Metal-Coordinated Ligand Radicals: Molecular and Electronic Structure and Reactivity

Bhowmik, Saumitra (2022) Metal-Coordinated Ligand Radicals: Molecular and Electronic Structure and Reactivity. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Saumitra Bhowmik (15RS038)) 15RS038.pdf - Submitted Version Restricted to Repository staff only Download (24MB)

Abstract

The work presented in this thesis, titled as “Metal-Coordinated Ligand Radicals: Molecular and Electronic Structure and Reactivity” primarily deals with (i) synthesis, (ii) studies of redox properties, and (iii) assignment of electronic structure of the transition metal complexes containing redox non-innocent ligands (NIL). Chapter 1 provides a good starting point elaborating the scope of the investigations. A brief overview of the history and gradual development of redox non-innocent ligands has been discussed. Chapter 2 describes synthesis and detailed characterization of the one-electron reduced [Fe(L)]– (S = 5/2) where H₄L= N,N′-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diamino(diphenyldithio) ethane. Its two redox related congeners: viz. the parent molecular complex, [Fe(L)] and the one-electron oxidized [Fe(L)]⁺ (S = 1/2) were described previously by our group. Interestingly, a redox-driven spin-state switchover from ls-Fe(III) to hs-Fe(III) was identified in the reduced iron complex, which is an uncommon phenomenon among the Fe-coordinated NILs. Chapter 3 describes the synthesis and molecular structural properties of cobalt complexes of the same ligand, H4L used in the previous Chapter 2. Several spectroscopic techniques combined with density functional theory were used to assign the electronic structure of the isolated and its redox related congeners. Partial valence tautomerism in the solid and solution phases of these cobalt complexes has been elaborated. In Chapter 4, two Ni(II) and two Pd(II) complexes are isolated using another noninnocent pentadentate azo-appended 2-aminophenol ligands. The formation of phenoxazine-ring by treating the complex with oxidizing agents and the possible reaction mechanism on the transformation has been discussed. Chapter 5A contains the C–H bond activation and successive ring-hydroxylation in Ni(II) and Pd(II) complexes with azophenyl-salicylaldimine ligands. Herein chemical transformations of an anionic tridentate ONN' ligand into a di-anionic tetradentate ONN'O' ligand is noted. Plausible reaction mechanism has been proposed which involve stepwise ortho-metalation and ring-hydroxylation reactions. EPR and absorption spectrum analyses have been used to explore the redox characteristics of all complexes in Chapter 5B. The nature of coulometrically generated one-electron oxidized and one-electron reduced species have been discussed. The studies of the ring-hydroxylation process in the copper complex of the tridentate azophenyl-salicylaldimine ligand have been reported in Chapter 5C. Detailed investigation of Xray, redox and EPR studies are used to understand the electronic structure of the isolated copper complexes. Notably, the ring-hydroxylation took place in the presence of an acetate salt and the oxygen of acetate is argued to be the oxygen source of ring-hydroxylation. The compounds, reported herein, have been characterized fully using several physicochemical techniques. The key compounds have been structurally characterized. The experimental work has been carried out competently.

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