Effect of Ligand Structure on Olefin Polymerization by a Metallocene

Mar 27, 2015 - We have carried out a systematic computational study on olefin polymerization by metallocene/borate catalysts, using three metallocenes...
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Effect of Ligand Structure on Olefin Polymerization by a Metallocene/Borate Catalyst: A Computational Study Anniina Laine,† Betty B. Coussens,*,‡ Janne T. Hirvi,† Alexandra Berthoud,§ Nic Friederichs,∥ John R. Severn,⊥ and Mikko Linnolahti*,† †

Department of Chemistry, University of Eastern Finland, Joensuu Campus, FI-80101 Joensuu, Finland DSM Chemical Technology R&D B.V., NL-6160 MD Geleen, The Netherlands § LANXESS Elastomers B.V., NL-6160 BC Geleen, The Netherlands ∥ SABIC Europe, NL-6160 AH Geleen, The Netherlands ⊥ DSM Ahead B.V., NL-6160 MD Geleen, The Netherlands ‡

S Supporting Information *

ABSTRACT: We have carried out a systematic computational study on olefin polymerization by metallocene/borate catalysts, using three metallocenes: Cp2ZrMe2 (Cp), rac-SiMe2-bis(1-(2-Me-(4-PhInd))ZrMe2 (4-PhInd), and rac-SiMe2-bis(1-(2-Me-(4,5-BenzInd))ZrMe2 (4,5-BenzInd). Detailed reaction pathways, including the structure of the catalytically active ion pair, anion displacement, chain propagation, and chain termination steps, are reported for ethene homopolymerization, alongside with investigation of ethene−propene copolymerization reactions. Initially, all catalysts form inner-sphere ion pairs ([L2ZrMe]+−[B(C6F5)4]−) with a direct Zr−F interaction, which is weak enough to be displaced by the incoming monomer. In comparison to Cp, the bulky and electron-rich 4-PhInd and 4,5BenzInd show higher barriers for anion displacement but lead to relative stabilization of the resulting π complexes. 4-PhInd enables the most feasible propene uptake, and both catalysts suppress the chain termination reactions relative to Cp. The borate counterion is shown to have a minor influence after the catalyst activation step.

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INTRODUCTION

The overall mechanisms of catalyst activation are not precisely understood. In the case of MAO, two mechanisms have been proposed: (1) abstraction of a leaving group from the precatalyst by a Lewis acidic site of MAO2 and (2) abstraction of an AlMe2+ end group from the MAO by the precatalyst followed by dissociation of AlMe3 by the incoming monomer.13,14 Recent computational studies have suggested mechanism 2 to dominate by thermodynamic considerations.15 In any case, all computational studies dealing with the MAO activator suffer from its elusive structure, thereby requiring the use of model systems. In that respect boron activators, forming well-defined structures, are a more practical choice for theoretical investigations of the process. Both the B(C6F5)3 and [B(C6F5)4]− activators have been employed in previous studies.16−19 They both abstract a ligand from the precatalyst, leading to a [catalyst]+[activator]− ion pair: B(C6F5)3 by direct means and [B(C6F5)4]− by a reaction between a dimethyl metallocene complex and a borate salt according to eq 1.7,8

Single-site α-olefin polymerization catalysis is an industrially important application of Group 4 organometallic complexes,1 particularly of metallocenes. The metallocene complexes need an activator to form a catalytically active [L2MMe]+[A]− ion pair.2 The catalytic properties of the resulting ion pair are highly dependent on both the ligand framework of the metallocene cation [L2MMe]+ and the structure of the counterion [A]−.2−4 Typical activators used in the process include methylaluminoxane (MAO),5,6 tris(pentafluorophenyl)borane (B(C6F5)3),7 and organoborates such as [CPh3]+[B(C6F5)4]−, the last giving rise to the weakly coordinating tetrakis(perfluoroaryl)borate counterion [B(C6F5)4]−.7,8 Regarding catalyst properties, the stability of the ion pair plays a key role.9,10 Experimental data suggest that weak coordination of the counterion is usually beneficial for catalytic activity.11 On the other hand, the counterion needs to stay close to the metallocene cation in order to compensate for its positive charge.12 Optimally, the counterion provides the needed stabilization for the electron-deficient metallocene cation but is easily displaced by the incoming monomer. © XXXX American Chemical Society

