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.

Synthesis, characterization and application of oxovanadium(iv) complexes with [NNO] donor ligands: X-ray structures of their corresponding dioxovanadium(v) complexes

“…77 Band appearing at 402 nm is attributed to the LMCT transitions originating from the p orbital of phenolate oxygen to the empty d orbital of vanadium(IV) center. 78 Moreover, d-d transition in complex 1 is apparent (500 and 660 nm) when the concentration of 1 was increased to 3.0 Â 10 À3 M. 79 Similarly, UV-visible spectrum of 2 (Fig. S6, † le) exhibits a band at 349 nm corresponding to p-p* transition of azomethine chromophore.…”

“…80,81 The intense band observed at 402 nm can be attributed to the charge transfer transition (LMCT) from the p orbital of the phenolate oxygen to empty d orbital of vanadium(IV). 78 However, the bands due to d-d transitions could not be distinguished properly as they are probably under the tail of much stronger LMCT band at ca. 465 nm (shoulder).…”

Heterogeneous vanadium Schiff base complexes in catalytic oxidation reactions

“…34 More importantly, vanadium complexes coordination chemistry has been explored due to the structural features in polynuclear and mononuclear complexes and their catalytic use for epoxidation of olefins and oxidation of alcohol and sulfide. 35 Vanadium is an essential component of a catalyst that participates in different chemical processes such as olefin epoxidation, toluene to benzonitrile, oxidation of propylene and propane to oxidation of propane. Also, they are applied in the field of catalysis with many reviews and research paper reports.…”

A bimetallic synergistic effect on the atomic scale of defect-enriched NiV-layered double hydroxide nanosheets for electrochemical phenol hydroxylation

“…The features in Csp 2 –H bond activation from a series of V-based catalysts have been explored because of the unsaturated 3d orbitals and strong coordination effects. 8–12 Vanadium complexes were explored as catalysts for the oxidative bromination of phenol using 30% H 2 O 2 as an oxidant. 10 In these processes, the catalysts exhibited poor stability using H 2 O 2 as an oxygen donor, which increased their economic cost and hindered their further improvement and practical application.…”

“…8–12 Vanadium complexes were explored as catalysts for the oxidative bromination of phenol using 30% H 2 O 2 as an oxidant. 10 In these processes, the catalysts exhibited poor stability using H 2 O 2 as an oxygen donor, which increased their economic cost and hindered their further improvement and practical application. Li et al utilized BiVO 4 as a photoanode for extracting phenol derivatives, confirming that vanadium sites were electrocatalytically active, which precluded the need for a redox mediator in solution.…”