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Article

Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO2 Reduction

by
Dominik Krisch
1,
He Sun
1,
Kevinjeorjios Pellumbi
2,3,
Kirill Faust
4,
Ulf-Peter Apfel
2,3 and
Wolfgang Schöfberger
1,*
1
Institute of Organic Chemistry, Johannes Kepler University (JKU), Altenberger Straße 69, 4040 Linz, Austria
2
Inorganic Chemistry I—Bioinorganic Chemistry, Ruhr University Bochum, Universitätsstraße 150, 44801 Bochum, Germany
3
Department of Electrosynthesis, Fraunhofer Institute for Environmental, Energy and Safety Technology UMSICHT, Osterfelder Straße 3, 46047 Oberhausen, Germany
4
Institute of Catalysis (INCA), Johannes Kepler University (JKU), Altenberger Straße 69, 4040 Linz, Austria
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(5), 545; https://doi.org/10.3390/catal12050545
Submission received: 21 April 2022 / Revised: 12 May 2022 / Accepted: 15 May 2022 / Published: 17 May 2022
(This article belongs to the Section Electrocatalysis)
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Abstract

:
We report herein the preparation and characterization of six readily assembled bis-coordinated homoleptic silver(I) N,N′-bis(arylimino)acenaphthene (BIAN) complexes of general structure [Ag(I)(BIAN)2]BF4 and the influence of the electronic properties of the ligand substitution pattern on their performance in electrochemical CO2 reduction (CO2R). All the explored catalysts displayed substantial current enhancements in carbon-dioxide-saturated solvents dependent on the ligated BIAN and no significant concurrent H2 evolution when utilizing 2% H2O as a proton source. Additionally, preliminary studies, employing a drop-casted ink of 0.4 mg cm−2 [Ag(I)(4-OMe-BIAN)2]BF4 (Ag4) immobilized onto carbon paper gas diffusion electrodes in a flow cell with 1M KHCO3 aqueous electrolyte, resulted in a propitious Faradaic efficiency of 51% for CO at a current density of 50 mA cm−2.

Graphical Abstract

1. Introduction

For decades, the global community has been facing an environmental crisis, resulting in the need to switch from outdated to new, more efficient, energy sources and a more effective way of tackling rising carbon dioxide emissions. The activation of small molecules, such as H2O [1,2,3,4], O2 [1,5,6,7], N2 [8,9,10], and CO2 [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34], in a cost- and energy-efficient way has become one of the key topics in catalysis research.
The main issue concerning the activation of these molecules is the kinetic barrier that needs to be overcome for the catalyzed reaction to take place [16]. Therefore, there is a research focus on the design of novel ligand systems whose metal complexes can be used for the direct electrocatalytic reduction of CO2 to valuable chemicals, such as carbon monoxide [18,19,20,21,22], formate [20,23,24], methanol [25,26,27], ethanol [28,29], methane [18,30], ethylene [28,31,32,33], and acetate [20,34,35].
While a variety of N-type ligands (e.g., porphyrinoids, bipyridine, salen, pincer ligands, etc.) and their metal complexes have been studied previously, the application of BIAN (N,N′-bis(arylimino)acenaphthene) ligated metallocomplexes for electrochemical conversion of carbon dioxide has been scarce [20]. Hitherto, only one family of BIAN ligated complexes for electrocatalytic CO2 reduction with Re(I)(Ar-BIAN)(CO)3Hal (Ar = phenyl, mesityl or 2,6-diisopropyl; Hal = Cl or Br) has been reported [22,36]. Homogenous application in an acetonitrile + 2% H2O mixture of the most active derivative Re(I)(2,6-diisopropyl-BIAN)(CO)3Cl led to the production of CO with a maximum Faradaic efficiency of 24% after 4000 s of electrolysis at −1.85 V vs. NHE [22].
Additionally, only a few reports on silver BIAN complexes have been published in the literature, almost exclusively focusing on their synthesis and characterization with none of the studied compounds being utilized for catalytic applications of any sort [37,38,39].
The research presented here illustrates the first implementation of molecular silver(I) complexes for electrocatalytic CO2 reduction. The studied redox-active, homoleptic Ag(I) bis-BIAN coordination compounds were readily prepared in high yields and subsequently characterized through various spectroscopic methods (NMR, UV-vis, IR), X-ray photoelectron spectroscopy (XPS), ESI-HRMS, single-crystal X-ray diffraction and electrochemical studies. The metal complexes were examined homogeneously under argon and CO2, and, heterogeneously, at a carbon paper gas diffusion electrode. The “non-innocent” nature of the BIAN ligands was reflected in the ability of the auxiliary ligands to participate directly in the catalytic cycle by accepting and donating electrons. All the explored complexes displayed alterations in their cyclic voltammograms under CO2. Addition of H2O, a proton source, to the solutions under carbon dioxide exhibited a steep increase in cathodic current when employing the corresponding BIAN-ligated silver(I) complexes. We, furthermore, demonstrated that altering the substituents at the aryl-moieties of the BIAN significantly affected the catalytic current during CO2 electroreduction. Finally, we established the applicability and stability of heterogenized [Ag(4-OMe-BIAN)2]BF4 (Ag4) over 1 h of galvanostatic electrolysis on GDEs at current densities up to 200 mA cm−2 in aqueous 1 M KHCO3 electrolytes with high selectivity towards CO2R for the generation of CO.

