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dc.contributor.authorAmanullah, Sk
dc.date.accessioned2022-05-15T06:37:47Z
dc.date.available2022-05-15T06:37:47Z
dc.date.issued2022-05-16
dc.identifier.urihttp://hdl.handle.net/10754/677888
dc.description.abstractReduction of CO2 to value-added chemicals is a logical way to store solar energy and address the increasing concerns about the higher abundance of the greenhouse gas, CO2, in the atmosphere.1 An important issue in CO2 reduction is selectivity as CO2 can be reduced to CO, HCOOH, CH3OH, CH4, or other C2 and C3 hydrocarbons, all of which have commercial values. However, for practical implementation of any of these carbon fixation strategies, the reduction should be selective. A key pursuit arising from the previous investigations of CO2 reduction is the possibility of obtaining the desired selectivity in CO2 reduction by incorporating second-sphere interactions in the ligand design.2 In fact, installing second-sphere hydrogen-bonding interactions has allowed fast reduction of CO2 to CO, but selectivity for the formation of HCOOH has yet to be demonstrated. Although frequently used to explain the observed reactivities, only rarely have the intermediates been experimentally observed and characterized.3 As a result, there is a lack of insight into the electronic structure of the M-COOH intermediates, which is key to understanding their reactivities. After CO2 binding to the metal-centre followed by the protonation, the M-COOH intermediate, can lead to either CO or HCOOH depending on the site of protonation on this M-COOH intermediate. When the O-center is protonated, CO is released, while when the C-center is protonated, HCOOH is formed. The fate of this protonation can be determined either by controlling the pKa of the C or O centers or by using second sphere interactions to direct the protonation. Alternatively, the pKa of the C and/or O centers can be tuned by controlling the Lewis acidity of the metal center via ligand design or choice of metal.4 Despite being attractive catalysts, iron porphyrins inevitably favor the reduction of CO2 to CO and not HCOOH. Additionally, the Fe center has to be reduced to its formal Fe(0) state to bind CO2, and the reduction requires proton sources like, PhOH, which considering its pernicious environmental effects, may be detrimental to the practical utilization of these complexes for CO2 reduction. Generally, over-potential can be lowered by installing electron-withdrawing groups around the porphyrin ring. Unfortunately, no known iron porphyrin or their analogues (without auxiliary ligands) are known to reduce CO2 selectively to HCOOH until date.1-2 \r\n We demonstrate that an iron chlorin, a saturated analogue of porphyrin, having four electron-withdrawing groups and a pendant amine second sphere residue reduces CO2 selectively to HCOOH under both electrochemical and chemical conditions in its Fe(I) state using H2O as a proton source.5 The solid-state structure and solution 1D and 2D NMR data of an analogous Ni(II) complex confirm the presence of the pendant amine group near the active site. Two intermediates involved in the reduction are identified using a combination of FTIR, resonance Raman, and Mössbauer spectroscopy. The ligand design is envisaged to place the second sphere residue closer to the active site, and this helps shuttle protons to the catalytic center, which (a) allow CO2 binding to the Fe(I) state resulting in only a meager overpotential, (b) stabilize both Fe(III)COOH and Fe(II) COOH intermediates, and (c) result in electrocatalytic CO2 reduction to HCOOH with 88% selectivity under chemical and 97% under electrochemical conditions. References: 1. a) Appel et al, Chem. Rev. 2013, 113, 6621-6658; b) Francke et al, Chem. Rev. 2018, 118, 4631-4701 2. Amanullah et al, Chem. Soc. Rev. 2021, 50, 3755-3823 3. Mondal et al, J. Am. Chem. Soc. 2015, 137, 11214 11217 4. Saha et al, Acc. Chem. Res. 2022, 55, 134-144 5. Amanullah et al, J. Am. Chem. Soc. 2021, 143, 13579-13592"
dc.titleActivating 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)
dc.typePoster
dc.conference.dateMay 15,2022
dc.conference.nameCarbon Capture and Utilization challenges and opportunities in Catalysis
dc.conference.locationTHUWAL, SAUDI ARABIA
refterms.dateFOA2022-05-29T13:01:32Z


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