prd, the four-membered Ni−Cα− Cβ−H ring has essentially the same bond lengths and angles as its ethyl analogue, 2b-agos (Figure 2); likewise, 2b-prd is 4684

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essentially isostructural with the ethyl complex 2a-agos. Conclusions based upon investigation of the ethyl complexes will therefore apply more generally to the higher n-alkyl analogues and thus to repeated insertions. Ethylene insertion in 2b-C2H4 is much more exoergic (ΔG°rxn = −12.3 kcal/mol) than insertion in 2a-C2H4 (−2.9 kcal/mol), as the α-carbon of the n-butyl group is trans to O in 2b-prd. The gas-phase activation free energy of ethyl migration in 2aC2H4 via 2a-TS is relatively high at 27.0 kcal/mol (Figure 3), which argues against the involvement of this pathway at ambient temperature. In contrast, the barrier to alkyl migration in the higher energy isomer (from 2b-C2H4 to 2b-TS) is only 11.9 kcal/mol. (Note that for the nontruncated Ph2P∧O complexes, the calculated barriers are quite similar at 26.2 and 9.8 kcal/mol, respectively.) Most importantly, although 2bC2H4 is above 2a-C2H4, the associated transition state (2b-TS) is consistently more than 10 kcal/mol below 2a-TS. As mentioned in the Introduction, this large difference between the barriers of ethyl to ethylene migration in the P∧O isomers has been noted in previous computational studies.19−22 For example, a comparable difference between the two barriers shown in Figure 3 was calculated by Morokuma and co-workers for a P∧O ligated Pd system, but the individual barriers were larger by about 4 kcal/mol than the Ni reaction barriers investigated here.48 This effect had also been calculated in cationic Ni(II) complexes, such as those having a chelating salicylaldiminato (N∧O) ligand.49 In order for 2b-TS (and TS’s for higher alkyl analogues) to be chemically relevant in the context of ethylene polymerization, 2b-C2H4 must be kinetically accessible from 2a-C2H4. Prior computational studies found that facile isomerization can take place via an intramolecular mechanism involving a nearly tetrahedral transition state.19,20 Our calculations of this mechanism afford 2-TSisomztn with a cis−trans isomerization barrier of 15.0 kcal/mol (Figure 3). On the free energy surface, 2-TSisomztn is just slightly below 2b-TS, ΔΔG°⧧ = +1.5 kcal/ mol (ΔΔE⧧ = −0.4 kcal/mol; Table 1), thus demonstrating the accessibility of an isomerization pathway that is at least competitive with low-energy 2b-TS. At the CCSD(T) level, 2-TSisomztn is 6.1 kcal/mol lower than 2b-TS (Table 1). Other isomerization mechanisms might offer even lower barriers, but a more elaborate investigation that would consider, for example, solvent-assisted or dissociative pathways, which do have precedence,50,51 is beyond the scope of the present study. The data used to validate the B3LYP/6-311G(2d,2p) results in Figures 2 and 3 are collected in Table 1. Briefly, there is overall good agreement between the B3LYP/6-311G(2d,2p) energies and the energies obtained at the M06, CCSD, and CCSD(T) levels using the 6-311++G(2d,2p) basis set. In comparison to all other computational levels applied here, calculations at the B3LYP/6-311++G(2d,2p) level appear to underestimate the energy of ethylene coordination. In fact, at this level, ethylene dissociation from 2a-C2H4 into 2a-agos is predicted to be exoergic by 5.0 kcal/mol. On the other hand, the M06 level appears to overestimate the barrier of isomerization of 2a-C2H4 (16.8 kcal/mol) and predicts 2TSisomztn to be 5.4 kcal/mol above 2b-TS, in disagreement with the CCSD(T) results, which place 2-TSisomztn 6.1 kcal/mol below 2b-TS. When 1,2-ethanediol is applied as the model solvent, solvation systematically lowers the energy of most species relative to 2a-C2H4 by a small amount (1−3 kcal/mol), with the exception of the T-shaped species, which are typically