2. Results and Discussion

2.1. Synthesis and Characterization

The employed BIAN ligands have been reported before, except for L3 and L6, and were synthesized via the methodologies established by the groups of Ragaini and Milani, with modified workup procedures for L3, L5 and L6, as described in the Section 3 (vide infra) [40,41]. In general, stirring acenaphthenequinone and zinc(II) chloride in acetic acid (and toluene for L1 and L6) at elevated temperatures, followed by addition of a slight excess of the respective aniline, led to the formation of BIAN-ligated zinc(II) dichloride precipitate after 15–45 min of reflux. Ensuing demetallation with potassium oxalate resulted in the free BIAN ligands.
The studied homoleptic Ag(I) bis-BIAN complexes were prepared in analogy to a literature procedure, where Ag2 has been reported [39]. By treating dichloromethane (DCM) solutions of [Ag(I)(ACN)4]BF4 (ACN = acetonitrile) with two equivalents of the BIAN ligand, the pertinent bis-chelated [Ag(I)(BIAN)2]BF4 complexes Ag1–Ag6 were isolated in excellent yields of 85–98% (Scheme 1). They were obtained as orange (Ag1, Ag2), red (Ag4, Ag5) or purple (Ag3, Ag6) powders after concentration and subsequent precipitation induced by the addition of n-pentane.
Novel BIAN ligands and all silver complexes have been fully characterized by 1H, 13C and 19F NMR, ESI-HRMS, UV-vis and IR spectroscopy. Due to the multifarious substitution patterns and consequential polarities of the products, however, multiple solvents had to be utilized for these measurements, limiting the direct comparability of some of the obtained data. Solution NMR experiments in CDCl3, CD3CN and DMSO-d6 confirmed that the investigated silver catalysts all exhibited diamagnetic properties, as was to be expected from d10 metal complexes.
UV-vis spectroscopy revealed that, despite metalation, no intense long-wavelength metal-to-ligand charge-transfer (MLCT) processes occurred in the prepared complexes Ag1–Ag6 and, predominantly, the intra-ligand centered π-π* and n-π* transitions of the aryl, acenaphthene and imine moieties were observed in the acetonitrile solutions [39,42,43]. Nevertheless, these absorptions bands were altered upon coordination to a varying degree, indicating at least some participation of the Ag orbitals with the aforementioned excitations [38]. The UV-vis absorption maxima and corresponding logarithmic extinction coefficients are listed in Table 1. The most significant alteration of an absorption band was monitored for L6, where λmax of the most bathochromic absorption shifted from 517 nm to 537 nm upon coordination to the silver metal center. In general, the respective lowest energy absorptions were shifted bathochromic in correlation with the increasing electron-donating properties of the BIANs, from 308 (L1) up to 517 nm (L6), suggesting the energetic convergence of HOMO and LUMO in these systems. The only aberration, with L3, can be explained by the incorporation of an extended π-system via the pyrene moiety. This trend also indicated that 4-diethylamino-substituted ligand L6 was, as would be anticipated, considerably more electron-donating than 4-dimethylamino-BIAN reported in the literature, the equivalent absorption maximum of which is located at 448 nm [44]. The UV-vis spectroscopic data were obtained predominantly with acetonitrile solutions, except for L3 (DCM), due to its limited solubility and Ag1. Measurements of Ag1 were conducted in tetrahydrofuran (THF), since the highly electron-deficient ligand L1 has proven to bind weakly to the silver center and is almost instantaneously displaced by rather strongly coordinating solvent molecules, e.g., in ACN.
The recorded high resolution ESI mass spectra clearly displayed the predicted singly charged cationic complexes missing the non-coordinating BF4− anion [M-BF4−]+.
X-ray quality crystals of Ag4 were obtained via slow diffusion of n-pentane into a dilute DCM solution of the pertinent complex. The derived molecular structure is depicted in Figure 1.
The observed distorted tetrahedral coordination at the Ag(I) metal center in the solid state (64.040(63)° torsion angle between the BIAN planes) was in accordance with the structure published for Ag2 and was expected to similarly expand onto the other prepared catalysts [39]. All the Ag-N bond lengths of Ag4 that crystallized in the orthorhombic space group (Ccca) were equivalent with 2.327(2) Å. Additionally, this extended to the BIAN C=N and imine C-C bonds with bond lengths of 1.261(4) Å and 1.527(6) Å, respectively. These values were consistent with reported bond lengths of neutral BIAN ligands, further verifying the general structure that was depicted in Scheme 1 [37,38,39,45,46,47]. They were nearly identical with the values reported for the molecular structure of the free ligand L4 [48], illustrating the absence of any particularly strong synergistic interaction from the Ag center to the BIAN, such as extensive π-back bonding, which was in agreement with the data obtained from UV-vis spectroscopy (vide supra). Furthermore the bite angle of the BIAN at the metal center was determined to be 73.30(11)° and the 4-methoxyphenyl moieties were almost perpendicularly (87.336(104)°) twisted out of plane of the acenaphthenequinone backbones. Unfortunately, crystals of other Ag complexes could not yet be obtained with sufficient quality or decomposed during the crystallographic measurements.

2.2. Electrochemical Characterization

Cyclic voltammetry for electrochemical characterization and electrocatalytic CO2 reduction experiments was performed at ambient temperatures and scan rates of 100 mV s−1 with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte, typically applying cathodic potentials first. Concentrations of 1 mM catalyst in acetonitrile were studied, except for Ag2, where 0.5 mM solutions were examined in consequence of its limited solubility. Additionally, measurements of Ag1 were conducted in dimethylformamide (DMF) due to the stability limitations elucidated in the previous section. All potentials are given vs. NHE, which were calculated after determination of the ferrocene/ferrocenium half-potential in each mixture as internal standard (listed in Table S2) [49,50,51].
Cyclic voltammograms (CVs) of the explored complexes under argon were primarily defined by the known distinctive redox non-innocence of BIANs [36,38,43,52,53,54,55]. The first reduction generating a radical anion typically delocalized between the imine-moieties occurred between −0.82 (Ag1) and −1.32 V (Ag2). By further increasing the cathodic potential, a second reduction at the BIAN ligands to the pertinent diamines appeared at −1.06 (Ag1) to −1.90 V (Ag2 and Ag4). The data for the electrochemical characterization in anhydrous acetonitrile or DMF under argon is summarized in Table 2.
Notably, 4-diethylamino-substituted BIAN L6 displayed an oxidation reaction that was fully reversible (E1/2 = 0.67 V), which is extraordinary for this type of α-diimine ligand [38,44,54,55,56].
The only considerable discrimen of the free BIAN ligands and the respective silver complexes was an oxidation reaction between +0.50 V (Ag6) and +0.70 V (Ag3), which was not observed for Ag1 in DMF, with the potentials thereof inversely correlating with the electronic donation of the coordinating ligand. This property was also noted in the electrochemical characterizations of Ag(I) bis-BIAN nitrate complexes that were studied by the Tkachenko group [38]. The oxidative current, however, appeared only if the reductive potentials were applied first, as solely anodic sweeps did not feature in this electrochemical process (see Figure S72). Additionally, this signal was also present in the CV of [Ag(ACN)4]BF4 at +0.80 V, but with a many-fold increased current. A CV response overlay of L6, Ag6 and [Ag(ACN)4]BF4 in acetonitrile under argon is depicted in Figure 2. A tentative explanation would be that some amount of Ag0 was formed during the cathodic sweeps, which was then re-oxidized upon applying anodic potentials. The substantial decrease in current of this oxidative process for Ag2–Ag6 indicated the stabilization of the silver(I) metal center within the BIAN-chelated complexes.

