Metal complex-assisted activation of small molecules. From NO to superoxide and peroxides

Abstract

Transition metal centres are one of the primary targets for nitric oxide (NO), superoxide (O2(-)) and hydrogen peroxide (H2O2), which are small molecules present in a biological milieu, and of industrial and environmental interest. Coordination to a metal centre modulates their redox behaviour in such a way that they become activated for an inner-sphere oxidation or reduction, depending on the electronic and redox properties of a particular transition metal ion. Since the related redox reactions play multiple roles in physiological and pathophysiological processes, as well as in chemical catalysis in terms of synthetic applications and exhaust gas purification, the elucidation of the mechanisms of the elementary reaction steps behind these complex processes is of fundamental importance. This review concentrates on our work in this area, where by applying low temperature and high pressure kinetic and thermodynamic techniques we shed more light on the mechanisms of the particular reaction steps involved in the activation of NO, O2(-) and various peroxides. The studies include work on solvent exchange reactions that control the binding of small molecules to the metal centre and subsequent electron-transfer processes. We paid special attention to different iron and manganese complexes with heme and non-heme ligand systems.

Cellular Fates of Manganese(II) Pentaazamacrocyclic Superoxide Dismutase (SOD) Mimetics: Fluorescently Labeled MnSOD Mimetics, X-ray Absorption Spectroscopy, and X-ray Fluorescence Microscopy Studies

“…8 Recently the enantiomer of M40403, named GC4419 (Scheme 1), has entered phase 2 trial for protection against chemoradiation therapy-induced oral mucositis (NCT02508389). 9 MnPAMs cycle between the Mn(II) and Mn(III) states as they dismutate superoxide, 4,[10][11][12][13] mimicking the MnSOD catalytic cycle. Although MnPAMs were considered selective for superoxide dismutation, it is now known that Mn(II)pyane (Scheme 2) also disproportionates nitric oxide in vitro, 14 in cells 15 and spermatozoa.…”

Intrinsic Properties of Nitric Oxide Binding to Ferrous and Ferric Hemes

“…In contrast, iron(III)-heme proteins and model complexes have revealed large and positive activation entropies and activation volumes consistent with a dissociative ligand substitution mechanism for both NO binding and release. 12 In the gas phase, we face iron(II)-and iron(III)-heme ions that are both four coordinate at the metal core. The metal center presents exactly the same environment and any difference in kinetics and equilibrium data is therefore a consequence of the different oxidation state and electronic structure.…”

N2.3−: Filling a Gap in the N2n− Series

“…This lack of interest is surprising, however, in view of the isoelectronic relationship of N 2 C 3À with the hyperoxide (superoxide) radical anion O 2 C À which has a long-established structural [8] and coordination chemistry [9] and which is widely researched because of its biological and industrial relevance. [10] Using dysprosium and yttrium complex fragments, Evans and co-workers have now observed the three-electron reduced N 2 as a bridging ligand in complexes 1-4 ( Figure 1). [11] The compounds were obtained from N 2 [12] N 2 is a potentially non-innocent [13a] (or "suspect" [13b] ) ligand, capable of acting in different oxidation states (N 2 , N 2 2À , N 2 4À ) in coordination compounds.…”