Covalent-Organic Frameworks Composed of Rhenium Bipyridine and

Subscriber access provided by UNIV TEXAS SW MEDICAL CENTER

Energy, Environmental, and Catalysis Applications

Covalent Organic Frameworks Composed of Rhenium Bipyridine and Metal Porphyrins: Designing Heterobimetallic Frameworks with Two Distinct Metal Sites Eric M Johnson, Ralf Haiges, and Smaranda C. Marinescu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07795 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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 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 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.

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 20 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

ACS Applied Materials & Interfaces

Covalent Organic Frameworks Composed of Rhenium Bipyridine and Metal Porphyrins: Designing Heterobimetallic Frameworks with Two Distinct Metal Sites Eric M. Johnson, Ralf Haiges, Smaranda C. Marinescu* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States *email: [email protected] Key words: Cobalt porphyrin, iron porphyrin, rhenium bipyridine, covalent organic framework, CO2 reduction ABSTRACT The incorporation of homogeneous catalysts for CO2 reduction into extended frameworks has been a successful strategy for increasing catalyst lifetime and activity, but the effects of the linkers on catalysis are underexplored. In this work, a novel rhenium bipyridine complex was synthesized for the purpose of designing a covalent-organic framework (COF) with both metalloporphyrin and metal bipyridine moieties. Investigation of the rhenium complex as a homogeneous catalyst shows a faradaic efficiency of 81(8)% for the electrocatalytic conversion of CO2 to CO upon the addition of methanol as the proton source. Treatment of the rhenium complex with tetra(4-aminophenyl)porphyrin (TAPP) under Schiff base conditions produces the desired COF, as indicated by powder X-ray diffraction (PXRD) studies. Metallation of the porphyrins was accomplished through post-synthetic modification with CoCl2 and FeCl3 metal precursors. The retention of the PXRD peaks and appearance of new Co and Fe peaks in the corresponding X-ray photoelectron spectroscopy (XPS) spectra suggest the successful incorporation of a secondary metal site into the framework. Cyclic voltammetry measurements 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

display increases in current densities when the atmosphere is changed from N2 to CO2. Controlled potential electrolyses show that the cobalt post metallated COF has the highest activity towards CO2 reduction, reaching a faradaic efficiency of 18(2)%. INTRODUCTION Metal- and covalent organic frameworks (MOFs and COFs, respectively) are classes of crystalline coordination polymers that exhibit high porosities and surface areas as well as modular structures.1-7 Because of these properties, MOFs and COFs are attractive materials for applications such as molecular sensing,8-12 gas absorption and separation,13-19 and catalysis.7,20-23 The use of MOFs and COFs for catalytic purposes has been extensively explored as the modular nature of these materials has facilitated the incorporation of known molecular catalysts through numerous available synthetic methods. The integration of molecular catalytic units into extended frameworks has often resulted in improved efficiency, stability, and durability in comparison to the corresponding molecular analogues.24-26 Additionally, important catalytic properties such as selectivity can be modulated by tuning the structural, electronic, and physical properties of the frameworks through direct synthesis27 or post synthetic modification.28 This synthetic tunability in conjunction with the improved durability of the molecular catalytic units following integration into the framework environment grants the catalyst-modified frameworks benefits of both homogeneous and heterogeneous catalysts. Although MOFs exhibit several attractive qualities, the low electrical conductivity of the MOFs has limited their ability to catalyze electrocatalytic reactions.29,30 The recent development of two-dimensional frameworks with extensive -delocalization in the ab plane, and the incorporation of redox-active ligands that can facilitate charge transport, has been a major breakthrough for the field of electrocatalytic MOFs.29,30 MOFs and COFs have been recently utilized for multiple types of electrocatalysis including hydrogen evolution, oxygen evolution, oxygen reduction, and carbon dioxide reduction.31 Porphyrins and bipyridines have proven to be useful building blocks for these frameworks, performing a variety of transformations. Electrocatalytically competent COFs composed of 5,10,15,20-tetra(4aminophenyl)porphyrincobalt(II) active sites were prepared using 1,4-benzenedicarboxaldehyde (COF-366) and biphenyl-4,4'-dicarboxaldehyde (COF-367) as linkers.32 These COFs were reported to reduce CO2 to CO under aqueous conditions with faradaic efficiencies up to 90%, 2 ACS Paragon Plus Environment

