Synergistic Effects of Imidazolium-Functionalization on fac-Mn(CO)3

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Synergistic Effects of Imidazolium-Functionalization on fac-Mn(CO) Bipyridine Catalyst Platforms for Electrocatalytic Carbon Dioxide Reduction Siyoung Sung, Xiaohui Li, Lucienna M. Wolf, Jeremy R. Meeder, Nattamai S. Bhuvanesh, Kyle A. Grice, Julien A. Panetier, and Michael Nippe J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13657 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Journal of the American Chemical Society

Synergistic Effects of Imidazolium-Functionalization on fac-Mn(CO)3 Bipyridine Catalyst Platforms for Electrocatalytic Carbon Dioxide Reduction Siyoung Sung,a Xiaohui Li,b Lucienna M. Wolf,c Jeremy R. Meeder,a Nattamai S. Bhuvanesh,a Kyle A. Grice,c Julien A. Panetier,*b and Michael Nippe*a a

Department of Chemistry, Texas A&M University, 3255 TAMU, College Station, Texas 77843, USA. E-mail: [email protected] b Department of Chemistry, State University of New York at Binghamton, Binghamton, New York, 13902, USA. E-mail: [email protected] c Department of Chemistry and Biochemistry, DePaul University, 1110 West Belden Ave, Chicago, Illinois 60614, USA ABSTRACT: The electrocatalytic reduction of carbon dioxide (CO2) could be a powerful tool to generate chemical fuels and feedstock molecules relevant to the chemical industry. One of the major challenges for molecular catalysts remains the necessity of high overpotentials, which can be overcome by identifying novel routes that improve the energetic reaction trajectory of critical intermediates during catalysis. In this combined experimental and computational study we show that imidazolium functionalization of molecular fac-Mn(CO)3 bipyridine complexes results in CO2 reduction at mild electrochemical potentials in the presence of H2O. Importantly, our studies suggest that imidazolium groups in the secondary coordination sphere promote the formation of a local hydration shell that facilitates protonation of CO2 reduction intermediates. As such, we propose a synergistic relationship between the functionalized catalyst and H2O which stands in contrast to other systems in which the presence of H 2O frequently has detrimental effects on catalysis.

Introduction Conversion of inexpensive and (over)abundant carbon dioxide (CO2) to value-added products, such as chemical fuels, is an attractive strategy to mitigate current energy and environment-related issues.1-3 However, utilization of CO2 as a C1 feedstock is fundamentally challenging due to its high stability4 and much effort has been devoted to developing efficient catalytic systems for CO2 reduction.4-14 In particular, transition metal-based molecular electrocatalysts have been intensively studied due to their high product selectivity as well as the feasibility to probe their reaction mechanisms and to gain insight into fundamental aspects of CO2 conversion.4, 6-14 Additionally, the highly tunable nature of molecular catalysts allows for systematic catalyst design alterations and comparative reactivity studies, which can uncover important design principles for catalyst optimization. Some of the most extensively studied molecular catalysts for CO2 reduction include Fe porphyrins15-19, Ni cyclams20-26, and Re27-37 or Mn34,38-44 tricarbonyl species. Over the past decades these classes of molecular catalysts were fine-tuned and optimized to display excellent catalytic properties. However, despite the advent of numerous highly active and selective catalysts, molecular catalysts that can operate at mild reducing potentials are still very rare.18,42-44 Contrary to the current synthetic systems, biological systems such as carbon monoxide dehydrogenases (CODHases) catalytically interconverts CO2 to CO near the thermodynamic potential. As shown in Figure 1a, X-ray diffraction studies on the C cluster of Ni,Fe-CODHases by Dobbek and co-workers reveal that Ni and Fe ions cooperatively activate CO2. More im-

portantly, the protonated imidazolium and amine groups of histidine and lysine residues stabilize the metal carboxylate and carboxylic acid intermediates via hydrogen bonding interactions and facilitate protonation and C-O bond cleavage.45 As a result, Ni,Fe-CODHases are able to reduce CO2 to CO with minimum energy penalty. In analogy to the protein residues in enzymes, synthetic chemists incorporated various functional groups in the secondary coordination sphere of transition metalbased molecular catalysts to improve their catalytic properties. Previously incorporated functional groups include amine 46,47, amide19, ether42,44, imidazolium36,48, phenol15-17,42, urea49, and thiourea37 moieties and they are similar in that each functional group is able to participate in hydrogen bonding. The hypothesized key involvements of these functional groups during catalysis are summarized in Figure 1b and 1c and are generally divided into two classes depending on the reaction stage. Functional groups in the first class stabilize the metal carboxylate intermediate by i) hydrogen bonding interactions between the NH moiety and O atom(s) of M-CO2-4,19,23,37 or ii) through-space electrostatic interactions between the positive charges of substituents and the negative charge developed in the coordinated CO2.18 At this reaction stage, direct protonation of the metal carboxylate by the tethered functional group was also proposed. 37 In the second reaction stage, functional groups in the secondary coordination sphere facilitate protonation of the metal carboxylic acid intermediate and subsequent C-O bond

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pyridyl tricarbonyl complexes. Systematic ligand alterations allow to establish the relative importance of various hydrogen atoms of the imidazolium ring in catalysis. Our computational studies suggest that the imidazolium moiety plays a key role in each reaction step throughout catalysis by promoting hydrogen bonding interactions as shown in Figure 1d. Finally, electrochemical investigations reveal that the catalytically active species can be generated at very mild electrochemical potentials and considerable catalytic currents can be observed at potentials as positive as -1.55 V vs Fc0/+ in the presence of H2O without the need for a strongly acidic proton source. Results and Discussion Syntheses and Characterization Scheme 1. Synthetic routes to imidazolium-functionalized Mn complexes.

