α-Agostic Interactions and Growing Chain Orientation for Olefin

Dec 21, 2015 - Growing chain orientational preference is one of the key factors determining the stereoselectivity of olefin polymerization and related...
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α‑Agostic Interactions and Growing Chain Orientation for Olefin Polymerization Catalysts Giovanni Talarico† and Peter H. M. Budzelaar*,‡,§ †

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Via Cintia, 80126 Napoli, Italy Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada § Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands ‡

S Supporting Information *

ABSTRACT: Growing chain orientational preference is one of the key factors determining the stereoselectivity of olefin polymerization and related reactions such as carboalumination; α-agostic interactions are an important component of this preference. Using insertion of ethene in the M−CH3 bond as a model, the intrinsic orientational preference was evaluated for a number of important catalyst types through determination of the energy profile for methyl group rotation at the insertion TS. Ti and Zr metallocenes show a pronounced preference (Ti, 10 kcal/mol; Zr, ∼6 kcal/mol) for a classical α-agostic arrangement with a single short M···CH contact and an elongated C−H bond; on CH3 rotation the agostic elongation mostly disappears. In contrast, for all non-metallocene systems studied the orientational preference is much smaller or even opposite that of metallocenes. Moreover, on CH3 rotation the agostic C−H bond elongation gets spread out over two C−H bonds rather than disappearing. These results point to greater chain orientation flexibility for nonmetallocene catalysts.

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Chart 1. Schematic View of 1-Olefin (C1H2C2HR) Insertion into the Primary Growing Chain (C3H2P)a

INTRODUCTION

Olefin insertion is one of the elementary reactions of organometallic chemistry and is also the basis for metalcatalyzed olefin polymerization. Polyolefins are among the most important bulk chemicals produced by the petrochemical industry. For polymerization of 1-olefins, one of the key properties affecting product quality is stereoregularity. Stereoregular 1-olefin polymers are invariably made using transitionmetal catalysis. Since the discovery by Ziegler of Ti-catalyzed ethene polymerization1 and the subsequent finding by Natta of stereoselective propene polymerization,2 the field has made impressive progress. This is reflected in the development of increasingly stereoselective catalysts, both heterogeneous (through optimization of support and internal and external donors)3,4 and homogeneous (ever more sophisticated metallocenes5,6 and post-metallocenes7,8). The generally accepted mechanism of stereoregulation for primary propagation, first proposed by Corradini,9,10 can be summarized as follows. (1) Two cis coordination sites are involved: one is occupied by the chain, while the other binds the incoming monomer.11−14 (2) At the transition state (TS), the growing chain assumes one of two conformational chiral15 orientations, indicated by the sign of the dihedral angle C1−M−C3−P in Chart 1. In the original proposal this was attributed to interaction of the chain with the environment of the metal center.9 Later work by others has emphasized the role of α-agostic interactions in limiting chain orientation.16−21 © XXXX American Chemical Society

a

The chain assumes a chiral conformation imposed by site (L) chirality and selects the olefin enantioface with R anti to P. α-Agostic interactions with a C3−H bond are indicated by dotted lines.

(3) Steric hindrance (by the ligand L) is the crucial factor that favors one of these two orientations and hence induces stereoselectivity. (4) The incoming monomer avoids the growing chain, resulting in the preferred anti insertion. This interaction transmits the stereopreference from the ligand via the chain to the monomer. Received: October 14, 2015

