Mechanistic investigations into electrocatalytic nitrate reduction by a cobalt complex reveal the critical role played by the flexible, redox-active ligand.
This
work reports a combined experimental and computational mechanistic
investigation into the electrocatalytic reduction of nitrite to ammonia
by a cobalt macrocycle in an aqueous solution. In the presence of
a nitrite substrate, the Co(III) precatalyst, [Co(DIM)(NO2)2]+ (DIM = 2,3-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,3-diene),
is formed in situ. Cyclic voltammetry and density
functional theory (DFT) calculations show that this complex is reduced
by two electrons, the first of which is coupled with nitrite ligand
loss, to provide the active catalyst. Experimental observations suggest
that the key N–O bond cleavage step is facilitated by intramolecular
proton transfer from an amine group of the macrocycle to a nitro ligand,
as supported by modeling several potential reaction pathways with
DFT. These results provide insights into how the combination of a
redox active ligand and first-row transition metal can facilitate
the multiproton/electron process of nitrite reduction.
The mechanism of water oxidation performed by a recently discovered manganese pyridinophane catalyst [Mn(PyNBu)(HO)] is studied using density functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a Mn-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O-O bond formation is found on the quintet potential energy surface and involves an intramolecular coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin Mn ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a critical role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O-O bond formation via a radical coupling reaction with a calculated reaction barrier of only 14.7 kcal mol. This mechanism of O-O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.
Acyliridium porphyrins were synthesized by the reactions of aryl aldehydes with iridium(III) porphyrin
chloride and methyl under solvent-free conditions with high yields. Selective aldehydic carbon hydrogen
bond activation (CHA) was observed without any aromatic CHA in all the cases. Mechanistic investigation
on the reactions with Ir(ttp)Cl(CO) suggested that (ttp)Ir cation was a likely intermediate of aldehydic
CHA, whereas the CHA with Ir(ttp)Me underwent an oxidative addition or σ bond metathesis pathway.
These reactions provided a facile synthesis of (arylacyl)iridium porphyrins.
Rhodium(III) porphyrin chloride reacted with aryl aldehydes in solvent-free conditions to give acyl rhodium porphyrins. Selective aldehydic without any aromatic carbon-hydrogen bond activation (CHA) was observed. At lower temperature, reduction and side products were found. Alkanals reacted poorly. On the other hand, Rh(III) porphyrin methyl reacted more cleanly with both aryl and alkyl aldehydes. These reactions provided a facile, convenient synthesis of acyl rhodium porphyrins. These activations are unique CHA by high-valent Rh(III) species. Preliminary mechanistic experiments suggested that the rhodium(III) porphyrin chloride initially formed a cationic rhodium(III) porphyrin via chloride dissociation and then underwent oxidative addition or heterolysis to yield the product. On the other hand, rhodium(III) porphyrin methyl underwent either oxidative addition or σ bond metathesis.
Porous microcarriers have aroused increasing attention recently, which can create a protected environment for sufficient cell seeding density, facilitate oxygen and nutrient transfer, and well support the cell attachment and growth. In this study, porous microcarriers fabricated from the strontium-substituted hydroxyapatite- graft-poly(γ-benzyl-l-glutamate) (Sr10-HA- g-PBLG) hybrid nanocomposite were developed. The surface grating of PBLG, the micromorphology and element distribution, mechanical strength, in vitro degradation, and Sr ion release of the obtained Sr10-HA- g-PBLG porous microcarriers were investigated, respectively. The grafting ratio and the molecular weight of the grafted PBLG of Sr10-HA- g-PBLG could be effectively controlled by varying the initial ratio of BLG-NCA to Sr10-HA-NH. The microcarriers exhibited a highly porous and interconnected microstructure with the porosity of about 90% and overall density of 1.03-1.06 g/cm. Also, the degradation rate of Sr10-HA-PBLG microcarriers could be effectively controlled and long-term Sr release was obtained. The Sr10-HA-PBLG microcarriers allowed cells adhesion, infiltration, and proliferation and promoted the osteogenic differentiation of rabbit adipose-derived stem cells (ADSCs). Successful healing of femoral bone defect was proved by injection of the ADSCs-seeded Sr10-HA-PBLG microcarriers in a rabbit model.
Selective carbonyl carbon (C(O)) and α-carbon (C(methyl)) bond activation of acetophenones was discovered by the high-valent, iridium(III) 5,10,15,20-tetrakis-4-tolylporphyrinato carbonyl chloride (Ir(ttp)Cl(CO)), which also acted as a Lewis acid in catalyzing the aldol condensation of acetophenones together with release of the coproduct water. Preliminary mechanistic studies suggest that both aliphatic and aromatic carbon−hydrogen bond activation products are kinetic products, which can be converted by reaction with water to iridium porphyrin hydride (Ir(ttp)H) via iridium porphyrin hydroxide (Ir(ttp)OH). Both Ir(ttp)OH and Ir(ttp)H were the possible intermediates to cleave the C(O)−C(methyl) bond of acetophenones and to generate iridium porphyrin acyl complexes as the thermodynamic products.
A series of Mn(II) complexes of differently substituted pyridinophane ligands, (PyNR)MnCl (R = Pr, Cy) and [(PyNR)MnF](PF) (R = Pr, Cy,Bu) are synthesized and characterized. The electrochemical properties of these complexes are investigated by cyclic voltammetry, along with those of previously reported (PyNMe)MnCl and the Mn(III) complex [(PyNMe)MnF](PF). The electronic structure of this and other Mn(III) complexes is probed experimentally and theoretically, via high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy ab initio quantum chemical theory (QCT), respectively. These studies show that the complexes contain relatively typical six-coordinate Mn(III). The catalytic activity of these complexes toward both HO disproportionation and HO oxidation has also been investigated. The rate of HO disproportionation decreases with increasing substituent size. Some of these complexes are active for electrocatalytic HO oxidation; however this activity cannot be rationalized in terms of simple electronic or steric effects.
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