An Investigation of Electrocatalytic CO2 Reduction Using a Manganese Tricarbonyl Biquinoline Complex

The subject of this study [fac-Mn(bqn)(CO)3(CH3CN)]+ (bqn = 2,2′-biquinoline), is of particular interest because the bqn ligand exhibits both steric and electronic influence over the fundamental redox properties of the complex and, consequently, its related catalytic properties with respect to the activation of CO2. While not a particularly efficient catalyst for CO2 to CO conversion, in-situ generation and activity measurements of the [fac-Mn(bqn)(CO)3]− active catalyst allows for a better understanding of ligand design at the Mn center. By making direct comparisons to the related 2,2′-bipyridyl (bpy), 1,10-phenanthroline (phen), and 2,9-dimethyl-1,10-phenanthroline (dmphen) ligands via a combination of voltammetry, infrared spectroelectrochemistry, controlled potential electrolysis and computational analysis, the role of steric vs. electronic influences on the nucleophilicity of Mn-based CO2 reduction electrocatalysts is discussed.


INTRODUCTION
The catalytic reduction of CO 2 into useful C-1 chemical feedstocks offers one potential strategy to develop a carbon-neutral alternative to our current dependence on fossil fuels. A major challenge for the catalysis community is to develop molecular catalysts capable of coupling electrochemical reduction with protonation of the CO 2 substrate toward useful C-1 products. One such example is the proton-coupled two-electron reduction of CO 2 to CO, which comes at a cost of just −0.52 V vs. SHE in water at pH 7 (Equation 1) (Arakawa et al., 2001), while in dry acetonitrile at pH 0 (Equation 2), the standard electrode potential for this reaction has been estimated, using two different thermodynamic cycles, as −0.13 V vs. Fc +/0 (Matsubara et al., 2015) or −0.12 V vs. Fc +/0 (Pegis et al., 2015). CO 2(g) + 2H + (aq) + 2e − ⇌ CO (g) + H 2 O (l) E 0 = −0.52 V vs. SHE (pH 7) (1) CO 2(g) + 2H + (CH3CN) + 2e − ⇌ CO (g) + H 2 O (CH3CN) E 0 = −0.13 V vs. Fc +/0 (pH 0) (2) This seemingly simple reaction is limited by slow kinetic parameters, however, which demands an appropriate catalyst to access a kinetically efficient pathway for CO 2 utilization -albeit at the cost of an electrochemical overpotential (η) (Appel and Helm, 2014). Of the homogeneous transition metal-based molecular catalysts used in this field (Francke et al., 2018;Sinopoli et al., 2018;Stanbury et al., 2018), manganese(I) polypyridyl tricarbonyl catalysts of the type [fac-Mn I (N ∧ N)(CO) 3 X] n (where N ∧ N = polypyridyl ligand, X = Br − (n = 0) or CH 3 CN (n = +1)), and their analogs, have been of keen interest due to their high selectivity, low cost, and low overpotential (Grills et al., 2018). Recently we have reported on tuning both the inner coordination sphere (McKinnon et al., 2019) and second coordination sphere (Ngo et al., 2017) of the ligand to optimize catalytic efficiency and selectivity for CO formation, notably against the competitive two-electron two-proton coupled redox transformations of CO 2 to HCO 2 H and H + to H 2 . More subtle modification on the periphery of the bpy ligand is also known to strongly influence the reduction potentials observed for these catalysts. This is evident, for example, with the inductive electron donating influence of the dtbpy (dtbpy = 4,4 ′ -t Bu 2 -bpy) ligand, which shifts the reduction potential 0.11 V more negative relative to the simple bpy analog (Smieja et al., 2013). Similarly, while maintaining the same inner coordination sphere of [fac-Mn(N ∧ N)(CO) 3 L] n catalysts but introducing redox non-innocence on the ligand backbone, a recent report on [fac-MnBr(phen-dione)(CO) 3 ] (phen-dione = 1,10-phenanthroline-5,6-dione) demonstrated vastly different electrochemistry, maintaining a high selectivity for CO evolution, compared to the analogous bpy and phen (phen = 1,10-phenanthroline) complexes, notwithstanding significant ligand-based redox activity at the dione functional group to generate a bis-carboxylate phenanthroline intermediate species (Stanbury et al., 2017).
In the current study we have focused on a rather simple modification of the polypyridyl ligand, but one which allows us to directly investigate both steric and electronic influences in the [fac-Mn(bqn)(CO) 3 (CH 3 CN)] + ([4-CH 3 CN] + ) pre-catalyst, where bqn = 2,2 ′ -biquinoline (Figure 1). While fundamental in approach, a simple systematic extension of the π-conjugated system of the polypyridyl ligand has not yet been reported for Mn(I) CO 2 reduction electrocatalysts. Four pre-catalysts of the general structure [fac-Mn(N ∧ N)(CO) 3 (CH 3 CN)] + are here investigated using the benchmark 2,2 ′ -bipyridyl (bpy) ligand ([1-CH 3 CN] + ) alongside the 1,10-phenanthroline (phen) ([2-CH 3 CN] + ), 2,9-dimethyl-1,10-phenanthroline (dmphen) ([3-CH 3 CN] + ), and aforementioned bqn ligands. Although the phen ligand does not have as extensive a π-conjugation as the bqn ligand, it serves a critical role in this study by bridging the gap between the sterically related dmphen and bqn ligands. Indeed, a direct comparison of the sterically and electronically expanded bqn ligand to the sterically related dmphen ligand probes the question of whether steric or electronic effects dominate in determining the redox, and ultimately catalytic, properties of this class of CO 2 reduction electrocatalyst. As demonstrated below using a combination of IR-SEC and computational studies, the combined electronic and steric influences of the π-extended bqn ligand hinder formation of a [4-4] 0 dimer, facilitating a concerted twoelectron ECE mechanism for the generation of [4] − . However, this comes at the cost of a change in product selectivity for the [4-CH 3 CN] + pre-catalyst to favor H 2 evolution in the presence of excess TFE. Selective CO 2 activation is observed for all catalysts here studied upon in-situ generation of a three-electron reduced [fac-Mn (−1)