Figure 1. CO2 activation by biological systems and synthetic systems.

cleavage recurrently adopting the hydrogen bonding motifs (Figure 1c). For example, Bocarsly proposed intramolecular protonation by the pendant phenol group and the following facile dehydration step.42 Rochford and Marinescu proposed hydrogen bonding networks encompassing the pendant functional group, metal carboxylic acid, and an exogenous proton source.44,47 We also note that intermolecular (as compared to intramolecular) interactions between redox active metal sites and extraneous functional groups have recently been suggested by Grills et al. who have shown improved catalytic CO2 reduction activity of Re(bpy)(CO)3Cl catalyst systems in the presence of imidazolium-based ionic liquids.50,51 We are particularly interested in evaluating intramolecular imidazolium-CO2 interactions in a systematic manner and have previously reported on imidazolium-functionalized Re bipyridyl tricarbonyl species (Figure 1d, left), which display significantly improved catalytic properties in comparison to the unfunctionalized Re(bpy)(CO)3Cl molecule.36 We showed that i) the positively charged imidazolium moieties anodically shift catalytic half-wave potentials as well as pre-catalyst reduction potentials, ii) the C2-H group of the imidazolium moiety appears to accelerate dissociation of Cl- upon reduction, and iii) the imidazolium group alters catalytic response of Re(bpy)(CO)3Cl in the presence of H2O. Although we unequivocally demonstrated that the incorporation of imidazolium groups results in beneficial effects for catalysis, discrete interactions of the imidazolium moieties with the substrate-bound catalytic intermediates and exogenous proton sources are yet to be investigated. Herein, we report a detailed experimental and computational study of the electrocatalytic reduction of CO2 to yield CO by a series of imidazolium-functionalized Mn bi-

The series of investigated imidazolium-functionalized Mn complexes were prepared as shown in Scheme 1. The imidazoliumtethered bipyridine ligands L1PF6 – L4PF6 were synthesized according to a slightly modified procedure previously reported by our group.36 Refluxing MnBr(CO)5 with the appropriate cationic ligand in diethyl ether under N2 in the dark yields imidazolium-functionalized mono-cationic Mn complexes 1PF6 – 4PF6 as bright orange solids. All ligands and complexes were fully characterized by 1H- and 13C-NMR, FT-IR spectroscopy, and elemental analysis (SI). {Mn[bpyMe(ImMe)](CO)3Br}PF6 (1PF6) and {Mn[bpyMe(ImtBu)](CO)3Br}PF6 (4PF6) feature hydrogen atoms on C2, C4, and C5 of the imidazolium moiety with different substituents on nitrogen. To investigate their discrete roles in catalysis systematically, the imidazolium hydrogen atoms were replaced with a methyl group at C2 in {Mn[bpyMe(ImMe2)](CO)3Br}PF6 (2PF6) and at C2, C4, and C5 in {Mn[bpyMe(ImMe4)](CO)3Br}PF6 (3PF6). The previously reported mesityl substituted complexes Mn(mes2bpy)(CO)3Br (5) and


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Figure 2. Solid-state molecular structures of (a) 1+, (b) 3+, and (c) 4+ (no co-crystallized solvents were observed) Color code: navy = Mn, brown = Br, red = oxygen, blue = nitrogen, gray = carbon. Hydrogen atoms and the counter anion have been omitted for clarity.

[Mn(mes2bpy)(CO)3(NCCH3)]OTf (6OTf) (OTf = -OSO2CF3) were also prepared as important reference catalysts. Kubiak and co-workers had reported high catalytic activity of complexes 5 and 6OTf, in which the bulky mesityl groups shut down the detrimental dimerization of Mn0(bpy)(CO)3 species (catalyst deactivation pathway).40 Single crystals of 1PF6, 3PF6, and 4PF6 suitable for X-ray diffraction (XRD) experiments were obtained from concentrated CH3CN solutions via slow vapor diffusion of diethyl ether and their solidstate molecular structures are provided in Figure 2. Despite repeated attempts, a high-quality single crystal of 2PF6 was not accessible. Results from XRD studies of 4PF6 do fully support the anticipated connectivity but were of too low quality for a detailed discussion of structural parameters. We therefore limit the structural discussion to 1PF6 and 3PF6. Complexes 1PF6 and 3PF6 crystalize in the space group P21/c and P212121 with four molecules in the unit cell. Solidstate molecular structures of 1+ and 3+ feature an octahedral Mn(I) ion with Mn-N, Mn-Ceq, Mn-Cax, and Mn-Br distances of 2.082[2] Å (3+: 2.097[4] Å), 1.811[3] Å (3+: 1.803[5] Å), 1.825[3] Å (3+: 1.822[5] Å), and 2.5293(4) Å (3+:2.5275[9] Å), respectively. These values are similar to those reported for 5 (2.087[2] Å, 1.813[3] Å, 1.795[3] Å, 2.5298[6] Å)40 and MnBr(CO)3(bpy) (2.0474[19] Å, 1.817[3] Å, 1.835[3] Å, 2.5225[4] Å).52 C2-H unit of the methylimidazolium group in 1PF6 does not show any intra- or intermolecular interactions with bromide anions, which stands in contrast to previously observed intermolecular interactions between the C2-H unit and a