Table 1. Effects of the Level of Theory on the Relative Energies of the Species Shown in Figures 2 and 3a B3LYP/6311G(2d,2p) gas phase 2a-C2H4 2-TSisomztn 2b-C2H4

0.0 15.5 4.5

2a-TS 2b-TS 2a-prd 2b-prd

26.5 15.0 −3.9 −9.1

2a-agos + C2H4 2b-agos + C2H4 2a-T + C2H4 2b-T + C2H4

13.6

a

ethanediol solvent

gas phase, 6-311++G(2d,2p) basis set B3LYP

M06

Square-Planar Complexes 0.0 0.0 0.0 13.3 15.1 16.8 3.5 4.5 5.2 Insertion TS's and Products 25.1 23.7 21.5 14.8 14.0 11.4 −6.0 −8.6 −8.3 −10.2 −13.3 −12.8 Dissociation Products 11.7 7.5 14.5

CCSD

CCSD(T)

0.0 13.8 4.9

0.0 7.1 2.0

24.4 14.3 −9.0 −19.3

24.0 13.2 −7.0 −13.2

11.2

14.7

19.2

16.2

12.4

19.4

16.7

19.2

25.7

20.2

15.3

25.1

19.3

24.1

37.6

31.0

25.8

36.6

30.7

34.1

Potential energy (E) values are in kcal/mol, relative to 2a-C2H4.

stabilized by ca. 6 kcal/mol. The energetic effects are even smaller with the less polar solvent toluene. We examine the transition-state geometries to rationalize the large difference in migration barriers computed for 2a-C2H4 and 2b-C2H4. Both 2a-TS and 2b-TS are characterized by a considerable lengthening in the migrating Ni−ethyl bond from its equilibrium position: Ni−C distances are 2.16 and 2.12 Å in the TS's vs 1.94 Å in either reactant. In addition, the symmetrical metal−ethylene bond of the reactants is rotated by 90° and slipped into an asymmetrical bonding mode characterized by one long (ca. 2.2 Å) and one short (ca. 1.92 Å) Ni−C bond distance, consistent with formation of an incipient n-butyl group. The shorter of the TS Ni−C(ethylene) bond distances is essentially equal to the distance of the full Ni− C(ethyl) σ bond in the reactants. In spite of the significant extent of Ni−ethyl and Ni−ethylene structural distortions, the distance between the α-carbon of the migrating ethyl and the ethylene carbon to which it is migrating remains quite long in the TS's: 1.98 (2a-TS) or 2.07 Å (2b-TS), indicating the absence of any significant degree of C−C bond formation in either TS. The very different barriers to migration computed for 2a-C2H4 and 2b-C2H4 may be rationalized by a qualitative assessment of the differences in the energy components expected for each of the identified Ni−ethyl and Ni−ethylene distortions that take place in the activation process. In the reaction of 2a-C2H4, the ethyl group migrates from a site trans to the oxygen of the P∧O ligand, whereas in 2b-C2H4 ethyl migrates from a position trans to the phosphino group. In view of the weaker trans influence of an oxygen donor compared to that of the phosphino group, the Ni−ethyl bond dissociation component of the barrier is expected to be much more favorable in 2b-C2H4 than in 2a-C2H4. Accordingly, in the transformation from 2b-C2H4 to 2b-TS, the Ni−P bond contracts dramatically, from 2.37 to 2.16 Å; likewise, the Ni−O bond contracts, albeit much more modestly, from 1.95 Å in 2aC2H4 to 1.89 Å in 2a-TS. The increased binding energy associated with these bond length reductions is expected to be 4685

