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.