rhenium bound chloride anion in the closely related ReCl(CO)3 complex of L1+.36 However, we do note that the C5-H unit in 1+ may have a weak intramolecular interaction with the bromide anion (d(C5-H∙∙∙Br) = 3.67 Å). Molecular geometries and relative orientations of appended functional groups are of course subject to crystal packing effects and therefore we should not over-interpret this observation. However, the presence of the imidazolium moiety also results in drastically altered redox properties and catalytic activity, as described below. Electrochemistry under Ar To investigate how imidazolium moieties in the secondary coordination sphere affect the electrochemical properties of MnBr(CO)3(bpy-R) systems, cyclic voltammograms (CVs) of complexes 1+ - 4+ were recorded under an Ar atmosphere in CH3CN solution. Since the four new complexes show almost identical CVs under these conditions, we will limit the in-depth discussion to 1+ (CVs of 2+ - 4+ are provided in the SI). As shown in Figure 3a, complex 1+ displays a sharp reduction with a peak potential at -1.49 V vs Fc0/+ (All potentials are reported in reference to Fc0/+ unless otherwise noted). The current density of the reduction is almost twice as large as that of the oxidation and no additional reduction features were observed up to -2.39 V (Figure S15). Thus, the reduction feature can be assigned to two 1 e- reductions of 1+ with concomitant dissociation of Br- to generate the neutral species 1(red2) via a likely ECE mechanism (Figure 3c; vide infra). We note that the reversibility of the 2 e- redox couple of 1+ can be increased in the presence of superstoichiometric amounts of bromide ions (in the form of NBu4Br) which is consistent with formation of 1+ after re-oxidation of 1(red2) (Figure S20). The coalesced two 1 e- reductions were previously observed for complexes 5 and 6+ featuring mesityl substituents40 and differ from the well separated (>300 mV) two 1 e- reductions of unsubstituted MnBr(CO)3(bpy-R) complexes.38 To understand the smaller current density of re-oxidation, multiple CVs were scanned consecutively within the appropriate potential window. As shown in Figure 3b, the reduction peak current gradually decreases to the value expected for a 1 e- reduction while the oxidation peak current remains constant, resulting in a quasi-reversible 1 e- redox couple after multiple scans. As will be discussed later, the one-electron re-oxidation stands in contrast to the redox properties of complexes 5 and 6+. In order to explain the experimental data, density functional theory calculations were performed on 1+, 5 and 6+. In our computational studies, we considered involvement of either the C2-H or C5-H groups of the imidazolium moiety during redox changes in 1+. Although small differences in energies and redox potentials were found, the calculation results for either unit involvement are qualitatively the same (Figure S21). Here, we will only discuss the computational results considering the C2-H involvement of the imidazolium moiety. As a general observation, all calculated redox potentials are slightly more negative than the experimentally determined values (i.e. E2 (exp.) = -1.49 V vs E2 (calc.) = -1.55 V). However, the qualitative order of the redox couples is in good agreement with the experimental results. As shown in Figure 3c, we propose that the initial reduction of 1+ (E1 (calc.) = -1.58 V) yields the neutral species 1(red1-Br) (S = 1/2) which features one unpaired e- in the



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Figure 3. (a and b) CVs of 1+ (1.0 mM) recorded under an Ar atmosphere in CH3CN with 0.1 M NBu4PF6 as a supporting electrolyte. CVs in (a) were recorded at a scan rate of 100 mVs-1 for 2 scans and CVs in (b) were recorded at a scan rate of 1000 mVs-1 for 5 scans. (c) A proposed reduction pathway of 1+ (see Figure S119 for optimized geometries of all species) with calculated reduction potentials (in V vs Fc0/+) and binding energies (in kcal/mol).

π* orbital of the bipyridine ligand (Mulliken spin population on Mn, Mn = 0.03). Upon reduction, the bromide ligand readily dissociates (G = -1.5 kcal/mol) and the mono-cationic species 1(red1)+ is produced. The five-coordinate Mn ion is computed to feature a formally Mn(0) metal center ( Mn = 1.10) while the bpy ligand is diamagnetic. Importantly, the calculated reduction potential for 1(red1)+ (E2 (calc.) = -1.55 V) is 30 mV less negative than E1, which explains the experimentally observed coalesced two 1 e- reductions in CVs. The second reduction yields a neutral species 1(red2), which features Mn(0) (Mn = 0.89) and a bipyridine π*-radical. Re-oxidation of 1(red2) by 1 e- regenerates the mono-cationic 1(red1)+ which could bind CH3CN to give a six-coordinate 1(red1-CH3CN)+. It is worth noting that CH3CN binding to 1(red1)+ is calculated to be uphill by 8.3 kcal/mol, which implies that this process might be kinetically slow. Previous computational studies by Carter and co-workers have also shown that CH3CN binding at [Mn(CO)3(bpy)] was uphill by 6.1 kcal/mol.34 Upon solvent binding, the electron density is redistributed and 1(red1-CH3CN)+ contains a Mn(I) ion (Mn = 0.01) and a bpy π*-radical. Our calculations show that 1(red1-CH3CN)+ can be further oxidized (E4 (calc.) = -1.42 V) to give a di-cationic species 12+-CH3CN, which contrasts the experimental results (re-oxidation by 1 e- not 2 e-s). We tentatively propose that the experimental observation of the oxidation of 1(red1-CH3CN)+ to 12+-CH3CN might be prohibited by the kinetic challenge to access 1(red1-CH3CN)+ due to slow CH3CN binding. However, a small but gradually growing reduction feature at ca. -1.39 V in Figure 3b might be attributed to the reduction of 12+-CH3CN. We also probed computationally the direct oxidation of 1(red1)+ to 12+-5coord. However, this oxidation is calculated to occur at a much more positive potential (E3 (calc.) = -0.83 V) than E4 and implies that the direct oxidation is unlikely.

