Modifying the Rate of Rhenium(Diimine)-Mediated Electrochemical Carbon Dioxide Reduction via the Addition of a Redox-Active Functional Group Near the Active Site

The development of electrocatalysts
that efficiently
valorize carbon
dioxide (CO2) is of ongoing interest. To that end, there
is interest in the advancement of molecular catalysts that can promote
reactions that address reaction bottlenecks. Examples of emerging
catalyst designs include ligands with ancillary groups that support
proton transfer reactions or ligands with charged groups that promote
electrostatic interactions that facilitate key reaction steps. Such
designs have considerably improved CO2 reduction rates
with respect to unmodified parent complexes. However, examples where
the ligand framework could provide more than one catalysis-assisting
function are rare. Herein, we use a (diimine)Re(I)-fac(CO)3 complex with an N-methylated terpyridine
ligand to demonstrate that the placement of a cationic and redox-active
group proximal to the Re active sites can improve CO2 reduction
rates. We observe a substantial improvement in observed rate constants
with respect to the unmethylated terpy complex. However, the role
of the methylpyridinium group is not as simple as pure redox mediation
or electrostatic effects. Density functional calculations support
the idea that both the redox reactivity of the entire ligand and the
presence of only partial positive charge near the Re site can contribute
to the observed CO2 reduction properties. The results are
an example of how ligand designs that incorporate combinations of
ancillary groups with different properties can be used to promote
electrocatalytic reactions.