SYNTHESIS, REACTIVITY AND CATALYTIC ACTIVITY OF VANADIUM AND MOLYBDENUM COMPLEXES

Abstract

Vanadium is 20th most abundant ductile transition metal. It is a vital element for certain bio organisms like fungi, bacteria and tunicates. Vanadium has been identified in naturally occurring vanadium-dependent haloperoxidase enzymes, Ascophyllum nodosum, Corallina officinalis and Curvularia inequalis. Among oxidation sates varying from –III to +V, the oxidation states +IV and +V are the most stable ones at the confined active sites of the enzymes. Therefore, in order to mimic the activity of these enzymes, several bioinspired oxidovandium(IV & V) complexes have been studied. Molybdenum is also a micronutrient essential for life. It is found in oxidation states ranging from –II to +VI in chemical compounds. Higher oxidation states are more relevant in biological systems. Molybdenum presents in many enzymes like xanthine oxidase, aldehyde oxidase and sulphite oxidase in various oxido-forms. These Mo containing oxido species are capable of transferring their oxido ligand to organic substrates. While both metal ions are normally stabilized with at least one M=O (M = Mo or V) bond, molybdenum generally forms stable complexes in +V and +VI oxidation states and vanadium prefers +IV and +V oxidation states. Complexes of these metal ions exist in different nuclearity and their syntheses are mostly governed by the structure of the ligand, starting metal precursor and solvent used for the syntiesis. The coordination chemistry, biological activity and catalytic applications (homogeneous as well as heterogeneous) of these metal ions have also been studied extensively in these stable oxidation states. While going though literature, it has been observed that mononuclear and binuclear complexes of these metal ions are very common while trinuclear complexes with tricompartmental structures are less studied. Therefore, it is resonable to undertake the systematic study on the chemistry of vanadium and molybdenum of having tricompartmental open structures and explore their catalytic potential. The complete thesis has been divided into the following chapters. First chapter is introductory one, containing their occurrence in nature and biological systems. Updated literature based on model complexes as well as general coordination chemistry

and applications of vanadium and molybdenum complexes as potential catalysts have also been included. Chapter 2 describes the three tridentate ONO sites each having dibasic Schiff bases. These ligands were obtained from the condensation of the triketone 2,4,6-triacetylphloroglucinol (H3ptk) with four different hydrazides, benzoyl hydrazide (bhz), furoyl hydrazide (fah), isonicotinoyl hydrazide (inh) and nicotinoyl hydrazide (nah): H6ptk(bhz)3 (2.I), H6ptk(fah)3 (2.II) H6ptk(inh)3, (2.III) and H6ptk(nah)3 (2.IV). These ligand precursors, each tris(ONO) donors, are tricompartmental building blocks able to form trinuclear complexes having C3 symmetry. Reaction of 2.I-2.IV with [VIVO(acac)2] lead to the formation of [{VIVO(H2O)}3(ptk(bhz)3)] (2.1), [{VIVO(H2O)}3(ptk(fah)3)] (2.2), [{VIVO(H2O)}3(ptk(inh)3)] (2.3), [{VIVO(H2O)}3(ptk(nah)3)] (2.4). In methanol / aqueous solutions of M2CO3 (M+ = Na+, K+ and Cs+) these complexes are slowly converted into dioxidovanadium(V) compounds, namely: [M3(VVO2)3{ptk(bhz)3}(H2O)6] [M+ = K+ (2.5), Na+ (2.9), Cs+ (2.13)], M3[(VVO2)3{ptk(fah)3}]6H2O [M+ = K+ (2.6), Na+ (2.10), Cs+ (2.14)], M3[(VVO2)3{ptk(inh)3}]6H2O [M+ = K+ (2.7), Na+ (2.11), Cs+ (2.15)] and M3[(VVO2)3{ptk(nah)3}]6H2O [M+ = K+ (2.8), Na+ (2.12), Cs+ (2.16)]. All ligand precursors and complexes are characterized by various techniques: FT-IR, UV/Visible, EPR, NMR (1H, 13C and 51V), elemental analysis, thermal studies, cyclic voltammetry (CV) and single-crystal X-ray analysis. X-ray diffraction studies of complexes K2.7[{(VVO2)3ptk(fah)3}]·11.5H2O·MeOH (2.6a), Cs3[{(VVO2)3ptk(bhz)3}]·7H2O (2.13a) and Cs3[{(VVO2)3ptk(nah)3}]·7.3H2O (2.16a) reveal their distorted square pyramidal geometry by coordinating through phenolate oxygen (of ptk), azomethine nitrogen and enolate oxygen (of hydrazide) atoms. The reactivity of the complexes (2.5-2.16) and their catalytic potential were screened towards their peroxidase mimetic activity in the oxidation of dopamine to aminochrome driven by H2O2 as an oxidant. The conversion of dopamine to aminochrome with different catalysts was monitored by HPLC showing high activity under mild conditions with good conversions within 1 h. Kinetic studies using compounds (2.13 2.16) as catalyst precursors reveal that the reaction follows a Michaelis-Menten like kinetics. This chapter has been published and has reference. Dalton Trans., 2020, 49, 2589–2609. Four trinuclear dioxidomolybdenum(VI) complexes, [{MoVIO2(H2O)}3ptk(bhz)3] (3.1), [{MoVIO2(H2O)}3ptk(fah)3] (3.2), [{MoVIO2(H2O)}3ptk(inh)3] (3.3), and [{MoVIO2(H2O)}3ptk(nah)3] (3.4), based on the tritopic central 2,4,6-triacetylphloroglucinol

