Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity
A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp MIIICl{(MeIm)2CHCOO}] and [MI(diene){(MeIm)2CHCOO}] (Cp* = 1, 2, 3, 4, 5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1, 5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)2CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the ¿2C, C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp MIII complexes, whereas in the MI(diene) complexes it is at the flagpole position of the metallacycle. The complexes [RhI(diene){(MeIm)2CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, bowsprit and flagpole, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of ~17 kcal·mol-1 was determined by two-dimensional exchange spectroscopy NMR measurements for [RhI(cod){(MeIm)2CHCOO}]. Reaction of zwitterionic Cp MIII complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp MIIICl{(MeIm)2CHCOOMe}]+ or [Cp MIIICl{(MeIm)2CHCOOH}]+ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [MI(diene){(MeIm)2CHCOOMe}]+ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic MI(diene) complexes with methyl triflate. In contrast, reaction of [IrI(cod){(MeIm)2CHCOO}] with HBF4·Et2O (Et = ethyl), CH3OTf, CH3I, or I2 gives cationic iridium(III) octahedral complexes [IrIIIX(cod){(MeIm)2CHCOO}]+ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from ¿2C, C' to ¿3C, C', O coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions
