Probing diruthenium σ-alkynyl bonding interactions via substituent effects. Linear free energy relationships in dinuclear compounds VI

Several paramagnetic tetrakis(diarylformamidinato)diruthenium(II,III) complexes bearing one phenylethynyl ligand at the axial position, Ru2(ArNC(H)NAr)4(CCPh) (Ar as p-ClC6H4 (2), m-ClC6H4 (3), m-CF3C6H4 (4), 3,4-Cl2C6H3 (5), and 3,5-Cl2C6H3 (6)), were prepared and characterized. Cyclic voltammetry studies of 2–6 revealed the presence of two one-electron redox couples: Ru26+/Ru25+ and Ru25+/Ru24+, and the half-wave potential for the latter correlates linearly with the Hammett constant (σ) of the aryl substituent. Molecular structures of 3 and 6 were determined by X-ray crystallography. The nature of Ru2–CC bonding interaction was predominantly Ru–Cα σ-bonding based on the substituent-dependence of υ(CC), which decreases linearly with the increase of the Hammett constant σ of substituent.