STUDIES ON TRINUCLEAR OCTAKIS(METHYLTHIO)TETRAAZAPORPHYRIN COMPLEXES

Abstract

Tetraazaporphyrins (TAP's) belong to the family of tetrapyrrolic macrocycle compounds. Structurally they are homologous to porphyrins, wherein meso (=CH-) groups are replaced with aza (-N=) bridges. The Lindsay macrocyclization of dinitriles offers a convenient method of synthesis of tetraazaporphyrins with a variety of peripheral substituents and thus provides an excellent opportunity to explore the subtle effects of ligand environment on the properties of the metallo-porphyrinic systems. Furthermore the tetraazaporphyrins and their metal complexes exhibit reversible oxidations and reductions, strong visible region absorbance and fluorescence emission characteristics that coupled with weak axial coordination abilities make them an attractive class of compounds for potential applications in fabricating ion and molecule sensors, photodynamic therapy of cancer, optical data storage devices, photovoltaic cells, dyes, pigments, non-linear optical materials and in designing of photoresponsive receptors and molecular switches. Although several hybrid oligomeric multichromophore metallo-porphyrin arrays have been synthesized by binding of polypyridyl ruthenium(II) moieties through axial coordination or through meso pyridyl attachments, similar hybrid systems with metallotetraazaporphyrins are less studied. Since, the metallo-tetraazaporphyrins and bis(bipyridyl)ruthenium(II) both are proven chromophores with characteristic luminescence and redox properties, it would be quite interesting to explore their mutual effects on electronic, redox and luminescence behaviours of the two moieties. Thus in the present study, it was aimed to synthesize (a) metal free or free-base octakis(methylthio)- tetraazaporphyrin, (OMTTAP) or {(MeS)8TAP) and its transition metal complexes with different bivalent transition metal ions viz., Mg2+, Mn2+, Co2+, Ni2+, Cu2+ and Zn2+ and (b) to bind one and two (bpy)2Ru", (phen)2Ru" and/or Cp(PPh3)Ru" moieties to their periphery (where, bpy =2,2'-bipyridine, phen = 1,10-phenanthroline and Cp =q5-C5H5"). As some of the [M{(MeS)8TAP}] complexes are reported in the literature, a total of 7 monomeric {(MeS)8TAP} complexes including metal free and three new ones for comparison; 14 new corresponding dinuclear complexes and 21 new trinuclear complexes with each trinuclear complex in two geometrical configurations have been synthesized. The spectral (electronic absorption, emission, 'H NMR, mass and ESR) and electrochemical characteristics ofthese compounds have been investigated. The structural parameters of representative complexes have been investigated using theoretical methods. The application of selected complexes in analytical determination of aqueous aniline have also been investigated. The thesis has been divided into seven chapters. The first chapter surveys syntheses and spectral studies and application background literature of the metal free tetraazaporphyrins and the metallo-tetraazaporphyrins and in that context poses the scope and purpose of this work. All relevant and up-to-date background information is included in this chapter to facilitate easy understanding of this work. It is of introductory nature and gives an overview of the structural diversity of tetraazaporphyrins and their derivatives. Particularly, the spectral studies, crystallography, electrochemistry, catalytic behaviour and theoretical calculation on tetraazaporphyrins are briefly surveyed in this chapter. The spectral studies include electronic absorption, emission, IR, ]H NMR and ESR spectral data. In their electronic absorption spectra the metallo-tetraazaporphyrins exhibit two strong absorption bands the Q band at longer wavelength and the Soret band at shorter wavelength like their metallo-porphyrin congeners. However, unlike that in metalloporphyrins, the Q band is relatively intense in metallo-tetraazaporphyrin complexes. In the metal free tetraazaporphyrin the Q band is observed to be split. The IR spectra of metallotetraazaporphyrins are not very informative due to presence of numerous bands but could be used as fingerprint for a particular compound. In case of the metal free tetraazaporphyrins, the v(N-H) stretching band is generally observed at ca. 3300 cm"1. They also exhibit 'H NMR signals at very upfield positions due to -NH protons. The Mn2+, Co2+ and Cu2+ tetraazaporphyrin complexes are paramagnetic and exhibit ESR signals. The electrochemical oxidation-reduction behaviour of metal free tetraazaporphyrins as well as metallotetraazaporphyrins has been little investigated. Although