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

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 CO sub(2). Exposing solutions or solid samples of the ion pair [Cp* sub(2)Sc][HB(C sub(6)F sub(5)) sub(3)] 1 sub(CIP), in which the highly electrophilic decamethyl-scandocene cation and [HB(C sub(6)F sub(5))] super(-) as a potentially reactive source of hydride equivalents are associated, to CO sub(2) selectively produces ion pair [Cp* sub(2)Sc][HCO sub(2)B(C sub(6)F sub(5)) sub(3)] 2 sub(CIP). The results of solution and solid state structural analysis of 2 sub(CIP) imply ionic association of [Cp* sub(2)Sc] super(+) and [HCO sub(2)B(C sub(6)F sub(5)) sub(3)] super(-) rather than B(C sub(6)F sub(5)) sub(3)- adduct formation to neutral Cp* sub(2)Sc-formate. In the presence of B(C sub(6)F sub(5)) sub(3) co-catalyst and excess triethylsilane, the formation of 2 sub(CIP) from 1 sub(CIP) initiates the catalytic deoxygenative hydrosilation of CO sub(2) to CH sub(4). 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 [HCO sub(2)B(C sub(6)F sub(5)) sub(3)] super(-) dissociation to give 2 sub(SIP) through formation of separated ion pairs [Cp* sub(2)Sc( Kappa super(2) -(Et sub(3)SiO) sub(2)CH sub(2))][HCO sub(2){B(C sub(6)F sub(5)) sub(3)} sub(n)], n= 1, 2. DFT calculations show that the formation 2 sub(CIP) from the reaction of 1 sub(CIP) with CO sub(2) 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.