2.3. Homogeneous Electrocatalysis

To probe the capabilities of the prepared catalysts Ag1–Ag6 towards mediating electrocatalytic CO2 reduction, cyclic voltammograms, employing carbon-dioxide-saturated solvents, with and without the addition of 2 or 20% (for Ag4 and Ag6) of water as a proton source, were recorded, under otherwise identical conditions to those described in the previous section. The introduction of CO2 into the anhydrous ACN or DMF solutions of the silver catalysts led to an increased cathodic current, except for Ag3, typically arising between the first and second ligand-centered reductions. In addition, the transitions became irreversible, confirming a reaction at the electrogenerated species. Although this enhancement for Ag1–Ag5 was in the range of factors 1.5–3, Ag6 exhibited a remarkable 15-times higher cathodic current in the presence of CO2 without a proton source at −1.90 V. The addition of 2% H2O further augmented the detected reductive currents for Ag1–Ag5, however, not when employing Ag6, where a slight decrease was noticed, yet still surpassing the other explored catalysts in this study under these conditions. A feasible explanation is that the tertiary para-amino functionalities of the BIAN ligand in Ag6 were protonated to some extent at weakly acidic pH values which were paralleled by CO2 saturation in the aqueous electrolytes [57]. This alteration resulted in an inversion of the electronic properties of the ligated BIAN, hence becoming electron withdrawing, which as a result partially deactivated the catalyst. The cathodic current densities of L6 and Ag1–Ag6 at −1.90 V under the various examined conditions are listed in Table 3.
As depicted in Figure 3 and summarized in Table 3, the catalytic currents in the organic solvent with 2% water mixtures exhibited a positive correlation with increasing electron-donating properties of the employed ligand. Even though the maximum CO2 concentration at ambient conditions in DMF was lower than in ACN (0.20 M vs. 0.28 M) [58,59], it can clearly be seen that Ag1, incorporating the severely electron withdrawing 3,5-bis-CF3-BIAN L1, produced the lowest cathodic current, despite still exhibiting catalytic activity (Figure S80). This trend, furthermore, pertained to the onset potentials shifting from ca. −1.65 V (Ag1) up to −1.35 V for Ag6 employing the particularly strongly donating 4-diethylamino-BIAN L6.
The currents observed under anhydrous conditions and in the solvent–water mixtures differed since the presence of protons allowed for alternative reduction mechanisms and, hence, distinct kinetics. It is well known that, without external proton sources, electrocatalytic CO2 reduction can proceed either via reductive disproportionation forming carbonate and CO, or the more seldom discussed route via dimerization of the radical anion CO2•− to oxalate, whereas the addition of protons affords varying pathways to a plethora of reduction products, such as CO, formic acid, methanol, methane, and higher carbon products, e.g., ethylene, ethanol and acetic acid [20,29,34,60,61,62,63].
Crucial to note is that none of the prepared Ag(I) bis-BIAN complexes displayed notable catalytic H2 evolution in solvent mixtures with 2% water under argon in the examined potential range. This fact renders them as potent CO2 reduction catalysts utilizing water as an environmentally benign proton source. CV response overlays of Ag4 and Ag6 under different experimental conditions are shown in Figure 4A,C.
Electrochemical studies of the bare BIAN ligand L6 and the precursor complex [Ag(ACN)4]BF4 verified that both the ligand and the coordinated silver metal center were necessary to form catalytically active species, since none of these compounds individually displayed catalytic CO2 reduction capabilities in ACN/H2O mixtures comparable to those of the respective chelated silver BIAN complexes (compare Figure 4B and Supplementary Materials). Although the CVs for [Ag(ACN)4]BF4 did suggest a considerable increase in cathodic current under CO2 in the presence of water at potentials below −1.5 V, when scanning over the entire potential region, this current rapidly decreased when the scan range was anodically limited to +0.5 V vs. NHE (Figure S76). This was in contrast to the catalysts Ag1–Ag6, which did exhibit stable currents during purely cathodic cycles even with 20% water in ACN (e.g., Figure 4D). This data, in agreement with the heterogeneous CO2R experiments performed with Ag4, clearly indicated that the electrocatalytic CO2 reduction was mediated by Ag1–Ag6, while for [Ag(ACN)4]BF4, merely an undefined stoichiometric alteration of the compound, which could be reverted upon oxidation at ca. +0.6 V (in ACN + 20% H2O), occurred. A similar decrease in the cathodic current was also observed for the BIAN ligand L6 (Figure S70).
The CVs in ACN + 20% H2O mixtures for Ag4 and Ag6 did exhibit slightly diminished reversibility of the observed redox peaks listed in Table 2, in addition to the ligand-centered reductions arising at less negative potentials (e.g., Figure S94). Nonetheless, electrochemical measurements conducted under an argon atmosphere indicated that no substantial hydrogen evolution by virtue of water electrolysis was mediated by the explored catalysts, as no significant current increase was observed under these conditions (see, e.g., Figures S91 and S92). The decrease in detected catalytic currents in CO2 saturated solvents, compared to the experiments conducted with 2% H2O, was to be anticipated, considering the significantly reduced solubility of CO2 in water compared to ACN (0.033 M vs. 0.28 M) [58]. It is interesting to note that the catalytic currents of Ag4 and Ag6 converged and were nearly identical in ACN with 20% H2O, further verifying the prior statement regarding the presumed deactivation of Ag6 induced by chemical alteration of 4-diethylamino-BIAN at weakly acidic pH values, along with the onset potentials of both catalysts being shifted anodic to ca. −1.2 V.
As a final remark, due to the explored catalysts incorporating 18-electron metal centers, it seems counterintuitive, at first, for their catalytic activity to stem from anything other than an outer sphere CO2 reduction mechanism [20,63,64,65]. However, solid state molecular structures, inter alia, of five coordinate bis-chelated Ag(I) BIAN nitrate complexes, along with pentaligated [{MeB [3-(Mes)pz]3}Ag(C2H4)] (Mes = mesitylene; pz = pyrazolate), formally bearing 20 electrons at the metal center, have been reported [38,66]. Hence it appears coherent with this that CO2 would be able to ligate to the tetracoordinated silver center in the explored compounds Ag1–Ag6, generating a reactive intermediate during electrocatalytic reduction.