Synthesis and Structural Characterization
Initial metathesis of [MnBr(CO) 5 ] with silver triflate was completed as a first step to produce the [Mn(CO) 5 (OTf)] intermediate (Scheiring et al., 2000), allowing for the subsequent straightforward isolation of pure [fac-Mn I (N ∧ N)(CO) 3 (OTf)] products. Microwave reflux of [Mn(CO)  CN] + complexes in neat acetonitrile provides a very useful comparison of structural and electronic properties of all four complexes using a basic knowledge of point-group symmetry and Mn(dπ)-CO(π * ) back-bonding, respectively ( Table 2). The bpy and phen complexes, [1-CH 3 CN] + and [2-CH 3 CN] + , each exhibit pseudo-C 3v point group symmetry with identical FTIR spectra composed of a sharp, symmetric ν(CO) stretching mode at 2,050 cm −1 and a second broad, lower frequency asymmetric stretching mode at 1,958 cm −1 . Consistent with voltammetry and computational analysis presented below, the identical ν(CO) stretching modes illustrates the negligible electronic influence of the π-extended phen ligand in comparison to the bpy system. This observation is consistent with related reports of the analogous bromide complexes (Kurtz et al., 2015;Stanbury et al., 2017;Tignor et al., 2018). In contrast, the dmphen complex [3-CH 3 CN] + exhibits a descent in symmetry to pseudo-C s , evident in a breaking of degeneracy for its lower frequency asymmetric ν(CO) stretching modes, which occur at 1,959 and 1,944(sh) cm −1 . This suggests a sterically induced distortion of the fac-Mn(CO) 3 core, likely by the 2,10-dimethyl substituents of the dmphen ligand. Furthermore, the higher frequency symmetric ν(CO) stretching mode of [3-CH 3 CN] + exhibits a 4 cm −1 shift to lower frequency at 2,046 cm −1 , consistent with an inductive electron-donating influence of the two methyl substituents at dmphen, likely due to increased Mn(dπ)→CO(π * ) back-bonding. Interestingly, the bqn complex, [4-CH 3 CN] + exhibits an FTIR spectral profile with ν(CO) = 2,047 and 1,959 cm −1 , indicative again of pseudo-C 3v symmetry, similar to [1-CH 3 CN] + and [2-CH 3 CN] + . However, with the high frequency symmetric ν(CO) stretching mode occurring at 2,047 cm −1 , this suggests that the inductive electron-donating influence of the bqn ligand is similar to that of the dmphen ligand in [3-CH 3 CN] + . Experimental FTIR spectral profiles are consistent with calculated spectra, as illustrated in Figure 2.    CN] + recorded in acetonitrile displaying characteristic ν(CO) stretching modes. Density functional theory-computed IR spectra (gray) are included for comparison (empirically-derived frequency scaling factor = 0.9633, FWHM = 12 cm −1 ).