the analogous metal free porphyrin exhibits two oxidation and two reduction waves, similar behaviour in tetraazaporphyrins have not been reported. The second chapter describes the synthesis of three new compounds i.e. octakis(methylthio)tetraazaporphyrin, {(MeS)8TAP} complexes with Mn2+, Co2+ and Zn metal ions, while synthesizing other 4 literature reported compounds metal free II2{(MeS)8TAP} and its complexes with Mg2+, Ni2+ and Cu2+ metal ions for comparison. The spectral and electrochemical characteristics of all seven compounds have been described. [Mg{(MeS)8TAP}] was synthesized by the reaction of bis(methylthio)- maleonitrile and magnesium butoxide under reflux in butanol. It was detnetallated by warming [Mg{(MeS)8TAP}] in chloroacetic acid at 70 °C to obtain the metal free H2{(MeS)8TAP}. Metal insertion reactions were carried out in C6H5C1/DMF (2:1 v/v) mixtures. The reaction between metal free {(MeS)8TAP) and corresponding metal acetates yielded the desired products. They were purified by column chromatography and were characterized using IR, UV-visible, fluorescence, 'H NMR, ESR, mass spectral studies, cyclic voltammetry and elemental analyses. Energies and relative intensities of the Qband in these complexes were found to depend on the nature ofthe metal ion in the {(MeS)8TAP} cavity. All these complexes show a strong emission from S2 level. All compounds exhibited prominent {(MeS)8TAP} centered reversible oxidation and reduction waves, beside the metal centered oxidations in [Mn{(MeS)8TAP}] and [Co{(MeS)8TAP}] exhibited one metal centered reversible oxidation wave between the {(MeS)8TAP) centered oxidations and reduction waves. The third chapter describes the synthesis and column chromatographic purification of 14 bichromophoric dyads possessing one (bpy)2Ru" or (phen)2Ru" and one H2{(MeS)8TAP} or [M{(MeS)8TAP}] moiety (where, M=Mg2+, Mn2+, Co2+, Ni2+, Cu2+ and Zn2+). Complexes were characterized using IR, UV-visible, fluorescence, 'H NMR, mass spectral studies, cyclic voltammetry and elemental analyses. The electronic spectra of the dinuclear complexes exhibited significant changes in the intensity of the Soret and Q bands as a consequence of binding of a peripheral ruthenium moiety. The ruthenium polypyridyl MLCT absorption was nearly indistinguishable and was masked under the [M{(MeS)8TAP}] absorptions. All these complexes exhibited a new oxidation wave due to the Ru2+/Ru3+ process of the peripheral moiety but the two bpy/phen reduction waves were not very distinguishable. The fluorescence excitation/emission spectra of dyads are indicative of effective excited state electronic communication energy transfer between the chromophore centers. Compounds exhibited two groups of 'H NMR signals, a group of several multiplets between 6.0-10.5 ppm and two signals below 5.5 ppm in acetonitrile-d3. The former arose due to bpy-H/phen-H while the latter arose due to the methyl (-CH3) group and -NH group. Due to the flexibility -S-CH3 chains fold over in such a way as to lie over the {(MeS)8TAP} plane and thus experienced anisotropic shielding due to the {(MeS)8TAP) in ring current. All these dinuclear complexes exhibited molecule ion peaks in the ESI mass spectra. In the fourth chapter synthesis spectral and electrochemical investigation of cis and trans isomers of 21 new triads possessing a central metal free H2{(MeS)8TAP} or [M{(MeS)8TAP}] and two (bpy)2Run, (phen)2Ru" or Cp(PPh3)Ru" peripheral moieties are described. The trinuclear complexes possessed bent (k -S ,S ,S ,S )[Ru ]2 and linear (k4-S2,S3,S12,S13)[Rui1]2 arrangement of the peripheral metallo-chromophore units. Complexes were characterized using IR, 'H NMR, UV-visible, mass spectral and electrochemical data. The tetraazaporphyrin's Soret and Q band absorptions were hypsochromic shifted upon (k -S,S')[Ru] coordination. The nature and magnitude of the shift depends upon nature of the peripheral Ru units and their mutual relative positions of attachment to {(MeS)8TAP} core. These complexes exhibited good electrode activity with multiple reversible redox waves in cyclic voltammetry scans. Unlike two reversible reductions in precursor [M{(MeS)8TAP}] complexes, the trinuclear complexes exhibited a single reversible reduction wave between Em -0.761 and -0.550 V vs. Ag/AgCl. The shift in reduction potential depends upon nature of the peripheral Ru2+ units and their mutual relative positions of attachment to {(MeS)8TAP} core. In the anodic scans, linear trinuclear complexes exhibited two successive one electron oxidations, the first due to Ru2+/Ru3+ and second due to {(MeS)8TAP}/{(MeS)8TAP}+ processes, while the bent trinuclear complexes exhibited three successive one electron oxidations, besides the Ru2+/Ru3+. Due to stabilization of the (bpy)2/(phen)2Ru" HOMO's the Ru2+/Ru3+ oxidations in (Kf-S,S')Ru(bpy)2/(phen)2 complexes were observed at significantly less positive potentials than that in corresponding saturated thioether complexes. All complexes showed a strong [M{(MeS)8TAP}] centered S2 emission with two component decay profile and rapid intercomponent excitation energy transfer (k 107 —108 s"1). The Soret as well as the 480-490 nm excitations both led to emission maxima in the visible region that showed two component decay, with a shorter lifetime (~3 ns) of [{(MeS)8TAP}Zn]* S2 and a longer lifetime (~9 ns) of Ru2+* component. As the two chromophores in the hybrid systems possessed comparable excited state energies with moderate energy barrier (< 8.5 kJmof1), emissions from both components were observed. The Ru2f emissions contributed the most in dinuclear complexes that declined upon formation of the trinuclear complexes. The hybrid iv dyad and triads possessed stronger emissions than the precursor complexes 1-7. Also the bent (k4-S2,S3,S7,S8)[Ru"]2 trinuclear complexes possessed stronger emissions than the linear (k4-S2,S3,S12,S13)[Ru"]2 isomers due to relatively efficient collisional deactivation in the latter. The fifth chapter describes the application of [M{(MeS)8TAP}] as fluorometric reagent for determination of aniline residue in natural water samples. The fluorescence emission spectra of 5.0x10"6 M solutions of complexes were investigated in presence of l.OxlO"3 - 5.0x10"6 M aniline in dichloromethane solutions. It was found that an aniline concentration >10"4 Mcompletely quenched all emissions, while for aniline concentrations between 10"5 - 10"6 M emission intensities of the complexes were modulated at all wavelengths. The emission maxima for 490 nm excitation exhibited large Stokes shift which was observed between 700-800 nm. This emission was most sensitive to micromolar concentrations of aniline in dichloromethane and exhibited linear response. The best response was observed for [Zn{(MeS)8TAP}] complex as fluorometric probe. Thus it was used for the determination recovered aniline concentrations from aqueous solutions. Analysis was tolerant to 10"3 Mconcentrations of coordinating cations Fe +and Cu +while 10"1 molar concentrations of alkali and alkaline earth metal ions. In the sixth chapter redox induced linkage isomerization ofthioether coordination in model compounds are described in order to assess suitability of peripheral thioether (bpy)2Ruu and (phen)2Run coordinated, linear and bent, triads in device applications for redox triggering. The acetate bearing dithioether, sodium di(2-carboxymethylsulfanyl)- maleonitrile, L1 upon reaction with [(bpy)2RunCl2]-2H20, [(phen)2Ru"Cl2]-2H20, [(bpy)2RumCl2]+, [(phen)2RuinCl2]+ in methanol formed (k2-SS')L' and (k2-00')L' coordinated complexes in Ru2+ and Ru3+ species respectively. Four other Ru3+ complexes with di(benzylsulfanyl)maleonitrile, L2, and with acetate ligands were also synthesized. In the cyclic voltammetry, (k2-S,S')L" complexes exhibited quasireversible oxidation waves at 1.01 and 1.02 V vs. Ag/AgCl over GC electrode in DMF, while the corresponding (k2-0,0')L' complexes exhibited reversible oxidation at E1/2 0.59 and 0.58 V, respectively, under identical conditions. This is unlike the voltammetric behaviour ofthe Ru2+ and Ru + L2 complexes, wherein the (k2-S,S')L2 complexes exhibited identical voltammograms with single reversible one electron waves at E1/2 0.98 and 0.92 V, respectively under identical conditions. The voltammograms of Ru2+-L2 complexes also become irreversible in presence of nearly four molar equivalent of sodium acetate. Hence, the irreversible redox behaviour of complexes (k2-S,S')L' has been interpreted in terms of rapid linkage isomerization, i.e. shift in (k2-S,S')L' to (k2-0,0')L' coordination, following the Ru2+/Ru3+ electrode process. In the seventh chapter structural parameters from theoretical geometry optimization and the molecular orbital energies of representative [M{(MeS)8TAP}], dyad and triad complexes are described. The ground state molecular geometry of the complexes were optimized using universal force field (UFF) molecular mechanics method. It was compared with the PM3 refinement of molecular geometry for [Mg{(MeS)8TAP}] complex. Theoretical information on molecular orbital energies and probable electronic transitions were obtained using ZINDO method. Eighth chapter summarizes some of the important findings of this study.