Electrocatalytic Reduction of Carbon Dioxide with a Manganese(I

Article pubs.acs.org/Organometallics

Electrocatalytic Reduction of Carbon Dioxide with a Manganese(I) Tricarbonyl Complex Containing a Nonaromatic α‑Diimine Ligand Qiang Zeng,⊥ Joanne Tory, and František Hartl* Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, U.K. ABSTRACT: The 2e reduced anion [Mn(CO)3(iPr-DAB)]− (DAB = 1,4diazabuta-1,3-diene, iPr = isopropyl) was shown to convert in the presence of CO2 and a small amount of water to the unstable complex [Mn(CO)3(iPrDAB)(η1-OCO2H)] (OCO2H− = unidentate bicarbonate) that was further reductively transformed to give a stable catalytic intermediate denoted as X2, showing νs(OCO) 1672 and 1646 (sh) cm−1. The subsequent cathodic shift by ca. 650 mV in comparison to the single 2e cathodic wave of the parent [Mn(CO)3(iPr-DAB)Br] triggers the reduction of intermediate X2 and catalytic activity converting CO2 to CO. Infrared spectroelectrochemistry has revealed that the high excess of CO generated at the cathode leads to the conversion of [Mn(CO)3(iPr-DAB)]− to inactive [Mn(CO)5]−. In contrast, the five-coordinate anion [Mn(CO)3(pTol-DAB)]− (pTol = 4-tolyl) is completely inert toward both CO2 and H2O (solvolysis). This detailed spectroelectrochemical study is a further contribution to the development of sustainable electro- and photoelectrocatalysts of CO2 reduction based on abundant first-row transition metals, in particular manganese.

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INTRODUCTION The growing concentration of CO2 in the atmosphere has created an immense demand for efficient ways to capture and utilize this gas, belonging to the greatest human wastes contributing to global warming. One of the contemporary general trends is to convert light into electricity, which is then used to reduce CO2 on the way to fuel production (solar fuels).1 Catalytic strategies consider the feasible 2e reduction of CO2 producing either CO, further utilized in the water-gas shift reaction (WGSR), or formate/formic acid, serving as a hydrogen storage material, a potential precursor of methanol, or a basis of actual fuel cells.2 Finding sustainable and efficient electrocatalysts of CO2 reduction operating at potentials close to thermodynamic values and at high turnover frequencies (TOF), similar to those of natural enzymes, has been a challenging task for a few decades.3 Group 7 transition-metal complexes having the formula fac[M(CO)3(R-bpy)X] (M = Mn, Re; R-bpy = substituted 2,2′bipyridine (bpy); X = halide; the prefix fac will be omitted hereafter) have attracted significant attention in this respect. Electrochemical reduction of the Re complexes generates catalysts converting CO2 to CO mainly via a 2e route. The fivecoordinate anion [Re(CO)3(R-bpy)]− has been identified as the catalyst, reacting with CO2 by direct oxidative addition, resulting in the intermediate [Re(CO)3(R-bpy)(CO2H)] (CO2H = C-bound hydroxycarbonyl) detectable with stopped-flow infrared spectroscopy.4−6 The Re complexes have also been used with success as catalytic centers capable of photoreducing CO2 in the presence of a sacrificial electron donor. The hydroxycarbonyl intermediate [Re(CO)3(R-bpy)(CO2H)] has again been identified in the photocatalytic cycle,7 while the concurrent formate complex [Re(CO)3(R-bpy)(OCHO)], formed by CO2 insertion into the Re−H bond in © XXXX American Chemical Society