Cyclic Voltammetry Under Non-catalytic Conditions
Electrochemical characterization of each complex was carried out under inert conditions (1 atmosphere of argon), in the absence of auxiliary Brønsted acid, prior to screening for catalytic activity. Each complex displays an irreversible Mn(II/I) oxidation event within 20 mV of each other in the range of +1.02 to +1.04 V vs. the ferricenium/ferrocence (Fc +/0 ) pseudoreference (Figure 3, Table 1). This suggests that there is little difference in electron density between each Mn-center upon varying the polypyridyl ligand across the four complexes. The very subtle differences observed in the v(CO) stretching modes by FTIR spectroscopy are unlikely to be reproduced by cyclic voltammetry. Furthermore, voltammetry data is more complex due to the in-situ generation of the Mn(II) oxidation state. We will focus on the reduction properties of these complexes forthwith due to their greater relevance toward our subsequent catalytic studies. The electrochemical properties of [1-CH 3 CN] + have been recently reported (Grills et al., 2018;McKinnon et al., 2019). Three sequential one-electron cathodic peaks are observed at E pc = −1.48 V, −1.83 V and −2.94 V with only the third reduction exhibiting quasi-reversible behavior (E 1/2 = −2.90 V, E p = 81 mV at υ = 0.1 V s −1 ) vs. Fc +/0 . The phen complex, [2-CH 3 CN] + exhibits a first one-electron irreversible reduction at E pc = −1.48 V vs. Fc +/0 , identical to that observed for [1-CH 3 CN] + , further supporting the electronic similarity of bpy and phen at least in the [fac-Mn(N ∧ N)(CO) 3 L] n class of complexes (Tignor et al., 2018). This first reduction has been previously established (Grills et al., 2018) as ligand-based according to an electrochemical-chemical (EC) reaction scheme whereby, upon one-electron reduction of the bpy π * orbital, rapid CH 3 CN dissociation occurs with a concurrent shift in radical character from the ligand to form the neutral five-coordinate 17-valence electron intermediate, [fac-Mn 0 (bpy)(CO) 3 ] ([1] 0 ). This metastable Mn(0) complex, in the absence of any steric hindrance , rapidly forms the Mn 0 -Mn 0 bound 18-valence electron [fac-Mn 0 (bpy)(CO) 3 ] 2 dimer, [1-1] 0 . While unequivocal evidence of [fac-Mn 0 (N ∧ N)(CO) 3 ] dimer formation is presented below via infrared spectroelectrochemical (IR-SEC) studies, its oxidation is often evident in the reverse anodic scan in cyclic voltammetry, observed here at −0.61 V and −0.58 V for [1-1] 0 and [2-2] 0 , respectively. Thus, the second reduction event for [fac-Mn(N ∧ N)(CO) 3 L] n complexes is often attributed to an irreversible two-electron EC event whereby the dimer is reductively cleaved to generate two equivalents of the two-electron reduced [fac-Mn 0 (N ∧ N •− )(CO) 3 ] − anion, now established as the principal active catalyst for CO 2 activation by this class of complex via the most common reduction-first pathway (Riplinger et al., 2014). Reduction of the phen based dimer, [2-2] 0 is observed at E pc = −1.86 V, suggesting that [2] − is more nucleophilic than the benchmark bpy analog, [1] − generated at the slightly more positive potential of E pc = −1.83 V. Similar to its bpy analog, the native [2-CH 3 CN] + precursor exhibits a quasi-reversible third reduction event at E pc = −2.72 V (E 1/2 = −2.66 V, E p = 114 mV at υ = 0.1 V s −1 ). This third quasi-reversible reduction has rarely been discussed in the literature but computational analysis here suggests that it is a predominantly ligand based reduction giving rise to a 19-electron [fac-Mn 0 (N ∧ N 2− )(CO) 3 ] 2− dianion. The related dmphen complex, [3-CH 3 CN] + exhibits unexceptionally similar voltammetry in comparison to both its bpy and phen analogs. Its first reduction again involves a ligand based EC pathway leading to formation of the [3-3] 0 dimer. This EC reaction is shifted negatively by 10 mV to E pc = −1.49 V, relative to formation of both [1-1] 0 and [2-2] 0 , consistent with the inductive donating character of the 2,9-dimethyl substituents observed by FTIR studies. Although there is weak evidence of dimer oxidation in the full scan voltammogram presented in Figure 3, reversing the scan after just one-electron reduction clearly demonstrates a significant dimer oxidation peak at E pa = −0.65 V ( Figure S5).
Reductive cleavage of the [3-3] 0 dimer to form the two-electron reduced five-coordinate [fac-Mn 0 (dmbpy •− )(CO) 3 ] − anion ([3] − ), however, occurs at a more positive potential of −1.78 V. This observation suggests that the steric influence of dmphen slightly hinders electronic coupling between the Mn(0) and dmphen •− radical centers in [3] − , rendering it slightly less nucleophilic as a result. The third reduction wave for [3] −/2− is observed at −2.86 V and, in contrast to its bpy and phen analogs, is completely irreversible. Noticeably in Figure 3, the bqn complex, [4-CH 3 CN] + exhibits a single, reversible, concerted two-electron reduction event at E pc = −1.30 V (E 1/2 = −1.28 V, E p = 53 mV at υ = 0.1 V s −1 ) followed by a single additional quasi-reversible oneelectron reduction event at E pc = −2.51 V (E 1/2 = −2.48 V, E p = 71 mV at υ = 0.1 V s −1 ) vs. Fc +/0 . Equally important here is the lack of any evidence for dimer oxidation in the reverse anodic scan of [4-CH 3 CN] + . This electrochemical behavior, at least in its peak profile, is uncannily similar to previously reported bulky 6,6 ′ -substituted bpy ligands investigated at the identical [fac- Ngo et al., 2017). Mn 0 -Mn 0 formation is known to favor a staggered structure of both bpy ligands in [1-1] 0 (Machan et al., 2014), and thus far only the inclusion of steric bulk orthogonal to the plane of the polypyridyl ligand has been demonstrated to prevent dimer formation Ngo et al., 2017). Thus, it is appropriate here to question whether the redox behavior exhibited by [4-CH 3 CN] + is in fact due to an electronic influence of this relatively electron deficient ligand, or rather, alternatively is the result of a steric influence of the bqn ligand. The bqn ligand is, after all, π-extended from the same 6,6 ′ -positions as previously reported 6,6 ′ -orthoganol-sterically bulky bpy stystems, albeit here non-orthogonal to, but in the same plane as, the core bpy structure. This question is addressed explicitly via computational analysis presented below.

