Synthesis, spectroscopy and electrochemistry of tetrakis(μ-N,N ′-diarylformamidinato)di(phenylethynyl)diruthenium(III) †,‡

The compounds Ru2[(XC6H4)NCHN(XC6H4)]4(CCPh)2 (X = p-OMe, H, p-Cl, m-Cl, m-CF3, 3,4-Cl2 or 3,5-Cl2) have been synthesized and characterized. These diamagnetic diruthenium(III) compounds display three (quasi)reversible redox couples: Ru27+–Ru26+ (A), Ru26+–Ru25+ (B) and Ru25+–Ru24+ (C). The electrode potential for each couple across the series linearly correlates with the Hammett constant (σ) of the substituent according to the following equation: ΔE½ = E½(X) – E½(H) = ρ(8σ) with ρ = 72.0, 92.4 and 80.5 mV for A, B and C, respectively, and the average HOMO–LUMO gap for the solvated diruthenium compounds is estimated as 1.35 eV. Consistent with the proposed ground-state configuration π4δ2π*4, a very long Ru–Ru bond (2.5554 Å) was revealed by an X-ray diffraction study of the compound with X = p-Cl, where unusual structural distortions in both the bridging and axial ligands were also observed. The origin of the distortions is attributed to the second-order Jahn–Teller effect, as elucidated from an MO analysis based on Fenske–Hall calculations.