A monolith immobilised iridium Cp* catalyst for hydrogen transfer reactions under flow conditions

DOI: 10.1039/C4OB02376E

An immobilised iridium hydrogen transfer catalyst has been developed for use in flow based processing by incorporation of a ligand into a porous polymeric monolithic flow reactor. The monolithic construct has been used for several redox reductions demonstrating excellent recyclability, good turnover numbers and high chemical stability giving negligible metal leaching over extended periods of use.

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Insights into the iridium-catalyzed water oxidation mechanism from a DFT study

Dr. David Balcells, Prof. Odile Eisenstein, Prof. Robert H Crabtree, Agusti Lledos Departament de Quimica, Universitat Autonoma de Barcelona, Bellaterra, Spain; Institut Charles Gerhardt, Universite Montpellier 2, Montpellier, France; Department of Chemistry, Yale University, New Haven, United States

The development of a new energy model is a major challenge in modern chemistry. The climate change and the raise of oil prices prompt the development of clean and cheap energy resources. In this field, artificial photosynthesis is one of the most promising solutions.¹ The catalytic oxidation of water to dioxygen is a fundamental part of this process. The mononuclear iridium complex Cp*Ir(ppy)(Cl) (ppy = phenylpyridine) is one of the most efficient catalysts reported for this reaction (Figure).² DFT calculations support the oxo complex Cp*IrO(ppy) as the active species. The electronic structure of this complex is characterized by having the antibonding p*(Ir=O) orbitals half-occupied. The calculations suggest that the reaction mechanism consists of an intermolecular attack of water to the oxo ligand. This reaction involves the formation of the O-O bond and a proton transfer, which is assisted by the molecules of water solvating the catalyst.

Figure. Iridium-catalyzed water oxidation.

References
(1) Hammarström, L.; Hammes-Schiffer, S. Acc. Chem. Res. 2009, 42, 1859-1860.
(2) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H. J. Am. Chem. Soc.2009, 131, 8730-8731.

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The water-soluble iridium complex {Cp*Ir[6,6′-(OH)₂bpy](H₂O)}[OTf]₂(Cp*=η⁵-pentamethylcyclopentadienyl, bpy=2,2′-bipyridine) was found to be a general and highly efficient catalyst for the N-alkylation of the poor nucleophilic sulfonamides with alcohols as alkylating agents in water. The presence of OH units in the bpy ligand is crucially important for the catalytic activity of the iridium complex. Mechanistic investigations revealed that the catalytically active species is a ligand-metal bifunctional iridium complex bearing an N,N′-chelated 2,2′-bipyridinated ligand and an aqua ligand. Notably, the present catalytic system and the proposed mechanism provide a new horizon and scope for the development of “hydrogen autotransfer (or hydrogen-borrowing) processes”.