2.4. Heterogeneous Electrocatalysis

Currently, few materials have achieved a direct transition from homogeneous catalysis to industrially relevant gas diffusion electrodes (GDEs) [19,67]. Concretely, GDEs shorten the diffusion pathways of CO2 to the catalytic centers compared to H-type cells involving liquid electrolytes, achieving current densities greater than 50 mA cm−2 under aqueous conditions [68]. Since Ag4 has proven pristine catalytic activity and stability towards the generation of CO2R products when employed homogeneously in high water content solvents, it was selected for preliminary studies on gas diffusion electrodes in a 1 M KHCO3 electrolyte.
For the fabrication of the GDEs, catalytic inks consisting of a mixture of carbon-black (33 wt.%) and Ag4, bound by Nafion, were drop-casted on the surface of an H23C6 carbon paper GDL, at a catalytic loading of 0.4 mg cm−2 Ag4. Investigation of the pertinent GDEs under 50 mA cm−2 to 200 mA cm−2 demonstrated that in aqueous electrolytes, Ag4 still possessed high selectivity towards CO (Figure 5). The constant half-cell potentials (Figure 5B) during galvanostatic electrolysis provided further evidence of the robustness and stability of the explored catalytic system.
At lower current densities of 50 mA cm−2 and 100 mA cm−2, CO was generated with a Faradaic efficiency (FECO) of 51% and 47%, respectively. Furthermore, regarding industrially relevant current densities of 200 mA cm−2, Ag4-GDEs remained highly active with an FECO of 42% at half-cell potentials of ca. −1.4 V vs. RHE. The remaining percentages of Faradaic efficiency at higher current densities could have been associated with losses of gaseous products in the catholyte reservoir, underlying the need for improvement regarding the wettability of the electrode surface.
The obtained results clearly demonstrated the promising perspective of Ag-BIAN electrocatalysts towards industrially relevant applications. Previous reports have already shown that optimal CO2RR activity can be achieved by tailoring, not only the catalyst, but also the reactive environment [69,70], electrode architecture [71] and surrounding parameters [72] in a synergistic fashion. Consequently, our future investigations will focus on optimizing BIAN-coated GDEs in state-of-the art CO2 electrolyzers.

3. Materials and Methods

3.1. Synthesis and Characterization

All chemicals were purchased from Alfa Aesar (Kandel, Germany), Strem (Bischheim, France), BLDpharm (Shanghai, China), Merck (Darmstadt, Germany) or Sigma-Aldrich (Vienna, Austria) and were used without further purification unless otherwise noted. Anhydrous DCM, THF and ACN were obtained from a molar sieve MB-SPS-7, M. Braun inert gas-System GmbH (Garching, Germany) under argon atmosphere. All deuterated NMR solvents were purchased from Euriso-Top (Fluorochem, Hadfield, United Kingdom). Proton (1H NMR), carbon (13C NMR) and fluorine (19F NMR) spectra were recorded on a Bruker (Billerica, MA, USA) DRX 500 MHz spectrometer equipped with a cryoprobe (TXI) and on a Bruker Advance 300 MHz NMR spectrometer. The chemical shifts were given in parts per million (ppm) on the delta scale (δ) and were referenced to the used deuterated solvent for 1H NMR and 13C NMR. High-resolution mass spectra were obtained utilizing an Agilent (Santa Clara, CA, USA) 6520 Q-TOF mass spectrometer with an ESI source and an Agilent G1607A coaxial sprayer. UV-Vis absorption spectra were collected on a Varian CARY 300 Bio spectrophotometer (Agilent, Santa Clara, CA, USA) from 200 to 900 nm. IR spectra were recorded on a Bruker ALPHA II FT-IR instrument. All spectral data can be found in the supporting information.

3.1.1. General Procedure for the Preparation of BIAN Ligands

BIAN ligands L1–L4 and L6 were synthesized via the methodology established by the Ragaini group with modified workup procedures for L3 and L6 as described below [40,41]. In general, acenaphthenequinone and zinc(II) chloride (2.7 equiv.) were stirred in acetic acid (and toluene for L1 and L6) at 80 °C under argon for 30 min. Subsequently, 2.2 equiv. of the respective aniline were added and the mixtures refluxed for 15 (L2, L4), 30 (L3) or 45 min (L1, L6), leading to the formation of BIAN ligated zinc(II) dichloride precipitate, which was collected via suction filtration. The residues were washed with cold HOAc and Et2O before being extracted with DCM and saturated aqueous potassium oxalate solution. Finally, the organic layers were dried over Na2SO4 and concentrated in vacuo to obtain the free BIAN ligands in yields comparable to the literature.

