Octahedral Bipyridine and Bipyrimidine Dioxomolybdenum(VI) Complexes: characterization, Application in Catalytic Epoxidation, and Density Functonal Mechanistic Study

摘要

Complexes of the general formula [MoO_2X_2L_2] (X = Cl, Br, Me; L_2 = bipy, bpym) have been prepared and fully characterized, including X-ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO_2Cl_2(4,4'-bis(hexyl)-2,2'-bipyridine)] has been synthesized. The reaction of the complexes with tert-butyl hydroperoxide (TBHP) is an equilibrium reaction, and leads to Mo~(VI) #eta#~1-alkylperoxo complexes that selectively catalyze the epoxidation of olefins. Neither the Mo-X bonds nor the Mo-N bonds are cleaved during this reaction. These experimental results are supported by theoretical calculations, which show that the attack of TBHP at the Mo center through the X-O-N face is energetically favored and the TBHP hydrogen atom is transferred to a terminal oxygen of the Mo=O moiety. After the attck of the olefin on the Mo-bound peroxo oxygen atom, epoxide and tert-bytyl alcohol are formed. The latter compound acts as a competitive inhibitor for the TBHP attack, and leads to a significant reduction in the catalytic activity with increasing reaction time.