MONONUCLEAR AND BINUCLEAR MOLYBDENUM AND TUNGSTEN COMPLEXES CONTAINING ASYMMETRIC BRIDGING LIGANDS - EFFECTS OF LIGAND CONJUGATION AND CONFORMATION ON METAL-METAL INTERACTIONS

We have prepared the new monodentate ligands 4-(4-methoxyphenyl)pyridine, 1-(4-pyridyl)-2-(4-methoxyphenyl)ethene, 1-(4-pyridyl)-2-(3-methoxyphenyl)ethene, and 1-(3-pyridyl)-2-(4-methoxyphenyl)ethene (L5-L8); demethylation of the methoxy group in each case afforded the new bridging bidentate ligands HL1-HL4, which contain one pyridyl and one phenolate donor. Attachment of a MoL*(NO)Cl [L* = hydrotris(3,5-dimethylpyrazolyl)borate] moiety to the pyridyl groups of L5-L8 gave the 17-electron complexes [Mo(NO)L*CIL'] (L' = L5-L8, complexes 5-8). Reaction of HL1-HL4 With [M(NO)L*Cl2] (M = Mo, W) afforded the mononuclear 16-electron complexes [M(NO)L*CIL'] (M = Mo, L' = L1-L4, complexes 1-4; M = W, L' = L1-L4, complexes 13-16), in which the phenolate terminus of L1-L4 is attached to the metal and the pyridyl group is pendant in each case. The pyridyl groups were N-methylated with CH3I to afford [9]+-[12]+ and [17]+-[20]+, respectively. Alternatively, a second MoL*(NO)Cl could be attached to the pendant pyridyl groups of 1-4 to give the binuclear complexes [Mo(NO)L*Cl(mu-L')Mo(NO)L*Cl] (L'= L1-L4, complexes 21-24), which contain a 16-electron Mo center at the phenolate terminus and a 17-electron Mo center at the pyridyl terminus. Electrochemical studies showed that the anodic shift of the 16e-17e reduction due to N-methylation in [9]+-[12]+ and [17]+-[20]+ varies with the ligand in the order L1 > (L2 almost-equal-to L4) > L3. This is explained by the extent to which the relative substitution patterns of the component aromatic rings allow communication between the halves; the halves of L3 appear to be electrochemically isolated, and this is supported by the electronic spectra. A similar pattern for the electrochemical interactions was observed in binuclear complexes 21-24. EPR studies showed that in 21-24 the unpaired electron is localized at the pyridyl-substituted Mo center, which is expected considering the difference in redox potentials between the two ends. However on one-electron reduction of the 16-electron phenolate-substituted Mo center to give binuclear complexes with two 17-electron centers, the spectra indicate that the two electrons are in fast exchange on the EPR time scale with both electrons coupled to both nuclei. The crystal structure of 5 shows that the 4-(4-methoxyphenyl)pyridine ligand is nearly planar, probably because charge transfer from the electron-donating methoxy group to the electron-accepting pyridyl group results in a quinonoidal contribution to the ligand structure.