Cobalt(I) Pincer Complexes as Catalysts for CO₂ Hydrogenation to Formate

Abstract

Carbon dioxide hydrogenation with a base to generate formate salts can provide a means of storing hydrogen in an energy-dense solid. However, this application requires catalytic CO 2 hydrogenation, which would ideally use an earth-abundant metal catalyst. In this article, six new (CNC)Co I L 2 pincer complexes were synthesized and fully characterized including single-crystal X-ray diffraction analysis on four new complexes. These complexes contain an imidazole-based (1 R ) N-heterocyclic carbene (NHC) ring or a benzimidazole-based NHC ring (2 R ) in the CNC pincer. The R group is para to N on the pyridine ring and has been varied from electron-withdrawing (CF 3 ) to donating (Me, OMe) substituents. The L-type ligands have included CO and phosphine ligands (in PPh3 2 and PMe3 2). Thus, two known Co complexes (1, 1 OMe ) and six new complexes (1 Me , 1 CF3 , 2, 2 OMe , PPh3 2, and PMe3 2) were studied for the CO 2 hydrogenation reaction. In general, the unsubstituted CNC pincer complexes bearing two carbonyl ligands led to the highest activity. The best catalyst, 2, remains active for over 16 h and produces a turnover number of 39,800 with 20 bar of 1:1 CO 2 /H 2 mixture at 60 °C. A computational study of the mechanism of CO 2 hydrogenation is also reported.