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

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journal contribution
posted on 09.07.2007, 00:00 by Chao-Yu Wang, Ching-Feng Fu, Yi-Hong Liu, Shei-Ming Peng, Shiuh-Tzung Liu
Coordination of iridium(I) metal ions with a pyridinyl imidazol-2-ylidene ligand (pyNC-R) [R = Me, mesityl(2,4,6-trimethylphenyl)] that processes bulky substituents has been investigated. The iridium carbene complexes [(C-pyNC-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 pyNC becomes feasible, resulting in the formation of [C,N-(pyNC-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-(pyNC-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.