DOI: 10.1002/chem.201304889

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& Oligomerization Mechanisms

Neutral Nickel Ethylene Oligo- and Polymerization Catalysts: Towards Computational Catalyst Prediction and Design Wouter Heyndrickx,[a, b] Giovanni Occhipinti,[a] and Vidar R. Jensen*[a]

nate between oligo- and polymerization catalysts. In agreement with experiment, we found the Gibbs free energy difference between the overall barrier for the most facile propagation and termination pathways to be close to 0 kcal mol1 for the ethylene oligomerization catalysts I and V, whereas values of at least 7 kcal mol1 in favor of propagation were determined for the polymerization catalysts III and IV. Because of the shared intermediates between the termination and branching pathways, we have been able to identify the preferred cis/trans regiochemistry of b-H elimination and show that a pronounced difference in s donation of the two bridgehead atoms of the bidentate ligand can suppress hydride formation and thus branching. The degree of rationalization obtained here from a handful of key intermediates and transition states is promising for the use of computational methods in the screening and prediction of new catalysts of the title class.

Abstract: DFT calculations have been used to elucidate the chain termination mechanisms for neutral nickel ethylene oligo- and polymerization catalysts and to rationalize the kind of oligomers and polymers produced by each catalyst. The catalysts studied are the (k2-O,O)-coordinated (1,1,1,5,5,5-hexafluoro-2,4-acetylacetonato)nickel catalyst I, the (k2-P,O)-coordinated SHOP-type nickel catalyst II, the (k2N,O)-coordinated anilinotropone and salicylaldiminato nickel catalysts III and IV, respectively, and the (k2-P,N)-coordinated phosphinosulfonamide nickel catalyst V. Numerous termination pathways involving b-H elimination and b-H transfer steps have been investigated, and the most probable routes identified. Despite the complexity and multitude of the possible termination pathways, the information most critical to chain termination is contained in only few transition states. In addition, by consideration of the propagation pathway, we have been able to estimate chain lengths and discrimi-

Introduction

tion metals are better suited for such copolymerization than the early transition metals employed in classic Ziegler–Natta catalysts. A major breakthrough was realized by Brookhart and co-workers with the development of the cationic palladium adiimine catalysts,[12] which are capable of copolymerizing ethylene and methyl acrylate,[13, 14] the latter being a challenging comonomer to incorporate.[15–18] A second important milestone was reached with the development of neutral palladium phosphane–sulfonate catalysts,[19] which incorporate methyl acrylate into a linear copolymer instead of forming the branched copolymer obtained by using the a-diimine catalysts. The palladium phosphane–sulfonate catalysts were later found to incorporate several other polar monomers, amongst them acrylonitrile,[20] vinyl acetate,[21] vinyl fluoride,[22] and vinyl ethers.[23] However, the activities and/or amounts of incorporated polar monomer generally remain low. The use of more weakly binding donor ligands (which dissociate before insertion) was found to improve activity and to allow for increased methyl acrylate incorporation and even methyl acrylate homopolymerization.[16, 17] Next, the introduction of high steric pressure at the axial sites has been found to enhance methyl acrylate incorporation and to give longer chains, a strategy that has been demonstrated for the cationic palladium a-diimine catalysts[24, 25] and the palladium phosphane–sulfonate catalysts.[26, 27] This design element is a recurring theme in olefin polymerization with nickel and palladium catalysts: Whereas catalysts without significant

Olefin oligo- and polymerization are extensively studied organometallic reactions of considerable commercial importance.[1] Proceeding through the same essential elementary step, namely that of consecutive migratory olefin insertion into a metalalkyl bond, oligo- and polymerization differ only in the number of incorporated olefins.[1] Over the last decades there has been a spectacular development in the capabilities and applications of oligo- and polymerization catalysts,[2–7] and long-standing goals in oligo- and polyolefin synthesis continue to be pursued to this day.[8] Apart from the quest for ever more active catalysts, one such long-standing objective is the random copolymerization of nonpolar and polar monomers, which holds the promise of novel polymer structures and properties.[9–11] Because of their lower oxophilicity, late transi[a] Dr. W. Heyndrickx, Dr. G. Occhipinti, Prof. Dr. V. R. Jensen Department of Chemistry, University of Bergen Allgaten 41, N-5007 Bergen (Norway) Fax: (+ 47) 55589490 E-mail: [email protected] [b] Dr. W. Heyndrickx Department of Inorganic and Physical Chemistry, Ghent University Krijgslaan 281 (S3), B-9000 Gent (Belgium) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201304889. Chem. Eur. J. 2014, 20, 1 – 18

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Full Paper steric pressure near the axial sites produce oligomers,[2] the chain length increases with increasing steric pressure.[28–30] Neutral nickel catalysts are a particularly interesting and appealing family of late-transition-metal olefin oligo- and polymerization catalysts. These catalysts show high activities combined with substantial polar functional group tolerance. In some cases, this polar functional group tolerance allows for the copolymerization of nonpolar olefins with certain polar olefins,[31–42] or for polymerization in the presence of polar additives or even in aqueous media.[43–45] Despite the many types of neutral (and zwitterionic) nickel catalysts,[46–59] copolymerization involving methyl acrylate only became possible when using a bimetallic neutral nickel complex in which the two nickel centers are in close proximity.[60] Two prominent examples of highly active, single-component neutral nickel polymerization catalysts are the (k2-N,O)-coordinated anilinotropone[61–63] and salicylaldiminato catalysts.[31–33, 64] Another prominent member of this catalyst family is the neutral (k2-P,O)-coordinated SHOP (Shell Higher Olefin Process)-type nickel catalyst, which has a long history in the oligomerization of olefins and is used industrially as a single-component catalyst for the production of linear a-olefins from ethylene.[65, 66] As a versatile catalyst, the SHOP-type catalyst is also capable of producing polyethylene when combined with a phosphane scavenger[67–69] and even copolymers of ethylene with a-olefins and certain polar monomers.[35] The same versatility is displayed by the structurally similar (k2-P,O)-coordinated phosphanylphenolate catalysts.[34, 70–73] Oligomeric branched internal olefins rather than linear a-olefins can be synthesized by using the neutral (k2-P,N)-coordinated phosphinosulfonamide nickel catalysts, which have an activity comparable to that of the SHOP-type catalysts.[74] Termination pathways are essential for the rationalization and prediction of the behavior of oligo- and polymerization catalysts. Indeed, the ratio of the termination and propagation rates determines whether dimers, oligomers, or polymers are obtained. Furthermore, because branching pathways are often part of termination pathways, knowledge of the termination pathways of a given catalyst should also shed light on the degree of branching to be expected from that catalyst. Although the propagation and chain-branching pathways are generally well established for nickel and palladium ethylene oligo- and polymerization catalysts,[12, 30, 63, 75–80] the termination pathways appear to remain more elusive and controversial. An example is provided by the cationic nickel a-diimine catalysts, for which experimental evidence in favor of chain termination by associative displacement was found, that is, the dissociation of an olefinic chain after coordination of ethylene to a hydride complex produced by b-H elimination (BHE). This evidence included an increase in chain termination rate with increasing ethylene concentration and a fairly low barrier to associative ligand exchange.[12, 76] However, several computational studies point towards a preference for a termination pathway involving b-H transfer (BHT) to the monomer,[81–84] whereas others show a preference for associative displacement after BHE.[85, 86] It should be noted that, on the basis of the experimental evidence, a termination pathway involving BHT cannot be exclud&

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ed, because it would also produce the same ethylene concentration dependency and is not incompatible with a fairly low barrier to associative exchange.[30] Also, both these termination mechanisms are impeded by the introduction of steric bulk at the axial sites. At this point it can also be noted that the BHT to the monomer can proceed in two distinct ways: With or without the involvement or assistance of the transition metal.[87, 88] Focusing on neutral nickel catalysts, several computational investigations aimed at understanding termination pathways have been undertaken. For the salicylaldiminato catalysts, a stepwise BHT pathway via a stable pentacoordinate bisolefin hydride complex was investigated along with the less favorable dissociative displacement, that is, the dissociation of an olefinic chain before the coordination of ethylene to a hydride complex formed after BHE.[89] The possibility of an associative displacement termination pathway or a direct BHT pathway was not considered.[89] In a couple of early works,[90, 91] the termination mechanism of the neutral nickel oligomerization catalyst (1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)nickel (or (1,1,1,5,5,5-hexafluoro-2,4-acetylacetonato)nickel), often conveniently abbreviated as “Ni(HFacac)”,[92] was explored. It was found that the dissociative displacement route was unlikely due to the high energy of the resulting tricoordinate hydride complex, whereas the stepwise BHT pathway (the so-called monomer-assisted BHE) was found to be the dominating termination pathway.[90, 91] Again, no attempts to evaluate the dissociative displacement or direct BHT pathways were made. Summarizing, it appears that the previous computational investigations typically have been focused on a limited number of the many possible termination mechanisms. Two examples exist in which termination pathway energies for nickel catalysts have been used to rationalize and/or predict the behavior of these catalysts: 1) The contrasting behavior of several nickel catalysts, including cationic nickel iminophosphonamide and amidinate catalysts, was rationalized by evaluating energy differences between bis-olefin hydride complexes rather than those of the transition states[93] and 2) the oligomer distribution of cationic (k2-P,N)-coordinated nickel ethylene oligomerization catalysts was predicted by energy differences between the BHE transition states for the alkyl agostic complex, despite the fact that this transition state was not shown to be rate-limiting and there could be other, more favorable, termination pathways.[94, 95] Even though good results were obtained with these rather approximate approaches, it seems clear that there is a need for more comprehensive strategies to establish the intimate mechanisms of termination for the various catalysts, not least if theory is going to play a role in prediction and tuning of nickel oligo- and polymerization catalysts. Promising results in this respect were recently obtained for neutral nickel catalysts involving alkyl-phosphane intermediates,[96] which provided a fundamental insight into the oligomerization behavior of the SHOP-type catalyst, and contrasted with single-component polymerization catalysts such as the salicylaldiminato and anilinotropone catalysts on one hand, and the pyrazolonato–phosphane catalyst producing