The re-oxidation of the two electron reduced species 1(red2) by only one electron is in stark contrast to the two-electron reoxidation of the corresponding species 5(red2)- (which will be discussed in detail below) and are an interesting consequence of imidazolium functionalization. To further support our electrochemical and computational analyses, we performed spectroelectrochemical (IR-SEC)52 investigations of 1+ in the infrared region during electrochemical events. The spectrum of 1+ displays signals at 2027, 1934, and 1925 cm-1 (Figure 4a) which are characteristic of carbonyl stretching frequencies of facMn(I)(CO)3 species (2023, 1936, and 1913 cm-1 for Mn(mes2bpy)(CO)3Br).40 Applying a potential of -1.98 V results in the formation of a new species which we assign as 1(red2). The two major signals at 1896 and 1801 cm-1 occur at similar energies to those previously reported for other two-electron reduced [Mn(bpy)(CO)3]- species (1909 and 1808 cm-1 for [Mn(mes2bpy)(CO)3]-).40 The much less intense features at 1967 and 1865 cm-1 may indicate the formation of small amounts of Mn-Mn bonded dimeric species. It is interesting to note that the formation of this dimer appears to be significantly hindered as compared to previously reported Mn-bpy complexes39 even though no very bulky substituents were included into the ligand backbone. It is likely that the cationic nature of 1(red1)+ reduces dimerization significantly. Importantly, oxidation of the electrochemically produced species 1(red2) at 1.48 V results solely in the formation of a species with a characteristic band at 1919 cm-1 and no significant growth of a signal at around 2020 cm-1 is observed (Figure 4b). Thus, re-oxidation of 1(red2) is a single electron event to yield proposed species 1(red1)+ and not the two-electron oxidized species


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Figure 4. (a) Infrared signatures of complex 1+ and spectroscopic changes during its two-electron reduction to yield species 1(red2). (b) One-electron oxidation of electrochemically generated 1(red2) results solely in formation of 1(red1)+. (c) Re-reduction of 1(red1)+ by one electron to species 1(red2).

Figure 5. Two consecutive CVs of 1.0 mM (a) 5 and (b) 6+ at a scan rate of 750 mVs-1 recorded under an Ar atmosphere in CH3CN with 0.1 M NBu4PF6 as a supporting electrolyte. (c) A proposed reduction pathway of 5 with calculated reduction potentials (in V vs Fc0/+) and binding energies (in kcal/mol).

12+-CH3CN or 12+-5coord. (which would be expected to display stretching modes around 2020 cm-1). It is also important to note that species 1(red1)+ can be reversibly re-reduced by one electron to yield again species 1(red2) (Figure 4c). These results are in very good agreement with the above proposed mechanism (Figure 3) for the reduction of 1+ under Ar. To highlight the changes in redox properties caused by the intramolecular imidazolium groups in 1+, we also investigated experimentally and computationally the redox properties of 5 and 6+. As shown in Figure 5a, complex 5 displays a reduction event with a peak potential at -1.79 V under an Ar atmosphere at a scan rate of 750 mVs-1. According to the previous report, this reduction is expected to involve two electrons per molecule of 5.40 On the reverse scan, we observe an anodic feature with a peak potential at -1.55 V which is assigned to two 1 e- oxidations. In the immediate second scan, a new reduction feature appears at -1.61 V while the initial reduction current density decreases. As shown in Figure 5a and b, the peak potential of new reductive feature for 5 is similar to that of the reversible 2 electron redox couple observed for 6+. We interpret these observations as shown in Figure 5c. Reduction of 5 (E1’(calc.) = -1.78 V) results in the formation of 5(red1-Br)- which features a bpy

π*-radical and a formally Mn(I) ion (Mn = 0.02). Similarly to 1(red1-Br), Bromide dissociation from 5(red1-Br)- initiates intramolecular charge transfer to furnish the Mn(0) species 5(red1) (Mn = 1.08), which can be reduced at less negative potential than E1’ (E2’ (calc.) = -1.76 V) to produce a Mn(0) bpy π*-radical species 5(red2)- (Mn = 0.98). 5(red2)- can be re-oxidized by 1 e- to 5(red1) from which CH3CN binding (G = 7.5 kcal/mol) can yield 6(red1). 6(red1) can be further oxidized by 1 e- to produce 6+ at -1.61 V (E4’). One interesting aspect is that CH3CN binding is calculated to be thermodynamically more favorable for 5(red1) than for 1(red1)+ by 0.8 kcal/mol. Although the relative computed free energy is within the computational error range, our spectroelectrochemistry results suggest that CH3CN binding might be thermodynamically more accessible for the neutral 5(red1) than for the mono-cationic 1(red1)+ since the former species is free of the charge balancing anion. As a result, the reversible 2 e- redox couple can be experimentally observed for 6+ (Figure 5c). Direct oxidation of 5(red1) to 5+-5coord. is unlikely to occur due to a highly oxidizing potential (E3’ (calc.) = -0.96 V).



