Molybdenum complexes electrochemical behavior

To explore the influence of coordination environment on rates and mechanisms of electron transfer reactions at molybdenum centers, we have investigated [17-20] the electrochemical behavior of several families of oxomolybdenum (VI) complexes. The compounds are illustrated in Figure 4. The first three examples are seven-coordinate complexes and exhibit pentagonal bipyramidal geometry. These compounds contain a... 

Chromium nitrosyl complexes, [Cr(S2CNR2)3(NO)], are somewhat less thermally stable than their molybdenum and tungsten counterparts heating [Cr(S2CNMc2)3(NO)] in toluene for 3 h leads to the formation of cis-[Cr(NO)2(S2CNMe2)2] and [Cr(S2CNMe2)3] (752). Somewhat contradictory reports siuround the electrochemical behavior of [Cr(S2CNR2)3(NO)]. Connelly and co-workers (752) report that they show no redox chemistry between 1.5 V,... 

The electrochemical reversibility of the M(VI)/M(V) couple for the complexes with sterically hindered ligands contrasts with the reported behavior of the [MoO(SPh)4] complex, which exhibited electrochemical irreversibility for the Mo(V)/Mo(VI) step but a reversible Mo(IV)/ Mo(V) couple. The sterically hindered aromatic substituent groups stabilize the molybdenum(VI) complex and decrease relative to the thiophenol derivative. The molybdenum(VI) species can also be isolated by chemical oxidation. [MoO(PFTP)4] was prepared by chemical redu-tion of [MoO(PFTP)4] . The presence of the electron-withdrawing substituents on the aromatic thiolate increases E ei relative to the thio-phenolate derivative. Evidently the properties of these last complexes are influenced primarily by the electron-withdrawing characteristics of the fluorine substituents rather than by steric factors (33). 

All three members of the electron transfer chain [Mo(CO)2(S2C2Me2)]" (n = 0,1, 2) were isolated and characterized. The complexes have trigonal prismatic geometries, and structural, spectral, and electrochemical properties that are consistent with the redox orbital having predominantly sulfur-ligand based character (>80%) behavior consistent also with DFT calculations. Closely related benzene- and 2,4-di-i-butyl-benzene-dithiolene complexes, [Mo(LL )(bdt)2] (L = 0, L = PPha, L = L = PMeR2 (R = Me or Ph)) were obtained by thioether S-dealkylation of appropriate dttd (22) complexes of molybdenum carbonyls and oxo species. 

The mechanism of electrochemical reduction of seven-coordinate MoO complexes is illustrated in Figure 6. Each electron transfer is accompanied by a change in coordination number of the metal. This alters the energy of the redox orbital in such a way that transfer of two electrons becomes possible at neatly identical potentials. The mechanism in Figure 6 may not be relevant to tite behavior of molybdenum hydroxylases, but it does illustrate that coodination reactions can sustain multielectron redox events at transition metal centers. 

---