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Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO2 Reduction Alexander Wilting,† Thorsten Stolper,‡ Ricardo A. Mata,*,‡ and Inke Siewert*,† †

Universität Göttingen,, Institut für Anorganische Chemie, Tammannstraße 4, D-37077 Göttingen, Germany Universität Göttingen,, Institut für Physikalische Chemie, Tammannstraße 6, D-37077 Göttingen, Germany

‡

S Supporting Information *

ABSTRACT: Herein, we present the reduction chemistry of a dinuclear α-diimine rhenium complex, 1, [Re2(L)(CO)6Cl2], with a proton responsive ligand and its application as a catalyst in the electrochemical CO2 reduction reaction (L = 4-tert-butyl-2,6bis(6-(1H-imidazol-2-yl)-pyridin-2-yl)phenol). The complex has a phenol group in close proximity to the active center, which may act as a proton relay during catalysis, and pyridine-NH-imidazole units as α-diimine donors. The complex is an active catalyst for the electrochemical CO2 reduction reaction. CO is the main product after catalysis, and only small amounts of H2 were observed, which can be related to the ligand reactivity. The ic/ip ratio of 20 in dimethylformamide (DMF) + 10% water for 1 points to a higher activity with regard to [Re(bpy)(CO)3Cl] in MeCN/H2O, albeit 1 requires a slightly larger overpotential (bpy = 2,2′-bipyridine). Spectroscopic and theoretical investigations revealed detailed information about the reduction chemistry of 1. The complex exhibits two reduction processes in DMF, and each process was identified as a two-electron reduction in the absence of CO2. The first 2e− reduction is ligand based and leads to homolytic N−H bond cleavage reactions at the imidazole units of 1, which is equal to a net double proton removal from 1 forming [Re2(LH−2)(CO)6Cl2]2−. The second 2e− reduction process has been identified as an O−H bond cleavage reaction at the phenol group, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction of [Re2(LH−3)(CO)6Cl]2−. In the presence of CO2, the second reduction process initiates catalysis. The reduced species is highly nucleophilic and likely favors the reaction with CO2 instead of O−H bond cleavage.

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INTRODUCTION

reduction over H2 formation, which is thermodynamically favored.4 The mechanism of the CO2 reduction catalyzed by [Re(bpy)(CO)3Cl] and its derivatives has been a subject of study for the past few decades. The first detailed mechanism was proposed by Meyer and co-workers in 1985, with slight revisions to date.5 The catalysis proceeds via the one- or the two-electron reduction pathway. The first reduction step of [Re(LNN)(CO)3Cl] is always ligand based and produces the radical anion [Re(LNN)(CO)3Cl]•− (LNN = 2,2′-bipyridine (bpy) and 4,4′-dimethyl-2,2′-bipyridine (dmp)).6 Such a ligand based reduction step leads to small shifts of the CO-frequencies of about −20 to −30 cm−1 (Scheme S2). Subsequently, the radical loses chloride yielding [Re(LNN)(CO)3]•.5,6 Kubiak suggested from IR measurements of [Re(tBu-bpy)(CO)3Cl] that the chloride ion dissociation initiates a ligand to metal charge transfer (LMCT) from the tBu-bpy ligand.6d This LMCT leads to a more pronounced shift of the CO bands of about −40 to −50 cm−1 with respect to the starting material (Scheme S2). In the one-electron reduction pathway, [Re-

The electrochemical reduction of CO2 has been investigated in the last few decades. It represents a key step toward the use of CO2 as a powerful and cheap fuel source or as chemical feedstock.1 The possible reduction products of CO2 are found to be very similar in energy (Scheme 1), thus warranting the development of selective redox catalysts. Additionally, the highenergy radical CO2•− must be bypassed in the process. One well-explored catalyst family in this respect is the group 7 tricarbonyl-diimine complexes. Since the pioneering work of Lehn and co-workers on [Re(bpy)(CO)3Cl], this catalyst family has been investigated intensively.3 The advantage of [Re(bpy)(CO)3Cl] and its derivatives is the preference for CO2 Scheme 1. CO2 Reduction Potentials [V] vs SCE in Water at pH2 = 7

Received: January 20, 2017 Published: March 20, 2017 © 2017 American Chemical Society

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DOI: 10.1021/acs.inorgchem.7b00178 Inorg. Chem. 2017, 56, 4176−4185