Infrared Spectroelectrochemistry (IR-SEC)
To gain structural insight into electrochemical activation of these pre-catalysts, IR-SEC was carried out in 0.1 M [Bu 4 N][PF 6 ] acetonitrile supporting electrolyte under 1 atmosphere of argon ( Table 2). IR-SEC is a powerful investigative tool that takes advantage of the structural specificity of IR spectroscopy for transition metal carbonyl complexes and the ability to prepare in-situ catalytic intermediates simply by gradually stepping the potential of the working electrode (Kaim and Fiedler, 2009;Machan et al., 2014). At open-circuit potential, i.e., resting potential of the native pre-catalyst, [1-CH 3 CN] + and [2-CH 3 CN] + both display identical ν(CO) stretching modes as presented earlier (Figure 2), confirming negligible influence of the 0.1 M [Bu 4 N][PF 6 ] electrolyte on their FTIR spectra. When the potential was biased beyond the first-reduction at −1.60 V  Figure S10) and at 1,857, 1,880, 1,935, and 1,977 cm −1 from [2-CH 3 CN] + ( Figure S11). These data are consistent with an earlier report of the [1-1] 0 dimer (Hartl et al., 1995), and the peak profile associated with related Mn 0 -Mn 0 dimers previously observed via IR-SEC (Grills et al., 2018), and they are also consistent with our computational frequency analysis. A similar assignment is made for [2-2] 0 which exhibits almost identical ν(CO) stretching frequencies (vide supra). Upon biasing the working electrode potential further negative (−2.00 V vs. Fc +/0 ) beyond the second Mn 0 -Mn 0 /Mn − based reduction for both [1-CH 3 CN] + and [2-CH 3 CN] + , loss of the four dimer stretches was observed (Figures S10, S11) with concurrent growth of two new ν(CO) stretching modes at lower wavenumber, corresponding to the five-coordinate two-electron reduced [fac    (Figure S13). This is consistent with an earlier report of poor stability of the simpler phen-based dimer, [2-2] 0 (Stanbury et al., 2017). As a concerted two-electron reduction event occurs for [4-CH 3 CN] + , the potential was set at −1.65 V vs. Fc +/0 for FIGURE 4 | IR-SEC spectra recorded on [3-CH 3 CN] + at the resting potential (blue), upon one-electron reduction (green) and upon two-electron reduction (red).  (Bourrez et al., 2011). In contrast to their Re counterparts, which have been established for some time to promote CO 2 reduction in the absence of a proton donor, Mn(I) polypyridyl based electrocatalysts typically require the presence of a proton donor to promote intermediate metallocarboxylic acid formation due to the poorer nucleophilicity of Mn vs. Re catalysts (Riplinger and Carter, 2015 (Figure 6). Although excess Brønsted acid was not added in these experiments, the reagent grade acetonitrile that was used still contains 0.17 M (0.3 %) residual H 2 O, which was recently demonstrated to be sufficient to facilitate proton-coupled CO 2 reduction to form CO (McKinnon et al., 2019). This observation is logical due to the relatively larger overpotential applied, especially as the active catalyst under these experimental conditions is predicted by computation (vide infra) to be the more nucleophilic three-electron reduced dianion.
Without knowledge of the in-situ pH of the electrolyte solution, the half-wave potential of the catalytic wave (E cat/2 ) can be used to make a relative comparison of overpotentials under these experimental conditions. As expected, π-extension with the bqn ligand in pre-catalyst [4-CH 3 CN] + gives rise to a significant positive shift of catalytic current with E cat/2 = −2.42 V, whereas the bpy, phen and dmphen pre-catalysts exhibit E cat/2 = −2.85, −2.64, and −2.74 V, respectively. One difficulty encountered under these experimental conditions was in attempting to establish steady-state catalytic conditions with respect to the rate-limiting consumption of CO 2 within the electrochemical double-layer. As discussed later, this became less problematic once excess Brønsted acid was added. As such, TOF max could not be satisfactorily determined under these experimental conditions. Thus, the TOF value is reported for each catalyst in Table 3 using Equation 3 and the i cat /i p ratio determined at a scan rate of 0.1 V s −1 , to aid their side-byside comparison, , n p is the number of electrons involved in the non-catalytic Faradaic current response (responsible for the non-catalytic Faradaic current, i p , as described by the Randles-Sevcik equation (Bard and Faulkner, 2001), and n cat is the number of electrons required for catalysis (two electrons for the reduction of CO 2 to CO as shown in Equations 1 and 2). In our calculations of TOF, the first one-electron reduction wave (n p = 1) was used for reference to determine the non-catalytic Faradaic current (  vs. 8/4 for n p = 1 or 2, respectively). However, this should, with ideal Randles-Sevcik behavior (i p ∝ n 3/2 p ) of the twoelectron reduction event, be completely offset by the 8× smaller (i cat /i p ) 2 ratio when n p = 2. Thus, in theory, kinetic analysis of [4-CH 3 CN] + can be conducted using either the concerted two-electron reduction event at E pc = −1.30 V (n p = 2) or the subsequent one-electron reduction event at E pc = −2.51 V (n p = 1) for reference in determining i cat /i p . Using data recorded at υ = 0.1 V s −1 , Equation 3 results in different TOF values of 25 s −1 (i cat /i p = 4.0, n p = 2) or 14 s −1 (i cat /i p = 8.5, n p = 1), indicating non-ideal behavior of the concerted twoelectron reduction event, which is unsurprising considering this is an ECE mechanism and not a pure two-electron concerted EE mechanism. Scan rate analysis of this two-electron reduction event for [4-CH 3 CN] + clearly demonstrates an increase in peak separation between its cathodic and anodic waves ( Figure S9). More importantly, the 2.83-fold increase in current for a twoelectron event, predicted by the Randles-Sevcik equation (i p ∝ n 3/2 p ) assuming similar diffusion coefficients for each redox state, is in fact found to be significantly smaller (2.13-fold) when we compare the i p values of [4-CH 3 CN] + at E pc = −1.30 (n p = 2) and E pc = −2.51 V (n p = 1). The TOF value of 25 s −1 (i cat /i p = 4.0, n p = 2) is therefore an overestimate of this catalyst's efficiency, and the value of TOF = 14 s −1 is henceforth quoted for [4-CH 3 CN] + (i cat /i p = 8.5, n p = 1) recorded at υ = 0.1 V s −1 . In summary, the benchmark bpy-based pre-catalyst, [1-CH 3 CN] + (i cat /i p = 11.3) exhibits the highest TOF in this study of 25 s −1 at υ = 0.1 V s −1 , followed closely by [3-CH 3 CN] + at 22 s −1 (i cat /i p = 10.7), the bqn-based system [4-CH 3 CN] + at 14 s −1 (i cat /i p = 8.5) and finally [2-CH 3 CN] + at just 11 s −1 (i cat /i p = 7.4).

