A family of mononuclear molybdenum-(VI), and -(IV) oxo complexes with a tridentate (ONO) ligand

Abstract

The salicylhydrazone of anthranilhydrazide (H2L) reacted with MoO2(acac)2 in refluxing alcohols to yield compounds of the general formula MoO2L(ROH) (where R = CH3, C2H5, n-C3H7, n-C4H9). The complex MoO2L(C2H5OH) is found to undergo facile reaction with heterocyclic bases to form MoO2L(Q)-type complexes where Q = pyridine, 4-picoline, imidazole and substituted imidazoles. Single crystal X-ray structural studies on the ligand H2L (1) and the complexes MoO2L(C2H5OH) (2) and MoO2L(Imz) (3) indicate that the free ligand exists in its keto form, but in the two complexes 2 and 3 the ligand coordinates to the MoO22+ core through the enolate oxygen, the phenolate oxygen and the azomethine nitrogen. Reaction of MoO2L(C2H5OH) with PPh3 in CH3CN under dry dinitrogen, in the absence and presence of bipy, produced MoIVOL (2a) (brown) and MoIVOL(bipy) (2b) (green) respectively along with Ph3PO. This reaction is reminiscent of the oxotransfer reaction from the MoO2+2 core of a molybdoenzyme to the substrate PPh3. Complexes 2 and 3 crystallized in the P21/n and P21/c space groups respectively and for both of them Z = 4. The structures clearly show that 2 and 3 have distorted octahedral coordination environments in which the Mo–O(ethanol) bond of 2 and the Mo–N (imidazole) bond of 3 are significantly longer than is usually observed. This is indicative of the weak bonding of ethanol and imidazole to the MoO22+ core and points to the inherent weakness of the sixth coordination position of the coordination polyhedron of the MoO22+ core in MoO2L(C2H5OH) and MoO2L(Imz).