Molybdenum complexes redox potentials

Cyclic voltammetry and controlled-potential electrolysis are the techniques that have been used to investigate the electrochemistry of oxo-chromium and oxo-molybdenum corrolates. The data have been related to those obtained for similar porphyrin complexes. Redox potentials are reported in Table 17. 

For both complexes, reversible metal-centered one-electron oxidations and reductions have been observed. The products of such redox processes have been examined by monitoring the EPR and electronic spectra obtained by controlled-potential electrolysis and, in the case of the molybdenum complex, have been identified as [Mo(IV)0(TMTEC)]+ and [Mo(VI)0(TMTEC)] ions. 

Data on the redox potentials of germylenes, stannylenes, plumbylenes and their complexes are scarce. In fact, only the electrochemistry of dihalogermylenes, dihalostan-nylenes and their complexes with Lewis bases338 as well as with chromium, molybdenum and tungsten pentacarbonyles339 has been studied. 

Molybdenum and tungsten complexes with three crown ether benzenedithio-lene ligands (21) have been reported (105) and the effect of alkali ion binding has been probed by CV (106). Upon binding with Li+, Na+, or K+, positive shifts in the redox potential have been observed for all complexes. This observation suggests that the tris(crown ether benzodithiolene) complexes of Mo and W may potentially be useful as sensors for alkali metal cations (106). 

Like the corresponding chromium complex shown in Scheme 7-3, the molybdenum-carbene complex (j/ -C5H4Me)Fe jy -C5H3[l-(OEt)C=Mo(CO)5](2-Me) undergoes a single one-electron oxidation in DME solution. The redox potential ( ° = -I- 0.73 V) is nearly coincident with that of the chromium complex (see Table 7-4) [36]. 

The gradual substitution of the phenyl groups in the phosphine ligand for ferro-cenyl subunits affords the diferrocenyl- and triferrocenyl-phosphinepentacarbonyl-molybdenum complexes, respectively. With respect to the redox pathway shown in Fig. 7-11, each added ferrocenyl ligand involves the appearance of a further one-electron oxidation [50]. The relevant redox potentials are given in Table 7-7. 

Table 7-9. Formal electrode potentials (vs. SCE) for the redox processes exhibited by the ferro-cenyl-molybdenum complexes shown in Scheme 7-7 and related mononuclear species...

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

The last series of tungsten complexes that have been electrochemically studied belong to the 3,5-dimethylpyrazolyl-borato complexes of the type shown in Scheme 7-7. Also in this case, they exhibit the one-electron oxidation of the ferrocenyl centre and the one-electron reduction of the molybdenum fragment [55], The relevant redox potentials are summarized in Table 7-13. 

Nitrogenase, as must now become clear, is a complex enzyme of two component proteins which requires ATP, a reductant, a reducible substrate, Mg " " as an activator, and an anaerobic environment to function. To this complexity must be added the difficulty that the component proteins have no enzymatic half reactions . There are, perhaps, four main questions to decide about the mechanism (1) the role(s) of the two component proteins (2) the role(s) of ATP (3) the nature of the active site(s) and (4) the mechanism of N2 reduction. Despite the complexities and difficulties mentioned above, progress in the last 15 years has partly answered all these questions. The Fe protein mediates an ATP-dependent electron transfer from the donor to the MoFe protein which contains the active site. MgATP binds and induces a conformational change in the Fe protein which lowers its redox potential. FeMoco, the molybdenum cofactor, which may be part of the active site of N2 reduction, has been isolated and partly characterized, while an intermediate in N2 reduction has recently been discovered (Thorneley ct al., 1978). The next part of this chapter describes the evidence for these claims. This evidence involves the noncatalytic reactions of the individual proteins, their... 

As detailed above, the observation of thiol/thione and zwitterionic resonance forms for the pyrrolo-S2BMOQO ligand supported a hypothesis that the PDT could potentially function as a complex non-innocent ligand in order to modulate the molybdenum redox potential during catalysis. The potential for multiple oxidation states being available to the PDT originates from... 

Redox potential-structure relationships in metal complexes. Part 2. The influence of trans-substituents upon the redox properties of certain dinitrogen complexes of molybdenum and tungsten and some carbonyl analogues inner-sphere versus outer-sphere electron transfer in the alkylation of co-ordinated dinitrogen J.Chem.Soc. Dalton Trans./ (1980)/ 121-127... 

Butler, G., Chatt, J., Leigh, G.J. and Pickett, C.J. Structure-redox potential relations of metal complexes Part 1. Organodiazenido-complexes of molybdenum Neighbouring-group influence on redox potential J.Chem.Soc. Dalton Trans., (1979), 113-116... 

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