Special Issue: Mike Lappert Memorial Issue Received: November 24, 2014

A

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Organometallics L 2MMe2 + [CPh3]+ [B(C6F5)4 ]− → [L 2MMe]+ [B(C6F5)4 ]− + CPh3Me

(1)

Comparison of the two boron activators has shown that [B(C6F5)]4− makes a more weakly coordinated ion pair, leading to stronger agostic interactions from the polymer chain to the metal, to higher exothermicity of the process, and to lower activation barriers.16 Both counterions can make two classes of ion pairs with the activated catalyst, which differ in distance between the ions. In the inner sphere, the counterion is in close contact with the catalyst metal center, whereas in the outer sphere the counterion is displaced from the close contact to the metal. [B(C6F5)4]−, being more weakly coordinated, has relatively stronger preference toward the outer sphere ion pairs.16,17 In a theoretical study reported herein, we explore the effects of the more weakly coordinating borate counterion [B(C6F5)4]− on the metallocene-catalyzed polymerization pathways, making comparisons to the “naked cation” approximation that omits the presence of a counterion. Ethene homopolymerization, ethene−propene copolymerization, and chain termination reactions are considered for three zirconocenes, namely Cp2ZrMe2 (Cp), rac-SiMe2-bis(1-(2-Me-(4-PhInd))ZrMe2 (4PhInd), and rac-SiMe2-bis(1-(2-Me-(4,5-BenzInd))ZrMe2 (4,5-BenzInd) (Figure 1) to simultaneously evaluate the effects of ligand structures on the process.

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RESULTS AND DISCUSSION Structure of the [L2ZrMe]+[B(C6F5)4]− Ion Pair. The starting point of the study was the activation of the dimethylated metallocene complexes, hence omitting the preceding catalyst alkylation step.20 We first studied the structure of the catalytically active ion pair [L2ZrMe]+[B(C6F5)4]− in the absence of monomer. On the basis of experimental observations, the position of the counterion in the catalytic ion pair can be characterized as being either inner sphere ([L 2 ZrMe] + −[B(C 6 F 5 ) 4 ] − ) or outer sphere ([L2ZrMe]+···[B(C6F5)4]−) (Figure 2).17,21 In practice, the classification is not clear cut for weakly coordinating counterions such as [B(C6F5)4]−, because of a large number of relative positions/orientations of the cation and the anion.22 Several relative orientations, lying close in energy, were located, on the basis of which orientation was selected for representation of both inner- and outer-sphere ion pairs (Figure 2). The relative stabilities of the ion pairs were calculated from eq 1 and are given in Table 1 for each of the studied zirconocenes relative to L2MMe2 + [CPh3]+[B(C6F5)4]−. Independent of the catalyst ligand structure, the most stable structure was characterized with a direct Zr−F interaction, corresponding to the inner-sphere ion pair ([L2ZrMe]+− [B(C6F5)4]−). While the structure of the catalytically active [L2ZrMe]+[B(C6F5)4]− ion pair has not been experimentally characterized, crystal data for a thorium complex, [Cp*2ThMe]+[B(C6F5)4]−, shows a similar metal−F interaction.23,24 The 4-PhInd catalyst gives relatively the most stable ion pair, which is due to an additional stabilization by π stacking between the counterion and a phenyl substituent of the catalyst. The outer-sphere ion pair is made feasible by bending of the ancillary ligand toward the metal,25 thereby stabilizing the cation. For plain cyclopentadienyl ligands such stabilization is not possible, and hence no outer-sphere ion pair was located for the Cp catalyst.

Figure 1. Zirconocene precatalysts included in the study.