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Full Paper shorter oligomers than the SHOP-type catalysts[47, 97] on the other. Following up on the comprehensive approach taken in our previous study,[96] we present herein a systematic density functional theory (DFT) study of five neutral nickel catalysts: The Ni(HFacac) I, the SHOP-type II, the anilinotropone III, the salicylaldiminato IV, and the phosphinosulfonamide catalyst V (see Scheme 1). The selection of catalysts was inspired by their dif-

of C1- or Cs-symmetric bidentate ligands, as is the case for all catalysts in this work with the exception of the bidentate ligand of I (C2v symmetric; see Scheme 1), it is necessary to consider both the cis and trans isomers. All energies in this work will be reported relative to the most stable ethylene p complex 1 a, in which the propyl chain is positioned trans to the most weakly s-donating ionic bridgehead atom X of the bidentate ligand (oxygen in the case of all catalysts except the phosphinosulfonamide catalyst V for which it is nitrogen).[104] The observation that the strongly s-donating alkyl ligand in square-planar polymerization catalysts tends to prefer the position trans to the weaker s-donating ionic bridgehead atom X of the bidentate ligand has been firmly established computationally[17, 89, 96, 102, 105–111] as well as by X-ray crystallography[61, 64, 65] for catalysts carrying (k2-P,O)- and (k2-N,O)-coordinated ligands. However, it is also well established that the most feasible insertion occurs from the least stable ethylene complex, that is, from 1 b, proceeding via transition state 1 b_TS.[17, 89, 96, 102, 105–111] The catalysts studied in this work conform these earlier observations, although the relative stability of 1 a compared with 1 b here was found to be slightly (by 0.1 kcal mol1) in favor of 1 b for IV (see Table 1). After insertion via 1 b_TS, an agostic complex is formed with the extended alkyl chain trans to X (cf. 2 a). Next, to continue propagation, ethylene is taken up via the ethylene association transition state (cf. 1 a_2 a_TS) to form the p complex (cf. 1 a) thereby closing the cycle. Hence, for the feasibility of the propagation, it is important that the relative positions of the ethylene and alkyl ligand can be changed by means of a cis/trans isomerization. Two types of cis/trans isomerizations were investigated here: One for ethylene p complexes proceeding via 1 a_1 b_TS and one for b-agostic ethylene-free complexes via 2 a_2 b_TS. The latter, which has previously been reported by Zeller and Straßner for IV,[102] consistently involves higher activation barriers than the former and will play no important role in this work. It can be noted that very shallow minima were found for most catalysts near the transition state for the cis/trans isomerization of the ethylene complexes 1 a and 1 b. These minima will be completely neglected in this work due to their limited importance. Before entering into the details of the individual catalysts, we will start by discussing Scheme 2, which introduces our terminology and covers the most important termination pathways. Scheme 2 shows that, starting from the ethylene p complex 1 (either 1 a or 1 b), ethylene can be ejected and a b-agostic bond can be formed in a concerted fashion to arrive at the propyl agostic complex 2. From there, b-H elimination to form the propene hydride complex 3 may take place. These steps are part of the conventional BHE termination pathway,[47, 48, 53, 62, 63, 89] which is completed either by the dissociation of propene (dissociative displacement or dissociative BHE) to arrive at the hydride complex 12 or by associative displacement by ethylene (associative displacement or associative BHE). The associative displacement pathway was found to give an intermediate in the form of a pentacoordinate bis-olefin hydride complex, 8, in which the two olefin molecules are parallel and the dihedral angle between the C=C double bond of the olefins and the NiH bond is close to 908. This conformer

Scheme 1. The neutral nickel catalysts studied: The Ni(HFacac) catalyst (I), the SHOP-type catalyst (II), the anilinotropone catalyst (III), the salicylaldiminato catalyst (IV), and the phosphinosulfonamide catalyst (V). Anth represents a 9-anthryl substituent and Ph(iPr)2 indicates a 2,6-diisopropylphenyl group. Unless otherwise stated, R represents a propyl group in the calculated structures. The ligand L does not play a significant role in this work because it is dissociated or abstracted.

ferences in behavior. Whereas catalysts III and IV produce high-molecular-weight polyethylene under typical polymerization conditions, the former with a higher tendency to branching than the latter, catalyst II produces low-molecularweight polymer. Branched oligomers are produced by catalyst V, whereas catalyst I mainly produces linear oligomers. With this contribution, we hope to assess the mechanisms for termination and branching to such an extent that the understanding acquired may serve as a solid basis for the prediction of new neutral nickel ethylene oligo- and polymerization catalysts. Moreover, detailed insight obtained for NiII-based systems could be an important stepping stone to other square-planar d8 systems, such as those based on PdII and RhI. With this insight, the rational development of highly active catalysts for ultra-high-molecular-weight polyethylene or the selective production of butene or lower oligomers, both with arbitrary degrees of branching, can be targeted.

Results and Discussion Overview of the intermediates and transition states Scheme 2 gives an overview of the intermediates and reaction steps studied in this work. As can be seen, the growing polymer chain was modeled by a propyl ligand. A propyl group is able to form the most important a- and b-agostic structures (e.g., see refs. [86, 98–102]), which allows for modeling the b-H elimination reactions in a realistic environment.[103] In the case Chem. Eur. J. 2014, 20, 1 – 18

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Scheme 2. Intermediates and reactions relevant to the termination, branching, and propagation pathways of neutral nickel ethylene oligo- and polymerization catalysts I–V (see Scheme 1) as studied in this work. Grey coloring indicates unfavorable intermediates and transition states. For intermediates and transition states not located for all five catalysts, a list of the catalysts for which these data are available is given in parenthesis, positioned below the label of the corresponding intermediates and above the reaction arrows (for transition states), respectively. For stationary points that were not investigated, or could not be located, for any of the catalysts, the corresponding parentheses contain “na”; see Table 1 for details. Beyond 8 a intermediates of type a are deemed accessible due to the analogy between the cis/trans isomerization connecting 1 a and 1 b via 1 a_1 b_TS, which is facile, and that between 11 a and 11 b. Note that the left and right sections of the scheme become equal for L = X, as is the case for I, which carries a C2v-symmetric bidentate ligand.

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Full Paper more facile than the other steps shown in Scheme 2, such as chain rearrangement, b-H elimination, and dissociation reactions, we have omitted them. A direct, as opposed to stepwise, BHT termination pathway containing only a single elementary step involves the agostic complex 4 in which the b-H is transferred directly to the ethylene through the involvement of the transition metal. The reaction starts from 4 and can be envisioned as a rotation of ethylene to become parallel to the NiH or NiL bond rather than to the NiX bond, followed by the transfer of b-H, via a square-pyramidal transition state 4_10_TS (not shown), leading to the agostic complex 10. From there, the ethyl propene complex 11 and eventually the ethyl agostic complex 14 can be reached, thereby completing the termination. In addition, because most of these termination pathways share a common intermediate, namely the bis-olefin hydride complex 8, combinations of the BHE and BHT pathways are possible due to crossover; for example, the conventional BHE pathway to eliminate hydrogen and transfer it to the metal, followed by a transfer of this hydrogen to ethylene. Alternatively, a stepwise BHT pathway can be followed leading to the bis-olefin hydride complex, followed by propene dissociation. Concluding, a multitude of termination pathways are possible and we have summarized them graphically in Scheme 3 as well as in the following list of reaction pathways.

was consistently found to be more stable than the conformers in which propene or ethylene is parallel to the NiH bond, namely 7 or 6, respectively. This ordering of relative energy is similar to those found in previous computations performed on a-diimine catalysts.[82–84, 86, 93, 101] The olefin rotations connecting the minima of 6, 7, and 8 were, in general, found to be facile. Alternatively, associative displacement can proceed via the less stable bis-olefin hydride conformer 6, in which ethylene is parallel to the NiH bond, followed by a rotation of ethylene to form 8. Termination is then complete after dissociation of the propene from the bis-olefin hydride complex 8, which results in the ethylene hydride complex 13. Several BHT to monomer termination pathways were investigated. We will distinguish between a stepwise BHT pathway, in which hydrogen is transferred to ethylene via intermediates, and a direct BHT pathway, in which hydrogen is transferred to ethylene in one elementary step. Again starting from the ethylene p complex 1, two possible stepwise BHT pathways were found. The first involves a rotation of the propyl ligand to create a b-agostic interaction in an axial position in complex 5 (see Scheme 2), followed by a b-H elimination to form the bisolefin hydride complex 6, in which the ethylene is parallel to the NiH bond. Upon rotation of ethylene, the more stable conformer of the bis-olefin hydride complex 8 can be formed. The minimum 5 containing a b-agostic interaction in an axial position has often been described as part of the BHT termination pathway in previous reports on cationic nickel catalysts.[81, 82, 93] In this work, however, 5 could only be located for catalyst II and the combined propyl chain rearrangement/b-H elimination transition state connecting 1 and 6 will generally be designated 1_6_TS for all catalysts with the exception of II. A seemingly less appreciated alternative involves the rotation of the propyl chain to an axial position to create a b-H interaction in the plane of the bidentate ligand, trans to one of its bridgehead atoms, similar to a transition state previously described in the context of neutral nickel catalysts for phosphane complexes.[96] Such a rotation results in the agostic complex 4, which has been reported for cationic nickel a-diimine catalysts.[85] After b-H elimination, this also forms a bis-olefin hydride complex, 7, in which the propene is parallel to the NiH bond, and a subsequent rotation of propene leads to the more stable bis-olefin hydride complex 8. This is the second stepwise BHT pathway. From the bis-olefin hydride complex 8 the stepwise BHT can proceed by actually transferring the hydrogen to ethylene via a transition state leading to intermediate 10 displaying a b-agostic interaction in the plane of the bidentate ligand or to intermediate 9 with a b-agostic interaction in an axial position. Just as for the intermediate 5 displaying a similar b-agostic interaction in an axial position, intermediate 9 was only found for catalyst II. After rearrangement of the propyl ligand to destroy the b-H interaction, the ethyl propene complex 11 is formed. Termination is then complete after dissociation of the propene resulting in the ethyl agostic complex 14. Note that the equivalents of the bis-olefin hydride complexes 6 and 7 beyond 8 have been omitted. Indeed, the formation of 9 and 10 requires propene and ethylene rotation, respectively, from 8. Because rotation of these olefins is generally Chem. Eur. J. 2014, 20, 1 – 18

