Reactions of Phenylhydrosilanes with Pincer–Nickel Complexes: Evidence for New Si–O and Si–C Bond Formation Pathways

Abstract

This contribution presents evidence for new pathways manifested in the reactions of the phenylhydrosilanes PhnSiH4-n with the pincer complexes (POCsp(2)OP)Ni(OSiMe3), 1-OSiMe3, and (POCsp(3)OP)Ni(OSiMe3), 2-OSiMe3 (POCsp(2)OP = 2,6-(i-Pr2PO)2C6H3; POCsp(3)OP = (i-Pr2POCH2)2CH). Excess PhSiH3 or Ph2SiH2 reacted with 1-OSiMe3 to eliminate the disilyl ethers Ph(n)H(3-n)SiOSiMe3 (n = 1 or 2) and generate the nickel hydride species 1-H. Subsequent reaction of the latter with more substrate formed corresponding nickel silyl species 1-SiPhH2 or 1-SiPh2H and generated multiple Si-containing products, including disilanes and redistribution products. The reaction of 1-OSiMe3 with excess Ph2SiH2/Ph2SiD2 revealed a net KIE of ca. 1.3-1.4 at room temperature. Treating 1-OSiMe3 with excess Ph3SiH also gave 1-H and the corresponding disilyl ether Ph3SiOSiMe3, but this reaction also generated the new siloxide 1-OSiPh3 apparently via an unconventional σ-bond metathesis pathway in which the Ni center is not involved directly. The reaction of excess PhSiH3 and 2-OSiMe3 gave polysilanes of varying solubilities and molecular weights; NMR investigations showed that these polymers arise from Ni(0) species generated in situ from the reductive elimination of the highly reactive hydride intermediate, 2-H. The stoichiometric reactions of 2-OSiMe3 with Ph2SiH2 and Ph3SiH gave, respectively, siloxides 2-OSiPh2(OSiMe3) and 2-OSiPh3. Together, these results demonstrate the strong influence of pincer backbone and hydrosilane sterics on the different reactivities of 1-OSiMe3 and 2-OSiMe3 toward Ph(n)SiH(4-n) (dimerization, polymerization, and redistribution vs formation of new siloxides). The mechanisms of the reactions that lead to the observed Si-O, Si-C, and Si-Si bond formations are discussed in terms of classical and unconventional σ-bond metathesis pathways.

Dehydrogenative Coupling of a Tertiary Silane Using Wilkinson's Catalyst

“…The most conventional used method for the production of high molecular weight polysilanes is the Wurtz‐type condensation of halosilanes in the presence of alkali metals , . The transition metal‐catalyzed Si–Si bond formation has also been thoroughly studied aiming to obtain selective catalysts active under softer reaction conditions . Early transition metal (Group 4) catalysts have been most intensively studied because of their higher catalytic activity, whereas late transition metal complexes, such as Wilkinson's catalyst, can exhibit competing catalytic activities between homodehydrocoupling and substituent redistribution reactions.…”

“…For early transition metal catalysts a mechanism based on successive, concerted, 4‐center transition states is accepted,, whereas for late transition metal catalysts the proposed mechanism consists of consecutive oxidative addition/reductive elimination cycles . Nevertheless, in the later case, the possibility of a mechanism involving σ‐bond metathesis steps has also been considered , , . Furthermore, most recently the dehydrocoupling of primary and secondary silanes, where silylene intermediates could be involved,, has been proposed.…”

“…[1,2] The transition metal-catalyzed Si-Si bond formation has also been thoroughly studied aiming to obtain selective catalysts active under softer reaction conditions. [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Early transition metal (Group 4) catalysts have been most intensively studied because of their higher catalytic activity, whereas late transition metal complexes, such as Wilkinson's catalyst, can exhibit competing catalytic activities between homodehydrocoupling and substituent redistribution reactions. For this reason the latter complexes have been less investigated as catalysts in this type of reactions.…”

“…[8][9][10] Nevertheless, in the later case, the possibility of a mechanism involving σ-bond metathesis steps has also been considered. [11,14,15] Furthermore, most recently the dehydrocoupling of primary and secondary silanes, where silylene intermediates could be involved, [19,20] has been proposed. Herein, we report an efficient method for the dehydrogenative coupling of a tertiary silane using Wilkinson's catalyst and propose an active catalytic species and a deactivation process based on stoichiometric evidences.…”

Nickel and Palladium Complexes with Reactive σ‐Metal‐Oxygen Covalent Bonds

“…Zargarian has explored similar reactions of nickel siloxide complexes with silanes. These may entail not only the metal‐siloxide linkage, but also MO‐SiR 3 bonds . As mentioned in the introduction (see Chart 1), nucleophilic attack of reactive palladium or nickel hydroxide species on organoboron reagents is involved in the key transmetallation step that takes place in the Suzuki‐Miyaura reaction…”