Vanadium(V) and Molybdenum(VI) Complexes Containing ONO Tridentate Schiff Bases and Their Application as Catalysts for Oxidative Bromination of Phenols

Abstract

Condensation of 2,4-dihydroxyacetophenone with isonicotinoyl hydrazide and nicotinoyl hydrazide in an equimolar ratio in methanol results in the formation of two stable ONO donor ligands, H2dhap-inh (I) and H2dhap-nah (II), respectively. These ligands react with [VIVO(acac)2] (Hacac=acetylacetone) in 1:1 molar ratio to give the corresponding neutral dioxidovanadium(V) complexes, [VVO2(Hdhap-inh)] (1) and [VVO2(Hdhap-nah)] (2), after overnight aerial oxidation in the presence of K2CO3. In the absence of K2CO3, they result in (μ-O){bisoxidovanadium(V)} complexes, [{VVO(dhap-inh)}2(μ-O)] (3) and [{VVO(dhap-nah)}2(μ-O)] (4). Treatment of neutral dioxidovanadium(V) complexes, 1 and 2, with H2O2 yields oxidoperoxidovanadium(V) complexes K[VVO(O2)(dhap-inh)(H2O)] (5) and K[VVO(O2)(dhap-nah)(H2O)] (6). The ligands also react with [MoVIO2(acac)2] in 1:1 molar ratio to form [MoVIO2(dhap-inh)]n (7) and [MoVIO2(dhap-nah)]n (8). The reaction of oxidoperoxidomolybdenum(VI) species, generated in situ by the reaction of MoO3 with H2O2 with ligands I and II, gives the oxidoperoxidomolybdenum(VI) complexes, [MoVIO(O2)(dhap-inh)(MeOH)] (9) and [MoVIO(O2)(dhap-nah)(MeOH)] (10), respectively. All synthesized complexes are characterized by elemental analysis, thermogravimetric, electrochemical, spectroscopic (IR, UV–Vis, 1H, 13C and 51V NMR) and single crystal X-ray diffraction (for 1, 2 and 8). Complexes are explored as catalysts for the oxidative bromination of phenol, a model oxidative halogenation reaction, in the presence of HClO4 and KBr, using 30% H2O2 as oxidant. Under optimized reaction conditions, the product selectivity follows the order: 4-bromophenol>2-bromophenol, both for molybdenum and vanadium complexes. Products characterized by GC analysis showed very good conversion of phenol. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim