Conversion of Carbon Dioxide to Methanol Using a C–H Activated Bis(imino)pyridine Molybdenum Hydroboration Catalyst

Using a multistep synthetic pathway, a bis­(imino)­pyridine (or pyridine diimine, PDI) molybdenum catalyst for the selective conversion of carbon dioxide into methanol has been developed. Starting from (Ph2PPrPDI)­Mo­(CO), I2 addition afforded [(Ph2PPrPDI)­MoI­(CO)]­[I], which features a seven-coordinate Mo­(II) center. Heating this complex to 100 °C under vacuum resulted in CO loss and the formation of [(Ph2PPrPDI)­MoI]­[I]. Reduction of [(Ph2PPrPDI)­MoI]­[I] in the presence of excess K/Hg yielded (κ6-P,N,N,N,C,P-Ph2PPrPDI)­MoH following methylene group C–H activation at the α-position of one PDI imine substituent. The addition of CO2 to (κ6-P,N,N,N,C,P-Ph2PPrPDI)­MoH resulted in facile insertion to generate the respective η1-formate complex, (κ6-P,N,N,N,C,P-Ph2PPrPDI)­Mo­(OCOH). When low pressures of CO2 were added to solutions of (κ6-P,N,N,N,C,P-Ph2PPrPDI)­MoH containing pinacolborane, the selective formation of H3COBPin and O­(BPin)2 was observed along with precatalyst regeneration. When HBPin was limited, H2C­(OBPin)2 was observed as an intermediate and (κ6-P,N,N,N,C,P-Ph2PPrPDI)­Mo­(OCOH) remained present throughout CO2 reduction. The hydroboration of CO2 to H3COBPin was optimized and 97% HBPin utilization by 0.1 mol % (κ6-P,N,N,N,C,P-Ph2PPrPDI)­MoH was demonstrated over 8 h at 90 °C, resulting in a methoxide formation turnover frequency (TOF) of 40.4 h–1 (B–H utilization TOF = 121.2 h–1). Hydrolysis of the products and distillation at 65 °C allowed for MeOH isolation. The mechanism of (κ6-P,N,N,N,C,P-Ph2PPrPDI)­MoH mediated CO2 hydroboration is presented in the context of these experimental observations. Notably, (κ6-P,N,N,N,C,P-Ph2PPrPDI)­MoH is the first Mo hydroboration catalyst capable of converting CO2 to MeOH, and the importance of this study as it relates to previously described catalysts is discussed.