A

DOI: 10.1021/acs.organomet.5b00866 Organometallics XXXX, XXX, XXX−XXX

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Organometallics This model does not consider direct interactions between the monomer and the chiral site, although recent studies suggest such interactions are not negligible.22 A crucial factor in this mechanism is a “binary choice” for the chain orientation between two well-defined positions (+ or −). This makes it relatively easy to develop or improve stereoselective homogeneous catalysts by simply introducing steric hindrance (represented by L in Chart 1) near one of the two possible chain positions. If chain orientation were “free”, i.e. with no intrinsic preference for any particular orientation, then a much larger volume of space would need to be blocked by the ligand in order to achieve stereoselectivity, and the job of catalyst designers would be correspondingly harder. However, it is not clear at all from the available literature how strong this orientational ± preference is or how it depends on the ligand(s) and/or the strength of α-agostic interactions. Analysis is complicated for realistic active site models by the fact that both the 1-olefin monomer and the growing chain introduce steric effects which may obscure any intrinsic preference. In the present paper, we attempt to separate out the intrinsic chain orientation preference by analyzing the simplest possible model of olefin polymerizationinsertion of ethene in the metal−methyl bondfor a large number of earlyand late-transition-metal catalysts. The selection of systems contains representative examples of virtually all known olefin polymerization catalysts for which the nature of the active species is well-established. Ti systems are summarized in Scheme 1, Zr systems in Scheme 2, and late-transition-metal

Scheme 2. Zr Catalyst Models Studied

Scheme 1. Ti Catalyst Models Studied

Scheme 3. LTM Catalyst Models Studied

systems (Ni/Pd/Co) in Scheme 3. The early-transition-metal (ETM) selection covers metallocenes (Cp2Ti,23 SiCp2Ti,24 Cp2Zr,25 SiCp2Zr,26−28 CCp2Zr,29,30 CCpFluZr,31 EthInd2Zr,32 EthTHInd 2 Zr, 33 SiInd 2 Zr, Si 2 MeInd 2 Zr, SiBzInd 2 Zr, Si2MeBzInd2Zr, Si4PhInd2Zr, Si2Me4PhInd2Zr,34,35 RescZr36), half-metallocenes (CGCTi,37−40 CpPhimTi,41 CpGuanTi,42 CGCZr,37−40 CGCFluStarZr,43 CpAmidZr44), and non-metallocenes (TacnImidTi,45 C2AmTi,46,47 C3AmTi,48 FITi,49−51 KolZr,52,53 FIZr,49−51 PyAmZr54). For late transition metals (LTM), α-diimine catalysts (DIMNi, DIMPd,55 AceDIMNi, AceDIMPd56), a phenoxy-imine system (SalAldNi57), and a cyclopentadienylcobalt system (CpPCo58) were included. In cases where bulky ligands were involved, minimal models (not shown in the schemes; indicated by a suffix of “_min”) were also included to allow assessment of steric effects.

Finally, it is worth noting that for ETM there is strong experimental evidence for α-agostic interactions during insertion,20 while for LTM no clear evidence has been reported.

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METHODS

Geometries were fully optimized at the TPSSh59/def-TZVP60,61 level using the program Turbomole62 in combination with an external optimizer (PQS Optimize for transition states,63,64 BOptimize for B

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Organometallics second-order saddle points65). A small-core effective core potential was used for Zr and Pd. This choice of functional has been found to perform fairly well for geometries of transition-metal complexes66,67 and was shown in earlier work to produce satisfactory geometries for calculating olefin insertion energy profiles at both early and late transition metals.68,69 The nature of all stationary points was confirmed by vibrational analyses showing exactly one (transition state, TS) or two (second-order saddle point, SOSP) imaginary vibrational modes. For a discussion of the relevance of SOSP, see the Supporting Information. In the text we cite pure electronic energies. The Supporting Information also includes calculated enthalpy and free energy values, but we consider those less important in the present context. The SOSPs we are studying do not have significance as species actually involved in a chemical reaction, only as a measure for the magnitude of an intrinsic geometric preference. In any case, all data are included in the Supporting Information and trends in E, H, and G are rather similar.

Figure 2. Methyl group rotation. The cone rotation angle φ measures the rotation of the methyl group around its axis relative to the agostic reference orientation (left, 0°), up to the nonagostic orientation (right, 60°).

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RESULTS AND DISCUSSION Approach. As mentioned above, we restrict ourselves in the present work to insertion of ethene into the M−CH3 bond in order to eliminate complications due to steric effects. Ethene insertion has been studied by many groups.70−84 The basic Cossee mechanism involves an olefin π complex which progresses via a four-center transition state to the product. The Green−Rooney or α-agostic assistance modification of this mechanism emphasizes the role of α-agostic interactions at the transition state.85 Given this interaction, there are indeed only two possible chain orientations (+ or −, Figure 1). We are interested in quantifying the preference for such orientations over e.g. a “central” position (Figure 1, middle).

