Homoleptic and heteroleptic diorganoselenides containing pyrazole functionalities. Synthesis, characterization, and antioxidant activity

The diorganoselenides [2-(R2NCH2)C6H4](pzCH(2)CH(2))Se [R = Me (1), Et (2), R-2 = O(CH2CH2)(2) (3); pz = pyrazole] and [2-{(CH2O)(2)CH}C6H4](pzCH(2)CH(2))Se (4) were prepared by reacting pzCH(2)CH(2)Br with the corresponding alkali metal organoselenolate. Compound [2-(O=CH)C6H4](pzCH(2)CH(2))Se (5) was obtained by deprotection of compound 4 under acidic conditions, whereas compound (pzCH(2)CH(2))(2)Se (6) was prepared by reacting pzCH(2)CH(2)Br with Na2Se. The antioxidant activity of compounds 1-3 and 6 in the catalytic reaction between H2O2 and PhSH was investigated by a combination of experimental methods (multinuclear NMR, UV-vis spectroscopy, cyclic voltammetry) and theoretical calculations. The formation of the intermediate selenoxide species 1a-6a was evidenced by multinuclear NMR experiments (H-1, C-13{H-1}, Se-77{H-1}) that revealed the existence of intramolecular N -> Se interactions. Density functional theory (DFT) calculations showed that these interactions are mostly covalent in nature. For compound 1, an autocatalytic mechanism is proposed.