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)pyridi ne, 1-(4-pyridyl)-2-(4-methoxyphenyl)ethene, 1-(4-pyridyl)-2-(3-methox yphenyl)ethene, and 1-(3-pyridyl)-2-(4-methoxyphenyl)ethene (L5-L8); d emethylation of the methoxy group in each case afforded the new bridgi ng bidentate ligands HL1-HL4, which contain one pyridyl and one phenol ate donor. Attachment of a MoL(NO)Cl [L* = hydrotris(3,5-dimethylpyra zolyl)borate] moiety to the pyridyl groups of L5-L8 gave the 17-electr on complexes [Mo(NO)LCIL'] (L' = L5-L8, complexes 5-8). Reaction of H L1-HL4 With [M(NO)LCl2] (M = Mo, W) afforded the mononuclear 16-elect ron complexes [M(NO)LCIL'] (M = Mo, L' = L1-L4, complexes 1-4; M = W, L' = L1-L4, complexes 13-16), in which the phenolate terminus of L1-L 4 is attached to the metal and the pyridyl group is pendant in each ca se. The pyridyl groups were N-methylated with CH3I to afford [9]+-[12] + and [17]+-[20]+, respectively. Alternatively, a second MoL(NO)Cl co uld be attached to the pendant pyridyl groups of 1-4 to give the binuc lear complexes [Mo(NO)LCl(mu-L')Mo(NO)L*Cl] (L'= L1-L4, complexes 21- 24), which contain a 16-electron Mo center at the phenolate terminus a nd a 17-electron Mo center at the pyridyl terminus. Electrochemical st udies showed that the anodic shift of the 16e-17e reduction due to N-m ethylation 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 ext ent to which the relative substitution patterns of the component aroma tic rings allow communication between the halves; the halves of L3 app ear to be electrochemically isolated, and this is supported by the ele ctronic spectra. A similar pattern for the electrochemical interaction s was observed in binuclear complexes 21-24. EPR studies showed that i n 21-24 the unpaired electron is localized at the pyridyl-substituted Mo center, which is expected considering the difference in redox poten tials between the two ends. However on one-electron reduction of the 1 6-electron phenolate-substituted Mo center to give binuclear complexes with two 17-electron centers, the spectra indicate that the two elect rons are in fast exchange on the EPR time scale with both electrons co upled to both nuclei. The crystal structure of 5 shows that the 4-(4-m ethoxyphenyl)pyridine ligand is nearly planar, probably because charge transfer from the electron-donating methoxy group to the electron-acc epting pyridyl group results in a quinonoidal contribution to the liga nd structure.