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

The complex [(CNC)Rh-Mes(PMe2Ph)]PF6 (1) has been found to be an effective catalyst for solventless formic acid (FA) dehydrogenation, affording exclusively H-2 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 TOFmax values up to 5869 h(-1). Additionally, we observed that the addition of water dramatically increased the catalytic activity in FA dehydrogenation, yielding TOFmax values up to 10 150 h(-1). Additionally, VT kinetic NMR experiments allowed us to estimate the activation energy (Delta G(double dagger) = 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)Rh-Mes(kappa(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)Rh-Mes(PMe2Ph)H-2]PF6 (1-H-2), 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 kappa(O)-formate intermediate 2, which subsequently undergoes a FA-assisted isomerization to the kappa(H)-formate species. Further hydride abstraction generates the dihydrido intermediate 1-H-2, which releases H-2 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.

Automated summary

In this study, the complex [(CNC)Rh-Mes(PMe2Ph)]PF6 (1) was found to be an effective catalyst for solventless formic acid (FA) dehydrogenation, producing only H2 and CO2 as decomposition products. The addition of a base as a co-catalyst, particularly HCOONa, significantly enhanced the catalyst efficiency. Water was also found to greatly increase the catalytic activity. Kinetic experiments and DFT calculations provided insights into the reaction mechanism, revealing the crucial steps and the nature of intermediate species involved.