Page 2 of 20

Page 3 of 20 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


turnover numbers up to 290,000, and an overpotential of 0.55 V.32 Optimization of the electrochemical set-up and introduction of linkers containing electron-withdrawing groups results in an increased current density up to 65 mA/mg.33 Cobalt34 and iron35 porphyrin based MOFs synthesized through atomic layer deposition and electrophoretic deposition, respectively, were also identified as electrocatalysts for CO2 reduction. The cobalt porphyrin MOF produced CO with a 76% faradaic efficiency and 1,400 turnovers per site under the same conditions as the cobalt porphyrin COF.34 The activity of the MOF was maintained for up to 7 hours, showing increased lifetime relative to the molecular species.34 The iron porphyrin MOF produced a mixture of CO and H2 with a total faradaic efficiency of 100% at an overpotential of 0.65 V with a turnover number of 1,520 upon the addition of a proton source.35 Rhenium bipyridine-based MOF thin films have been shown to catalyze the electrochemical reduction of CO2 to CO with faradaic efficiencies around 93% and current densities up to 2 mA/cm2 with an overpotential of 0.65 V.36 MOFs based on iron porphyrin37 and nickel porphyrin38 moieties have been used as oxygen evolution and oxygen reduction catalysts, respectively, and cobalt bipyridine-based COFs39 and ruthenium bipyridine-based MOFs40 have been investigated as oxygen evolution catalysts. These examples highlight the synthetic ease with which well-known molecular porphyrin and biypridine-based catalysts can be incorporated into a framework structure. While selection of the appropriate catalytic active site to facilitate the desired transformation is vital, the identity of the organic linker used for framework formation influences several important properties that dictate the viability of the generated COFs as electrocatalysts. A large library of organic linkers have been utilized in the syntheses of catalytically active COFs to improve structural stability41 in aqueous acidic/alkaline media and increase the accessible surface area,32,41 which is of particular importance for substrate and product diffusion. Reasonably conductive frameworks can make use of variations in the linkages to tune the active site, providing increased control over activity and selectivity.33 Issues of poor conductivity can be lessened in certain cases by selecting linkers which orient metal centers in a manner which allows for charge hopping,37,38 or by incorporating redox active linkages.30 While this growing catalog of organic linkers aims to maximize catalysis at the metal center, the linkers themselves are traditionally catalytically inactive. An underexplored area of COF catalysis is the utilization of only catalytically active building blocks, which will allow for the introduction of two unique metal sites. Metalloporphyrin sites can be connected through metallobipyridine linkages to 3 ACS Paragon Plus Environment


