Effect of Redox Active Ligands on the Electrochemical Properties of

May 22, 2019 - Additional information detailing the Mn K-edge EXAFS analysis and DFT and TD-DFT calculation details on the various complexes investiga...
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Article Cite This: Inorg. Chem. 2019, 58, 7453−7465

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Effect of Redox Active Ligands on the Electrochemical Properties of Manganese Tricarbonyl Complexes Benjamin D. Matson,†,‡,# Elizabeth A. McLoughlin,†,# Keith C. Armstrong,† Robert M. Waymouth,*,† and Ritimukta Sarangi*,‡ †

Department of Chemistry, Stanford University, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California, United States

‡

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ABSTRACT: The synthesis, structural characterization, and electrochemical behavior of the neutral Mn(azpy)(CO)3(Br) 4 (azpy = 2-phenylazopyridine) complex is reported and compared with its structural analogue Mn(bipy)(CO)3(Br) 1 (bipy = 2,2′-bipyridine). 4 exhibits reversible two-electron reduction at a mild potential (−0.93 V vs Fc+/0 in acetonitrile) in contrast to 1, which exhibits two sequential one-electron reductions at −1.68 V and −1.89 V vs Fc+/0 in acetonitrile. The key electronic structure differences between 1 and 4 that lead to disparate electrochemical properties are investigated using a combination of Mn−K-edge X-ray absorption spectroscopy (XAS), Mn−Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) on 1, 4, their debrominated analogues, [Mn(L)(CO)3(CH3CN)][CF3SO3] (L = bipy 2, azpy 5), and two-electron reduced counterparts [Mn(bipy)(CO)3][K(18-crown-6)] 3 and [Mn(azpy)(CO)3][Cp2Co] 6. The results reveal differences in the distribution of electrons about the CO and bidentate ligands (bipy and azpy), particularly upon formation of the highly reduced, formally Mn(−1) species. The data show that the degree of ligand noninnocence and resulting redox-activity in Mn(L)(CO)3 type complexes impacts not only the reducing power of such systems, but the speciation of the reduced complexes via perturbation of the monomer−dimer equilibrium in the singly reduced Mn(0) state. This study highlights the role of redox-active ligands in tuning the reactivity of metal centers involved in electrocatalytic transformations.

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INTRODUCTION The development of electrocatalysts that reduce protons to H2 or carbon dioxide (CO2) to liquid fuels is essential for the storage of electricity sourced from renewable resources (e.g., solar, wind). There are numerous reports of efficient and stable catalysts that electrocatalytically reduce CO2 to carbon monoxide (CO).1 Although first row transition metal complexes can perform this electrocatalytic transformation, the best electrocatalysts are predominantly based on Pd,2−6 Ru,7 Rh,8 and Re.9−15 The development of Earth abundant CO2 electroreduction catalysts that operate at low overpotentials (90%) still remains a considerable challenge. One notable step toward this goal has been the development of Mn(bipy-R)(CO)3(Br) (bipy-R = substituted 2,2′-bipyridine) complexes, a number of which have been shown to reduce CO2 to CO in the presence of exogenous or endogenous weak acids.17−22 Mn(bipy)(CO)3(Br) (1) was first reported by Deronzier and co-workers to electrocatalytically reduce CO2 to CO with high selectivity in the presence of weak acid at −1.90 V vs Fc+/0 in acetonitrile.17 One electron reduction of 1 at −1.68 V vs Fc+/0 in acetonitrile results in rapid and irreversible loss of Br and subsequent dimerization to © 2019 American Chemical Society

form [Mn(bipy)(CO)3]2. Reduction of this dimeric species at −1.89 V vs Fc+/0 correlates with the observed electrocatalytic potential and is further shown to yield the anionic complex [Mn(bipy)(CO)3]− 3, which binds CO2 in the presence of weak acid.22 The hypothesized dependence of the electrocatalytic potential on dimer reduction has motivated attempts to eliminate dimerization in order to lower the operating potential. Kubiak and co-workers installed the sterically bulky tert-butyl and mesityl groups onto the bipyridine backbone.21,22 The resulting mesityl substituted complex, Mn(bipy-mes)(CO)3(Br) (bipy-mes = 6,6′-dimesityl-2,2′-bipyridine), undergoes a concerted two-electron reduction at −1.55 V vs Fc+/0 in acetonitrile. Although the addition of mesityl groups successfully prevented dimerization, electrocatalysis is reported at −2.10 V vs Fc+/0 in acetonitrile. Electrochemical and spectroelectrochemical studies suggest that the electrocatalytically limiting step22 is the reduction of the bound CO2 species, Mn(bipy-mes)CO)3(COOH), and not the formation of the anionic Mn(bipy-mes)(CO) 3− species. This is Received: March 5, 2019 Published: May 22, 2019 7453

DOI: 10.1021/acs.inorgchem.9b00652 Inorg. Chem. 2019, 58, 7453−7465

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Inorganic Chemistry

reduced complex [Mn(azpy)(CO)3][Cp2Co] 6 was prepared by mixing the Mn(azpy)(CO)3(Br) precursor 4 in tetrahydrofuran with cobaltocene (Cp2Co). Complexes 3 and 6 were characterized by 1H NMR (Figures S2 and S5), 13C NMR (Figures S3 and S6), FT-IR (Figures S16 and S17), and ESIMS. X-ray quality crystals of 6 were grown by vapor diffusion of diethyl ether into a saturated tetrahydrofuran solution. X-ray quality crystals of 3 were grown by vapor diffusion of pentane into a saturated tetrahydrofuran solution of the complex. Infrared Spectroscopy. Fourier-transform infrared spectroscopy (FT-IR) was performed on Mn(+1)−Br complexes 1 and 4, as well as the two-electron reduced complexes 3 and 6. DFT calculations were used to aid in the assignment of CO stretching features (Figure S43). The spectra for 1 and 4 show three distinct CO oscillator strengths (1: νCO = 2025, 1935, 1920 cm−1; 4: νCO = 2035, 1960, 1940 cm−1, Figures S16 and S17). In both cases, the higher energy feature (νCO = 2025 cm−1 in 1, νCO = 2035 cm−1 in 4) is assigned to the axial CO that is trans to the bromide ligand, and the two lower energy features (νCO = 1935, 1920 cm−1 in 1; νCO = 1960, 1940 cm−1 in 4) are assigned to the equatorial COs that are trans to the bidentate ligand.34 The blue-shift of all bands on going from 1 to 4 indicates a relative strengthening of the C−O bond and corresponding weakening of the M−CO interaction. The IR spectra of the two-electron reduced complexes 3 (νCO = 1910, 1805 cm−1) and 6 (νCO = 1960, 1855 cm−1) show two CO vibrations (Figures S16 and S17) of considerably lower frequency than those of 1 and 4. The red-shift in the CO vibrations upon two-electron reduction of 1 and 3 indicates an increase in back-bonding interaction from the reduced Mn species. The IR spectra of the bipy complexes reported herein are similar to those of substituted bipy complexes previously reported.21,22 Electrochemistry. The cyclic voltammograms (CVs) of 1, 2, 4, and 5 were measured under an atmosphere of N2 in dry acetonitrile with 0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4) as a supporting electrolyte and referenced to the ferrocene couple (Fc+/0). The CVs of bromide species 1 and 4 are shown in Figure 2. The CVs of the Mn(bipy) complexes 1