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asymmetry in the chelating ligand on the overall rate of olefin insertion and chain growth. The kinetics of a given system depends upon the free energy difference between the resting state and the rate-determining TS. The asymmetrical chelate seems to afford a low-energy TS (2b-TS) due to a favorable disposition of STI and WTI chelating groups relative to the substrate “ligands” (i.e., the migrating ethyl group and the slipped ethylene). Offsetting this effect, however, the asymmetrical chelate also offers a low-energy match in the resting state configuration (2a-C2H4), which also has an STI substrate ligand (ethyl) and a relatively WTI ligand (ethylene). In the following section we therefore introduce related ancillary ligands, particularly symmetrical ones. We consider replacement of the P and O groups individually, with emphasis exclusively on the resting state and the rate-determining TS (2a-C2H4 and 2b-TS, respectively). Complexes with Symmetrical O∧O and P∧P Ligands. We chose the truncated analogue of the acac anion, 1,3dioxopropan-2-ide (Figure 4), as a symmetrical WTI∧WTI chelate designed to have two O-coordinating groups while preserving the overall charge and the Ni oxidation state of the P∧O complexes. As STI∧STI analogues, we consider both the monoanionic bis-phosphine Peters-type ligand 58 bis(phosphinomethyl)dihydroborate and the planar 1,2-diphosphinocyclopentadienyl anion. The barrier to ethyl migration in O∧O complex 3-C2H4 is 22.0 kcal/mol, about 10 kcal/mol greater than the insertion barrier for P∧O complex 2b-C2H4 (11.9 kcal/mol) but 5 kcal/ mol less than the corresponding barrier in 2a-C2H4 (27.0 kcal/ mol; Figure 3). This data suggests that replacing a phosphine with an O donor in the position trans to the ethyl group disfavors the insertion step by ca. 10 kcal/mol, while the same substitution at the position trans to ethylene favors insertion by ca. 5 kcal/mol (Scheme 3). These results are fully consistent with calculations of insertion reactions involving bis(phosphinomethyl)dihydroborate P∧P complex 4-C2H4: replacing O with P in 2a-C2H4 at the position trans to ethyl lowers the barrier by 10.5 kcal/mol, while substitution at the position trans to ethylene raises the barrier by 4.6 kcal/mol (Scheme 3). The 22.0 kcal/mol barrier computed for ethyl migration in O∧O complex 3-C2H4 is significantly greater than the barrier via 2b-TS in the P∧O system, even after taking into account that the reactant 2b-C2H4 is 4.6 kcal/mol above the calculated resting state 2a-C2H4. For the purpose of considering overall activation energies, we view 3-C2H4 and 3-TS as species in which we have replaced the phosphino groups in 2a-C2H4 and 2b-TS, respectively, with O-coordinating groups (even though they are not directly connected, 2a-C2H4 and 2b-T nevertheless form the relevant resting state and low-energy TS in the P∧O system). The phosphino group in 2a-C2H4 and 2b-TS is trans to ethylene and to a migrating ethyl group, respectively. The migrating ethyl is a weaker trans-influence group than bound ethylene, as indicated by the Ni−P distances trans to these groups in 2a-C2H4 and 2b-TS (2.16 and 2.20 Å, respectively) and, independently, by the structures of 4-C2H4 and 4-TS (Ni− P distances of 2.21 and 2.18 Å, respectively). Thus, the higher barrier for migration in the O∧O complex (4-C2H4 → 4-TS) in comparison with 2a-C2H4 → 2b-TS in the P∧O system can again be rationalized on the basis of simple trans-influence considerations: the trans O group, relative to a trans P group, favors the reactant (with bound ethylene) more than it favors the transition state with its migrating ethyl group, which exerts a very weak trans influence. This is illustrated in Scheme 4.