produce COFs with two distinct metal sites. By substituting catalytically inactive linkages with active ones, COFs with two distinct metal sites can be produced, which will be beneficial in formulating design principles for the development of electrocatalytic MOFs. Herein, 2,2'-bipyridine-5,5'-dicarbaldehyde and 2,2'-bipyridine-5,5'dicarbaldehydetricarbonylchlororhenium(I) are synthesized, electrochemically analyzed, and integrated into a porphyrin-bipyridine bifunctional COF through a Schiff base reaction with 5,10,15,20-tetra(4-aminophenyl)porphyrin. Post synthetic modification with cobalt and iron chloride salts results in the incorporation of the corresponding metal ions into the porphyrin binding pocket, producing a COF with two distinct metal sites. Given the extensive literature on the electrochemical CO2 reduction using rhenium bipyridine, cobalt porphyrin, and iron porphyrin molecular species,42-49 the electrocatalytic CO2 reduction activity was explored in aqueous media. RESULTS AND DISCUSSION Synthesis and Characterization of Rhenium Complex The bipyridine-based linker was synthesized by radical bromination of 5,5'-dimethyl-2,2'bipyridine through the use of azobisisobutyronitrile (AIBN) and N-bromosuccinimide (NBS), which produced the dibrominated product, 5,5'-bis(bromomethyl)-2,2'-bipyridine (Scheme S1). Attempts to convert the 5,5'-bis(bromomethyl)-2,2'-bipyridine into the corresponding dialdehyde through the reported procedure using Bredereck’s reagent followed by sodium periodate50 were unsuccessful. An alternative synthetic route using modified Sommelet reaction conditions (see SI for details) yielded 2,2'-bipyridine-5,5'-dicarbaldehyde (1) as a white solid.51 The 1H NMR spectrum (Figure S1) of 1 in DMSO-d6 reveals peaks corresponding to the aldehyde proton at δ 10.14 ppm (s) and the aromatic protons at δ 9.22 (d), 8.66 (d), and 8.43 (dd) ppm. Refluxing 1 with pentacarbonylchlororhenium(I) in toluene generates the rhenium complex 2 as a red solid (Equation 1).52 X-ray quality crystals of 2 were grown from a mixture of dichloromethane and hexanes. The crystal structure of 2 (Figure 1) shows bidentate coordination of the bipyridine ligand to the rhenium metal center with Re-N bond lengths of 2.172(4) and 2.184(4) Å. The rhenium center maintains three of the five carbonyl groups, with two carbonyls oriented in the equatorial plane and the third in an axial coordination. The chloride ligand occupies the other axial position. 4 ACS Paragon Plus Environment

Page 4 of 20

Page 5 of 20 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


O

O

Re(CO)5Cl

N O

N

N

Cl O

N

Re OC

(1)

CO CO

2

1

Figure 1. X-ray crystal structure of 2. Selected hydrogen atoms have been omitted from the bipyridine ligand for clarity. The 1H NMR spectrum of 2 displays four peaks at δ 10.23 (s), 9.47 (d), 9.04 (d), and 8.73 (dd) ppm (Figure S3). The aromatic protons of the bipyridine ligand are deshielded due to the presence of the rhenium center. The peaks in the 13C NMR spectrum of 2 (Figures S4) are also deshielded in comparison to the free bipyridine ligand (Figure S2) with the aromatic carbons shifting to higher ppm values upon formation of 2. Additionally, two new peaks appear at δ 197 and 189 ppm, which correspond to the axial and equatorial carbonyl ligands. The Fourier transform infrared (FT-IR) spectrum of 2 (Figure 2) reveals two intense peaks at 2025 and 1897 cm-1, which are not present in 1 (Figure S5). These peaks are characteristic of metal-carbonyls, with the 2025 cm-1 peak corresponding to the fully-symmetric stretching mode of the carbonyl ligands, while the broad peak at 1897 cm-1 is due to the coalesced in-phase and out-of-phase stretching modes. The positions of these peaks are consistent with previously reported rhenium bipyridine complexes with carboxylate groups in the 5 and 5' positions, Re(bpydc)(CO)3Cl (bpydc = 2,2'-bipyridine-5,5'-dicarboxylate), which displays three peaks at 2022, 1920, and 1910 cm-1.53

