Efficient solventless dehydrogenation of formic acid by a CNC-based rhodium catalyst

Abstract

The complex [(CNC)^MesRh(PMe₂Ph)]PF₆ (1) has been found to be an effective catalyst for solventless formic acid (FA) dehydrogenation, affording exclusively H₂ and CO₂ as decomposition products. The effect of the addition of a base as a co-catalyst was studied, and it was found that HCOONa was the most efficient additive in terms of catalyst efficiency with a catalyst loading of 0.016 mol%, reaching TOF_max values up to 5869 h⁻¹. Additionally, we observed that the addition of water dramatically increased the catalytic activity in FA dehydrogenation, yielding TOF_max values up to 10 150 h⁻¹. Additionally, VT kinetic NMR experiments allowed us to estimate the activation energy (ΔG^‡ = 18.12 ± 1.17 kcal mol⁻¹) of the FA dehydrogenation catalysed by 1. Stoichiometric NMR experiments, aimed to shed light on the nature of possible catalytic intermediates, allowed us to detect and further isolate the Rh^III hydrido formate complex [(CNC)^MesRh(κ^O-OC(O)H)(PMe₂Ph)H]PF₆ (2), which originates from an oxidative addition of FA to 1; additionally, we could detect a bis(hydrido) Rh^III complex [(CNC)^MesRh(PMe₂Ph)H₂]PF₆ (1-H₂), which is another operative intermediate in the catalytic FA dehydrogenation by 1. DFT calculations performed on the catalytic FA dehydrogenation perfectly accounted for the gathered experimental data; the approach of a FA molecule to 1 leads to O–H oxidative addition producing the κ^O-formate intermediate 2, which subsequently undergoes a FA-assisted isomerization to the κ^H-formate species. Further hydride abstraction generates the dihydrido intermediate 1-H₂, which releases H₂ upon interaction with another FA molecule closing the catalytic cycle. The rate-limiting step in the catalytic process corresponds to the hydride abstraction step, which agrees with the KIE values estimated by NMR experiments.