SYNTHESIS, SPECTROSCOPY AND ELECTROCHEMISTRY OF RUTHENIUM(II) COMPLEXES OF TRIDENTATE PYRIDYLPYRAZOLE LIGANDS - PREDOMINANCE OF ELECTRONIC OVER STERIC EFFECTS

The preparations and properties of bis complexes of ruthenium(II) containing various combinations of the tridentate ligands 2,6-bis(pyrazol-1-ylmethyl)pyridine (L1) and di- and tetra-methyl substituted derivatives (L2 and L3) are described. The absorption spectral properties of the complexes are thoroughly analysed. Full assignments have been made for the H-1 NMR spectra of two representative complexes in CD3CN and the origins of the co-ordination-induced shifts are discussed. Cyclic voltammetric experiments (MeCN solutions) reveal reversible one-electron Ru(III)-Ru(II) redox couples in the potential range 1.00-1.06 V vs. saturated calomel electrode (SCE). The occurrence of ligand-based irreversible reductions at low potentials (E(p,c) values lie in the range: -1.90 to -2.20 V vs, SCE) reveals that these pyrazole-rich ligands are very poor pi-acceptors. The formal potentials of the Ru(III)-Ru(II) couples decrease (by ca. 8 mV per methyl group) as the number of methyl groups in the ligands are increased. The decreased potential step size for [RuL(L')]2+ (L,L' = L1-L3) relative to those observed for non-sterically hindered ruthenium(II) complexes reveals that in the present system steric effects due to 3-Me substituent(s) contribute to the observed effect but that electronic factors predominate over steric effects.