5 ACS Paragon Plus Environment


Figure 2. FT-IR spectrum of 2. The black box highlights the carbonyl stretching frequency region. Electrochemical Studies of Molecular Species Electrochemical experiments were conducted in dimethylformamide (DMF) solution with 0.5 mM of 1 or 2 and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte. The cyclic voltammogram (CV) of the ligand 1 under a nitrogen atmosphere shows two reversible one-electron reductions with E1/2 of -1.08 and -1.38 V versus SCE (Figure 3A). Complex 2 also displays two reversible one-electron reductions under nitrogen similar to the unmetallated ligand; however, these peaks are positively shifted to -0.56 and -0.90 V versus SCE. The peak current densities of these reversible couples increase linearly with respect to the square root of the scan rate (Figures S6-S9), as expected for freely diffusing species in solution. The reduction potentials of 2 are positively shifted by roughly 800 mV in comparison to previously reported reduction potentials for the well-established CO2 reduction catalysts, Re([2,2'-bipyridine]-4,4'-tBu)(CO)3Cl (-1.45 V and -1.83 V versus SCE) and Re(bpy)(CO)3Cl (-1.34 V and -1.73 V versus SCE).54 Additionally, while 2 demonstrates two reversible couples that are ~340 mV apart, other known rhenium bipyridine complexes display a quasi-reversible first reduction event, followed by an irreversible second reduction.54


Page 6 of 20

Page 7 of 20 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


Figure 3. Electrochemical studies of 1 and 2 in DMF. (A) Overlay of the cyclic voltammograms of 1 (0.5 mM, blue) and 2 (0.5 mM, red) under an N2 atmosphere and (B) cyclic voltammograms of 2 (0.5 mM) under N2 (purple) and CO2 (green) atmospheres. (C) Cyclic voltammogram of 2 (0.5 mM) under 1 atm of CO2 with increasing concentrations of MeOH (from 0 to 4 M). Cyclic voltammetry experiments were performed in DMF with 0.1 M TBAPF6 at a scan rate of 20 mV/s. (D) Controlled potential electrolysis of 1 (blue), 2 (red), and a blank solution (dashed black) at -2.0 V versus SCE in DMF with 4 M MeOH under a CO2 atmosphere. When the solution of 2 was saturated with CO2, the reversible couples were unperturbed, but a current increase was observed at a potential of approximately -1.60 V versus SCE (Figure 3B). Re([2,2'-bipyridine]-4,4'-tBu)(CO)3Cl and Re(bpy)(CO)3Cl display current increases near similar potentials (-1.83 V and -1.73 V versus SCE, respectively) in the presence of CO2, which was assigned to the onset of CO2 reduction with these rhenium complexes.54 The current increase observed in the CV experiments of 2 under CO2 is indicative of an interaction between 2 and CO2. In comparison, ligand 1 displays negligible increase in current when the atmosphere is 7 ACS Paragon Plus Environment


changed from nitrogen to CO2 (Figures S10 and S11), indicating that the free ligand does not react with CO2. Acid titrations were conducted in DMF under CO2 to determine if the availability of excess protons enhanced the catalytic activity of 2. When methanol is used as the proton source, current increases are observed upon successive additions of MeOH with a five-fold increase occurring at 4 M MeOH (Figure 3C). A negligible current increase was observed for ligand 1 in the presence of CO2 and 4 M MeOH (Figure S14). Additionally, species 1 and 2 display low current increases under nitrogen and 4 M MeOH (Figures S12 and S13), indicating that CO2 is required for the current increases observed. Controlled potential electrolysis (CPE) experiments were performed in 0.1 M TBAPF6 DMF solution at -2.0 V versus SCE under an atmosphere of CO2, to identify and quantify the reduction products. In the absence of an added proton source, 1 and 2 produced no quantifiable amount of gaseous products after 4 hours of electrolysis. Upon the addition of 4 M MeOH, complex 2 selectively produces 73.7 μmol of CO (17.5 C), corresponding to a faradaic efficiency (FE) of 81(4)% (Figure 3D and Table S1). Other CO2 reduction products, such as formate/formic acid, were not detected. Upon completion of electrolysis, the electrode was placed in a fresh solution without the complex. The resulting cyclic voltammogram resembled the bare electrode, which suggests no deposition of active species onto the electrode surface is occurring (Figure S15). CPE conducted under a N2 atmosphere and 4 M MeOH produced no CO (Figure S16 and Table S1). To determine if 2 is responsible for the generated CO, control experiments were performed at -2.0 V versus SCE with the free ligand 1 (0.5 mM) in the presence of CO2 and 4 M MeOH (Figure 3D and Table S1). No CO was produced following these experiments, indicating that the presence of the rhenium center in 2 is required for CO2 reduction. These control experiments suggest that CO is only produced in the presence of 2, CO2, and an added proton source (MeOH). Rhenium bipyridine complexes are known to reduce CO2 to CO with faradaic efficiencies near unity. The decrease in FE of complex 2 (81(8)%) could be due to the electrochemical reduction of the aldehyde moiety. Aromatic aldehydes such as benzaldehyde were reported to generate radicals under reductive conditions, which led to dimerized products with quinoid structures.55-58 Therefore, the incorporation of the rhenium-bipyridine catalyst 2 into an extended framework is a viable method for eliminating the reactive aldehyde moieties through the 8 ACS Paragon Plus Environment

