Oxygen atom transfer between DMSO and benzoin catalyzed by cis-dioxidomolybdenum(vi) complexes of tetradentate Mannich bases

Abstract

The reaction of dibasic tetradentate ONNO donor Mannich bases derived from ethylenediamine and 2,4-di-tert-butylphenol (H₂L¹I), 2,4-di-methylphenol (H₂L²II), 2-tert-butyl-4-methylphenol (H₂L³III), 2,4-di-chlorophenol (H₂L⁴IV) and 2-naphthol (H₂L⁵V) with [Mo^VIO₂(acac)₂] (Hacac = acetylacetone) in a 1 : 1 molar ratio in refluxing MeOH gave the corresponding cis-dioxidomolybdenum(VI) complexes [Mo^VIO₂(L¹)] 1, [Mo^VIO₂(L²)] 2, [Mo^VIO₂(L³)] 3, [Mo^VIO₂(L⁴)] 4, and [Mo^VIO₂(L⁵)] 5, respectively. All complexes were characterized by elemental analysis, various spectroscopic (FT-IR, UV/Vis, ¹H and ¹³C NMR) techniques and single-crystal X-ray analysis (of 1, 2, 3 and 5). These complexes adopt a distorted six-coordinated octahedral geometry where the ligands act in a tetradentate manner, coordinating through the two O_phenolate and two N_amine atoms in a cis-α type binding mode involving coordination with one of the N_amine atoms in the apical position and one O_oxido terminal oxygen atom in the equatorial position. These complexes catalyze oxygen atom transfer between benzoin and dimethyl sulfoxide (DMSO) in acetonitrile at 80 °C. The formation of benzil could easily be monitored by HPLC. The electronic effect caused by the para substituent and the steric effect caused by the ortho substituent influence the formation of benzil and therefore the conversion varied between 66 and 99% in 18 h of reaction time under optimized conditions; the most active catalyst was [Mo^VIO₂(L⁵)]. A similar trend has also been obtained with 4-chlorobenzoin. The pseudo first order rate constant for [Mo^VIO₂(L⁵)] was found to be 0.0998 h⁻¹. During catalytic reaction, the formation of the binuclear intermediate and its fast decay into the initial dioxidomolybdenum(VI) complex were established by time dependent UV/Vis studies.