Comparative Density Functional Theory Study of Cinchonidine and Hydrogen Coadsorption on Platinum Group Metals (Rh, Ir, Pd, and Pt) and Its Implications on Surface Chiral Site Formation

Abstract

Coadsorption of hydrogen and cinchonidine (CD), the chiral modifier most frequently applied to bestow chiral sites onto platinum group metals, has been studied theoretically. In focus was the stability and configuration (spatial orientation) of CD adsorbed on the (111) surfaces of Rh, Ir, Pd, and Pt. On all these metals, CD is shown to be adsorbed with the quinoline moiety laying nearly parallel to the surface, likely through πbonding. At high H-coverages, the position of the quinoline ring becomes tilted and the binding energy of CD decreases. While the configuration of adsorbed CD on Rh, Ir, and Pt is comparable, on Pd, it is significantly different, particularly as regards the position of the vinyl group of the quinuclidine moiety, which lays flatly on the surface and is involved in the adsorption bonding. On all metals, hydrogen coadsorption notably influences the stability and configuration of adsorbed CD. Generally, the stability of adsorbed CD decreases with increasing hydrogen coverage. The metals also show different propensities for H-transfer from the surface to adsorbed CD. Analyses of density of states projected on the orbitals of CD adsorbed on the (111) metal surfaces provide insights into the adsorption mechanisms depending on the H-coverage.

Influence of N-Substituents on the Adsorption Geometry of OH-Functionalized Chiral N-Heterocyclic Carbenes

“…It should be noted that exposure to reaction conditions involves the addition of various factors that can also affect the adsorption geometry of the ligands and their interaction with the prochiral reactant. It was demonstrated that the surface adsorption of hydrogen, , solvent molecules, , coadsorption of modifiers, , and reactants − can alter the reaction mechanism and the adsorption geometry of surface ligands. It cannot be excluded that similar factors will also modify the adsorption geometry of NHC ligands during the enantioselective α-arylation reaction.…”

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

“…However, it was shown that in aprotic solvents and the gas-phase, the proton can also be delivered by the Pt-surface (as a spill-over, one might say), 89,90 although Hahn and Baiker described the process consistently as H atom transfer. 91 Mechanistically, the chemistry of these modifying chiral ligands on the surface could be closely related to heterolytic cleavage. However, in the vast literature on ligand-induced, heterogeneous, asymmetric hydrogenations, heterolytic cleavage does not seem to play a role in the discussions and we mention this catalysis only here in the introduction.…”

Coadsorption and Interaction of Quinolines and Hydrogen on Platinum Group Metals and Gold: A First-Principles Analysis

“…The adsorption of quinolines and derivatives thereof on metals and metal oxides plays an important role in many fields, including heterogeneous catalysis, − environmental pollution, − and corrosion inhibitors . Spectroscopic investigations − and density functional theory (DFT) studies − have shed some light on the adsorption modes and energetics of quinolines on platinum group (PG) metals. Particularly, substantial experimental work has been expended to examine the adsorption of cinchona alkaloids, derivatives of quinoline, which are applied to impart chirality to PG metal catalysts for the enantioselective hydrogenation of activated ketones. − A comprehensive and critical review of adsorption studies of various aromatic and N-heteroaromatic compounds on metals by first-principles DFT published till 2009 has been provided by Jenkins .…”