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 Mo^IVOL (2a) (brown) and Mo^IVOL(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).