Voltammetry Under 1 Atm CO 2 in the Presence of Trifluoroethanol
To further probe the catalytic activity of [1-CH 3 CN] + , [2-CH 3 CN] + , [3-CH 3 CN] + and [4-CH 3 CN] + , the voltammetry conditions were altered by adding incremental amounts of a non-aqueous proton source. For the purpose of this study, 2,2,2-trifluoroethanol (TFE, pK a(CH3CN) = 35.4(est.) Lam et al., 2015) was added as a Brønsted acid to observe and optimize proton-coupled catalytic CO 2 reduction. The addition of at least 1.5 M TFE results in a plateau of catalytic current for [1-CH 3 CN] + , [2-CH 3 CN] + and [3-CH 3 CN] + when monitored at υ = 0.1 V s −1 (Figure 7, Figures S16-S19)  Interestingly, a weak grow-in of catalytic current can also be observed at E cat/2 = −1.48 V for both [1-CH 3 CN] + and [2-CH 3 CN] + (a similar maximum is less discernable for [3-CH 3 CN] + ). This lower overpotential weak catalytic current occurs directly from the one-electron reduced species and is thus attributed to catalytic activity of the Mn 0 -Mn 0 dimer intermediate. Indirect support of this hypothesis is the fact that such weak catalytic current is not observed with the bqn precatalyst, [4-CH 3 CN] + which we have already established, via IR-SEC studies, does not form a Mn 0 -Mn 0 dimer intermediate. While appearance of this lower energy catalytic pathway is promising, the very weak current observed, and thus the minimal TOF (<1 s −1 ) prompted us to focus on the more efficient reduction-first pathway for these complexes. Furthermore, this mechanism has already been the focus of other independent studies (Bourrez et al., 2014;Neri et al., 2019).
In contrast to the above mentioned studies in the absence of excess Brønsted acid, the presence of optimal TFE concentrations benefitted the pursuit of steady-state catalytic conditions, thus allowing for an estimation of a maximum turnover frequency (TOF max ), at least in the case of [1-CH 3 CN] + and [2-CH 3 CN] + . A collection of scan-rate dependent voltammetry data is provided in the Supporting Information for all precatalysts (Figures S20-S23) with kinetic analysis summarized in Table 3. Pre-catalysts [1-CH 3 CN] + and [2-CH 3 CN] + perform comparably with TOF max values estimated at 75 s −1 under steady-state catalytic conditions. Pre-catalyst [3-CH 3 CN] + still exhibits a scan-rate dependent current upon increasing scan rate >0.1 V s −1 , thus preventing pure kinetic steady-state conditions, and hence TOF max , to be obtained. Thus, an estimated TOF value of 67 s −1 is reported for [3-CH 3 CN] + recorded at a scan rate of υ = 0.1 V s −1 (Figure S22). Results from the addition of TFE to [4-CH 3 CN] + were rather complicated due to the growin of an additional pre-wave at approx. −2.42 V which prevented accurate determination of E cat/2 or TOF max (Figure S23). At least from a qualitative perspective, it can be stated that TFE addition does give rise to a modest increase in the observed catalytic current maximum (Figure 8). Unfortunately, this phenomenon of multiple catalytic waves raises concerns about product selectivity and competitive side reactions, which are borne true following controlled potential electrolysis experiments, discussed below, which conclusively confirm hydrogen evolution as not just a competitive but a dominant process under these experimental conditions with [4-CH 3 CN] + . Mechanistic details of competitive hydrogen evolution vs. CO 2 reduction are discussed in more detail below via computational analysis.