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1) Dissociative BHE or dissociative displacement: 1!1_2_ TS!2!2_3_TS!3!12 (or 1!1_3_TS!3…)[112] 2) Associative BHE or associative displacement: 1!1_2_TS! 2!2_3_TS!3!3_8_TS!8!8_13_TS!13 (or 1!1_3_ TS!3…)[112]

Scheme 3. Possible b-H elimination (left) and b-H transfer to monomer (right) pathways for chain termination. See Scheme 2 for the line structures associated with the labels. The mixed BHE/BHT, BHT/BHE, and BHT/BHT pathways (pathways 7–9) are not shown explicitly but can be envisioned to switch side (left to right for BHE/BHT, right to left for BHT/BHE) or arrow type (for BHT/BHT) at the common intermediate 8 (see Scheme 2).

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Full Paper 3) Alternative associative BHE or associative displacement: 1!1_2_TS!2!2_3_TS!3!3_6_TS!6!6_8_TS!8! 8_13_TS!13 (or 1!1_3_TS!3…)[112] 4) Stepwise BHT by an axial agostic interaction: 1!1_5_TS! 5!5_6_TS!6!6_8_TS!8!8_9_TS!9!9_11_TS! 11!14 5) Stepwise BHT by an in-plane agostic interaction: 1!1_4_ TS!4!4_7_TS!7!7_8_TS!8!8_10_TS!10!10_ 11_TS!11!14 6) Direct BHT: 1!1_4_TS!4!4_10_TS!10!10_11_TS! 11!14 7) Mixed BHE/BHT: 1!1_2_TS!2!2_3_TS!3!3_8_TS! 8!8_9_TS!9!9_11_TS!11!14 (or 1!1_3_TS! 3…,[112] or …!8!8_10_TS!10!10_11_TS!11!14, or …!3!3_6_TS!6!6_8_TS!8!…) 8) Mixed BHT/BHE: 1!1_5_TS!5!5_6_TS!6!6_8_TS! 8!8_13_TS!13 (or 1!1_4_TS!4!4_7_TS!7!7_8_ TS!8!…) 9) Mixed BHT/BHT: 1!1_5_TS!5!5_6_TS!6!6_8_TS! 8!8_10_TS!10!10_11_TS!11!14 (or 1!1_4_TS! 4!4_7_TS!7!7_8_TS!8!8_9_TS!9!9_11_TS! 11!14)

From the bis-olefin hydride complex 8, the termination pathways fork and a manifold of routes can then be followed downhill, all passing through transition states that are lower in energy than 3_6_TS and 1_4_TS (21.1 kcal mol1). Hence all the termination pathways featuring an initial BHT step via 1_4_ TS are realistic, that is, the stepwise BHT by an in-plane agostic interaction (pathway 5), the direct BHT (pathway 6), and the mixed BHT/BHE and BHT/BHT (pathways 8 and 9; see Scheme 3). Some of the termination pathways featuring an initial BHE step are also realistic, such as the mixed alternative associative exchange (pathway 3) and the mixed BHE/BHT (pathway 7). In summary, for I, only three of the nine termination pathways can be considered as unrealistic: The dissociative BHE (pathway 1), the associative ligand exchange with ethylene perpendicular to the NiH bond (pathway 2), and the stepwise BHT through an axial agostic interaction (pathway 4). All this taken together indicates that both the ethylene hydride complex 13 (by the mixed BHT/BHE termination pathway) and the ethyl agostic complex 14 (by the other termination pathways) may be involved in the first step of the catalytic cycle. From the above, we can conclude that the potential kinetic bottlenecks along the most likely termination pathways occur prior to the formation of the bis-olefin complexes 6–8. This means that the transition states before these common intermediates, such as the chain rearrangement/b-H elimination transition states 1_4_TS and 1_6_TS are of higher energy than their counterparts further down the route to termination, 10_ 11_TS and 8_11_TS, respectively. The lower energy of the latter two transition states is probably at least partially caused by the propene-ethyl ligand combination being more stable than the ethylene-propyl combination, as can be seen from the increased stability (by 2.5 kcal mol1) of 11 over 1. A similar effect is noticed for the ethylene association transition state 3_ 8_TS, which is higher in energy than its analogue beyond 8, namely the propene dissociation transition state 8_13_TS. Another example of this is the higher energy (by 1.8 kcal mol1) of the ethylene hydride complex 3 relative to the propene hydride complex 13. Because the ethyl and propyl ligands can be expected to be more similar than the ethylene and propene ligands, the explanation for these observations must primarily be rooted in differences between the two olefinic ligands. In fact, these trends basically reflect the higher stability of propene relative to ethylene, which manifests itself in a DG of 4.7 kcal mol1 for the reaction propene + ethane!ethylene + propane, as was calculated by using the current methods. The b-H elimination reactions via 4_7_TS and 1_6_TS eliminate one of two nonequivalent hydrogen atoms. Depending on which hydrogen is eliminated, the complex formed either bears a propene molecule coordinated through the Re face (or formally the Re-Re face because both the sp2-hybridized carbon atoms of propene are bound to the metal through their Re face according to the prochirality rules of Hanson)[113] or the Si face (or formally the Si-Si face;[113] see Figure 1). In the case of I, disregarding 3_6_TS for a moment, if one of the two hydrogen atoms is significantly more prone to elimination, this could restrict the possible isomers of the bis-olefin hydride complex 8. Because olefin rotations generally require fairly

“Ni(HFacac)” catalyst I Termination From Schemes 2 and 3 it can be seen that chain termination can be initiated by three possible transition states: 1_4_TS, 1_ 6_TS, and 1_2_TS. The chain rearrangement transition state for BHT termination, 1_4_TS (stepwise and direct), shows a slightly lower energy (21.1 kcal mol1; Table 1) than that of 1_ 6_TS (stepwise; 21.9 kcal mol1). However, the initial ethylene dissociation step via 1_2_TS (18.2 kcal mol1) is lower in energy than both 1_4_TS and 1_6_TS, which indicates that the agostic propyl complex 2 may be involved in termination. Indeed, after a relatively facile b-H elimination via 2_3_TS (17.1 kcal mol1), the bis-olefin hydride intermediate 8 may be formed by ethylene binding parallel to the NiH via 3_6_TS (21.1 kcal mol1), followed by olefin rotation via 6_8_TS (19.8 kcal mol1). The direct formation of 8 via 3_8_TS (22.2 kcal mol1) is less favored, and hence the alternative associative displacement (pathway 3) will be favored over the normal associative displacement (pathway 2; see Scheme 3). Considering the BHT termination pathways, 8 can be generated by olefin rotation via 6_8_TS and by b-H elimination via 4_7_TS (18.2 kcal mol1), followed by olefin rotation via 7_8_TS (20.2 kcal mol1). Dissociative ligand exchange from 3 (pathway 1) leads to the hydride 12, which is impossibly (> 45 kcal mol1) high in energy. Furthermore, direct BHT (pathway 6) via 4_10_TS (20.6 kcal mol1) is a realistic pathway because it involves a transition state of lower energy than that of 1_4_TS, the energy of which is on a par with that of the other termination bottleneck 3_6_TS. In contrast, the energy of 1_6_TS is higher than that of both 1_4_TS and 3_6_TS, which suggests that the former will not be important for termination. &

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Full Paper Table 1. Gibbs free energies for the stationary points shown in Scheme 2.[a,b]

Table 1. (Continued)

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1a 1 a_TS 1 a_1 b_TS 1 a_2 a_TS 1 a_4 a_TS 1 a_6 a_TS 1 a_7 a_TS 2a 2 a_2 b_TS 2 a_3 a_TS 2 a_4 a_TS 3a 3 a_6 a_TS 3 a_8 a_TS 4a 4 a_7 a_TS 4 a_10 a_TS 5a 5 a_6 a_TS 6a 6 a_8 a_TS 7a 7 a_8 a_TS 8a 8 a_10 a_TS 8 a_11 a_TS 8 a_13 a_TS 9a 10 a 10 a_11 a_TS 11 a 12 a 13 a 14 a 1b 1 b_TS 1 b_2 b_TS 1 b_4 b_TS 1 b_5 b_TS 1 b_6 b_TS 1 b_7 b_TS 2b 2 b_3 b_TS 2 b_4 b_TS 3b 3 b_6 b_TS 3 b_8 b_TS 4b 4 b_7 b_TS 4 b_10 b_TS 5b 5 b_6 b_TS 6b 6 b_8 b_TS 7b 7 b_8 b_TS 7 b_13 b_TS 8b 8 b_9 b_TS 8 b_10 b_TS 8 b_11 b_TS 8 b_13 b_TS 9b 10 b

