Synthesis and CO₂ Insertion Chemistry of Uranium(IV) Mixed-Sandwich Alkyl and Hydride Complexes

A series of U­(IV) mixed-sandwich alkyls of the form [U­(COTTIPS2)­Cp*R] (R = Me, CH2Ph, CH2TMS, CH­(TMS)2; COTTIPS2 = C8H6(SiiPr3-1,4)2; Cp* = C5Me5; TMS = SiMe3) have been synthesized and structurally characterized, and their reactivity toward H2 and CO2 has been investigated. The alkyls R = Me, CH2Ph, CH2TMS react at room temperature with a stoichiometric amount of CO2 to form κ2-carboxylate complexes. Reaction of all four alkyls with H2 yields a monomeric, terminal hydride complex, [U­(COTTIPS2)­Cp*H], which is unstable with respect to hydrogen loss and reacts with CO2 to give the κ2-formate complex [U­(COTTIPS2)­Cp*­(κ2-O2CH)]. Additionally, a common decomposition product of the alkyls and hydride complexactivation of a Cp* methyl group to give a “tucked-in” alkylhas been isolated and structurally characterized and its insertion chemistry toward CO2 has been examined.