Titanium(IV) Alkoxide-Carbamate Complexes: Synthesis and Catalytic Potential in H2O2-Oxidation of Organic Sulfides

Novel titanium alkoxide-carbamate complexes, i. e. [Ti(OiPr)2(O2CNR2)2]m (R=Et, 1; R=iPr, 2; R=Bn=CH2Ph, 3), were synthesized in 87–94 % yields by using two distinct synthetic approaches, i. e. the reaction of [Ti(OiPr)4] with [Ti(O2CNR)4] (1 : 1 molar ratio) in dichloromethane solution, and the addition of [Ti(OiPr)4] to amine/CO2 mixture in toluene (amine/Ti ratio=2). The liquid products 1–3 were characterized by elemental analysis, IR and NMR spectroscopy, and the structure of 1 was optimized by DFT calculations. Combined, experimental and theoretical outcomes suggest a trinuclear structure for 1–3, with the carbamates acting as bridging bidentate ligands. Compound 2 revealed the most effective of the series in promoting the oxidation of a series of organic sulfides into the corresponding sulfones, working generally better than 1 and 3 and the homoleptic counterparts [Ti(OiPr)4] or [Ti(O2CNEt2)4]. However, [Ti(OiPr)4] exhibited the best performance in the conversion of tetrahydrothiophene to tetrahydrothiophene 1,1-dioxide, instead [Ti(O2CNEt2)4] was the most suitable catalyst for the mono-oxygenation of dibenzylsulfide. NMR and DFT studies indicate that mononuclear bis-methanol adducts, bearing the solvent molecules trans-located with respect to the isopropoxides, are plausible active species in the catalytic cycle.