Electrochemical and EPR spectral studies of mono- and bimetallic (trispyrazolylborato)molybdenum and tungsten complexes with extended di-phenol bridging ligands; evidence for electron exchange below the fast limit

Abstract

Mononuclear diphenolato complexes [M(NO)L*X(OQOH)] and binuclear complexes [{M(NOL)L*X} 2(μ-OQO)] {M = Mo, W; L* = [HB(3,5-Me 2C 3HN 2) 3] −; X = Cl, I; Q = C 6H 4, 4,4′-C 6H 4C(O)C 6H 4, 4,4′-C 6H 4S(O) 2C 6H 4 or C 6H 4C(O)(C 6H 4) 2C(O)C 6H 4} were prepared. Some have been reported before; the new complexes were thoroughly characterized. Electrochemical interactions between the metal centers in the binuclear complex with the three longer bridging ligands are small. The binuclear molybdenum complexes (which contain two 16-electron metal centres) with 4,4′-OC 6H 4C(O)C 6H 4O or 4,4′-OC 6H 4S(O) 2C 6H 4O as bridging ligands were reduced to dianionic species (two 17-electron centres), whose EPR spectra indicate pairwise electron exchange at or near the fast limit; with the longer bridging ligand OC 6H 4C(O)(C 6H 4) 2C(O)C 6H 4O the more complex EPR spectrum of the reduced dianionic complex indicates that electron exchange is no longer “fast”, but occurs at an intermediate rate. With the shortest bridging ligand OC 6H 4O the electrochemical interaction is sufficiently large to allow selective preparation of the mixed-valence 16-electron/17-electron species, whose EPR spectrum indicates that it is valence trapped; fast exchange is only initiated when both metal centres have a 17-electron configuration. The EPR spectra of the corresponding bimetallic tungsten dianions cannot be used to examine electron exchange processes since they show no resolved hyperfine signals.