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Inorganic Chemistry (bpy)(CO)3]• reacts with CO2 to form the complex [Re(bpy)(CO)3(CO2)]. The latter then reacts in a bimolecular process forming CO, CO32−, and [Re(bpy)(CO)3(L)]n+, (L = Cl−, n = 0 or L = MeCN, n = 1).5d The highly reactive [Re(bpy)(CO)3]• radical may also be stabilized by a sixth ligands such as MeCN, which impedes the reaction with CO2. It can also dimerize to yield [Re(bpy)(CO)3]2.6 [Re(bpy)(CO)3]2 has typical CO stretching frequencies at 1988, 1951, 1887, and 1859 cm−1 (Scheme S2, Figure S19). Fujita and co-workers suggested a very unfavorable equilibrium for the LMCT in [Re(dmb)(CO)3(THF)] in THF, which results in a small rate constant for the dimer formation (dmb = 4,4′-dimethyl-2,2′bipyridine).7 The dimer can re-enter the catalytic cycle by reduction, but its reduction potential is lower than the one of the monomer.5d,6b In the two-electron pathway, the radical is usually stabilized by a sixth ligand and catalysis is initiated upon further reduction of [Re(bpy)(CO)3(L′)] yielding [Re(bpy)(CO)3]−, which reacts with CO2 to produce CO, [Re(bpy)(CO)3(L)]n+, and XO (L = Cl−, n = 0 or L = MeCN, n = 1, X = O2− acceptor, L′ = usually π-acceptor ligand such as MeCN, P(OEt)3).5,6c,8 Recently, Muckerman, Schaefer, and co-workers investigated the reaction mechanism of [Re(bpy)(CO)3]• and CO2 by density functional theory (DFT) calculations and proposed a detailed picture of the one-electron reduction pathway.9 When two [Re(bpy)(CO)3]• radicals are in close proximity, they react with 1 equiv of CO2 to produce the dimer [Re2(bpy)2(CO)6(CO2)]. In the presence of an additional CO2 molecule, [Re2(bpy)2(CO)6(CO2)] forms a formyl carbonate dimer, which rearranges to the carbonate bridged dimer and CO. According to this proposed mechanism, a dimeric species may enhance catalytic activity during the one-electron pathway. Indeed, Kubiak and co-workers recently observed an enhanced bimolecular catalytic activity through a supramolecular arrangement of two [Re(LNN)(CO)3Cl] units.10b The complex catalyzed the reductive disproportionation of CO2 to CO and CO32− at the first reduction potential with higher efficiencies than the parent complex. However, a higher catalytic efficiency was not measurable at the second reduction potential. A few further dinuclear Re-complexes have been investigated as redox catalysts in the electrochemical or photochemical CO 2 reduction.10 In the two-electron pathway, CO is not formed by disproportionation but by protonation of reduced CO2 yielding CO2H, which is further reduced to CO and water (Scheme 1). Thus, a proton relay in close proximity to the reaction center could be very beneficial, as it may facilitate intramolecular (rather than intermolecular) proton transfer to the bound CO2 upon reduction.11 A remarkable example for such enhanced catalytic activity was presented recently by Savéant, Costentin, and co-workers. They introduced phenol substituents near the metal center in a catalytically active iron-tetraphenylphorphyrin, effectively speeding up catalysis.11a These studies have prompted us to investigate in detail the dinuclear rhenium complex 1, which has a proton relay in close proximity to the metal center, as a potential catalyst in the electrochemical CO2 reduction (Figure 1). The ligand of 1 likely shows a noninnocent behavior, and this may initiate reactivity at the NH-functions. Fujita and co-workers recently investigated the redox chemistry of [Re(4-dbp)(CO)3Cl], which also bears a noninnocent, proton responsive ligand (4dbp = 4,4′-dihydroxy-2,2′-bipyrdine).12 The authors observed homolytic OH-bond cleavage upon 2e− reduction and

Figure 1. Left: Dinuclear rhenium complexes with a proton relay in close proximity to the metal center. Right: Mononuclear complex (3), which was investigated by Warren and co-workers as a redox catalyst in the electrochemical CO2 reduction reaction.14

formation of [Re(4-dbpH−2)(CO)3]− (Scheme S3). The product shows small redshifts of the CO bands in relation to the starting material (about −10 cm−1 per 1e− step), since the redox state of neither the ligand nor the metal changes. The resulting 4-dbpH−2 ligand is just a slightly better σ-donor than 4-dbp. A similar behavior seems feasible in 1, since Hartl and co-workers proposed a homolytic NH-bond cleavage upon 1e− reduction of [Re(bpy)(CO) 3 (im)] yielding [Re(bpy)(CO)3(imH−1)]− (im = imidazole, imH−1 = imidazolate).13 During our studies, a paper has been published by Warren and co-workers, in which they described the reactivity of a very similar, mononuclear complex (Figure 1, 3).14 3 is not active in the electrochemical CO2 reduction in MeCN or dimethylformamide (DMF).14,15

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EXPERIMENTAL SECTION

General. Manipulations of air-sensitive reagents were carried out in a MBraun glovebox or by means of Schlenk-type techniques involving the use of a dry nitrogen or argon atmosphere. Millipore water was degassed by bubbling argon through it before use. All reagents were purchased as reagent grade or with higher quality and used without further purification. Acetonitrile was dried over P4O10 and purified by distillation. Dry DMF was purchased from Sigma-Aldrich (227056). The solutions were purged 20 min with CO2 before the CO2 reduction experiments (CO2 from Air Liquide, Quality 5.3). Electrochemical Studies. Electrochemical measurements were recorded with a Gamry Instruments Reference 600 or Reference 600+ using degassed Millipore water, dry DMF, or dry acetonitrile. A common three electrode setup was used with a glassy carbon working electrode (GC: CH Instruments, ALS Japan; A = 7.1 mm2), a platinum wire as a counter electrode, and a silver wire as pseudo reference electrode. nBu4NPF6 or nBu4NCl were used as conducting salts, I = 0.1 M, and the complex concentration was about 1 mM. All data were referenced vs the Fc+/0 redox potential. Since NHE/SCE referencing is also quite common in CO2 reduction catalysis, we also measured the Fc+/0 redox potential vs SCE in DMF/water mixtures (Table S3). iR compensation was performed by the positive feedback method, which is implemented in the PHE200 software of Gamry. We used different custom-made gastight cells for the electrochemical measurements. In the CPE and coulometry experiments, the counter electrode was separated from the bulk solution by a sample holder with a porous glass frit. The amounts of CO and H2 in the CPE experiments were determined by a Shimadzu GC-2014 equipped with a TCD detector and a ShinCarbon ST 80/100 Silco column. Methane was used as an internal standard in order to determine nH2 and nCO. The faradaic efficiency was determined by n(measured)H2/CO/(Q/2F), Q = electric charge. Calibration curves for CH4/H2 and CH4/CO were determined separately by injecting known quantities of the mixtures. IR-SEC experiments (infrared-spectroelectrochemistry) were conducted in an OTTLE cell.16 The cell is equipped with a platinum working electrode, pseudo-Ag-reference and a platinum counter electrode. The IR spectra 4177


