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

Raja Pal, Thomas L. Groy, Ryan Trovitch

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23 Scopus citations

Abstract

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<inf>2</inf> 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>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)MoH following methylene group C-H activation at the α-position of one PDI imine substituent. The addition of CO<inf>2</inf> to (κ<sup>6</sup>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)MoH resulted in facile insertion to generate the respective η<sup>1</sup>-formate complex, (κ<sup>6</sup>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)Mo(OCOH). When low pressures of CO<inf>2</inf> were added to solutions of (κ<sup>6</sup>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)MoH containing pinacolborane, the selective formation of H<inf>3</inf>COBPin and O(BPin)<inf>2</inf> was observed along with precatalyst regeneration. When HBPin was limited, H<inf>2</inf>C(OBPin)<inf>2</inf> was observed as an intermediate and (κ<sup>6</sup>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)Mo(OCOH) remained present throughout CO<inf>2</inf> reduction. The hydroboration of CO<inf>2</inf> to H<inf>3</inf>COBPin was optimized and 97% HBPin utilization by 0.1 mol % (κ<sup>6</sup>-P,N,N,N,C,P-<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>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)MoH mediated CO<inf>2</inf> hydroboration is presented in the context of these experimental observations. Notably, (κ<sup>6</sup>-P,N,N,N,C,P-<sup>Ph2PPr</sup>PDI)MoH is the first Mo hydroboration catalyst capable of converting CO<inf>2</inf> to MeOH, and the importance of this study as it relates to previously described catalysts is discussed.

Original languageEnglish (US)
Pages (from-to)7506-7515
Number of pages10
JournalInorganic Chemistry
Volume54
Issue number15
DOIs
StatePublished - Aug 3 2015

ASJC Scopus subject areas

  • Inorganic Chemistry
  • Physical and Theoretical Chemistry

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