Download or read book Dirhodium(II, II) Complexes as Electrocatalysts for CO2 Reduction written by Hemanthi D. Manamperi and published by . This book was released on 2020 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Formic acid, HCOOH, is an important product that results from the 2e−/2H+ reduction of CO2, since it has applications in fields that include the preservative and textile industries and it can also serve as a fuel that is carbon neutral. Partial paddlewheel dirhodium complexes represent a robust platform to develop CO2 reduction catalysts, as these complexes possess multiple low energy metal- and ligand-centered unoccupied frontier molecular orbitals to store redox equivalents. Therefore, the present work focuses on the design of a series of partial paddlewheel Rh2II,II complexes to better understand the steric and electronic requirements imposed by the ligand environment around the Rh2II,II core on CO2 reduction catalysis. Two Rh2II,II complexes were synthesized which contain the deprotonated 6-hydroxy-2-methylpyridine (mhp-) as the bridging ligand in cis-H,T-[Rh2(mhp)2(L)2]2+, with L = 1,10-phenanthroline (phen; Rh2-phen2) and dipyrido[3,2-f:2′,3′- h]quinoxaline (dpq; Rh2-dpq2) as chelating diimine ligands. Although Rh2-phen2 and Rh2-dpq2 feature similar molecular structures, selective electrocatalytic conversion of CO2 to HCOOH was achieved only with the former, resulting in 60 ± 5 x 10−6 mol of HCOOH and 4.3 ± 0.4 x 10−6 mol of H2, whereas Rh2-dpq2 resulted in only 7 ± 0.3 x 10−6 mol of H2. Mechanistic studies point to an axial Rh2II,I−H hydride as an intermediate with both complexes, however, H/D kinetic isotope effect (KIE) experiments suggest the insertion of CO2 in to the Rh2II,I−H bond takes place only in Rh2-phen2. In addition, the reduction of the dpq ligands in Rh2-dpq2 is followed by protonation of the ligand pyrazine nitrogen atoms, making the electrons provided by the dpq-centered reduction(s) unavailable for catalytic events taking place at the dirhodium core. The protonation of the reduced dpq ligands is also expected to affect the hydricity of the Rh2-H intermediate, thus affecting the subsequent CO2 insertion step. A second series of Rh2II,II complexes were synthesized by varying the steric hindrance near the axial sites, cis-H,T-[Rh2(L)2(phen)2][BF4]2, (L = trifluoroacetamidate (1) and N-tolyl-acetamidate (2)), and cis-[Rh2(Me-DtolF)2(phen)2][BF4]2 (Me-DTolF = N′-bis(tolyl)ethanimidamidate; 3). Selective electrocatalytic conversion of CO2 to HCOOH was achieved with complexes 1 - 3, however, with varying efficiency. An unexpected Rh2-Rh2 aggregation was observed with complex 1 upon one-electron reduction, a process attributed to the presence of an axial interaction between singly reduced species afforded by the sterically open axial sites in 1. In fact, this chemical event had a negative impact on the CO2 reduction, as evidenced by the reduced product formation with 1 (81±2 x 10−6 mol HCOOH and 1.8 ± 0.4 x 10−6 mol H2) as compared to 160 ± 6 x 10−6 mol HCOOH and 0.17 ± 0.06 x 10−6 mol H2 observed with 2 as the catalyst. Within the series, complex 3 exhibits the least reactivity resulting 16 ± 1 x 10−6 mol of HCOOH and 5.2 ± 0.7 x 10−6 mol H2, which can be partially attributed to the presence of sterically blocked axial sites that can interfere with substrate binding and/or hydride transfer step. The accumulated electrochemistry and spectroelectrochemistry data point to the presence of a Rh2II,I−H hydride as an intermediate and hydride transfer as the rate determining step.