ic5b01102_si_001.pdf (1.49 MB)
Conversion of Carbon Dioxide to Methanol Using a C–H Activated Bis(imino)pyridine Molybdenum Hydroboration Catalyst
journal contribution
posted on 2015-08-03, 00:00 authored by Raja Pal, Thomas L. Groy, Ryan J. TrovitchUsing
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.