120°. Methyl group orientations are indicated as A (“agostic”, |φ| < 15°), NA (“nonagostic”, |φ| > 45°), or INT (“intermediate”, 15°10 kcal/mol) for the agostic orientation of the Me group. Ti catalysts with only a single Cp group (CGCTi, CpGuanTi, CpPhimTi) show a strongly reduced preference (2−6 kcal/ mol), octahedral systems (FITi) show even less, and true tetrahedral systems (C2AmTi, C3AmTi) have virtually no orientational preference. (2) Zirconocenes again show the highest preference of all Zr complexes, but the preference (5−6 kcal/mol) is reduced from that found for Ti systems. Trends for Zr are also similar to those for Ti but are somewhat attenuated: half-metallocenes have a preference of about 3 kcal/mol and octahedral and fivecoordinate systems have even less. (3) For the late transition metals Ni and Pd there is a clear and fairly constant preference of about 2 kcal/mol for nonagostic TS geometries. This is not related to ligand bulk, since minimal models produce virtually identical values. There appears to be very little difference between cationic (α-diimine ligands) and neutral (phenoxy-imine ligands) Ni complexes. Many ETM systems conform qualitatively to the profile of SiCp2Zr (Figure 3A; structures in Figure 4A,B), with a nonagostic SOSP near φ = 60° and an agostic TS near φ = 0°. However, the ideal agostic orientation has perfectly eclipsed methyl and ethene C−H bonds, which introduces some repulsion. For several systems, this results in a preference for slightly less symmetric insertion TSs, the perfectly symmetric structure now becoming an SOSP.71 An example of this is CCp2Zr (profile Figure 3B, structures Figure 4C−E). At the other end of the scale, we find rather flat profiles where neither agostic nor nonagostic structures are strongly preferred. This is illustrated by the C2AmTi system, where the ideal agostic and nonagostic geometries are both SOSPs (Figure 4F,H), and the “true” TSs (Figure 4G) are intermediate; the complete rotation profile (Figure 3C) falls within a band of less than 1 kcal/mol.

Figure 1. − and + chain orientations enforced by α-agostic interactions and a “central” position that would be possible in the absence of such interactions.

Thus, we evaluated the energy profiles for methyl rotation at the surface separating reactant from product; the low points correspond to transition states (TS: one imaginary mode corresponding to insertion), while the high points are secondorder saddle points (SOSP: two imaginary modes, one corresponding to insertion and the other to methyl rotation). We have previously reported how SOSP-TS energy differences can be useful in characterizing the robustness of reaction paths.86 In the present case, small differences (easy methyl rotation) imply little intrinsic preference for any specific orientation; see the Supporting Information for further discussion of SOSPs. Geometries were characterized by calculation of the CH3 group cone rotation angle φ (Figure 2), which measures the rotation of the CH3 group around its cone axis away from the ideal agostic orientation (φ = 0°) toward the ideal nonagostic orientation (φ = ±60°); for a precise definition of this and a few other useful descriptors see the Supporting Information. Since methyl rotation has a 3-fold degeneracy, φ has a period of C

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Organometallics Table 1. Relative Energies and Geometrical Details for Selected M(CH3)(C2H4) Insertions C−H (Å) system Cp2Ti SiCp2Ti CGCTi

CpPhimTi TacnImidTi C2AmTi

Cp2Zr SiCp2Zr CCp2Zr

Si2Me4PhInd2Zr CGCZr CpAmidZr KolZr PyAmZr DIMNi SalAldNi DIMPd CpPCo a

stat Pt

φ (deg)

geometrya

Erel (kcal/mol)

SOSP TS SOSP TS SOSP SOSP TS SOSP TS TS SOSP TS SOSP TS SOSP SOSP TS TS SOSP TS SOSP SOSP TS SOSP SOSP TS SOSP TS SOSP TS TS SOSP TS SOSP SOSP TS TS SOSP SOSP TS

