Structure and Reactivity Correlations of High-valent Manganese and Cobalt Complexes

Abstract

Earth-abundant transition metals have attracted a great attention for decades due to their versatile nature that enables them to carry out a variety of reactions in metalloenzymes. Accordingly, various model systems have been developed to investigate reactivities of earth-abundant transition metals in different platforms, towards the pursuit of efficient catalysts in industrial applications. Although there are well-established structure-reactivity relationships for a couple of earth-abundant transition metals, more information is needed to better understand their reactivities and design selective metal catalysts with earth-abundant transition metals. Therefore, recent developments as well as experimental and theoretical results on structure-reactivity relationships of high-valent manganese and cobalt model complexes are delivered in this dissertation.Formation, characterization, and reactivity investigations on high-valent bis(μ-oxo)dimanganese complexes and mononuclear manganese oxo complexes were investigated. Equatorial ligand field effects on O–H bond activation reactivity of MnIIIMnIV(μ-O)2 complexes were studied. Two mononuclear species, [MnII(OTf)(N4py)](OTf) and [MnII(OTf)(DMMN4py)](OTf), were used to prepare [MnIIIMnIV(µ-O)2(N4py)2]3+ and [MnIIIMnIV(µ-O)2(DMMN4py)2]3+, respectively, with H2O2 and a base. Structural features are compared between the two bis(μ-oxo)dimanganese complexes, which are nearly identical. Reduction potentials of the two bis(μ-oxo)dimanganese complexes are also comparable. Reactivity studies towards O–H bond activation with the two bis(μ-oxo)dimanganese complexes show marginal differences, presumably due to the divided equatorial ligand field effects by the bis(μ-oxo) bridge. Formation of a high-valent MnIV-oxo species was explored using [MnII(OTf)(DMMN4py)](OTf), different amounts of ceric(IV) ammonium nitrate (CAN), and water in acetonitrile. Upon the addition of 2 equiv. CAN to [MnII(OTf)(DMMN4py)](OTf), a broad near-IR band was instantly shown, which is a characteristic feature of [MnIV(O)(DMMN4py)]2+. This intermediate decayed to [MnIIIMnIV(µ-O)2(DMMN4py)2]3+ (at room temperature) or [MnIVMnIV(µ-O)2(DMMN4py)2]3+ (at 0 °C). Addition of 4 equiv. CAN to [MnII(OTf)(DMMN4py)](OTf) also generates a broad near-IR band; however, the intermediate decays to a new chromophore, which might be a CeIV-bound MnIV-oxo complex. Additional experiments are needed to further characterize this new chromophore. Two novel mononuclear MnII complexes were prepared to examine equatorial ligand field effects within a wide range of the ligand field strength. The C–H bond activation reactivity of [MnIV(O)(N3pyQ)]2+ follows an oxidative reactivity trend that shows a linear correlation between the reactivity and equatorial ligand field strength. The oxidation state and electronic structure of a high-valent cobalt complex, [CoIV(ONO2)2(NNN)], was studied using electron paramagnetic resonance (EPR) and computational methods. This high-valent cobalt complex shows an unusual high spin density at the Co center. The results of single- and multireference computations suggest that the unpaired electron is located at dz2 orbital, which may facilitate concerted proton-electron transfer (CPET) reactions between [CoIV(ONO2)2(NNN)] and substrates. Transition state (TS) structures with different proton accepting oxygens on the nitrate ligands were computationally investigated. Calculated activation parameters from those TSs are comparable to the experimental values for ethylbenzene oxidation reaction. However, a notable deviation was observed between experimental and calculated activation parameters for 9,10-dihydroanthracene (DHA) oxidation reaction. Preliminary multireference calculation data show more features of each transition state, which provides insight to propose the most probable TS for the oxidation reactions of [CoIV(ONO2)2(NNN)] and substrates.