Nitrate and nitroarene hydrogenations catalyzed by alkaline-earth nickel phosphide clathrates

Abstract

The alkaline-earth-containing nickel phosphide clathrates AeNi₂P₄ (Ae = Ba, Sr) are investigated as catalysts for the reduction of nitrate or nitroarenes in aqueous or ethanolic solution, respectively. While AeNi₂P₄ clathrates are inactive in their bulk polycrystalline form, they become active in nitrate hydrogenation after size reduction by either grinding or ball milling. However, while the clathrate structure remains intact after manual grinding, ball milling is of limited use as it results in significant clathrate degradation. Ground AeNi₂P₄ catalysts are also active in nitroarene hydrogenation. Condensation products such as azoxy- and azo-benzenes form early (4 h) but anilines accumulate after long reaction times (24 h). Unexpectedly, BaNi₂P₄ partially devinylates nitrostyrene to nitrobenzene. Overall, BaNi₂P₄ is more active than SrNi₂P₄ in both nitrate and nitroarene hydrogenation. These results showcase the potential utility of clathrates in a growing number of catalytic transformations.