Page 8 of 20

Page 9 of 20 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


formation of imines by Schiff based reactions necessary to generate the covalent-organic frameworks of interest. Immobilization of Rhenium Complex via COFs The incorporation of the rhenium bipyridine catalyst 2 into a covalent-organic framework was accomplished by treating 2 with 5,10,15,20-tetra(4-aminophenyl)porphyrin (TAPP) in a mixture of o-dichlorobenzene, 1-butanol, and 6 M acetic acid (1:1:0.2) in a sealed ampule under vacuum, which led to the formation of COF-Re. An analogous derivative, whereby the unmetallated bipyridine ligand 1 was used as the linker in the synthesis, yielded a second framework denoted COF-Bpy. This framework is similar to the CuP-BPyPh COF reported previously which contains a copper porphyrin and a bipyridine linker59 and serves as a useful structural comparison for COF-Re. This synthetic strategy allows for the preparation of a framework containing a mixture of linkers 1 and 2. TAPP was treated with a 1:1 ratio of 1 and 2 to produce COF-Mix.

OC CO OC Re Cl OC CO OC Re Cl NH

N

N

NH

N

N

N

NH2 NH N N HN

H 2N

N

N

N

N

N

N

N M N

N

N

N

HN

N

N

N

M

N

HN N

NH2

N

N

N

H 2N

N

Cl

OC CO Cl Re CO O N N

CO Re

CO

Re N

CO

N

Cl

CO CO

N

MCln

Cl

Re CO

Re N

CO

N

CO CO

N

CO N

N

OC CO Cl Re CO OC CO Cl Re CO

O

N

N

N

NH

N

N N

N N

Cl

CO

N

NH

CO

N

N

HN

N N

N

N

N

N N

N M

N

N

M

N

N

HN

COF-Re

M = Co

COF-Re_Co

M = Fe

COF-Re_Fe

Scheme 1. Syntheses of COF-Re, COF-Re_Co, and COF-Re_Fe. The crystalline structure of the synthesized frameworks was confirmed by powder X-ray diffraction (PXRD) studies. The PXRD patterns of COF-Re and COF-Mix show major peaks at 2θ values of 3.1, 4.3, and 6.2° corresponding to 28.3, 20.6, and 14.2 Å, respectively (Figure S18 and Figure S19). These patterns are comparable to those of the 2-D cobalt porphyrin framework constructed from cobalt-metallated TAPP and biphenyl-4,4'-dicarboxaldehyde (COF-367).32 The



