Abstract
A highly robust immobilized [Cp*IrCl2]2 precatalyst on Wang resin for transfer hydrogenation, which can be recycled up to 30 times, was studied using a novel combination of X-ray absorption spectroscopy (XAS) at Ir L3-edge, Cl K-edge, and K K-edge. These culminate in in situ XAS experiments that link structural changes of the Ir complex with its catalytic activity and its deactivation. Mercury poisoning and "hot filtration" experiments ruled out leached Ir as the active catalyst. Spectroscopic evidence indicates the exchange of one chloride ligand with an alkoxide to generate the active precatalyst. The exchange of the second chloride ligand, however, leads to a potassium alkoxide-iridate species as the deactivated form of this immobilized catalyst. These findings could be widely applicable to the many homogeneous transfer hydrogenation catalysts with Cp*IrCl substructure.
Highlights
Mechanistic understanding of the deactivation pathways, and possible reactivation, is vitally important in the application of catalytic reactions in industry
We report a mechanistic study into the deactivation of a common Cp*Ir transfer hydrogenation precatalyst
We recently reported an immobilized transfer hydrogenation precatalyst [Cp**IrCl2]2, (Cp** = immobilized Cp* ligand) on a Wang resin support, which was reused up to 35 times in the presence of KOtBu, which will become important in the catalyst deactivation that follows (Scheme 2), using iPrOH as Received: December 22, 2014
Summary
Mechanistic understanding of the deactivation pathways, and possible reactivation, is vitally important in the application of catalytic reactions in industry. We recently reported an immobilized transfer hydrogenation precatalyst [Cp**IrCl2]2, (Cp** = immobilized Cp* ligand) on a Wang resin support, which was reused up to 35 times in the presence of KOtBu, which will become important in the catalyst deactivation that follows (Scheme 2), using iPrOH as Received: December 22, 2014
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