Subscriber access provided by UNIV OF CALIFORNIA SAN DIEGO LIBRARIES
Communication -
+
Intermediates Involved in the 2e/2H Reduction of CO to CO by Iron(0) Porphyrin 2
Biswajit Mondal, Atanu Rana, Pritha Sen, and Abhishek Dey J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b05992 • Publication Date (Web): 20 Aug 2015 Downloaded from http://pubs.acs.org on August 21, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society
Intermediates Involved in the 2e-/2H+ Reduction of CO2 to CO by Iron(0) Porphyrin Biswajit Mondal‡, Atanu Rana‡, Pritha Sen, Abhishek Dey* Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India 700032 Supporting Information Placeholder ABSTRACT: The reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates. The resonance Raman data of intermediate I, which could only be stabilized at -95oC, indicates that it is a Fe(II)-CO22adduct and is followed by an another intermediate II at -80oC where the bound CO2 in intermediate I is protonated to form a Fe(II)-COOH species. While the initial protonation can be achieved using weak proton sources like MeOH and PhOH, the facile heterolytic cleavage of the C-OH bond in intermediate II requires strong acids.
Mitigation of CO2 and assimilating it into useful chemicals is an urgent challenge in the context of growing concerns over the green house effect of CO2 and depleting fossil fuels.1,2-4Substantial focus is on the effective reduction of CO2 into CO and other essential products.5-7 The direct reduction of CO2 to CO2.- is highly energy demanding and occurs at fairly negative potential (-1.9 V vs SHE in acetonitrile).1, 8, 9 Kubiaket al. demonstrated the reduction of CO2 to CO with Re based molecular catalysts.10-12 They have further improved their catalyst by introducing proton channel for enhancing the kinetics of the CO2 reduction process.10, 13, 14 Sauvage used Ni-cylam and heme complexes for the reduction of CO2 to CO in the 1980’s.15 Saveant investigated the electrochemistry of CO2 reduction with Fe porphyrin in details16, 17 and has modified this approach of CO2 reduction by Fe porphyrin, with the iron in its formal zero valent state, with the incorporation of a weak proton donor (like phenol) appended to the periphery of the porphyrin macrocyle to act as a proton relay during CO2 reduction and thereby increasing the efficiency of the catalyst.4, 18-21 By analysing the electrocatalytic response, a plausible mechanism for the reduction of CO2 by Fe(0) porphyrin was offered. CO2 binds an Fe(0) centre and upon subsequent proton and electron transfer, CO is released (Scheme 1). It was proposed that a concerted proton-electron transfer bond cleavage (CPETBC: electron transfer from the central atom concerted with proton transfer and breaking of one C-O bond) of a Fe-COOH intermediate species is the rate determining step in organic solvents. However, till date, no such intermediate has been isolated and spectroscopically characterized. Vibrational spectroscopy can be used to characterize the M-CO2adducts. The CO vibrations are intense in the FTIR and Kubiak has used in situ FTIR to show the formation of [M-Ligand-CO] and [M-LigandCO2OH] as the end product of CO2 reduction.22 Peters et al. also characterized a Cobalt bound CO2 adduct with FTIR.23 Over the last few year iron porphyrins which facilitate H-bonding in the distal pocket through triazoles have been reported.24 This hydrogen
bonding interaction has been demonstrated to increase the rate and selectivity of O2 reduction by stabilizing intermediates involved in the process.24 In this paper, two intermediates involved in the reduction of CO2 to CO by Fe(0) porphyrins which are stabilized by 2nd sphere H-bonding pocket of the porphyrin ligand, are trapped at cryogenic temperatures and characterized using vibrational spectroscopy. Isotopic labelling of CO2 by 13C indicates the presence of Fe-C bonds in both. The role of proton source is evaluated using MeOH, PhOH and p-CH3C6H4SO3H. The results indicate that the heterolytic cleavage of a C-OH bond in a FeII-COOH intermediate is rate determining under these conditions. Scheme 1: Proposed mechanism of CO2 reduction to CO with Feporphyrins.
A
B
Figure 1. A) The schematic representation of the complex. B) Cyclic voltammogram of FeEs4 in 4% H2O containing acetonitrile (black) vs catalytic CO2 reduction of same using 30 eq. of PhOHCO2 in 4% H2O, acetonitrile mixture (red). CO2 binds Fe-porphyrin complexes in its Fe(0) state as proposed by Saveant et al. Accordingly, in acetonitrile solvent, electroreduction of CO2 observed with FeEs4 (Fig. 1A). The potential of CO2 reduction indicates that the formally Fe(0) oxidation state (Fig. 1B) is the catalytically active species. Iron porphyrin with Fe(0) can be obtained, in solution, by the chemical reduction of Fe(III) porphyrin with sodium anthracenide.25-28 The crystal structure of this dark green species shows an intact porphyrin macrocycle. In our pursuit,
ACS Paragon Plus Environment
Journal of the American Chemical Society
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
we used Na-Hg in THF-MeOH solution for the reduction of the FeEs4 to its formal Fe(0) oxidation state and obtained a dark green solution which is spectroscopically identical to that obtained using sodium anthracenide (regeneration of the Fe(III)porphyrin took 3 eqv of ferrocenium; Fig S2). Upon bubbling CO2 in the solution of Fe(0), the green colour changes to red and the Soret band at 439 nm shifts to 421 nm (Fig. S1: red) indicating the oxidation of Fe(0) porphyrin by CO2. The resonance Raman (rR) spectroscopy of the Fe(0)Es4 shows a 4 band at 1325 cm-1and 1336 cm-1 (Fig. 2A: blue) and 2 band at 1542 cm-1. The rR features of the species is different from those of Fe(III) (4 at 1362 cm-1, 2 at 1555 cm-1) and Fe(II) (4 at 1342 cm-1 and 2 at 1542 cm-1) porphyrins (Fig. 2A: red). On passing CO2 over a Fe(0) solution at -950C, a transient intermediate is observed (t1/2
