Alkene Hydrosilylation Using Tertiary Silanes with α-Diimine Nickel

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Alkene Hydrosilylation Using Tertiary Silanes with α‑Diimine Nickel Catalysts. Redox-Active Ligands Promote a Distinct Mechanistic Pathway from Platinum Catalysts Iraklis Pappas, Sean Treacy, and Paul J. Chirik* Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: Combination of the readily available α-diimine ligand, ((ArNC(Me))2 Ar = 2,6-iPr2−C6H3), (iPrDI) with airstable nickel(II) bis(carboxylates) generated a highly active catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation of alkenes with a variety of industrially relevant tertiary alkoxy- and siloxy-substituted silanes. A combination of the method of continuous variations with stoichiometric studies identified the formally Ni(I) hydride dimer, [(iPrDI)NiH]2 as the nickel compound formed following reduction of the carboxylate ligands. For the hydrosilylation of 1-octene with (EtO)3SiH, a rate law of [Ni]1/2[1-octene][(EtO)3SiH] in combination with deuterium-labeling studies establish dissociation of the nickel hydride dimer followed by fast and reversible alkene insertion into (iPrDI)NiH, consistent with turnover-limiting C−Si bond formation. The hydrosilylation of 1-octene with triethoxysilane, a reaction performed commercially in the silicones industry on a scale of >5 000 000 kg/year, was conducted on a 10 g scale with 96% yield and >98% selectivity for the desired product. Silicone cross-linking, another major industrial application of homogeneous hydrosilylation, was also demonstrated using the air-stable nickel and ligand precursors. KEYWORDS: nickel, silicone, hydrosilylation, diimine, carboxylate, hydride, mechanism

T

he hydrosilylation of alkenes with tertiary silanes promoted by platinum catalysts such as Pt2{[(CH2 CH)SiMe2]2O}3 (Karstedt’s catalyst) and H2PtCl6·6H2O (Speier’s catalyst) is an enabling technology in the silicones industry used in the manufacture of release coatings, adhesives, surfactants, moldings, and other consumer products.1 Discovery of practical catalysts based on earth abundant metals is a longstanding challenge owing to the high cost and deleterious environmental profile associated with platinum mining.2 Highly active and selective iron3 and cobalt4 catalysts have been reported, but many lack the robustness, ease of handling, and air stability associated with platinum. In cases where air-stable metal precursors are used, catalyst activation with reactive borohydride, alkyl lithium, or Grignard reagents is required.5 Often these highly reducing activators are incompatible with trialkoxysilanes promoting disproportionation to silicates and pyrophoric silane gas.6 Mechanistic studies on industrially preferred Karstedt’s catalyst established the so-called Chalk−Harrod pathway, whereby rate-limiting olefin insertion into a Pt−H is followed by relatively rapid C−Si bond formation (Figure 1).7 In this pathway, the platinum catalyst operates by a Pt(0)−Pt(II) redox couple with the reduced platinum species stabilized by alkene ligands.8 An alternative possibility, dubbed the modified Chalk−Harrod pathway9 invokes insertion into a metal-silyl and is often implicated in catalytic dehydrogenative silylation reactions.10 With platinum catalysts, competing alkene isomerization is deleterious as the internal olefin byproducts are © 2016 American Chemical Society

Figure 1. Comparison of Chalk−Harrod, modified Chalk−Harrod, and the mechanistic pathway likely operative in this work.

unreactive toward hydrosilylation and likely arise from Pt nanoparticles formed from catalyst decomposition.11 For catalysts with earth-abundant transition metals to be considered as practical alternatives to current platinum technology, emphasis should be placed on those compounds that are reactive toward commercially relevant silanes and siloxanes, promote the hydrosilylation of internal alkenes to terminally functionalized products, and are easily prepared and handled. Received: April 21, 2016 Revised: May 24, 2016 Published: May 24, 2016 4105