greater for the Ni−P bond and should contribute toward the smaller barrier in 2b-C2H4. The second component to the migratory insertion barrier relates to ethylene “slippage”. On the basis of the equilibrium geometries (Figure 3), a priori one might expect ethylene slippage to be energetically more favorable in 2a-C2H4, where the olefin is trans to P and exhibits longer Ni−C distances. However, (perhaps counterintuitively) when ethylene slips to the insertion-TS position, the incipient α-alkyl carbon exerts a much stronger trans influence than that of π-bound ethylene, as indicated by a substantial lengthening of the respective trans Ni−ligand bond lengths. Specifically, in the insertion reaction of 2a-C2H4 the Ni−P bond elongates from 2.20 to 2.29 Å in 2a-TS, and in the reaction of 2b-C2H4 the Ni−O bond increases from 1.90 to 1.94 Å. Thus, ethylene slippage (in contrast to ethyl migration) results in a weakening of the Ni− ligand bond in the trans position. This should be associated with an energetic cost, which is greater in the case of a transcoordinated Ni−P bond, and contributes further toward a larger barrier in 2a-C2H4. Related effects of the ancillary ligands have been invoked to account for calculated large differences in the activation energies of alkyl to CO migratory insertion in isomeric octahedral complexes.52,53 Also, previous experimental54,55 and theoretical56,57 studies have shown that asymmetric metal−olefin coordination polarizes the π-electron density of the olefin toward the metal center, presumably accounting for the high trans influence resulting from this coordination mode. Thus, in the migratory insertion TS's a significant degree of dissociation of the ethyl group (weakening its trans influence) and slippage of the ethylene ligand (increasing its trans influence) are observed. The analysis suggests the value of adopting an abstract view of the TS as a coordination compound possessing, in addition to the chelating ligand, two ligands in the form of a largely dissociated (WTI) alkyl and a slipped (STI) ethylene. Accordingly, just as 2a-C2H4 is clearly an energetically more favorable species than 2b-C2H4 on the basis of trans-influence considerations, 2b-TS should be significantly lower in energy than 2a-TS (Scheme 2). Scheme 2. Relative Favorability of 2a-C2H4 vs 2b-C2H4 and of 2a-TS vs 2b-TS Illustrated in Terms of Mutual trans Influences

Furthermore, assuming that 2b-C2H4 is kinetically accessible from 2a-C2H4, 2b-TS will be the distinctly favored TS for olefin insertion (regardless of the thermodynamics of 2a-C2H4 vs 2bC2H4). Related trans-effect arguments have been considered by Morokuma to account for the relative reactivity of isomers in a Pd(P∧O) system.48 The conclusion from the above analysis does not, however, answer the central question of this work: namely, the effect of 4686

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Scheme 3. Comparison of the Ethylene Insertion (Ethyl Migration) Barriers (ΔG°⧧) in Analogous O∧O, P∧O, and P∧P Complexes

Scheme 4. Effect of trans Influence on Relative Insertion Barriers

Figure 4. Geometries (in Å and deg) and Gibbs free energies (kcal/ mol, T = 298 K and P = 1 atm) for alkyl migratory insertion and ethylene dissociation in [(X∧X)Ni(ethyl)(ethylene)] complexes. Values in parentheses are free energies relative to the respective Ni−C2H4 complex.

very similar to those calculated for 4-C2H4, except that the insertion barrier was 16.4 kcal/mol and hence essentially equal to the barrier for the 2a-C2H4/2b-TS transition. We believe that the principal value of our analysis is its simplicity, but of course, therein also lie limitations. In particular, we have implicitly assumed that the TS's for the insertions of all complexes are completely analogous. However, 2b-TS, for example, presumably represents an earlier TS than 2a-TS and is probably earlier than the symmetrically ligated TS's. More generally, the nature of the migrating ethyl-slipped ethylene combination is presumably less well defined than the ethyl−ethylene ligand combination in the reactants; perhaps the former pair is able to adopt a more asymmetric configuration to take advantage of the asymmetric environment afforded by the P∧O chelate. This may account for the energy of 2b-TS being comparable to that of 4-TS even though, on the basis of the arguments presented above, the latter could be expected to be significantly lower. We might also question our