(H3ptk) based ligands, H6ptk(bhz)3 (2.I), H6ptk(fah)3 (2.II), H6ptk(inh)3 (2.III) and H6ptk(nah)3 (2.IV) (Hbhz = benzoylhydrazide, Hfah = 2-furanoylhydrazide, Hinh = isonicotinoylhydrazide and Hnah = nicotinoylhydrazide), respectively, are reported in Chapter 3. All of the synthesized ligands, as well as their complexes, have been characterized by elemental, thermal and electrochemical analyses, spectroscopic techniques (FT-IR, UV-Vis, 1H and 13C NMR), and single crystal X-ray studies of [{MoVIO2(H2O)}{MoVIO2(MeOH)}2ptk(bhz)3(H2O)]·2H2O·1.25MeOH (3.1a) and [{MoVIO2(EtOH)}3ptk(fah)3]·3EtOH (3.2a). Each pocket of the ligands acts as a dibasic tridentate, coordinating through two oxygen atoms and one nitrogen atom to each metal center. Due to the presence of tridentate binding pockets in ligands, each metal center conserves its octahedral structure by coordinating with water molecule in synthesized complexes or by other solvent(s) in the crystal structures. These complexes were evaluated for the epoxidation of terminal and internal alkenes in the presence of H2O2 using NaHCO3 as promoter. Under the optimized reaction conditions, all alkenes resulted in the formation of corresponding epoxide selectively in good yield and high turnover number. This chapter has been published and has reference. Eur. J. Inorg. Chem., 2018, 2952–2964. Chapter 4 presents trinuclear cis-dioxidomolybdenum(VI) [{MoVIO2(H2O)}3L1] (4.1), [{MoVIO2(H2O)}L2] (4.2), complexes, [{MoVIO2(H2O)}L3] (4.3), [{MoVIO2(H2O)}3L4] (4.4), [{MoVIO2(H2O)}3L5] (4.5), [{MoVIO2(H2O)}3L6] (4.6), and [{MoVIO2(H2O)}3 L6] (4.7) with tris(H2ONO) donor ligands [H6 L1 (4.I), H6 L2 (4.II), H6 L3 (4.III), H6L4 (4.IV), H6L5 (4.5), H6L6 (4.VI), and H6L7 (4.VII)] prepared from benzene-1,3,5 tricarbohydrazide (bthz) and corresponding salicylaldehyde (sal). All ligands and complexes were characterized by various spectroscopic techniques like FT-IR, UV-visible, NMR (1H and 13C) spectroscopy, electrochemical study, elemental analysis, thermogravimetry study and single crystal X-ray diffraction [for ligand 4.III, and complexes 4.1 and 4.5]. Complexes characterized by X-ray diffraction study and further supported by DFT in the solid-state show that each metal center in the trinuclear system has a distorted octahedral geometry. Cyclic voltammetric (CV) along with differential pulse voltametric (DPV) confirms that both the systems (i.e. ligands as well as complexes) show irreversible redox behavior in nature in DMF. In the presence of hydrogen peroxide as oxidant, these complexes show excellent catalytic potential towards the one-pot three components (ethyl acetoacetate, benzaldehyde (or its derivatives) and ammonium acetate) dynamic covalent assembly, via Hantzsch reaction. Under solvent free conditions as high as 98% iii Ph.D. Thesis

conversion along with 100% selectivity towards diethyl 2,6-dimethyl-4-phenyl-1,4 dihydropyridine-3,5-dicarboxylate (1,4-DHP) has been achieved in 1 h. Though solvent does not improve the conversion, they do influence on the selectivity of the products. However, with the elapse of time the conversion of dihydropyridine to diethyl 2,6-dimethyl-4-phenylpyridine-3,5 dicarboxylate derivative occurs and complete in ca. 10 h with distinct color change, showing the importance of the catalysts. A suitable reaction pathway for the catalytic reaction has been provided based on spectroscopic and DFT studies. This chapter has been published and has reference. Polyhedron, 2020, 186, 114617.