Mechanistic Study of Pd(II) Pincer-Catalyzed Acceptorless Alcohol Dehydrogenation: The Crucial Role of the K+ Counterion

Abstract Square-planar pincer complexes featuring d8 metal centers represent an important class of molecular catalysts for dehydrogenation, borrowing hydrogen transformations, and cross-coupling reactions owing to their well-defined coordination environments and tunable electronic properties. However, their coordinatively saturated nature challenges conventional catalytic pathways that rely on vacant site activation, and the fundamental principles governing hydrogen transfer in these systems remain incompletely understood. Herein, using the Pd-ONO pincer complex reported by [ Kavin, S. P.; Ramesh, R.Dalton Trans.2023, 52(29), 10038–10044]. as a model system, we systematically investigated the role of K+ counterions in β-hydride elimination and their site-dependent coordination effects through density functional theory (DFT) calculations. Three competing pathways were evaluated, including classical inner-sphere β-hydride elimination, ligand-assisted β-hydride elimination, and K+-assisted outer-sphere hydride transfer. The results reveal that the K+-assisted outer-sphere pathway exhibits the lowest activation barrier (26.3 kcal/mol), substantially outperforming the conventional inner-sphere and ligand-assisted mechanisms. Moreover, K+ preferentially coordinates at the phenoxide oxygen site, leading to enhanced stabilization compared with coordination at the oximato oxygen site. Distortion/interaction analysis demonstrates that the favorable kinetics originate from reduced structural deformation in the outer-sphere transition state, while energy decomposition analysis identifies strong electrostatic stabilization arising from simultaneous K+ coordination with multiple ligand oxygen atoms as the dominant factor lowering the reaction barrier. This work uncovers the previously underappreciated synergistic role of alkali-metal counterions in β-hydride elimination of d8 pincer complexes and provides new insights into secondary coordination sphere regulation of transition-metal catalysis.