Combining Bimetallic Scaffolds and Metal-Coordinated Phenoxyl Radicals for Multi Electron Transformations: A Step Beyond Nature
Final Report Abstract
For enabling two-electron (or multi-electron) substrate transformations, natural metalloenzymes pursue two main strategies, namely (i) the use of two or more redox-active metal ions that work in concert within the active site, or (ii) the use of a single redox-active metal ion in combination with an organic redox cofactor, usually a redox-active ligand. A prominent example exploiting the latter strategy is the mononuclear copper enzyme galactose oxidase (GOase) that features a single copper ion shuttling between oxidation states +I and +II and a metal-bound, cysteinelinked tyrosine residue that operates as an additional one-electron cofactor shuttling between tyrosine/tyrosinate and tyrosyl radical forms; this allows the enzyme to achieve the two-electron oxidation of primary alcohols to aldehydes coupled to dioxygen reduction to hydrogen peroxide. Bioinspired coordination chemistry aims at emulating functional principles of nature in artificial systems. Taking GOase as a blueprint, prior work has introduced metal-bound redox-active phenolates in the ligand scaffolds of mononuclear complexes; a popular ligand class in this context are the so-called salen (N,N'-bis(salicylidene)ethylenediamine) ligands. The aim of this project was to combine, in a single system, both of nature’s strategies, viz., to develop and investigate novel binuclear coordination compounds that can undergo multiple metal- and ligand-based redox processes. To that end a novel class of bimetallic complexes has been established, based a tailor-made binucleating ligand scaffold that can be viewed as a pyrazolate-expanded variant of the prominent mononucleating salen-type ligands. A comprehensive set of dicopper(II) and dinickel(II) complexes hosting exogenous carboxylate or pyrazolate coligands within the bimetallic pocket has been fully characterized structurally and spectroscopically. These bimetallic systems provide multiple potential oxidation sites, viz. the two central metal ions and the two coordinating phenolato groups at the ligand periphery. It has been demonstrated that the loci of the first and second oxidation are mostly on the phenolates, yielding coordinated phenoxyl radicals. However, the electronic structures are dramatically dependent on the experimental conditions, specifically on the presence of potentially coordinating solvent, anions or other ligands, and may include significant metal(III) contributions in the oxidized species. While elucidation of the electronic structures of the oxidized species is thus particularly challenging, such complex situation also offers interesting perspectives for modulating the properties and reactivities of the new systems. On this basis, future work should target complexes with labile groups within the bimetallic cleft, which is expected to enable cooperative and multi-electron redox transformations of substrates. The project benefitted from complementary methodological expertise of the collaborating research groups at the University of Göttingen (Germany) and IIT Kanpur (India), and the German PhD student involved in the project spent two months at the partner institute in India.
Publications
- Neutral, Cationic, and Anionic Low-Spin Iron(III) Complexes Stabilized by Amidophenolate and Iminobenzosemiquinonate Radical in N,N,O Ligands. Inorg. Chem. 2014, 53, 36-48
A. Rajput, A. K. Sharma, S. K. Barman, D. Koley, M. Steinert, R. Mukherjee
(See online at https://doi.org/10.1021/ic401985d) - Preorganized Anion Receptors Exhibiting Anion-p Interactions & Binuclear Copper and Nickel Complexes of Non-Innocent Pyrazolate-Based Ligands – An Electronic and Reactivity Study. Dissertation. Sierke-Verlag, Göttingen 2016, ISBN 13:978-3-86844-788-0
A. Bretschneider
