Molybdenum complexes Schiff bases

Interesting results of NMR study of Schiff base molybdenum(VI) ds-dioxo complexes, derivatives of a or p-D-glucosamines [36] have been published by Zhao et al.87... 

The reaction between MoX3 (X = Cl, Br), aniline and 2,6-diacetylpyridine in butanol yields [MoO py(anil)2 X2], where py(anil)2 is a tridentate Schiff base ligand formed in situ.im These complexes are isomorphous with [VO py(anil)2 X2] and, therefore, are considered to have octahedral coordination of the molybdenum. 

Note that these compounds are not enantiomers, but true diastereomers with different properties, and they may be separated by fractional crystallization. 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 bkJentate ligand had been ethylenediamine, bipyridine, or the oxalate ion, there would have been a mirror plane and no duality at the molybdenum. 

Figure 20.9 (a) Protonation and reduction of an immobilized moiybdenum(Vi) nitrido complex 43 to the imido molybdenum(V) complex 44 and (b) immobilized Schiff base vanadyl complex 45. 

Hexacoordinated mixed-ligand dinitrosyl complexes of molybdenum(O) of the composition [Mo(NO)2(L)(OH)] with a variety of Schiff base ligands have been reported [3]. Such complexes are readily obtained by a single-step reaction between molybdate(VI) and the ligand in presence of hydroxylaminehydrochloride in a dimethylformamide solvent. A ds-octahedral structure has been observed in these complexes, as shown in Figure 12. 

Mo(C5Pli4R)02Br, where R = 2,5-dimethoxyphenyl, shows vMo=0 bands at 926 and 895 cm. In Mo 0(02)(TMP), where TMP = 5,10,15,20-tetramesi-tylporphyrinato, vMo=0 is at 912 cm , with Vs/VasMo-02 at 503 and 574 cm i. 3 Resonance Raman data (vMoO, vMoOMo) were used to probe the catalytic mechanism of molybdenum 0x0transferases. " M0O2L complexes, where L Schiff bases prepared by condensation of an aryl hydrazine with ethylaceto-acetate, etc., have two VM0O2 bands due to cis-Mo02 units. ... 

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