SYNTHESIS, SPECTRAL, ELECTROCHEMICAL AND CATALYTIC STUDIES OF N/S MACROCYCLE COMPLEXES

Abstract

The field of macrocyclic chemistry has undergone enormous expansion during the past few decades. A macrocycle is defined as a cyclic compound with nine or more atoms (including all hetero atoms) in the ring along with three or more donor (ligating) atoms. Several types of macrocyclic ligands containing different ligating atoms or combination there of ie; aza (N), thia (S) or oxa (O) can be modified to accommodate specific metal ions by fine tuning the ligand design features like macrocyclic ring size, type of donor atoms, ring size, degree of saturation, peripheral substitution and availability of vacant coordination sites. Hence, it is not surprising that they are being seen as promising materials with potential applications in diverse areas as MRI contrast media, chemical sensors, bio sensors, biomedical diagnosis, as pigments and dyes and for optical data storage. The macrocyclic complexes show various types of biological activity such as antibiotic, antifungal and antibacterial and are used as radiotherapeutic agents. They also have potential applications as catalysts for various chemical reactions. Catalysts have played an important role in the development of modern chemical technologies. They activate the chemical reaction under relatively mild conditions through the bonding of reactant molecules with definite functional groups called "active sites", where they react and finally detach from the catalyst for the next cycle. Transition metal complexes display good catalytic properties when they are used in pure form or immobilized on solid supports like polymers, alumina, silica, zeolites, etc. as well. The thesis entitled "Synthesis, spectral, electrochemical and catalytic studies ofN/S macrocycle complexes" mainly deals with the synthesis of tetraazamacrocycle and tetrathiamacrocycle complexes and their characterization by various physicochemical techniques. Catalytic activities towards oxidation of various substrates have also been carried out under optimized reaction conditions to achieve maximum conversion of substrates. For convenience the present thesis has been divided in to five chapters. in The first chapter is introductory in nature, and presents a historical view of macrocyclic chemistry as it developed during the last few decades and gives an overview of the structural diversity of their derivatives in that context. All relevant background information is included in this chapter to facilitate easy understanding of this work. A brief introduction to catalysts and heterogenization of homogeneous catalysts has also been described. The literature on immobilized metal complexes and their catalytic applications for various reactions has also been reviewed. The second chapter deals with the synthesis of two new tetraazamacrocycle complexes [Cu{Me4Bzo2[14]aneN4}]Cl2 and [Cu{Me4Bzo2[14]aneN4}](C6H50)2 (where Me4Bzo2[14]aneN4 = 5,7,12,14-tetramethyldibenzo[Z>,/][l,4,8,l 1Jtetraazacyclotetradecane). The tetraazamacrocycle copper complex was synthesized by the demetallation reaction of [Ni{Me4Bzo2[14]aneN4}]Cl2 followed by coordination of the copper(II) ion with the free base macrocycle ligand. The [Cu{Me4Bzo2[14]- aneN4}]Cl2 complex thus obtained was converted to phenoxide derivative, [Cu{Me4Bzo2[14]aneN4}](C6H50)2, by metathesis reaction and both products were characterized using spectral and elemental analyses data. Electronic spectra of the chloro and phenoxo complexes showed a very weak absorption band at Amax 520 and 515 nm, respectively, besides the ligand centered strong absorption bands in the UV region. It is known that copper(II) N4-macrocycle complexes exhibit weak d-d absorption bands in the visible region. The structural parameters of the complex cation [Cu{Me4Bzo2[14]aneN4}]2+ in these complexes, deduced through DFT computations, suggest a relatively rigid framework and a saddle-shaped geometry for the complex in which the copper(II) ion sits off the N4 plane. In the electrochemical investigation the complex [Cu{Me4Bzo2[14]aneN4}](C6H50)2 exhibited a new irreversible oxidation wave, particularly in the presence of traces of water in acetonitrile, due to catalyzed electro-oxidation of the phenoxide counter ions. The [Cu{Me4Bzo2[14]aneN4}]Cl2 was found to catalyze chemical oxidation of phenol by H202 to dihydroxy benzenes. The reaction products were analyzed using gas chromatography. Catalysis was highly stereoselective and led to predominant formation of catechol as the major product, 93.0% at 50 °C and the remainder formed hydroquinone with no appreciable detection of other oxidized phenol derivatives. IV Product selectivity decreased with increasing reaction temperature or increasing the relative amount of the oxidant. Reaction conditions for the maximum oxidation of phenol have been optimized by varying some parameters such as reaction temperature, amount of catalyst, oxidant and substrate concentration, reaction media and reaction time. In the third chapter, the synthesis of two new tetraazamacrocycle complexes, namely, 7,16-dinicotinoyl[Ni{Me4(4-MeBzo)2[14]tetraeneN4}] (Ni-Nic) and 7, 16- diisonicotinoyl[Ni{Me4(4-MeBzo)2[14]tetraeneN4}] (Ni-Iso) (where [Ni{Me4(4-Me- Bzo)2[14]tetraeneN4}] = 5,7,12,14-tetramethyl-di-4-methylbenzo[/>,/][ 1,4,8,1 l]tetraazacyclotetradecahexaenatonickel( II)) by acylation of [Ni{Me4(4-MeBzo)2[14]tetraene- N4}] and their characterization using spectral and microanalytical data is described. Upon acylation the electronic spectra underwent a slight change ie; the absorption band observed at 