In the case of the symmetrical P∧P ligand we must consider replacement of the O donor atoms of 2a-C2H4 and 2b-TS by a phosphino group, trans to ethyl and the slipping ethylene, respectively (Scheme 4). Although the slipping ethylene has a fairly strong trans influence, a comparison of the Ni−P bond lengths in 2b-C2H4 and 2a-TS (2.37 vs 2.29 Å) and in 4-C2H4 and 4-TS (2.29 vs 2.25 Å) indicates that the trans influence of the bound ethyl group is relatively stronger. On this basis we would predict that the P∧P complexes would undergo ethyl migration with an energy barrier lower than that of 2a-C2H4 (proceeding via 2b-TS). The calculated ethyl migration barrier for 4-C2H4 is 15.8 kcal/mol, only slightly less than the kinetically relevant 16.5 kcal/mol energy difference between 2aC2H4 and 2b-TS. Calculations with the 1,2-bis(phosphino)cyclopentadienyl anion (5-C2H4; Figure 4) produced results 4687

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an ethyl group undergoing β-agostic bonding effectively exerts a much smaller trans influence than the α-carbon of the nonagostic ethyl. Conversely, the strength of the agostic bonding interaction should be increased by the presence of a stronger trans-influence ligand positioned trans to the ethyl αcarbon. The data for the O∧O and P∧P species in Figure 4 are in full support of this view. First, the Ni−Cα−Cβ−H ring geometries are quite similar in the O∧O complex 3-agos and the P∧P complexes 4-agos and 5-agos; for example, the Cβ−H (1.17, 1.18, and 1.19 Å) and the CβH−Ni (1.64, 1.65, and 1.63 Å) bond distances are essentially equal in the three complexes. Furthermore, the energies needed to transform the agostic species to the purely nonagostic T-rot configuration are also comparable for the O∧O and P∧P complexes: 14.1, 14.9, and 16.8 kcal/mol (Figure 4). In contrast, the agostic P∧O complexes have energies and structures that differ significantly from each other and from those of the O∧O and P∧P complexes (e.g., CβH−Ni distances of 1.70 and 1.59 Å for 2a-agos and 2bagos, respectively, and binding energies of 12.0 and 17.9 kcal/ mol relative to 2-T-rot). Thus, the strength of the agostic interaction is favored by a stronger trans-influence ligand positioned trans to the ethyl α-carbon, while it is disfavored by a stronger trans-influence ligand positioned trans to the Cβ-H unit coordinating to the metal (as is the case with any typical trans interaction). These two factors cancel when comparing O∧O and P∧P complexes, while they apply additively in the case of the P∧O complexes (favorably in the case of 2b-agos and unfavorably in the case of 2a-agos). While the energies required to overcome the agostic binding are very similar in the O∧O and P∧P complexes (ca. 15 kcal/ mol), the ethylene binding energies to the three-coordinate (hypothetical nonagostic) species are very different, as would be expected on the basis of the respective trans influences of P and O. Ethylene binding to the nonagostic O∧O complex 3-Trot is 20.7 kcal/mol exergonic in comparison with 6.5 and 10.2 kcal/mol for the P∧P complexes 4-T-rot and 5-T-rot, respectively. Thus, although the energy difference between the P∧P ethylene complexes and their respective insertion TS's is similar to that for the P∧O system, the high stability of the agostic speciesrelative to the ethylene complexescan have a significant effect on the kinetics of insertion under catalytic conditions. An Extreme Case: Si∧O. A silyl group with a negative charge on Si is expected to exert a stronger trans influence than a neutral phosphine. To test some of the inferences reached above, we therefore consider reactions of model Si∧O complexes, shown in Figure 5. The isomeric 6a-C2H4 and 6b-C2H4 complexes differ by 12.5 kcal/mol (favoring the former); this difference is much greater than that of their P∧O analogues (4.6 kcal/mol), consistent with a greater disparity between the trans-influence strength of the donors in Si∧O compared to those in P∧O. In accord with the proposition that a stronger trans-influence group, when trans to the ethyl αcarbon, favors the β-agostic interaction, 6b-agos exhibits a remarkably elongated Cβ−H bond (1.28 Å), a very short CβH− Ni distance (1.54 Å), and a very short Cα−Cβ bond distance (1.45 Å). Furthermore, rotation of the ethyl group in 6b-agos into 6b-T is calculated to have a remarkably high energy of 22.1 kcal/mol, as compared with 16.7 kcal/mol for 2b-agos (P∧O), 13.3 kcal/mol for the O∧O complex, and only 5.9 kcal/mol for the lower energy Si∧O isomer 6a-agos. The free energy of ethylene dissociation from 6a-C2H4 to give 6a-agos is nearly