−51.7 5.5 −60.0 0.1 −54.1 −0.3 9.6 −53.1 6.5 −5.1 54.3 −30.4 0.9 25.9 55.2 −60.0 −1.9 −1.6 59.9 −5.9 −0.1 60.0 −4.8 58.1 −57.0 12.6 −40.0 24.1 −57.0 16.7 −1.1 59.9 −55.8 18.9 28.4 59.8 −56.5 −2.8 −56.1 13.7

NA A NA A NA A A NA A A NA INT A INT NA NA A A NA A A NA A NA NA A INT INT NA INT A NA NA INT INT NA NA A NA A

10.75 0.00 10.35 0.00 2.59 0.07 0.00 5.90 0.00 0.00 2.79 0.00 0.61 0.24 0.84 6.53 0.00 0.00 5.17 0.00 0.02 3.75 0.00 5.15 1.69 0.00 3.05 0.00 5.17 0.00 0.00 3.15 0.00 2.63 1.76 0.00 0.00 2.39 3.10 0.00

1.092 1.132 1.091 1.134 1.112 1.136 1.134 1.100 1.130 1.121 1.102 1.129 1.136 1.131 1.113 1.099 1.139 1.139 1.101 1.140 1.140 1.104 1.134 1.103 1.111 1.135 1.114 1.123 1.116 1.130 1.135 1.104 1.092 1.090 1.095 1.089 1.091 1.094 1.087 1.090

1.089 1.087 1.091 1.086 1.101 1.088 1.089 1.094 1.087 1.090 1.095 1.096 1.088 1.093 1.107 1.099 1.087 1.087 1.101 1.088 1.087 1.104 1.087 1.098 1.106 1.090 1.093 1.092 1.112 1.089 1.089 1.104 1.091 1.087 1.086 1.089 1.091 1.085 1.085 1.087

1.085 1.086 1.085 1.086 1.084 1.087 1.087 1.084 1.087 1.087 1.085 1.086 1.088 1.086 1.084 1.085 1.087 1.087 1.084 1.087 1.087 1.084 1.087 1.085 1.085 1.087 1.085 1.086 1.097 1.086 1.088 1.087 1.084 1.087 1.083 1.085 1.084 1.084 1.084 1.084

∑elong (Å)

max elong (Å)

0.008 0.047 0.009 0.048 0.039 0.053 0.053 0.021 0.046 0.040 0.025 0.052 0.054 0.052 0.046 0.025 0.054 0.055 0.029 0.057 0.057 0.034 0.051 0.027 0.044 0.054 0.033 0.044 0.068 0.048 0.054 0.038 0.014 0.012 0.011 0.011 0.013 0.010

0.006 0.046 0.005 0.048 0.026 0.050 0.048 0.014 0.044 0.035 0.017 0.043 0.050 0.045 0.027 0.013 0.053 0.053 0.015 0.054 0.054 0.018 0.048 0.017 0.025 0.049 0.028 0.037 0.030 0.044 0.049 0.018 0.008 0.006 0.011 0.005 0.007 0.010

Based on cone rotation angle: A = agostic (|φ| < 15°), NA = nonagostic (|φ| > 45°), INT = intermediate situation (15° < |φ| < 45°).

For some of the LTM complexes (in particular the Ni examples), pronounced out-of-plane distortions are observed in the (unfavorable) agostic orientation. While φ is still adequate as a description of the orientation of the Me group relative to the metal atom, it no longer provides a complete picture of the insertion geometry. For these highly distorted structures we find φ values of 20−40° instead of the ideal 0°; at the same time, the distortion avoids eclipsing CH3 and monomer C−H bonds, and this may well be the main cause of the distortion. C−H Bond Elongation. Table 1 includes individual C−H bond lengths and the sum of all the bond length elongations as well as the largest individual elongation of any of the three C− H bonds, relative to the appropriate reference value (the Supporting Information provides data for all compounds studied). An agostic interaction is usually accompanied by a significant elongation of the C−H bond. While there is still some