1-Pyrene-BIAN (L3)

According to the literature procedure [40], but with an adapted workup: subsequent to extraction with aq. K2C2O4 and evaporation, the dark residue was triturated with n-heptane/ethyl acetate (3:1) thrice to remove residual 1-aminopyrene. The solid was collected via suction filtration, washed with n-heptane/ethyl acetate (3:1) and dried in vacuo to obtain 1-pyrene-BIAN L3 as a dark red-purple powder in 68% yield (2.66 mmol, 1.54 g).

4-Diethylamino-BIAN (L6)

According to the literature procedure [40], but with an adapted workup: subsequent to refluxing the reaction mixture in HOAc/toluene for 45 min, it was cooled in a refrigerator for 1 h. The solution was decanted, and the dark, sticky residue triturated with Et2O and filtered five times. Afterwards, it was dissolved in MeOH (blue color) and extracted with DCM and saturated aqueous K2C2O4. The now purple organic phase was dried over Na2SO4, filtered and concentrated in vacuo to obtain 4-diethylamino-BIAN L6 as a dark purple solid in 66% yield (5.17 mmol, 2.46 g).

3,4,5-Trimethoxy-BIAN (L5)

Synthesized in analogy to a reported procedure [41] with modified workup: 3,5-bis-CF3-BIAN zinc(II) dichloride complex (1.20 g, 1.58 mmol) was dissolved in 80 mL MeOH, 3,4,5-trimethoxyaniline (868 mg, 4.74 mmol, 3 eq.) added and the solution stirred at rt overnight, during which it turned from orange to red. Subsequently, the reaction mixture was evaporated to dryness, and the residue taken up in DCM and extracted with saturated aqueous K2C2O4 solution. After concentration in vacuo, the oily residue was triturated with n-heptane, the remaining solid was dissolved in DCM, extracted five times with 0.1 M HCl and finally washed with water. Afterwards it was dried over Na2SO4, filtered and concentrated in vacuo to obtain 3,4,5-trimethoxy-BIAN L5 as a red solid in 66% yield (1.04 mmol, 531 mg).

3.1.2. General Procedure for the Preparation of [Ag(I)(BIAN)2]BF4 Complexes Ag1–Ag6

Ag(I) bis-BIAN complexes were prepared analogously to a literature procedure, where Ag2 has been reported [39]. DCM solutions (10 mL) of [Ag(I)(ACN)4]BF4 (0.2 mmol) were treated with dropwise addition of BIAN ligand (0.4 mmol, 2 equiv.) in 20 mL DCM, leading to immediate color changes. The resulting mixtures were stirred at room temperature under argon in the dark overnight, before the reaction volumes were reduced to ca. 5 mL under reduced pressure. Subsequently, n-pentane was added, the suspensions cooled for 1 h and the precipitated solids collected via suction filtration. After washing the residues with n-pentane they were dried in vacuo to obtain Ag1–Ag6 as orange (Ag1, Ag2), red (Ag4, Ag5) or purple (Ag3, Ag6) powders in 85–98% yield (see Scheme 1).

3.2. Homogeneous Electrochemistry

Tetrabutylammonium hexafluorophosphate (TBAPF6) for the electrochemical measurements was purchased from Sigma-Aldrich and recrystallized twice from absolute ethanol (Acros Organics, Vienna, Austria), dried under high vacuum and stored under argon prior to use. Anhydrous ACN was obtained from a molar sieve MB-SPS-7, M. Braun Inert Gas System GmbH (Garching, Germany) under argon atmosphere. Anhydrous DMF was purchased from Acros Organics. Aqueous solutions for electrochemical experiments were prepared using high purity water (18 MΩ). Homogeneous electrochemical measurements were conducted in a low volume Gamry cell using a three-electrode setup at 25 °C under argon or CO2, respectively, utilizing a Pine WaveDriver 20 DC Bipotentiostat (Equilabrium, Lyon, France). The investigated solutions were prepared from dry, degassed solvents and contained 0.1 M supporting electrolyte (TBAPF6) and 1 mM of the respective complexes or ligands, except for [Ag(2,6-dipp-BIAN)2]BF4 Ag2, where 0.5 mM solutions were examined in ACN due to its limited solubility. Complex Ag1 was characterized in DMF, since coordinating solvents, such as ACN, cause ligand substitution. For all electrochemical measurements, a glassy carbon (Pine, 65 mm long, 6.4 mm OD PCTFE shroud, 3 mm OD disk) as working electrode, platinum wire (Pine, 65 mm long, 7 mm OD epoxy tube shroud, 0.58 cm2 approximate surface area) as counter electrode and a non-aqueous pseudo-Ag/AgCl reference electrode, were used. Prior to each measurement, the glassy carbon WE was polished with a 0.05 μm alumina suspension (deagglomerated, Allied-high tech products) on a MicroCloth polishing pad (Buehler, PSA, Tokyo, Japan). Subsequently, the electrode was cleaned in an ultrasonication bath in pure water for 3 min and finally rinsed with pure water and acetonitrile to remove any excess of alumina particles. Following each experiment, ferrocene was added as an internal standard. The converted potentials against Fc/Fc+ were transformed to V vs. NHE with the conversion of +0.630 V vs. NHE in ACN and +0.590 V vs. NHE in DMF according to literature [49,50,51]. If not otherwise noted, CVs were recorded at a scan rate of 100 mV/s. Plots of the obtained data can be found in the supporting information.

3.3. Heterogeneous Electrochemistry

3.3.1. Fabrication of Gas Diffusion Electrodes

For the catalytic inks, 10 mg of Ag4 along with 5 mg of Ensaco 250G carbon black (Imerys, Paris, France) were dispersed along with 50 μL of 5 wt.% Nafion 117 solution (Sigma Aldrich, Taufkirchen, Germany) in 5 mL of ethanol. Prior to drop-casting the Ag4-containing inks were sonicated for 30 min. Under constant stirring, drop-casting was performed onto the surface of a 22 mm Freundenberg H23C6 GDL (Freundenberg, Weinheim, Germany) placed onto a heated vacuum plate at 80 °C, until a loading of 0.4 mg cm−2 of Ag4 was achieved. The catalytic loading was determined by the mass difference prior to and after drop-casting.

3.3.2. Electrochemical Measurements

The Ag4-loaded gas diffusion electrodes were measured in an in-house flow electrolyzer, equipped with parallel titanium flow-fields. Ni-foam (Goodfellow, Hamburg, Germany) was used as the anode, an RHE (Gaskatel, Kassel, Germany) as the reference electrode and an Ag4-GDE as the working electrode, with the electrolyte 1M KHCO3 circulating at a rate of 10 mL min−1. The flow compartment possessed a ø 16 mm opening, thus exposing a 2 cm2 area of the GDE as the active geometric area with the help of PTFE gaskets.
A CO2 feed of 22.5 mL min−1 was provided to the back-side of the GDE in a flow-by mode, controlled by a mass-flow controller (Bronckhorst, Düsseldorf, Germany). To detect possible changes in the gaseous products streams due to carbonate build-up or due to gas-forming reactions, 10 vol.% Ar (2.5 mL min−1) was added to the CO2 feed as an internal standard. A differential pressure of 40 mbar was applied with the help of a backpressure regulator (Equilibar, Fletcher, NC, USA). Electrolysis was performed utilizing a Gamry (Haar, Germany) 1010B Potentiosat/Galvanostat for 1 h. All the potential readings were iR-corrected and reported against the RHE. The mean values represented at least three separate measurements, each performed with a new Ag4-GDE.

3.3.3. Product Analysis

The composition of gaseous products was determined with the help of an online Shimdazu (Kyoto, Japan) QP2020 GC-MS, equipped with a Supelco Carboxen 1010 Plot column (Sigma Aldrich, Taufkirchen, Germany). Gas samples were taken every 20 min for a total of 1 h. For the quantification of acids, an HPLC 1200 (Agilent, Santa Clara, CA, USA) with a BinPump1200, an autosampler ALS1200 and a DAD detector were used, employing a Phenomenex (Aschaffenburg, Germany) Rezex ROA-Organic Acid H+ column at a flow rate of 0.2 mL min−1. Before analysis, electrolysis samples were diluted in a 1:5 ratio in 5M H2SO4. Quantification of alcohols was performed with the help of a Shimadzu GCMS-QP2020, equipped with an HS-20 trap system and a SH-Rtx-200MS column.

Calculation of the Faradaic Efficiency

For the gas product quantification, the Faradaic efficiency was calculated as follows:
F E = z F i · x p r o d · p F v R T · 100 %
where z is the number of the transferred electrons for the formation of each product, F is the Faraday constant, n i is the number of moles of the respective product, i is the total applied current and xprod is the mole of fraction of product (xmeas) in the gas stream corrected by the internal standard Ar. Moreover, Fv is the initial CO2 volume flow, p represents the absolute CO2 pressure, R the gas constant and T the temperature,
x p r o d = x m e a s · x A r , i n p u t x A r , o u p u t
For liquid products, the following equation was used for the calculation of the FE:
F E i = z · F · n i i · t · 100 %
where z is the number of the transferred electrons for the formation of each product, F is the Faraday constant, n i is the number of moles of the respective product, i is the total measured current and t is the time during which the current i was passed through the electrolyte.

4. Conclusions

In summary, we have reported the synthesis and characterization of six homoleptic silver(I) bis-BIAN complexes and assessed their catalytic activity in electrochemical CO2 reduction. The explored compounds Ag1–Ag6, of general structure [Ag(I)(BIAN)2]BF4, were readily assembled via two-step syntheses from commercially available starting materials, in good yields, without the prerequisite of dried solvents or manipulations utilizing thorough Schlenk techniques.
Each of the studied complexes Ag1–Ag6 was shown to mediate the catalytic transformation of carbon dioxide upon the application of cathodic potentials in ACN or DMF and ACN/DMF with 2% H2O, establishing them as the first described molecular CO2 reduction catalysts comprising silver metal centers. Juxtaposition of the detected catalytic currents showed that the strongly electron-donating BIANs ameliorated the efficacy of electrochemical CO2 conversion. Additionally, no significant H2 evolution was observed for any of the employed catalysts in 2 vol.% aqueous electrolytes under argon at potentials up to −2 V vs. NHE.
Preliminary tests employing 0.4 mg cm−2 Ag4, immobilized with an ink as a heterogeneous catalyst on carbon paper GDEs with 1M KHCO3 aqueous electrolyte, produced CO at an auspicious 51% FECO at 50 mA cm−2 and 42% at 200 mA cm−2 under as yet non-optimized conditions. These results greatly emphasized the potency of the reported Ag BIAN complexes for electrochemical CO2 reduction, rendering them particularly appealing for further research and future applications.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal12050545/s1: The characterization and spectral data of L1–L6 and Ag1–Ag6 (1H NMR, 13C NMR, 19F NMR (for L1 and Ag1), UV-vis, IR, HR-MS, XPS); all homogeneous electrochemical data acquired for L6, [Ag(ACN)4]BF4 and Ag1–Ag6; the description, as well as the results, of the X-ray crystallographic measurements of Ag4 (CCDC 2167847); and the GC-MS chromatograms of the heterogeneous electrochemistry.