reminiscent of similar transformations in the related Re(bipyR)(CO)3(X) systems reported by Kubiak and co-workers.13,14 Here, we examine the effect of the bidentate ligand on tuning the reduction potential of electrocatalytically relevant species. We posited that replacement of the bipyridine scaffold with 2-phenylazopyridine (azpy) would allow for reduction at less negative potentials, as observed in previous studies of CpCo(L) complexes23 (Cp = cyclopentadiene, L = bipy, azpy) and Re(L)(CO)3(X) complexes (L = bipy,9−15 azpy24−26). We show that the Mn(+1)Br complex (4) bearing the azpy ligand exhibits a reversible two-electron reduction at a very mild potential in contrast to the analogous bipy species 1. Mn−Kedge X-ray absorption spectroscopy (XAS), Mn−Kβ X-ray emission spectroscopy (XES), and density functional theory (DFT) provide insights on the key differences between the Mn(bipy) and Mn(azpy) complexes 1−6 (Figure 1) and

Figure 1. Structures of Mn(L)(CO)3 (L = bipyridine (bipy), 2phenylazopyridine (azpy)).

suggest that the highly redox active27−30 azpy ligand plays a critical role in preventing dimerization. The findings reported here provide new insight into the role of the bidentate ligands in this class of complexes, which have the potential to enable the design of new catalysts with improved electrocatalytic properties.

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RESULTS AND DISCUSSION Synthesis and Characterization. The complexes examined in this work are shown in Figure 1. Mn(bipy)(CO)3(Br) (1)17,31−33 and Mn(azpy)(CO)3(Br) (4) were prepared according to literature procedures. [Mn(bipy)(CO) 3 (CH3CN)][CF3SO3] (2) and [Mn(azpy)(CO)3(CH3CN)][CF3SO3] (5) were synthesized based on the previously reported synthesis of [Mn(bipy-R)(CO)3(CH3CN)][CF3SO3] (R = mesityl, t-butyl).21,22 The relevant Mn(L)(CO)3(Br) precursors 1 (L = bipy) and 4 (L = azpy) were reacted with silver trifluoromethanesulfonate ([Ag][CF3SO3]) in refluxing acetonitrile overnight, to form 2 and 5, respectively, which were characterized by 1H NMR (Figures S1 and S4, respectively) and ESI-MS. The two-electron reduced complex [Mn(bipy)(CO)3][K(18-crown-6)] 3 was prepared based on the previously reported synthesis of [Mn(bipy-R)(CO)3][K(18-crown-6] (R = mesityl, t-butyl).21,22 The Mn(bipy)(CO)3(Br) precursor 1 (1 equiv) was mixed with potassium-intercalated graphite (KC8) (2.3 equiv) in tetrahydrofuran in the presence of 18crown-6, which inhibits potassium coordination to the carbonyls of the anionic Mn complex. The two-electron

Figure 2. Cyclic voltammograms of 1 and 4 in 0.1 M Bu4NBF4 in acetonitrile (1 mM Mn). Scan rate 100 mV/s.

and 2 feature two irreversible one-electron reduction waves separated by ca. 200 mV and ca. 400 mV, respectively (Figures S14 and S15, Table S3). The first one-electron reduction occurs at −1.68 V and −1.49 V vs Fc+/0 for 1 and 2, respectively (Table S3). As previously reported,17 upon oneelectron reduction, the coordinated bromide or CH3CN ligand dissociates from the Mn center, and the resulting unsaturated, 7454

DOI: 10.1021/acs.inorgchem.9b00652 Inorg. Chem. 2019, 58, 7453−7465

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Inorganic Chemistry

The two-electron reduction of 4 is reminiscent of the electrochemical behavior of Mn(bipy-mes)(CO)3(Br).22 Notably, two-electron reduction of 4 occurs at a potential ca. 600 mV more positive than the two-electron reduction of Mn(bipymes)(CO)3(Br) (−0.93 V compared to −1.55 V vs Fc+/0 in acetonitrile).3,4 It is also notable that bromide dissociation from 4 only happens after two-electron reduction. In both the bipy-mes and bipy systems, bromide dissociation (chemical step, C) occurs between two single electron transfer steps (E) in an ECE type mechanism (Figure 3).17,21,22 Overall, 6 can be formed at ca. 960 mV more positive potential than that required to form the analogous bipy complex 3 and ca. 600 mV more positive potential than that required to form the analogous bipy-mes anion. This is consistent with previous studies on analogous Re complexes, where the first reduction potential shifts from −1.74 V to −0.80 V vs Fc+/0 in acetonitrile upon replacing the bipyridine ligand with the azopyridine ligand.11,24,26 The ligand substitution in the Mn complexes is different from that in the Re complexes, however, in that it changes this one-electron reduction into a twoelectron reduction. X-ray Crystallography. To correlate the differences in electrochemical properties of 1 and 4, we sought to evaluate the similarities and differences of their geometric and electronic structures. Accordingly, X-ray crystallographic (Figures S7 and S8; Tables S1 and S2), EXAFS derived (Figures S18−S23; Tables S4 and S5), and DFT derived bond metrics were compared (Table 1). The agreement between the X-ray crystallographic and DFT derived values, the latter being calculated in the presence of a universal solvation model (see Theoretical Methods section), indicates that these trends are robust in both the solid and solution state. X-ray crystallography shows that 1 and 4 are octahedral with geometries and bond lengths typical of this class of complexes (Table 1).17,21,22,32,33 The Mn−CO bond lengths are almost identical in the two complexes; the Mn−C distance to the axial CO ligand is 1.803 and 1.805 Å in 1 and 4, respectively, whereas the average Mn−C distance to the equatorial COs is 1.812 and 1.818 Å in 1 and 4, respectively. The main structural difference between the two complexes is the average Mn−N bond length, which is 2.048 Å in 1 and 2.019 Å in 4. Two-electron reduction of 1 and 4 results in loss of the bromide ligand and yields the coordinately unsaturated fivecoordinate anionic complexes 3 and 6 (Figures S7 and S8), respectively. The anionic complexes are structurally different from the parent bromide complexes 1 and 4 but are similar to previously reported anionic Mn(L)(CO)3 complexes.21,22,35 Upon two-electron reduction of 1 and 4, the average Mn−N bond lengths decrease by ∼0.06 Å, and the Mn−C bonds to the COs decrease by ∼0.03 Å in both 3 and 6. This shortening of the Mn−N bonds suggests the presence of π-bonding interactions in the L ligand, as well as the CO ligand. This is explored in more detail in the Ground State DFT Calculations section. Mn K-Edge XAS. To understand the electronic structure differences between 1 and 4 that lead to their disparate electrochemical properties, Mn K-edge XAS measurements were performed. Extended X-ray absorption fine structure (EXAFS) data were measured on all complexes reported in this study to confirm the integrity of the samples and compare the results with crystallographic data (Figures S18−S23; Tables S4 and S5). The bromide ligand in 1 and 4 has a dramatic effect on the Mn K-edge XAS spectra (red-shift to lower energy) and