discussion about the precise nature and cause of such interactions,17 this geometric indicator seems to be generally accepted as valid. For the ETM complexes studied here, in the agostic orientation, we find thatas expectedthe agostic C− H bond is significantly elongated while the remaining two bonds are normal. It makes no difference whether the perfectly eclipsed agostic orientation corresponds to a TS (SiCp2Ti) or a SOSP (C2AmTi): it is the spatial arrangement of the C−H bond relative to the metal that determines its length. Relevant geometrical details for these two systems are shown in Figure 5A,C. One might perhaps expect that in the “nonagostic” orientation all three C−H bonds would be normal, but this turns out not to be the case. Rather, both C−H bonds close to the metal are elongated. For most systems except metallocenes, the sum of the two elongations in the nonagostic orientation is comparable to the single elongation in the agostic orientation. D

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Organometallics

Figure 5. Geometric details for M(CH3)(C2H4) fragments of representative structures: SiCp2Ti, A (TS, φ = 0°) and B (SOSP, φ = 60°); C2AmTi, C (SOSP, φ = 0°), D (TS, φ = 36°), and E (SOSP, φ = 58°). Bond lengths are given in Å; values at H atoms indicate M− C3−H bond angles (deg).

while for other systems the agostic interaction is merely spread out and the energy cost is much lower. Population Analysis. For a few selected systems we used Wiberg bond indices (WBI87−89) to analyze bonding within the M(CH3) moiety (see Table 2). This analysis confirms the picture obtained from bond lengths. C−H bonds without agostic interactions typically have a C−H WBI of 0.91−0.92 and a M-H WBI of ∼0.01. For cone rotation angles close to 0°, there is a single agostic hydrogen with a lowered C−H WBI (0.72−0.81) and a correspondingly higher M−H WBI; the sum of the C−H and M−H WBIs is virtually the same as that for the remaining two nonagostic hydrogens. The agostic M−H WBI is somewhat larger for Ti (0.12−0.17) than for Zr (0.11−0.13). At the other end of the scale, for cone rotation angles of around 60° there are two C−H bonds with lowered C−H WBI and increased M−H WBI. Again, the sum of the C−H and M−H WBIs is more or less the same for each of the methyl C−H hydrogens. The sum of the two increased M−H WBIs for φ ≈ 60° is comparable to the single large M−H WBI for φ ≈ 0°. Interestingly, in most but not all cases the M−C WBI is lowest for the agostic orientation (i.e., φ ≈ 0°). There does not seem to be a correlation between the magnitude of the agostic C−H WBI and the M−C WBI. The only clear trend is that Zr− C WBIs are systematically smaller than Ti−C WBIs. Metallocenes vs Other Systems. What causes these significant difference between metallocenes and most other systems? We can think of two different causes, which likely both contribute. In the first place, there must be an electronic component. C− H bond lengths are rather insensitive to steric factors. The strong elongation of one C−H bond for φ ≈ 0° metallocenes indicates a strong electronic interaction, while the much smaller total elongation on rotation indicates the loss of this interaction. Agostic interactions involve donation of electron density from the C−H bond. Metallocenes have strongly donating Cp ligand π orbitals saturating the out-of-plane90 metal d orbitals but much weaker donation into the in-plane acceptor orbitals.91 As a consequence, in metallocenes donation from an in-plane C−H bond can be strong, while after CH3 rotation donation from out-of-plane C−H bonds is much weaker. Apparently, other systems do not have such a clear difference in donation to in-plane and out-of-plane metal acceptor orbitals. A second contribution likely comes from sterics. In the agostic geometry, the single in-plane C−H bond fits neatly into the Cp2M wedge. In contrast, the nonagostic arrangement has

Figure 3. Representative shapes of methyl group rotation energy profiles: (top) SiCp2Zr, A(TS), and B(SOSP); (middle) CCp2Zr, C(SOSP), D(TS), and E(SOSP); (bottom) C2AmTi, F(SOSP), G(TS), and H(SOSP). Stationary points are indicated by black dots. For structure drawings, see Figure 4.

Figure 4. Structures of stationary points for methyl group rotation profiles in Figure 3: SiCp2Zr, A (TS) and B (SOSP); CCp2Zr, C (SOSP), D (TS), and E (SOSP); C2AmTi, F (SOSP), G (TS), and H (SOSP). Ar groups are omitted for clarity.

