Site-directed anchoring of an N-heterocyclic carbene on a dimetal platform: evaluation of a pair of diruthenium(I) catalysts for carbene-transfer reactions from ethyl diazoacetate

Site-directed anchoring of naphthyridine-functionalized N-heterocylic carbene (NHC) is achieved on a metal−metal singly bonded diruthenium(I) platform. Room-temperature treatment of 1-isopropyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazolium bromide (PIN·HBr) with Ru2(CH3COO)2(CO)4 in acetonitrile affords the unsupported compound Ru2(CO)4(κ2C2, N1-PIN)2Br2 (1). Judicious alteration in the NHC ligand resulted in the bridged compound Ru2(CO)4(CH3COO)(μ2-κ2C2,N1-BIN)Br (2) (BIN = 1-benzyl-3-(3-phenyl-1,8-naphthyrid-2-yl)imidazol-2-ylidene). X-ray analysis revealed the chelate binding of PIN on each ruthenium at equatorial sites for 1, and the bridge-chelate binding of BIN spanning the diruthenium core for 2. The catalytic utilities of the BArF (tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) salts of these compounds are evaluated toward carbene-transfer reactions from ethyl diazoacetate including aldehyde olefination, cyclopropanation, and X−H (X = O, N) insertions. 1-BArF is clearly shown to be the superior catalyst. DFT calculations are undertaken to understand the influence of NHC binding on the electronic structures of the “Ru2(CO)4” core and to rationalize the lower activity of 2-BArF