neutral Mn(bipy)(CO)3 complex rapidly dimerizes. A oneelectron reduction corresponding to the reduction of this dimeric species [Mn(bipy)(CO)3]2 is observed at −1.89 V vs Fc+/0 in both 1 and 2 to generate [Mn(bipy)(CO)3]− 3 (Table S3).17 Replacement of the bipyridine with the azopyridine ligand has a substantial effect on the electrochemical properties. The azpy complex 4 exhibits an apparent two-electron reduction at −0.93 V vs Fc+/0 (Figure 2) and an additional one-electron reduction at −2.50 vs Fc+/0 (Figure S9). Chemical reduction of 4 with two equivalents of cobaltocene generates the anionic [Mn(azpy)(CO)3]− 6. Upon scanning anodically, two oneelectron oxidation waves are observed at −0.83 V and −0.69 V vs Fc +/0 . The CVs of the independently synthesized acetonitrile-bound cationic complex [Mn(azpy)(CO) 3 (CH3CN)][CF3SO3] 5 (Figures S12 and S13) exhibit a reversible redox feature at −0.67 V vs Fc+/0, which suggests that the oxidative feature at −0.69 V vs Fc+/0 corresponds to oxidation of the neutral acetonitrile-bound complex Mn(azpy)(CO)3(CH3CN). Scan rate studies of 4 reveal that this oxidative feature at −0.69 V vs Fc+/0 increases relative to the oxidative feature at −0.83 V vs Fc+/0 with increasing scan rate (Figure S10). This observation suggests that the oxidative feature at −0.83 V vs Fc+/0 is due to bromide reassociating with the Mn center upon oxidation and suggests that bromide loss upon reduction is rapid, which is further supported by titration experiments with tetrabutylammonium bromide (Bu4NBr) (Figure S11). The results indicate an EEC mechanism (Figure 3) in which two consecutive electron transfer steps (E) are followed by the dissociation of bromide (chemical step, C).

Figure 3. Proposed mechanisms for the two-electron reduction of 1 (top) and 4 (bottom), showing isolable intermediates (black) and putative intermediates (gray). Reduction potentials are reported in V vs Fc+/0 in acetonitrile. 7455

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Inorganic Chemistry Table 1. Mn−L Distances (Å)a Obtained from Experimental and Theoretical Methods 1 Mn−N Mn−Cb Mn−Cc

2

EXAFS

DFT

EXAFS

DFT

XRD

EXAFS

DFT

2.043 2.052 1.803 1.809 1.814

2.04

2.05 2.05 1.80 1.81 1.81

2.03

2.06 2.06 1.81 1.82 1.82

1.959 1.982 1.787 1.777 1.785

1.98

1.99 1.99 1.77 1.78 1.78

1.79 1.79

1.79 1.79

4 Mn−Nazod Mn−Npyre Mn−Cb Mn−Cc N−N

3

XRD

5

1.75 1.75 6

XRD

EXAFS

DFT

EXAFS

DFT

XRD

EXAFS

DFT

2.019 2.018 1.805 1.821 1.814 1.274

2.01 2.01 1.78 1.78

2.02 2.03 1.80 1.82 1.82 1.28

2.00 2.00 1.80 1.80

2.04 2.04 1.82 1.83 1.82 1.32

1.889 1.972 1.789 1.786 1.794 1.378

1.94 1.94 1.78 1.78

1.92 1.99 1.78 1.79 1.80 1.37

Errors in bond distances are ±0.02 Å for EXAFS and ±0.01 Å for DFT. See Tables S1 and S2 for XRD data. bAxial CO ligand (trans- to Br, CH3CN, and/or vacant coordination site). cEquatorial CO ligands (trans- to L2). dAzo nitrogen in azpy. ePyridine nitrogen in azpy. DFT metrics were obtained from structures optimized using appropriate solvation models (see Theoretical Methods section). a

reduction from a formally Mn(+1) to a Mn(−1) species. For the azpy species, the rising edge energies occur at 6551.6 eV for the cationic complex 5 and 6551.3 eV for the anionic complex 6. The ∼0.3 eV decrease in rising-edge energy position upon two-electron reduction from 5 to 6 is consistent with the trend observed for the bipy series but is notably lower in magnitude. A typical metal-centered reduction is expected to red-shift the rising edge energy position by ∼1 eV per electron. Thus, the relatively small change in the rising-edge energy position upon two-electron reduction of 5 to 6 and, to a lesser degree, 2 to 3 suggests that the reduction is not completely metal centered and the CO and bidentate ligands (both azpy and bipy) play a noninnocent role in the redox chemistry of these compounds. The roughly 3-fold increase in the risingedge energy shift from azpy to bipy is indicative of important electronic structure differences between these two congeners. The inset of Figure 4 shows the expanded Mn K-pre-edge region. The pre-edge occurs due to an electric dipoleforbidden, quadrupole-allowed 1s → 3d transition, which gains intensity through metal 3d−4p mixing mechanisms.37,40−43 The energy position of the pre-edge is mostly affected by ligand-field strength at the Mn center.44 The intensity weighted average energy (IWAE) position for the Mn K-pre-edge feature in the bipy complex 2 occurs at 6541.4 eV. Upon two-electron reduction of 2 to 3, the IWAE position redshifts by ∼1.1 to 6540.3 eV, which indicates a decrease in ligand field on going from the six-coordinate, formally Mn(+1) species to the five-coordinate (via loss of the CH3CN ligand), formally Mn(−1) species. In contrast, the azopyridine analogue 6 red-shifts by 0.4 eV from 6541.0 eV in 5 to 6540.6 eV in 6. Mn Kβ XES. X-ray emission spectroscopy is a powerful technique to probe the nature of the occupied states in transition metal complexes.45−52 In this technique, Mn−Kedge excitation is followed by photon emission from the Mn porbitals and/or the Mn d-manifold. For this study, emission from the Mn 3p level (Kβ mainline emission) and Mn 3d manifold (Kβ valence to core emission, KβV2C) was monitored. Kβ mainline emission has been shown to be sensitive to the spin state of the complex,47 whereas KβV2C spectra provide

obfuscates changes in the rising-edge region due to change in Mn oxidation state. Therefore, the cationic acetonitrile-bound analogues 2 and 5 were used. It is important to note that the trends in the Mn K-edge data and the electrochemical properties of 1, 2, 4, and 5 (Figures S26−28; Table S3) are qualitatively similar, suggesting that the changes in electronic structure upon reduction of the acetonitrile species are similar to those for the brominated species. Consequently, the XAS comparisons presented henceforth will focus on the acetonitrile bound species 2 and 5 and the two-electron reduced anionic analogues 3 and 6, respectively. The experimental data for 1 and 4 are shown in Figure S24. The Mn K-rising edge region (∼6539 eV for Mn foil) is dominated by Mn 1s → 4p+ continuum transitions and therefore strongly modulated by the Zeff (effective nuclear charge) on Mn.36−39 An increase in Zeff results in a greater stabilization of the 1s levels and hence a blue-shift in the rising edge position. As shown in Figure 4, the rising-edge energy position, measured at the first derivative inflection point, occurs at 6551.4 eV for the cationic bipy complex 2 and 6550.4 eV for the anionic bipy complex 3, which represents a 1.0 eV decrease in rising-edge energy position upon two-electron