395 nm in the precursor complex was shifted to lower wavelength. A strong IR peak due to C=0 stretching mode is observed in the region 1650-1670 cm"1.In 'H NMR spectra pyridine proton signals are observed between 5 7.30-9.50 ppm and olefinic proton signals at 7 and 16 position disappeared upon acylation. The substituted pyridine proton signals shifted down field by the deshilding effect of benzene rings, Also the (12H, CH3) proton signals show an upfield shift upon acylation because these 12 methyl protons lie in the shielding zone produced by the magnetic anisotropy of the pyridine rings. Electrochemical studies on these macrocycle complexes were carried out using glassy carbon electrode in dry acetonitrile/benzene (1:1 v/v) mixture using tetraethyl ammoniumperchlorate (TEAP) as electrolyte. Electrocatalytic oxidation of phenol on glassy carbon electrode modified by the synthesized macrocycle complexes has been studied. Solution of phenol in doubly distilled water with phosphate buffer (pH 7.3) as supporting electrolyte exhibits oxidation peak at 0.57 V at bare glassy carbon electrode. When glassy carbon electrode was modified by the complex Ni-Nic, oxidation of phenol was observed at a lower potential of 0.52 V while in case of Ni-Iso, it was observed at 0.51 V shifting to lower potential by 0.06 V with enhanced peak current. All tetraazamacrocycle complexes were found to catalyze chemical oxidation of phenol by H202 to catechol and hydroquinone and it is found that Ni-Iso gave the best performance. Catechol as the major product and hydroquinone as the minor one were characterized under optimal values of some parameters such as amount of catalyst, reaction temperature, oxidant and substrate concentration, reaction media and reaction time. In the forth chapter, neat 7,16-diacetyl[M{Me4(Bzo)2[14]tetraeneN4}], {M = Cu(II) and Ni(II)} tetraazamacrocycle complexes were synthesized by the acylation of [M{Me4(Bzo)2[14]tetraeneN4}], {M = Cu(II) and Ni(II)}. These tetraazamacrocycle complexes were encapsulated in the cavity of zeolite-Y using the template synthesis method. These neat and zeolite encapsulated tetraazamacrocycle complexes have been characterized by using various physico-chemical techniques viz., spectroscopic (electronic and IR) studies, thermal analysis, metal analysis, field emission scanning electron micrographs and X-ray diffraction patterns. These heterogenized tetraazamacrocycles were used for the catalytic oxidation of benzyl alcohol and styrene. Zeolite encapsulated 7,16-diacetyl[Cu{Me4(Bzo)2[14]tetraeneN4}]-NaY and [CulMe^Bzo}?- [14]tetraeneN4}]-NaY tetraazamacrocycle complexes have been used as heterogeneous catalysts for the solvent free oxidation of benzyl alcohol using H202 as oxidant. Oxidation of benzyl alcohol gives benzaldehyde as major product while benzoic acid and benzyl benzoate are formed as minor reaction products. Reaction conditions for the maximum conversion of benzyl alcohol have been optimized by varying the concentration of oxidant, amount of catalyst, temperature of reaction mixture and reaction time. The zeolite encapsulated 7,16-diacetyI[Ni{Me4(Bzo)2[14]tetraeneN4}]- NaY and [Ni{Me4(Bzo)2[14]tetraeneN4}]-NaY tetraazamacrocycle complexes have been used for the oxidation of styrene using H202 as oxidant in acetonitrile. Oxidation of styrene generally gives five oxidation products namely styrene oxide, benzaldehyde, benzoic acid, phenylacetaldehyde and l-phenylethane-1,2- diol. However, in the present case styrene oxide and benzaldehyde were characterized as the major oxidation product along with the other oxidation products mentioned above. Reaction conditions for maximum oxidation of styrene have been optimized by varying some parameters such as reaction temperature, amount of catalyst, oxidant and substrate concentration and reaction time. VI The fifth chapter describes the synthesis of two new tetrathiamacrocycle complexes of Ni(II) and Cu(II) possessing four peripheral pendant cyano groups in high yield by the metal-template cyclization of Na2S2C2(CN)2 and 1,3- dichloropropane under in reflux DMF. The complexes [Ni{(CN)4[14]eneS4}]Cl2 and [CuCl2{(CN)4[14]eneS4}] so synthesized underwent demetallation upon reaction with CN" or H2S respectively and metal free tetrathiamacrocycle (CN)4[14]eneS4 was obtained. Both [Ni{(CN)4[14]eneS4}]Cl2 and [CuCl2{(CN)4[14]eneS4}] reacted with [CpRu(PPh3)2Cl] to form cationic dinuclear complexes [Ni{(CN)4[14]eneS4}- {RuCp(PPh3)2][PF6]3 and [CuCl2{(CN)4[14]eneS4}{RuCp(PPh3)2][PF6], which were isolated as PF6" salts. All complexes, as well as the metal-free tetrathiamacrocycle, were characterized using IR, UV-vis., 'H NMR, mass spectral studies, cyclic voltammetry and elemental analysis. Redox characteristics of these complexes were also investigated using cyclic voltammetry. Complexes [Ni{(CN)4[14]eneS4}]Cl2 and [CuCl2{(CN)4[14]eneS4}] exhibit a strong band at 2234 and 2209 cm"1 respectively due to v(CN) vibrations. Upon binding of the additional RuCp(PPh3)2 fragment the symmetry and rigidity of the macrocycle framework is perturbed, resulting in a shift of the v(CN) band to lower frequencies. In these dinuclear complexes this band is observed at 2186 and 2191 cm"1 respectively. In the 'H NMR spectra of [Ni!(CN)4[14]eneS4}]Cl2and the (CN)4[14]eneS4 two signals were observed at 5-2.7 and -3.7 ppm due to CH2 and S-CH2- protons. Upon binding the "RuCp(PPh3)2" group to the CN nitrogen, a complex multiplet was observed between 8 7.0-8.0 ppm due to the phenyl protons. Three signals were observed at 5 4.1,4.4 and 4.6ppm and are attributed to the cyclopentadienyl protons.