Studies on bis(halogeno) dioxomolybdenum(VI)-bipyridine complexes: Synthesis and catalytic activity

Dioxomolybdenum(VI) complexes with the general formula [MoO ₂Cl₂L₂] (L₂=3,3′-dimethyl-2, 2′-bipyridine, 5,5′-dimethyl-2,2′-bipyridine, 6,6′-dimethyl-2,2′-bipyridine, 4,4′-dibromo-2,2′- bipyridine, 5,5′-dibromo-2,2′-bipyridine, 5,5′-diamino-2, 2′-bipyridine; 5,5′-dinitro-2,2′-bipyridine; 5,5′-di-ethoxycarbonyl-2,2′-bipyridine; 6-phenyl-2,2′- bipyridine; 2,2′:6′,2″-terpyridine) have been prepared and characterised. [MoO₂Cl₂(5,5′-di-ethoxycarbonyl-2, 2′-bipyridine)] has been examined by single crystal X-ray analysis. The complexes were applied as homogenous catalysts for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) as oxidising agent. The new compounds show an overall high activity and are highly selective catalysts in the epoxidation of cyclooctene. The stability of the complexes and differences in the catalytic activity can be clearly attributed to electronic contributions of the functional groups on bipyridine ligands and to steric restrictions. DFT calculations have assisted in a better understanding of the stability of the complexes and are in agreement with experiment. The influence of the terminal oxo ligands and the Lewis base ligands on the Mo center keep the compounds on quite a stable level of electron density.