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

Abstract

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-BAr^F 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-BAr^F.