Sulfide Oxidation by 2,6-Bis[hydroxyl(methyl)amino]-4-morpholino-1,3,5-triazinatodioxomolybdenum(VI): Mechanistic Implications with DFT Calculations for a New Class of Molybdenum(VI) Complex.

Sulfide oxidation is accomplished by a new class of dioxomolybdenum(VI) catalyst ( 1 ) that uses the tridentate 2,6-bis[hydroxyl(methyl)amino]-4-morpholino-1,3,5-triazine ligand to form a five-coordinate molybdenum(VI) center. Resonance Raman spectra show that the dioxo groups on the Mo(VI) oxygens readily exchange with water in an acetonitrile media that allows ¹⁸ O labeling of catalyst 1 . The model oxidation reaction was the conversion of thioanisole ( 2 ) to the corresponding sulfoxide with 4% of 1 using an equimolar amount of H ₂ O ₂ in MeCN- d ₃ . Oxygen-18 labeling experiments with either ¹⁸ O-labeled 1 or ¹⁸ O-labeled H ₂ O ₂ are consistent with a sulfide oxygenation pathway that uses a η ¹ -Mo(OOH) hydroxoperoxyl species ( 3 ). The hypothesized intermediate 3 is initially formed in a proton transfer reaction between 1 and H ₂ O ₂ . Oxidation is hypothesized via nucleophilic attack of the sulfide on 3 that is supported from a Hammett linear free-energy relationship for para-derivatives of 2 . A Hammett reactivity constant (ρ) of -1.2 ± 0.2 was obtained, which is consistent with other ρ values found in prior sulfide oxidation reactions by group 6 complexes. An Eyring plot of the 2 oxidation by 1 gives an E _a of 63.0 ± 5.2 kJ/mol, which is slightly higher than that of a similar oxidation of 2 by the molybdenum(VI) complex, oxodiperoxo (pyridine-2-carboxylato)molybdate(VI) bis(pyridine-2-carboxylic acid) monohydrate ( 5 ). Computational modeling with density functional theory (DFT) of the complete reaction profile gave enthalpy and entropy of activations (64 kJ/mol and -120 J/mol·K, respectively) within 1 standard deviation of the experimental values, further supporting the hypothesized mechanism.