DG [kcal mol ] III IV

I

II

0.0 19.6 –[d] 18.2 21.1 21.9 –[g] 10.7 –[d] 17.1 –[e] 14.5 21.1 22.2 16.7 18.2 20.6 –[g] –[g] 18.6 19.8 18.6 20.2 15.4 18.4 20.9 19.1 –[g] 14.3 18.0 2.5 47.2 12.7 5.4 0.0 19.6 18.2 21.1 –[g] 21.9 –[g] 10.7 17.1 –[g] 14.5 21.1 22.2 16.7 18.2 20.6 –[g] –[g] 18.6 19.8 18.6 20.2 –[g] 15.4 –[g] 18.4 20.9 19.1 –[g] 14.3

0.0 20.1 13.0 7.6 10.3 –[g] –[g] 3.3 32.1 21.7 –[e] 15.5 –[e] –[e] 9.0 –[e] 18.9 0.8 –[g] –[e] –[e] –[e] –[e] 20.4 18.6 –[e] –[e] –[e] 7.4 8.6 3.6 47.5 14.4 1.6 2.5 12.7 16.5 18.6 5.7 –[g] –[g] 7.8 6.9 16.9 7.0 11.8 14.3 13.5 14.0 –[f] 5.1 15.4 11.7 12.4 11.1 13.4 –[g] 7.0 14.8 13.5 –[g] 13.8 2.2 12.0

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0.0 21.3 12.1 16.8 –[e] 27.9 –[g] 8.8 31.5 19.4 –[e] 15.8 –[e] –[e] –[e] 26.3 30.6 –[g] –[g] –[e] –[e] –[e] –[e] 26.0 26.2 –[e] –[e] –[g] –[e] –[e] 2.8 38.6 15.3 5.8 0.8 15.3 22.7 23.1 –[g] 24.8 –[g] 9.3 15.4 23.3 9.2 21.2 22.6 22.1 22.3 –[f] –[g] –[g] 21.1 22.3 19.3 21.3 –[g] 19.4 –[g] 21.0 23.6 18.4 –[g] 18.6

0.0 –[c] 6.9 11.6 –[e] 27.1[h] –[g] 3.0 28.8 18.9 –[e] 14.7 –[e] –[e] –[e] 25.1 29.8 –[g] –[g] 25.2 –[e] –[e] –[e] 24.9 24.9 –[e] –[e] –[g] –[e] –[e] 4.1 33.8 13.8 0.0 0.1 11.2 18.3 21.4 –[g] 22.3[h] –[g] 5.4 12.4 19.4 6.5 17.9 18.9 17.8 17.7 –[f] –[g] –[g] 16.7 19.3 –[g] –[g] –[g] 15.3 –[g] 18.2 21.0 15.8 –[g] 16.5

V

10 b_11 b_TS 11 b 12 b 13 b 14 b

0.0 18.1 11.9 9.5 –[f] –[f] 22.0 2.0 26.0 17.3 –[e] 13.4 –[e] –[e] –[f] –[e] 23.9 –[f] –[f] –[e] –[e] –[e] –[e] 22.3 22.9 –[e] –[e] –[f] –[e] –[e] 2.5 42.8 10.9 1.3 6.2 15.8 15.2 –[f] –[g] 19.1 16.3 5.4 10.4 –[f] 9.1 17.0 19.4 –[f] –[f] –[f] –[g] –[g] 17.0 –[f] 14.6 15.4 13.8 15.9 –[f] –[f] 20.1 –[f] –[f] –[f]

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18.0 2.5 47.2 12.7 5.4

15.3 1.2 23.7 7.8 2.8

DG [kcal mol1] III IV 20.9 1.4 38.1 9.2 6.3

17.3 1.7 31.0 7.0 1.7

V –[f] 1.8 23.1 6.7 2.2

[a] Italicized text indicates unfavorable intermediates and transition states. [b] In toluene at 313.15 K for the anilinotropone (III), salicylaldiminato (IV), and phosphinosulfonamide (V) catalysts and at 353.15 K for the Ni(HFacac) (I) and the SHOP-type (II) catalysts. [c] Geometry optimizations consistently yielded a stationary structure with two imaginary frequencies. [d] Does not exist for symmetry reasons. [e] Not investigated due to the uncompetitively high energy of 8 a, which suggests that this stationary point is of limited importance. [f] Could not be located due to a very high barrier or no interconnecting path, see text for additional details. [g] A well-defined transition state could not be located due to a flat potential energy surface, see text for additional details. [h] 1_3_TS instead of 1_6_TS for this catalyst; see ref. 112.

Figure 1. Illustration of the two possible diastereomers for the bis-olefin complex 7 occurring after b-H elimination via 4_7_TS. Depending on which b-agostic hydrogen is abstracted, the resulting propene will be coordinated via its Si (top) or Re (bottom) face. Similar considerations can be made for bH elimination via 1_6_TS.

little energy (illustrated, for example, by 6_8_TS or 7_8_TS), only the two Re and Si diastereomers (Figure 1) are truly distinct for the bis-olefin hydride complexes. Changing the coordination face of propene can by no means be assumed a facile process. In other words, which b-H is eliminated initially is likely to affect the further course of the reaction. Our calculations show that both isomers will be accessible for the bisolefin complex because the chain rearrangement 1_4_TS shows almost no preference (0.4 kcal mol1, see 1_4_TS versus 1_4_TS_alt in the Supporting Information) for one or other isomer. Therefore further calculation of the termination pathway can involve either isomer. On the other hand, 1_6_TS does show some preference (2.5 kcal mol1, see 1_6_TS versus 1_6_TS_alt in the Supporting Information) for the Re coordination of propene (see the Supporting Information and Figure 1). Because the termination via 3_6_TS is also competitive, one would also have to consider the association transition states 7

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Full Paper for the different isomers of the hydride propene complex 3 to determine the accessible isomers of the bis-olefin hydride complex 8. However, because both isomers already occur from 1_4_TS, this was not done. The data in Table 1 shows the small energy difference between the propagation pathway via 1_TS (19.6 kcal mol1) and the most facile termination pathway via 1_4_TS or 3_6_TS (21.1 kcal mol1), which reflects the tendency of the catalyst towards ethylene oligomerization rather than polymerization. The overall energy difference DDG° between the bottleneck termination and propagation transition states is 1.5 kcal mol1 in favor of propagation. Considering the fact that catalyst I produces mainly lower C4–C10 oligomer fractions,[92] this DDG° is likely overestimated somewhat. Nevertheless, “chemical accuracy” ( 1 kcal mol1) cannot be expected from the present methods, and it must be concluded that the preference for oligomerization is captured remarkably well by the calculations. Branching It is interesting to note that the rather low barriers to ethylene association/dissociation via 1_2_TS (18.2 kcal mol1) and b-H elimination via 2_3_TS (17.1 kcal mol1) relative to propagation via 1_TS (19.6 kcal mol1) indicate that the propene hydride 3 (14.5 kcal mol1) will be readily accessible. This hydride is a necessary intermediate in the conventional branching pathway and hence the low barriers are consistent with the approximately 20 % content of branched oligomers in the C4–C24 fractions.[92, 114] It is also a necessary intermediate in the olefin isomerization, which I has been found to catalyze.[92] It should also be noted that associative ligand displacement via 3_6_TS is significantly less facile than b-H elimination via 2_3_TS, which is another necessary requirement for branching and isomerization to occur via the conventional mechanism because it allows for insertion of propene into the NiH bond (the reverse b-H elimination) with opposite regiochemistry (2,1-insertion of propene into the NiH bond) before termination can occur (see Scheme 4). This indicates that an alkyl ligand with a secondary carbon atom can easily be formed, and this alkyl species is capable of propagating and introducing a branch, or terminating, thereby resulting in an internal olefin. A more quantitative assessment of the branching content would also require determination of the ethylene insertion barriers for the isopropyl chain and comparison with the insertion of n-propyl, tasks which fall outside the scope of this investigation.

Scheme 4. Conventional branching mechanism for the introduction of a methyl branch into polyethylene. Longer chains and isomerization requires the involvement of another b-agostic hydrogen.

which excludes pathway 5. BHT-based routes involving an axial agostic interaction are also more facile than ethylene association via 1 b_2 b_TS (16.5 kcal mol1), which excludes the BHEinitiated pathways (pathways 1–3 and 7). Recall that the intermediate 5 b, with an agostic interaction in the axial position, could only be found for II and not for the other catalysts studied in this work. After b-H elimination via 5 b_6 b_TS and ethylene rotation via 6 b_8 b_TS to form 8, termination may be completed by several routes involving barriers that are lower than that of 5 b_6 b_TS. From 8 b the complex may undergo another BHT involving an axial agostic interaction, 8 b_9 b_TS (14.8 kcal mol1, pathway 4). Again, it can be noted that the bottleneck transition state for the BHT termination pathway occurs before the bis-olefin complex 8. Moreover, the initial barrier for creating the b-agostic interaction in the axial position 1 b_5 b_TS is very low (5.7 kcal mol1), which is why the analogous transition state 9 b_11 b_TS has not been calculated. Other routes, such as mixed BHT/BHE (pathway 8) or mixed BHT/BHT involving stepwise BHT with an in-plane agostic interaction after 8 (pathway 9) also involve bottleneck transition states, 8 b_13 b_TS (13.8 kcal mol1) and 10 b_11 b_ TS (15.3 kcal mol1), of lower energy than 5 b_6 b_TS and are likely to play a role, alongside pathway 4, in chain termination for the SHOP-type catalyst II. Based on the high energy (20.4 kcal mol1) of the most stable bis-olefin complex with hydrogen trans to phosphorus (8 a), which is already higher than the highest point along the most facile termination pathway (5 b_6 b_TS), most a-sided ter-