In other words, for non-metallocenes methyl rotation from φ = 0 to 60° does not remove the agostic interaction but spreads it out over two C−H bonds (Figure 5E). For metallocenes, on the other hand, the total elongation is reduced by 50−90% (depending on the system) on rotation (Figure 5B). This loss in agostic interaction parallels the much larger destabilization of the φ = 60° metallocene geometries, and it seems reasonable to assume these two aspects are connected. In other words, methyl rotation from φ = 0 to 60° mostly cancels agostic interactions and costs a great deal of energy for metallocenes, E

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Organometallics Table 2. Wiberg Bond Indices for M(CH3) Moieties in Selected Systemsa system SiCp2Ti SiCp2Zr CGCTi_min

CGCZr_min C2AmTi_min

C3AmTi_min

CCp2Zr

stat pt

φ (deg)

typeb

M−C

C−H1

C−H2

C−H3

M−H1

M−H2

M−H3

(M+C)−H1c

(M+C)−H2

(M+C)−H3

TS SOSP TS SOSP SOSP TS SOSP SOSP TS TS SOSP TS SOSP SOSP SOSP TS SOSP TS SOSP

0.1 60.9 −1.6 60.7 −7.3 19.8 62.5 −1.1 43.3 −35.5 −0.1 33.7 58.1 −59.9 0.0 26.2 −0.1 −5.9 60.0

A NA A NA A INT NA A INT INT A INT NA NA A INT A A NA

0.9198 0.8098 0.6273 0.6964 0.8123 0.8566 0.9742 0.5502 0.6551 0.7805 0.6985 0.7711 0.8166 0.8224 0.7151 0.7675 0.6280 0.6336 0.7255

0.7237 0.8798 0.7887 0.8553 0.7414 0.7434 0.8141 0.8147 0.8293 0.8000 0.7806 0.7997 0.8419 0.8468 0.7770 0.7893 0.7901 0.7905 0.8519

0.9264 0.8799 0.9134 0.8557 0.9128 0.9023 0.8194 0.9131 0.8777 0.8925 0.9174 0.8959 0.8501 0.8473 0.9188 0.9054 0.9130 0.9111 0.8521

0.9264 0.9155 0.9139 0.9075 0.9147 0.9142 0.9107 0.9138 0.9057 0.9077 0.9175 0.9082 0.9051 0.9071 0.9188 0.9119 0.9131 0.9133 0.9082

0.1725 0.0466 0.1273 0.0584 0.1566 0.1546 0.0977 0.1068 0.0924 0.1120 0.1287 0.1126 0.0764 0.0714 0.1316 0.1214 0.1277 0.1272 0.0628

0.0093 0.0466 0.0109 0.0582 0.0166 0.0279 0.0931 0.0132 0.0430 0.0342 0.0111 0.0315 0.0693 0.0711 0.0105 0.0236 0.0109 0.0124 0.0627

0.0093 0.0134 0.0103 0.0139 0.0139 0.0109 0.0134 0.0126 0.0129 0.0112 0.0109 0.0111 0.0124 0.0120 0.0104 0.0102 0.0108 0.0101 0.0141

0.8962 0.9264 0.9160 0.9137 0.8980 0.8980 0.9118 0.9215 0.9217 0.9120 0.9093 0.9123 0.9183 0.9182 0.9086 0.9107 0.9178 0.9177 0.9147

0.9357 0.9265 0.9243 0.9139 0.9294 0.9302 0.9125 0.9263 0.9207 0.9267 0.9285 0.9274 0.9194 0.9184 0.9293 0.9290 0.9239 0.9235 0.9148

0.9357 0.9289 0.9242 0.9214 0.9286 0.9251 0.9241 0.9264 0.9186 0.9189 0.9284 0.9193 0.9175 0.9191 0.9292 0.9221 0.9239 0.9234 0.9223

a H1, or H1 and H2, are the agostic hydrogens. bCH3 group orientation: A = agostic, NA = nonagostic, INT = intermediate. c(M+C)−H: sum of C− H and M−H WBIs.

