Electrochemical and electron–electron exchange interactions in binuclear molybdenum complexes containing the bridging ligand 4-(imidazol-1-yl)phenol

Abstract

A series of mono- and bi-nuclear complexes containing {M(NO)LX}[M = Mo or W, X = Cl or I, L = tris(3,5-dimethylpyrazol-1-yl)hydroborate] groups attached to either or both ends of the potentially bridging ligand 4-(imidazol-1-yl)phenol (HL¹) have been prepared, in order to examine both electrochemical and electron–electron exchange interactions across the bridging ligand. Electrochemical studies have shown that the imidazolyl ligand shifts the redox potentials of the {Mo(NO)LX}(X = Cl or I) core cathodically to the extent that the 17e–18e reductions cannot be observed. As with the similar bridging ligand 4-(4-pyridyl)phenol, the electrochemical interaction between the two 16e–17e processes in the binuclear complex is weak. The binuclear molybdenum complexes of L¹ contain one 17-electron (imidazolyl terminus) and one 16-electron (phenolate terminus) metal centre, and are clearly valence-trapped according to the EPR spectra. However one-electron reduction to the 17e–17e biradical initiates a strong electron–electron exchange interaction, whereby both of the unpaired electrons are coupled equally to both molybdenum nuclei, despite the weak electrochemical interaction.