Conversion of Carbon Dioxide to Methanol Using a C鈥揌 Activated Bis(imino)pyridine Molybdenum Hydroboration Catalyst
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- 作者：Raja Pal ; Thomas L. Groy ; Ryan J. Trovitch
- 刊名：Inorganic Chemistry
- 出版年：2015
- 出版时间：August 3, 2015
- 年：2015
- 卷：54
- 期：15
- 页码：7506-7515
- 全文大小：578K
- ISSN：1520-510X
文摘
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 掳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 (魏6-P,N,N,N,C,P-Ph2PPrPDI)MoH following methylene group C鈥揌 activation at the 伪-position of one PDI imine substituent. The addition of CO2 to (魏6-P,N,N,N,C,P-Ph2PPrPDI)MoH resulted in facile insertion to generate the respective 畏1-formate complex, (魏6-P,N,N,N,C,P-Ph2PPrPDI)Mo(OCOH). When low pressures of CO2 were added to solutions of (魏6-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 (魏6-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 % (魏6-P,N,N,N,C,P-Ph2PPrPDI)MoH was demonstrated over 8 h at 90 掳C, resulting in a methoxide formation turnover frequency (TOF) of 40.4 h鈥? (B鈥揌 utilization TOF = 121.2 h鈥?). Hydrolysis of the products and distillation at 65 掳C allowed for MeOH isolation. The mechanism of (魏6-P,N,N,N,C,P-Ph2PPrPDI)MoH mediated CO2 hydroboration is presented in the context of these experimental observations. Notably, (魏6-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.