SHOP-type catalyst II Termination The possible termination pathways for catalyst II are somewhat more discriminating than those for I (see Table 1). The most facile termination pathway involves the stepwise BHT with an axial agostic interaction with an overall barrier of 15.4 kcal mol1 for the b-H elimination transition state 5 b_6 b_TS, which excludes pathways other than 4, 8, and 9. In comparison, the stepwise BHT with in-plane agostic interaction involves an overall barrier of 18.6 kcal mol1 via 1 b_4 b_TS, &

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Full Paper and the b-agostic complex 1 b_2 b_TS and 2 b. Although the C1- or Cs-symmetric bidentate ligand can also modulate the insertion barrier, the effect is larger for the rate-limiting transition states leading to branching, 2 a_3 a_TS and 1 b_2 b_TS. For example, the destabilization of 2 a_3 a_TS and 1 b_2 b_TS for II relative to I can be compared in Figure 2, in which energies are shown relative to 1 b_TS. This finding can be extended to the other catalysts with C1- or Cs-symmetric bidentate ligands III, IV, and V (Figure 2), for which 2 a_3 a_TS and 1 b_2 b_TS are destabilized for all catalysts relative to I. The least difference is observed for V, in which the phosphane and amide moieties can both be considered strongly s-donating.

mination pathways (pathways 1–9, except for 1 and 6) are uncompetitive. A direct BHT (pathway 6), with transition state 4 a_10 a_TS (18.9 kcal mol1), is the most facile a-sided termination pathway, but nevertheless still uncompetitive. The a-sided dissociative displacement (pathway 1) is excluded based on the very high energy of 12 a (47.5 kcal mol1). Whereas the b-H elimination transition state 2 a_3 a_TS, in which a hydride is created trans to the phosphorus, is higher in energy than its counterpart 2 b_3 b_TS, not unexpectedly, the transition states that do not include hydrogen trans to phosphorus, for example, 1 a_2 a_TS, 1 a_4 a_TS, and 10 a_11 a_TS, are more stable than their counterparts 1 b_2 b_TS, 1 b_4 b_TS, and 10 b_11 b_ TS, in which propyl is trans to phosphorus. Regarding the different isomers accessible to the bis-olefin hydride complex 8 b, 5 b_6 b_TS has a relatively clear preference (by 1.5 kcal mol1) for leading to one of the two isomers, namely the Si diastereomer (see 5 b_6 b_TS_alt in the Supporting Information for the Re diastereomer). Again, rotations of propene will be facile, reducing the number of distinct isomers from four to two (see Figure 1 and associated text). The reported bis-olefin hydride complex 8 b and the following termination pathways are all consistent with this distinct isomer of 8 b. For the alternatives to 5 b_6 b_TS leading to termination (4 b_ 7 b_TS and 1 b_2 b_TS), only the most stable transition states and minima are reported. However, 4 b_7 b_TS, in contrast to 5 b_6 b_TS, does not discriminate between the two transition states leading to the different isomers. One peculiarity of the SHOP-type catalyst II, already noted in earlier work,[96] is that the cis/trans isomerization barrier of 13.0 kcal mol1 between the two ethylene complexes via 1 a_ 1 b_TS is comparable to the most facile ethylene insertion barrier via 1 b_TS (12.7 kcal mol1). The calculated value for DDG° (between 1 b_TS and 5 b_6 b_TS) is 2.1 kcal mol1 in favor of propagation, that is, higher than the value for the oligomerization catalyst I and consistent with the fact that low-molecularweight polyethylene (Mw = 104) is obtained with the SHOP-type catalyst.[47]

Figure 2. Gibbs free energy profiles for propagation via 1 b_TS and hydride formation via 1 b_2 b_TS or 2 a_3 a_TS for the catalysts studied in this work. Note that the energies are given relative to 1 b_TS to ensure facile comparison between the transition states leading to propagation on one hand and to hydride formation on the other.

Part of the reason why the C1- or Cs-symmetric bidentate ligands do not suppress ethylene insertion to the same extent as branching pathways has to do with the fact that the effects of the difference in trans influence are more pronounced in (agostic) alkyl complexes than in complexes also containing an olefin ligand. To illustrate, the difference in stability between the alkyl complexes 2 a and 2 b is larger by 2 kcal mol1 than the corresponding difference between the olefin complexes 1 a and 1 b. Starting ethylene insertion from the latter requires little activation and results in an alkyl complex with R positioned trans to X, that is, similar to 2 a. Conversely, insertion starting from the most stable (by 2.5 kcal mol1) olefin complex 1 a results in the least stable (by 4.5 kcal mol1 if taken as the difference between 2 a and 2 b) nickel alkyl complex, one in which R is positioned trans to L. In a Hammond sense,[115] 1 b_ TS should then be considered a relatively early transition state, resembling 1 b more than 1 a_TS resembles 1 a. Insertion from 1 a requires more activation and structural changes to reach a transition state that to a greater extent resembles the comparably unstable alkyl product similar to 2 b. Concluding, consideration of the differences in s donation and trans influence helps explain why C1- or Cs-symmetric bidentate ligands steer olefin insertion into a preferred, viable route, whereas the same ligands more uniformly suppress a range of different pathways to hydrogen elimination and branching.

Branching A C1- or Cs-symmetric bidentate ligand with different s-donating properties on each side of the ligand will have a substantial effect on the conventional branching mechanism via the b-H elimination transition states 2 a_3 a_TS and 2 b_3 b_TS. Although hydride elimination from the most stable b-agostic complex 2 a is high in energy because of the creation of a strongly s-donating hydride ligand trans to the stronger sdonating phosphorus of the bidentate ligand, the hydride elimination from the least stable b-agostic complex 2 b is also disfavored, but now because of ethylene dissociation via 1 b_ 2 b_TS. The fact that the latter barrier is relatively high can be attributed to the reduced stability of the agostic complex 2 b in which the propyl ligand is trans to the stronger s-donating phosphorus of the bidentate ligand. Hence, the strong s-donation on one side of the bidentate ligand hinders both conventional b-H elimination pathways by destabilization of either the b-H elimination itself (2 a_3 a_TS) or the ethylene dissociation Chem. Eur. J. 2014, 20, 1 – 18

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Full Paper Following the rationale outlined above, the degree of branching should increase with decreasing difference in s-donating capabilities of the bridgehead atoms of the bidentate ligand, a trend that is visible from the ordering of the energies of 2 a_3 a_TS and 1 b_2 b_TS relative to that of 1 b_TS (see Figure 2), that is, I < V < II, III, IV. Considering other examples of neutral nickel catalysts, the phosphane–sulfonate catalyst stands out by the pronounced difference between the s donation of the phosphane moiety (strong) and the sulfonate moiety (very weak) of the bidentate ligand. An experimental branch content of around 10/1000 C in polyethylene produced by the phosphane–sulfonate catalyst[51, 59] compares very well with the branching experimentally found for IV,[31, 33] which is the catalyst least likely to form the hydride 3 a or 3 b. Furthermore, substituting the aryl substituents of the phosphorus in the bidentate ligand of the phosphane–sulfonate catalyst by cyclohexyl groups, which increases the s-donating capability of phosphorus without significantly affecting that of oxygen, leads, as expected (because the difference in s donation increases), to less branching.[51] From Figure 2, the difference between III and IV in tendency to form branches (III generally produces more branches)[61, 62, 116] is also reflected in the energy difference (3.0 kcal mol1, with energies relative to 1 b_TS, as illustrated in Figure 2) between the rate-limiting transition states that lead to the hydride, that is, 2 a_3 a_TS for III and 1 b_2 b_TS for IV. It can be noted that the stability of the p complex resting state 1 a does not influence the competition between propagation and the formation of the potentially branch-producing hydride 3 a or 3 b because these will be determined by the relative energies of the transition states involved, as explained above. An increase in the stability of 1 a would likely lead to a drop in activity, however. And if neutral nickel catalysts are used as single-component catalysts, the binding constant of the ligand L must also be determined for a reliable assessment of the absolute activity. Also, increasingly branched chains can still be obtained by reducing ethylene pressure, albeit with a drop in activity, because that would increase the concentration of the agostic complexes 2 a and 2 b relative to 1 a without affecting the reaction rate constants. Finally, an additional factor favoring the formation of linear chains with II originates from the cumbersome access to 2 b and 3 b for this catalyst, and when 3 b is not formed that often, the probability for termination exceeds the probability for the introduction of a branch. Indeed, termination, that is, easy ethylene association via 3 b_6 b_TS (11.8 kcal mol1), which could take place to form 6 b and 8 b and eventually lead to facile BHT-induced termination via 8 b_9 b_TS (14.8 kcal mol1; see above), appears to have a lower overall barrier than that of branching, that is, reinsertion of propene into the NiH bond (the reverse b-H elimination) with opposite regiochemistry (2,1-insertion), followed by rate-limiting ethylene capture (cf. 1 b_2 b_TS, 16.5 kcal mol1) and propagation.