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Inorganic Chemistry were recorded with a Bruker Vertex 70 IR spectrometer. The UV/visSEC spectra were recorded with an Avantes ava-spec 2048 spectrometer. All electrochemical data reported in the paper are background corrected. Synthesis. The ligand synthesis has been described previously.17 1: A flask was charged with 4-tert-butyl-2,6-bis(6-(1H-imidazol-2-yl)pyridin-2-yl)phenol (1 equiv) and [Re(CO)5Cl] (2 equiv) in dry toluene (20 mL). A reflux condenser was attached to the flask, and the mixture was heated to reflux for 8 h. The solution was allowed to cool to room temperature, and the solvent volume was reduced in vacuo. A yellow precipitate was collected by filtration, washed with Et2O and pentane, and dried in vacuo. 1 can be recrystallized from acetone (yield typically 65%). 1H NMR (DMSO-d6, 300 MHz): major isomer δ [ppm] = 14.3 (s, NH, 2H), 8.42−8.19 (m, CHAr, 4H), 7.72 (d, J = 1.4 Hz, CHAr, 2H), 7.60 (s, OH, 1H), 7.51−7.45 (m, 4H), 7.37 (s, CHPhenol, 2H), 1.32 (s, tBu, 9H); minor isomer δ [ppm] = 14.3 (s, NH, 2H), 9.03 (s, OH, 1H), 8.42−8.19 (m, CHAr, 4H), 7.70 (d, J = 1.4 Hz, CHAr, 2H), 7.51−7.45 (m, 4H), 7.30 (s, CHPhenol, 2H), 1.28 (s, tBu, 9H). ESI-MS (MeOH): m/z = 1013.1 ([M−Cl]+), 1071.0 ([M + Na]+). IR (KBr): ṽ [cm−1] = 2957 (w), 2933 (w), 2024 (vs, CO), 1920 (vs, CO), 1895 (vs, CO), 1616 (m), 1558 (m), 1496 (w), 1477 (s), 1413 (w), 1245 (m), 1114 (m), 955 (w), 887 (w), 817 (m), 775 (m), 764 (m), 754 (m), 727 (w), 702 (w), 651 (m). C32H24Cl2N6O7Re2·0.5Et2O (determined by NMR) calcd.: C: 37.6; H: 2.7; N: 7.8; Cl: 6.5; found: C: 37.6; H: 3.0; N: 7.8; Cl: 7.1. Computational Details. DFT calculations were carried out to characterize the synthesized Re-catalysts and corresponding reduction products. Structure optimizations were carried out at the BP86-D3/ def2-TZVP18 level of theory (with Becke-Johnson type damping) with the Stuttgart/Dresden pseudopotential ECP60MWB for the Re atoms.19 The nature of the stationary points was confirmed by frequency calculations. Optimisations with the continuum solvation model COSMO (DMF as solvent) were also carried out.20 The latter structures were used for the infrared spectra calculations. The electronic energies were refined at the B3LYP*-D3/def2-TZVP21 level of theory (the B3LYP* functional corresponds to a modified version with 15% exact exchange) to obtain reduction potentials. The latter single point calculations included solvent effects through the COSMO model. The zero-point energy corrections as well as the remaining corrections to the 298.15 K free energy (by 1 atm) were obtained with the BP86 functional as mentioned above, under the rigid rotor harmonic oscillator approximation. Spin states corresponding to antiferromagnetically coupled, localized electrons were treated using the broken symmetry formalism as proposed by Neese.22 RI23 approximations were applied in the GGA calculations, while the RIJCOSX24 method was used in the hybrid functional runs. The reduction potentials were computed using the expression ΔE = −ΔG/ neF, whereby ne is the number of electrons transferred, F is the Faraday constant and ΔG the free energy difference between the two species. All electronic structure calculations were carried out with the Orca 3.0.3 program package.25

Figure 2. Molecular structure of the main isomer of 2. Hydrogen atoms were omitted for clarity. Thermal ellipsoids were set at the 50% level.

Table 1. Selected Bond Lengths [Å] of 2 from the X-ray Structure with Estimated Standard Deviations in Parentheses and of the Calculated Structures of 1 and 2 atoms

2 (X-ray)

1 (calc.)

2 (calc.)

Re3−N1 Re3−N2 Re3−Cl1 Re3−C29 Re3−C30 Re3−C31 Re4−N4 Re4−N5 Re4−Cl2 Re4−C32 Re4−C33 Re4−C34

2.237(2) 2.146(3) 2.4773(7) 1.921(3) 1.895(3) 1.912(3) 2.234(2) 2.134(3) 2.4918(8) 1.923(4) 1.898(3) 1.894(4)

2.270 2.169 2.472 1.928 1.914 1.916 2.257 2.171 2.497 1.933 1.915 1.916

2.256 2.157 2.487 1.936 1.911 1.908 2.235 2.154 2.522 1.936 1.914 1.907

unit are disordered (occupancy factors: 0.88/0.12, Figure S2), which explains the two isomers observed by NMR spectroscopy. There is a hydrogen bond between the disordered Cl2 atom of the Re(CO)3Cl-fragment and the phenolic O atom (cf. d(Cl2···O1) = 3.06 Å). The distance between the Cl1 atom and the O1 atom is considerably longer (cf. d(Cl1···O1) = 3.75 Å). One diimine unit (Re3) and the phenol unit are almost perpendicular to each other, while the second one (Re4) is slightly tilted due to the hydrogen bond (torsion angle of 85° and 77°, respectively). The Re−N distances ranging from 2.13 to 2.24 Å, which is in the typical range of ReI−Ndiimine distances and the N−Re−N angles of 74.7° and 74.8°, respectively, are also very similar to well-known [Re(LNN)(CO)3Cl] derivatives.26 No crystal structure of 1 could be obtained. Because of the coincident spectroscopic features of 1 and 2 (vide infra) as well as computed stable minima, very similar structure and isomeric forms are considered for 1. The structures of both compounds were optimized at the BP86-D3/def2-TZVP level of theory and compared to the available X-ray structure. The structural parameters of 2 are in close agreement to the latter. We also obtained a stable minimum for the unmethylated complex 1 with a conformation and structure similar to 2. As one will observe later, the same conformation and structure can be used to describe compound 1 in DMF solution, bearing a good agreement in the CO stretching frequencies measured. Optical Spectroscopy. The IR spectra of 1 shows three CO stretching bands in DMF (Table 2, Figure S3). The CO stretching frequencies of 1 and 2 are identical, which supports