intensity of these peaks is slightly diminished in COF-Bpy, with only the peak at 3.1° being observed in the PXRD pattern (Figure S17). Attempts to directly synthesize a metalloporphyrin-based COF with either 1 or 2 as the linker proved unsuccessful. Therefore, a post synthetic modification strategy was employed whereby the free-base porphyrins were post metallated with either cobalt or iron precursors. Insertion of metal ions into free-base porphyrin frameworks has been previously accomplished through the use of post metallation techniques35 and analogous conditions are utilized here. COF-Re was heated in DMF at 80 °C in the presence of excess cobalt(II) or iron(III) chloride salts for 24 hours generating COF-Re_Co and COF-Re_Fe, respectively. The PXRD patterns of COF-Re_Co and COF-Re_Fe are identical to the pattern of COF-Re (Figures 4, S20, and S21) confirming the preservation of the ordered structure following post metallation. When the bipyridine sites in the framework are all fully occupied by rhenium as in COF-Re, the resultant post metallated product retains its long-range order. However, when COF-Bpy and COF-Mix are post metallated with iron or cobalt, the resulting product loses crystallinity as evidenced by the loss of the diagnostic peaks in the PXRD pattern. Modelling was performed with Materials Studio to understand the stacking orientation in COFRe and in the post metallated frameworks. Of the various stacking modes explored, the two space groups (P21212 and PCC2) that more closely matched the experimental structure were both in an eclipsed conformation. In the P21212 space group, the rhenium moieties are fully eclipsed, while in the PCC2 space group, the bipyridine moieties are eclipsed, but the orientation of the rhenium tricarbonyl moieties alternate between adjacent layers (Figures S22-S25). These space groups display predicted peaks that align well with the experimental major peak at 2θ of 3.1°, as well as with the peak at 6.2°. The peak at 4.3° is associated with the orientation of the rhenium moieties, as frameworks without rhenium lack this peak in the predicted PXRD patterns, and moreover, this peak is absent in the experimental PXRD pattern of COF-Bpy.


Page 10 of 20

Page 11 of 20 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


Figure 4. PXRD patterns of COF-Bpy (green), COF-Re (yellow), COF-Re_Co (light red), and COF-Re_Fe (black). The incorporation of cobalt and iron ions into COF-Re was confirmed by X-ray photoelectron spectroscopy (XPS). Analysis of the COF-Re Re 4f spectrum (Figure 5A) showed two peaks with binding energies of 44.2 and 41.9 eV, corresponding to Re 4f5/2 and 4f7/2. These peaks and binding energies are similar to those of the previously reported rhenium(I) bipyridine complexes.36,60,61 There are also three nitrogen peaks present in the N 1s region at 400.5, 399.6, and 398.1 eV (Figure S27) which are assigned to the three unique nitrogen environments in the framework.

Figure 5. XPS analyses of COFs. (A) Re 4f XPS spectrum of COF-Re. (B) Co 2p XPS spectrum of COF-Re_Co. (C) Fe 2p XPS spectrum of COF-Re_Fe. XPS spectra of COF-Re_Co and COF-Re_Fe show retention of the rhenium peaks (Figures S28B and S29B). Additional peaks corresponding to Co 2p (Figure 5B) and Fe 2p (Figure 5C) are observed. The cobalt region in COF-Re_Co displays two peaks at 795.6 and 11 ACS Paragon Plus Environment


780.1 eV, corresponding to Co 2p1/2 and 2p3/2 of a Co(II),62 respectively. The iron region in the COF-Re_Fe displays two peaks at 724.4 and 710.5 eV, corresponding to Fe 2p1/2 and 2p3/2 of Fe(III), respectively.63 These values are consistent with reported Co62-64 and Fe63,65 porphyrin molecular species. The retention of the Re peaks and the appearance of Co and Fe peaks is a strong indicator of successful incorporation of two distinct metal centers in the COF. The N 1s region is unchanged from COF-Re to COF-Re_Co (Figure S28A) and COF-Re_Fe (Figure S29A), respectively. The vibrational stretching modes of the carbonyls in the COFs were analyzed by FT-IR and compared to the stretches observed for 2 (Figures 2, 6, and S30-S33). The FT-IR spectrum of COF-Bpy shows no CO stretches as expected. On the other hand, the FT-IR spectrum of COF-Re displays CO stretching frequencies at the same positions (2025 and 1897 cm-1) as in complex 2, indicating the retention of the molecular structure of 2 upon incorporation into the extended COF structure. Identical CO stretching frequencies are also present in COF-Re_Co and COF-Re_Fe, further indicating that rhenium bipyridine moieties persist after post synthetic modification. Additionally, other regions of the IR spectra show strong similarities, suggesting structural retention even after post synthetic modification.

