MULTICHROMOPHOREMETALLO-TETRAAZAPORPHYRIN ARRAYS POSSESSING Ru(II) MOIETIES

Abstract

The tetraazaporphyrins, also known as azaporphyrin or porphyrazines are a class of aromatic, macrocyclic compounds that are composed of four pyrrole rings joined together through aza bridges. As the tetraazaporphyrins could conveniently be synthesized with a diverse variety of peripheral substituents, they provide an excellent opportunity to explore the subtle effects of ligand environment on the properties of the metallo-porphyrinic systems. The tetraazaporphyrins and their metal complexes show several unique optical, electrochemical and catalytic properties, which make them a very interesting class of compounds. They have potential applications in the areas of optical data storage, photodynamic therapy of cancer, photovoltaic cells, dyes, pigments, non-linear optical materials and chemical sensors. They are also useful for making magnetic toner ink for electrographic recording, making of light switches and in biomedical diagnosis. Several oligomeric multichromophore metallo-porphyrin arrays have been synthesized by binding polypyridyl ruthenium(II) moieties through axial coordination or through meso pyridyl coordination. Although the metallo-tetraazaporphyrins have been synthesized with a variety of peripheral substituents and peripherally coordinated metal moieties, the multichromophore tetraazaporphyrin arrays have so far been little explored. Thus in this present study, it was aimed to synthesize (a) free-base octakis(benzylthio)tetraazaporphyrin, (OBTTAP) and its transition metal complexes with different bivalent transition metal ion viz. Mg \ Mn +, Co +, Ni +, Cu + and Zn2+ and (b) to bind one, two or four (bpy)2Ru", (phen)2Ru" and/or Cp(PPh3)Ru" moieties to their periphery (where bpy =2,2'-bipyridine, phen = 1,10-phenanthroline, Cp =r|5-C5H5'). A total of 55 new compounds have been synthesized. The spectral (electronic absorption, emission, H n.m.r, mass, and e.s.r.) and electrochemical characteristics of these compounds have been investigated. The structural parameters, using theoretical methods as well as the catalytic properties of a few selected complexes have also been investigated. The thesis has been divides in the seven chapters. First chapter describes the scope and purpose of this work. It also includes syntheses and spectral background of the free-base tetraazaporphyrins and the metallo-tetraazaporphyrins that is essential for and easy understanding of this work. It is of introductory nature and gives an overview of the structural diversity of tetraazaporphyrins and their derivatives. The spectral studies, crystallography, electrochemistry, catalytic behavior and theoretical calculation on tetraazaporphyrins so far made are briefly surveyed in this chapter. The spectral studies include electronic absorption, emission, i.r., 'H n.m.r and e.s.r. spectral data. In their electronic absorption spectra the metallo-tetraazaporphyrins exhibit two strong absorption bands the Qband at longer wavelength and the Soret band at shorter wavelength like their metallo-porphyrin congeners. However, unlike that in metallo-porphyrins, the Soret bands in the metallotetraazapoiphyrins are more intense than the Q-bands. In the free-base tetraazaporphyrin the Qband is observed splitted. The i.r. spectra of metallo-tetraazaporphyrins are not much informative due to presence of numerous bands but couldbe used as fingerprint for a particular compound. In case of the free-base tetraazaporphyrins, the v(N-H) stretching band is generally observed at ca. 3300 cm"1. They also exhibit 'H n.m.r signals at very upfield positions due to -NH protons. The Mn +, Co +and Cu2+ tetraazaporphyrin complexes are paramagnetic and exhibit e.s.r. signals. The electrochemical oxidation-reduction behavior of free-base tetraazaporphyrins as well as metallo-tetraazaporphyrins have been little investigated. Although the analogous free-base 11 porphyrin exhibit two oxidation and two reductions waves but similar behaviour in tetraazaporphyrins have not been reported. In the second chapter syntheses spectral and electrochemical studies of seven new compounds viz. free-base octakis(benzylthio)tetraazaporphyrin, (OBTTAP) and its complexes with Mg , Mn , Co , Ni , Cu and Zn metal ions have been described. The [Mg(OBTTAP)] was synthesized by the reaction of bis(benzylthio)maleonitrile and magnesium butoxide in refluxing butanol. It was demetallated by reacting [Mg(OBTTAP)] in chloroacetic acid at 70 °C in order to obtain the free-base OBTTAP. Metal re-insertion reactions were carried out in PhCl-DMF (2:1 v/v) mixtures. The reaction between free-base OBTTAP and corresponding metal acetates yielded the desired products. They were purified by column chromatography and were characterized using i.r., u.v.