Generation of a Mn(IV)–Peroxo or Mn(III)–Oxo–Mn(III) Species upon Oxygenation of Mono- and Binuclear Thiolate-Ligated Mn(II) Complexes

Synthesis and Characterization of Manganese Dithiocarbamate Complexes: New Evidence of Dioxygen Activation

(1) Background: Metal dithiocarbamate compounds have long been the subject of research due to their ease of formation, excellent properties and potential applications. However, manganese complexes with dithiocarbamates, to our knowledge, have never been used for medical imaging applications. With the aim of developing a new class of mononuclear manganese(II)-based agents for molecular imaging applications, we performed a specific investigation into the synthesis of mononuclear bis-substituted Mn(II) complexes with dithiocarbamate ligands. (2) Methods: Synthesis in either open or inert atmosphere at different Mn(II) to diethyldithiocarbamate molar ratios were performed and the products characterized by IR, EA, ESI-MS and XRD analysis. (3) Results: We found that only under oxygen-free atmospheric conditions the Mn(II) complex MnL₂, where L = diethyldithiocarbamate ligand, is obtained, which was further observed to react with dioxygen in the solid state to form the intermediate superoxo Mn(III) complex [MnL₂(η²-O₂)]. The existence of the superoxo complex was revealed by mass spectroscopy, and this species was interpreted as an intermediate step in the reaction that led the bis-substituted Mn(II) complex, MnL₂, to transform into the tris-substituted Mn(III) complex, MnL₃. A similar result was found with the ligand L’ (= bis(N-ethoxyethyl)dithiocarbamate). (4) Conclusions: We found that in open atmosphere and in aqueous solution, only manganese(III) diethyldithiocarbamate complexes can be prepared. We report here a new example of a small-molecule Mn(II) complex that efficiently activates dioxygen in the solid state through the formation of an intermediate superoxide adduct.

A combined experimental and theoretical study on a single, unsupported oxo-bridged Mn(III,III) dimer coordinated to two iminobenzosemiquinone π-radical anions

Ligand H2LAP comprises a non-innocent 2-aminophenol unit and an innocent bis(pyridin-2-ylmethyl)amine unit. The ligand, upon reaction with an equivalent amount of Mn(ClO4)2·6H2O in the presence of Et3N under air in MeOH, provided a mono(oxo)-bridged dinuclear Mn2 complex ({[(LISQ)MnIII-O-MnIII(LISQ)][(ClO4)]2}; 1). X-ray crystal structure analysis of complex 1 revealed that in the dicationic unit, the physical oxidation state of each Mn ion was +III and the 2-aminophenol unit of ligand H2LAP was in its one-electron oxidized iminobenzosemiquinone form. 1H-NMR measurement of complex 1 confirmed that the complex acquired a diamagnetic ground state (St = 0). Thus, antiferromagnetic couplings among the paramagnetic centers were realized. The UV-Vis-NIR spectrum of complex 1 was consisted of ligand-to-metal charge-transfer transitions in the visible region, while ligand-to-metal and metal-to-ligand charge-transfer transitions were noticed in the near-infrared region due to the presence of iminobenzosemiquinone radical units. The cyclic voltammogram of the complex showed three one-electron oxidation waves and two one-electron reduction waves. While the first two oxidation processes were metal-based, the two successive reductions were ligand-centered. DFT-based theoretical studies confirmed the assignment.

Steric control of dioxygen activation pathways for MnII complexes supported by pentadentate, amide-containing ligands

Dioxygen activation at manganese centers is well known in nature, but synthetic manganese systems capable of utilizing O2 as an oxidant are relatively uncommon. These present investigations probe the dioxygen activation pathways of two mononuclear MnII complexes supported by pentacoordinate amide-containing ligands, [MnII(dpaq)](OTf) and the sterically modified [MnII(dpaq2Me)](OTf). Dioxygen titration experiments demonstrate that [MnII(dpaq)](OTf) reacts with O2 to form [MnIII(OH)(dpaq)](OTf) according to a 4 : 1 Mn : O2 stoichiometry. This stoichiometry is consistent with a pathway involving comproportionation between a MnIV-oxo species and residual MnII complex to form a (μ-oxo)dimanganese(iii,iii) species that is hydrolyzed by water to give the MnIII-hydroxo product. In contrast, the sterically modified [MnII(dpaq2Me)](OTf) complex was found to react with O2 according to a 2 : 1 Mn : O2 stoichiometry. This stoichiometry is indicative of a pathway in which a MnIV-oxo intermediate abstracts a hydrogen atom from solvent instead of undergoing comproportionation with the MnII starting complex. Isotopic labeling experiments, in which the oxygenation of the MnII complexes was carried out in deuterated solvent, supported this change in pathway. The oxygenation of [MnII(dpaq)](OTf) did not result in any deuterium incorporation in the MnIII-hydroxo product, while the oxygenation of [MnII(dpaq2Me)](OTf) in d3-MeCN showed [MnIII(OD)(dpaq2Me)]+ formation. Taken together, these observations highlight the use of steric effects as a means to select which intermediates form along dioxygen activation pathways.

