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

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)­Cl2]−), which featured cationic as well as anionic Ir centers. Electrochemical
analysis of [2]+[Ir­(COD)­Cl2]− 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)­Cl2]− under an atmosphere of carbon monoxide
generated the corresponding Ir carbonyl complex [(1)2Ir­(CO)2]+[Ir­(CO)2(Cl)2]− ([3]+[Ir­(CO)2Cl2]−). 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]+[BArF4]− 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.