Manamperi, Hemanthi D. Witt, Suzanne E. Turro, Claudia Selective Electrocatalytic Conversion of CO<sub>2</sub> to HCOOH by a Cationic Rh<sub>2</sub>(II,II) Complex The electrocatalytic reduction of CO<sub>2</sub> by <i>cis</i>-H,T-[Rh<sub>2</sub>(mhp)<sub>2</sub>(L)<sub>2</sub>]­[BF<sub>4</sub>]<sub>2</sub>, where mhp<sup>–</sup> = the deprotonated 6-methyl-2-hydroxypyridine anion and L = 1,10-phenanthroline (phen; <b>Rh<sub><b>2</b></sub><b>-phen</b></b>) and dipyrido­[3,2-<i>f</i>:2′,3′-<i>h</i>]­quinoxaline (dpq; <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub>), was investigated. The <i>cis</i>-H,T-[Rh<sub>2</sub>(mhp)<sub>2</sub>(L)<sub>2</sub>]<sup>2+</sup> architecture is composed of two electron-rich mhp<sup>–</sup> bridging ligands, two electron-accepting diimine chelating ligands, L, and the redox-active Rh<sub>2</sub>(II,II) bimetallic core. <b>Rh</b><sub><b>2</b></sub><b>-phen</b><sub><b>2</b></sub> and <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub> display metal-centered Rh<sub>2</sub><sup>II,II/II,I</sup> 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 CH<sub>3</sub>CN under N<sub>2</sub>. A second metal-centered reduction is observed at −1.70 and −1.52 V vs Ag/AgCl in <b>Rh</b><sub><b>2</b></sub><b>-phen</b><sub><b>2</b></sub> and <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub>, respectively. Under a CO<sub>2</sub> atmosphere and 3 M H<sub>2</sub>O as the proton source, both complexes display catalytic currents near the third reduction couple. Although both <b>Rh</b><sub><b>2</b></sub><b>-phen</b><sub><b>2</b></sub> and <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub> possess comparable electronic structures and steric environments, they exhibit surprisingly different selectivity and efficiency in bulk electrolysis experiments. <b>Rh</b><sub><b>2</b></sub><b>-phen</b><sub><b>2</b></sub> is both more selective and efficient for the reduction of CO<sub>2</sub> to HCOOH than H<sup>+</sup> to H<sub>2</sub> than <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub>. 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 <b>Rh</b><sub><b>2</b></sub><b>-phen</b><sub><b>2</b></sub> as compared to <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub>. In <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub>, 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 <b>Rh</b><sub><b>2</b></sub><b>-dpq</b><sub><b>2</b></sub> 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. BF;CO 2 atmosphere;Rh 2;HCOOH;redox-active Rh 2;dpq 2;pyrazine unit;phen 2;CO 2;CH 3 CN;Cationic Rh 2;electron-accepting diimine chelating ligands;Selective Electrocatalytic Conversion;dirhodium core;deprotonated 6- methyl -2-hydroxypyridine anion;bulk electrolysis experiments;II;3 M H 2 O 2019-10-01
    https://acs.figshare.com/articles/journal_contribution/Selective_Electrocatalytic_Conversion_of_CO_sub_2_sub_to_HCOOH_by_a_Cationic_Rh_sub_2_sub_II_II_Complex/9924026
10.1021/acsaem.9b01283.s001