To summarize the most important findings of our comparative studies on redox properties of the imidazolium-functionalized complexes 1+ - 4+, and complexes 5 and 6+ with mesityl groups under Ar: (i) Complexes 1+ - 4+ exhibit the coalesced two one-electron reductions at more positive potentials (-1.49 V – 1.53 V, Figure 3a and Figure S15-S18) than 5 (-1.79 V), and 6+ (-1.61 V). (ii) The 2 e--reduced neutral species 1(red2) can be reversibly oxidized by only one-electron likely due to slow CH3CN binding to 1(red1)+, while the analogous monoanionic species 5(red2)- can be reversibly oxidized by 2 e-s. Reactivity towards CO2 and electrocatalytic CO2 reduction at mild potentials

Figure 6. CVs of 1.0 mM 1+ (red) and 6+ (black) recorded under an CO2 atmosphere at a scan rate of 100 mVs-1 in CH3CN with 0.1 M NBu4PF6 as a supporting electrolyte.

To test the reactivity of the imidazolium-functionalized Mn complexes towards CO2, CVs were measured in CO2-saturated CH3CN solutions. CVs of 1+ - 4+ under a CO2 atmosphere are essentially unaltered from those recorded under an Ar atmosphere (Figure 6 and Figure S15-S18). This result is consistent with previous reports that exogenous proton sources are required to observe catalytic turnover for Mn tricarbonyl complexes. Importantly, 2 e- reduction peaks of 1+ - 4+ are observed

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approximately at -1.50 V vs Fc0/+ suggesting that catalytically active species of 1+ - 4+ can be generated at 100 mV less negative potentials compared to 6+ (Figure 6). The anodic potential shift of 1+ - 4+ can be attributed in part to the coulombic/electrostatic effect of the cationic charge of the imidazolium moieties. Addition of commonly utilized organic proton sources, such as trifluoroethanol (Figure S43) resulted mostly in significant catalytic current enhancements at potentials more negative than -1.8 V (1+, 2+, 3+) or -2.0 V (6+), which is in good agreement with previous reports.34,44 In contrast, with cumulative addition of H2O as a proton source, the imidazolium-functionalized Mn complexes 1+ - 4+ display several distinctive features in their CVs under CO2 (Figure 7). First, the initial reduction peaks gradually shift anodically up to a potential of -1.35 V vs Fc0/+ in the presence of high H2O concentrations. It is worth noting that the reduction of pre-catalyst as positive as -1.35 V is unprecedented. Previously reported Mn tricarbonyl complexes require potentials of at least -1.5 V to observe the initial reduction peak even in the presence of strong Bronsted acids such as phenol. Upon completion of the pre-catalyst reduction, a catalytic plateau current is observed in the potential region of -1.5 to -1.8 V. Importantly, the height of the catalytic plateau currents follows the trend 1+ ≈ 4+ > 2+ > 3+ (Figure 7). This observation is in good agreement with our previous hypothesis that the hydrogen atoms of the imidazolium moieties, specifically the C2-H groups, may have important effects on catalysis via hydrogen bonding like interactions. In contrast to the imidazolium-functionalized Mn complexes, 6+ shows no catalytic current enhancement under identical conditions at these potentials and requires much more negative operating potentials of around -1.95 V (Figure 7e). Lastly, at very negative potentials (starting at -2.0 V), the imidazolium-functionalized complexes 1+ - 4+ display another large current enhancement which may indicate a second catalytic regime at strongly negative potentials. The catalytic peak currents in this regime follow the same order as those observed at milder potentials, in e.g.: 1+ ≈ 4+ > 2+ > 3+.


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Figure 7. CVs of 1.0 mM (a) 1+, (b) 2+, (c) 3+, (d) 4+and (e) 6+ recorded under an CO2 atmosphere at a scan rate of 100 mVs-1 in CH3CN with 0.1 M NBu4PF6 as a supporting electrolyte with cumulative addition of H2O as a proton source. (f) Comparison of the current traces of all complexes recorded at 10.5 M H2O. For precise potential referencing, CVs were measured in the presence of Fc and potentials of each CV were referenced respectively.

Controlled Potential Electrolysis Experiments Table 1. Summary of controlled potential electrolysis experiments

To confirm that the currents observed in CVs for the imidazolium-functionalized Mn complexes are indeed due to catalytic CO2 reduction, controlled potential electrolysis (CPE) experiments were carried out. All CPE experiments were performed with a CH3CN solution of 1.0 mM catalyst, 0.1 M NBu4PF6, and ca. 0.05 mM ferrocene at either -1.82 V vs Fc0/+ (for 1+, 2+, 3+, and 6+) or -1.56 V vs Fc0/+ (for 1+, 2+, 3+, and 4+) for 1 h in the presence of 9.25 M H2O as a proton source. Headspace analyses by gas chromatography (GC) after CPE experiments showed that the only detectable reduction product was CO for all tested complexes. At an applied potential of -1.82 V, the imidazoliumfunctionalized complexes 1+ - 3+ consumed significantly more charge than 6+ with higher Faradaic efficiencies (~70%) as summarized in Table 1. Complexes 1+-4+ displayed slightly attenuated activity at the fairly positive applied potential of -1.56 V with comparable faradaic efficiencies. Current traces over time shown in Figure 8 also validate the improved catalytic activity of complexes 1+ - 4+ as compared to 6+ although a slight decrease in current of 1+ was observed over time. The CPE results

