Investigation on transition metal-radical complexes for oxidative cleavage of aromatic CC bond, aliphatic CN bond and radical-to-metal electron transfer phenomena

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Aminophenol-containing non-innocent ligands and their corresponding transition metal complexes are very useful for enzymatic functional mimicking; homogeneous catalysis, e.g. CF3 group transferring, CN bond forming reactions; molecular oxygen activation; etc. In these regards, three new aminophenol-based redox active ligands [H2GanAP, H4LPiperazine(AP/AP) and H2LAP(Ph)] and their corresponding transition metal complexes have been investigated. Reaction of ligand H2GanAP with an Fe(II) or Fe(III) perchlorate salt in the presence of dioxygen provided complex 2A which led to a biomimetic CC bond cleavage of the aromatic ring of the 2-aminophenol unit. On the other hand, a different reactivity (no CC bond cleavage of the aromatic ring) was observed when Mn(II) perchlorate salt was employed in the reaction thereby provided complex 2C. The difference in the reactivity emphasized the importance of the spin and the nature of the metal-radical interactions in controlling the dioxygen reactivity of transition metal complexes involving non-innocent ligand. The piperazine based ligand H4LPiperazine(AP/AP) provided an octahedral Co(III) complex (3A) where the coordinated ligand was present in non-radical aminophenolate form due to the presence of hydrogen bonding interaction. Contrarily, iminosemiquinone radical was generated for both the Cu(II) complex(3B and 3C) when there was no such hydrogen bond interactions. Thus, the effect of hydrogen bonding and the orientation of the ligand framework have been demonstrated for the formation of radical and non-radical transition metal complex. A facile redox process of H2LAP(Ph)-containing five coordinate diradical Co(III) complexes (4C and 4D) and four coordinate monoradical Co(III) complex (4A) and the redox interconversion have been achieved in presence of sunlight. The complexes were being utilized in the catalytic conversion of isocyanate to urea. This study established the potential of radical-coordinated transition metal complexes in photocatalysis. One step synthesis of five-coordinate square pyramidal Cu(I)-bis(iminoquinone) complex (5B) has also been successfully carried out by using non-innocent ligand H2LAP(Ph) and excess amount of CuCl22H2O salt via geometry-driven iminosemiquinone radical to Cu(II) center electron transfer. Furthermore, it was found that complex (5B) oxidized KO2 to dioxygen molecule.
Supervisor: Chandan Mukherjee