Decamethylscandocinium-hydrido-(perfluorophenyl)borate: fixation and tandem tris(perfluorophenyl)borane catalysed deoxygenative hydrosilation of carbon dioxide

Abstract

This work was directed at studying the capability of structurally defined, strongly Lewis-acidic metal centers to effect catalytic reductive fixation of the small molecule substrate CO₂. Exposing solutions or solid samples of the ion pair [Cp*₂Sc][HB(C₆F₅)₃] 1_CIP, in which the highly electrophilic decamethyl-scandocene cation and [HB(C₆F₅)]⁻ as a potentially reactive source of hydride equivalents are associated, to CO₂ selectively produces ion pair [Cp*₂Sc][HCO₂B(C₆F₅)₃] 2_CIP. The results of solution and solid state structural analysis of 2_CIP imply ionic association of [Cp*₂Sc]⁺ and [HCO₂B(C₆F₅)₃]⁻ rather than B(C₆F₅)₃-adduct formation to neutral Cp*₂Sc-formate. In the presence of B(C₆F₅)₃ co-catalyst and excess triethylsilane, the formation of 2_CIP from 1_CIP initiates the catalytic deoxygenative hydrosilation of CO₂ to CH₄. The roles of ion pairs 1 and 2, borane co-catalyst, and silane in the catalytic reaction were studied mechanistically by NMR spectroscopy. Intermediately formed 3,3,7,7-tetraethyl-3,7-disila-4,6-dioxanonane product was found to exert an accelerating effect on the overall reaction rate by promoting [HCO₂B(C₆F₅)₃]⁻ dissociation to give 2_SIP through formation of separated ion pairs [Cp*₂Sc(κ²-(Et₃SiO)₂CH₂)][HCO₂{B(C₆F₅)₃}_n], n = 1, 2. DFT calculations show that the formation 2_CIP from the reaction of 1_CIP with CO₂ is exoergic and without significant energy barriers. This work lays the basis for future studies of reactive ion pairs of this kind in the context of small molecule chemistry.