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Anilinotropone catalyst III Termination It can be seen from Scheme 2 and Table 1 that there are several pathways likely to lead to termination for catalyst III. For all the studied b-sided termination pathways, the rate-limiting transition states are located before the bis-olefin hydride complex, that is, 1 b_2 b_TS (22.7 kcal mol1), 1 b_4 b_TS (23.1 kcal mol1), and 1 b_6 b_TS (24.8 kcal mol1). The first two are very close in energy and can essentially be considered degenerate. This suggests that the operative termination pathways will be manifold. Whereas the higher energy of 1 b_6 b_TS makes the possibility of the stepwise BHT through an axial agostic interaction (pathway 4) seem less likely, dissociative displacement (pathway 1) can be excluded outright due to the very high energy of the hydride 12 b (38.1 kcal mol1). All the other pathways starting from 1 b seem feasible, that is, associative BHE or associative displacement, both regular and alternative, stepwise BHT through axial and in-plane agostic interactions, mixed BHE/BHT, mixed BHT/BHE, and mixed BHT/BHT (pathways 2, 3, 5, and 7–9). For the a-sided termination pathways, transition states for bH elimination (4 a_7 a_TS and 1 a_6 a_TS) of rather high energy are found, effectively shutting off the BHT pathways. The high energy of the bis-olefin hydride intermediate 8 a (26.0 kcal mol1), already higher than the bottleneck transition states of the b-sided termination pathways via 1 b_2 b_TS (22.7 kcal mol1) and 1 b_4 b_TS (23.1 kcal mol1), suggests that most of the a-sided termination pathways (pathways 1—9, except for 1 and 6) will be uncompetitive. The direct BHT pathway (pathway 6) via 4 a_10 a_TS is uncompetitively high in energy (30.6 kcal mol1) and the dissociative displacement (pathway 1) can be excluded based on the high energy of the hydride 12 a (38.6 kcal mol1). Both 1 b_4 b_TS and 1 b_6 b_TS have rather clear (> 2 kcal mol1) preferences for one of either transition states leading to different diastereomers for the bis-olefin hydride complex 8 b (see 1 b_4 b_TS_alt and 1 b_6 b_TS_alt in the Supporting Information for the alternative, competing diastereomer). Herein we report the preferred diastereomer (Re-coordinated propene) from 1 b_4 b_TS and the subsequent steps for this particular diastereomer, because 1 b_6 b_TS leads to a somewhat less stable bis-olefin hydride isomer. However, both isomers should be easily accessible. As mentioned already, the rather low barriers for the rotation of propene and ethylene, 7 b_8 b_ TS and 6 b_8 b_TS, reduce the number of isomers from four to two. For the anilinotropone catalyst III, the cis/trans isomerization 1 a_1 b_TS (12.1 kcal mol1) was found to require less energy than the most facile propagation via 1 b_TS (15.3 kcal mol1). The energy difference in the overall barrier, DDG°, between the most facile propagation (via 1 b_TS) and termination pathways (via 1 b_2 b_TS) is then 7.4 kcal mol1, which correctly reflects the tendency of III to polymerize ethylene.

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Full Paper mol1; see ref. 112) for b-H elimination, 1 b_4 b_TS (21.4 kcal mol1) for chain rearrangement, and 1 b_2 b_TS (18.3 kcal mol1) for dissociation (Figure 3). It should also be noted that, for this catalyst, a route from 1 to 6 could not be located, either for b- or for a-sided complexes.[112] The lack of such a reaction is presumably due to the steric pressure of the ligand, which leads to ethylene dissociation upon b-hydride elimination[89] to form the tetracoordinate hydride complex 3. Of the three routes to termination starting from 1 b, the one via 1 b_ 2 b_TS is the most facile with a barrier of 18.3 kcal mol1. Continuation of the termination process by b-H elimination trans

Branching Our calculations show that the most likely route to the formation of a secondary carbon atom involves b-H elimination via 2 a_3 a_TS (19.4 kcal mol1) from the most stable b-agostic complex 2 a, despite the fact that this transition state is higher in energy than that for the b-H elimination in which a hydride is created trans to oxygen 2 b_3 b_TS (15.4 kcal mol1). The reason for this lies in the ethylene dissociation barrier via 1 b_ 2 b_TS, which is already as high in energy (22.7 kcal mol1) as the rate-limiting steps of the termination pathways, represented by 1 b_4 b_TS and also 1 b_2 b_TS itself. This means that the b-agostic complex 2 b and the hydride 3 b are practically inaccessible without entering termination; hence no branching can originate from this seemingly more favorable b-H elimination via 2 b_3 b_TS. Instead, branching will be initiated by ethylene dissociation from 1 a via 1 a_2 a_TS (16.8 kcal mol1), which requires considerably less energy than dissociation from 1 b, followed by the less favorable b-H elimination via 2 a_3 a_ TS (see also Figure 2). Note that this finding contrasts the claims of Michalak and Ziegler, who, for III, stated that “The DFT results clearly demonstrate that the energetically preferred pathways for chain isomerization start for the higher energy cis/trans isomers”, and who used these results in stochastic modeling of the polymer branching.[107] Their conclusions with respect to the involvement of cis/trans isomers originate from the fact that they did not explicitly include ethylene dissociation as part of the chain isomerization. A related example of the influence of ligand dissociation and association barriers was demonstrated in the discussion of the linearity of the polyethylene produced by the palladium sulfonate-phosphane catalyst. Although Haras et al. first neglected the dissociation barrier of the neutral ligand,[108] it was subsequently shown by Noda et al. to play a role in branching.[109] The process from 3 a towards termination must occur via 8 a because dissociative displacement and direct BHT are uncompetitive (see above). Complex 8 a is relatively high in energy (26.0 kcal mol1) compared with 2 a_3 a_TS (19.4 kcal mol1), which suggests that reinsertion of propene into the NiH bond and subsequent ethylene association to form 1 a, possibly with iPr instead of nPr, could be faster than termination via 8 a, in accordance with the occurrence of branching found in the polyethylene microstructure (see Scheme 4). This consideration is based on the assumption that b-hydrogen atoms of primary and secondary carbon atoms are eliminated with similar barriers. Although Michalak and Ziegler found such barriers to be within 1 kcal mol1,[107] no firm conclusion regarding the competitiveness of the route to branching involving reinsertion into NiH to form iPr can be made here because b-H elimination (or propene insertion into NiH) involving secondary carbon atoms was not explicitly studied.

Figure 3. Ethylene association/dissociation transition state 1 b_2 b_TS of IV. Indicated interatomic distances are given in angstroms.

to oxygen via 2 b_3 b_TS is more facile (12.4 kcal mol1) than the initial transition state 1 b_2 b_TS, which also implies that 3 b can be reached more easily via 2 b than directly from 1 b via 1 b_3 b_TS. The subsequent association of ethylene to form 8 b is also reasonably facile, either directly (18.9 kcal mol1) or via an additional ethylene rotation crossing, 3 b_6 b_TS and 6 b_8 b_TS (17.9 and 19.3 kcal mol1). This indicates that 1 b_ 4 b_TS is not involved in the most facile pathway to the formation of the bis-olefin hydride complex 8 b, nor in the preferred termination pathway. All the BHT-initiated termination pathways (pathways 4–6, 8 and 9) may thus be regarded as less relevant. The very high energy of 12 b again excludes a dissociative displacement (pathway 1). The relatively low energy for propene dissociation via 8 b_13 b_TS and completion of the termination suggests that the alternative associative displacement is the dominating termination pathway (pathway 3). Another alternative could be BHT via 8 b_10 b_TS and 10 b_11 b_ TS, with an overall barrier of 18.2 kcal mol1, that is, lower than the bottleneck transition states leading to 8 b, 3 b_8 b_TS and 6 b_8 b_TS. This makes the mixed BHE/BHT (pathway 7) a viable termination pathway. On the other hand, the alternative BHT termination sequence from 8 b, via 8 b_11 b_TS (21.0 kcal mol1), seems unfavorable. Neither of the combined effective barriers to propagation (1 b!1 b_TS) and termination (1 b!3 b_8 b_TS) involve ethylene dissociation or dissociation. The observed longer polymer chains resulting from increasing

Salicylaldiminato catalyst IV Termination Three transition states possibly leading to termination can be distinguished starting from 1 b, namely 1 b_3 b_TS (22.3 kcal Chem. Eur. J. 2014, 20, 1 – 18

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Full Paper ethylene pressure[31] thus could be the result of reduced termination rates from phosphane complexes.[96] In a pioneering computational study on nickel-based salicylaldiminato catalysts, the termination pathways for a- and b-sided complexes, that is, complexes in which the growing polymer chain is situated trans to oxygen and trans to nitrogen, respectively, were found to involve similar barriers for a (smaller) model catalyst, and only the a-sided route to termination was followed for the full, realistic catalysts.[89] In this work, termination reactions starting from the ethylene complex 1 a are generally (for all the catalysts II–V) found to involve barriers that are higher than those for 1 b. The salicylaldiminato catalyst IV is no exception in this respect, as seen from the uncompetitively high energies of 4 a_7 a_TS and especially the common bis-olefin hydride intermediate 8 a (24.9 kcal mol1) in addition to 4 a_10 a_TS (29.8 kcal mol1) and 12 a (33.8 kcal mol1). All reported minima and transition states beyond 1 b_2 b_TS are in accord with the isomer from that transition state. Furthermore, the corresponding cis/trans isomerization via 1 a_ 1 b_TS (6.9 kcal mol1) was found to be more facile than the most facile propagation via 1 b_TS (11.2 kcal mol1), which means that the difference between the overall barrier (DDG°) to propagation (via 1 b_TS) and termination (via 3 b_8 b_TS) is 7.7 kcal mol1, a value similar to that for III, and which correctly reflects the tendency of this catalyst to polymerize ethylene.

this steric pressure; the b-agostic intermediates 4 b, 5 b, 9 b, and 10 b could nevertheless not be located for this catalyst. Termination of the growing chain from the ethylene complex 1 b can commence along three pathways: Ethylene dissociation via 1 b_2 b_TS (15.2 kcal mol1), b-H elimination via 1 b_ 6 b_TS (19.1 kcal mol1), and combined chain rearrangement and b-H elimination via 1 b_7 b_TS (16.3 kcal mol1; see Figure 4). Of these three transition states, 1 b_2 b_TS has the lowest energy and the other two exceed the energy required for propagation via 1 b_TS (15.8 kcal mol1). However, further down the BHE termination routes (pathways 1–3 and 7) starting with 1 b_2 b_TS, the barriers for ethylene association 3 b_ 6 b_TS (17.0 kcal mol1) or 3 b_8 b_TS (19.4 kcal mol1) are higher in energy than that of 1 b_7 b_TS. From 7 b, propene may readily dissociate via 7 b_13 b_TS (13.8 kcal mol1) to complete the chain termination. The corresponding transition state from 8 b, 8 b_13 b_TS, could not be located. Therefore the pre-

Branching Despite their structural similarity, an important difference between catalysts III and IV is observed. Considering the conventional mechanism for branching via 1 a_2 a_TS and 2 a_3 a_TS, or alternatively via 1 b_2 b_TS and 2 b_3 b_TS (see above and Figure 2), the highest point along these two alternative routes for III is the ethylene dissociation via 1 b_2 b_TS, which makes 1 a_2 a_TS and 2 a_3 a_TS preferred. In contrast, for IV the b-H elimination transition state 2 a_3 a_TS is the highest, and thus 1 b_2 b_TS and 2 b_3 b_TS are preferred. In other words, the branching mechanism for IV involves a b-H elimination that results in a hydride trans to oxygen. This contrasts with the work of Chan et al.,[89] whose conclusions for BHE termination mechanisms, due to the consideration of only alternatives starting from 1 a, were based on 2 a_3 a_TS. From these findings it can be stated that a priori prediction based on chemical knowledge of the regiochemistry of b-H elimination in the branching and termination procedures is difficult and requires the consideration of the ethylene dissociation barriers.