Anion Displacement by the Incoming Monomer. The incoming olefin monomer can approach the [L2ZrMe]+[B(C6F5)4]− ion pair from either the f ront or the back (Figure 3). Because the inner-sphere ion pair, [L2ZrMe]+−[B(C6F5)4]− (IPMe in Figure 3), is energetically favored over the outersphere ion pair, the monomer first needs to displace the borate counterion for the insertion reactions. The anion displacement (TSd) leads to π coordination of the olefin monomer (π). The energetics of the anion displacement step for both f ront and back directions, along with the ethene monomer insertion B

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Figure 2. Inner-sphere and outer-sphere ion pairs of [L2ZrMe]+[B(C6F5)4]−.

Table 1. Energies (ΔG in kJ mol−1) for the Formation of the Ion Pairs by Eq 1, Relative Stabilities (ΔGrel in kJ mol−1) of the Ion Pairs with Respect to L2MMe2 + [CPh3]+[B(C6F5)4]−, and Major Nonbonding Interactions of the Ion Pairs inner sphere [L2ZrMe]+−[B(C6F5)4]− Cp 4-PhInd 4,5-BenzInd

ΔG

ΔGrel

−145.5 −167.7 −147.5

0.0 −22.2 −2.0

a

outer sphere [L2ZrMe]+···[B(C6F5)4]−

Zr−F

C6−C6b

C6(c)−C6(c)

2.34 2.28 2.27

3.12

3.68

c

ΔG

ΔGrela

Zr−F

C5−C6d

C5(c)−C6(c)e

−91.4 −90.8

−54.1 −54.7

4.74 4.18

3.22 3.10

3.48 3.46

Relative to the inner sphere [Cp2ZrMe]+−[B(C6F5)4]− ion pair. bThe closest C−C distance between a phenyl and a C6F5 ring. cDistance between the centroids of phenyl and C6F5 rings. dThe closest C−C distance between a Cp and a C6F5 ring. eDistance between the centroids of Cp and C6F5 rings. a

barrier for anion displacement is lower for the f ront direction, in accordance with previous reports.16,18,27−30 This is because, when the ethene approaches from the back direction, it pushes the methyl group of the zirconocene toward the coordination site occupied by the counterion, thus causing steric congestion at the transition state. On comparison of the three

steps that will be discussed later, are given in Figure 4. Solvent effects were initially considered for the Cp catalyst by the polarizable continuum model approach26 using toluene as a solvent. The solvent systematically stabilizes the reaction intermediates but has little effect on the relative stabilities and are hence omitted from Figure 4. For each catalyst, the C

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Figure 3. Schematic presentation of the f ront and back routes for anion displacement and chain propagation for the first ethene insertion.

zirconocenes, the energy for the anion displacement transition state (TSd) for the more feasible f ront direction increases in the order Cp < 4-PhInd < 4,5-BenzInd. The order has a steric origin. A catalyst with a spatially less demanding ligand framework can more easily make room for the incoming ethene. Ethene π coordination (π), which follows the anion displacement, again leads to a number of minima for each catalyst. In the preceding step, not involving the monomer, each of the zirconocenes favor the inner-sphere ion pair with direct Zr−F interaction. However, the incoming monomer is capable of displacing the weakly nucleophilic counterion from the immediate contact to the metal, thereby generally leading to a preference of the outer-sphere ion pair for the ethene π complex.16,17 With respect to Cp, the π complexes of 4-PhInd and 4,5-BenzInd are significantly stabilized by the electron-rich aromatic ligand frameworks, which facilitate the cation−anion separation.4 The cation-stabilizing effect of the ligands in 4PhInd and 4,5-BenzInd is also clearly seen in the naked cation model (Figure 4c), where the catalytic intermediates are systematically stabilized by ca. 40 kJ/mol with respect to Cp. Chain Propagation. Two consecutive f ront/f ront and back/back ethene insertions were studied for each catalyst for the preferred outer-sphere ion pair (Figure 4 and Table 2). The insertion proceeds through a planar, four-centered transition state (TSi), resulting in a propyl chain stabilized by a γ-agostic interaction (γ). Furthermore, the γ-agostic propyl products systematically show stabilizing π-stacking interactions between Cp and C6F5 rings. The agostic interaction is broken either by formation of an inner-sphere ion pair (IPPr) or by π coordination of a second ethene. The insertion of the second ethene then proceeds in the same way as the insertion of the first ethene. Basically, the weak coordination of the borate anion in the outer-sphere ion pairs accounts for the chain migration in the case of front insertions. Regarding transition state energies for ethene insertion (TSi), both pathways show an increase in the order 4-PhInd ≈ 4,5BenzInd < Cp. This is in line with experimental observations that large, electron-rich ligand structures facilitate the polymerization reaction.3,4,31 The subsequent steps in the polymerization pathways repeat the same relative stability order with a lower transition state for insertion of the second monomer.32

