Complexes of cis-dioxomolybdenum(VI) with unsymmetrical tripodal NO < inf > 3 < /inf > -donor ligands: Synthesis, characterization and catalytic applications

Abstract

A series of cis-dioxomolybdenum(VI) complexes of general formula cis-[MoO2(HLn)] (1-4) have been synthesized in 80-85% yields by reacting equimolar amounts of [MoO2(acac)2] (acac- = acetylacetonate) with 2,2′-(2-hydroxy-3,5-R1,R2-benzylazanediyl)diethanols (H3Ln, n = 1-4) in methanol. Characterization of the complexes has been performed by elemental analysis, spectroscopic (IR, UV-Vis, 1H and 13C NMR) and electrochemical measurements. The molecular structures of all four complexes have been determined by single-crystal X-ray diffraction studies. In each of these analogous complexes, the metal centre is in a distorted octahedral NO5 coordination sphere assembled by the single edge shared 5,5,6-membered chelate rings forming NO3-donor (HLn)2- and two cis oriented oxo groups. Crystal structures of the complexes reveal formation of discrete centrosymmetric dimeric species via a pair of reciprocal intermolecular O-HO hydrogen bonding interactions. Spectroscopic data of all the complexes are consistent with their molecular structures. In the cyclic voltammograms, the redox-active complexes display a quasi-reversible to irreversible metal centred reduction with the cathodic peak potential in the range -0.92 to -1.12 V (versus Ag/AgCl). All the complexes have been evaluated for their catalytic activities in oxidative bromination reactions of styrene and salicylaldehyde and in benzoin oxidation reaction.