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RESULTS AND DISCUSSION Complex Synthesis and Characterization. The ligand can be synthesized in a multistep synthesis as reported previously.17 The reaction of the ligand and [Re(CO)5Cl] in refluxing toluene led to the formation of a yellow precipitate, which was isolated and purified. Proton NMR spectroscopy of the complex indicated the formation of two isomers and ESI spectrometry revealed the formation of a binuclear rhenium complex having the general formula [Re2L(CO)6Cl2], 1. The purity of the bulk material was confirmed by elemental analysis. The methyl imidazole derivative of 1, 2, has been characterized by a single crystal X-ray diffraction experiment. The result is depicted in Figure 2 and Figure S2. Selected bond lengths and angles are shown in Table 1 and Table S2. The facial isomer of 2 is formed, in which three CO ligands occupy one face of the octahedron. The trans oriented Cl2/C34O7 ligands of one Re 4178


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at higher scan rates, we observed a growing shoulder at a slightly lower reduction potential than the second reduction wave (Figure S6). The potentials and the ip ratios of the first and second wave were rather insensitive to the scan rate, which indicates minimal an EE mechanism and/or an ECE mechanism with a very fast chemical reaction. The first reduction event is slightly shifted but still irreversible when a Pt working electrode was used instead of a GC working electrode. In order to obtain further information on the structures of the reduced species, combined theoretical and (spectro)electrochemical studies were conducted. UV/vis-spectroelectrochemical (UV/vis-SEC) measurements showed growing shoulders at 385 and 345 nm upon the first 2e− reduction (red species in Figure 3, Figure S8). IR-spectroelectrochemical

Table 2. IR Stretching Frequencies of 1 and 2 in DMF as Well as of 3 and [Re(bpy)(CO)3Cl] ṽ(CO)

a

complex

A′(1)a

A′(2)a

A″a

1 2 314 [Re(bpy)(CO)3Cl]6b

2016 2016 2017 2019

1905 1904 1909 1917

1886 1886 1886 1895

Pseudo Cs symmetry.

the assumption that both complexes have the same structure. The CO stretching frequencies of 1 are slightly lower than the ones of [Re(bpy)(CO)3Cl], likely because of the stronger σdonor and weaker π-acceptor character of the pyridineimidazole unit and, thus, larger π-back-donation.27 The strength of σ-bonding interaction can be estimated from the pKa of the heterocycles (imidazole: basic pKa of 6.99, pyridine: pKa of 5.2528). The CO stretching frequencies of 1 and 2 are very similar to those of 3.14 On the basis of the DFT calculations carried out for comparison with the crystal data structure, we reoptimized the structure of 1 with the COSMO model to approximate the DMF solvation effect. The obtained geometry differs only slightly from that obtained for the gas phase. The computed harmonic vibrational spectrum reveals two close transitions at higher frequencies for the symmetric CO stretches (average of 1985 cm−1), and several other transitions in the range 1878− 1844 cm−1 (Figure S21). No empirical scaling factors have been applied to the results. A fair agreement between the experimental spectrum and the computed harmonic values is observed, with a good estimate for the separation between the higher frequency band and the remaining asymmetric CO stretch transitions. The absorption spectra of 1 and 2 in DMF show one strong absorption at 318 nm (1)/323 nm (2), and shoulders at ∼375 nm (1)/∼380 nm (2), respectively (Figure S4). Likely, the shoulders correspond to MLCT transitions,29 while the strong absorption bands correspond to intra ligand bands, since the ligand exhibits similar transitions.30 Further intra ligand bands are masked by N,N-dimethylformamide (UV/vis cutoff wavelength at 300 nm). Calculations on the computed structure of 1 again reveal close agreement to the measured values. A CAMB3LYP/def2-TZVP31 calculation of the first excited states gives visible transitions at 310 and 371 nm. Inspection of the corresponding natural transition orbitals (NTOs)32 confirm the tentative assignment provided above. (Spectro)electrochemical Characterization of 1. The CV of 1 shows two irreversible reduction waves in the potential window of DMF at a scan rate of 100 mV s−1 (GC working electrode, Figure S5, potential window of DMF: −3.3 to −1.2 V vs Fc+/0). 3 exhibited only one irreversible reduction process in MeCN.14 The first reduction wave belongs to a two-electron reduction as determined by coulometry. Upon applying a potential which matches the potential of the first reduction process, 2.1 electrons were injected (i dropped from 200 μA to 2 μA, Figure S7). We assigned this to a single electron reduction of each rhenium entity, because the diimine unit and the phenol rings are aligned almost perpendicular to each other. The current of the second reduction process is slightly higher than the one of the first process, which indicates that the second reduction also belongs to a multielectron reduction process, i.e., two or three electrons. When we collected the CVs

Figure 3. UV/vis-SEC and IR-SEC of 1 under argon atmosphere in DMF, I = 0.1 M nBu4NPF6. Blue: 1; red: 1red1, that is, the 2e− reduced species of 1; green: 1red2, which is the species that formed upon the second reduction process of 1; brown: intermediate.

measurements (IR-SEC) in DMF/nBu4NPF6 showed a shift of the CO stretching frequencies by about −11 cm−1 for the CO band higher in energy and −18 cm−1 for the two bands lower in energy during the first 2e− reduction (red species in Figure 3, Figure S9, Table 3). The small shifts indicated that the Table 3. IR Stretching Frequencies of 1, 1red1, the Intermediate, and 1red2 in DMFa

a

complex (electrolyte)

ṽ(CO)

1 (nBu4NPF6) 1 (nBu4NCl) 1red1 (nBu4NPF6) 1red1 (nBu4NCl) Int (nBu4NPF6) Int (nBu4NCl) 1red2 (nBu4NPF6) 1red2 (nBu4NCl)

2016, 1905, 1886 2016, 1904, 1884 2005, 1888, 1871 2006, 1887, 1872 2003, 1997, 1883, 1861 2002, 1886, 1857 1986 (w), 1967, 1852, 1829 1986 (w), 1968, 1852, 1825

