Selective Electrocatalytic Conversion of CO₂ to HCOOH by a Cationic Rh₂(II,II) Complex

The electrocatalytic reduction of CO2 by cis-H,T-[Rh2(mhp)2(L)2][BF4]2, where mhp– = the deprotonated 6-methyl-2-hydroxypyridine anion and L = 1,10-phenanthroline (phen; Rh2-phen) and dipyrido[3,2-f:2′,3′-h]quinoxaline (dpq; Rh2-dpq2), was investigated. The cis-H,T-[Rh2(mhp)2(L)2]2+ architecture is composed of two electron-rich mhp– bridging ligands, two electron-accepting diimine chelating ligands, L, and the redox-active Rh2(II,II) bimetallic core. Rh2-phen2 and Rh2-dpq2 display metal-centered Rh2II,II/II,I reduction waves at −0.36 and −0.29 V vs Ag/AgCl, followed by a reduction event localized on the phen and dpq ligands at −1.15 V and −1.07 vs Ag/AgCl, respectively, in CH3CN under N2. A second metal-centered reduction is observed at −1.70 and −1.52 V vs Ag/AgCl in Rh2-phen2 and Rh2-dpq2, respectively. Under a CO2 atmosphere and 3 M H2O as the proton source, both complexes display catalytic currents near the third reduction couple. Although both Rh2-phen2 and Rh2-dpq2 possess comparable electronic structures and steric environments, they exhibit surprisingly different selectivity and efficiency in bulk electrolysis experiments. Rh2-phen2 is both more selective and efficient for the reduction of CO2 to HCOOH than H+ to H2 than Rh2-dpq2. The difference in catalytic activity between the two complexes is attributed to the greater electron density closer to the dirhodium core upon reduction of the diimine ligand in Rh2-phen2 as compared to Rh2-dpq2. In Rh2-dpq2, the dpq-based reduction is expected to be mainly localized at the distal pyrazine unit and to exert a less pronounced effect on subsequent reactivity taking place at the dirhodium core. In addition, the reduction of the dpq ligand in Rh2-dpq2 is followed by protonation of the nitrogen atoms on the pyrazine unit, thus reducing its ability to store and then supply a redox equivalent required for catalysis at the dirhodium core. The present work provides structural and electronic guidelines for the design of selective and efficient bimetallic catalysts.