Group 13 metal catalysts for hydrogen activation and hydrogenation

Catalysts with aromatization/dearomatization metal–ligand cooperation paradigm are of increasing interest in the field of hydrogen activation and hydrogenation, with PNP (2,6-bis[(phosphino-methyl]pyridine) as a representative class of ligand. It is highly desired to design PNP catalysts using main group metals to replace transition metals. Herein, a theoretical study of the properties of Group 13 metal (Al, Ga, In) PNP catalysts for hydrogen activation and hydrogenation was performed to comprehensively compare and analyze their catalytic mechanism and reactivity. In sharp contrast to traditional transition metal PNP catalysts that operate an aromatization/dearomatization metal–ligand cooperation, the Group 13 metal PNP catalysts adopt a tetrahedral geometry and prefer a metal-alkoxide promoted cooperation for the hydrogen activation and hydrogenation. The hard and soft acids and bases matching between the metal centers and the alkoxide is found to play a key role in the catalytic properties. PNP-In is found to be the most promising catalyst for hydrogen activation and hydrogenation, which has a proper reaction barrier (26.7 kcal·mol−1) during hydrogen activation because the In center could serve as a relatively softer acceptor for the hydride. The alkoxide bound on the softer Indium center has stronger basicity to deprotonate the PNP ligand or to obtain a proton from the H2 molecule. However, the harder Ga or Al center interacts more strongly with the alkoxide group, hampering the metal-alkoxide promoted protonation during hydrogenation. The bonding analysis and orbital energy analysis indicated that [(PNPtBu)InH(Me)]+ hydride also has a proper activity for the hydrogenation process. In contrast, although [(PNPtBu)AlH(Me)]+ has a much higher activity for hydride transfer, the Al-catalyzed hydrogenation is limited by the benzyloxide bound on the harder Al center, which is less active to assist the H − H bond cleavage or deprotonate the PNPtBu ligand. The Ga catalyst performs in between the best In catalyst and the least active Al catalyst. This understanding of the mechanism and reactivity of Group 13 metal catalysts should be helpful for the development of main group metal catalysts for hydrogen activation and hydrogenation.