I = 0.1 M nBu4NPF6 or nBu4NCl.

reductions are not metal based6,13 and thus corroborated the assumption that both ReLNN(CO)3Cl entities of 1 were reduced at the same potential and not one entity twice. The IR spectra did not show further changes over time after the first reduction process. No dimer formation has been observed by IR spectroscopy, since no CO stretching frequency at around ∼1950 cm−1 appeared (Scheme S2).6,7,33 The observed ṽ(CO) shifts of −11 and −18 cm−1 were smaller than the shifts which are typically observed for ligandbased reductions of [Re(LNN)(CO)3Cl] complexes, but also larger than the shifts which Fujita and co-workers as well as 4179


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Inorganic Chemistry Scheme 2. Possible Reduction Products of 1 and Their DFT Calculated Harmonic CO Wavenumbersa

a

Bold, symmetric modes, italic for asymmetric modes. Red arrows represent reduction processes, green arrows represent follow-up chemical reactions, and blue arrows indicate deprotonation steps.

of 1 and its reversibility were not altered in the presence of an ∼100 fold excess of chloride ions, i.e. nBu4NCl (Figure S5). Furthermore, an IR-SEC experiment in 0.1 M nBu4NCl/DMF led to the same CO stretching frequencies for 1red1 as in the presence of DMF/nBu4NPF6 (Figure S10). The further reaction to C is therefore also rejected as a possible structure for 1red1. Comparing the computed harmonic shifts (ωCO) to the measured ṽ(CO) shifts of −11 and −18 cm−1, there is only one structure in close agreement, compound D (computed shifts of about −12, −18 cm−1). The latter corresponds to a net double proton removal from 1, thereby increasing the formal electronic charge of the complex (Scheme 2, Scheme S3). Neither the redox state of the imidazole-pyridine units nor of the metal ions changed upon reduction due to the concomitant H• atom splitting and likely H2 formation. This falls is in line with a small shift of the CO bands. In fact, the addition of 2 equiv of LDA to a solution of 1 led to a product which has the same CO frequencies as D (Figure S11). The UV/vis spectra of 1H−2 showed the same characteristics as the UV/vis spectra of 1red1. We can exclude phenol deprotonation, because phenolate formation leads to a new, pronounced red-shifted UV/vis band.30 Such an EC mechanism for 1 may also explain the shoulder after the second reduction in the CV at higher scan rates: This shoulder may be ascribed to the “electron-only” reduction product, [Re2(L2•−)(CO)6Cl2]2−, which would have a higher reduction potential than the EC product [Re2(LH−2)(CO)6Cl2]2−.34 Finally, we investigated 2’s reduction by IRSEC, since 2 has no N−H bonds. The first reduction of 2 led to more pronounced shifts of the CO bands by −28, −34, and −34 cm−1, respectively (Figure S12). Such shifts are similar to those calculated for the ligand based reduction of 1 (A in Scheme 2) and typically observed for a first, ligand based reduction of [Re(LNN)(CO)3(Cl)] complexes (cf. Scheme S2, Scheme 2).

Hartl and co-workers observed upon reductive, homolytic X−H bond cleavage.12,13 Therefore, we calculated the CO stretching frequencies of tentative reduction products by DFT (for the spectra see Supporting Information). Some possibilities were ruled out based on the general properties of the measured spectra. 1red1 can only be attributed to species whereby the same changes in coordination happen to both centers. Otherwise, one would observe a splitting of the first band corresponding to the symmetric CO stretches. Reasonable products are shown in Scheme 2. No symmetry constraints were applied. The reduction of 1 should yield the diradical A at first. This is in line with its computed reduction potential of −1.94 V relative to ferrocene, which is in close agreement with the measured potential of −2.0 V. A could lose two chloride ions yielding B as previously reported for similar complexes.5,6 The DMF adduct of B, C, also seemed feasible as well as the formation of D upon homolytic N−H bond breaking and H2 formation. Calculated spectra for compounds whereby one chloride is substituted by a DMF solvent molecule led to the appearance of an extra band with no correspondence in the experimental IR-spectra, and thus these possibilities were removed from our scheme. In order to better compare the calculated frequencies to the experimentally observed bands, the following discussion will be based on band shifts deduced from the broadened theoretical spectra (provided in the Supporting Information). In the case of C, one would be dealing with a net ligand reduction (Figure S20), with a CO band shift of about −36 cm−1 with respect to 1. The electron density in B shifts close to the metal center with respect to C. However, this leads only to a CO band shift of a few wavenumbers with respect to A or C (Figure S20). Independent of the shifts, B was also shown to be unlikely by electrochemical measurements: The first reduction potential 4180


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Inorganic Chemistry

CO2 Reduction Catalysis Employing 1. Upon purging the DMF solution of 1 with CO2, a catalytic wave appeared at the potential of the second reduction process (Figure 4). Since

