LIGAND-CONTROLLED SYNTHESIS, REACTIVITY AND OXO-TRANSFER KINETICS OF OXOMOLYBDENUM-(VI) AND OXOMOLYBDENUM-(IV) COMPLEXES

Trifunctional (ONS) dianionic Schiff-base ligands L2-[H-2L = S-methyl 3-(2-hydroxyphenyl)methylenedithiocarbazate or 5-R substituent derivat ives (R = H, Me, Cl, Br or NO2) or a naphthyl derivative], in sharp co ntrast to the S-benzyl analogues (H-2L') form only Mo-O-->Mo bridged o ligomers [(MoO2L)n] in EtOH or MeOH, irrespective of the substituent R . However, these substituents R control the position of the v(Mo-O-->M o) vibration, the Mo(VI)-Mo(V) redox couple, the ligand-to-metal charg e-transfer transition, as well as the chemical shift of the azomethine proton signal and the asymmetric v(Mo=O) vibration in solution, for b oth the polymeric complexes and, where relevant, the donor molecule (D ) co-ordinated monomers [MoO2L(D)] [D = pyridine (py), dimethylformami de (dmf) or MeSO]. Reaction of [MoO2L] with PPh3 in CHCl2, MeOH or MeC N or in donor solvents D (dmf or py) produced oxomolybdenum(IV) deriva tives, [MoO(L)] or [MoOL(D)], respectively. The kinetics of oxo transf er from MoO22+ to PPh, occurs in a second-order process. The rate cons tant of the oxo-transfer reaction from the polymer [(MoO2L)n] (R = H) to the PPh3 substrate is almost-equal-to 10(2) times higher than that of the corresponding monomer [MoO2L(D)]. Both [MoO(L)] and [MoOL(dmf)] react with the Me2SO substrate in CH2Cl2 or dmf in a two-stage proces s. The first involves the equilibrium formation of a Me2SO adduct whil e during the second stage an intramolecular oxo transfer occurs from M e2SO to the MoO core via the elimination of Me2S. The rate constant of the reverse oxo transfer (k-1) is almost identical for both the polym er [MoO(L)] and monomer [MoOL(dmf)] but the equilibrium constant, K, f or the formation of the Me2SO complexed species is slightly higher for [MoOL(dmf)] than for [MoO(L)].