[Re(CO)3(R-bpy)(H)], has been said to inhibit the catalytic CO formation.8 An attachment of [Re(CO)3(R-bpy)X] to a proper photosensitizer in a supramolecular light-harvesting dyad allows for triggering efficient photocatalytic CO 2 reduction on excitation even with visible light (λexc >500 nm).9,10 The catalytic activity of the related manganese complexes [Mn(CO)3(R-bpy)Br] has been discovered only recently.11−15 The obvious driving force behind this research seeking more viable catalyst alternatives has been the vast abundance of Mn in the Earth’s crust, the much lower price of the corresponding compounds, and their less negative reduction potentials. Differently from [Re(CO)3(R-bpy)X], the analogous Mn(Rbpy) catalysts have been shown to produce CO exclusively in the presence of Brønsted acids (H2O, alcohols). The negligible catalytic activity of [Mn(CO)3(R-bpy)Br] (R = H) in dry organic solvents is in line with our earlier observations made with IR spectroelectrochemistry.16 Recently published results show that the catalytic species are both the 2e reduced fivecoordinate anion [Mn(CO)3(R-bpy)]− and the formally 1e reduced Mn(0) dimer [Mn(CO)3(R-bpy)]2. The reaction of CO2 with [Mn(CO)3(R-bpy)]− produces the Mn(I) hydroxycarbonyl intermediate [Mn(CO)3(R-bpy)(CO2H)], which was detected with IR spectroelectrochemistry for R-bpy = 6,6′-dimesityl-2,2′-bipyridine (Mesbpy).14 The catalytic reduction of CO2 in the latter case is triggered by the reduction of [Mn(CO)3(Mesbpy)(CO2H)], which requires a significant overpotential in comparison to the initial reduction of the Special Issue: Organometallic Electrochemistry Received: April 15, 2014

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precursor [Mn(CO) 3 (Mesbpy)Br] to generate [Mn(CO)3(Mesbpy)]−. For R-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine (tBu-bpy) the formate complex [Mn(CO)3(tBu-bpy)(OCHO)], produced by pulse radiolysis of [Mn(CO)3(tBubpy)Br] in the presence of Bu4N[OCHO], has also been claimed to act as a catalyst precursor, converting to [Mn(CO)3(tBu-bpy)]2 upon 1e reduction.15 The catalytic activity of the dimers [Mn(CO)3(R-bpy)]2 (R-bpy = 4,4′-disubstituted bpy; R = H, tBu, Me) has recently been supported by the independent conversion of [Mn(CO)3(Me-bpy)]2 in the reaction with CO2 to the low-spin Mn(II) hydroxycarbonyl complex mer-[Mn(CO)3(Mebpy)(CO2H]+ detectable by pulsed EPR spectroscopy.12 However, the onset of the catalytic wave in the cyclic voltammograms of the corresponding parent species [Mn(CO)3(R-bpy)Br] does not coincide with the first (formally 1e) cathodic wave, where the precursor is rapidly converted to [Mn(CO)3(R-bpy)]2, but with the negatively shifted second cathodic wave, where [Mn(CO)3(R-bpy)]2 produces 2e reduced [Mn(CO)3(R-bpy)]−.11 This observation points to the dominant catalytic activity of the latter anions. The dimer may be responsible for the weaker catalytic effect observed already at the cathodic potential corresponding to the initial reduction of [Mn(CO)3(R-bpy)Br].11 It is noteworthy that the active catalysts [Mn(CO)3(Rbpy)]− have been claimed in recent reports11,13−15 to form only by the electrochemical reduction of dimers [Mn(CO)3(Rbpy)]2 at cathodic potentials more negative (Ec2 < Ec1) than those of the parent complexes [Mn(CO)3(R-bpy)Br], as described by eqs 1−4.13