Figure 4. Normalized Mn K-edge XAS spectrum of 2, 3, 5, and 6 (as labeled) showing the rising edge and pre-edge regions. Inset: Expansion of the pre-edge region. 7456

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Inorganic Chemistry insights into the ligand field interactions present about the Mn center.45,46 The Mn Kβ mainline XES data for 2, 5, and their comparison to the two-electron reduced analogues 3 and 6 are presented in Figure S25.53 Two-electron reduction of 2 to 3 is accompanied by a small blue-shift in the energy of the Kβ mainline emission (Figure S25; 6489.3 to 6489.8 eV; ΔE = 0.5 eV), while the two-electron reduction of 5 to 6 leads to a smaller blue-shift (Figure S25; 6489.5 to 6489.8 eV; ΔE = 0.3 eV). This red-shift in the Kβ mainline data is likely the result of a difference in ligand field strength on going from a six- to fivecoordination species upon two-electron reduction.54 However, the S = 0 nature of all four complexes studied limits the information content of the Kβ mainline in general and does not allow detailed comparison of the two ligands. The Kβ valence to core region (KβV2C) provides detailed insights into the Mn/ligand electronic structure interactions. Consequently, KβV2C data were measured for 2, 3, 5, and 6 and are presented in Figure 5. 2 shows three distinguishable

feature (∼6538 eV) in complexes 3 and 6 is consistent with the appearance of new Mn−L/Mn−CO π-interactions, which gain intensity in the KβV2C spectra as a result of increased 4pmixing in the five-coordinate reduced species (see Figures S38−S41). The rich KβV2C spectral shape thus indicates distinct bonding environments for the Mn−L and Mn−CO interactions, which are further analyzed using computational methods. Computational Studies. To shed light on the Mn K-edge XAS and KβV2C XES spectral changes, we performed spinunrestricted DFT calculations on 2, 5, and their two-electron reduced analogues using the ORCA quantum chemical code.55 The NMR spectroscopic characterization presented above indicates that 2 and 5 are low-spin (S = 0, 3d6), formally Mn(+1) complexes, and 3 and 6 are low-spin (S = 0, 3d8), formally Mn(−1) complexes. DFT calculations on high-spin states of each form were significantly higher in energy, consistent with the experimental spectroscopic data. The pure functional TPSS was shown to accurately reproduce structural parameters (judged by comparison to the EXAFS/ XRD data; Table 1) and reduction potentials (Table S3; Figure S42). This method has also been successfully applied to transition metal complexes.56−58 To experimentally validate the choice of theoretical method, time-dependent DFT (TDDFT) and ground state DFT (ORCA) calculations were performed to theoretically predict the Mn K-pre-edge and Mn KβV2C spectra, respectively (vide infra). Thus, the chosen level of theory is both structurally and electronically calibrated to experimental data. DFT and TD-DFT Simulated Spectra. TD-DFT calculated pre-edge features were compared to the experimental Mn K-edge XAS pre-edge spectra. Intensity weighted average energies (IWAEs) and average intensities (AIs) from theoretical simulations and experimental spectra of 2, 3, 5, and 6 were derived by fitting the pre-edge features and subtracting the rising edge background. For theoretical simulations, the first five predicted transitions with significant Mn character (>5%) were used to generate the TD-DFT derived spectra. Figures S29−S32 show these calculated and experimentally derived spectra overlaid with selected TD-DFT predicted orbital contributions. As shown in Figure S37, the IWAEs and AIs for the calculated spectra display good linear relationships with the experimental values and support the accuracy of the chosen level of theory in comparing relative energies and intensities. While the complexity of the Kβ V2C spectra makes quantitative evaluation of the DFT derived values difficult, we note that the theoretically predicted spectra show excellent qualitative agreement with the experimental spectra, as shown in Figures S33−S36. Comparisons of DFT predicted spectra with the experimental data for 2 and 3, as well as 5 and 6, show that the noted experimental changes caused by two-electron reduction are well represented by theory. Particularly important is the fidelity with which the high-energy feature in the spectra of anions 3 and 6 are reproduced. Although the solvato Mn (+1) complexes 2 and 5 proved ideal for Mn K-edge XAS studies, Br complexes 1 and 4 are better suited to electrochemical analysis (vide supra) due to the ease at which exogenous bromide concentrations can be controlled. To directly probe the differences in electrochemical properties of the bipy and azpy complexes using DFT methods, analysis henceforth focuses on the brominated analogues. Note that although the Mn K-edge XAS data are

Figure 5. KβV2C spectra (A) bipy complexes 2 and 3 and (B) azpy complexes 5 and 6 (B).

features at 6528.3, 6531.7, and 6534.0 eV (Figure 5A). The spectra change dramatically upon two-electron reduction to 3. The two low-energy features in 2 red-shift to 6527.5 and 6531.5 eV in 3 (Figure 5A), and two additional higher-energy features appear at 6537.0 and 6538.0 eV. The KβV2C spectrum of 5 is qualitatively similar to its bipy counterpart and displays similar spectral changes upon twoelectron reduction (Figure 5). As for 2, the spectrum of 5 includes features at 6528.5 eV, 6531.7, and 6534.5 eV (Figure 5B). Upon two-electron reduction to 6, the lower energy features are red-shifted to 6527.8 and 6531.3 eV. A high energy feature appears at 6537.3 eV. Notably, this feature appears as a single peak, whereas the corresponding feature in 3 occurs as a doublet (Figure 5A). The appearance of this high energy 7457

