Design of Iron Coordination Complexes as Highly Active Homogenous Water Oxidation Catalysts by Deuteration of Oxidation-Sensitive Sites

Abstract

The nature of the oxidizing species in water oxidation reactions with chemical oxidants catalyzed by α-[Fe(OTf)2(mcp)] (mcp = N, N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine, OTf = trifluoromethanesulfonate anion), (1α) and β-[Fe(OTf)2(mcp)], (1β) has been investigated. Mössbauer spectroscopy provides definitive evidence that 1α and 1β generate oxoiron(IV) species as the resting state. Decomposition paths of the catalysts have been investigated by identifying and quantifying ligand fragments that form upon degradation. This analysis correlates the water oxidation activity of 1α and 1β with stability against oxidative damage of the ligand via aliphatic C-H oxidation. The site of degradation and the relative stability against oxidative degradation is shown to be dependent on the topology of the catalyst. Furthermore, the mechanisms of catalyst degradation have been rationalized by computational analyses, which also explain why the topology of the catalyst enforces different oxidation sensitive sites. This information has served for creating catalysts where sensitive C-H bonds have been replaced by C-D bonds. Deuterated analogs, D4-α-[Fe(OTf)2(mcp)] (D4-1α), D4-β-[Fe(OTf)2(mcp)] (D4-1β) and D6-β-[Fe(OTf)2(mcp)] (D6-1β) were prepared, and their catalytic activity has been studied. D4-1α proves to be an extraordinarily active and efficient catalyst (up to 91% of O2 yield); it exhibits initial reaction rates identical to its protio analogue, but it is substantially more robust towards oxidative degradation and yields more than 3400 TON (n(O2)/n(Fe)). Altogether evidence that the water oxidation catalytic activity is performed by a well-defined coordination complex and not by iron oxides formed after oxidative degradation of the ligands.

Post Synthetic Defect Engineering of UiO-66 Metal-Organic Framework with Iridium(III)-HEDTA Complex and Application in Catalysis for Water Oxydation

“…Alternatively, it might be hypothesized that the oxidative potential of iridium inIrEDTA@UiO-66 is slightly higher than in the molecular precursor and hybrid material IrEDTA@TiO2, thus asking for a higher Ce 4+ /Ce 3+ ratio in order to reach the appropriate "Nernstian" potential for WO. [36] Both the explanations are consistent with the observation that the addition of a second aliquot of CAN restores the catalytic activity.…”

Post Synthetic Defect Engineering of UiO-66 Metal–Organic Framework with An Iridium(III)-HEDTA Complex and Application in Water Oxidation Catalysis

“…It can be hypothesized some Ce 4+ might undergo an exchange with the Zr 4+ ions of MOF, becoming not available anymore for driving the oxidative splitting of water. Alternatively, it might be hypothesized that the oxidative potential of iridium inIrEDTA@UiO-66 is slightly higher than in the molecular precursor and hybrid material IrEDTA@TiO 2 , thus asking for a higher Ce 4+ /Ce 3+ ratio in order to reach the appropriate "Nernstian" potential for WO [41,42]. Both the explanations are consistent with the observation that the addition of a second aliquot of CAN restores the catalytic activity.…”

Oxido‐Hydroxido‐ and Oxido‐Aminato‐Osmium(V) Complexes with a Cyclohexanediamine‐Based Tetradentate Ligand as Active Oxidants for Dihydroxylation and Aminohydroxylation of Alkenes

“…BPMCN can adopt three conformations, cis ‐ α, cis ‐ β , and planar, in an octahedral metal complex. Among the three conformations, the cis ‐ α one is the most energetically stable, so that most of complexes with BPMCN so far been reported take the cis ‐ α structure . In this study, we obtained both cis ‐ α and cis ‐ β isomers of the dichloro‐osmium(III) complexes, 1 and 2 , by the treatment of (NH 4 ) 2 [Os IV Cl 6 ] with BPMCN.…”