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

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, H(2)dhap-inh (I) and H(2)dhap-nah (II), respectively. These ligands react with [(VO)-O-IV(acac)(2)] (Hacac=acetylacetone) in 1:1 molar ratio to give the corresponding neutral dioxidovanadium(V) complexes, [(VO2)-O-V(Hdhap-inh)] (1) and [(VO2)-O-V(Hdhap-nah)] (2), after overnight aerial oxidation in the presence of K2CO3. In the absence of K2CO3, they result in (mu-O){bisoxidovanadium(V)} complexes, [{(VO)-O-V(dhap-inh)}(2)(mu-O)] (3) and [{(VO)-O-V(dhap-nah)}(2)(mu-O)] (4). Treatment of neutral dioxidovanadium(V) complexes, 1 and 2, with H2O2 yields oxidoperoxidovanadium(V) complexes K[(VO)-O-V(O-2)(dhap-inh)(H2O)] (5) and K[(VO)-O-V(O-2)(dhap-nah)(H2O)] (6). The ligands also react with [(MoO2)-O-VI(acac)(2)] in 1:1 molar ratio to form [(MoO2)-O-VI(dhap-inh)](n) (7) and [(MoO2)-O-VI(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, [(MoO)-O-VI(O-2)(dhap-inh)(MeOH)] (9) and [(MoO)-O-VI(O-2)(dhap-nah)(MeOH)] (10), respectively. All synthesized complexes are characterized by elemental analysis, thermogravimetric, electrochemical, spectroscopic (IR, UV-Vis, H-1, C-13 and V-51 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.