Mechanistic insight into the borrowing hydrogen reaction catalysed by a Pd MLC catalyst: unveiling the ligand-to-ligand hydrogen transfer pathway

Abstract

Metal–ligand bifunctional catalysis plays a pivotal role in various catalytic hydrogenation and dehydrogenation processes. However, elucidating the catalytic mechanisms of bifunctional catalysts containing non-d⁶ metal centers presents a formidable challenge due to their versatile coordination modes. This study delves into the hydrogen borrowing reaction catalyzed by a Pd(II) metal–ligand bifunctional catalyst featuring a 2-pyridonate type ligand. Through comprehensive DFT calculations, we have identified the ^tBuO⁻ coordinated complex (B-IM0) as the active species responsible for initiating the reaction. Remarkably, we propose a novel mechanism in which ^tBuO⁻ serves as an ancillary ligand, facilitating proton transfer during alcohol dehydrogenation. In the crucial β-H elimination reaction, the catalyst employs a distinctive ligand-to-ligand hydrogen transfer (LLHT) mechanism, bypassing the formation of a stable Pd–H intermediate. This LLHT mechanism is also applicable to chalcone hydrogenation. Intrinsic reaction coordinate analysis reveals the collaborative involvement of both the ligand and metal center in activating the C–H bond during the LLHT process. Calculations of Nuclear Independent Chemical Shift (NICS) parameters suggest that ligand protonation restores the aromaticity of the quinolin-2-ol ligand, serving as the driving force in this intricate mechanism. Our findings underscore the specificity of the metal–ligand bifunctional catalyst with a non-d⁶ metal center. These insights are poised to inspire the design of future catalysts with enhanced capabilities and specificity in related catalytic processes.