Syntheses of tervalent iridium complexes of octaethylporphyrin; influence of axial ligands on their ultra-violet–visible, infrared, and nuclear magnetic resonance spectra, and redox potentials

Abstract

Tervalent iridium complexes of 2,3,7,8,12,13,17,18-octaethylporphyrin (oep), [Ir^III(oep)(CO)X](X = Cl, ClO₄, BF₄, CN, Br, or CH₃), and [Ir^III(oep)(L)CH₃][L = none, N-methylimidazole (mim), pyridine (py), NH₃, or CN] were isolated or generated in situ. For the carbonyl complexes [Ir^III(oep)(CO)X] the carbonyl stretching frequencies [ν(CO)] and the wavenumbers of the absorption peaks of electronic spectra are both found to be approximately in the order of decreasing electron-donating tendency of the axial ligand, i.e. CH₃^–, Br^–∼ Cl^–, ClO₄^–∼ BF₄^–, except in the case of the cyanide ion, where various types of complexity are conceivable. Throughout the whole series there is a good linear correlation between the chemical shift of the meso protons and the reversible redox potential Ir^I–Ir^III as shown in cyclic voltammetry in the range ca.+0.4–+1.3 V depending on the axial ligands. These results are explained in terms of the change of the electron density caused by chelation of the axial ligands depending on their electron-donating tendency, and the order of such tendency is inferred to be : CO < none < py < NH₃ < mim < CN^– < ClO₄^–∼ BF₄^– < Cl^–∼ Br^– < CH₃^–.