Photochemical and electrochemical investigation of cobalt phtalocynanine – cobaltoximes complexes

Supramolecular chemistry has relied on more or less rigidly organized, synthetically built up molecular receptors for effecting molecular recognitian, catalysis, light harvesting, transport processes and for setting up molecular devices. The use of macrocyclic structures like phthalocyanines was dictated by the need to achieve better control over the geometry and rigidity of molecular receptors. Phthalocyanines have been prepared in the laboratory from phthalonitriles, phthalimides, phthalic anhydrides, diiminoisoindolines, and phthalic acids as the starting material. The center of the phthalocyanine system may consist of two hydrogen atoms or a metal center. On the phthalocyanine macrocyle, there are sixteen hydrogen atoms on the four benzene rings; each hydrogen atom may be replaced by another substituent. To date, more than seventy different elements, have been used as central atoms in Pcs. Pc ligand can be coordinated to nearly all of the metals. The coordinated number of square planar Pc ring is four. The phthalocyanines coordinated to the metals which prefer higher coordination number have square pyramidal or octahedral structures. The phthalocyanines which are heteroatomic systems due to their thermal and chemical stability, electrical, optical or liquid-crystal properties have considerable importance not only in theoretical field, but also in many different area of application. On the basis that phthalocyanines have complementary absorptions in the visible region and exhibit distinct redox and photophysical properties, hetero-arrays of these macrocycles would be of particular interest. Apart from the potential use as novel optoelectronic materials, these systems can also serve as models for the study of energy and electron-transfer processes in artificial photosynthetic systems. In addition, both classes of these tetrapyrrole derivatives are efficient photosensitizers for photodynamic therapy. Phthalocyanines are synthetic tetrapyrrolic rings consisting of a planar macrocycle with an 18 -electron system and comprising four isoindole units linked together through their 1,3-positions by aza bridges. Rich coordination chemistry of phthalocyanines allows researchers to use them as high technology materials. For these reasons, they find wide application in materials science as catalysts, as photosensitizers in cancer therapy, in optical-data storage systems, in chemical sensors, as liquid crystals, in nonlinear optics and in Langmuir-Blodgett films. Hydrogen has a clean alternative energy carrier for the future because of its high calorific value and since it only produces water upon combustion. It is believed that hydrogen will substitute fossil fuels as one of the next generations of clean energy sources. However, the major challenge for using hydrogen energy as a storable fuel is its unavailability in nature. The development of renewable and cost-effective approaches for H2 generation will address both the environmental needs and the energy independence. Photo-catalytic H2 generation utilizing solar energy has attracted considerable attention because of its potential applications in clean energy production, recycling of a renewable resource, reducing the consumption of fossil fuels and the environmental protection. Despite the great interest in terms of reactivity and physicochemical characteristics of oximes and their metal complexes in the medical field, bio-organic systems, catalysis, electrochemical sensors, electrooptic and current practices in large, there are very few studies with tetrapyrole/ cobaltdioxime combined systems on the production of H2 in the literature. In those relatively few papers, photocatalyst/ light absorber systems containing a porphyrin ring and cobaltoximes are tested for photocatalytic H2 generation and very good results were obtained. The main aim of this study is to make use of the very close similarity between phthalocyanine and porphyrin structures with higher thermal and optical stability of the phtalocyanines, to carry out similar hydrogen evolution studies with cobaltoximes/ phthalocyanine couples. Therefore, in this study, number of Co(II)phtalocyanine ve cobaltdimethylglyoxime compounds have been synthesized. Firstly, the phthalocyanine precursor 4-(4-pyridyloxy)-phtalonitrile was synthesized by substitution reaction of 4-hydroxypyridine and 4- nitrophtalonitrile in the DMF at 25 oC in the presence anhydrous potassium carbonate (K2CO3). The next step was the cyclotetramerization of this precursor with zinc asetate Zn(CH3COO)2 in 2- dimethylaminoethanol. When the same reaction conditions were applied with CoCl2 as the metal salt, tetrakis (4-pyridyloxy)phatocyaninatocobalt(II) (3) was synthesized. As the second component in this catalysis system, the vic-dioxime complex, chloro (pyridyl)bis(dimethylglyoximato)cobalt(III)[CoDMG] (4), was synthesized from cobalt (II) chloride hexahydrate and dimethylglioxime by bubling air through the raction mixture in the presence of pyridine and 2-dimethylaminoethanol for 2 hours. In the last stage of this study, the supramolecular structure (5) with phthalocyanine in the inner core and for Co(III) oxime complexes attached through pyridyl substituents was synthesised through the reaction of phthalocyanine (3), cobalt (II) chloride hexahydrate and dimethylglioxime by bubling air through the reaction mixture in the presence of pyridine and 2-dimethylaminoethanol for 2 hours. All these new compounds have been characterized by spectroscopic methods. Cobalt phthalocyanine bearing redox active tetra Co-DMG substituents ((Co-DMG)4- CoPc) was electrochemically characterized in order to determine its functionalities for practical usages as effective electrocatalysts. (Co-DMG)4-CoPc went two oxidation and three reduction reactions in solution. Both of the oxidation reactions and two of the reduction reactions were determined as metal based electron transfer reactions. Tetra Co-DMG groups of the complex showed a four-electron reduction process at - 1.12 V due to reduction of CoII center of Co-DMG to CoI. Multi-electron and metal and/or ring based redox processes showed worthy of the complex for using as an electrocatalyst. Shifting of the proton reduction reaction toward the positive potentials indicated electrocatalytic activities of the complex for the hydrogen evolution reaction(HER). Metallophthalocyanines (MPcs) were used as functional materials in different electrochemical technologies due to their rich redox activities. Redox activities of MPcs can be enhanced by incorporating redox active metal ions into the cavity of the Pc ring and/or by substituting with redox active moieties. MPcs having metal based electron transfer reaction generally behave as active species, thus CoPc-(Co-DMG)4 (5) bearing redox active CoII ions in the Pc ring and tetra redox active Co-DMG was synthesized and tested as possible electrocatalyst for hydrogen evolution reaction (HER). When glassy carbon electrod (GCE) is coated with CoPc-(Co-DMG)4(5), potential for HER has been reduced by 360 mV which indicates that CoPc-(Co-DMG)4 (5) is a promising catalyst for hydrogen evolution reaction.