Synthesis, structure studies and electrochemistry of molybdenum(VI) Schiff base complexes in the presence of different donor solvent molecules

The reactions between bis(acetylacetonato)dioxomolybdenum(VI) and Schiff base ligands derived from 5-chlorosalicylaldehyde or 3-ethoxy-salicylaldehye, and 3-methoxy-benzoic hydrazide (m-anisic hydrazide), 2-furoic hydrazide or 2,4-dihydroxy-benzoic hydrazide in the presence of donor solvents yielded cis-dioxomolybdenum(VI) complexes with the general formula MoO2L(D), where L = tridentate Schiff base ligand and D = dimethylsulfoxide, hexamethylphosphoramide, dimethylformamide, imidazole or methanol. The complexes were characterized by elemental analysis, electronic spectra, IR, 1H and 13C NMR spectroscopies, thermogravimetric analysis, cyclic voltammetry, and the molecular structures of five of the dioxomolybdenum complexes were elucidated by single crystal X-ray diffractometion studies. In general, the complexes adopt an octahedral environment around the Mo center with a cis-oxo configuration. The other coordination sites are occupied by the imino nitrogen, phenoxyl oxygen, hydroxyl oxygen of the tridentate Schiff base and the donor atom of the solvent molecule. The structural data revealed that the labile coordination site, which is occupied by N or O atoms from the donor solvents, has a longer Mo–O or Mo–N bond distance.

Oxo-molybdenum chemistry is of great interest due to the presence of such units in the active sites of many enzymes, such as nitrogenase, aldehyde oxidase, xanthine oxidase, sulfite oxidase, nitrate reductase and xanthine dehydrogenate. Dioxomolybdenum complexes also play a pivotal role as catalysts in many important industrial processes, such as the oxidation of alcohols, olefin epoxidation and olefin metathesis. This work describes the synthesis, electrochemistry and structural features of a number of mononuclear dioxomolybdenum complexes containing dibasic tridentate Schiff base ligands. The synthesis and structures of dinuclear and polynuclear dioxomolybdenum complexes as well as the structure–catalytic activity relationship of these dioxomolybdenum complexes will be reported in subsequent research work.Figure optionsDownload as PowerPoint slideHighlights
► Several new cis-dioxomolybdenum(VI) complexes have been prepared.
► The complexes have been characterized by various spectroscopy methods and X-ray diffractometry methods.
► All the complexes show irreversible redox behavior.