N,N′-Dialkylation Catalyzed by Bimetallic Iridium Complexes Containing a Saturated Bis-N-Heterocyclic Carbene (NHC) Ligand

Reaction of the bis-aminophosphinimine [m-C6H4(HNCH2CH2NPPh3)] with W(CO)6 afforded [m-C6H4{(CNCH2CH2NH)W(CO)5}2] (2), which underwent N-alkylation with benzyl bromide to yield [m-C6H4{(CNCH2CH2NCH2Ph)W(CO)5}2] (3). A carbene transfer reaction from W(0) to Ir(I) proceeded smoothly via the reaction of 3 with [Ir(COD)Cl]2 under mild conditions to give the diiridium(I) carbene complex [m-C6H4{(CNCH2CH2NH)Ir(CO)2Cl}2] (4). Ligand substitution of 4 with an excess of PPh3 produced the phosphine complex 5. All complexes have been characterized by spectroscopic and elemental analyses. Complexes 2 and 5 were further confirmed by X-ray diffraction studies. Complex 4 is an efficient catalyst for the reductive N,N′-dialkylation of phenylenediamines with alcohols. The mechanistic pathway of the catalysis involving the possible synergistic effect between two metal centers is discussed.