[Mn(CO)3 (R‐bpy)]− + [Mn(CO)3 (R‐bpy)Br] → [Mn(CO)3 (R‐bpy)]2 + Br −

The first crystal structure of [Mn(CO)3(α-diimine)] has been reported for α-diimine = bpy,20 followed by those with tBu-bpy13 and more sterically hindered Mesbpy14 ligands. The strongly delocalized π bonding within the Mn(α-diimine) metallacycle (the π-HOMO and π*-LUMO), stabilizing the five-coordinate geometry, was studied with DFT and TDDFT methods for α-diimine = 2,2′-bipyridine (bpy) and N,N′diisopropyl-1,4-diazabuta-1,3-diene (iPr-DAB).20 The latter nonaromatic α-diimine ligand inspired us in this work to investigate the reduction path of the complex [Mn(CO)3(iPrDAB)Br] (1) with cyclic voltammetry and IR spectroelectrochemistry in the presence of CO2, with the aim of identifying the product(s) acting as catalysts for CO2 reduction. It is noteworthy that the electrochemical reduction of complex 1 yields ultimately the 2e reduced anion [Mn(CO)3(iPr-DAB)]− (2) instead of the dimer [Mn(CO)3(iPr-DAB)]2 (3) already at the parent cathodic wave, as in this case the electrode potentials Ec1 and Ec2 are almost identical (eqs 1 and 4).17 A similar 2e (ECE) cathodic wave has recently been reported for [Mn(CO)3(Mesbpy)Br].14 This difference in the reduction paths of complex 1 and [Mn(CO)3(R-bpy)Br] (R = H, tBu, Me) was assumed to lead to the lower overpotential needed to catalytically reduce CO2 with 1 as the catalyst precursor. However, the actual spectroscopic monitoring of the cathodic area during the electrolysis revealed a reactivity of anion 2 different from that expected. The molecular structures of complexes 1−3 are depicted in Chart 1. As a reference

[Mn(CO)3 (R‐bpy)Br] + e− → [Mn(CO)3 (R‐bpy)Br]− (at Ec1)

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Chart 1. Schematic Molecular Structures of the Studied Mn(iPr-DAB) Carbonyl Complexes

(1)

[Mn(CO)3 ((R‐bpy)Br]− → [Mn(CO)3 (R‐bpy)] + Br − (2)

2[Mn(CO)3 (R‐bpy)] → [Mn(CO)3 (R‐bpy)]2

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[Mn(CO)3 (R‐bpy)]2 + 2e− → 2[Mn(CO)3 (R‐bpy)]− (at Ec 2 < Ec1)

(4)

complex, we selected [Mn(CO)3(pTol-DAB)Br] (pTol = 4tolyl) with a lower LUMO energy in order to study the effect of tuning the electronic properties of the R-DAB ligand on the catalytic activity toward the CO2 reduction.

However, Hartl and co-workers17 have proven on grounds of combined cyclic voltammetric and IR spectroelectrochemical results that 2e reduced species [Mn(CO)3(R-bpy)]− form already at the first cathodic wave E c 1 of the parent [Mn(CO)3(R-bpy)Br] via an ECE mechanism (eqs 1, 2, and 5). The electroinduced dimerization then does not occur by the coupling reaction of the five-coordinate radicals (eq 3) but instead by the zero-electron coupling reaction described by eq 6; this process can be slowed down or completely hindered at sufficiently low temperature or by reducing the experimental time scale upon rapid potential scanning. In addition, Hartl and co-workers17 have also explained the mechanism of unusual photochemical generation of [Mn(CO)3(α-diimine)]− 18 which may be of principal importance for exploiting the Mn carbonyls at photocatalysis of CO2 reduction to CO or formic acid.19

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[Mn(CO)3 (R‐bpy)] + e− → [Mn(CO)3 (R‐bpy)]− (at Ec1)

EXPERIMENTAL SECTION

Materials. All electrochemical and spectroelectrochemical measurements were conducted under an atmosphere of dry argon gas, using standard Schlenk techniques. Solvents were freshly distilled under dry nitrogen gas from purple benzophenone/Na (ketyl) radicals (tetrahydrofuran, THF) or P 2O5 (acetonitrile, MeCN). The supporting electrolyte tetrabutylammonium hexafluorophosphate (TBAPF6) was recrystallized twice from absolute ethanol and dried overnight under vacuum at 80 °C. Solutions were saturated with CO2 at normal pressure by bubbling on a frit. The studied complexes fac[Mn(CO)3(R-DAB)Br] (R-DAB = N,N′-di-R-1,4-diazabuta-1.3-diene, R = isopropyl (iPr, 1), 4-tolyl (pTol)) and the corresponding R-DAB ligands were synthesized and purified according to literature methods.21,22 Their purity was checked by IR spectroscopy (in THF, on a Bruker Vertex 70v FT-IR spectrometer) and 1H NMR spectroscopy (in CDCl3, on a Bruker NanoBay spectrometer).