The overall process is spontaneous, somewhat favoring the f ront direction. The energy profiles are notably similar to that for the naked cation model, suggesting the counterion primarily acts as a spectator once it has been displaced by the incoming monomer. Comparison of anion displacement and ethene insertion steps shows that in the case of Cp the transition state lies higher for the insertion, while the transition state for anion displacement lies higher for 4-PhInd and 4,5-BenzInd. It is not evident which step is the actual rate-limiting step in the olefin polymerization process.33−36 Propene Homopolymerization, Ethene−Propene Copolymerization, and Chain Termination. We continued to investigate the propene homopolymerization and ethene− propene copolymerization pathways along with chain termination for each of the three catalysts (Figure 5). The study was carried out for two consecutive f ront insertions, as it was shown above to be the favored route for ethene. Insertion of propene can take place in four orientations, deciding the stereo- and regiochemistry of the produced polymer.37 As the 4-PhInd and 4,5-BenzInd catalysts produce highly stereo- and regioregular polymer,38,39 we focused on the primary 1,2-insertion leading to regioregular isotactic polymers. Monomer insertion is affected by the previously inserted monomer,37 and hence the copolymerization step was studied for the second insertion step taking place after either ethene or propene insertion. The molecular weight of the resulting polymer is ultimately determined by the ratio of insertion and termination reactions. Chain termination by β-hydrogen transfer to ethene was chosen as the termination mechanism, since it is equivalent in composition to the insertion reactions and thus enables direct comparisons. The chain growth and termination reactions were analyzed on the basis of the energies of transition states for second monomer insertion and for termination after the first monomer insertion (Figure 6). The differences in the transition state energies (ΔΔGrel*) for ethene vs propene insertion and monomer insertion vs chain termination are given in parts a and b of Figure 7, respectively, for each catalyst. The results obtained by the naked cation model are included for comparison. Independent of the chain end, both with and without the presence of counterion, propene uptake in comparison to D

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Figure 5. Schematic presentation of transition state structures (left) and products (right) for homopolymerization of ethene and propene, copolymerization of ethene and propene, and chain termination by βhydrogen transfer to monomer. Figure 4. Energetics for anion displacement from the [L2ZrMe]+− [B(C6F5)4]− ion pair by ethene monomer, followed by chain growth by insertion of two consecutive monomers: (a) f ront direction; (b) back direction; (c) naked cation model. For nomenclature, see Figure 3. The energies are calculated from eq 1 and are given relative to the inner-sphere [Cp2ZrMe]+−[B(C6F5)4]− ion pair.

ethene chain end, and the approximation generally produces a smaller ΔΔGrel* for ethene vs propene insertions in the absence of steric hindrance of the borate counterion. The effect is less pronounced for the sterically least demanding Cp. The high affinity of 4-PhInd for propene has been observed experimentally.39,40 Generally, however, the model system does not quantitatively reproduce the experimentally observed comonomer affinities.39 This may be due to the rotation of the polymer chain requiring more energy than the ethene insertion. The unattainability of quantitative correlations was

ethene uptake is the most feasible with 4-PhInd (Figure 7a). With the naked cation approximation, propene uptake is actually even favored over ethene uptake in the case of the Table 2. Tabulated Energetics (kJ mol−1) of Figure 4 Cp

