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

Abstract

International audienceThis 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] 1CIP, 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] 2CIP. The results of solution and solid state structural analysis of 2CIP 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 2CIP from 1CIP 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 2SIP 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 2CIP from the reaction of 1CIP 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

Boron-mediated activation of carbon dioxide

“…The subsequent report by the group of Piers substantiated such mechanism with the support of theoretical calculations by Maron and Eisenstein. With the use of the same hydroborate species but with a (Cp*) 2 Sc countercation 90, the first activation step is much easier, leading to compound 40 (cf section 3) [46]. Solution state studies proved that compound 40 is in equilibrium with (Cp*) 2 Sc(O 2 CH) and free B(C 6 F 5 ) 3 , explaining that catalysis proceeds to completion without additional B(C 6 F 5 ) 3 .…”

“…In 2013, Piers, Maron, Eisenstein et al described that the hydroborate H(B(C 6 F 5 ) 3 ) anion with the strong Lewis acidic scandium countercation Cp* 2 Sc + readily activate CO 2 (1 atm) at room temperature to form compound 32 (scheme 7) [46]. Different factors explains this enhanced reactivity compare to systems giving rise to 25 (12 h, 100°C, 1 atm of CO 2 ) and 26 (few hours, r.t., 4 atm of CO 2 ).…”

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

“…In recent years, the combination of unquenched Lewis pair reactivity and transition‐metal chemistry has received considerable attention, by developing transition‐metal‐based FLPs . The benefits of using transition metals include increased reactivity in small molecule activation, reactivity not observed with main group frustrated Lewis pairs and the advantage of an easy variation of the FLP components, for example, by ligand modifications …”

Selective Hydrosilation of CO2 to a Bis(silylacetal) Using an Anilido Bipyridyl‐Ligated Organoscandium Catalyst

“…[3] Selectivity in CO 2 reduction is thus a key consideration for both homo-and heterogeneous catalysts. [6] The first transfer of hydride to CO 2 (step a) is mediated by an ion pair in which the cation is partnered with the hydridoborate counteranion [HB(C 6 F 5 ) 3 ] À . [6] The first transfer of hydride to CO 2 (step a) is mediated by an ion pair in which the cation is partnered with the hydridoborate counteranion [HB(C 6 F 5 ) 3 ] À .…”