Upon applying a potential which matches the potential of the second reduction process, a new transition band at 470 nm occurred in the UV/vis-SEC spectra (green species in Figure 3, Figure S8). No transition bands in the near-infrared region were observed. IR-SEC measurements in DMF/nBu4NPF6 showed the formation of an intermediate which exhibits CO bands at slightly lower frequencies than 1red1 (brown species in Figure 3, Table 3) and a final product which showed shifts of the CO stretching wavenumbers with respect to 1red1 by about −38 cm−1, −36 cm−1, and −42 cm−1, respectively (Table 3, green species in Figure 3, Figure S9). The shifts of the CO bands of the intermediate depend on the anion of the conducting salt. They were less prominent when we employed DMF/nBu4NCl. Also, the first band did not split into two bands (Table 3, Figure S9), which could indicate that its formation is accompanied by chloride dissociation in DMF/nBu4NPF6. Two reaction pathways seemed feasible upon the reduction of 1red1 (= D, Scheme 2). A metal or ligand based 2e− reduction could lead to E, which may lose chloride forming F. On the other hand, an initial 1e− reduction of D yields K, which could undergo a homolytic O−H bond cleavage and a Cl− loss yielding [Re2(LH−3)(CO)6Cl]2− (G), similar to the NHfunction. G can undergo a further 1 or 2e− reduction process forming the final product H, I, J, or M (Scheme 2). The calculated structures and IR spectra of E and F indicated metal based reductions, which led to large CO band shifts of about −50 to −70 cm−1 (Figure S21). That is in line with experimental reports on [Re(LNN)(CO)3] species (Scheme S2).6 However, only the computed IR spectrum of G is in close agreement with the experimentally observed spectra of the intermediate. The splitting of the band highest in energy and the small shift substantiates an O−H bond cleavage. In the optimized structure we obtained, the phenol unit coordinates to one of the Re centers. In order to substantiate this reaction pathway, we added 3 equiv of LDA as a base to 1, and in fact, the IR spectra of the resulting species was reminiscent of the one of the intermediate (Figure S13, Scheme S3). From there, a 1 or 2e− reduction seems feasible, maybe accompanied by chloride loss. The computed structure H exhibited large density accumulation at the metal centers, and the shifts of the CO bands were much larger than those experimentally observed. The spectrum of M also showed too large shifts to fit with the experimental observations. Only J, whereby a 1e− reduction takes place, falls in agreement with our measurements. The pattern for the symmetric CO stretches region is particularly elucidative. In J, a chloride is replaced by a DMF solvent molecule, probably due to the less favorable electrostatic interaction between the chloride ion and the reduced complex. The final reduction product 1red2 should then likely have one Re center coordinated by a solvent molecule, while the other center coordinates to the deprotonated phenol. This leads to two separate higher frequency bands, the weaker band at 1986 cm−1 corresponding to CO stretches at the DMF coordinated center. Inspection of the computed spin density (both BP86 and B3LYP*) of [Re2(LH−3)(CO)6(DMF)]2− (doublet) reveals that the unpaired electron is distributed over both entities. A visual comparison between the measured spectra and the computed BP86 harmonic wavenumbers with the appropriate assignments is provided in Figure S14. A constant shift of 31 cm−1 was applied to all computed values to facilitate the interpretation of the frequency shifts.

Figure 4. CV of 1 in DMF/water mixtures under argon atmosphere (left) and under CO2 atmosphere (right).

it has been observed that Brønsted acids accelerate catalysis,35 we investigated the redox processes in the presence of various amounts of water. The current under argon did not show significant changes in water/DMF mixtures, which suggested that 1 and, in particular 1red1, are stable in water and do not catalyze proton reduction, e.g., through reprotonation of the ligand in 1red1 by water (Figure 4). The potential of the first redox wave was independent of the water concentration, whereas the second redox process slightly shifted toward less negative potentials (Table S5). 1 was also active in CO2 reduction catalysis in DMF/water. Addition of 5% of water led to a decrease of the catalytic current with respect to CO2 reduction catalysis in pure DMF, while the catalytic current in DMF and 10% water was similar to the one observed in pure DMF (Table 4). The onset of the catalytic wave shifted in the presence of water, and the half peak potential appears between the two redox waves (Table 5, Figure 4). Table 4. Absolute Currents [μA] of the First Reduction Processes of 1 in the Absence of CO2 and of the Catalytic Process in the Presence of CO2, Solvent: DMF + Various Amounts of Water, ν = 100 mV s−1a [%] H2O ([M] H2O)

0

5 (2.6)

10 (5.1)

|ip,1| at {E} |ic| at {E} ic/(ip,1/2.8)b

13 {−2.11} 90 {−2.77} 19

7.3 {−2.11} 41 {−2.55} 16

7.2 {−2.11} 52 {−2.56} 20

a

The values in curly brackets {} refer to the potential [V] at which the values were taken. bSince ip,1 is a 2e− process, we divided ip,1 by a factor of 2.8 for the calculations of the ic/ip ratio, see eq 1 and discussion.

Table 5. Half and Maximum Peak Potential [V] of the First Irreversible Reduction Wave of 1 as Well as the Half Peak Potential36 and Maximum Peak Potential of the Catalytic Wave [V] under CO2 Atmosphere; Solvent DMF + Various Amounts of Water; E vs Fc+/0, ν = 100 mV s−1

4181

[%] H2O ([M] H2O)

0

5 (2.6)

10 (5.1)

E11/2 E1′max Ecat1/2 Ecatmax η

−2.03 −2.12 −2.61 −2.77 1.9

∼−2.1 −2.24 −2.55 1.1

−2.22 −2.56 1.0


Article

Inorganic Chemistry

constant, R = gas constant, v = scan rate; T = temperature, S = substrate concentration.45 The first reduction process was used in order to estimate ip, since the irreversibility seemed to be chemical rather than electrochemical (Table 4, Table 5). The process became slightly reversible at very high scan rates (Figure S17). As ip is a 2e− reduction process, we divided ip by n3/2, n = number of electrons, in order to allow for comparison with mononuclear complexes such as [Re(bpy)(CO)3Cl], where ip belongs to a one-electron process (eqs 1 and 3). The catalysis is first order in 1, as determined by concentration-dependent measurements (Figure S18). The current at the maximum of the catalytic wave under CO2 atmosphere has been taken for ic (Table 4, Table 5). The ic/ip ratio of 1 is about 20 in the absence and presence of 10% water (e.g., 5.1 M). The activity of 1 appears to be higher than the activity of [Re(bpy)(CO)3(py)]+ and [Re(tBubpy)(CO)3(MeCN)](OTf). Wong and co-workers determined an ic/ip ratio of 3.2 for the bipyridine complex at low water concentrations (0.57 M) and of 11.4 at larger water concentrations (10.4 M) in acetonitrile, while Kubiak and coworkers reported on an ic/ip ratio of 9.0 for the tert-butylbipyridine complex in a 3.1 M solution of water in MeCN at the same scan rate.35c,44 On the other hand, the overpotential of 1 is slightly larger than the one of [Re(bpy)(CO)3(L)]. In order to obtain further information about the catalysis, we conducted IR-SEC measurements under CO2 atmosphere in DMF. Upon applying a potential of ∼ −2.0 V, 1red1 is formed (ṽ(CO) = 2005, 1888, 1868 cm−1). Under catalysis at an applied potential of ∼ −2.6 V, 1red1 is the main species, and the intermediate G is formed only to a minor extent (Figure S15). An IR-SEC experiment under CO2 atmosphere in DMF/water mixtures revealed exclusive formation of 1red1 (Figure 5),

