Synthesis and reactivity studies of high valent iron cyanide complexes

Chaubal, Tanishqa Ajay (2025) Synthesis and reactivity studies of high valent iron cyanide complexes. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS Dissertation of Tanishqa Ajay Chaubal (20MS201)) 20MS201_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (4MB)

Abstract

In biological systems, a variety of Fe-containing metalloenzymes facilitate the oxidation of saturated hydrocarbons via oxidative C–H bond activation. This process is commonly attributed to a hydrogen atom transfer (HAT) mechanism, in which high-valent metal–oxo or bridging oxo species abstract a hydrogen atom from the inert C–H bond. More recent efforts have focused on the capability of M-X (X = OH, OR, N3, Cl, F, ONO2) and metal-imido entities in oxidative C-H bond activation, demonstrating that a terminal M=O is not a prerequisite for hydrocarbon oxidation. In this study, we report the synthesis of two high-valent Fe(IV)–cyanide complexes: a five-coordinate [FeIV(CN)(bTAML)] and a six-coordinate [FeIV(CN)2(bTAML)] complex. Both were comprehensively characterized using UV-Vis, EPR, IR, and mass spectroscopies, along with single-crystal X-ray diffraction (SC-XRD) for structural analysis. These complexes exhibited the ability to oxidize O–H bonds in a range of phenols and C–H bonds in various hydrocarbons at room temperature. Notably, the six-coordinate complex showed significantly enhanced (~250 times higher) hydrogen atom abstraction (HAA) reactivity compared to its five-coordinate counterpart. Kinetic isotope effect (KIE) measurements revealed a substantial KIE of ~13, confirming that HAA is the rate-limiting step. Hammett analysis of both complexes yielded positive ρ values, indicating that the basicity of the complex plays a key role in facilitating HAT. Interestingly, no correlation was found between ln(k₂) and either the bond dissociation energy (BDE) or redox potential (E) of the substrates. However, an inverse relationship was observed between ln(k₂) and the pKa of the substrates, suggesting that the HAA mechanism in these high-valent Fe-CN complexes is primarily pKa-driven HAT.

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