DOI: 10.1021/acscatal.6b01134 ACS Catal. 2016, 6, 4105−4109

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ACS Catalysis Based on periodicity,12 nickel has long been recognized as an inexpensive and terrestrially abundant alternative to platinum. The nickel analog of Karstedt’s catalyst, Ni2{[(CH2CH)SiMe2]2O}3 as well as other common nickel precursors such as Ni(COD)2 and Ni(acac)2 (COD = 1,5-cyclooctadiene; acac = acetylacetonate) promote dehydrogenative silylation.13 More recent efforts have focused on nickel(II) complexes bearing indenyl,14 allyl,15 phosphine,16 amido,17 salicylaldiminato (sp),18 naphthyridine diimine (NDI),19 and anionic pincertype ligands20 for catalytic alkene hydrosilylation. Notably, Hu and co-workers have recently described a nickel catalyst with broad substrate scope and excellent selectivity for olefin hydrosilylation using Ph2SiH2.20a Despite the successful demonstration of anti-Markovnikov addition, all of these catalysts utilize primary or secondary silanes; commercially relevant and less expensive tertiary silanes produced either no reaction or lead to significant quantities of dehydrogenative silylation products.21 The presence of residual Si−H bonds decreases product stability and hence commercial utility. Here we describe nickel catalysts derived from straightforward combination of a readily available and inexpensive α-diimine ligand with nickel(II) bis(carboxylates) that generate a rare example of a highly active and anti-Markovnikov selective catalyst for the hydrosilylation of terminal alkenes with tertiary silanes. All of the catalyst precursors are air-stable and no exogeneous reductants or activators are required, representing a significant advance in hydrosilylation catalysis with earthabundant transition metals. Determination of the rate law, isolation of a nickel hydride complex, and deuterium-labeling experiments support a metal-hydride pathway that is distinct from the mechanism established in platinum catalysis and highlights the chemistry enabled with redox-active ligands. Given the industrial importance of the hydrosilylation of 1octene with (EtO)3SiH and precedent with iron and cobalt catalysts,22 this reaction was used to evaluate combinations of nickel precursors and ligands for catalytic activity (Table S1). Nickel(II) bis(carboxylates) were attractive as the metal source due to their low cost, air-stability, and known ability of carboxylate ligands to undergo substrate-induced activation with main group hydrides to generate active first row transition metal catalysts.23,4a Using the hydrocarbon soluble Ni(2-EH)2, (2-EH = 2-ethylhexanoate) evaluation of 13 total ligands (5 mol % loading, 12 h, 23 °C, see Table S1) identified several ligand combinations with promising hydrosilylation activity. Pure σ-donating ligands such as 1,2-bis(diphenylphosphino)ethane (dppe) or tetramethylethylenediamine (tmeda) produced either no reaction or products derived from dehydrogenative silylation. By contrast, potentially redox-active 2,2′bipyridine (bpy), the aryl-substituted pyridine(imine) (iPrPI) and the related aryl-substituted α-diimine, (iPrDI) all promoted anti-Markovnikov hydrosilylation. Using this straightforward protocol, the hydrosilylation of 1-octene with (EtO)3SiH was conducted on a 10 g scale with 0.1 mol % loading of iPrDI/ Ni(2-EH)2 and produced >99% of the desired noctylSi(OEt) product (Figure 2).24 These results are noteworthy in light of the success of redox-active, tridentate pyridinediimines in supporting high activity iron and cobalt hydrosilylation catalysts.25 The observation of high yields coupled with the ease of synthesis of α-diimine ligands resulted in selection of iPrDI for subsequent studies. Efforts were devoted to optimization of the catalytic reaction, identification of the active nickel species, exploration of the silane scope, and elucidation of the

Figure 2. (Top) Scope of nickel catalyzed hydrosilylation. Ten-gram scale hydrosilylation using air-stable nickel catalyst precursors. (Bottom) Silicone fluid cross-linking catalyzed by [Ni] at ppm loadings. aAdded as a 0.15 M toluene solution containing 1 equiv. iPrDI and 6 equiv. (EtO)3SiH.