Author Contributions

Conceptualization: D.K. and W.S.; methodology: D.K., H.S., K.P., U.-P.A. and W.S.; validation: D.K., H.S. and U.-P.A.; formal analysis: D.K., H.S., K.P. and K.F.; investigation: D.K., H.S., K.P. and K.F.; writing—original draft preparation: D.K., K.P. and W.S.; writing—review and editing: D.K. and W.S.; visualization: D.K., K.P. and K.F.; supervision: U.-P.A. and W.S.; funding acquisition: U.-P.A. and W.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Austrian Science Fund (FWF Standalone Project P32045), the European Union through the EFRE INTERREG IV ETC-AT-CZ program (project M00146, “RERI-uasb”). the German Federal Ministry for Economic Affairs and Energy (projects “ElkaSyn—Steigerung der Energieeffizienz der elektrokatalytischen Alkoholsynthese”, grant 03ET1642C, and “E4MeWi—Energie-Effiziente Erneuerbare-Energien basierte Methanol-Wirtschaft”, grant 03EI3035A-D), the Deutsche Forschungsgemeinschaft (under Germany’s Excellence Strategy—EXC-2033—Project number 390677874), and the Fraunhofer Internal Programs under Grant No. Attract 097-602175. Open Access Funding by the Austrian Science Fund (FWF).

Data Availability Statement

Not applicable.

