Bidentate ligands, molybdenum

Reactions of the metallocene derivatives of molybdenum with pyrazole lead to the mononuclear complexes of the type 22. Structure 22 shows that it cannot be used as a ligand for the preparation of dinuclear complexes owing to geometric constraints [80JOM( 197)291 83JOM(253)53]. In acetone, an unusual complex 23 is formed [83JOM(253)53]. The bidentate ligand is the product of the reaction of pyrazole and acetone. 

The chiral ligand (44) was prepared starting from the cyclic a-amino acid (S)-proline80). Recently, similar chiral catalysts and related molybdenum complexes involving optically active N-alkyl-P-aminoalcohols as stable chiral ligands and acetylacetone as a replaceable bidentate ligand, were designed for the epoxidation of allylic alcohols with alkyl hydroperoxides which could be catalyzed by such metal complexes 8,). 

In seven-coordinate molybdenum peroxo complexes of the type Mo(0)(02)2L2, there is the problem of dissociation of the complex in the case of monodentate ligands like pyridine, HMPA, DMF or H20. Bidentate ligands like 2,2 -bipyridine, on the other hand, tend to give insoluble complexes. A solution to this problem was presented by Thiel in 1997 and 1998282 283. jjis group utilized the Mo-diperoxo complex 160 bearing a pyrazolylpyridine ligand. 

Bis[dw-butyLphosphano] tellurium replaces norbornadiene, cycloheptatriene, acetonitrile, and dimethyl(methylene)oxosulfurane in chromium, molybdenum, and tungsten complexes. The P —Te compound acts as a bidentate ligand with the two phosphorus atoms coordinated to the metal atom1. The tetracarbonylchromium complex [R = CH(CH3)J can be recrystallized without loss of tellurium2. 

These complexes are excellent models for theoretical studies. The octacyano complexes of molybdenum and tungsten are stable and inert toward substitution reactions and therefore very suitable for theoretical studies of redox reactions and application of the Marcus theory. The photoreactivity of these systems is also proving to be important. The 0X0- and nitridocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) are very good candidates for kinetic studies of substitution reactions with both mono- and bidentate ligands and are of interest especially in view of the large variations in the observed reactivity. 

The reaction of [MoO(H20)(CN)4] with 1,10-phenanthroline (211) and 2,2 -dipyridyl (215) has also been studied by means of reaction kinetics. It is clear that there are still uncertainties and questions regarding the mechanism of the reaction of these bidentate ligands For example, why is the reaction of the molybdenum complex faster at high pH values at which there is less of the aqua and more of the hydroxo) complex in solution (211) Scheme 5 was proposed for reaction with 2,2 -bipyridyl (215). 

Note that these compounds are not enantiomers, but true diastereomers with different properties, and they may be separated by fractional crystaiiization. The asymmetric carbon atom has an 5 configuration in both diastereomers, but the chirality about the molybdenum atom is different. Thus the asymmetric carbon aids in the resolution of the molybdenum center, but its presence is not necessary for the complex to be chiral. It is merely necessary for the Schiff base to be unsymmetric, i.e., have one pyridine nitrogen and one imino nitrogen. If the bidentate ligand had been ethylenediamine, bipyridine, or the oxalate km. there would have been a mirror plane and no ctuali at the molybdenum. 

Early transition metals (Ti, V, Cr triads) often coordinate several peroxo species, leaving the metal in formally a very high oxidatipn state (e.g., Cr(02)4 , a Cr ion). The M—O2 links have more ionic character, with the 17-peroxo groups acting as bidentate ligands. Titanium and molybdenum(II) porphyrins bind, respectively, one and two dioxygen molecules in this manner. " ... 

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