METAL METAL INTERACTIONS ACROSS SYMMETRICAL BIPYRIDYL BRIDGING LIGANDS IN BINUCLEAR 17-ELECTRON MOLYBDENUM COMPLEXES

A series of 17-electron mononuclear complexes [Mo(NO)L(Cl)X] and their binuclear counterparts [{Mo-(NO)LCl}2(mu-X)] [L = tris(3,5-dimethylpyrazolyl)hydroborate; X = 3,3'-dimethyl-4,4'-bipyridine (3,3'-dmbipy), 1,2-bis(4-pyridyl)acetylene (bpac), 4,4'-azopyridine (azpy), 1,4-bis[2-(4-pyridyl)ethenyl]-benzene (bpeb) or 1,4-bis(4-pyridyl)benzene (bpb)] have been prepared. Electrochemical studies show that the reduction potentials of the mononuclear complexes are sensitive to the degree of unsaturation in the monodentate ligand X, whereas the oxidation potentials are virtually constant. This suggests that the redox orbital involved in the reductions have considerable ligand-based character whereas the oxidations are more strongly metal-centred. This is supported by the electrochemical properties of the binuclear complexes, where the oxidation potentials are in every case coincident but the splitting between the reduction potentials of the equivalent molybdenum centres varies from 0.16 V (X=bpeb) to 0.56 V (X=bpac). By contrast the splitting of the redox potentials of pentaammineruthenium(II) fragments at either end of 'extended' 4,4'-bipyridine analogues of this type is an order of magnitude smaller. This strong interaction between [Mo(NO)LCl] moieties is in part due to a planar conformation of the bridging ligands, even when they are in principle capable of free rotation, since changing the bridging ligand from 4,4'-bipyridine to 3,3'-dmbipy (which cannot be planar due to the steric effects of the methyl groups) results in a decrease in the splitting of the reduction potentials from 0.77 to 0.38 V. The EPR spectra of the binuclear complexes all show that the two unpaired electrons (one at each 17-electron molybdenum centre) are in fast exchange across the bridging ligand at room temperature.