Octahedral bipyridine and bipyrimidine dioxomolybdenum(VI) complexes: Characterization, application in catalytic epoxidation, and density functional mechanistic study

Complexes of the general formula [MoO₂X₂L₂] (X = Cl, Br, Me; L₂ = bipy, bpym) have been prepared and fully characterized, including X-ray crystallographic investigations of all six compounds. Additionally, the highly soluble complex [MoO₂Cl₂(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^v1, η¹-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 attack of the olefin on the Mo-bound peroxo oxygen atom, epoxide and tert-butyl 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.