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

Abstract

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: ΔE_½ = E_½(X) – E_½(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.