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

Several paramagnetic tetrakis(diarylformamidinato)diruthenium(II,III) compl exes bearing one phenylethynyl ligand at the axial position, Ru-2(ArNC(H)NA r)(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 voltamme try studies of 2-6 revealed the presence of two one-electron redox couples: Ru-2(6+)/Ru-2(5+) and Ru-2(5+)/Ru-2(4+), and the half-wave potential for t he latter correlates linearly with the Hammett constant (sigma) of the aryl substituent. Molecular structures of 3 and 6 were determined by X-ray crys tallography. The nature of Ru-2-C=C bonding interaction was predominantly R u-C-alpha sigma-bonding based on the substituent-dependence of v(C=C), whic h decreases linearly with the increase of the Hammett constant sigma of sub stituent. (C) 1999 Elsevier Science S.A. All rights reserved.