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Cyclic Voltammetry. Cyclic voltammograms were recorded using an EG&G PAR Model 283 potentiostat operated with M770 v.4.23 software. An airtight, single-compartment, three-electrode cell was equipped with a platinum microdisk (r = 0.2 mm) or glassy carbon disk (r = 2 mm) working electrode polished with 0.25 μm diamond paste, a coiled platinum wire auxiliary electrode, and a coiled Ag wire pseudoreference electrode. The ferrocene/ferrocenium (Fc/Fc+) redox couple served as an internal reference for the determination of electrode potentials. The studied samples contained a 10−3 M Mn complex and 10−1 M Bu4NPF6 supporting electrolyte. Spectroelectrochemistry. IR spectroelectrochemical experiments were performed using a Bruker Vertex 70v FT-IR spectrometer equipped with a DLATGS detector. UV−vis spectroelectrochemistry was performed using a Scinco S-3100 diode-array spectrophotometer (190−1100 nm spectral range). Thin-layer UV−vis and IR spectroelectrochemical measurements were carried out at 293 K (THF, MeCN) using an optically transparent thin-layer spectroelectrochemical (OTTLE) cell23 equipped with Pt minigrid working and auxiliary electrodes, an Ag microwire pseudoreference electrode, and CaF2 windows. The course of spectroelectrochemical experiments was monitored by thin-layer cyclic voltammetry conducted with a PA4 potentiostat (Laboratory Devices, Polná, Czech Republic). The studied samples contained 10−3 M (UV−vis spectroelectrochemistry) or 3 × 10 −3 M (IR spectroelectrochemistry) Mn complex and 3 × 10−1 M Bu4NPF6 as the supporting electrolyte.

1). Infrared and UV−vis spectroelectrochemistry provide further evidence for this ECE(C) cathodic path of 1 (see below). Anion 2 is formed from both parent 1 and dimer 3 less negatively by ca. 300 and 400 mV than the CO2 reduction catalyst [Mn(CO)3(bpy)]− from [Mn(CO)3(bpy)X] (X = Cl, Br) and [Mn(CO)3(bpy)]2, respectively.11,17 Importantly, the absence of an anodic response of anion 2 at −0.28 V in the cyclic voltammogram of 1 recorded in CO2-saturated MeCN (Figure 1) points to an efficient reaction of 2 with CO2; this reaction, however, is noncatalytic. The product could not be identified with conventional cyclic voltammetry due to its poorly discernible response in the cathodic potential region. Addition of 5% H2O to the MeCN electrolyte caused a largely diminished cathodic response of 1 and the appearance of a new cathodic wave at −1.28 V (Figure 2). A similar

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RESULTS AND DISCUSSION Cyclic Voltammetry. The cyclic voltammogram of [Mn(CO)3(iPr-DAB)Br] (1) in dry MeCN (Figure 1) shows the

Figure 2. Cyclic voltammograms of ca. 10−3 M [Mn(CO)3(iPrDAB)Br] (1) in MeCN/5% H2O (solid line) and MeCN/5% H2O saturated with CO2 (dashed line). The solutions contained 10−1 M TBAPF6.. Conditions: Pt microdisk, v = 100 mV s−1, 293 K.