Figure 6. FT-IR spectra of COF-Bpy (green), COF-Re (yellow), COF-Re_Co (light red), and COF-Re_Fe (dark red).

The accessible surface area of the activated COFs was measured using the BrunauerEmmett-Teller (BET) technique (see SI for details). The frameworks COF-Re, COF-Re_Co, 12 ACS Paragon Plus Environment

Page 12 of 20

Page 13 of 20 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


and COF-Re_Fe display similar surface areas with values of 618.30, 615.97, and 723.12 m2/g, respectively (Table S2, Figures S34-S36). These surface areas are comparable with those of other structurally similar porphyrin-based COFs.33,59 Thermogravimetric analysis measurements indicate negligible decreases in mass percent up to 300 °C, indicating that the materials have been freed of residual solvent (Figure S37-S39). ICP analyses of the frameworks indicate a Re:Co ratio of 2 to 0.53 and a Re:Fe ratio of 2 to 0.31 for COF-Re_Co and COF-Re_Fe, respectively. The expected ratio is 2 rhenium centers per 1 cobalt or iron center, suggesting that cobalt incorporation in COF-Re_Co is 53.4%, while the iron incorporation in COF-Re_Fe is 30.7%. While previously reported post metallations of porphyrin-based frameworks led to near unity incorporation of metal ions,35 it has also been shown that full incorporation is unnecessary as all metal sites may not be electrochemically active.32 The activity of the COF-Re, COF-Re_Co, and COF-Re_Fe towards the electrocatalytic reduction of CO2 was explored in pH 7.2 aqueous phosphate buffer solutions with 0.5 M KHCO3. The conditions used here have been previously reported for successful CO2 reduction using cobalt porphyrin containing frameworks (COF-366 and COF 367).32 The COF-modified electrodes were prepared by dropcasting the materials (0.5 mg) onto a 2 cm  1 cm strip of carbon fabric. Based on the ICP-OES results, the estimated loading for COF-Re_Co is 4.66  10-7 mol of Re and 1.24  10-7 mol of Co, while the loading for COF-Re_Fe is 5.15  10-7 mol of Re and 7.90  10-8 mol of Fe. Under a nitrogen atmosphere, none of the COFs displayed any observable redox features. The CVs of COF-Re, COF-Re_Co, and COF-Re_Fe demonstrated a slight positive shift in onset potential in the presence of CO2 with COF-Re_Fe exhibiting a current increase as well (Figure 7). These current increases are similar to those previously reported for similar porphyrin frameworks.32,33



Figure 7. Cyclic voltammograms of (A) COF-Re, (B) COF-Re_Co, and (C) COF-Re_Fe in pH 7.2 aqueous phosphate buffer solutions with 0.5 M KHCO3 under N2 (blue) and CO2 (red) atmosphere. Scan rate = 100 mV/s. Controlled potential electrolyses were conducted in pH 7.2 aqueous phosphate buffer solutions with 0.5 M KHCO3 under a CO2 atmosphere for one hour at -1.1 V vs SHE to determine if any gaseous products are formed (Figure S40, Table S3). No CO was generated for COF-Re and the bare carbon fabric. COF-Re_Co produced 12.7 μmol of CO, corresponding to a faradaic efficiency of 18(2)%. H2 was also formed during the electrolysis with a faradaic efficiency of 55(5)%. COF-Re_Fe produced 5.3 μmol of CO under the same conditions, corresponding to a faradaic efficiency

Covalent-Organic Frameworks Composed of Rhenium Bipyridine and

Recommend Documents