Activating the Fe(I) State of Iron Porphyrinoid with Second-Sphere Proton Transfer Residues for Selective Reduction of CO₂ to HCOOH via Fe(III/II)–COOH Intermediate(s)

Abstract

The ability to tune the selectivity of CO2 reduction by first-row transition metal-based complexes via the inclusion of second-sphere effects heralds exciting and sought-after possibilities. On the basis of the mechanistic understanding of CO2 reduction by iron porphyrins developed by trapping and characterizing the intermediates involved (J. Am. Chem. Soc.201513711214), a porphyrinoid ligand is envisaged to switch the selectivity of the iron porphyrins by reducing CO2 from CO to HCOOH as well as lower the overpotential to the process. The results show that the iron porphyrinoid designed can catalyze the reduction of CO2 to HCOOH using water as the proton source with 97% yield with no detectable H2 or CO. The iron porphyrinoid can activate CO2 in its Fe­(I) state resulting in very low overpotential for CO2 reduction in contrast to all reported iron porphyrins, which can reduce CO2 in their Fe(0) state. Intermediates involved in CO2 reduction, Fe­(III)–COOH and a Fe­(II)–COOH, are identified with in situ FTIR-SEC and subsequently chemically generated and characterized using FTIR, resonance Raman, and Mössbauer spectroscopy. The mechanism of the reaction helps elucidate a key role played by a closely placed proton transfer residue in aiding CO2 binding to Fe­(I), stabilizing the intermediates, and determining the fate of a rate-determining Fe­(II)–COOH intermediate.

Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins

“…(Thio)ureas are known to be highly effective motifs for selective binding of bicarbonate and other oxyanions, and thus have been incorporated into diverse frameworks such as amide-linked macrocycles, 24 cyclopeptide cages, 25 and tripodal 26 and linear [27][28][29][30][31] ligand scaffolds for recognition and sensing applications. Indeed, previous reports from our laboratory 6,32,33 and others [34][35][36][37][38][39][40][41][42][43][44][45][46][47] have established the ability of second coordination sphere pendants to enhance CO2RR catalysis. As such, we reasoned that functionalization of Fe-TPP with a properly-positioned urea moiety would promote advantageous interactions with bicarbonate through templated hydrogen bonding interactions (Figure 1).…”

Dissection of Light‐Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction

“…It is worth noting that a recent report from A. Dey and coll . also described the activation of CO 2 at the Fe I oxidation state of a modified Iron‐chlorin catalyst that catalyzes the selective reduction of CO 2 to formic acid [47] …”

Self-Assembled Liposomes Enhance Electron Transfer for Efficient Photocatalytic CO2 Reduction

“…Furthermore, the most active catalyst, 5,10,15,20-(tetra-N-hexadecyl-4-pyridinium)porphyrin cobalt(II) ( CoP L ), is comprehensively studied on transparent electrodes using in situ UV–vis–NIR and resonance Raman spectroelectrochemistry to understand its catalytic behavior, an approach that still remains scarce. 26 − 32 In combination with density functional theory (DFT), these methods reveal important reaction intermediates during CO 2 reduction and an unusual precatalytic four-electron charging mechanism that precedes its catalytic activity.…”