Synthesis, spectroscopy and electrochemistry of ruthenium(II) complexes of tridentate pyridylpyrazole ligands. Predominance of electronic over steric effects

Abstract

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 ¹H 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 p 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.