Redox-Active Ligands in Electroassisted Catalytic H⁺ and CO₂ Reductions: Benefits and Risks

Abstract

In the past decade, the use of redox-active ligands has emerged as a promising strategy to improve catalyst selectivity, efficiency, and stability in electroassisted H + and CO 2 reductions. Partial delocalization of the electrons within ligand-centered orbitals has been proposed to serve as an electron reservoir, as a catalytic trigger, or as a way to prevent deleterious low-valent metal center formation. However, conclusive evidence of these effects is still scarce, and open questions remain regarding the way redoxactive ligands may affect the catalytic mechanism. In this Perspective, advances in redox-active ligands in electroassisted catalytic H + and CO 2 reductions are discussed through recent representative examples.

Immobilization of “Capping Arene” Cobalt(II) Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

“…Last, the capping arene ligands provide π-active ligands similar to bipyridyls that are known shuttle redox equivalents during electrochemical processes. 66 Two Co II complexes, (6-FP)Co(NO 3 ) 2 (1) (6-FP = 8,8′-(1,2-phenylene)diquinoline) and (5-FP)Co(NO 3 ) 2 (2) (5-FP = 1,2-bis(N-7-azaindolyl)benzene), were synthesized by mixing the corresponding proligand 6-FP or 5-FP and Co(NO 3 ) 2 •6H 2 O in acetonitrile at room temperature. Complexes 1 and 2 were isolated as purple crystals in yields of 75% (1) and 45% (2) (Scheme 4).…”

Systematic Variation of 3d Metal Centers in a Redox-Innocent Ligand Environment: Structures, Electrochemical Properties, and Carbon Dioxide Activation

“…Here we note that Fe, Co, and Ni complexes are among the most prominent eCO 2 R catalysts. 17 While ligand-centered reductions at mildly negative potentials are commonly proposed for Fe, Co, or Ni complexes 17 , 25 , 30 , 31 , 50 , 62 , 63 (e.g., with polypyridinic ligands having delocalized, low-energy π* orbitals), in our series, a comparison with the other complexes (of Mn, Cu, and Zn) allows assignment of the first two reductions ( Figure 8 C) solely to metal-centered orbitals in Fe MeCN , Co Cl , and Ni Cl .…”

“… 24 − 27 The majority of these studies rely on the so-called “noninnocent” ligands because these ligands are often perceived as beneficial for the activity by sharing excess electron density (redox noninnocence) or relaying protons (chemical noninnocence). 28 − 30 The resulting complex interplay of ligand- and metal-centered processes renders deconvolution of the individual contributions rather challenging. 31 By contrast, systematic variations of the metal centers with redox-innocent ligands, such as pincer platforms, 32 − 39 remain insufficiently investigated in the frame of CO 2 electroreduction ( Figure 2 B).…”

“…binding affinity, define the prevailing mechanistic route, which is directly linked to the catalytic performance. However, only a limited number of studies report on the systematic variation of the metal center within an identical ligand framework in relation to CO 2 electroreduction (Figure A). − The majority of these studies rely on the so-called “noninnocent” ligands because these ligands are often perceived as beneficial for the activity by sharing excess electron density (redox noninnocence) or relaying protons (chemical noninnocence). − The resulting complex interplay of ligand- and metal-centered processes renders deconvolution of the individual contributions rather challenging . By contrast, systematic variations of the metal centers with redox-innocent ligands, such as pincer platforms, − remain insufficiently investigated in the frame of CO 2 electroreduction (Figure B).…”

Thiolate-Thione Redox-Active Ligand with a Six-Membered Chelate Ring via Template Condensation and Its Pt(II) Complexes

“…All three of these large groups of redox-active ligands have five-membered chelate rings. The use of noninnocent ligands in many areas of research continues to grow, and numerous reviews are available. − …”