are in good agreement with the comparative reactivity discussions above based on cyclic voltammetry and support the relative activity order of 1+ ≈ 4+ > 2+ > 3+ > 6+. To elucidate the slight decrease in catalytic current observed for 1+, CVs were recorded before and after CPE experiments (Figure S45, S50, and S55). Although a slight decrease in the plateau current at -1.8 V vs Fc0/+ was observed in the CV measured after the CPE experiment, a clear reduction peak at ca. 1.35 V vs Fc0/+ corresponding to the reduction of the pre-catalyst was observed suggesting that the current decrease during the CPE experiment should not be

Figure 8. Current traces during CPE experiments with a CO2saturated solution of 1+, 2+, 3+, 4+ and 6+ and an Ar-saturated solution of 1+ at an applied potential of -1.80 V or -1.56 V vs. Fc0/+.



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due to catalyst decomposition. To verify that the detected CO Mechanistic precedence was not generated by decomposition of 1+, a CPE experiment The previously reported mechanistic details for the electrowas also performed with an Ar-saturated solution under the catalytic conversion of CO2 to CO by conventional Mn(bpysame experimental conditions. Under an Ar atmosphere, 1+ disR)(CO)3X are summarized in Scheme 2. Entry into the catalytic played significantly lower currents compared to under CO 2 atcycle is hereby given by the two-electron reduced species which mosphere, which correspond to the reduction of the pre-catalyst binds CO2. Protonation of the CO2 adduct yields a metallocarand no CO was detected in the headspace. In conclusion, CPE boxylic acid intermediate, which can either be protonated and experiments confirm that the catalytic currents observed at mild subsequently reduced by one electron (protonation-first pathpotentials in CV of the imidazolium-functionalized Mn comway; Scheme 2a), or first reduced and then protonated (reducplexes were indeed due to catalysis and not pre-catalyst decomtion-first pathway, Scheme 2b). Either pathway generates a position. tetra-carbonyl species which upon reduction and CO dissociaThe fact that catalytic activity in the presence of H2O as the tion regenerates the initial reactive species. The protonationsole proton source can be achieved by the imidazolium-funcfirst pathway allows for catalysis to occur at significantly less tionalized complexes at very mild electrochemical potentials negative potentials than the reduction-first pathway, because stands in contrast to previously reported Mn(bpy-R)(CO)3 platthe mono-cationic tetracarbonyl species can be reduced at less forms. The order of catalytic rates strongly suggests that catalnegative potentials than the neutral metallocarboxylic acid. ysis is readily affected by the presence of intramolecular of C2Despite the energy advantage provided by the protonationH imidazolium groups. In the following we provide a mechafirst pathway, manganese catalysts that can operate at nistic discussion based on literature precedence and our own computational efforts to rationalize this unique scenario. Scheme 2. Comparison in protonation-first pathway and reduction-first pathway

low overpotentials via the protonation-first pathway are very rare. Since the first study using Mn bipyridyl complexes reported by Deronzier and co-workers in 201138, much effort has been devoted primarily to maximizing catalytic activity. For example, Kubiak and co-workers significantly improved catalytic activity of Mn(bpy)(CO)3Br by incorporating electron-donating tert-butyl groups at the 4,4’- positions of bipyridine.39 They also incorporated bulky mesityl groups on bipyridine, which prevent dimerization of the singly reduced species (catalyst deactivation pathway) to achieve high catalytic activity.40 However, both catalysts require strongly negative potentials (ca. -2.2 V vs Fc+/0) to observe catalytic activity, implying both catalyst systems follow the reduction-first pathway. In 2015, Bocarsly and co-workers reported a phenol-substituted Mn catalyst, which displays catalytic current enhancement at milder potentials (ca. -1.7 V vs Fc0/+).42 Computational investigation suggests that the phenol group in the secondary coordination sphere facilitates dehydration of the metallocarboxylic acid via intramolecular protonation. Interestingly, the reference catalyst bearing a methoxyphenyl-substituent displayed a small catalytic current enhancement at potentials as low as -1.6 V vs Fc0/+, suggesting that the ether group might also play a key role in enabling the

protonation-first pathway. A detailed study on promoting the protonation-first pathway by an ether functional group in the secondary coordination sphere was reported in 2017. Rochford and co-workers reported a Mn complex featuring 2,6-dimethoxyphenyl groups on 6- and 6’-position of bipyridine, which exhibits considerable current enhancement at ca. -1.6 V vs Fc0/+ in the presence of exogenous Bronsted acids such as TFE and phenol.44 Their computational studies suggest that the protonation-first pathway is accessible due to cooperative hydrogen bonding interactions among the methoxy group, -OH of metallocarboxylic acid, and exogenous Bronsted acid, which stabilize the transition state for C-O bond cleavage. As we will show in the following, our studies suggest that the catalytic current enhancement observed for 1+ at the mild potential of -1.55 V can be achieved via imidazolium-functionalization without the need to invoke the classical protonation-first pathway and is a consequence of synergistic interactions between the imidazolium group, discrete water molecules, and CO2 reduction intermediates. Computational Details


