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 saltmetathesis and hydride abstraction

Abstract

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(Ar_F′)₄]⁻ [Ar_F′ = 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(Ar_F′)₄]⁻.