Controlled Potential Electrolysis
Product selectivity of all four pre-catalysts in the absence and presence of TFE under 1 atmosphere of CO 2 was investigated by controlled potential electrolysis (CPE) with in-situ gas chromatography analysis used for CO and H 2 quantification over a time period of 4 h. The potential bias applied in each case corresponded to E cat/2 for specific experimental conditions as summarized in Table 3 for each catalyst. The tabulated Faradaic efficiency (FE) data in Table 4 represent the peak CO selectivity observed at a single time interval over the duration of the experiment, whereas the turnover number (TON) tabulated for both CO and H 2 represents the total TON over the entire course of the experiment. Plots summarizing all electrolysis data are provided in Figures S28-S35. Ultimately, all pre-catalysts yielded CO product regardless of the conditions employed. Each of the pre-catalysts, [1-CH 3 CN] + , [2-CH 3 CN] + , and [3-CH 3 CN] + exhibited a clear selectivity for CO production in the absence or presence of TFE with only [4-CH 3 CN] + yielding significant H 2 in the presence of 1.5 M TFE. Unfortunately, however, in most cases there is FE that is unaccounted for. This is especially true for pre-catalysts [2-CH 3 CN] + and [3-CH 3 CN] + which is likely due to rapid decomposition, consistent with their poor TONs for CO or H 2 evolution. Pre-catalyst [1-CH 3 CN] + performed significantly better, exhibiting percentage FE's of 62:2 CO:H 2 in the absence of TFE, which increased to 84:2 in the presence of 2.0 M TFE. Efforts to quantify any formate (HCO − 2 ) production to account for the full FE of all catalysts were in vain. This is likely due to the low turnover numbers of these catalysts, which as a whole performed quite poorly under the experimental conditions employed; a recognized problem for the [fac-Mn(N ∧ N)(CO) 3 ] − class of electrocatalysts under homogeneous conditions (Grills et al., 2018). Indeed, two prior CPE studies of the [fac-MnBr(phen)(CO) 3 ] pre-catalyst have each highlighted the poor performance of this system with the FE CO ranging from 18 to 57% (Stanbury et al., 2017;Tignor et al., 2018). Interestingly, the bqn-derived pre-catalyst, [4-CH 3 CN] + exhibited the greatest FE CO of 98% in the absence of TFE, albeit with a very low TON of just 3. However, with such a low TON we cannot rule out catalyst decomposition as a contributing factor. As anticipated from its irregular voltammetry behavior, As discussed below in the computational section, this shift in product selectivity is possibly a consequence of the reduced nucleophilicity of the two-electron reduced [4] − active catalyst, in large part due to the lower lying π * orbitals of the bqn ligand.

Computational Analysis
Density functional theory (DFT) calculations at the M06 level of theory Truhlar, 2008a,b, 2010) in conjunction with the SMD continuum solvation model (Marenich et al., 2009) for acetonitrile were performed to examine the catalyst activation and electrocatalytic CO 2 reduction mechanisms of [1-CH 3 CN] + , [2-CH 3 CN] + , and [4-CH 3 CN] + . Mechanistic calculations on [3-CH 3 CN] + were omitted for the sake of brevity due to its similarity in behavior to [2-CH 3 CN] + . The results are summarized in Schemes 1 and 2 along with tabulated energetics (Tables 5, 6). Further details on the electronic structures of selected reaction intermediates are provided in Supporting Information.

Catalyst Activation and Dimer Formation
The activation of each catalyst starts with one-electron reduction of the solvent-coordinated  Typically observed as an EC mechanism in voltammetry (E 1 G 1 in Scheme 1), subsequent dissociation of the acetonitrile ligand ( G 1 ) leads to localization of the unpaired spin on the Mn center, generating a formally Mn 0 species ( [Mn] Figure S37). This difference is partly attributed to ligand-induced steric effects that lead the Mn center to be outof-plane with respect to the ligand in the case of [4] 0 (for [4] 0 , C bridge -N-Mn-N is 23.2 • , compared to 1.1 • for [1] 0 ). It should be noted that dimerization of [4] 0 is even further suppressed due to the predicted equilibrium between hexacoordinate [4-CH 3 CN] 0 and pentacoordinate [4] 0 , even if the latter forms during electrocatalysis. As a result of the computational analysis described above, [1-CH 3 CN] + and [2-CH 3 CN] + are predicted to form [1] 0 and [2] 0 , respectively, via an EC process followed by fast dimerization to form [1-1] 0 and [2-2] 0 , respectively, which can then be reduced via sequential one-electron processes, or a concerted two-electron reduction process, to generate

CO 2 Binding and CO Evolution
Next, we turn our attention to CO 2 binding to the twoelectron reduced [Mn] − catalyst and subsequent steps of electrocatalytic CO 2 reduction (Scheme 2). Similar to earlier reports (Riplinger et al., 2014), CO 2 binding to [1] − is computed to be uphill ( G 3 = 8.6 kcal/mol) and is driven by proton transfer from a Brønsted acid to generate [1-CO 2 H] 0 (pK a1 = 24.8  (Figure S38), results in increased reactivity toward CO 2 to generate [4-CO 2 ] 2− ( G 4 = 10.8 kcal/mol), and subsequent protonation yields [4-CO 2 H] − (pK a3 = 29.4) (Scheme 2). It should be noted that the spin density ( Figure S38) and total electron density difference ( Figure S39) plots indicate that the third reduction is predominantly ligand centered in [4] 2− and best characterized as [Mn 0 -bqn 2− ] 2− , although the additional negative charge is shared between the metal center and the ligand in the case of