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Density functional theory (DFT) calculations were run with Gaussian 09 Rev. E 01.53 Geometry optimizations were carried out at the unrestricted B97X-D54 level of theory in solution (acetonitrile,  = 35.688) using the SMD approach.55 The Def2TZVP basis set was used for Mn and Fe while the Def2-SVP basis set was employed for all other atoms (denoted BS1).56 Additional single-point calculations were run in solution using the Def2-TZVPP basis sets for all atoms except Br in which the Def2-TZVPPD basis set was employed (denoted BS2). 56,57 Exchange correlation integrals were evaluated with a quadrature grid of 99 radial shells and 590 angular points per shell. Minimum energy and transition state geometries were confirmed as such through analytical frequency calculations, having no imaginary frequencies and a single imaginary frequency, respectively. Transition states were further characterized through intrinsic reaction coordinate (IRC) calculations and subsequent geometry optimizations.58 All computed structures were confirmed to have no internal instabilities by using the “stable=opt” and “scf=qc” keywords. All reported redox potentials were calculated using the direct approach in which free energies of the reactants and products are calculated directly in solution (acetonitrile using the SMD approach), rather than by a thermodynamic cycle involving gas phase energies (see supporting information for additional details).59 All potential reported herein (in V) are versus the Fc+/0 redox couple. All computed free energies include the zero-point

vibrational energy corrections as well as thermal corrections and entropies computed by standard statistical thermodynamic methods at 298.15 K. Finally, an applied potential of  = -1.55 V vs Fc0/+ was used to model each electrochemical reduction step.34,60 This applied potential corresponds to the calculated redox potential for formation of the neutral species 1(red2) (Figure 3c). Computational Results In order to fundamentally understand the discrete role of C2H of the imidazolium moiety for CO2 to CO conversion by 1+, electronic structure calculations were performed. It should be noted here that only the computational results considering the C2-H involvement of the imidazolium moiety are discussed. Although the C4/C5-H units may play a role during catalysis (e.g. CO2 binding, first and second protonation steps), the examination of the catalytic mechanism involving protons on the backbone of the imidazolium ligand is beyond the scope of this work and a full computational study is underway. Entry into the catalytic cycle is provided by the association of the 2 e--reduced species 1(red2) with one molecule of CO2 to yield a loosely bound adduct I1 (G = +5.9 kcal/mol, Figure 9). The optimized geometry of I1 reveals that C2-H of the imidazolium moiety keeps the CO2 molecule in the vicinity of the reduced metal through hydrogen bonding type interactions (C2-H…..O = 2.46 Å, Figure S120), thereby facilitating nucleophilic attack

Figure 9. Computed free energy (kcal/mol) profile leading to the formation of cationic metallocarboxylic acid species I5, from I1 and CO2 in the presence of a (H2O)5 cluster (see Figures S120-S122 for optimized geometries of all species). All free energies are calculated with respect to the separated reactants (1(red2), CO2 and (H2O)5 cyclic configuration). The superscript represents the spin multiplicity of a given species (all singlet ground states). aI1 has an open-shell singlet ground state, = 0.65, while other species have a closed-shell singlet ground states, = 0.00. bThe computed electronic energy for TS2 (E = -18.9 kcal/mol) is higher in energy than that of I3 (E = -21.2 kcal/mol) and I4 (E = -20.1 kcal/mol). However, TS2 has a lower zero-point energy than I3 and I4, offsetting the higher potential energy of the transition state with respect to the intermediates.

on CO2. Formation of the metallocarboxylate intermediate I2 from I1 is slightly endergonic (G = +0.8 kcal/mol) and is calculated to be kinetically accessible with a moderate activation energy of +10.7 kcal/mol with respect to the preceding intermediate I1. The activation energy becomes +16.1 kcal/mol with respect to the separated reactants. Importantly, the C2-H group

stabilizes the activated CO2 molecule through hydrogen bonding in I2 (C2-H…..O = 1.68 Å, Figure S120). It should be mentioned that several transition states and metallocarboxylate species have been obtained (Figure S120) and only the lowest energy structures are depicted on Figure 9.



Protonation of I2 was considered by adding explicit water molecules.61 However, the use of water as the proton source leads to the formation of hydroxide anions (OH-), which we expect to be poorly solvated in solution (acetonitrile,  = 35.688) using the SMD approach. Previous studies have shown that OHcan accept three or four hydrogen bonds from water. 62 Therefore, in order to stabilize the electron density around the OH -, a total of four water molecules was employed leading to a OH(H2O)4 cluster. In this case, three water molecules are placed in the first solvation shell of the hydroxide anion while the fourth one is located in the second solvation shell, donating hydrogenbonds to two of the water molecules in the first solvation shell.62a Addition of a (H2O)5 cluster to I2 yields I3, which is uphill by 4.5 kcal/mol. Note that in this case the formation energy of the water cluster (5H2O → (H2O)5) has been neglected by referencing all free energies to the separated reactants (1(red2), CO2 plus (H2O)5 in a cyclic configuration).61 The optimized geometry of I3 (Figure 10) suggests that the water cluster participates in a delicate hydrogen bonding network, in which it acts as a hydrogen bond donor to the metal bound CO2anion (O…..H = 1.48 Å) and an acceptor to the C2-H moiety of imidazolium (C2-H…..O = 1.96 Å). The resulting pre-positioning of the (H2O)5 cluster is appropriate to facilitate the cleavage and formation of the O-H bond, which allows for the conversion of I3 to the metallocarboxylic acid intermediate I4. Previous computational works have demonstrated that this step is significant uphill.25,63 However, our calculations suggest that the process is thermoneutral due to the stabilization of OH- via hydrogen bonding interactions from the water cluster and imidazolium ligand (Figure 10). After formation of I5, the reaction trajectory bifurcates and the metallocarboxylic acid can proceed via either a reductionfirst or a hydration-first pathway (Figure 11). In the reductionfirst mechanism, the metallocarboxylic acid I5 is reduced at -

