FACILE SYNTHESIS OF VANADIUM, MANGANESE AND MOLYBDENUM PORPHYRIN COMPLEXES AND THEIR CATALYTIC APPLICATIONS

Abstract

Metalloporphyrins play a tremendous role in various biological processes including photosynthesis reaction in chlorophyll, oxygen transfer and storage in heme, oxidation reactions by cytochrome P450, and many more. Metalloporphyrins are bioinspired catalysts, and their activities inspire scientists to design and develop novel metalloporphyrins for various aspects. Synthetic porphyrin and metalloporphyrin have vast applications in various fields such as dye-sensitized solar cell (DSSC), nonlinear optics (NLO), photodynamic therapy (PDT), sensing, biomedical in various diseases, bioinspired catalysis etc. The bioinspired catalytic reactions shown by non-porphyrin containing as well as heme-containing haloperoxidases and related functional models for the synthesis of halogenated materials are found to be “greener” and “sustainable” since it requires KBr as the source of bromine along with hydrogen peroxide (green reagent) as oxidant. Though metalloporphyrins have strong catalytic potentials in organic transformations, not much effort has been made for the type of organic transformations by synthetic metalloporphyrins. Thesis has been divided in seven chapters: Chapter 1 deals with the general introduction of tetrapyrrolic pigments and the literature on porphyrins. This chapter also included recent synthetic developments and applications of metalloporphyrins (mainly vanadium, manganese and molybdenum-based porphyrins) in the field of catalysis in various organic transformations. Chapter 2 includes the synthesis of β-octabromo-meso-tetraphenylporphyrin, [VIVO(TPPBr8)] (2.2) by self-catalytic oxidative bromination of [VIVO(TPP)] (2.1) in excellent yield at 25 °C and confirmed by X-ray structure. Remarkably, 2.2 also mimics the vanadium bromoperoxidase (VBPO) enzyme for the oxidative bromination of phenol derivatives in water with very high TOF values (22.7–29.1 s–1) and its recyclability. Further, 2.2 was utilized for the selective epoxidation of various olefins in excellent yields with high TOF values (2.4–3.6 s–1). We have also carried out oxidative bromination of phenol derivatives and epoxidation reaction of olefins using [VIVO(TPP)] (2.1) as a catalyst under similar reaction conditions. It was found that 2.2 shows superior catalytic performance as compared to 2.1 due to its robust structure and electron-withdrawing bromo substituents. Chapter 3 includes the synthesis of 2,3,12,13-tetracyano-5,10,15,20-tetraphenylporphyrinatooxidovanadium(IV) {[VIVO(TPP(CN)4)], 3.2} by the nucleophilic substitution of β-bromo groups of the corresponding 2,3,12,13-tetrabromo-5,10,15,20- tetraphenyl-porphyrinatooxidovanadium(IV) {[VIVO(TPPBr4)], 3.1} using CuCN in quinoline. Both complexes show biomimetic catalytic activity similar to enzymes haloperoxidases and efficiently brominate various phenol derivatives in the presence of KBr, H2O2 and HClO4 in the aqueous medium. Among the two complexes, 3.2 exhibits excellent catalytic activity with high turnover frequency (35.5–43.3 s–1) due to the strong electron-withdrawing nature of the cyano groups attached at β-positions and its moderate nonplanar structure as compared to 2.1 (TOF = 22.1–27.4 s–1). Notably, this is the highest turnover frequency value observed for any porphyrin system. The selective epoxidation of various terminal alkenes using complex 3.2 has also been carried out and the results are good specifying the importance of electron-withdrawing cyano groups. Catalysts 3.1 and 3.2 are recyclable through the corresponding stable [VVO(OH)(TPPBr4)] [VVO(OH)(TPP(CN)4)] intermediate species, respectively, through which catalytic activity proceeds. Chapter 4 includes a sustainable approach for the synthesis of β-brominated Mn porphyrins, [MnIII(Br)(TPPBr4)] (4.2), [MnIII(Br)(TPPBr6)] (4.3) and [MnIV(Br)2(TPPBr8)] (4.4) by self-catalytic haloperoxidases mimicking the activity of [MnIII(Br)(TPP)] [bromo (meso-tetraphenylporphyrinato)manganese(III)] (4.1) in aqueous medium under different mild and controlled reaction conditions. These polybrominated porphyrin complexes have been synthesized by precisely tweaking important parameters (e.g. amount of catalyst, H2O2, HClO4 and KBr). This method is safer and applicable under milder reaction conditions than the conventional procedures for β-bromination of porphyrins. These complexes are characterized by various spectroscopic techniques, including UV–Visible spectroscopy, elemental analysis, MALDI–TOF mass spectrometry, cyclic voltammetry, DFT calculations and single crystal X-ray diffraction analysis of 4.2, 4.3 and 4.4. Complex 4.4 is the first example of the trans-(Br–MnIV–Br) core. These complexes have been explored as catalysts for the bromination of various phenol derivatives, a functional model of haloperoxidases, using H2O2 as an oxidant and KBr as a brominating agent in a mild acidic aqueous medium at room temperature. The relative order of reactivity depends on the steric hindrance due to the β-brominated group and thus follows the order: 4.2 > 4.1 > 4.3 > 4.4. After identifying the reaction intermediates using mass spectrometry, a suitable catalytic reaction mechanism has been proposed for the bromination of substrates. Chapter 5 includes a systematic analysis of the effect of para-substituents (H, Cl, Br and OMe) on the meso-phenyl group in oxidovanadium(IV)-R-tetraphenylporphyrins ([VIVO(TPP)] (R = H, 5.1), [VIVO(TCPP)] (R = Cl, 5.2), [VIVO(TBPP)] (R = Br, 5.3) and [VIVO(TMPP)] (R = OMe, 5.4)) on their properties and catalytic oxygen atom transfer (OAT) for oxidation of benzoin to benzil using DMSO as well as 30% aqueous H2O2 as the sacrificial oxygen source.Electrochemical and theoretical (DFT, density functional theory) study is in good agreement with the influence of these substituents on the catalytic property of these complexes. Complex [VIVO(TCPP)], 5.2 displayed the best catalytic activity for the conversion (92%) of benzoin to benzil in 30 h with > 99% product selectivity if DMSO was used as an oxygen source, whereas excellent conversion ~ 100%) of benzoin to benzil was noticed in 18 h with 95% product selectivity when 30% aqueous H2O2 was used as a source of oxygen. Experimental and simulated EPR studies confirmed the +4 oxidation of vanadium in these complexes. The structure of 5.2, 5.3 and 5.4, confirmed by single crystal X-ray diffraction method, are domed shape and the displacement of VIV ion from the mean porphyrin plane follows the order: 5.2 (0.458 Å) < 5.3 (0.459 Å) < 5.4 (0.479 Å). To the best of our knowledge, this is the first detailed oxygen atom transfer (OAT) catalytic study using oxidovanadium(IV) porphyrins for oxidizing benzoin to benzil using DMSO as well as aqueous H2O2 as the sacrificial oxygen source. Chapter 6 includes the synthesis of trans-[MoVIO2(TPP)] (trans-dioxidomolybdenum(VI)-meso-tetraphenylporphyrin, 6.1) and trans-[MoVIO2(TCPP)] (trans-dioxidomolybdenum(VI)-meso-tetra(p-chlorophenyl)porphyrin, 6.2) by metalation of free-base porphyrins with Mo(CO)6 in 1,2,4-trichlorobenzene at 190 ℃. Both complexes are utilized as haloperoxidases mimicking oxidative bromination of phenols using KBr, H2O2 and HClO4 as green brominating agents with very high TOF values (up to 16.2 s–1 for catalyst 6.2). The self-catalytic behaviour of 6.1 produce [MoVIO2(TPPBr4)] (6.3) under mild reaction condition using KBr and H2O2 as brominating agents, in the biphasic reaction medium (CH2Cl2:H2O, 2:3, v/v). In addition, complexes 6.1 and 6.2 efficiently catalyze the selective oxidation of 2-methylnaphthalene to produce vitamin K3. This is possibly the first report presenting trans-dioxidomolybdenum(VI) based porphyrin as haloperoxidases mimicking oxidative bromination of phenols (along with self-catalytic behaviour) and selective oxidation of 2-methylnapthalene to Vitamin K3. Chapter 7 includes the summary and all the conclusions based on achievements.