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

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 CO2 hydrogenation, which would ideally use an earth-abundant metal catalyst. In this article, six new (CNC)(CoL2)-L-I 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 (CF3) 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 CO2 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 CO2/H-2 mixture at 60 degrees C. A computational study of the mechanism of CO2 hydrogenation is also reported.