behavior was also observed for [Mn(CO)3(bpy)Br] in this electrolyte11 and was ascribed to solvolysis producing [Mn(CO)3(bpy)(Sv)]+ (Sv = MeCN, H2O). We attribute the cationic complex tentatively to [Mn(CO)3(iPr-DAB)(H2O)]+ on the grounds of IR spectroelectrochemical monitoring of its reduction (see below) that yielded a product different from dimer 3 observed17 for [Mn(CO)3(iPr-DAB)(MeCN)]+. Bubbling CO2 through the MeCN/5% H2O electrolyte led to almost complete replacement of the wave at −1.28 V due to [Mn(CO)3(iPr-DAB)(H2O)]+ with a new wave at −1.48 V (Figure 2) belonging to an as yet unassigned tricarbonyl complex X1 absorbing in the IR region at 2042 and 1946 (br) cm−1. This behavior most likely corresponds to an autocatalytic (electron-transfer-catalyzed) substitution reaction that was not investigated in detail at this stage. In comparison to Figure 2, very similar cyclic voltammograms of [Mn(CO)3(iPr-DAB)Br] in MeCN/5% H2O were recorded also at a glassy carbon (GC) disk electrode, which allowed us to explore a wider cathodic potential window (Figure 3). In the CO2-saturated MeCN/5% H2O electrolyte the cathodic wave of the new tricarbonyl complex X1 is shifted at GC to −1.40 V. When the cathodic scan was continued to ca. −2.1 V, an inset of a catalytic process was observed, which has its origin in the reduction of a bicarbonate complex, denoted as X2, formed at the cathodic wave of complex X1. More details about this electrocatalytic behavior are presented below in Spectroelectrochemistry.

Figure 1. Cyclic voltammograms of ca. 10−3 M [Mn(CO)3(iPrDAB)Br] (1) in dry MeCN (solid line) and MeCN saturated with CO2 (dashed line). The solutions contained 10−1 M TBAPF6. Conditions: Pt microdisk, v = 100 mV s−1, 293 K. The prominent anodic wave at −1.18 V corresponds to the oxidation of [Mn(CO)3(iPr-DAB)]− (2). The crossing of the cathodic and anodic curves between −1.2 and −1.4 V is characteristic for the 2e ECE cathodic path leading to 2.

following characteristic features: (a) an irreversible 2e cathodic wave at −1.58 V vs Fc/Fc+ that generates the five-coordinate anion [Mn(CO)3(iPr-DAB)]− (2) and (b) an irreversible 2e anodic wave at −1.18 V on the reverse scan, which corresponds to the oxidation of [Mn(CO)3(iPr-DAB)]− to regenerate the parent complex. Both ECE processes result ultimately in the formation of the dimer [Mn(CO)3(iPr-DAB)]2 (3) (by a zeroelectron coupling reaction between 1 and 2); however, it is hardly observable on the forward cathodic scan due to its concomitant 2e reduction to 2 at −1.64 V.17 On the reverse anodic scan the oxidation of 3 is perceptible at −0.28 V (Figure C

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Figure 3. Cyclic voltammograms of ca. 10−3 M [Mn(CO)3(iPrDAB)Br] (1) in MeCN/5% H2O (solid and dashed lines) and MeCN/5% H2O saturated with CO2 (dotted line). The solutions contained 10−1 M TBAPF6. Conditions: glassy carbon (GC) disk, v = 100 mV s−1, 293 K.

Figure 5. Electronic absorption spectra: (blue) complex 1 prior to reduction within an OTTLE cell; (green) anion 2 (550 and 480 nm) in a mixture with subordinate dimer 3 (744 nm) formed in dry MeCN/Bu4NPF6; (red) bicarbonate complex X2 formed by the reduction of complex 1 in MeCN saturated with CO2.

The reference complex [Mn(CO)3(pTol-DAB)Br] in dry MeCN/Bu4NPF6 reduces irreversibly at −1.18 V vs Fc/Fc+. The significantly negative potential shift of 400 mV on the substitution of the pTol-DAB ligand with less electron withdrawing iPr-DAB is close to the potential difference of 350 mV reported in the literature24 for the corresponding Re complexes. The reduction of [Mn(CO)3(pTol-DAB)Br] generates the five-coordinate anion [Mn(CO)3(pTol-DAB)]− (as confirmed by IR spectroelectrochemistry; see below), which is oxidized on the reverse anodic scan at −0.97 V. In sharp contrast to the case for complex 1, no change in the cyclic voltammetric response of [Mn(CO)3(pTol-DAB)Br] was observed in the MeCN electrolyte containing 5% H2O and/ or dissolved CO2 gas. The same applies also to the reduction of [Mn(CO)3(pTol-DAB)Br] in THF, at both the Pt and GC working microelectrodes. Spectroelectrochemistry. The reduction of complex 1 was conducted in dry MeCN/Bu4NPF6 electrolyte within an OTTLE cell, and the course was monitored with IR (Figure 4) and UV−vis (Figure 5) spectroscopy. The recorded spectra are in full agreement with the cyclic voltammetric response of 1

