Push or Pull? Proton Responsive Ligand Effects in Rhenium Tricarbonyl CO₂ Reduction Catalysts

Abstract

Proton responsive ligands offer control of catalytic reactions through modulation of pH-dependent properties, second coordination sphere stabilization of transition states, or by providing a local proton source for multiproton, multielectron reactions. Two fac-[Re(I)(α-diimine)(CO)3Cl] complexes with α-diimine = 4,4'- (or 6,6'-) dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts for the reduction of carbon dioxide. Consecutive electrochemical reduction of these complexes yields species identical to those obtained by chemical deprotonation. An energetically feasible mechanism for reductive deprotonation is proposed in which the bpy anion is doubly protonated followed by loss of H2 and 2H(+). Cyclic voltammetry reveals a two-electron, three-wave system owing to competing EEC and ECE pathways. The chemical step of the ECE pathway might be attributed to the reductive deprotonation but cannot be distinguished from chloride dissociation. The rate obtained by digital simulation is approximately 8 s(-1). Under CO2, these competing reactions generate a two-slope catalytic waveform with onset potential of -1.65 V vs Ag/AgCl. Reduction of CO2 to CO by the [Re(I)(4DHBP-2H(+))(CO)3](-) suggests the interaction of CO2 with the deprotonated species or a third reduction followed by catalysis. Conversely, the reduced form of [Re(6DHBP)(CO)3Cl] converts CO2 to CO with a single turnover.

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy–R)(CO)3Br] Complexes

“…This new species can be reasonably assigned as a singly‐deprotonated neutral species, [Mn(pdbpy–H + )(CO) 3 ] ( 1 b ), formed by reductive deprotonation of the starting 1 a upon R1. According to this interpretation, the newly formed bands fit well with the spectroscopic data of Re and Mn complexes that contain the 4‐dhbpy ligand (4,4′‐dihydroxy‐2,2′‐bipyridyl), for which a reductive deprotonation mechanism was reported . In particular, a shift in the high‐energy CO band from 2019 to 2012 cm −1 (7 cm −1 redshift) was observed as a result of the transformation of [Re(4‐dhbpy)(CO) 3 Cl] into the singly‐deprotonated complex [Re(4‐dhbpy–H + )(CO) 3 Cl] − (Table S1) .…”

Fluorescence probes for prokaryotic and eukaryotic cells using Re(CO)3+complexes with an electron withdrawing ancillary ligand

“…These results present similar values than those published by different authors using analogous complexes [72][73][74][75][76][77][78][79][80][81]. Interphase: Pt|1.C1, rhenium oxidations appear at E p 1.51 V, which seems to be electrochemically irreversible, and a second of reversible character at E ½ 1.90 V. On the other hand C2 presents same peaks at E p 1.46 V and E ½ 1.86 V, respectively.…”

“…eletrochemical-chemical mechanism (EEC) (Manbeck et al[73]) with bromide dissociation as the chemical step (v and vii), Scheme 3: Reaction mechanism for C1 and C2 reversible reductions.The kinetic description for L-containing complexes (L is (E)-2((3-amino-pyridin-4-ylimino)-methyl)-4,6-diterbutylphenol ancillary ligand) is quite similar, without the bromide elimination steps. Reduction processes may be assigned to be of the same nature than those described for C1 and C2.…”

Experimental and theoretical studies of the ancillary ligand (E)-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-di-tert-butylphenol in the rhenium(i) core

“…S8, ESI †). 41 Regardless, if these reduction processes may be attributed specifically to a particular moiety, by the shape and position of the peaks it seems that the electrochemical reaction follow a typical electrochemical-electrochemical-chemical reaction mechanism (EEC) previously described by Manbeck et al 37 with bromide dissociation as the chemical step: [38][39][40] Other authors, using bpy ligands, have reported the same reduction values with different degrees of reversibility, because the redox potentials for the rhenium complexes depend on the nature of the ligands.…”

“…S7, ESI †). [34][35][36][37] In C1, rhenium oxidation appears at E p 1.47 V according to the value reported by Hasselmann et al 15 Also, the reversible reduction of deeb is present at E 1/2 À0.94 V, which is more distinguishable in the working window study (Fig. [34][35][36][37] In C1, rhenium oxidation appears at E p 1.47 V according to the value reported by Hasselmann et al 15 Also, the reversible reduction of deeb is present at E 1/2 À0.94 V, which is more distinguishable in the working window study (Fig.…”