mechanism of turnover. Notably, addition of (EtO)3SiH to an equimolar mixture of (iPrDI) and Ni(2-EH)2 generated a blue solution, (λmax = 515 nm) suggesting ligand complexation, activation of the carboxylate groups by the silane, and formation of the active Ni species. Using the UV−vis absorbance at 515 nm, the method of continuous variations (“Job plot”)26 was applied to determine the optimal number of equivalents of silane required for catalyst activation (Figure S6). A reaction stoichiometry corresponding to a 6:1 ratio of [(EtO)3SiH]:[Ni] was established. Both (EtO)3SiOSi(OEt)3 and 2-ethylhexanol were observed by GCMS as byproducts of the activation procedure suggesting that Si−O−Si bond formation is likely the driving force behind catalyst activation. Application of the optimized [(EtO)3SiH]:[Ni] ratio on a preparative scale resulted in isolation of a deep blue (λmax = 515 nm; ε515 = 3225 M−1 cm−1) diamagnetic solid identified as [(iPrDI)NiH]2, a compound previously identified by Yang and co-workers.27 To account for the formal reduction from Ni(II) to Ni(I), a Toepler pump28 experiment was conducted and 0.45 equiv of H2 were collectedconsistent with liberation of the expected 0.5 equiv of dihydrogen en route to [(iPrDI)NiH]2 (see SI). This compound was also independently prepared and isolated in 42% yield following treatment of (iPrDI)NiBr2 with two equivalents of NaBEt3H, and the spectroscopic data for material obtained from the two routes were identical. These experiments unequivocally demonstrate that the in situ protocol generates [(iPrDI)NiH]2 in solution. The high activity, selectivity, and scalability of isolated or in situ generated [(iPrDI)NiH]2 prompted additional studies into the scope of the catalytic reaction. Efforts were devoted to alkoxy and siloxysilanes, as these substrates are the most relevant to commercial hydrosilylation and have not been successfully used in nickel-catalyzed reactions. Each catalytic 4106

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formation of [(iPrDI)NiH]2 were observed. Significant isomerization to internal octene isomers occurred over the course of the experiment but noctySi(OEt)3 eventually formed over the course of hours as the sole hydrosilylation product. The concentration of [(iPrDI)NiH]2 was unchanged throughout the reaction, implicating the dimeric nickel hydride as an off-cycle catalyst resting state. Deuterium-labeling studies were also conducted. Catalytic deuterosilylation of 1-octene with (EtO)3SiD in the presence of 0.5 mol % [(iPrDI)NiH]2 yielded the alkyl silane with deuterium in all positions of the alkyl chain as judged by 2H NMR spectroscopy (Figure 3). These experiments establish that β-H

