Application of Hemilabile Ligands to “At-Metal Switching” Hydrogenation Catalysis

Abstract

The paper describes the development of switchable catalysts: i.e., precatalysts that are activated by a reagent and the resulting active catalyst can be shut off with a second reagent. A concept is introduced involving oxidative addition of a rhodium(I) catalyst with trityl chloride and reductive activation of dichlororhodium(III) phosphines with cobaltocene. Part 1 of the paper describes the development of the catalytic platforms, which are 2-diphenylphosphinoanisole (PPh2An) complexes of Rh and Ir. Part 2 describes the proof of concept as applied to the hydrogenation of styrene, including mechanistic investigations. The rhodium catalysts were developed from Rh2Cl2(C2H4)4, which was converted to Rh2Cl2(C2H4)2(κ1-PPh2An)2 and RhCl(κ1-PPh2An)(κ2-PPh2An). This charge-neutral chloride is a precursor to [Rh(κ2-PPh2An)2]BArF4 and the precatalyst [RhCl2(κ2-PPh2An)2]BArF4. The iridium catalysts were developed from Ir2Cl2(coe)4, which reacts with PPh2An to give IrClH(κ2-PPh2C6H4OCH2)(κ2-PPh2An). This cyclometalated complex behaves equivalently to IrCl(PPh2An)2. IrClH(κ2-PPh2C6H4OCH2)(κ2-PPh2An) readily reacts with H2 to form IrClH2(κ1-PPh2An)(κ2-PPh2An), which is a viable precursor to the off-state catalyst [IrCl2(κ2-PPh2An)2]BArF4. In part 2, we demonstrate that the complexes [MCl2(κ2-PPh2An)2]BArF4 (M = Rh, Ir) are inactive for styrene hydrogenation, in contrast with the other M-PAn compounds. Especially in the case of Rh, the hydrogenation is well controlled by the addition of selected reagents. Details of oxidative addition/reductive activation (OA/RA) are elucidated using cyclic voltammetry and stoichiometric chemical redox experiments.