Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity

Abstract

A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*M^IIICl{(MeIm)₂CHCOO}] and [M^I(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 κ²C,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*M^III complexes, whereas in the M^I(diene) complexes it is at the flagpole position of the metallacycle. The complexes [Rh^I(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 [Rh^I(cod){(MeIm)₂CHCOO}]. Reaction of zwitterionic Cp*M^III complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*M^IIICl{(MeIm)₂CHCOOMe}]⁺ or [Cp*M^IIICl{(MeIm)₂CHCOOH}]⁺ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [M^I(diene){(MeIm)₂CHCOOMe}]⁺ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic M^I(diene) complexes with methyl triflate. In contrast, reaction of [Ir^I(cod){(MeIm)₂CHCOO}] with HBF₄·Et₂O (Et = ethyl), CH₃OTf, CH₃I, or I₂ gives cationic iridium(III) octahedral complexes [Ir^IIIX(cod){(MeIm)₂CHCOO}]⁺ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κ²C,C' to κ³C,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.