Figure 4. Combined chain rearrangement and b-H elimination transition state 1 b_7 b_TS of V. Indicated interatomic distances are given in angstroms.

ferred termination pathway for the phosphinosulfonamide catalyst can best be described as a mixed BHT/BHE route (pathway 8). The transition state 8 b_10 b_TS necessary for continuation of the stepwise BHT with an in-plane agostic interaction (pathway 5) also could not be located. The mixed BHT/BHT is excluded because of the high energy of 8 b_11 b_TS (20.1 kcal mol1). The analogous combined chain rearrangement and b-H elimination transition state 1 a_7 a_TS (22.0 kcal mol1) and the bis-olefin hydride intermediate 8 a, in which hydrogen is trans to phosphorus, are of markedly higher energy than their counterparts in which hydrogen is trans to nitrogen, which excludes them from playing a role in the termination. The direct BHT termination via 4 a_10 a_TS is also unfavorable, with an energy of 23.9 kcal mol1. Only the Si diastereomer of 1 b_7 b_TS is reported as a Recoordinated propene would create a significant amount of steric pressure between the propyl/propene methyl group and

Phosphinosulfonamide catalyst V Termination Of all catalysts studied, the (k2-P,N)-coordinated phosphinosulfonamide catalyst V arguably displays the highest steric pressure close to the axial sites, with very bulky substituents on both sides of the bidentate ligand. The flexibility of the backbone of the bidentate ligand can serve to alleviate some of &

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Full Paper the phenyl substituent of phosphorus (see Figure 4). Therefore only the Si-derived diastereomer is investigated further in the termination reaction. In summary, whereas the propagation barrier between 1 a and 1 b_TS for V is comparable to that of III, the tendency of V to produce oligomers is due to a much easier b-H elimination step via 1 b_7 b_TS, which is rate-limiting to termination, despite the steric pressure from the bidentate ligand. A pronounced energy difference of 6.2 kcal mol1 is seen between the two cis/trans isomers of the ethylene complexes 1 a and 1 b, with 1 a, in which the strongly donating propyl ligand is trans to the amide side of the bidentate ligand, being the most stable. These two ethylene complexes are connected by the cis/trans isomerization transition state 1 a_1 b_TS (11.9 kcal mol1). Considering the most stable insertion transition state 1 b_TS (15.8 kcal mol1), in which the propyl ligand is now trans to the phosphorus of the bidentate ligand, we conclude that cis/trans isomerization is considerably faster than insertion, and propagation will proceed through 1 b_TS. Gratifyingly, the barrier to insertion calculated herein, amounting to 15.8 kcal mol1, is close to the experimentally determined upper limit (17.2  0.1 kcal mol1[74]) for the barrier to insertion of ethylene into the Nimethyl bond. Furthermore, with a small and positive DDG° (0.5 kcal mol1, between 1 b_TS and 1 b_7 b_TS), the calculations predict that propagation is slightly faster than termination and that this catalyst should oligomerize rather than polymerize ethylene, in accordance with experiment.

From the bis-olefin hydride complex 7 b, propene rotation is facile, as indicated by the low barrier, 7 b_8 b_TS (15.4 kcal mol1), to the formation of complex 8 b. Propene reinsertion can thus potentially occur with different regiochemistry, giving rise to secondary carbon atoms capable of introducing methyl branches upon further propagation. Catalyst V typically produces oligomers containing, on average, one branch each, which indicates that the barriers to insertion, b-hydride elimination and olefin rotation, and chain release are of roughly comparable magnitude. And because “chemical accuracy” ( 1 kcal mol1) cannot be expected for the methods used here, this is basically the picture emerging from the calculations. Although the barriers to insertion, b-hydride elimination, and olefin rotation, are all within 1 kcal mol1 (15.4–16.3 kcal mol1), the stability of the weakly bound transition state for the dissociation of propene from 7 b (7 b_13 b_TS, 13.8 kcal mol1) is slightly overestimated. Taken literally, the low energy of 7 b_13 b_TS means that, once 7 b has been reached, the probability of reinsertion (possibly with modified regiochemistry) via 1 b_7 b_TS and 1 b_TS is too low. Consequently, b-hydride elimination to reach 7 b would, in most cases, also lead to release and thus termination of an unbranched chain. General considerations Considering the transition state highest in energy along the most facile termination pathways for the catalysts studied in this work, it can be noted that these occur before the bisolefin hydride complex 8 (see above). More specifically, the candidate bottleneck transition states are 1) the b-H elimination transition state 1 b_6 b_TS (or 5 b_6 b_TS) or 2 a_3 a_TS, 2) the chain rearrangement transition state 1 b_4 b_TS (or 1 b_ 7 b_TS), or 3) the ethylene dissociation transition state 1 b_2 b_ TS. This means that, despite the complexity and multitude of possible termination pathways (see Scheme 2), the most critical information pertaining to chain termination is contained in relatively few transition states.

Branching A peculiar aspect of the branching observed for the phosphinosulfonamide catalyst is that it seems to be independent of the ethylene pressure and temperature.[74] For many late-transition-metal ethylene oligo- and polymerization catalysts, a decrease in ethylene pressure and increase in temperature, both of which shift the equilibrium between the ethylene complexes (cf. 1 a and 1 b) and the alkyl agostic complexes (cf. 2 a and 2 b) towards the latter, increases the amount of branching. The different branching behavior observed for V is consistent with the mechanistic scenario provided by the calculations, which suggests that b-hydride elimination, and thus branching, might take place directly from the ethylene complex 1 b, that is, without the dissociation of ethylene. In such a scenario, variations in ethylene pressure and temperature would only be expected to have a minor influence on branching. As discussed above, b-hydride elimination can proceed with the hydrogen trans to the stronger s-donating phosphorus or the slightly less s-donating nitrogen of the bidentate ligand. When the reaction proceeds trans to phosphorus, ethylene dissociation via 1 b_2 b_TS, with a relative energy of 15.2 kcal mol1, is rate-limiting. When the reaction proceeds trans to nitrogen, b-H elimination itself constitutes the rate-limiting step, proceeding via 2 a_3 a_TS and with a relative energy of 17.3 kcal mol1. For catalyst V, b-H elimination in which ethylene is still coordinated, proceeding via 1 b_7 b_TS, is thus competitive, at least at higher ethylene pressure. Chem. Eur. J. 2014, 20, 1 – 18

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Conclusions In this work we have gained an insight into the many possible chain termination mechanisms for neutral nickel ethylene oligo- and polymerization catalysts. For the (1,1,1,5,5,5-hexafluoro-2,4-pentanedionato)nickel (or (1,1,1,5,5,5-hexafluoro-2,4acetylacetonato)nickel) oligomerization catalyst (“Ni(HFacac)”, I), which lacks any steric bulk around the nickel center, many energetically degenerate termination pathways were found. This was also the case for the more bulky anilinotropone catalyst III. The most facile termination pathway for the SHOP-type catalyst II was found to be the stepwise BHT through an axial agostic interaction, whereas for the salicylaldiminato catalyst IV, the alternative associative displacement and mixed BHE/ BHT reaction were found to be equally preferred. Finally, for the phosphinosulfonamide catalyst V, the mixed BHT/BHE pathway dominates. Combining this information with the overall barrier to propagation allows for rationalization of the chain lengths obtained with each catalyst. Indeed, the tendency of 13

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Full Paper gen led to a considerable speeding up of the calculations and is justified by the observation that the inner-shell orbitals are hardly altered upon molecule formation.

III and IV to polymerize ethylene to high-molecular-weight polyethylene is reflected in a significant Gibbs free energy difference, DDG°, between the rate-limiting transition states for termination and propagation that is at least 7 kcal mol1. As expected for the oligomerization catalysts I and V, DDG° values close to zero are found. For II, which produces shorter polyethylene chains than III and IV, the DDG° value falls correctly in between the DDG° values for the oligomerization catalysts I and V on one hand, and III and IV on the other. Because the investigated termination pathways share intermediates with common branching pathways, this work also provides information on chain branching. In particular, we found the preferred regiochemistry for b-H elimination to be as follows: The hydride is created trans to oxygen for II and IV and trans to nitrogen for III and V. In addition, we show that a difference in s-donating capacity of the two bridgehead atoms of the bidentate ligand can suppress hydride formation and thus branching. Finally, the large amount of information on chain length, degree of branching, and catalytic activity that can be extracted from calculations on a limited number of stationary points makes this work appear promising for cost-efficient, in silico screening of neutral nickel catalysts.

