Tuning Second Coordination Sphere Interactions in Polypyridyl–Iron Complexes to Achieve Selective Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide

Abstract

Tuning Second Coordination Sphere Interactions in Polypyridyl-Iron Complexes to Achieve Selective Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide. ChemRxiv. Preprint. The development of noble metal-free catalysts capable of electrochemically converting carbon dioxide (CO 2) selectively into value added compounds remains one of the central challenges in sustainable energy science. Here, we present a systematic study of Fe(II) complexes of the functionalized ligands bpy R PY2Me (bpyPY2Me = 6-(1,1-di(pyridin-2-yl)ethyl)-2,2′-bipyridine) in pursuit of water-stable molecular Fe complexes that are selective for the catalytic formation of CO from CO 2. Taking advantage of the inherently high degree of tunability of this ligand manifold, we followed a bio-inspired approach by installing protic functional groups of varying acidities (-H,-OH,-OMe,-NHEt, and-NEt2) into the ligand framework to systematically modify the second coordination sphere of the Fe center. This family of [(bpy R PY2Me)Fe(II)] complexes was characterized using single-crystal X-ray analysis, 1H NMR spectroscopy, and mass spectrometry. Comparative catalytic evaluation of this set of compounds via voltammetry and electrolysis experiments identified [(bpy NHEt PY2Me)Fe] 2+ in particular as an efficient, iron-based, non-heme CO 2 electro-reduction catalyst that displays significant selectivity for the conversion of CO 2 to CO in acetonitrile solution with 11 M H 2 O. We propose that the NH group acts as a local proton source for cleaving the CO bond in CO 2 to form CO. Interestingly, the complex with the most acidic functional group in the second coordination sphere, [(bpy OH PY2Me)Fe] 2+ , favors formation of H 2 over CO. Our results correlate the selectivity of water versus carbon dioxide reduction to the acidity of the second coordination sphere functional group and emphasize the continued untapped potential that synthetic molecular chemistry offers in the pursuit of next-generation CO 2 reduction electrocatalysts. File list (2) download file view on ChemRxiv Zee_Nippe_King_Chang_Long manuscript.pdf (2.73 MiB) download file view on ChemRxiv Zee_Nippe_King_Chang_Long SI.pdf (10.98 MiB)

Transition metal-based catalysts for the electrochemical CO2reduction: from atoms and molecules to nanostructured materials

“…According to computational DFT data, CO 2 RR is predicted to occur more favorably at the edge-hosted Fe-N 2+2 -C 8 configuration, due to the beneficial role of the neighboring C atoms with dangling bonds to facilitate the key *COOH dissociation step. 297 These data emphasize the importance of the specific interaction between the active site and the hosting environment in controlling the activity and selectivity of Fe-N-C SACs, suggesting analogous outer coordination sphere effects observed in homogeneous Fe systems with heme 112,120,[124][125][126]131 or non-heme ligands 267 (see Sections 3 and 4).…”

“…305, [343][344][345] An increase in the level of N doping was first predicted by first-principle calculations to facilitate the *COOH formation and *CO desorption steps in Fe-N 4 SACs. 346 In a similar fashion, the presence of additional N atoms near the Fe-N 4 was calculated to exert a synergistic effect which contributes to lower the barrier for the protonation of the bound *CO 2 À , resembling the stabilizing effect of pendant amine, 245,257,267 amide 125 or urea 126 groups in the second coordination sphere in molecular catalysis (see Sections 3 and 4). 339 From an experimental perspective, Yang et al studied the effect of the oxidation treatment and found improved CO 2 RR activity resulting from an increased amount of pyrrolic N. 345 In another recent investigation, the annealing of hemin (Fe source) in the presence of an excess of melamine (N-rich additive) and defective graphene led to atomically dispersed Fe-N 5 sites stabilized by pyrrolic N-doped graphene, acting as an additional axial ligand coordinated to Fe-N 4 moieties (Fig.…”

“…260 Recently, a series of Fe II complexes supported by polydentate bipyridyl-based platforms bearing different functional groups in the second coordination sphere, were explored for CO 2 RR. 267 Among them, the only system showing a high selectivity for CO production in aqueous CH 3 CN (FE CO = 81%; FE H 2 = 11%) was a derivative featuring a pendant secondary amine group (57). An increased acidity of the intramolecular proton relay (phenolic functionality) was found to dramatically enhance the H 2 evolution process.…”

“…421 It is interesting to note that in Section 3 we have discussed several examples from molecular catalysis in which pendant pyridine/pyridinium groups in the second coordination sphere exerted a boosting effect for the catalytic CO 2 RR to CO. 168,192,194 Moreover, a selectivity change induced by different groups in the second coordination sphere has been also observed in a number of homogeneous molecular catalysts. [267][268][269][270][271] Surfactant molecules containing highly hydrophobic long alkyl chains are commonly used as stabilizing agents during metal NPs synthesis. In some cases however, they may partially block the surface active sites limiting the catalyst performance 386 or be easily detached from the surface during prolonged electrolysis.…”

Aromatic foldamers as scaffolds for metal second coordination sphere design

“…For example, functional groups covalently appended to the rst sphere ligands may engage in useful second sphere interactions. [5][6][7][8][9][10][11][12][13] In other approaches, a cavity has been built around the metal center to surround some rst sphere ligands 14 using for example calixarenes [15][16][17] or cyclodextrins. 18,19 As an extension, an entire metal complex, that is, including its rst sphere ligands, may be encapsulated within the cavity of a synthetic receptor based, for example, on resorcinarenes 20,21 or self-assembled metallo-organic cages.…”