Mimicking Elementary Reactions of Manganese Lipoxygenase Using Mn-hydroxo and Mn-alkylperoxo Complexes

Manganese lipoxygenase (MnLOX) is an enzyme that converts polyunsaturated fatty acids to alkyl hydroperoxides. In proposed mechanisms for this enzyme, the transfer of a hydrogen atom from a substrate C-H bond to an active-site MnIII-hydroxo center initiates substrate oxidation. In some proposed mechanisms, the active-site MnIII-hydroxo complex is regenerated by the reaction of a MnIII-alkylperoxo intermediate with water by a ligand substitution reaction. In a recent study, we described a pair of MnIII-hydroxo and MnIII-alkylperoxo complexes supported by the same amide-containing pentadentate ligand (6Medpaq). In this present work, we describe the reaction of the MnIII-hydroxo unit in C-H and O-H bond oxidation processes, thus mimicking one of the elementary reactions of the MnLOX enzyme. An analysis of kinetic data shows that the MnIII-hydroxo complex [MnIII(OH)(6Medpaq)]+ oxidizes TEMPOH (2,2′-6,6′-tetramethylpiperidine-1-ol) faster than the majority of previously reported MnIII-hydroxo complexes. Using a combination of cyclic voltammetry and electronic structure computations, we demonstrate that the weak MnIII-N(pyridine) bonds lead to a higher MnIII/II reduction potential, increasing the driving force for substrate oxidation reactions and accounting for the faster reaction rate. In addition, we demonstrate that the MnIII-alkylperoxo complex [MnIII(OOtBu)(6Medpaq)]+ reacts with water to obtain the corresponding MnIII-hydroxo species, thus mimicking the ligand substitution step proposed for MnLOX.

Hydrolysis of Scandium Alkyl Derivatives Supported by a Pentadentate Diborate Ligand: Interconversion of Hydroxo and Oxo Complexes

Uncontrolled reaction of water with scandium alkyls (compounds <b>1-R</b>) supported by a dianionic, pentadentate ligand leads to rapid formation of an oxo-bridged dimer (<b>2</b>). Solid state samples can be exposed to ambient atmosphere to generate samples enriched in the bridging dihydroxo dimer <b>3</b>, which slowly converts to the m-oxo species with elimination of water. DFT computations show that <b>3</b> is actually more thermodynamically stable than <b>2</b>, but the reactivity of <b>3</b> with the water eliminated leads to its decomposition to <b>2</b> and several hydrolysis products. Some of these products were characterized by X-ray crystallography, specifically a hexameric scandium dihydroxo cluster (<b>4</b>) in which the pentadentate ligand has partially demetallated. Attempts to synthesize hydroxo complex <b>3</b> by protonation of <b>2</b> also lead to hydrolysis products.

Hydrolysis of Scandium Alkyl Derivatives Supported by a Pentadentate Diborate Ligand: Interconversion of Hydroxo and Oxo Complexes

Uncontrolled reaction of water with scandium alkyls (compounds <b>1-R</b>) supported by a dianionic, pentadentate ligand leads to rapid formation of an oxo-bridged dimer (<b>2</b>). Solid state samples can be exposed to ambient atmosphere to generate samples enriched in the bridging dihydroxo dimer <b>3</b>, which slowly converts to the m-oxo species with elimination of water. DFT computations show that <b>3</b> is actually more thermodynamically stable than <b>2</b>, but the reactivity of <b>3</b> with the water eliminated leads to its decomposition to <b>2</b> and several hydrolysis products. Some of these products were characterized by X-ray crystallography, specifically a hexameric scandium dihydroxo cluster (<b>4</b>) in which the pentadentate ligand has partially demetallated. Attempts to synthesize hydroxo complex <b>3</b> by protonation of <b>2</b> also lead to hydrolysis products.

Chemical bath synthesis of metal chalcogenide films. Part 39. Chemical bath deposition of ZnS films by thioacetamide

ZnS thin films are promising as a buffer layer in solar cells, which can be basis of photovoltaic cells, photoelectric sensors, and light-emitting diodes. For the preparation of thin ZnS films by chemical bath deposition, thioacetamide or thiourea is used as a chalcogenization agent, and ammonia, triethanolamine and sodium citrate are mainly used as ligands, carrying out the process in an alkaline medium. In the present work, in order to predict the conditions of hydrochemical deposition of ZnS films, we have analyzed ionic equilibria in two reaction systems “ZnCl2 – NH4OH – CH3CNH2” and “ZnCl2 – CH3CSNH2 – KHC8H4O4” that differ in acidity of the medium. An analysis of ionic equilibrium showed that in the first bath ~80% of the metal is in the form of a neutral hydroxo complex Zn(OH)2 at pH > 7, and in the second more than 98% of zinc is present as acetate complexes Zn(CH3COO)+ and Zn(CH3COO)2 in the range of pH from 0 to 7. The thermodynamic evaluation of the boundary conditions for the formation of zinc sulfide made it possible to conclude that a zinc sulfide film can be formed in both systems without the admixture of Zn(OH)2 hydroxide. ZnS films were obtained by hydrochemical deposition with thick about 100 nm from both systems. Using local energy-dispersive elemental analysis, it was found that the average ratio between the main elements of Zn and S in the layers obtained in an alkaline medium is 49.48 and 50.52 at.%, and in the synthesized from acidic solutions – 50.35 and 49.65 at.%. According to the data of electron microscopy, up to 85% of the agglomerates have an average size of 200-450 nm that formed from ZnS particles growing in an alkaline reaction bath. At the same time, there are aggregates whose dimensions reach 700 nm. The layers that deposited from relatively acidic solutions are distinguished by a higher degree of dispersion. Here up to ~90% of the film-forming particles is in the nanoscale range from 50 to 90 nm.