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*MCl{(MeIm)CHCOO}] and [M(diene){(MeIm)CHCOO}] (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)CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κC,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*M complexes, whereas in the M(diene) complexes it is at the flagpole position of the metallacycle. The complexes [Rh(diene){(MeIm)CHCOO}] (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 was determined by two-dimensional exchange spectroscopy NMR measurements for [Rh(cod){(MeIm)CHCOO}]. Reaction of zwitterionic Cp*M complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*MCl{(MeIm)CHCOOMe}] or [Cp*MCl{(MeIm)CHCOOH}] (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [M(diene){(MeIm)CHCOOMe}] (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic M(diene) complexes with methyl triflate. In contrast, reaction of [Ir(cod){(MeIm)CHCOO}] with HBF·EtO (Et = ethyl), CHOTf, CHI, or I gives cationic iridium(III) octahedral complexes [IrX(cod){(MeIm)CHCOO}] (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κC,C' to κC,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.

Mechanistic Insights on the Functionalization of CO2 with Amines and Hydrosilanes Catalyzed by a Zwitterionic Iridium Carboxylate‐Functionalized Bis‐NHC Catalyst

“…1 H/ 13 C‐HSQC, 1 H/ 13 C‐HMBC and 1 H/ 29 Si‐HMBC sequences were used to assign the 1 H and 13 C{ 1 H} spectra. The iridium(III) complexes [Cp*IrCl{(MeIm) 2 CHCOO}] ( 1 ) and [Cp*IrCl{(MeIm) 2 CH 2 }][PF 6 ] ( 5 ) were prepared following the reported procedure. Amine (pyrrolidine, piperidine, morpholine, Et 2 NH, Me( c ‐hexyl)NH and i Pr 2 NH) and silane reagents (HSiMe 2 Ph, HSiMePh 2 , HSiEt 3 and 1,4‐(HMe 2 Si) 2 (C 6 H 4 )) used in the catalytic experiments were purchased from commercial sources and dried over 4 Å molecular sieves prior to use.…”

“…As a continuation of our studies on the catalytic applications of functionalized bis‐NHC metal complexes, we have found that the zwitterionic complex, [Cp*IrCl{(MeIm) 2 CHCOO}] ( 1 ) (MeIm=3‐methylimidazol‐2‐yliden‐1‐yl), which features a carboxylate bridge‐functionalized bis‐NHC ligand, efficiently catalyzes the reduction of CO 2 with hydrosilanes to selectively afford the corresponding silylformates. The investigation of the reaction mechanism has disclosed the key role of the uncoordinated carboxylate fragment on the catalytic performance.…”

“…We have recently focused our attention on the potential of a carboxylate‐functionalized methylene‐bridged bis‐NHC ligand for the construction of a versatile metal‐ligand platform with application in catalysis . In this regard, we have shown that the carboxylate group at the linker provides hemilabile properties to the ligand, allowing for the stabilization of catalytic intermediates through the κ 3 ‐ C,C’,O ‐tridentate coordination mode .…”

“…In this regard, we have shown that the carboxylate group at the linker provides hemilabile properties to the ligand, allowing for the stabilization of catalytic intermediates through the κ 3 ‐ C,C’,O ‐tridentate coordination mode . However, in most complexes the bis‐NHC ligand exhibits a κ 2 ‐ C , C bidentate coordination mode with an uncoordinated functional group in the skeleton that has shown to be a reactive site …”

Molecular water oxidation catalysis by zwitterionic carboxylate bridge-functionalized bis-NHC iridium complexes

“…10,14a Our approach for the design of robust water oxidation catalysts is the use of bis-NHC ligands functionalized with a carboxylate group at the linker. 16 The carboxylate function may confer hemilabile properties to the ligand while imparting water solubility to the complexes. In addition, the potential coordination of the carboxylate to the metal centre might help for the stabilization of high-valent species likely involved in water oxidation catalysis.…”