DOI: 10.1021/acs.inorgchem.9b00652 Inorg. Chem. 2019, 58, 7453−7465

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Inorganic Chemistry

Mayer bond orders for 1, 3, 4, and 6.59 The charges are divided into three fragments: the Mn center, the bidentate ligand (L), and the total over the CO ligands. The discussion below focuses on the Mn-ligand bond orders. In 1, the elementary charge (e) on the Mn, L, and CO fragments is 0.05 e, 0.82 e, and −0.24 e, respectively. The positive charge on bipy is consistent with ligand σ-donation providing electrons for the Mn−Nbipy bonds. The small difference and opposite signs between the Mn and CO indicate a π-back-bonding interaction, which leads to more positive charge on Mn and more negative charge on the CO ligands. This model is supported by Mayer bond orders of ∼0.45 for the Mn−N interactions and ∼1.16−1.32 for the Mn−CO bonds, indicative of mostly single bond character in the Mn−N interactions and multiple bond character in the Mn−CO bonds (Table 2). This suggests that a strong σbonding interaction between the formally Mn(+1) and bipy is balanced by charge backdonation to the CO ligands via a πback-bonding interaction. In 4, the charge on the Mn, L, and CO fragments is 0.05 e, 0.52 e, and −0.18 e. Relative to 1, the Mn center is unchanged, while several changes occur in the CO fragment: the Mn−C bond elongates (Table 1), the C−O frequency blue-shifts (Figure S16), and the charge decreases (Table 2). This indicates a weakening of Mn−CO π-backbonding interaction in 4, balanced by a corresponding increase in the Mn−L π-bonding interaction as evidenced by the shortening of the Mn−N bonds (Table 1) and decrease in positive charge on L (Table 2). This change in relative πbonding between the CO and L moieties is an important bonding difference, highlighting the increased noninnocent nature of the azpy ligand relative to the bipy ligand and is consistent with FT-IR measurements. The two-electron reductions of 1 to 3 and 4 to 6 result in formally Mn(−1) complexes. Interestingly, the charge on the

obscured by the presence of the heavy Br-ligand in 1 and 4, this is not the case with Mn KβV2C XES. A comparison of KβV2C spectra of bipy complexes 1 and 3 as well as azpy complexes 4 and 6 (Figure S27) reproduce the trends presented in Figure 5, suggesting that the bromide complexes exhibit similar trends in ligand bonding changes upon twoelectron reduction to their CH3CN bound counterparts. Ground State DFT Calculations. To understand the origin of the observed trends in XAS, it is important to elucidate the distribution of and ensuing change in electron density over the metal and ligand fragments upon two-electron reduction. Table 2 summarizes the Hirshfeld charges and Table 2. Summary of Hirshfeld Charges and Mayer Bond Orders for 1−6 Hirshfeld charge (e)

Mayer bond order

CO

Mn−N

Mn-COaxb

Mn-COeqc

0.82

−0.24

1.32

−0.07

−0.15

−0.78

4

0.05

0.52

−0.18

6

0.01

−0.40

−0.61

0.45 0.45 0.51 0.54 0.51 0.41 0.42 0.74

1.16 1.17 1.38 1.36 1.25 1.15 1.37 1.25

Mn

L

1

0.05

3

a

1.25 1.28 1.29

a

For bipy calculations, N1 and N2 were chosen arbitrarily and their atom numbers were maintained for each calculation. For azpy calculations, the Mn−N bond orders referring to the azo nitrogen (Nazo) is shown in bold. bThe axial CO (COax) was taken as the CO trans- to the CH3CN ligand. cFor azpy calculations, Mn−CO bond orders referring to the CO trans- to the azo nitrogen is shown in italics.

Figure 6. (A) Frontier molecular orbital diagram of 1, 1-R, and 3, with energies plotted relative to the Mn 1s energy. (B) Frontier molecular orbital diagram of 4, 4-R, and 6, with energies reported relative to the Mn 1s energy. Kohn−Sham orbital densities and tabulated contributions from the Mn, Br− ligand, L ligand, and CO ligands are shown for the LUMO of 1 and 4, the SOMO of 1-R and 4-R, and the HOMO of 3 and 6. Kohn− Sham orbitals of the HOMO−2 and LUMO+2 in 1 and 4 (as labeled) are shown as examples of the five predominantly Mn−CO π-back-bonding interactions (HOMO−3, HOMO−2, HOMO−1, LUMO+1, and LUMO+2). 7458

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changes in orbital composition and/or energy observed across this series occur predominantly due to changes in the Mn− bipy π* interactions. Bipy complex 1 is an 18 e−, six-coordinate species and is thus formally Mn(+1), d6, and S = 0. Accordingly, the FMOs show splitting of the d-manifold into a fully occupied [dxy, dxz, dyz]60 set and an unoccupied [dz2, dx2‑y2] set, in accordance with its approximately octahedral character (Figure 6A). Kohn− Sham orbital populations confirm the predominantly Mn-CO π-back-bonding character (∼37−65% Mn; 24−35% CO) of these orbitals. The dz2 and dx2‑y2 valence orbitals are energetically close in the six-coordinate 1 (Figure 6A). Upon one-electron reduction to 1-R, the dz2 orbital is stabilized relative to the dx2‑y2 orbital (Figure 6A) because of weakening of the Mn−Br bond (vide infra) and mixes with the bipy π*, resulting in the SOMO in 1-R. This SOMO is best characterized as one still dominated by bipy π* character (now bipy●−), but with increased contribution from the Mn 3dz2 orbital, attributed to the presence of the added electron and the improved energetic overlap between the bipy●− π* and the slightly stabilized dz2 orbital. Because the dz2 orbital in 1 is involved in π-bonding with CO and σ-bonding with Br, a corresponding increase in CO and Br contribution occurs in the SOMO of 1-R. This conclusion is supported by a comparison of population densities: the LUMO in 1 is dominantly bipy centered (89.7% bipy, 2.9% Mn), while the SOMO of 1-R reveals the presence of decreased bipy orbital density (77.2%) and concomitant increase in contribution from Mn, CO, and Br (8.5%, 7.4%, and 6.9%, respectively; Figure 6A). Loss of Br and subsequent reduction of the dimeric intermediate leads to formation of the isolable anionic complex 3 (d8, 18 e−). The HOMO of 3 has substantially increased Mn dz2 character and strong overlap with the, now reduced, bipy π* (46.4% on bipy, 34.6% on Mn; Figure 6A). In the fully reduced species, a distinct Mn−bipy π-bonding interaction is observed. Given the large amount of bipy π* character in the LUMO of 1 and the SOMO of 1-R, this Mn−bipy π-bond is best described as one in which the bipy2− ligand is a π-donor. The HOMO of 3 also reveals increased Mn−CO π-bonding, consistent with the decrease in CO stretching frequency observed in FT-IR experiments (Figure S16). Thus, upon stepwise reduction of 1 to 3, the frontier valence orbital increasingly gains Mn dz2 character, and the π-bonding interactions from bipy and the CO ligands are stabilized (Figure 6A). The two-electron reduction of the azpy series reveals similar qualitative trends in charge flow and bond-order changes. However, clear quantitative differences that ultimately explain the difference in electrochemistry are observed. Note that the symmetry of the d-orbitals involved in the LUMO, SOMO, and HOMO are different than that observed in the bipy series because of the asymmetry of the azpy ligand. However, the dorbital splitting is similar (Figure 6B), which allows for a direct comparison to the bipy FMOs The LUMO of the six-coordinate, bromide species 4 (18 e−, 6 d , S = 0), reveals an L centric orbital (86.1% on azpy, 5.1% on Mn; Figure 6), similar to that in 1. Interestingly, the oneelectron reduction of 4 to 4-R does not alter the composition of the FMO accepting the electron (LUMO in 4, SOMO in 4R) and remains dominantly azpy based (86.1% azpy, 4.8% Mn and 9% Br− and CO) (Figure 6B). This trend indicates that

