Cationic Iridium Complexes Containing Anionic Iridium Counterions Supported by Redox-Active N-Heterocyclic Carbenes

Abstract

The first cationic iridium complex supported by two bulky, ferrocenylated N-heterocyclic carbenes, [(1)(2)Ir-(COD)(+)[Cl](-) ([2]Cl+](-); 1 = 1-ferrocenylmethyl-3-mesityl-imidazol-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 [(I)(2)Ir(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)(2)Ir(CO)(2)](+)[Ir(CO)(2)(Cl)(2)](-) ([3](+)[Ir(CO)(2)Cl-2](-)). Cyclic voltammetry (CV) measurements of the aforementioned complexes containing [(1)(2)Ir(COD)](+) showed that the iron-centered oxidations were concurrent, reflective of limited electronic coupling between the two ferrocenyl moieties. However, spectroelectrochemical analysis of [(1)(2)Ir(CO)(2)](+)[BAr4F](-) 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 v(CO) bands.