(see above) and with the previously reported17 reduction path of 1 in butyronitrile. The conversion of 1 (ν(CO) at 2026 and 1930(br) cm−1) to 2e reduced anion 2 at −1.58 V was confirmed by the appearance of the ν(CO) bands at 1921 and 1811(br) cm−1 and the π → π*(Mn-DAB) and dπ(Mn) → π*(DAB) electronic absorptions at 550 and 480 nm, respectively.20 The presence of the subordinate dimer 3, as indicated by the small ν(CO) bands at 1975, 1945, and 1886 (br) cm−1 in the IR spectrum and the dπ(Mn) → π*(DAB) absorption band at 744 nm in the UV−vis absorption spectrum,20 reflects the small difference of ca. 60 mV between the reduction potentials of 1 and 3. Briefly, the reductions of both complexes generate anion 2, although via different ECE mechanisms. Dimer 3 is then formed by the coupling reaction between 1 and 2 at potentials slightly less negative than that of the reduction of 3. The reduction of complex 1 in MeCN saturated with CO2 yields anion 2 only as a minor product (Figure 6, a small ν(CO) band at 1811 cm−1). This observation is in line with the cyclic voltammetric response shown in Figure 1, where the

Figure 4. FT-IR spectral changes accompanying the cathodic conversion of complex 1 (↓) to anion 2 (↑) via dimer 3 (↑↓). The tiny band at 2046 cm−1 (↑↓) belongs to [Mn(CO)3(iPr-DAB)(H2O)]+ formed due to a small amount of moisture in the MeCN/ Bu4NPF6 electrolyte. Conditions: electrolysis within an OTTLE cell at 293 K.

Figure 6. FT-IR spectral changes accompanying the cathodic conversion of complex 1 (↓) to intermediate X2 (↑) in the MeCN/ Bu4NPF6 electrolyte saturated with CO2. The sharp band at 1672 cm−1 probably belongs to the symmetric OCO stretch of bicarbonate anion coordinated in the dominant product. Conditions: electrolysis in within an OTTLE cell at 293 K. D

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the smaller ν(CO) values in comparison to those of parent 1 (ν(CO) at 2026 and 1930 (br) cm−1). The solvolysis of complex 1 in MeCN/5% H2O saturated with CO2 again produced initially [Mn(CO)3(iPr-DAB)(H2O)]+. However, the cationic aqua complex gradually converted into another, as yet unassigned, tricarbonyl species X1 showing ν(CO) absorption maxima at 2042 and 1943 (br) cm−1 (Figure 8); this process probably corresponds to the

anodic wave of 2 is largely diminished after bubbling the MeCN electrolyte with CO2, indicating that anion 2 reacts with CO2 efficiently. The corresponding IR spectra in Figure 6 reveal that the major product of the reduction of 1 in MeCN/CO2 is the tricarbonyl complex X2 showing ν(CO) bands at 2032 and 1924 (br) cm−1 accompanied by a medium-intensity sharp band at 1672 cm−1 with a shoulder at 1646 cm−1 belonging probably to the νs(OCO) mode of the anionic bicarbonate ligand OC(O)OH−.25 The intermediate X2 was also generated independently by electrochemical reduction of the separately synthesized complex [Mn(CO)3(iPr-DAB)(η1-OCO2H)] (η1OCO2H denotes a unidentate bicarbonate anion).26 No catalytic reduction of CO2 was observed at the cathodic wave of complex 1. We can safely conclude that the catalytic activity of 2e reduced anion 2 in the CO2 reduction is rapidly inhibited by the formation of the complex [Mn(CO)3(iPr-DAB)(η1OCO2H)], which transforms at the cathode to give the more stable transient X2. The deactivation of the Mn center remains a challenge for practical applications of this otherwise promising system. The catalytic reduction of CO2 then occurs at a more negative (