Study On The Preparation And Properties Of Corrole Cobalt(?)-oxo Complexes

Study on corrole macrocycle is one of the most active branches of modern porphyrin chemistry.Amongst the metallocorroles,cobalt corroles are extensively studied because of their ability to catalyze various type of chemical reactions such as hydroxylation,C-H bond activation,water oxidation,hydrogen reduction and oxygen atom transfer reactions.The most fascinating chemistry of cobalt corroles is the nature of active intermediates responsible for oxygen atom transfer in the catalytic oxidation of organic substrates.Mostly,cobalt-oxo transient intermediates are involved in these reactions.Notably,the factors controlling the stability and reactivity of cobalt-oxo intermediates need to understand,to gain further insight into the nature of these intermediates.This piece of research work is focused on the preparation and identification of highest valent cobalt???-oxo corroles with different electronic environment and their reactivity in oxygen transfer reaction to an alkene.Also,electrochemical catalytic activity of cobalt corroles towards hydrogen evolution was investigated.The main work of this thesis is highlighted below:This work has developed a novel,2-chloro-5,10,15-tris?2,4,6-triphenylphenyl?-corrole?1b?in an acceptable yield of 5.7%during the synthesis of its parent corrole?1a?,in which DDQ served as oxidant and chlorinating agent.The selective chlorination has a significant effect on photophysical properties such as fluorescence intensity,fluorescence life time,and fluorescence quantum yield.Three cobalt?III?corroles?2d-2f?depending on different meso-substituent were synthesized.All cobalt?III?corroles were completely characterized by U?-vis,MS,EXAFS and ¹H NMR spectroscopy.The characteristics Soret band for cobalt?III?corroles present around 380-390 nm.Cobalt???-oxo corroles?2g-2i?were prepared by the reaction of corresponding cobalt?III?complexes with KHSO₅ by two electron oxidation.All cobalt corroles were well characterized by U?-vis,MS,¹H NMR,electron paramagnetic resonance?EPR?spectroscopy,EXAFS and resonance Raman?rR?spectroscopy.The most notable U?-?is differences were the appearance of Co^??O peak at 420-425 nm.Mass spectroscopy analysis employing¹⁸O-labeled water demonstrate increase in two unit mass for all Co^??¹⁸O?2g-2i?complexes.Most importantly,resonance Raman characterization revealed the presence of triply bonded terminal high-valent Co^??O species?2g-2i?which display a diatomic Co?O vibration band around 840 cm^-1.EPR investigation exhibited the occurrence of cobalt+5 oxidation in all cobalt???-oxo?2g-2i?complexes.Extended x-ray absorption fine structure displayed increase in oxidation state from Co^III to Co^?.DFT calculations indicated that triplet state is least energy and most stable configuration among the three states in all cobalt???-oxo?2g-2i?complexes.Also,DFT calculated Co^??O stretching vibrations are well matched with experimental results.The stability of cobalt???-oxo corroles?2g-2i?were explored by their U?-vis spectroscopy.The self-decay rate constants in acetonitrile were in the following order:2i>2h>2g,i.e.,electron-poor cobalt???-oxo corrole decayed slower than electron-rich corrole.The observed pseudo-first-order rate constants for the oxygen atom transfer from cobalt???-oxo corroles to styrene follow order:2i>2h>2g.The electrochemical activity of three cobalt corroles?2d-2f?was investigated in the acetic acid and water system.The cyclovoltammetry analysis revealed that with increasing electron withdrawing substituent at meso position redox potential moves towards positive values.Turn over frequency?TOF?values depend on the electronic environment of the cobalt corroles?2d-2f?.Complex 2d and 2e have almost same value of TOF,while 2f displayed less catalytic performance in aqueous media.