A Mn(iv)-peroxo complex in the reactions with proton donors

Protons play an important role in promoting O-O or M-O bond cleavage of metal-peroxo complexes. Treatment of side-on O2-bound [PPN][MnIV(TMSPS3)(O2)] (1, PPN = bis(triphenylphosphine)iminium and TMSPS3H3 = 2,2',2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine) with perchloric acid (HClO4) in the presence of PR3 (R = phenyl or p-tolyl) results in the formation of neutral five-coordinate MnIII(OPR3)(TMSPS3) complexes (R = phenyl, 2a; p-tolyl, 2b), which are confirmed by X-ray crystallography. Isotope labelling experiments demonstrate that the oxygen atom in the phosphine oxide product derives from the peroxo ligand of 1. Reactions of 1 with weak proton donors, such as phenylthiol, phenol, substituted phenol and methanol, are also investigated to explore the reactivity of the MnIV-peroxo complex, leading to the isolation of a series of five-coordinate [MnIII(L)(TMSPS3)]- complexes (L = phenylthiolate, phenolate or methoxide). Mechanistic aspects of the reactions of the MnIV-peroxo complex with proton donors are discussed as well.

Improved Electrical Wiring of Glucose Oxidase Enzyme with an in-Situ Immobilized Mn(1,10-Phenanthroline)2Cl2-Complex/Multiwalled Carbon Nanotube-Modified Electrode Displaying Superior Performance to Os-Complex for High-Current Sensitivity Bioelectrocatalytic and Biofuel Cell Applications

The search for a new and efficient transducer that can electrically connect enzyme active sites, like flavin adenine dinucleotide in glucose oxidase (GOx), with the electrode surface is a cutting-edge research area. Currently, Os(bpy)-complex pendent polyvinylpyridine/polyvinyl imidazole/pyridinium hydrogel based chemically modified electrodes have been widely used for this purpose (bpy = 2,2'-bipyridine). Herein, we report, a [Mn₂^III(phen)₄(O)(Cl)₂]²⁺ complex/Nafion-immobilized carboxylic acid-functionalized multiwalled carbon nanotube modified glassy carbon electrode (GCE/f-MWCNT@Mn₂(Phen)₄O(Cl)₂-Nf, phen = 1,10-phenanthroline), prepared by an in-situ electrochemical method using the precursor, Mn(phen)₂Cl₂, as an efficient and low cost alternate to the Os-complex transducer, for the glucose oxidase enzyme (GOx) based bio-electro-catalytic system. The existence of the key active site, [Mn₂^III(phen)₄(O)(Cl)₂]²⁺, on the modified electrode was confirmed by physicochemical characterizations using transmission electron microscope, Raman, infrared, and UV-vis spectroscopes and electrospray ionization mass spectrometry techniques. The Mn-complex modified electrode showed a redox peak at E°' = 0.55 V vs Ag/AgCl in neutral solution with a surface excess (Γ_Mn) value of 5.6 × 10^-9 mol cm^-2. The GOx enzyme bioanode prepared by adsorbing GOx on the Mn-complex modified electrode has shown an efficient bioelectrocatalytic oxidation of glucose with a Tafel slope value of 111 mV dec^-1. Amperometric i-t analysis of glucose showed a calibration plot in a linear range of 50-550 μM and with current sensitivity of 316.7 μA mM^-1 cm^-2. The current sensitivity value obtained here is about 2-80 000 times higher than that of the Os(bpy)-complex based transducers used for GOx based bio-electro-catalytic applications. Utilizing this new bioanode system along with a Pt-based oxygen reduction electrode, a new biofuel cell was constructed and achieved a power density value 7.5 μW cm^-2.