An Iron Porphyrin Complex with Pendant Pyridine Substituents Facilitates Electrocatalytic CO₂Reduction via Second Coordination Sphere Effects

Abstract

A bispyridylamine-based hanging unit within the ligand framework of a newly synthesized iron porphyrin complex (Py 2 XPFe) can act, on the one hand, as a hydrogen bonding site to facilitate proton transfer in catalysis and, on the other hand, as coordination site for a second Lewis acidic metal center. The bispyridylamine group in close proximity of the iron porphyrin center is able to mediate electrocatalytic CO 2 reduction in anhydrous MeCN. The hydrogen bonding interactions within the hanging group affect the kinetics of catalysis likely through stabilization of the [Fe I (CO 2 H)] À intermediate, increasing the overall rate of catalysis when compared to the non-functionalized analog, TMPFe (TMP = tetramesitylporphyrin). The rate constants (k app ) of the reduction reaction were calculated using the FOWA method which resulted in a higher TOF max for the complex Py 2 XPFe compared with TMPFe in neat MeCN (1.7 × 10 2 vs. 1.1 × 10 1 s À 1 ). The addition of weak Brønsted acids to the reaction mixture (TFE or PhOH) shows an increase in the rate of catalysis for both complexes, yet the Py 2 XPFe analog displays higher TOF max at each relative acid concentration, suggesting the hanging group beneficially impacts the rate of catalysis in the presence of these proton sources. The addition of Lewis acidic Sc 3 + to Py 2 XPFe also results in an increase in current density of the CO 2 reduction reaction. Resonance Raman as well as 1 H-NMR spectroscopy indicates coordination to the pyridine substituents.

Potential Induced Protonation of the Second Coordination Sphere in a Hangman-type Iron Porphyrin Complex Promotes HER: Insights via in Situ Raman Spectroelectrochemistry

“…The RR spectrum of the Py 2 XPFe complex displays bands at 1360, 1452, and 1551 cm –1 belonging to the totally symmetric A 1g modes of the iron porphyrin and are assigned as the v 4 , v 3 , and v 2 vibrational modes, respectively. Primarily, the v 4 band is indicative of the oxidation state, and the v 2 band of the spin state of iron–porphyrins. , These marker bands at 1360 and 1551 cm –1 indicate that the grafted Py 2 XPFe complex exists in the formal Fe III state as a high-spin complex, in congruence with electrochemical and NMR data of Py 2 XPFe in solution . The broad band observed at approximately 1600 cm –1 corresponds to the symmetric vibrations of the Pyr-1 linker component (Figure S4).…”

Electrocatalytic CO2Reduction: from Discrete Molecular Catalysts to Their Integrated Catalytic Materials

“…Lessons from the concept of reaction microenvironment in enzymatic catalysis inspire the installation of functional moieties around the active site in proper spatial orientation and distance to interact with bound substrates and thus to exert boosting effects beyond inductive effects on ECR (Figure 3): [43] i) stabilizing key intermediates through intramolecular H-bonding between bound CO 2 species and pendent Lewis base sites (phenols and amine [44] ), or through electrostatic attraction interaction between bound CO 2 anion and positively charged residues (ammonium [45] and imidazolium cations [46] ), or through Lewis acidic metals; [47] ii) facilitating the protonation of CO 2 anion adduct by virtue of pendant proton donors or local proton source (phenols, [48] imidazolium, [49] thiourea, [50] and carboxylic acid [51] ) or by facilitating H-bonding network (amine-NH, [52] imidazolium-CH, [49] and PEG [53] ); iii) promoting the cleavage of CÀ O bond via H-bonding interaction. [44,54] This strategy represents a powerful tool for molecular catalyst optimization and has been effectively used in molecular catalysis to overcome the aforementioned trade-off.…”

Comprehensive Review for an Efficient Charge Transfer in Single Atomic Site Catalyst/Organic Polymers toward Photocatalytic CO2 Reduction

“…Their unique structures are appealing for adsorptive activation of CO 2 molecules, transfer of intermediates from support to the active SAC, and production of desirable products in CO 2 reduction, [ 51 ] and iii) their electronic properties can be tuned by changing the ligand or coordinating environment to enhance photocatalytic activity. [ 52–55 ]…”

“…Their unique structures are appealing for adsorptive activation of CO 2 molecules, transfer of intermediates from support to the active SAC, and production of desirable products in CO 2 reduction, [51] and iii) their electronic properties can be tuned by changing the ligand or coordinating environment to enhance photocatalytic activity. [52][53][54][55] Although there is a considerable review on the hybrid single atomic site catalyst/organic polymer system, [56][57][58][59][60][61][62] there is a lack of comprehensive review on the interaction between organic polymer and single atomic site catalyst for the rational fabrication of an efficient hybrid system. As light absorption and redox reactions occur at the two separate components, the electron transfer between organic polymer and single atomic site catalyst plays a significant role in the efficiency of photocatalytic CO 2 reduction.…”