In order to determine the reaction products of the catalysis, controlled potential electrolysis (CPE) experiments in DMF and 10% water were conducted. The formation of carbonate, bicarbonate, formate, oxalate, or formaldehyde was not observed by IR-SEC in MeCN (Figure S16). Formate, oxalate, or formaldehyde were also not observed by NMR spectroscopy after CPE in DMF-d7. CO was the main product as determined by GC, and small amounts of dihydrogen were formed (Table 6). The dihydrogen formation can be explained by the ligand reactivity upon 1red1 formation (vide supra), as it never exceeded a TON of 1. Table 6. Faradaic Efficiencies of the CO2 Reduction Catalysis Eap [V]

CO [%]

H2 [%]

t [min]

Q [C]

−2.6 vs Fc+/0 −2.6 vs Fc+/0 −2.6 vs Fc+/0

61 60 65

5 4 5

60 120 300

0.77 1.4 2.6

Catalytic Efficiency. The standard potential of the CO2/ CO redox couple has been determined to −0.73 V vs Fc+/0 in pure DMF.37 In order to estimate the overpotential in DMF/ water mixtures, the pKa of the Brønsted acid has to be considered according to the Nernst equation. Since H2CO3 is a stronger acid than H2O in DMF,11a the pKa of H2CO3 was used to estimate the reduction potential of the DMF/water/CO2 mixture and not the pKa of water (pKa(water) = 32 in DMSO,38,39 pKa(H2CO3) = 7.37 in DMF,11a pKa(H2CO3) = 6.3 and 10.3 in water, pKa(water) = 15.7 in water). DMF (DMF,water) E(CO = E(CO − 0.0592pK a(H 2CO3,DMF) 2 /CO) 2 /CO)

= −1.17V

From this we estimated an overpotential of 1.9 V in DMF and of about 1.0−1.1 V in DMF/water mixtures for the catalysis (Table 5).36,40 The catalysis proceeds under purely kinetic conditions but the substrate is consumed.41 The foot-of-the-wave analysis of Savéant and co-workers, which was developed for catalysts that do not display idealized “S-shaped” CVs, cannot be applied here, since the first EC process is at the foot of the wave.42 Thus, we used the ic/ip ratio in order to estimate the activity of 1.11c,43 TOF is usually defined as [S]x·kcat and, therefore, ic/ip serves as a TOF equivalent. It has been used previously as a benchmark of the activity and allows for a comparison with similar catalysts.11c,35c,44 The peak current ip of a reversible redox process is given by eq 1, and the catalytic current ic is given by eq 2.45 The division of eq 2 by eq 1 eliminates A, D, and Ccat: i p = 0.4463ACcat

n p3F 3·D RT

ν1/2

although we have also observed large background reactivity. It is likely that the initial reduction product of 1red1 is highly nucleophilic and favors CO2 reduction over OH-bond cleavage (Scheme S1). Thus, the OH function can still act as a proton relay under catalytic conditions, and this may also explain why the catalysis is not accelerated by additional water. Inspection of the computed spin density of [Re2(LH−2)(CO)6Cl2]3−, K, reveals that the unpaired electron is distributed over both entities, which may in turn explain why 1 catalyzes the CO2 reduction, while 3 is inactive.

(1)

1/2 ic = ncFACcatS x Dkcat

(2)

ic nc = ip 0.4463

(3)

RT x 1/2 S kcat n p3Fv

Figure 5. IR-SEC of 1 under CO2 atmosphere in DMF/water (10%). blue: 1 (start); orange: under catalysis (Eappl ∼ −2.6 V).

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CONCLUSION We investigated the dinuclear rhenium complex 1 under reductive conditions in the absence and presence of CO2. 1 is an active catalyst in the electrochemical CO2 reduction

A = electrode surface, Ccat = catalyst concentration, n = number of transferred electrons, F = Faraday constant, D = Diffusion 4182


