Synthesis of Iridium Pyridinyl N-Heterocyclic Carbene Complexes and Their Catalytic Activities on Reduction of Nitroarenes

Coordination of iridium(I) metal ions with a pyridinyl imidazol-2-ylidene ligand (pyN∧C-R) [R = Me, mesityl(2,4,6-trimethylphenyl)] that processes bulky substituents has been investigated. The iridium carbene complexes [(C-pyN∧C-R)IrCl(COD)] (COD = 1,5-cyclooctadiene) are prepared via transmetalation from the corresponding silver carbene complexes. Upon the abstraction of chloride, the chelation of pyN∧C becomes feasible, resulting in the formation of [C,N-(pyN∧C-R)Ir(COD)](BF4) (4). The coordinated COD of complex 4 can be replaced by carbon monoxide to yield the corresponding carbonyl species [C,N-(pyN∧C-R)Ir(CO)2](BF4). The labile nature of the pyridinyl nitrogen donor is readily replaced by acetonitrile, as is evidenced by the NMR study. All iridium complexes show catalytic activity on the hydrogen-transfer reduction of carbonyl and nitro functionalities. By manipulation of the reaction conditions, the iridium-catalyzed reduction of nitroarenes can selectively provide aniline or azo compounds as the desired product.