Acknowledgments

We gratefully thank Thomas Bögl from the Department of Analytical Chemistry at the JKU for carrying out ESI HR-MS measurements. W.S. acknowledges the financial support of the Austrian Science Fund (FWF Standalone Project P32045). The NMR spectrometers were acquired in collaboration with the University of South Bohemia (CZ) with financial support from the European Union through the EFRE INTERREG IV ETC-AT-CZ program (project M00146, “RERI-uasb”). K.P. acknowledges the Fonds of the Chemical Industry for a PhD fellowship. U.-P.A. is grateful for financial support from the German Federal Ministry for Economic Affairs and Energy (projects “ElkaSyn—Steigerung der Energieeffizienz der elektrokatalytischen Alkoholsynthese”, grant 03ET1642C, and “E4MeWi—Energie-Effiziente Erneuerbare-Energien basierte Methanol-Wirtschaft”, grant 03EI3035A-D). Furthermore, U.-P.A. is also grateful for the financial support by the Deutsche Forschungsgemeinschaft (under Germany’s Excellence Strategy—EXC-2033—Project number 390677874) and the Fraunhofer Internal Programs under Grant No. Attract 097-602175.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00545 sch001 550
Scheme 1. Preparation and structures of the evaluated Ag(I) bis-BIAN complexes Ag1–Ag6.
Scheme 1. Preparation and structures of the evaluated Ag(I) bis-BIAN complexes Ag1–Ag6.
Catalysts 12 00545 sch001
Catalysts 12 00545 g001 550
Figure 1. Solid-state molecular structure of Ag(I) bis-BIAN complex Ag4 in the crystal (Ccca). Thermal ellipsoids are drawn at the 30% probability level at 25 °C. Hydrogen atoms are omitted for clarity.
Figure 1. Solid-state molecular structure of Ag(I) bis-BIAN complex Ag4 in the crystal (Ccca). Thermal ellipsoids are drawn at the 30% probability level at 25 °C. Hydrogen atoms are omitted for clarity.
Catalysts 12 00545 g001
Catalysts 12 00545 g002 550
Figure 2. CV response overlay of 1 mM [Ag(ACN)4]BF4, BIAN ligand L6 and Ag(I) bis-BIAN complex Ag6 in ACN under argon at 100 mV s−1 scan rate.
Figure 2. CV response overlay of 1 mM [Ag(ACN)4]BF4, BIAN ligand L6 and Ag(I) bis-BIAN complex Ag6 in ACN under argon at 100 mV s−1 scan rate.
Catalysts 12 00545 g002
Catalysts 12 00545 g003 550
Figure 3. Homogeneous electrocatalysis of Ag(I) bis-BIAN complexes. Cathodic segment of cyclic voltammograms of 1 mM Ag1-Ag6 (except for 0.5 mM Ag2) in CO2 saturated ACN or DMF (Ag1) with 2% H2O at a scan rate of 100 mV s−1.
Figure 3. Homogeneous electrocatalysis of Ag(I) bis-BIAN complexes. Cathodic segment of cyclic voltammograms of 1 mM Ag1-Ag6 (except for 0.5 mM Ag2) in CO2 saturated ACN or DMF (Ag1) with 2% H2O at a scan rate of 100 mV s−1.
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Catalysts 12 00545 g004 550
Figure 4. Homogeneous electrocatalysis of BIAN ligand L6 and Ag(I) bis-BIAN complexes Ag4 and Ag6. (A) Comparison of CVs of Ag6 in ACN under argon (black), CO2 (red), in ACN with 2% H2O under argon (blue) and CO2 (green) at 100 mV s−1 scan rate. (B) CV response overlay of L6 (blue) and Ag6 (orange) in ACN with 2% H2O under CO2. (C) Comparison of CVs of Ag4 in ACN under argon (black), CO2 (red), in ACN with 2% H2O under argon (blue) and CO2 (green) at 100 mV s−1 scan rate. (D) Stability test of Ag6 in ACN with 20% H2O under CO2 scanning ten purely cathodic cycles not portending a decrease in catalytic current.
Figure 4. Homogeneous electrocatalysis of BIAN ligand L6 and Ag(I) bis-BIAN complexes Ag4 and Ag6. (A) Comparison of CVs of Ag6 in ACN under argon (black), CO2 (red), in ACN with 2% H2O under argon (blue) and CO2 (green) at 100 mV s−1 scan rate. (B) CV response overlay of L6 (blue) and Ag6 (orange) in ACN with 2% H2O under CO2. (C) Comparison of CVs of Ag4 in ACN under argon (black), CO2 (red), in ACN with 2% H2O under argon (blue) and CO2 (green) at 100 mV s−1 scan rate. (D) Stability test of Ag6 in ACN with 20% H2O under CO2 scanning ten purely cathodic cycles not portending a decrease in catalytic current.
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Catalysts 12 00545 g005 550
Figure 5. Performance of Ag(I) bis-BIAN complex Ag4 on H23C6 carbon paper GDL during the heterogeneous electrocatalytic CO2 reduction process. (A) Faradaic efficiencies after 1 h of galvanostatic electrolysis at different current densities in 1M KHCO3. (B) Obtained potential curves of galvanostatic electrolysis at different current densities in 1M KHCO3.
Figure 5. Performance of Ag(I) bis-BIAN complex Ag4 on H23C6 carbon paper GDL during the heterogeneous electrocatalytic CO2 reduction process. (A) Faradaic efficiencies after 1 h of galvanostatic electrolysis at different current densities in 1M KHCO3. (B) Obtained potential curves of galvanostatic electrolysis at different current densities in 1M KHCO3.
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Table
Table 1. UV-vis absorption data of the prepared BIAN ligands L1–L6 and corresponding Ag(I) bis-BIAN complexes Ag1–Ag6 in acetonitrile (except for L3 and Ag1).
Table 1. UV-vis absorption data of the prepared BIAN ligands L1–L6 and corresponding Ag(I) bis-BIAN complexes Ag1–Ag6 in acetonitrile (except for L3 and Ag1).
Compoundλmax/nm (log ε)
L1201 (4.65), 226 (4.71), 297 (3.95), 308 (3.95)
L2204 (4.99), 229 (4.97), 272 (3.94), 308 (3.93), 409 (2.96)
L3a245 (5.28), 270 (4.92), 280 (4.97), 348 (4.91), 384 (4.35), 484 (4.04)
L4212 (4.64), 229 (4.83), 291 (4.08), 424 (3.63)
L5208 (4.91), 228 (4.94), 300 (3.94), 429 (3.43)
L6208 (4.82), 229 (4.88), 265 (4.63), 306 (4.27), 317 (4.25), 517 (4.09)
Ag1b229 (5.35) 297 (4.58)
Ag2204 (5.20), 229 (5.19), 308 (4.23), 412 (3.30)
Ag3239 (5.35), 269 (5.02), 279 (5.06), 347 (5.01), 383 (4.45), 474 (4.11)
Ag4212 (4.92), 229 (5.12), 292 (4.36), 422 (3.88)
Ag5207 (5.12), 228 (5.17), 300 (4.22), 429 (3.72)
Ag6206 (5.05), 230 (5.09), 268 (4.86), 308 (4.51), 537 (4.38)
a in DCM. b in THF.
Table
Table 2. Electrochemical characteristics of catalysts Ag1–Ag6 compared with L4, L6 and the precursor complex [Ag(ACN)4]BF4 at 100 mV s−1 scan rate in ACN under argon. Due to large peak separations, except for the second cathodic reduction, cathodic (pc) and anodic (pa) peak potentials are otherwise given.
Table 2. Electrochemical characteristics of catalysts Ag1–Ag6 compared with L4, L6 and the precursor complex [Ag(ACN)4]BF4 at 100 mV s−1 scan rate in ACN under argon. Due to large peak separations, except for the second cathodic reduction, cathodic (pc) and anodic (pa) peak potentials are otherwise given.
EredEox
Compound E p c 1 /V E p a 1 /V E 1 / 2 2 /V E p a 1 /V E p a 2 /V E p c 2 /V
L4−1.20−0.79−1.78-1.25-
L6−1.27−0.94−1.74-0.690.64
[Ag(ACN)4]BF4---0.80--
Ag1a−0.82−0.72−0.99---
Ag2−1.32−1.08−1.820.67--
Ag3−0.98−0.89−1.830.701.30-
Ag4−1.20−0.84−1.820.531.32-
Ag5−1.11−0.66−1.760.521.35-
Ag6−1.26−0.93−1.730.500.810.71
a in DMF.
Table
Table 3. CV current densities (0.28 cm2 glassy carbon WE) of 1 mM BIAN ligand L6 and Ag(I) bis-BIAN complexes Ag1–Ag6 under argon or CO2 in anhydrous acetonitrile solutions or with addition of 2% H2O at a potential of −1.90 V vs. NHE and cathodic scan rate of 100 mV s−1.
Table 3. CV current densities (0.28 cm2 glassy carbon WE) of 1 mM BIAN ligand L6 and Ag(I) bis-BIAN complexes Ag1–Ag6 under argon or CO2 in anhydrous acetonitrile solutions or with addition of 2% H2O at a potential of −1.90 V vs. NHE and cathodic scan rate of 100 mV s−1.
ACNACN + 2% H2O
CompoundjAr/µA cm−2 j C O 2 /µA cm−2jAr/µA cm−2 j C O 2 / µA cm−2
L6−65−198−81−205
Ag1a−118−187−149−244
Ag2b−103−297−116−513
Ag3−180−156−255−700
Ag4−177−354−187−895
Ag5−166−488−149−1263
Ag6−159−2264−158−2058
a in DMF. b 0.5 mM catalyst solution.
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Krisch, D.; Sun, H.; Pellumbi, K.; Faust, K.; Apfel, U.-P.; Schöfberger, W. Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO2 Reduction. Catalysts 2022, 12, 545. https://doi.org/10.3390/catal12050545

AMA Style

Krisch D, Sun H, Pellumbi K, Faust K, Apfel U-P, Schöfberger W. Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO2 Reduction. Catalysts. 2022; 12(5):545. https://doi.org/10.3390/catal12050545

Chicago/Turabian Style

Krisch, Dominik, He Sun, Kevinjeorjios Pellumbi, Kirill Faust, Ulf-Peter Apfel, and Wolfgang Schöfberger. 2022. "Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO2 Reduction" Catalysts 12, no. 5: 545. https://doi.org/10.3390/catal12050545

APA Style

Krisch, D., Sun, H., Pellumbi, K., Faust, K., Apfel, U.-P., & Schöfberger, W. (2022). Tuning the Electronic Properties of Homoleptic Silver(I) bis-BIAN Complexes towards Efficient Electrocatalytic CO2 Reduction. Catalysts, 12(5), 545. https://doi.org/10.3390/catal12050545

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