An oxidorhenium(v) complex with an electron-withdrawing ligand: benefits and drawbacks for a dual role catalyst

Abstract

One very unique feature of oxidorhenium(V) complexes is their dual catalytic activity in both reduction of stable oxyanions like perchlorate ClO₄⁻ and nitrate NO₃⁻ as well as epoxidation of olefins. In our ongoing research efforts, we were interested to study how an electron-withdrawing ligand would affect both these catalytic reactions. Hence, we synthesized the novel bidentate dimethyloxazoline-dichlorophenol ligand HL1 and synthesized oxidorhenium(V) complex [ReOCl(L1)₂] (1). Then, catalytic experiments were conducted showing that non-redox epoxidation activity is indeed enhanced, but redox catalysis via oxygen atom transfer (OAT) activity was reduced for ClO₄⁻ and NO₃⁻ reductions. From one nitrate reduction experiment, a small amount of the singly-oxidized dioxidorhenium(VI) complex [ReO₂(L1)] (2) could be isolated, confirming the successful reduction sequence of nitrate to nitrite NO₂⁻ (2e⁻ reduction) to NO (1e⁻ reduction). Furthermore, ligand L1 displayed a richer than usually observed coordination chemistry, allowing for the isolation of complexes [ReOCl₂(SMe₂)(L1)] (trans-3a), [ReOCl₂(OPPh₃)(L1)] (3b) and [ReCl₃(OPPh₃)(L1)] (3c). Complexes 1 and 3a-b were tested in cyclooctene epoxidation, 1 was additionally investigated as an oxyanion reduction catalyst of perchlorate and nitrate. All compounds HL1, 1, 2 and 3a–c could be characterized by single-crystal X-ray diffraction, besides other routine analyses.