assumption that slipped ethylene exerts a weaker trans influence than the bound ethyl. Although this is indicated by comparison of the Ni−P bond lengths in the TS's with those in the ethyl− ethylene complexes, these two species are obviously not fully analogous (it is, of course, not possible in this case to keep the cis ligand constant, since slipped ethylene can only be accompanied by a migrating alkyl group). The fact that the slipped ethylene Ni−C bond length is shorter than that of bound ethyl may suggest that the former might exert at least a comparably strong trans influence. If we do not assume that slipped ethylene exerts a weaker trans influence than the fully bound ethyl, then we would not predict that an STI∧STI chelate would afford a lower barrier to insertion than an STI∧WTI chelate. Importantly, however, our other conclusions discussed above remain valid. The differences between the two isomeric TS's in the case of an STI∧WTI chelate are even greater, if slipped ethylene has a very strong trans influence. The prediction of a higher insertion barrier for the WTI∧WTI versus STI∧WTI chelate (Scheme 4, top) also remains valid, since this is based on the greater trans influence of bound ethylene versus migrating ethyl. Regardless of the fact that comparable barriers to insertion are calculated for the P∧P and P∧O ethyl−ethylene complexes, consideration of all the relevant species calculated suggests that the rate of ethylene insertion by the P∧P complexes would be slower than that by the P∧O system. Whereas the calculated resting states in both P∧O and O∧O systems are the fourcoordinate ethylene complexes, the calculations predict ethylene coordination in the case of the P∧P systems to be weak and P∧P systems would have the agostic ethyl complex (plus free ethylene) as the resting state. At the B3LYP/6-311G(2d,2p) level, ethylene coordination to the agostic ethyl species is calculated to be highly endoergic (ΔG°coord = +8.5 kcal/mol in the case of 4-agos); at the M06/6-311++G(2d,2p) level, ΔG°coord to 4-agos and 5-agos is +6.4 and +4.4 kcal/mol, respectively. The significant endoergicity of ethylene coordination to the STI∧STI complexes would much more than offset the slightly lower barrier of insertion found for the [(STI∧STI)Ni(ethyl)(ethylene)] complexes in comparison with the STI∧WTI analogues. As a result, the P∧O (STI∧WTI) system therefore appears more favorable for insertion than either the STI∧STI or WTI∧WTI systems, although for very different reasons. Origin of the Unexpectedly Strong Agostic Interaction in (P∧P)Ni(ethyl). The weak binding of ethylene to [(P∧P)Ni(ethyl)] (ΔG°diss = +8.5 kcal/mol for 4-agos) is in striking contrast with the O∧O system, for which ethylene binding to 3-agos is calculated to be quite exergonic (ΔG°diss = −6.5 kcal/mol): i.e., ΔΔG°diss = 15.0 kcal/mol (Figure 4; ΔΔG°diss = 12.4 kcal/mol at the M06 level). In the P∧O system, ΔG° for ethylene dissociation and agostic bond formation were essentially equal for the 2a-C2H4 and 2b-C2H4 isomers (1.1 and 1.5 kcal/mol, respectively) and approximately midway between the dissociation energies of the O∧O and P∧P ethylene complexes, although the exchange between ethylene and the agostic Cβ−H bond in this pair is also taking place trans to P in one case and trans to O in the other. Thus, the large difference between ethylene binding in the agostic O∧O and P∧P complexes cannot be explained solely in terms of the trans influence of O vs P as exerted on ethylene in comparison with, specifically, the Cβ−H terminal of the ethyl group. On the basis of the structural and energy data for the P∧O system presented in Figure 2, we inferred that the α-carbon of 4688

