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 κ2C,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
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.