IPMe TSd π TSi γ π TSi γ

f ront direction

back direction

0.0 51.5 44.9 96.6 35.6 3.4 31.0 −20.2

0.0 131.4 68.3 102.9 31.3 16.4 32.1 −44.6

4-PhInd naked cation model 0.0 −60.3 −23.0 −89.4 −109.5 −90.4 −163.7

4,5-BenzInd

f ront direction

back direction

naked cation model

f ront direction

back direction

naked cation model

−22.1 69.4 12.4 53.5 −7.1 −29.0 −12.7 −74.4

−22.1 106.5 25.4 61.1 −16.4 −19.8 −2.3 −80.3

−68.1

−2.0 92.1 12.9 47.1 −5.2 −31.4 −9.5 −81.1

−2.0 112.6 28.7 57.7 −15.0 −21.2 −6.8 −83.9

−62.0

E

−98.5 −61.7 −133.1 −151.3 −130.1 −201.3

−100.1 −64.1 −128.3 −148.1 −131.0 −201.1

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monomer insertion is strongly favored over the chain termination by transfer to monomer and is overestimated in comparison to experiments.39 The results are qualitatively reproduced by the naked cation approximation, which further outlines the weak coordination of the [B(C6F5)4]− counterion.

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CONCLUSIONS

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EXPERIMENTAL SECTION

We have reported a comparative computational study on the metallocene perfluoroarylborate ([B(C6F5)4]−) catalyzed ethene−propene homo- and copolymerization processes. Three catalysts were included in the study: Cp2ZrMe2 (Cp), rac-SiMe2-bis(1-(2-Me-(4-PhInd))ZrMe2 (4-PhInd), and racSiMe2-bis(1-(2-Me-(4,5-BenzInd))ZrMe2 (4,5-BenzInd). The first part of the study focused on the effects of the counterion and catalyst ligand structure on the ethene homopolymerization pathways. The results on ethene homopolymerization pathways were further applied to study the ethene−propene copolymerization and termination reactions by β-hydrogen transfer to ethene. Regarding ethene homopolymerization pathways, the polymerization process is initiated with an anion displacement step, where the first incoming monomer displaces the counterion from the inner-sphere [L2ZrMe]+−[B(C6F5)4]− ion pair to the outer-sphere ion pair. The anion displacement takes place preferably from the f ront direction, in line with previous computational results, and the transition state energy increases in the order Cp < 4-PhInd < 4,5-BenzInd. Large electron-rich aromatic ligands sterically hinder the anion displacement but, on the other hand, stabilize the outer-sphere ethene π complexes relative to the inner-sphere complexes. For 4PhInd and 4,5-BenzInd, anion displacement is the rate-limiting step, whereas for Cp it is the first ethene insertion. Beyond the formation of the ethene π complex, the relative stabilities of the reaction intermediates are not significantly affected by the ligand structure of the catalyst. The energetics of the chain propagation are also well produced by the naked cation approach, suggesting the outer-sphere borate counterion is largely a spectator after the catalyst has been activated. Regarding ethene−propene copolymerization, 4-PhInd enables the most feasible propene uptake, in agreement with experiments. The naked cation model, without the steric hindrance of the borate counterion, even shows a preference for propene uptake into the ethene chain end. Moreover, both 4PhInd and 4,5-BenzInd, with large, electron-rich aromatic ligands, suppress the termination reaction relative to the olefin insertion reaction, thereby enabling higher molecular weight production.

Figure 6. Schematic energy profile for competing homopolymerization (solid black), copolymerization (solid green), and chain termination (dashed orange) reactions.

Figure 7. Differences in the transition state energies (ΔΔGrel*, kJ mol−1): (a) propene insertion relative to ethene insertion; (b) chain termination relative to monomer insertion.

discussed in a previous work that used the naked cation approach.39 Note that the previous approach also gave a preference for ethene over propene insertion much greater than that suggested by the experiments. In that respect, the present work provides an improvement, but the improvement appears to be due to the use of a different functional (M062X vs B3LYP) rather than the presence of a counterion. Regarding insertion vs termination (Figure 7b), the termination reaction is the most competitive for Cp. The crowded ligand structures of 4-PhInd and 4,5-BenzInd disfavor the termination reaction, thereby facilitating higher molecular weight polymer production. The effect is particularly strong at the propene chain end, where the propene chain end facilitates the chain termination for Cp. In all other cases,

