Carbon Dioxide Hydrogenation Catalysed by Non-Noble Metal PNP Pincer Complexes

The catalytic reduction of CO2 is of great interest for its potential as hydrogen storage method and use of this abundant substrate as C-1 feedstock. In particular, the selective reaction of molecular hydrogen with CO2 offers the possibility to generate novel hydrogen carriers such as formic acid, a widely studied target molecule giving access into the chemical as well as the energetic value chain. In an effort to replace expensive noble metal-based catalysts with cheaper, earth-abundant counterparts, we report the first example of Mn(I)-catalysed hydrogenation of CO2 to HCOOH using PNP pincer ligands. The novel Mn(I) catalyst [Mn(PNP-iPr)(H)(CO)2] showed higher stability and activity than its Fe(II) analogue, which results are described for comparison. TONs up to 10000 and quantitative yields were obtained with the Mn complexes after 24 h using DBU as base. At catalyst loadings as low as 0.002 mol%, in the presence of LiOTf as co-catalyst, TONs greater than 30000 could be achieved, among the highest activities reported for non-noble metal catalysed CO2 hydrogenation to date. The results are complemented by detailed mechanistic studies by experimental techniques (NMR) and DFT calculations.