hydrosilylation reaction was conducted using the straightforward in situ protocol using 1 mol % each of Ni(2-EH)2 and (iPrDI) in a neat mixture of 1-octene and the appropriate silane at 23 °C and in all cases reached complete conversion (Figure 2). Our laboratory has previously reported multiple generations of Fe and Co catalysts for olefin hydrosilylation that are compatible with industrially relevant tertiary silanes.3i−l,4b The hydrosilylation reaction of 1-octene with HSiMe(OSiMe3)2 (MD’M) is a useful benchmark to compare the present Ni catalyst to the most active Fe and Co catalysts. With [( Me APDI)Fe(N 2 )] 2 (μ 2 -N 2 ) ( Me APDI = 2,6-(MeN CMe)2C5H3N), this reaction is complete (>98% of the yield anti-Markovnikov product) within 15 min at 23 °C with a catalyst loading of 0.004 mol %.3j The air-stable cobalt precatalyst, TFAPDICo(2-EH) (TFAPDI = 2,6-(3,4-dihydro2H-pyrrole) requires 1 mol % catalyst loading and 1 h at 23 °C to reach a similar level of completion.4b{(iPrDI)NiH}2 is markedly less activerequiring 1 mol % catalyst loading and 6 h at 23 °C to produce the anti-Markovnikov product in high yield. Despite the comparatively lower activity of the Ni catalyst, the ease with which the α-diimine scaffold can be modified and the convenience of the in situ activation protocol enables catalyst diversification (Table S1) and should provide a straightforward path to more active catalysts based on the specific application. A second major class of commercially important hydrosilylation reactions involves the cross-linking of silicone polymer fluids, whereby a polymeric vinyl silane is combined with a polymeric hydridosilane containing tertiary Si−H linkages to form silicone polymers that are widely employed as release coatings, fast setting molds, or extruded to form ubiquitous consumer products.29 The morphology of the crosslinked silicones prevents catalyst recovery and highlights the potential economic benefit of base metal catalysts. With a concentrated (∼0.15 M) toluene premix of (iPrDI), Ni(2-EH)2, and 6 equiv of (EtO)3SiH efficient hydrosilylation to yield cross-linked product was observed after 2 h at 80 °C (Figure 2). The cross-linked product is colorless, an important characteristic for commercial viability.30 The mechanism of alkene hydrosilylation promoted by [(iPrDI)NiH]2 was of interest to not only understand the remarkable catalytic performance but also to compare to the mechanism of platinum catalysts. Reaction of alkene with [(iPrDI)NiH]2 offers two possibilities: insertion into the nickelhydride, likely following dissociation into a monomer or asymmetric rupture of the bridging hydrides to form (iPrDI)Ni(alkene) and (iPrDI)NiH2. The latter path would be reminiscent of platinum catalysis. To distinguish these possibilities, 5 mol % of (iPrDI)Ni(η2-C2H4)31 was added to a mixture of 1-octene and (EtO)3SiH. After 12 h at 23 °C, 98% conversion. To eliminate the possibility that ethylene is a catalyst poison, the reaction of (EtO)3SiH with ethylene and 5 mol % Ni(2-EH)2/iPrDI was also performed and quantitative formation of (EtO)3SiCH2CH3 was obtained after 12 h. These experiments eliminate the catalytic competency of (iPrDI)Ni(alkene) compounds and suggest a distinct mechanistic pathway from platinum catalysts. Insight into the catalyst resting state was obtained by monitoring the hydrosilylation of 1-octene with (EtO)3SiH in benzene-d6 solution by 1H NMR spectroscopy. With 1 mol % each of Ni(2-EH)2 and iPrDI, resonances diagnostic with

Figure 3. (Left) Comparison of the one-step procedure (top) to the multistep procedure (bottom) for the synthesis of [iPrDINiH]2. (Right) Deuterium isotope incorporation studies related to catalytic hydrosilylation of 1-octene with (EtO)3SiD using [iPrDINiH]2. Conditions: 0.5 mol % [iPrDINiH]2, 6 h, 23 °C, neat.

elimination is fast and reversible relative to carbon−silicon bond formation and release of alkyl silane. The propensity for cationic α-diimine Ni(II) alkyls to undergo facile chain walking is well-established in the context of alkene polymerization;32 this property is preserved in the one-electron reduced examples described here. Hydrosilylation of 4-octene with (EtO)3SiH in the presence of 0.5 mol % [(iPrDI)NiH]2 was performed, yielding exclusive formation of noctylSi(OEt)3 (>98% conversion, 92% isolated yield) after 6 h at 23 °C offering a distinct advantage over platinum catalysis and previously reported Co catalysts for which isomerization/hydrosilylation is not operative.4b,33 Kinetic studies in the form of initial rates were performed to establish the rate law for the nickel-catalyzed hydrosilylation of 1-octene and (EtO)3SiH with [(iPrDI)NiH]2 in hexane solvent. The following rate law was established: rate = kobs[{(iPrDI)NiH}2 ]1/2 [1‐octene][(EtO)3 SiH]

