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 dinnine, PDI) molybdenum catalyst for the selective conversion of carbon dioxide into methanol has been developed. Starting from ((PDI)-P-Ph2PPr)Mo(CO), I-2 addition afforded [((PDI)-P-Ph2PPr)MoI(CO)][I], which features a seven-coordinate Mo(II) center. Heating this complex to 100 degrees C under vacuum resulted in CO loss and the formation of [((PDI)-P-Ph2PPr)MoI] [I]. Reduction of [((PDI)-P-Ph2PPr)MoI] [I] in the presence of excess K/Hg yielded (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)MoH following methylene group C-H activation at the alpha-position of one PDI imine substituent. The addition of CO2 to (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)MoH resulted in facile insertion to generate the respective eta(1)-formate complex, (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)Mo(OCOH). When low pressures of CO2 were added to solutions of (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)MoH containing pinacolborane, the selective formation of H(3)COBPin and O(BPin)(2) was observed along with precatalyst regeneration. When HBPin was limited, H2C(OBPin)(2) was observed as an intermediate and (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)Mo(OCOH) remained present throughout CO2 reduction. The hydroboration of CO2 to H(3)COBPin was optimized and 97% HBPin utilization by 0.1 mol % (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)MoH was demonstrated over 8 h at 90 degrees C, resulting in a methoxide formation turnover frequency (TOF) of 40.4 h(-1) (B-H utilization TOP = 121.2 h(-1)). Hydrolysis of the products and distillation at 65 degrees C allowed for MeOH isolation. The mechanism of (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)MoH mediated CO2 hydroboration is presented in the context of these experimental observations. Notably, (kappa(6)-P,N,N,N,C,P-(PDI)-P-Ph2PPr)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.