Mn fragment does not change significantly upon two-electron reduction, and the majority of the electrons are deposited in to the L and CO fragments (Table 2). Notably, the decrease in Mn Hirshfeld charge upon reduction of 1 and 4 is −0.12 e (∼240%) and −0.04 e (∼80%), respectively, suggesting a roughly 3-fold decrease in the amount of Mn based reduction in the azpy complex compared to bipy. This is consistent with the 3-fold decrease in Mn K-rising-edge shift on going from 4 to 6 relative to that observed on going from 1 to 3 (Figure 4). As mentioned above, the large majority of the added electrons are deposited in the L and CO fragments in 3 (−1.51 e) and 6 (−1.32 e). Similar to the trends observed in the formally Mn(+1) complexes 1 and 4, a relative difference in charge density is observed on the L and CO fragments in 3 and 6. In 3, the added two-electrons are somewhat more evenly distributed over the bipy and CO fragments relative to the azpy and CO fragments in 6. It is interesting to note that the charge on the two N atoms in the azo-group alone increased by −0.28 e−, accounting for a 17% increase in charge. This trend is consistent with a decrease in the N−N bond order from 1.4 to 0.8. Coupled with the short Mn−Nazo bond (Table 1) and increased Mn−Nazo bond order (0.41−0.74 upon two-electron reduction), this observation supports the conclusion that a very strong Mn−(NN) π-bonding interaction in 6 is the major cause of the decreased electron density on the Mn center and decreased Mn-CO π-back-bonding, relative to the bipy system. Frontier Molecular Orbital Diagrams. To understand how the difference in bonding affects the two-electron reduction in 1 and 4 and ultimately to understand the divergent electrochemical behavior observed between the bipy and azpy systems, DFT calculations were performed on the putative Mn(0) intermediates (1-R and 4-R for bipy and azpy, respectively), and the trends in the two one-electron reductions (Mn(1+) to Mn(0) and Mn(0) to Mn(1−)) were compared. The full list of DFT calculated bond lengths for 1-R and 4-R is in Table S7. 1-R and 4-R were modeled as the Mn(0) complexes Mn(L)(CO)3(Br)− (d7, S = 1/2). A comparison of the DFT calculated bond distances in 1 with 1R reveals slight elongation (Δavg = +0.007 Å) of the Mn−N bonds and slight contraction of the Mn−CO bonds (Δavg = −0.006 Å). The reduction of 4 to 4-R shows the same trends: contraction of the Mn−CO bonds (Δavg = −0.016 Å) and elongation of the Mn−N bonds (Δavg = +0.015 Å). However, the bond distance changes in 4 to 4-R are twice in magnitude, suggesting a much larger rearrangement within the L and CO ligands upon one-electron reduction. In contrast, the largest changes in bond length occur in the elongation of the Mn−Br distance from 1 to 1-R (Δ = 0.21 Å), and to a significantly lesser degree 4 to 4-R (Δ = 0.09 Å). Thus, the reduction of 1 to 1-R primarily affects the Mn−Br bond without significantly perturbing the Mn−L and Mn−CO interaction. The frontier molecular orbitals (FMOs) across the series 1 → 1-R → 3 and 4 → 4-R → 6 are shown in Figure 6 and allow for visualization of the electronic structure differences between bipy and azpy. Tracking the lowest occupied molecular orbital (LUMO), singly occupied molecular orbital (SOMO), and highest occupied molecular orbital (HOMO) upon stepwise one-electron reductions from 1 to 1-R to 3 provides key insight into changes in the highly covalent bonding of the redox active Mn-bipy interaction (Figure 6A, as labeled). It is important to note that bipy π* ligand orbitals are dominantly involved in the LUMO of 1, the SOMO of 1-R, and the HOMO of 3 (Figure 6A). This behavior suggests that the 7459

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Inorganic Chemistry the reduction occurs exclusively on the azpy ligand, with very little perturbation on the remaining Mn(CO)3(Br) moieties. One-electron reduction of 4-R converts the SOMO in 4-R to the HOMO in 6. This HOMO reveals that the azpy2− π* is now involved in strong Mn−azpy π-bonding interaction, which although qualitatively similar to that in 3, is quantitatively stronger, as evidenced by the FMO compositions and HOMO contour plot shown in Figures 6 and 7.

Figure 8. Calculated Hirshfeld charges and Mn−Br bond lengths for 1, 1-R, 4, and 4-R. Calculated spin densities are shown for S = 1/2 species 1-R and 4-R. Hirshfeld charge and spin density contributions are presented for the Mn center, L ligand, CO ligands, and Br− ligand.

(10% on Mn, 7% on Br; Figure 7) compared to 4-R (4% on both Mn and Br; Figure 7). Note that although the Hirshfeld charges in 1 and 4 suggest the notable stabilization of negative charge by the azpy ligand, this effect is enhanced in the reduced species 1-R and 4-R. The negative charge in 1-R is predominantly distributed between the Br and CO ligands (−0.58 e and −0.42 e, respectively; Figure 7). In contrast, 4-R shows the azpy ligand supporting significant charge (−0.20 e) at the expense of charge on the Br and CO ligands (−0.49 e and −0.35 e, respectively; Figure 7). Although the Hirshfeld charges for all fragments change by the same amount when going from 1 to 1-R and 4 to 4-R, the initial stabilization of negative charge by the azpy ligand in 4 leads to a one-electron reduced species (4-R) that has Mn−Br metrics more comparable to the Mn(+1) species 1 than to its isoelectronic Mn(0) counterpart, 1-R (Figure 7). These DFT results therefore suggest that the redox noninnocence of azpy necessitates the addition of two electron into 4 to induce Br loss, accounting for the observed lack of dimerization and change in electrochemical mechanism.

Figure 7. An overlay of FMO diagrams of (A) 1-R and 4-R, and (B) 3 and 6. Energies were calibrated to the respective Mn 1s energy. Bipy orbitals are shown in red, and azpy orbitals are shown in blue. Mixing of CO and bipy π* with Mn dz2 is represented with dashed lines, while that of CO and azpy π* with Mn dyz is shown with solid lines. In all cases the contributing orbitals from Mn, CO, and azpy, bipy are shown as a visual aid only.

