Molybdenum, oxo -, complexes

One approach to limit dimer formation in model complexes involves the use of bulky ligands and weakly coordinating solvents. Holm and coworkers [196,198-201] have studied oxygen atom transfer reactions of 2,6-bis(2,2-diphe-nyl-2-thioethyl)pyridinate [2] molybdenum oxo complexes. In contrast to the structurally similar dithiocarbamate molybdenum complexes, the tendency of the 2,6-bis(2,2-dipheny 1-2-thioethy l)pyridinate MoIV monoxo complex to undergo di-... 

Figure 1.6 a SMPO process b the catalytic oxidation of propene with tert-butyl hydroperoxide in the presence of a molybdenum-oxo complex. 

Efficient catalysts for direct episulfidation of alkenes by sulfur-atom donors are also diethyldithiocarbamate and dithiophosphonate molybdenum oxo complexes 302 and 303, respectively (Equation 43 Scheme 88 Table 13) <2003JA3871>. The most efficient sulfur-atom donor in this reaction is 2-phenylthiirane. The best results were obtained, in most cases, when 1 equiv of 2-phenylthiirane was used as sulfur-atom donor in deuterated benzene at 80 °C for 30 min. Reaction of /ra r-cyclooctene catalyzed by either of the oxomolybdenum complexes gave trans-epithiocyclooctane. When this reaction was carried out in the presence of 302, the yield of the corresponding thiirane was only 20%, whereas catalyst 303 converted the olefin in almost quantitative yield (Table 14). 

Table 13 Episulfidation of alkenes by 2-phenylthiirane in the presence of molybdenum oxo complexes...

Table 14 Episulfidation of frans-cyclooctene with 2-phenylthiirane catalyzed by the molybdenum oxo complexes 302 and 303...

Reaction of allenes with a sulfur-atom donor (e.g., 2-phenylthiirane, 2-methylthiirane, sodium tetrasulfide, or elemental sulfur) in the presence of molybdenum oxo complexes gave the appropriate vinylthiirane in moderate to quantitative yield (Scheme 89 Table 15) <2003JA3871>. 

The possible formation of molybdenum imido complexes during the reaction was also considered, but no conclusion concerning their possible role could be drawn. Note that a catalytic cycle in which a molybdenum complex reduces nitrobenzene (to aniline) forming an intermediate molybdenum-oxo complex has indeed been reported, but the reducing agent was a thiol, not CO... 

Bishop, M.W., Butler, G., Chatt, J., Dilworth, J.R, and height, G.J. (1979) The reactions of molybdenum oxo complexes with substituted hydrazines Part 5. The preparation of some organodiazenido complexes of molybdenum. /. Chem. Soc., Dalton Trans., 1843-1850. 

Seven and eight coordinate molybdenum complexes and related molybaenum(IV) oxo complexes with cyanide and isocyanide ligands. S. J. Lippard, Prog. Inorg. Chem., 1976, 21, 91-103 (44). 

F.E. Inscore, R. McNaughton, B.L. Westcott, M.E. Helton, R. Jones, I.K. Dhawan, J.H. Enemark, and M.L. Kirk, Spectroscopic evidence for a unique bonding interaction in oxo-molybdenum dithiolate complexes implications for sigma electron transfer pathways in the pyranopterin dithiolate centers of enzymes. Inorg. Chem. 38, 1401-1410 (1999). 

Owing to their involvement in bioinorganic processes and catalysis, much interest has been attracted by the chemistry of Mo and W 1,2-dithiolene complexes.39,40 Molybdenum oxo-1,2-dithiolene complexes can be typically... 

Oxo complexes dominate the chemistry of molybdenum in its higher oxidation states,5 80,92 93 and play a major role in the catalytic activity of molybdenum in biological systems (see Section 36.6.7). MoIV complexes have been characterized containing the monooxo (MoO 2+ and the trans-dioxomolybdenum(IV) Mo02 cores, but no compound has been isolated analogous to the ds-dioxomolybdenum(VI) species described in Section 36.5. Crystal structure determinations have been accomplished for a number of MoIV complexes, and some typical examples are listed in Table 3.80... 

Molybdenum(V) chemistry is dominated by oxo complexes many of these exist as dimers, but monomeric species can be isolated from strongly addic solutions or under nonaqueous conditions. Non-oxo compounds are also known, both as monomers and as dimers or polymers with halide or sulfur bridges. ESR spectroscopy has been used extensively to investigate the properties of monomeric Mov systems, and has shown the participation of this oxidation state in the reactions of the oxomolybdoenzymes (see Section 36.6.7). 

Monomeric non-oxo molybdenum(V) complexes have been isolated with nitride (N3 ), imide (NR2-) and amide (NRR -) ligands, which may act as models for intermediates in the degradation of dinitrogen in nitrogenase. However, no molybdenum(V) compounds are yet known which correspond to the diazenide or hydrazide analogues of the molybdenum(IV) complexes described in Section 36.4.2.3. 

The decomposition of the peroxometallacycle (72a) or (72b) occurs in a way different from that previously shown to occur in the epoxidation of alkenes by molybdenum-peroxo complexes (equation 26). The three possibilities are shown in equations (58)-(60) and involve (a) a [C-/6, C-a] hydride shift which directly produces the methyl ketone and the rhodium-oxo complex, or the hydroxo species from (72b equation 58) (b) a [C-/3,0-/3] hydride shift which gives enol (equation... 

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