In general, the accompanying products and reactants of transition states were identified by carrying out minor displacements of the transition vector in both directions, followed by geometry optimization. In addition, for a few of the elementary reactions most critical to chain termination, reactants, transition states, and products were also connected by intrinsic reaction coordinate (IRC) calculations. These calculations were performed by using the Gaussian 09 suite of programs[126] in which the IRC routines are very efficient and much improved compared with earlier versions of the Gaussian package. The basis sets used in the IRC calculations were identical to those described above for the geometry optimizations. Except for a small step size of 0.05 Bohr (half the default value), default values were adopted for the options of the IRC calculations. All reported minima and transition states are spin singlet states, consistent with experimental evidence for the diamagneticity of alkyl complexes of II,[47, 65] III,[63] IV,[64] and V,[74] and of hydride phosphane complexes of II.[127] The catalysts I–V have considerable conformational flexibility. For each stage of the reaction, a number of relative orientations of the ligand substituents, metal-coordinated olefins, and alkyl chain are possible. In this work, several such possibilities were explored in each case, the best (most stable and relevant for the reaction) of which have been presented herein. The main findings, in particular the nature of the rate-limiting transition states, appear to be robust towards conformational changes in the sense that outside the rate-determining transition region, a handful of conformations can, in each case, be reached with low interconversion barriers. This work has focused on identifying the kinetic bottlenecks and does not pretend to be complete with respect to exploration of the conformations in kinetically less important regions.

Computational Details Geometry optimization and calculation of thermochemical corrections All the geometry optimizations were performed by using the generalized gradient approximation (GGA) functional BP86,[117–120] as implemented in the Gaussian 03 suite of programs.[121] This functional has a reputation for obtaining reliable geometries of transition-metal compounds.[122] Numerical integrations were performed by using the default fine grid of Gaussian 03 (75 radial shells and 302 angular points per shell). Gaussian 03 default values were also adopted for geometry optimization criteria (maximum force 0.00045 a.u., RMS force 0.0003 a.u., maximum displacement 0.0018 , RMS displacement 0.0012 ). All the geometry optimizations were performed within C1 symmetry. All the optimized geometries were characterized by their Hessian eigenvalues. Thermal corrections to give enthalpies and Gibbs free energies were computed within standard ideal gas, rigid rotor and harmonic oscillator approximations following statistical mechanics textbook procedures. The temperatures used in the calculation of thermal corrections were those commonly employed in the respective polymerization experiments, 313.15 K for the anilinotropone (III), salicylaldiminato (IV), and phosphinosulfonamide (V) catalysts, and 353.15 K for the “Ni(HFacac)” (I) and SHOP-type (II) catalysts. The standard state for every reaction species has been chosen to be that of an diluted ideal solution of 1 mol L1 concentration (see the Supporting Information for details).

Single-point energy evaluations Single-point (SP) energy evaluations using the BP86 optimized geometries were performed by using several GGA functionals (BP86,[117–120] PBE,[128, 129] and BLYP)[117, 130, 131] as well as a popular and well-tested hybrid-GGA functional (B3LYP),[132, 133] as implemented in the Gaussian 03 suite of programs.[121] The SP energy evaluations using PBE, BLYP, BP86, and B3LYP were complemented with an empirical dispersion term proposed by Grimme,[134] more specifically the DFT-D3 correction[135] with Becke–Johnson (BJ) damping,[136] by using the DFTD3 program.[137] The resulting values are here termed DFT-D and have been shown to improve the accuracy over standard DFT significantly.[134, 138–142] In a recent contribution we noted good agreement with key experimental data on nickel-catalyzed ethylene oligomerization/polymerization catalysts, in particular for PBE-D.[96] Thus, unless explicitly noted, the energies reported herein are those of PBE-D. The SCF density convergence criterion was tightened 10-fold, to 105, compared with the Gaussian 03 default for SP calculations. Whereas the ECPs described above for the geometry optimizations were retained in the SP energy evaluations, the valence basis sets were improved. The valence basis sets for carbon, nitrogen, and oxygen were supplemented by single sets of diffuse s and p functions, obtained even-temperedly, and also by polarization d functions (ad = 0.72 for C; ad = 0.98 for N; ad = 1.28 for O). The resulting (5s5p1d) primitive basis sets for carbon and nitrogen were contracted to [4s4p1d], whereas the (5s6p1d) primitive basis set for oxygen was contracted to [4s5p1d]. For ruthenium, two primitive f functions (af = 0.4780, af = 1.6660) were added to the (8s7p6d)

Effective core potentials (ECPs) of the Stuttgart type were used for all non-hydrogen elements. The ECPs accounted for two inner electrons of carbon, nitrogen, and oxygen, and were used in combination with their corresponding [2s2p] (C, N) and [2s3p] (O) contracted valence basis sets.[123] Similarly, nickel was described by a 10electron ECP accompanied by a (8s7p6d)/[6s5p3d] contracted valence basis set.[124] Hydrogen atoms were described by a Dunning double-z basis set.[125] The use of ECPs for all elements but hydro-

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Full Paper primitive basis sets. The resulting (8s7p6d2f) primitive basis set was contracted to [7s6p4d2f]. Hydrogen atoms were described by a Dunning triple-z basis set[125] augmented by an even-tempered, diffuse s function (as = 0.043152) and a polarization p function (ap = 1.00).

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Solvent effects were estimated by using the continuum solvent model SM8,[143, 144] as implemented in Jaguar 7.6[145] with B3LYP[132, 133] and the LACVP* basis set, which implies the LANL2DZ[146] basis set for nickel and 6-31G* for the other elements. Toluene, as a widely used solvent in olefin polymerization, was chosen as solvent in the calculations. SM8 has been shown to provide very accurate Gibbs free energies of solvation, including those of neutral solutes in organic solvents.[143] The computational model described above, including the use of the PBE-D functional in combination with SM8 to estimate solvent effects, is identical to that employed in our recent work on chain termination for nickel alkyl phosphane intermediates.[96] Excellent agreement with experimental data was noted in the latter work.

Acknowledgements The Research Council of Norway is acknowledged for financial support through the GASSMAKS (Grants No. 182536 and 203379) and KOSK (177322) programs as well as for CPU and storage resources granted through the NOTUR (NN2506K) and NORSTORE (NS2506K) supercomputing programs. Yury Minenkov and Dr. Nicolas Merle are thanked for stimulating discussions. Keywords: density functional calculations · homogeneous catalysis · nickel · oligomerization · transition states [1] R. H. Crabtree in The Organometallic Chemistry of the Transition Metals, Wiley, Hoboken, 2005. [2] J. Skupin´ska, Chem. Rev. 1991, 91, 613 – 648. [3] G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem. 1999, 111, 448 – 468; Angew. Chem. Int. Ed. 1999, 38, 428 – 447. [4] V. C. Gibson, S. K. Spitzmesser, Chem. Rev. 2003, 103, 283 – 315. [5] S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100, 1169 – 1203. [6] H. Makio, T. Fujita, Acc. Chem. Res. 2009, 42, 1532 – 1544. [7] D. F. Wass, Dalton Trans. 2007, 816 – 819. [8] V. Busico, Dalton Trans. 2009, 8794 – 8802. [9] L. S. Boffa, B. M. Novak, Chem. Rev. 2000, 100, 1479 – 1493. [10] A. Nakamura, S. Ito, K. Nozaki, Chem. Rev. 2009, 109, 5215 – 5244. [11] A. Berkefeld, S. Mecking, Angew. Chem. 2008, 120, 2572 – 2576; Angew. Chem. Int. Ed. 2008, 47, 2538 – 2542. [12] L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc. 1995, 117, 6414 – 6415. [13] L. K. Johnson, S. Mecking, M. Brookhart, J. Am. Chem. Soc. 1996, 118, 267 – 268. [14] S. Mecking, L. K. Johnson, L. Wang, M. Brookhart, J. Am. Chem. Soc. 1998, 120, 888 – 899. [15] A. W. Waltman, T. R. Younkin, R. H. Grubbs, Organometallics 2004, 23, 5121 – 5123. [16] D. Guironnet, P. Roesle, T. Rnzi, I. Gçttker-Schnetmann, S. Mecking, J. Am. Chem. Soc. 2009, 131, 422 – 423. [17] D. Guironnet, L. Caporaso, B. Neuwald, I. Gçttker-Schnetmann, L. Cavallo, S. Mecking, J. Am. Chem. Soc. 2010, 132, 4418 – 4426. [18] A. Berkefeld, M. Drexler, H. M. Mçller, S. Mecking, J. Am. Chem. Soc. 2009, 131, 12613 – 12622. [19] E. Drent, R. van Dijk, R. van Ginkel, B. van Oort, R. I. Pugh, Chem. Commun. 2002, 744 – 745. Chem. Eur. J. 2014, 20, 1 – 18

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Received: December 13, 2013 Revised: March 31, 2014 Published online on && &&, 0000

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FULL PAPER & Oligomerization Mechanisms W. Heyndrickx, G. Occhipinti, V. R. Jensen* && – && Neutral Nickel Ethylene Oligo- and Polymerization Catalysts: Towards Computational Catalyst Prediction and Design

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Towards catalyst design: A DFT investigation of a series of nickel-based ethylene oligo- and polymerization catalysts shows that the balance between chain termination, propagation, and branching is determined by only a handful of intermediates and transition states,

which facilitates comparison and understanding of these catalysts as well as the design of new catalysts of this family (see figure; Anth represents a 9anthryl substituent and Ph(iPr)2 indicates a 2,6-diisopropylphenyl group).

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Neutral nickel ethylene oligo- and polymerization catalysts: towards computational catalyst prediction and design.

DFT calculations have been used to elucidate the chain termination mechanisms for neutral nickel ethylene oligo- and polymerization catalysts and to r...
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