Preparation and structure of bis(μ-acetato)dichlorodicarbonyldiiridium(II) complexes with Group 15 ligands and ESR and density functional theory studies of electronic structure of their cationic radicals

bis(μ-Acetato)dichlorodicarbonyldiiridium(II) complexes with Group 15 compounds as axial ligands, [Ir2(μ-O2CMe)2Cl2(CO)2L2] (L=PPh3: 1, PCy3: 2, P(OPh)3: 3, AsPh3: 4, SbPh3: 5) were synthesized. The Ir–Ir distances of complexes 2–5 (2.6200(9)–2.6936(7) Å) were longer than those of the complexes with MeCN, py or dmso ligands at the axial sites. Cyclic voltammograms of 1–5 show a chemically reversible one-electron oxidation wave of which E1/2 (vs. Fc+–Fc) values were between 0.22 for 2 and 0.75 V for 3 depending on the axial ligands. Oxidation by electrolysis and/or radiolysis of 1, 2 and 4 gave their cationic radicals. The ESR spectra of 1+, 2+ and 4+ at 77 K were pseudo-axially symmetric with g tensors of g⊥=2.15 and g∣∣=1.96, 2.18 and 1.95, and 2.20 and 1.96, respectively. Their hyperfine coupling indicates that their odd electron is delocalized equivalently onto the two axial phosphorous or arsenic atoms. The odd electron densities were estimated from the hyperfine coupling tensors as ρ≈0.1 on the P atoms of 1+ and 2+ and ρ≈0.15 on the As atoms of 4+. These results indicate that their singly occupied molecular orbital (SOMO) is the σIrIr orbital with σIrP* or σIrAs* character. DFT calculations for model complexes, [Ir2(μ-O2CH)2Cl2(CO)2(PH3)2]+ (6+) and [Ir2(μ-O2CH)2Cl2(CO)2(AsH3)2]+ (7+), gave electronic structures consistent with the ESR results.