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Finally, to mitigate the extreme differences in trans influence within the Si∧O chelate while maintaining the very strong transinfluence silyl group, we examine the silyl−amine system shown in Figure 6. Interestingly, for this system the geometry of the

Figure 5. Geometries (in Å and deg) and Gibbs free energies (kcal/ mol, T = 298 K and P = 1 atm) for alkyl migratory insertion and ethylene dissociation in the isomers of [(Si∧O)Ni(ethyl)(ethylene)]. Values in parentheses are Gibbs free energies relative to 6a-C2H4. Figure 6. Geometries (in Å and deg) and Gibbs free energies (kcal/ mol, T = 298 K and P = 1 atm) for alkyl migratory insertion and ethylene dissociation in the isomers of [(Si ∧NH2)Ni(ethyl)(ethylene)]. Values in parentheses are free energies defined relative to 7a-C2H4.

neutral (−0.3 kcal/mol), while that of 6b-C2H4 is slightly more exoergic (−3.3 kcal/mol). The barrier to insertion for 6a-C2H4 is calculated to be very high (34.2 kcal/mol). In the insertion TS the Ni−Si bond lengthens from 2.23 to 2.35 Å, reflecting the unfavorable effect of the slipped ethylene on the binding of a trans-positioned STI ligand. In marked contrast, transformation from 6b-C2H4 to 6b-TS involves a contraction of the Ni−Si bond from 2.37 to 2.22 Å and a barrier of only 4.5 kcal/mol. These barriers in the Si∧O system compare with 27.0 and 15.9 kcal/mol for the analogous P∧O complexes, highlighting the effect on the insertion barriers of the large difference in trans influence between the two coordinating groups. Although the activation energy for insertion in 6b-C2H4 is very small, the large energy difference between the reactant Si∧O ethylene complexes prior to insertion (12.5 kcal/mol) is much greater than that for the P∧O system (4.6 kcal/mol). The overall barrier from 6a-C2H4 (which in turn is 0.3 kcal/mol above 6a-agos) to 6b-TS is 17.1 kcal/mol, close to the overall barrier in the P∧O system (16.5 kcal/mol). Thus, no advantage is obtained in the overall insertion barrier upon substitution of the Si∧O ligand for the P∧O ligand. Data from the M06 calculations support the same conclusion. It is perhaps not surprising that our approach, comparing only the TS's and resting states of lowest energy, would break down in the limit where the higher energy ethyl− ethylene isomer (i.e., 6b-C2H4) approaches the relative energy of the insertion TS of another system, in this case 2b-TS, which is 16.5 kcal/mol above 2a-C2H4.

7b-C2H4 isomer is calculated to have ethylene in the plane of the molecule. Nonetheless, the calculated insertion barriers for the 7a-C2H4 and 7b-C2H4 isomers are 29.7 and 12.6 kcal/mol, respectively, intermediate between the corresponding barriers in the Si∧O system and close to those for the P∧O complexes (27.0 and 11.9 kcal/mol). This presumably reflects the similar difference in trans influence between the silyl and amino groups in comparison with the difference between P and O groups. The low-energy insertion TS (7b-TS) is 16.2 kcal/mol above the lower energy 7a-C2H4 reactant, very similar to the difference between P∧O species 2a-C2H4 and 2b-TS (16.5 kcal/mol). Thus, in accord with the arguments presented above, increasing the trans influence of both groups equally (P∧O vs Si∧NH2) appears to have little effect on the energy difference between the low-energy ethyl−ethylene isomer and the low-energy TS.

