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 (<sup>Ph2PPr</sup>PDI)­Mo­(CO), I<sub>2</sub> addition afforded [(<sup>Ph2PPr</sup>PDI)­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 [(<sup>Ph2PPr</sup>PDI)­MoI]­[I]. Reduction of [(<sup>Ph2PPr</sup>PDI)­MoI]­[I] in the presence of excess K/Hg yielded (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­MoH following methylene group C–H activation at the α-position of one PDI imine substituent. The addition of CO<sub>2</sub> to (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­MoH resulted in facile insertion to generate the respective η<sup>1</sup>-formate complex, (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­Mo­(OCOH). When low pressures of CO<sub>2</sub> were added to solutions of (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­MoH containing pinacolborane, the selective formation of H<sub>3</sub>COBPin and O­(BPin)<sub>2</sub> was observed along with precatalyst regeneration. When HBPin was limited, H<sub>2</sub>C­(OBPin)<sub>2</sub> was observed as an intermediate and (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­Mo­(OCOH) remained present throughout CO<sub>2</sub> reduction. The hydroboration of CO<sub>2</sub> to H<sub>3</sub>COBPin was optimized and 97% HBPin utilization by 0.1 mol % (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­MoH was demonstrated over 8 h at 90 °C, resulting in a methoxide formation turnover frequency (TOF) of 40.4 h<sup>–1</sup> (B–H utilization TOF = 121.2 h<sup>–1</sup>). Hydrolysis of the products and distillation at 65 °C allowed for MeOH isolation. The mechanism of (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­MoH mediated CO<sub>2</sub> hydroboration is presented in the context of these experimental observations. Notably, (κ<sup>6</sup>-<i>P</i>,<i>N</i>,<i>N</i>,<i>N</i>,<i>C</i>,<i>P</i>-<sup>Ph2PPr</sup>PDI)­MoH is the first Mo hydroboration catalyst capable of converting CO<sub>2</sub> to MeOH, and the importance of this study as it relates to previously described catalysts is discussed.