Synthesis of a Redox-Active NNP-Type Pincer Ligand and Its Application to Electrocatalytic CO2 Reduction With First-Row Transition Metal Complexes

Abstract

We report the synthesis of a rigid phosphine-substituted, redox-active pincer ligand and its application to electrocatalytic CO 2 reduction with first-row transition metal complexes. The tridentate ligand was prepared by Stille coupling of 2,8-dibromoquinoline and 2-(tributylstannyl)pyridine, followed by a palladium-catalyzed cross-coupling with HPPh 2 . Complexes were synthesized from a variety of metal precursors and characterized by NMR, high-resolution mass spectrometry, elemental analysis, and cyclic voltammetry. Formation of bis-chelated metal complexes, rather than mono-chelated complexes, was favored in all synthetic conditions explored. The complexes were assessed for their ability to mediate electrocatalytic CO 2 reduction, where the cobalt complex was found to have the best activity for CO 2 -to-CO conversion in the presence of water as an added proton source.

Electro‐ and Photochemical Reduction of CO2 by Molecular Manganese Catalysts: Exploring the Positional Effect of Second‐Sphere Hydrogen‐Bond Donors

“…It should be noted that no catalytic current enhancement was observed in N 2 -saturated TFE solutions suggesting that the catalysts are not active for proton reduction in the potential window studied. [23,24] In fact, comparing CVs of N 2 -sparged CH 3 CN/0.1 m Bu 4 NPF 6 solutions with added TFE in the presence ( Figure 4) and absence ( Figure S17) of catalyst shows that the Mn complexes actually suppress proton reduction at the glassy carbon electrode.…”

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

“…In the first step, 2,6-dibromopyridine and aniline were reacted following a reported palladium-catalyzed Buchwald− Hartwig coupling to produce N 2 ,N 6 -diphenylpyridine-2,6diamine, 40 which was isolated in 76% yield after purification by column chromatography on silica. Subsequently, diphenylphosphino moieties were installed by low-temperature lithiation of the diamine in tetrahydrofuran (THF) before 25,27 (B) complexes reported in the literature coordinated by redox-innocent PN 3 P ligand frameworks, 32−38 the addition of chlorodiphenylphosphine and stirring at 65 °C for 18 h. NMR spectroscopic analysis (Figures S1−S3), highresolution mass spectrometry (HRMS), and elemental analysis confirmed the structure and purity of ligand L received in 67% yield after workup (detailed synthetic procedures are given in the Experimental Section).…”

“…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 2 A). 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.…”

“…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).…”

DataSheet1.PDF

“…Therefore, it is necessary to explore the efficacy of Mn(I)NNN pincer complexes toward the activation of carbon dioxide. In many instances, aromatic systems are commonly used during the designing of metal pincer complexes with limited combinations (Figure 1) (Choi and Lee., 2020;Jessica et al, 2015;Talukdar et al, 2019). The aromatic systems are known to provide better thermal stability to the catalyst complex by offering steric bulk and hydrophobic groups to minimize the leaching of the metals.…”