Two-Electron Reduction vs Dimerization. The difference in the π-bonding character of the azpy and bipy ligands leads to different compositions and energies of FMOs upon sequential one-electron reduction of 1 and 4. The LUMO of 1 is 0.8 eV higher in energy than in 4, which agrees well with the 740 mV difference in the Mn(+1) reduction potential of the two complexes (Table S6). The DFT predicted potentials for the reoxidation of 3 and 6 are consistent with CV data and suggest that reduction of the Mn(0) intermediate should be more facile than the Mn(+1) for both bipy and azpy.61 Therefore, in the absence of dimer formation, 1 would be expected to undergo a two-electron reduction akin to 4, suggesting that the differences in electrochemical behavior are strongly coupled to the dimerization kinetics and/or the equilibrium concentrations of 1-R and 4-R. Electrochemical data suggest that Br− loss in Mn(bipy)(CO)3(Br)− 1-R is appreciably faster than in Mn(azpy)(CO)3(Br)− 4-R, supporting the assertion that the faster rate of dimerization in 1-R results in its dimerization-beforereduction behavior. Since dimerization is directly coupled to the loss of the bromide ligand in 1-R, it is useful to look at the change in Mn−Br interaction upon one-electron reduction of 1 and 4 to 1-R and 4-R, respectively. The differing bond lengths, Hirshfeld charges, and bond orders seen in 1-R compared to 4-R are particularly informative, given that their neutral Mn(+1) compounds 1 and 4 display more similar metrics (Table 1, Table S7, Figure 7). As mentioned before, the theoretical reduction of 1 to 1-R is accompanied by a 0.21 Å increase in Mn−Br length (2.57 Å to 2.78 Å; Figure 7), in contrast to the reduction of 4 to 4-R, which displays a relatively modest change of 0.09 Å (2.55 Å to 2.64 Å; Figure 7). These changes in Mn−Br length correlate with the increased spin density along the Mn−Br σ* in 1-R

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CONCLUSIONS It has been shown, via electrochemical analysis, that structurally similar Mn(L)(CO)3(Br) complexes (L = bipy, azpy) display significant differences in redox behavior. Bipy complex 1 undergoes an ECE mechanism in which bromide loss and dimerization occur between electron transfer steps, whereas azpy complex 4 undergoes an EEC mechanism in which bromide loss occurs after consecutive electron transfer steps, which suppresses dimer formation. Mn K-edge XAS rising edge and pre-edge features demonstrate that the two ligands show different behavior with regard to their redox noninnocence, with the azpy complexes demonstrating a distinctly lower amount of Mn centered reduction. Remarkable alignment of electrochemical, spectroscopic (XAS, XES, and FT-IR), and theoretical methods provides a fully consistent molecular orbital based bonding picture in which the presence of a highly redox active azpy ligand stabilizes a larger amount of negative charge when compared to isoelectronic bipy complexes. Extension of this DFT analysis to putative one-electron reduced species, 1-R and 4-R, demonstrates the role that this charge stabilization plays in preventing population of the Mn−Br σ* orbital in azpy species 4-R compared to bipy species 1-R. The subsequent decrease in Br− lability in azpy requires an extra electron transfer step to occur 7460

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Inorganic Chemistry prior to Br− loss, thus blocking dimerization of the Mn(0) intermediate. These consistent experimental and computational results show the role that ligand electronics rather than sterics can play in suppressing undesirable dimer formation, which highlights an alternate method to consider in catalyst design and improvement. A combined theory and spectroscopy-based investigation of CO2 reactivity with 1 and 4 are the focus of our future investigations.

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Synthesis of [Mn(bipy)(CO)3][K(18-crown-6)] (3). In a typical synthesis, a 15 mM solution of 1 was prepared in THF in an inert atmosphere glovebox. After the solution was cooled in a −30 °C freezer overnight, 18-crown-6 (2.1 equiv) and KC8 (2.5 equiv) were added, and the reaction was stirred for roughly 1 h in the glovebox as it warmed to room temperature. The solution was syringe filtered, and filtrate was evacuated in vacuo. The crude residue was redissolved in ∼2 mL of THF in the glovebox, and 15 mL of pentane was carefully layered over the THF solution. The solution was stored in a −30 °C freezer overnight and decanted the following morning. The resulting purple crystalline solid was dried under a vacuum. 1H NMR (500 MHz, CD3CN) δ 9.32 (d, J = 6.9 Hz 2H), 7.62 (d, J = 8.8 Hz, 2H), 6.57 (dd, J = 8.8, 7.6 Hz, 2H), 6.08 (dd, J = 5.9. 4.8 Hz, 2H). 13C NMR (126 MHz, CD3CN) δ 153.82, 139.68, 121.89, 119.72, 111.27. m/z 294.9921; found m/z 294.9910. The 1H NMR spectrum is provided in Figure S2, the 13C NMR spectrum is provided in Figure S3, and the FT-IR spectrum is provided in Figure S16. Synthesis of [Mn(azpy)(CO)3][Cp2Co] (6). In a typical synthesis, a 5 mM solution of 4 was prepared in THF in an inert atmosphere glovebox. After the solution was cooled in a −30 °C freezer overnight, cobaltocene (Cp2Co, 2 equiv) was added, and the reaction was stirred for roughly 1 h in the glovebox as it warmed to room temperature. The solution was syringe filtered, and filtrate was dried in vacuo. The crude residue was thoroughly washed with diethyl ether and collected via filtration. The resulting red crystalline solid was dried under a vacuum. 1H NMR (400 MHz, CD3CN) δ 9.03 (d, J = 6.3 Hz, 1H), 7.63 (d, J = 8.0 Hz, 2H), 7.26 (dt, J = 14.5, 8.2 Hz, 3H), 7.08 (t, J = 7.3 Hz, 1H), 6.96−6.85 (m, 1H), 6.07 (t, J = 6.4 Hz, 1H), 5.62 (s, 13H). 13C NMR (126 MHz, CD3CN) δ 152.37, 128.31, 128.06, 125.60, 106.52, 85.74. m/z 322.0019; found m/z 322.0015. The 1H NMR spectrum is provided in Figure S5, the 13C NMR spectrum is provided in Figure S6, and the FT-IR spectrum is provided in Figure S17. X-ray Crystallography. Single crystals for X-ray analysis were mounted on a Kapton loop using Parabar 10312 hydrocarbon oil. All measurements were made on a Bruker D8 Venture diffractometer with CCD detector and graphite monochromated Mo−Kα radiation (λ = 0.71073 Å). Frames corresponding to an arbitrary sphere of data were collected using ω-scans of 0.3° counted for a total of seconds per frame. Data were integrated using the Bruker SAINT soft-ware program62 to a maximum θ-value of 28.26°, analyzed for agreement and possible absorption using XPR, and corrected for Lorentz and polarization effects. Absorption corrections were applied using the SADABS program.62 No decay correction was applied. Structures were solved in Olex263 with the ShelXT structure solution program64 using intrinsic phasing and refined with ShelXL65 using least-squares minimization. Hydrogen atoms were included in ideal positions and refined isotropically in riding model with Uiso = 1.5Ueq(X) for methyl groups and Uiso = 1.2Ueq(X) for other atoms, where Ueq(X) are thermal parameters of parent atoms. Non-hydrogen atoms were refined anisotropically. Crystallographic data for 1 and 4 have been previously reported. Crystallographic data for 3 and 6 are presented in the Supporting Information (Tables S1 and S2, respectively). Crystals of 3 suitable for X-ray diffraction were obtained from vapor diffusion of pentane into a THF solution. Crystals of 6 suitable for X-ray diffraction were obtained from vapor diffusion of diethyl ether into a THF solution. The ORTEP for 3 and 6 are shown in Figures S7 and S8, respectively. X-ray Absorption Spectroscopy. The Mn K-edge X-ray absorption spectra of 1−6 were measured at the Stanford Synchrotron Radiation Lightsource (SSRL) on the unfocused 20-pole 2 T wiggler sidestation beamline 7-3 under nonstandard ring conditions of 3 GeV and ∼100 mA (low-alpha operations mode at SSRL). A Si(220) double crystal monochromator was used for energy selection. A Rhcoated harmonic rejection mirror was used on beamline 7-3 to reject components of higher harmonics. The monochromator was further detuned by 50% to eliminate components of higher harmonics. Complexes 1, 2, 4, and 5 were measured as solids, which were first homogeneously ground with BN as the dilutant and then placed in 1 mm Al-spacers for measurement. Complexes 3 and 6 were transferred

