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

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 ((Ph2PPr)PDI)Mo(CO), I2 addition afforded [((Ph2PPr)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 [((Ph2PPr)PDI)MoI][I]. Reduction of [((Ph2PPr)PDI)MoI][I] in the presence of excess K/Hg yielded (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH following methylene group C-H activation at the α-position of one PDI imine substituent. The addition of CO2 to (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH resulted in facile insertion to generate the respective η(1)-formate complex, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)Mo(OCOH). When low pressures of CO2 were added to solutions of (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)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 (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)Mo(OCOH) remained present throughout CO2 reduction. The hydroboration of CO2 to H3COBPin was optimized and 97% HBPin utilization by 0.1 mol % (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)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 (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)MoH mediated CO2 hydroboration is presented in the context of these experimental observations. Notably, (κ(6)-P,N,N,N,C,P-(Ph2PPr)PDI)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.

Recent Advances in Catalysis with Transition‐Metal Pincer Compounds

“…The catalytic conversion of CO 2 represents an environmentally friendly alternative to decrease its emissions. In this sense, several complexes based on pincer ligands have been developed . In general, precious transition metals show the highest catalytic activities in the hydrogenation of CO 2 ; however, in the last years some research groups have focused their attention on the use of non‐precious transition metals, such as Fe and Co …”

“…This is a desirable and attractive process but is challenging to achieve . Efficient homogeneous catalysts for this reaction have been reported only recently . Methanol can act as a hydrogen storage system (dehydrogenation) or as a direct fuel (combustion).…”

CO2 Reduction to Methanol in the Liquid Phase: A Review

“…A homogeneous catalyst is soluble in the solvent to easily access CO 2 in the condensed phase, promoting high catalytic activity. The homogeneous catalysts on CO 2 hydrogenation to methanol, including [Ru], [34] [Co], [35] [Ir], [36] [Mn], [37] [Fe], [38] [Mo], [39] [Ni], [40] [Re], [41] [Pd], [42] and frustrated Lewis pairs (FLPs) or metal‐free catalyst, [43] showed good catalytic capacity towards methanol production. The catalytic performance of the homogeneous catalysts and the reaction conditions were depicted and compared in Table 1.…”

Mild and Selective Carbon Dioxide Hydroboration to Methoxyboranes Catalyzed by Mn(I) PNP Pincer Complexes

“…Among early transition metals, Trovitch and co‐workers described the use of bis(imino)pyridine‐type molybdenum catalyst [(κ 6 − P,N,N,N,C,P ‐ Ph2PPr PDI)MoH] for CO 2 hydroboration with HBpin, with 97 % borane utilization using 0.1 mol % of catalyst, after 8 h at 90 °C in C 6 D 6 , and CH 3 OBpin formation with turnover frequency (TOF) of ca. 40 h −1 …”