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submitted on 16.07.2020 and posted on 16.07.2020by Jenny Yang, Tyler Kerr, Xinran S. Wang, Jeffrey Barlow
The catalytic reduction of CO2 to HCO2-
requires a formal transfer of a hydride (two electrons, one proton). Synthetic
approaches for inorganic molecular catalysts have exclusively relied on classic
metal hydrides, where the proton and electrons originate from the metal (via heterolytic
cleavage of an M-H bond). An analysis of the scaling relationships that exist in
classic metal hydrides reveal that hydride donors sufficiently hydridic to
perform CO2 reduction are only accessible at very reducing electrochemical
potentials, which is consistent with known synthetic electrocatalysts. By
comparison, the formate dehydrogenase enzymes operate at relatively mild
potentials. In contrast to reported synthetic catalysts, none of the major
mechanistic proposals for hydride transfer in formate dehydrogenase proceed
through a classic metal hydride. Instead, they invoke formal hydride transfer from
an orthogonal or bi-directional mechanism, where the proton and electron are
not co-located. We discuss the thermodynamic advantages of this approach for
favoring CO2 reduction at mild potentials, along with guidelines for
replicating this strategy in synthetic systems.