CONCLUSIONS
Through a systematic variation of the polypyridyl ligand from bpy to phen to dmphen and finally bqn, both stericand electronic-based ligand influences on the activity of a [fac-Mn(N ∧ N)(CO) 3 (CH 3 CN)] + class of CO 2 reduction precatalysts has been established, providing critical insight into the manipulation of CO 2 binding affinities and resulting product selectivity for these catalysts. Through a combination of IR-SEC and computational studies, the combined electronic and steric influences of the π-extended bqn ligand have been probed in [4-CH 3 CN] + , where formation of the [4-4] 0 dimer is hindered and a concerted two-electron ECE mechanism for the generation of [4] − is favored. Computations have revealed how the lower lying π * -orbitals of the bqn ligand have rendered this pentacoordinate [4] − intermediate inactive with respect to CO 2 , even in the presence of a Brønsted acid. In contrast to the bpy, phen, and dmphen derived catalysts, this has resulted in a shift in product selectivity for the [4-CH 3 CN] + pre-catalyst to favor H 2 evolution in the presence of excess TFE. However, electrochemical and computational investigations have established successful CO 2 activation following in-situ generation of a three-electron reduced [fac-Mn (−1) (N ∧ N •− )(CO) 3 ] 2− active catalyst, a first for any [fac-Mn(N ∧ N)(CO) 3 (CH 3 CN)] + pre-catalyst. Although at the cost of additional overpotential, this three-electron pathway results in increased reactivity toward CO 2 to generate the previously established [fac-Mn (I) (CO 2 H)(N ∧ N •− )(CO) 3 ] − intermediate, which subsequently propagates the catalytic cycle via the standard reduction-first pathway involving ratedetermining, proton-coupled C-OH bond cleavage.

Acetonitrile
(ACS reagent grade, 99.5%), bromopentacarbonylmanganese(I) (98%), potassium carbonate (>99%), silver trifluoromethanesulfonate (>99%), tetrahydrofuran (anhydrous, 99.9%) and 2,2,2-trifluoroethanol (>99%) were purchased from Sigma Aldrich and used as received. Dichloromethane (ACS reagent grade, >99.9%) and diethyl ether were purchased from Pharmco-Aaper (ACS reagent grade, >99.9%) and used as received. The water content in ACS reagent grade acetonitrile was confirmed by Karl-Fisher titration to be 0.17 M (0.3%). Tetrabutylammonium hexafluorophosphate (99%, Sigma Aldrich) was recrystallized thrice from ethanol and dried under vacuum prior to electrolyte preparation. Steady-state FTIR spectra were recorded on a Thermo Nicolet 670 FTIR spectrophotometer using a liquid cell with CaF 2 windows in spectrophotometric grade acetonitrile (99.5%, Sigma Aldrich) solvent. NMR spectra were recorded on an Agilent spectrometer operated at 399.80 MHz for 1 H nuclei. CD 3 CN was used as received from Sigma Aldrich and its residual 1 H solvent signal used as an internal reference for reporting the chemical shift (δ = 1.96 ppm). Voltammetry and bulk electrolysis were carried out on a CH Instruments 620E potentiostat. A custom threeelectrode cell was used for both voltammetry and bulk electrolysis experiments allowing airtight introduction of working, counter and reference electrodes as well as septa for gas purging. For cyclic voltammetry, glassy carbon (3 mm diameter) and Pt wire were used as working and counter electrodes, respectively, with 0.1 M [Bu 4 N][PF 6 ] in ACS reagent grade acetonitrile as the supporting electrolyte. A non-aqueous reference electrode was used to minimize ohmic potential drop at the solvent interface. This consisted of a Ag wire in 0.10 M [Bu 4 N][PF 6 ] acetonitrile supporting electrolyte isolated by a vycor frit and was calibrated in-situ using the ferricenium/ferrocene redox couple as an internal reference. Redox potentials (E) were determined from cyclic voltammetry as (E pa + E pc )/2, where E pa and E pc are the anodic and cathodic peak potentials, respectively. Where E could not be calculated due to irreversible behavior, E pc or E pa are reported accordingly. For CO 2 concentration dependent studies, gas cylinders were ordered from Airgas containing pre-mixed ratios of Ar:CO 2 (100:0, 80:20, 60:40, 50:50, 40:60, 20:80, 0:100). For controlled potential bulk electrolysis experiments a vitreous carbon (Duocell) working electrode soldered to a copper wire was used. A Pt gauze counter electrode was used, isolated from the main compartment by a fine porosity vycor tube+frit to minimize mass transfer resistance. Gas chromatography data were recorded on a custom Shimadzu GC-2014 instrument where a Ni "methanizer" catalyst was used to convert CO to CH 4 prior to quantification of CH 4 by the thermal conductivity detector. H 2 was simultaneously monitored by a flame ionization detector during the same injection. The GC was pre-calibrated for CO and H 2 quantification by mimicking bulk electrolysis conditions (i.e., 5 mL supporting electrolyte in the same cell, with electrodes, under 1 atm CO 2 ). Standard curves for H 2 and CO were generated using this cell where known volumes of the analyte gas (H 2 or CO) were injected and the solution stirred for 30 min to allow equilibration of the analyte between the electrolyte and headspace prior to GC injection.
General Synthesis of [fac-Mn(N ∧ N)(CO) 3 (OTf)] Complexes 1-4 Bromopentacarbonyl manganese(I) (100 mg, 0.36 mmol) and silver trifluoromethanesulfonate (93.5 mg, 0.36 mmol) were charged to a 50-mL round bottomed flask with approximately 25 mL of degassed dichloromethane under 1 atmosphere of argon. The solution was allowed to mix in the dark for 1 h at room temperature after which the AgBr precipitate was removed by filtration through celite. The dichloromethane solvent was subsequently removed by a rotary evaporator. After confirming quantitative transformation to the [fac-Mn(OTf)(CO) 5 ] intermediate by FTIR spectroscopy, the solid was transferred to a 10-mL microwave vial with 3 mL of tetrahydrofuran. To this solution was added a slight deficit of the appropriate ligand (0.33 mol). The vial was then sealed and reacted in a CEM Discovery microwave reactor at 70 • C for 10 min. Each reaction afforded a bright yellow-orange solution. The tetrahydrofuran was reduced in volume to roughly 0.5 mL on a rotary evaporator at which point the pure product was immediately precipitated by addition of excess diethyl ether. The solid was isolated via vacuum filtration, rinsed with diethyl ether and dried under vacuum. No further purification was necessary. [