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1.64 V vs Fc0/+ to yield species I5-R, which lies in the potential range where the catalytic plateau current is experimentally observed in CVs. From I5-R, addition of a (H2O)5 cluster forms I6-R (G = +12.6 kcal/mol). As previously observed, TS3 features a hydrogen bonding network (Figure S122) and occurs via a moderate activation energy (∆G‡ = +6.7 kcal mol-1) to generate the neutral tetracarbonyl intermediate I7-R. The activation barrier increases to 19.3 kcal/mol with respect to I5-R in which the reactants are infinitely separated. This value is similar to what Neese and co-workers reported (∆G‡ = +20.9 kcal/mol) for the heterocyclic C-O bond cleavage of CO2 by [NiI(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in the presence of water.25 It is worth noting that in the current work the water pentamer (i.e. cyclic configuration) is employed as reference for the protonation steps. This configuration was shown to be the lowest energy structure.61 However, our calculations show that the (H2O)5 cluster adopts a chair configuration, which is +6.4 kcal/mol higher in energy than the cyclic configuration at the B97XD(SMD)/BS2 level of theory. This implies that our computed energies for the protonation steps are most likely overestimated with respect to the experimental values. Alternatively, species I5 can be protonated in the presence of water via initial formation of I5-P (G = 13.1 kcal/mol). Once again, the hydrogen bonding network is used to facilitate the CO bond cleavage and yields I6-P (Figure S123). The activation barrier for formation of the tetracarbonyl intermediate is slightly higher in energy than in the reduction first pathway (∆G‡ = +9.6 kcal/mol cf ∆G‡ = +6.7 kcal/mol in the reductionfirst mechanism) but remains accessible. In general, protonation of metallocarboxylic acid like intermediates prior to 1 e--reduction are believed to be highly endergonic, which prompts most of the

Figure 10. Optimized geometries of critical intermediates I3 (left) and I4 (right). Computed energies are in kcal/mol, distances are given in angstroms and bond angles in degrees. Non-participating hydrogen atoms have been omitted for clarity.


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Figure 11. Computed free energy (kcal/mol) profile for the protonation-first and reduction-first pathway from intermediate I5 in the presence of a (H2O)5 cluster (see Figures S122- S124 for optimized geometries of all species). All free energies are calculated with respect to the separated reactants (1(red2), CO2 and (H2O)5 cyclic configuration). An applied potential of  = -1.55 V vs Fc0/+ was used as reference potential. The superscript represents the spin multiplicity of a given species. Note that all computed species have a Mulliken spin population on manganese of Mn = 0.0, corresponding to a formal oxidation state of +1 for Mn.

published Mn tricarbonyl species to follow the reduction-first pathway.38-40 One alternative pathway is to reduce I5-P to form I6-R (G = 1.4 kcal/mol) from which formation of the neutral tetracarbonyl intermediate I7-R is yielded via TS3, as previously discussed. It should be noted that our calculated redox potential (E = -1.62 V) for formation of I6-R from I5-P also matches the potential range where the catalytic plateau current is experimentally observed in CVs, implying that this pathway is competitive. After hydroxide removal, the cationic tetracarbonyl species I8 is formed (G = -4.7 kcal/mol). Interestingly enough, CO dissociation to regenerate the five-coordinate complex 1(red1) is calculated to be exergonic (G = -1.7 kcal/mol; Figure S125). This result contrasts with the idea that addition of one electron is usually required prior to CO dissociation to regenerate the active catalyst (see SI).34,64,65 Conclusion In conclusion, we have shown that the series of imidazoliumfunctionalized catalyst systems based on complexes 1+ - 4+ is capable of electrocatalytically reducing CO2 to CO in the presence of H2O at much more positive potentials than a highly active reference catalyst. This is achieved via synergistic interactions between the intramolecular imidazolium groups and H 2O

molecules, which result in local hydration and facilitate CO2 reduction. The relative activity order (1+ ≈ 4+ > 2+ > 3+) strongly suggests that the C2-H functionality of the imidazolium moiety plays a critical role during catalysis which is most likely due to its ability to form stronger hydrogen-bonding like interactions. Taken together, the combined experimental and computational results of this work provide a new way of thinking about tuning hydration shells around reactive metal sites to improve molecular electrocatalytic conversions.

Acknowledgements MN is grateful for general financial support by The Welch Foundation (A-1880). Electronic structure calculations were performed on the Spiedie cluster at Binghamton University, as well as the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562, under allocation TG-CHE180009.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.



Experimental, cyclic voltammetry, spectroelectrochemistry and controlled potential electrolysis data, as well as Cartesian coordinates and computed energies, including Figures S1−S125 and Tables S1−S2 (PDF) X-ray crystallographic data for 1PF6 (CIF) X-ray crystallographic data for 3PF6 (CIF) X-ray crystallographic data for 4PF6 (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

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Synergistic Effects of Imidazolium-Functionalization on fac-Mn(CO)3

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