It should be noted here that in typical cases the interactions in Corradini’s model (chain−monomer, ligand−chain, and ligand−monomer) each amount to less than 3 kcal/mol;22 thus, an intrinsic orientation preference of about 3 kcal/mol would be sufficient for a fairly stereoselective catalyst. This means half-metallocenes are fine, but octahedral, tetrahedral, or five-coordinate systems need special attention in ligand design. LTM systems pose their own special challenge. Intrinsically, they prefer the chain to be “central” (corresponding to φ angles close to 60°) and because of this the Corradini model does not apply. One obvious way to induce stereoselectivity for such systems is through direct ligand−monomer interaction at the insertion TS, but this is not easy. That, in combination with the tendency of many LTM systems to undergo chain walking, may explain why so far there are few examples of stereoselective LTM olefin polymerization catalysts. Ni complexes of C2vsymmetric α-diimine ligands have been shown to produce poorly syndiotactic polymers at low temperature by chain-end control.92 Using more elaborate C2-symmetric α-diimine Ni complexes, Coates was able to generate fairly isotactic blocks of polypropylene, although again low temperatures were required.93

two C−H bonds pointing directly into the nearby Cp rings, resulting in significant repulsion. This steric contribution easily explains why the “agostic preference” is larger for Ti than for Zr. For LTM systems, we find hardly any C−H elongation, demonstrating that there are no significant agostic interactions in either the φ = 0° or the φ = 60° orientation. This agrees with the idea, expressed by Scherer,17 that an agostic interaction requires “local Lewis acidity”: the Ni or Pd d orbitals close to the Me group, and in principle available for an agostic interaction, are all doubly filled, resulting in a clear lack of local Lewis acidity and a consistent preference for nonagostic geometries. It also partially explains the tendency toward outof-plane distortion with CH3 rotation: in the absence of any stabilizing electronic interaction, the φ = 0° orientation would be expected to be unfavorable, due to the very close approach of the “agostic” C−H bond to the metal. In view of the above, it becomes clear that one should be cautious in correlating “chain orientation” with “interaction”. For ETM metallocenes, φ = 0° corresponds to an agostic interaction, while φ = 60° mostly lacks this interaction. For nonmetallocenes, φ = 0° and φ = 60° (and all values in between) are agostic to a comparable degree. In addition, for LTM complexes, neither φ = 0° nor φ = 60° is associated with significant agostic interactions. With this caveat, we therefore use the terms “agostic orientation” (φ ≈ 0°) and “nonagostic orientation” (φ ≈ 60°) to indicate Me group orientation, without implying anything about the strength of the agostic interaction. Implications for Ligand Design. Metallocenes are special. No other class of ligands shows such a pronounced energetic preference for a well-defined α-agostic (φ = 0°) orientation at the insertion TS. In addition, according to the argument made in the Introduction, this means metallocenes should be the easiest ligands for use in rational catalyst improvement. Halfmetallocenes come next, and according to our results true tetrahedral systems such as diamides have the lowest preference, meaning that most likely rather elaborate ligand design would be needed to make such catalysts stereoselective.

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CONCLUSIONS

Agostic interactions are ubiquitous in the chemistry of electronpoor transition-metal alkyls. In particular, all insertions of C C, CC, and CX bonds into metal−alkyl bonds likely involve agostic interactions before and during insertion; this includes not only olefin polymerization but also reactions such as Zr-catalyzed carboalumination.94−97 The relatively rigid insertion geometry observed with metallocenes, caused by orientation dependence of α-agostic interactions, is atypical and may in part be responsible for the successful design of stereoselective variations, whereas for many other frameworks stereoselectivity remains mostly “hit and miss”. F

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Organometallics

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00866. Total energies, enthalpies and free energies, and geometrical details (PDF) Cartesian coordinates for ETM systems (XYZ) Cartesian coordinates for LTM systems (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for P.H.M.B.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the SABIC Netherlands (unnumbered grant) and NSERC (RGPIN 04766-14) is gratefully acknowledged. Monitoring of the transition state optimizations used in this work was greatly facilitated by software developed in the context of Dutch Polymer Institute project #641.

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REFERENCES

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