Oxidoperoxidomolybdenum(vi) complexes with acylpyrazolonate ligands: synthesis, structure and catalytic properties

Oxidoperoxido-molybdenum(vi) complexes containing acylpyrazolonate ligands were obtained by reaction of [Mo(O)(O)₂(H₂O)_n] with the corresponding acylpyrazolone compounds HQ^R. Complexes Ph₄P[Mo(O)(O₂)₂(Q^R)] (R = neopentyl, 1; perfluoroethyl, 2; hexyl, 3; phenyl, 4; naphthyl, 5; methyl, 6; cyclohexyl, 7; ethylcyclopentyl, 8) were obtained if the reaction was carried out with one equivalent of HQ^R in the presence of Ph₄PCl. Alternatively, neutral complexes [Mo(O)(O₂)(Q^R)₂] (R = neopentyl, 9; hexyl, 10; cyclohexyl, 11) were formed when two equivalents of HQ^R were used in the reaction. These complexes were isolated in good yields as yellow or yellow-orange crystalline solids and were spectroscopically (IR, ¹H, ¹³C{¹H} and ³¹P{¹H} NMR), theoretically (DFT) and structurally characterised (X-ray for 1, 2, 9 and 10). Compounds 1 and 9 were selected to investigate their catalytic behaviour in epoxidation of selected alkenes and oxidation of selected sulphides, while 10 and 11 were tested as catalyst precursors in the deoxygenation of selected epoxide substrates to alkenes, using PPh₃ as the oxygen-acceptor. Complexes Ph₄P[Mo(O)(O₂)₂(Q^R)] were shown to be poor catalyst precursors in oxidation reactions, while the activity of [Mo(O)(O₂)(Q^R)₂] species is good in all the studied reactions and comparable to related oxidoperoxido-molybdenum(vi) complexes. Complex [Mo(O)₂(Q^C6)₂], 12, was obtained by treatment of 10 with one equivalent of PPh₃, demonstrating that the first step in the epoxide deoxygenation mechanism was the oxygen atom transfer toward the phosphane.