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CONCLUSIONS First-principles electronic structure calculations have been used to study the insertion of ethylene into Ni(II)−ethyl bonds in square-planar [(X∧Y)Ni(ethyl)(ethylene)] complexes, where X∧Y is a variable bidentate ligand. We have focused on the question of how asymmetric X∧Y ligands, in particular the phosphinoenolate (P∧O) ligands used in the SHOP process for oligomerization, may confer intrinsic kinetic advantages in 4689

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Organometallics comparison to symmetrical analogues. The calculations largely neglect steric effects but are designed to offer insight into electronic effects, specifically the consequences of varying the trans influence of the coordinating groups. The resting state of the asymmetric [(STI∧WTI)Ni(ethyl)(ethylene)] complexes is the isomer with ethyl trans to the WTI ligand. Isomerization is kinetically facile, however (although endoergic), allowing insertion to proceed via a very favorable TS in which the ethyl group is migrating from the position trans to the STI ligand. The migrating ethyl group exerts a very weak trans influence, and the migration is thus strongly favored by a trans-positioned STI ligand. Accordingly, complexes with symmetrical WTI∧WTI ligands are calculated to have higher insertion barriers than the asymmetric complexes, since the migrating ethyl is necessarily trans to a WTI group in the TS. The same reasoning used above, based on loss of Ni−Et bonding in the insertion TS, leads to the expectation that [(STI∧STI)Ni(ethyl)(ethylene)] complexes should enjoy lower overall barriers to insertion. Indeed, complexes with monoanionic bidentate phosphines representing STI∧STI type ligands are calculated to have access to insertion TS's of lower energy than the asymmetric P∧O ligand, although the differences are very small. However, the STI∧STI complexes are predicted to have stable three-coordinate β-agostic [(STI∧STI)Ni(ethyl)] resting states. This is partially attributable to the strong trans influence of an STI group trans to ethylene favoring dissociation of ethylene from the square-planar complex. In addition, and much less expectedly, the presence of a strong trans-influence group, positioned trans to the ethyl α-carbon, is found to result in a strengthening of the β-agostic interaction. This low-energy agostic resting state of the (STI∧STI)Ni species disfavors the kinetics of the insertion reaction. Our results indicate that within the family of neutral Ni(II) complexes containing a chelating ligand, the P∧O class of chelates affords a combination of trans influences that may be very nearly optimal for the kinetics of insertion. This is not to suggest that other chelates could not afford a similar environment, nor should we generalize to assume that this near optimum applies to other metal centers or even to cationic Ni(II) complexes. Nevertheless, the fundamental factors influencing insertion kinetics elucidated in this work are probably applicable to a broad range of late-transition-metal centers that are active oligo- and polymerization catalysts.

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ACKNOWLEDGMENTS

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REFERENCES

F.H. thanks the University Research Board at the American University of Beirut. A.S.G. and K.K.-J. thank the National Science Foundation (Award CHE-1112456) for support. The XSEDE program is acknowledged for a grant of computer time (TG-CHE100062). A.S.G. thanks the Université Paul Sabatier and the Laboratoire de Chimie de Coordination-CNRS, Toulouse, and the Laboratoire de Chimie de Coordination of the Université de Strasbourg (UMR 7177 CNRS) for Invited Professorships. We thank Prof. Michel Etienne for valuable discussions.

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ASSOCIATED CONTENT

S Supporting Information *

Tables giving relative B3LYP/6-311G(2d,2p) and M06/6-311++G(2d,2p) electronic energies, enthalpies, entropies, and free energies, in the gas and solvent dielectric continuum, and Cartesian coordinates and absolute energies for the B3LYP/6-311G(2d,2p) minima and transition states used in the figures and text giving the complete ref 33. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.H.); [email protected] (K.K.J.); [email protected] (A.S.G.). Notes

The authors declare no competing financial interest. 4690

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