Density Functional Theory
All geometries were fully optimized at the M06 level of density functional theory Truhlar, 2008a,b, 2010) with the SMD continuum solvation model (Marenich et al., 2009) for acetonitrile as solvent using the Stuttgart [8s7p6d2f | 6s5p3d1f] ECP10MDF contracted pseudopotential basis set (Dolg et al., 1987) on Mn and the 6-31G(d) basis set on all other atoms (Hehre et al., 1986). Non-analytical integrals were evaluated using the integral = grid = ultrafine option as implemented in the Gaussian 16 software package (Frisch et al., 2016). The nature of all stationary points was verified by analytic computation of vibrational frequencies, which were also used for the computation of zero-point vibrational energies, molecular partition functions, and for determining the reactants and products associated with each transition-state structure (by following the normal modes associated with imaginary frequencies). Partition functions were used in the computation of 298 K thermal contributions to the free energy employing the usual ideal-gas, rigid-rotator, harmonic oscillator approximation (Cramer, 2004). Free-energy contributions were added to singlepoint, SMD-solvated M06 electronic energies computed at the optimized geometries obtained with the initial basis with the SDD basis set on Mn and the larger 6-311+G(2df,p) basis set on all other atoms to arrive at final, composite free energies.

Solvation and Standard Reduction Potentials
As mentioned above, solvation effects for acetonitrile were accounted for by using the SMD continuum solvation model. A 1 M standard state was used for all species in solution (except for acetonitrile as solvent for which the standard state was assigned as 19.14 M). Thus, the free energy in solution is computed as the 1 atm gas-phase free energy, plus an adjustment for the 1 atm to 1 M standard-state concentration change of RT ln (24.5), or 1.9 kcal/mol, plus the 1 M to 1 M transfer (solvation) free energy computed from the SMD model. Standard reduction potentials were calculated for various possible redox couples to assess the energetic accessibility of different intermediates at various oxidation states. For a redox reaction of the form where O and R denote the oxidized and reduced states of the redox couple, respectively, and n is the number of electrons involved in redox reaction, the reduction potential E o O|R relative to SCE was computed as where G o O|R is the free energy change associated with Equation 1 (using Boltzmann statistics for the electron) and E o ref is taken as 0.141 V (Keith et al., 2013), which is required for the conversion of calculated E o O|R vs. normal hydrogen electrode (NHE) in aqueous solution (E NHE = −4.281 V) (Kelly et al., 2006) to E o O|R vs. the saturated calomel electrode (SCE) in acetonitrile (E SCE = −4.422 V) (Isse and Gennaro, 2010). We obtained reduction potentials referenced to the ferricenium/ferrocene couple by using a shift of −0.384 V from E o O|R vs. SCE.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the manuscript/Supplementary Files.

AUTHOR CONTRIBUTIONS
MM, VB, and JR contributed to the synthesis, spectroscopic, and electrochemical characterization of all complexes studied. KN and DG contributed IR-SEC studies. ME contributed all computational studies. MM, ME, DG, and JR contributed equally to manuscript preparation.

SUPPLEMENTARY MATERIAL
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem. 2019.00628/full#supplementary-material Scan rate dependent voltammetry studies under inert and catalytic conditions; experimental and computational IR-SEC spectra; catalyst, acid and CO 2 concentration dependence; controlled potential electrolysis plots; computational spindensity plots, total electron density difference plots, and cartesian coordinates. These materials are provided in the supporting information document.