Cationic iridium complexes containing anionic iridium counterions supported by redox-active N-heterocyclic carbenes

摘要

The first cationic iridium complex supported by two bulky, ferrocenylated N-heterocyclic carbenes, [(1)_2Ir(COD)]~+[Cl]~- ([2]~+[Cl]~-; 1 = 1-ferrocenylmethyl-3-mesitylimidazol-2- ylidene, Fc-NHC; COD = cis,cis-1,5-cyclooctadiene), was synthesized and characterized. Treatment of the aforementioned complex with a mixture of [N(nBu)_4]~+[Cl]~- and [Ir(COD)(Cl)]_2 afforded [(1)_2Ir(COD)]~+[Ir(COD)(Cl)_2] ~- ([2]~+[Ir(COD)Cl_2]~-), which featured cationic as well as anionic Ir centers. Electrochemical analysis of [2]~+[Ir(COD)Cl_2]~- revealed that the complex displayed two reversible (iron- and iridium-centered) and two irreversible (iridium-centered) redox processes, which were assigned to the cationic and anionic components, respectively. Stirring [2]~+[Ir(COD)Cl _2]~- under an atmosphere of carbon monoxide generated the corresponding Ir carbonyl complex [(1)_2Ir(CO)_2] ~+[Ir(CO)_2(Cl)_2]~- ([3] ~+[Ir(CO)_2Cl_2]~-). Cyclic voltammetry (CV) measurements of the aforementioned complexes containing [(1) _2Ir(COD)]~+ showed that the iron-centered oxidations were concurrent, reflective of limited electronic coupling between the two ferrocenyl moieties. However, spectroelectrochemical analysis of [(1)_2Ir(CO) _2]~+[BAr _4~F]~- revealed that the electron density at the Ir centers supported by the Fc-NHC ligands decreased upon oxidation of the ferrocenyl groups, as evidenced by a ca. 10 cm~(-1) increase in the recorded νCO bands.