Methylene-Linked Bis-NHC Half-Sandwich Ruthenium Complexes: Binding of Small Molecules and Catalysis toward Ketone Transfer Hydrogenation

The complex [Cp*RuCl­(COD)] reacts with LH2Cl2 (L = bis­(3-methylimidazol-2-ylidene)) and LiBu n in tetrahydrofuran at 65 °C furnishing the bis-carbene derivative [Cp*RuCl­(L)] (2). This compound reacts with NaBPh4 in MeOH under dinitrogen to yield the labile dinitrogen-bridged complex [{Cp*Ru­(L)}2(μ-N2)]­[BPh4]2 (4). The dinitrogen ligand in 4 is readily replaced by a series of donor molecules leading to the corresponding cationic complexes [Cp*Ru­(X)­(L)]­[BPh4] (X = MeCN 3, H2 6, C2H4 8a, CH2CHCOOMe 8b, CHPh 9). Attempts to recrystallize 4 from MeNO2/EtOH solutions led to the isolation of the nitrosyl derivative [Cp*Ru­(NO)­(L)]­[BPh4]2 (5), which was structurally characterized. The allenylidene complex [Cp*RuCCCPh2(L)]­[BPh4] (10) was also obtained, and it was prepared by reaction of 2 with HCCC­(OH)­Ph2 and NaBPh4 in MeOH at 60 °C. Complexes 3, 4, and 6 are efficient catalyst precursors for the transfer hydrogenation of a broad range of ketones. The dihydrogen complex 6 has proven particularly effective, reaching TOF values up to 455 h–1 at catalyst loadings of 0.1% mol, with a high functional group tolerance on the reduction of a broad scope of aryl and aliphatic ketones to yield the corresponding alcohols.