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(3) For example: (a) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Efficient Photochemical Reduction of CO2 to CO by Visible Light Irradiation of Systems Containing Re(bpy) (CO)3X or Ru(bpy)32+−Co2+ Combinations as Homogeneous Catalysts. J. Chem. Soc., Chem. Commun. 1983, 536−538. (b) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Electrocatalytic Reduction of Carbon Dioxide Mediated by Re(bpy) (CO)3Cl (bpy = 2,2′-Bipyridine). J. Chem. Soc., Chem. Commun. 1984, 328−330. (c) Hawecker, J.; Lehn, J.; Ziessel, R. Photochemical and Electrochemical Reduction of Carbon Dioxide to Carbon Monoxide Mediated by (2,2′-Bipyridine)tricarbonylchlororhenium(I) and Related Complexes as Homogeneous Catalysts. Helv. Chim. Acta 1986, 69, 1990−2012. (d) Takeda, H.; Ishitani, O. Development of Efficient Photocatalytic Systems for CO2 Reduction Using Mononuclear and Multinuclear Metal Complexes Based on Mechanistic Studies. Coord. Chem. Rev. 2010, 254, 346−354. (e) Grice, K. A.; Kubiak, C. P. Recent Studies of Rhenium and Manganese Bipyridine Carbonyl Catalysts for the Electrochemical Reduction of CO2. Adv. Inorg. Chem. 2014, 66, 163−188. (f) Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.; Carter, E. A. Mechanistic Contrasts between Manganese and Rhenium Bipyridine Electrocatalysts for the Reduction of Carbon Dioxide. J. Am. Chem. Soc. 2014, 136, 16285−16298. (4) Keith, J. A.; Grice, K. A.; Kubiak, C. P.; Carter, E. A. Elucidation of the Selectivity of Proton-Dependent Electrocatalytic CO2 Reduction by fac-Re(bpy) (CO)3Cl. J. Am. Chem. Soc. 2013, 135, 15823−15829. (5) (a) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.; Meyer, T. J. One- and Two-Electron Pathways in the Electrocatalytic Reduction of CO2 by fac-Re(bpy) (CO)3Cl (bpy = 2,2′-Bipyridine). J. Chem. Soc., Chem. Commun. 1985, 1414−1416. (b) Breikss, A. I.; Abruña, H. D. Electrochemical and Mechanistic Studies of Re(CO)3(dmbpy)Cl and Their Relation to the Catalytic Reduction of CO2. J. Electroanal. Chem. Interfacial Electrochem. 1986, 201, 347−358. (c) Sullivan, B. P.; Bruce, M. R.-M.; O’Toole, T. R.; Bolinger, C. M.; Megehee, E.; Thorp, H.; Meyer, T. J. Electrocatalytic Carbon Dioxide Reduction. In Catalytic Activation of Carbon Dioxide; Ayers, W. M., Eds.; ACS Symposium Series 363; American Chemical Society: Washington, DC, 1988; pp 52−90. (d) O’Toole, T. R.; Sullivan, B. P.; Bruce, M. R.-M.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. Electrocatalytic Reduction of CO2 by a Complex of Rhenium in Thin Polymeric Films. J. Electroanal. Chem. Interfacial Electrochem. 1989, 259, 217−239 and references therein.. (6) (a) Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A. An in situ Infrared Study of CO2 Reduction Catalysed by Rhenium Tricarbonyl Bipyridyl Derivatives. J. Chem. Soc., Dalton Trans. 1992, 1455−1463. (b) Stor, G. J.; Hartl, F.; van Outersterp, J. W. M.; Stufkens, D. J. Spectroelectrochemical (IR, UV/Vis) Determination of the Reduction Pathways for a Series of [Re(CO)3(α-diimine)L′]0/+ (L′ = Halide, OTf−, THF, MeCN, n-PrCN, PPh3, P(OMe)3) Complexes. Organometallics 1995, 14, 1115−1131. (c) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J. Electrocatalytic Reduction of CO2 Using the Complexes [Re(bpy) (CO)3L]n (n = + 1, L = P(OEt)3, CH3CN; n = 0, L = Cl−, Otf−; bpy = 2,2′-Bipyridine; Otf− = CF3SO3) as Catalyst Precursors: Infrared Spectroelectrochemical Investigation. Organometallics 1996, 15, 3374−3387. (d) Smieja, J. M.; Kubiak, C. P. Re(bpy-tBu) (CO)3Cl-Improved Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical and Mechanistic Studies. Inorg. Chem. 2010, 49, 9283−9289. (7) Hayashi, Y.; Kita, S.; Brunschwig, B. S.; Fujita, E. Involvement of a Binuclear Species with the Re−C(O)O−Re Moiety in CO2 Reduction Catalyzed by Tricarbonyl Rhenium(I) Complexes with Diimine Ligands: Strikingly Slow Formation of the Re−Re and Re− C(O)O−Re Species from Re(dmb) (CO)3S (dmb = 4,4′-Dimethyl2,2′-bipyridine, S = Solvent). J. Am. Chem. Soc. 2003, 125, 11976− 11987. (8) Van Outersterp, J. W. M.; Hartl, F.; Stufkens, D. J. Variable Temperature IR Spectroelectrochemical Investigation of the Stability of the Metal-Metal-Bonded Radical Anions [(CO)5MnRe(CO)3(L)]•‑ (L = 2,2′-Bipyridine (BPY), 2,2′-Bipyrimidine (BPYM), 2,3-Bis(2Pyridyl)pyrazine (DPP)) and [(CO) 5 MnRe(CO) 3 (L)Re(Br)

reaction, yielding CO and traces of H2, the latter likely connected to ligand reactivity. The complex exhibits slightly higher activities than [Re(LNN)(CO)3Cl] (LNN = bpy, tBu-bpy) in MeCN/water mixtures. This might be attributed to the proton relay in close proximity to the reaction center. Spectroscopic and theoretical investigations revealed a detailed picture of the reduction chemistry of 1. The initial 2e− reduction is ligand based and induces NH bond cleavage yielding [Re2(LH−2)(CO)6Cl2]2−. The formation of a negatively charged ligand under reductive conditions is rather counterintuitive, but it is in agreement with the observed small shifts of the carbonyl stretching vibrations (−11 and −18 cm−1) and with independent IR and UV/vis spectroscopic measurements under basic conditions. The next reduction process initiates catalysis under CO2 atmosphere, as the reduced species is highly nucleophilic and favors the reaction with CO2. In the absence of CO2, the 2e− reduction of [Re2(LH−2)(CO)6Cl2]2− leads to a homolytic O−H bond cleavage reaction, removal of chloride ions from the coordination spheres of the metal ions, and a ligand-centered one-electron reduction yielding [Re2(LH−3)(CO)6(DMF)]2−. The extensive computed vibrational spectra featured in this work provide not only a framework for understanding the reactivity of the compounds under study, but can also be correlated with assignments made in previous studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00178. Further experimental, spectroscopic and calculated data (PDF) Crystallographic information file of 2 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*(I.S.) E-mail: [email protected]. *(R.A.M.) E-mail: [email protected]. ORCID

Ricardo A. Mata: 0000-0002-2720-3364 Inke Siewert: 0000-0003-3121-3917 Funding

This work was supported by funding from the DFG (SI 1577/2 (IS), SFB 1073 (AW Project C01, TS Project C03), INST 186/ 1086-1 FUGG (X-ray), the Fonds der Chemischen Industrie and the Dr. Otto Röhm Gedächtnisstiftung. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Dr. Christian Würtele for his help in Xray crystallography. IS thanks Prof. Dr. Franc Meyer for his support.

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REFERENCES

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