-visible, fluorescence, 'H n.m.r., e.s.r., mass spectral studies, cyclic voltammetry and elemental analyses. The energy and the intensity of the Q-band in these complexes were found to be dependent on nature of the metal ion present in the OBTTAP cavity. All these complexes show a strong emission from S2 level. The [Mn(OBTTAP)], [Co(OBTTAP)] and [Cu(OBTTAP)] exhibited a reversible oxidation wave at E1/2 -0.33, - 0.29 and 0.58 VVs Ag/AgCl respectively. The Mn2+, Co2+ and Cu2+ complexes are paramagnetic with magnetic moments (ieff= 4.35, 1.13 and 2.08 BM respectively. In the third chapter syntheses of bichromophoric dyads having one (bpy)2Ru , (phen)2Ru" or Cp(PPh3)Ru" and one OBTTAP or [M(OBTTAP)] moiety (where M= Mg2+, Mn2+,Co2+, Ni2+, Cu2+ and Zn2+) are described. A total of 21 new complexes were synthesized that were purified and characterized using i.r, u.v-visible, fluorescence, H n.m.r., mass spectral studies, cyclic voltammetry and elemental analyses. The electronic spectra of the dyads showed significant alteration in the intensity of the Soret and Q-bands due to binding of a peripheral in ruthenium moiety. The ruthenium polypyridyl MLCT absorption was observed as a comparatively weak band between OBTTAP centered, Soret and Q-bands. All these complexes exhibited a new oxidation wave due to the Ru'VRu1" process of the peripheral moiety. The fluorescence excitation/emission spectra of dyads with particularly (bpy)2Ru" and (phen)2Ru" peripheral moieties are indicative of excited state energy transfer between the chromophore centers. The 'H n.m.r data of these compounds are very informative. The dyads having (bpy)2Ru", (phen)2Ru" or Cp(PPh3)Ru" moieties exhibit two groups of 'H n.m.r signals, a group of multiple multiplets between 6.0-10.5 ppm and three signals below 5.5 ppm in acetone-d6. The former arose due to bpy-H/phen-H/Ph-H whiled the latter arose due to the benzyl -CH2 group and some Ph groups. Due to the flexibility of-S-CH2- chains some of the phenyl rings fold over in such a way as to lie over the OBTTAP plane and thus experienced shielding effect due to the OBTTAP ring current. The cyclopentadienyl and phenyl protons have markedly different chemical shifts and hence their intensity ratio is a good indicator of 1:1 coordination. In the ESImass spectral studies the molecular ion peaks were not observed. The MALDI-TOF mass spectra in 2,5-dihydrobenzoic acid (DHB) matrix exhibited base peaks at m/z at 567, 615, 691 that correspond to [Ru(phe)2+(DHB)]+, [Ru(bpy)2+(DHB)]+, [CpRu(PPh3)]+ fragments respectively. In the fourth chapter syntheses spectral and electrochemical investigation of 21 new pentads possessing a central free-base OBTTAP or [M(OBTTAP)] and four (bpy)2Ru", (phen)2Ru" or Cp(PPh3)Ru" peripheral moieties is described. These complexes were purified and characterized using i.r, u.v-visible, fluorescence, 'H n.m.r. mass spectra, cyclic voltammetry and elemental analyses. All these complexes exhibited strong inter chromophore electronic interaction as reflected by the changes in their spectral band parameters and the oxidation potential shifts. The electronic absorption spectra of these complexes showed bands arising in IV (bpy)2Ru", (phen)2Run or Cp(PPh3)Ru" and [M(OBTTAP)] moieties but are not simple composites of them. The fluorescence excitation/emission spectra of these complexes are indicative of energy channelization from peripheral units to the central [M(OBTTAP)] core. In the 'fl n.m.r. spectra the aryl protons signals are complex multiplets due to irregular stereochemistry and consequent varying magnetic environments. Three new triads possessing [Mg(OBTTAP)] and (bpy)2Ru"-(phen)2Ru", (phen)2Ru"-Cp(PPh3)Ru" or (bpy)2Ru"- Cp(PPh3)Ru" moieties have also been synthesized and studied. The fifth chapter describes the electrochemical behaviors of [M(OBTTAP)] modified silver and pyrolytic graphite electrodes (PGE) in acidic, neutral and basic media in presence of nitrite ions, are reported (where M= Mn2+, Co2+, Ni2+, Cu2+ and Zn2+). The [Co(OBTTAP)] and [Cu(OBTTAP)] complexes showed affinity for N02" ions. The former complex in acidic, neutral and alkaline solutions showed a low lying irreversible reduction wave at Epc~ - 0.2 V vs Ag/AgCl, probably due to formation of a penta-coordinated NO+ complex. The [Cu(OBTTAP)] modified electrode in acidic and neutral solutions also exhibited a similar low lying irreversible reduction wave. But in alkaline medium it formed a stable NO+ complex that was isolated and characterized as [Cu(OBTTAP)(NO)(OH)]. This complex exhibits two successive reductions the first reversible NO+ centered reduction at E1/2 - 0.26V vs Ag/AgCl and the second OBTTAP centered reduction at Epc - 0.96V vs Ag/AgCl. In the sixth chapter structural parameters from theoretical geometry optimization and the molecular orbital energies of some selected [M(OBTTAP)] complexes are described. The MO energies were calculated using ZINDO program. The ground state molecular geometry and heat of formation were obtained using MOPAC with PM3 parameters as well as using DGauss theory. Both programs gave similar geometry parameters, wherein the pyrrolic N-N distance across the OBTTAP cavity was found to be 4.1 A. In the optimized geometry it was observed that three of the phenyl groups due to folding at thioether "S" and -CH2 groups lie above the tetrapyrrohc ring. Geometry of the [M(OBTTAP)] suggests that in the event of collision between the [M(OBTTAP)] and solvent molecules the benzyl groups could act as cushion for the central tetrapyrrohc core. The energies of the electronic spectral bands obtained from ZINDO calculations are invariably higher than the actual empirical results. Seventh chapter summarizes some ofthe important findings ofthis study.