The observed fractional order in nickel is consistent with dissociation of the hydride dimer to (iPrDI)NiH, which undergoes fast and reversible insertion of 1-octene (Figure 4). Interaction of the nickel alkyl with the silane is turnoverlimiting. Based on the current data, both σ-bond metathesis and oxidative addition/reductive elimination are viable mechanistic pathways for product release and regeneration of the active catalyst. These observations are consistent with the deuteriumlabeling studies and the isomerization to internal alkenes when the catalytic reaction was monitored by 1H NMR spectroscopy in benzene-d6 solution. The presence of a redox-active α-diimine ligand raises questions as to the oxidation state of the nickel throughout the catalytic cycle. Because only [(iPrDI)NiH]2 was observed, DFT (B3LYP level) calculations were conducted on this compound and related intermediates. The electronic structure of diamagnetic [(iPrDI)NiH]2 is best described as two Ni(II) 4107

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Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b01134. Experimental procedures, deuterium-labeling experiments, method of continuous variations, Toepler pump results, 10-g scale hydrosilylation, silicone fluid crosslinking, kinetics experiments, computational details, and general protocols (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 4. Proposed mechanism for the hydrosilylation of 1-octene with (EtO)3SiH and [(iPrDI)NiH]2.

The authors declare the following competing financial interest(s): One patent related to this work has been filed: Chirik, P. J.; Pappas, I.; U.S. Patent Application filed November 2015 (unpublished).

centers each with one-electron reduced diimine ligands.34 The metrical parameters from the X-ray structure also support the bond distance perturbations associated with this electronic structure assignment. Unrestricted Kohn−Sham calculations of the corresponding monomer, (iPrDI)NiH, converge to a broken symmetry (2,1)35 solution with strong antiferromagnetic coupling (S = 0.72) between the S = 1 Ni(II) metal center and the one electron reduced α-diimine ligand (Figure 5).

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ACKNOWLEDGMENTS We thank Momentive Performance Materials Inc. (MPM) for financial support. We also acknowledge Drs. Kenrick Lewis (MPM), Aroop Roy (MPM), Keith Weller (MPM), Julie Boyer (MPM), and Jos Delis (MPM) for helpful discussions.

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REFERENCES

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Figure 5. (a) Spin density plot of (iPrDI)NiH. Mulliken spin populations are 1.5 (Ni) and −0.5 (iPrDI ligand). (b) Spin density plot of (iPrDI)Ni(n-octyl). Mulliken spin populations are 1.5 (Ni) and −0.6 (iPrDI ligand).

Similarly, the electronic structures of two putative nickel alkyl intermediates, (iPrDI)Ni(n-octyl) and (iPrDI)Ni(2-octyl), were computed and both converged to a BS (2,1) solution best described as Ni(II) with a one electron reduced α-diimine ligand. Electronic participation of the diimine ligand serves to stabilize the formally Ni(I) metal center in the putative nickel monohydride and alkyl complexes. The accessibility and stability of these 3-coordinate 15-electron complexes may explain their unprecedented reactivity compared to other Ni hydrosilylation catalysts. In summary, a highly active, anti-Markovnikov-selective, and scalable nickel catalyst has been discovered for the hydrosilylation of 1-octene with commercially relevant silanes and siloxanes. Both the ligand and metal precursors are air-stable and inexpensive, and no additional pyrophoric activators are required. Investigations into the olefin scope are forthcoming, but initial studies indicate that rapid olefin isomerization has deleterious implications for functional group compatibility. The mechanism of the nickel-catalyzed hydrosilylation is distinct from the established Chalk−Harrod mechanism established for currently used platinum alternatives and involves nickel(II) intermediates with one-electron reduced α-diimines. 4108

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DOI: 10.1021/acscatal.6b01134 ACS Catal. 2016, 6, 4105−4109