Investigation of the synthesis of {Mo(η5-C5H5)(CO)3}+ fragments partnered with the monoanionic carboranes [closo-CB11H11Br]−, [closo-CB11H6Br6]− and [closo-HCB11Me11]− by silver salt metathesis and hydride abstraction

Abstraction of iodide from [Cp(CO)3MoI] (Cp = η5-C5H5) using the silver salt of the weakly coordinating anion [closo-CB11H11Br]−, initially affords an intermediate dimeric complex, [MoCp(CO)3I·Ag(CB11H11Br)]2, which has a central {AgI}2 core appended by two carborane anions in the solid state. Prolonged reaction does result in elimination of AgI to form [MoCp(CO)3(CB11H11Br)], but not cleanly. This complex can be isolated in good yield by hydride abstraction from [Cp(CO)3MoH] using the new trityl salt [CPh3][closo-CB11H11Br]. Similarly, reaction of this hydride source with [CPh3][closo-CB11H6Br6] in CD2Cl2 results in hydride abstraction, but a mixture of products is formed, suggested to consist of the hydride bridged dimer, [{Cp(CO)3Mo}2(μ-H)][closo-CB11H6Br6], the anion coordinated complex [Cp(CO)3Mo(closo-CB11H6Br6)] and the solvent adduct [Cp(CO)3Mo(ClCD2Cl)][closo-CB11H6Br6]. Reaction of Ag[closo-HCB11Me11] with [Cp(CO)3MoX] (X = Cl, I) results in a intermediate complex which has a [Cp(CO)3MoX]6Ag2 central core (crystallographically characterised for X = I) with no interactions from the permethylated carborane anions. Prolonged reaction does result in partial AgX elimination to form the halide-bridged dimers [{CpMo(CO)3}2(μ-X)][closo-HCB11Me11]. Hydride abstraction from [Cp(CO)3MoH] using the new trityl salt [CPh3][closo-HCB11Me11] resulted in the clean formation of the hydride-bridged dimer [{Cp(CO)3Mo}2(μ-H)][closo-HCB11Me11]. The two anions [closo-CB11H6Br6]− and [closo-HCB11Me11]− have been compared with [B(ArF′)4]− [ArF′ = 3,5-bis(trifluoromethyl)phenyl] in Bullock's ionic hydrogenation of 3-pentanone using [Cp(CO)2(PPh3)Mo]+ catalysts. The permethylated anion slightly outperforms its hexabrominated congener in catalysis, but is still slower than [B(ArF′)4]−.