Dehydrogenation of formic acid mediated by a Phosphorus–Nitrogen PN³P-manganese pincer complex: Catalytic performance and mechanistic insights

The utilization of formic acid as a liquid organic hydrogen carrier has taken a vast interest lately because of several desirable properties. The state-of-the-art homogenous catalysts known for formic acid dehydrogenation are mainly based on noble metals such as iridium or ruthenium. 3d metals are considered to be an attractive alternative due to their abundance and low toxicity. Exploration of 3d metals has achieved exciting results mainly with iron-based catalysts; however, manganese has not received much attention, and only a few examples are available. Here we report a manganese complex [Mn(PN³P)(CO)₂]Br containing a pincer backbone, as an efficient catalyst for formic acid dehydrogenation. Under the optimized condition, the complex afforded a TON of 15,200. To the best of our knowledge, this is considered one of the best TON achieved using a manganese-based complex with excellent selectivity. Mechanistic studies suggested that the imine arm participates in the formic acid activation/deprotonation step, emphasizing the importance of metal-ligand cooperativity during substrate activation to promote catalytic efficacy.