Computational Details. All calculations were performed by the M062X41/def2-SVP42,43 level of theory using the Gaussian09 program package.44 For Zr, a 28-electron relativistic effective core potential was used to describe the core electrons.45 The M06 set of functionals is capable of addressing dispersive interactions, present in the studied ion pairs.46,47 Furthermore, the method selection allows comparisons to studies involving an MAO activator, where it has been shown to provide accuracy comparable to that of correlated ab initio methods.15,46 Frequency analysis was performed for all optimized structures, to obtain the Gibbs free energies (T = 298.15 K, p = 1.013 bar) and to confirm the nature of the stationary points as a geometry minimum or a transition state. F

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(20) Laine, A.; Linnolahti, M.; Pakkanen, T. A. J. Organomet. Chem. 2012, 716, 79−85. (21) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448−1464. (22) Correa, A.; Cavallo, L. J. Am. Chem. Soc. 2006, 12, 10952− 10959. (23) Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1991, 10, 840−842. (24) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 842−857. (25) Linnolahti, M.; Pakkanen, T. A.; Leino, R.; Luttikhedde, H. J. G.; Wilén, C.-E.; Näsman, J. H. Eur. J. Inorg. Chem. 2001, 2033−2040. (26) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999−3093. (27) Xu, Z.; Vanka, K.; Ziegler, T. Organometallics 2004, 23, 104− 116. (28) Lanza, G.; Fragalà, I. L.; Marks, T. J. Organometallics 2002, 21, 5594−5612. (29) Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2009, 28, 2609−2618. (30) Xu, Z.; Vanka, K.; Firman, T.; Michalak, A.; Zurek, E.; Zhu, C.; Ziegler, T. Organometallics 2002, 21, 2444−2453. (31) Möhring, P. C.; Coville, N. J. Coord. Chem. Rev. 2006, 250, 18− 35. (32) Song, F.; Cannon, R. D.; Bochmann, M. J. Am. Chem. Soc. 2003, 125, 7641−7653. (33) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223−237. (34) Landis, C. R.; Rosaaen, K. A.; Uddin, J. J. Am. Chem. Soc. 2002, 124, 12062−12063. (35) Song, F.; Cannon, R. D.; Bochmann, M. Chem. Commun. 2004, 10, 542−543. (36) Bochmann, M. J. Organomet. Chem. 2004, 689, 3982−3998. (37) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. Rev. 2000, 100, 1253−1346. (38) Ewen, J. A. J. Mol. Catal. A: Chem. 1998, 128, 103−109. (39) Friederichs, N.; Wang, B.; Budzelaar, P. H. M.; Coussens, B. B. J. Mol. Catal. A: Chem. 2005, 242, 91−104. (40) Busico, V.; Cipullo, R.; Segre, A. L. Macromol. Chem. Phys. 2002, 203, 1403−1412. (41) Zhao, Y.; Truhlar, D. Theor. Chem. Acc. 2008, 120, 215−241. (42) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119−124. (43) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, M. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian, Inc., Wallingford, CT, 2009. (45) Andrae, D.; Häussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123−141. (46) Boudene, Z.; De Bruin, T.; Toulhoat, H.; Raybaud, P. Organometallics 2012, 31, 8312−8322. (47) Ehm, C.; Antinucci, G.; Budzelaar, P. H. M.; Busico, V. J. Organomet. Chem. 2014, 772−773, 161−171.

ASSOCIATED CONTENT

S Supporting Information *

A file containing the computed Cartesian coordinates of all of the molecules reported in this study, which may be opened as a text file to read the coordinates or opened directly by a molecular modeling program such as Mercury (version 3.3 or later, http://www.ccdc.cam.ac.uk/pages/Home.aspx) for visualization and analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for B.B.C.: [email protected]. *E-mail for M.L.: mikko.linnolahti@uef.fi. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Academy of Finland (project 251448). The computations were made possible by use of the Finnish Grid infrastructure resources.

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DEDICATION This paper is dedicated to Prof. M. F. Lappert, mentor, “chemical” father, and fellow Maggot Racer.

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REFERENCES

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DOI: 10.1021/om501185x Organometallics XXXX, XXX, XXX−XXX