EXPERIMENTAL SECTION

Materials. All manipulations were carried out under an inert atmosphere of nitrogen or argon with the use of standard vacuum line, Schlenk, and glovebox techniques. Solvents were dried by standard methods and degassed via three freeze−pump−thaw cycles. Deuterated solvents for NMR were purchased from Cambridge Isotope Laboratories. All reagents were used as received. The synthesis of Mn(bipy)(CO)3Br 117 and Mn(azpy)(CO)3Br 431−33 was described previously. Instrumentation. 1H and 13C NMR spectra were recorded on Varian 300, 400, or 500 MHz spectrometers. All NMR spectra were taken at room temperature unless stated otherwise. Residual solvent proton and carbon peaks were used as reference. Chemical shifts are reported in parts per million (δ). High-resolution mass spectra were obtained by ESI-MS on a Thermo Fisher Exactive Orbitrap mass spectrometer. Cyclic voltammetry experiments were performed using a Bio-Logic SP-200 potentiostat or a WaveNow USB potentiostat (Pine Research Instrumentation) at ambient temperature in an inert atmosphere glovebox. A typical electrochemical cell consisted of a three-electrode setup using a glassy carbon working electrode (3 mm diameter, Bioanalytical Systems, Inc.), platinum wire auxiliary electrode, and Ag/AgNO3 nonaqueous reference electrode (Bioanalytical Systems, Inc.). All electrochemical experiments were performed with 0.1 M tetrabutylammonium tetrafluoroborate supporting electrolyte in acetonitrile, unless stated otherwise. The glassy carbon working electrode was polished between each scan. Potentials are referenced to the Fc+/0 couple (0.0 V). Synthesis of [Mn(bipy)(CO)3(CH3CN)][CF3SO3] (2). A solution of silver trifluoromethanesulfonate (AgOTf, 82 mg, 0.320 mmol) in acetonitrile (10 mL) was added dropwise to a solution of 1 (100 mg, 0.266 mmol) in acetonitrile (15 mL) with stirring under N2. Upon addition of the AgOTf solution, the color of the solution changed from orange to pale yellow with precipitation of AgBr. The reaction mixture was covered with foil to avoid light exposure and was refluxed under N2 overnight. The solvent was subsequently removed via rotary evaporation, yielding an orange-yellow oil that crystallized upon exposure to high-vac. The product was dissolved in a minimal amount of acetonitrile and passed through a short, basic alumina column using acetonitrile as the eluent. The yellow-colored fractions were combined, and the solvent was removed by rotary evaporation, yielding an orange-yellow solid. 1H NMR (400 MHz, CD3CN) δ9.14 (d, J = 5.4 Hz, 2H), 8.41 (d, J = 8.2 Hz, 2H), 8.23 (dd, J = 7.6, 6.1 Hz, 2H), 7.71 (dd, J = 7.9, 6.9 Hz, 2H), 1.96 (s, 3H). m/z 336.0181; found m/z 336.0168. The 1H NMR spectrum is provided in Figure S1. Synthesis of [Mn(azpy)(CO)3(CH3CN)][CF3SO3] (5). A solution of silver trifluoromethanesulfonate (AgOTf, 74 mg, 0.288 mmol) in acetonitrile (10 mL) was added dropwise to a solution of 4 (100 mg, 0.240 mmol) in acetonitrile (20 mL) with stirring under N2. Upon addition of the AgOTf solution, the color of the solution changed from purple to cherry red with precipitation of AgBr. The reaction mixture was covered with foil to avoid light exposure and was refluxed under N2 overnight. The solvent was subsequently removed via rotary evaporation, yielding a deep red oil that crystallized upon exposure to high-vac. The red solid was thoroughly washed with diethyl ether and collected via vacuum filtration. 1H NMR (500 MHz, CD3CN) δ 9.15 (d, J = 5.9 Hz, 1H), 8.73 (d, J = 7.6 Hz, 1H), 8.47 (td, J = 7.8, 1.4 Hz, 1H), 8.02−7.81 (m, 3H), 7.74 (dq, J = 14.9, 7.3 Hz, 3H), 1.96 (s, 4H). m/z 363.0290; found m/z 363.0277. The 1H NMR spectrum is provided in Figure S4. 7461

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Inorganic Chemistry into 2 mm Delrin XAS cells with 70 μm Kapton tape windows under synthesis conditions and were immediately frozen after preparation and stored under liquid N2. During data collection, all samples were maintained at a constant temperature of ∼10−15 K using an Oxford Instruments CF 1208 liquid helium cryostat. Data were measured to k = 14 Å−1 for 1, 2, 4, and 5 (transmission mode) using an ionization chamber detector placed directly downstream of the sample and to k = 12 Å−1 for 3 and 6 (fluorescence mode) using a Canberra Ge 30element array detector. Internal energy calibration was accomplished by simultaneous measurement of the absorption of a Mn foil placed between two ionization chambers situated after the sample. The first inflection point of the foil spectrum was fixed at 6539.0 eV. Owing to the low redox state of the complexes, no visual change in the rising edge energy position or the pre-edge features was observed over successive scans, indicating that all the samples were resistant to photoreduction/damage under experimental conditions. Data presented here are between two to four scan averages for the solid samples and 10−15 scan averages for the solution samples. Data were processed by fitting a second-order polynomial to the pre-edge region and subtracting this from the entire spectrum as background. A threeregion spline of orders 2, 3, and 3 was used to model the smoothly decaying postedge region. The data were normalized by subtracting the cubic spline and assigning the edge jump to 1.0 at 6555 eV using the Pyspline program. Data were then renormalized in Kaleidagraph for comparison and quantification purposes. Theoretical EXAFS signals χ(k) were calculated by using FEFF (Macintosh version 8.4). Structural model for 1, 3, 4, and 6 were obtained from crystal structures, and models for 2 and 5 were obtained from DFT geometry optimization. The input structures were successful in generating reasonable phase and amplitude parameters required to obtain a good fit. Data fitting was performed in EXAFSPAK. The structural parameters varied during the fitting process were the bond distance (R) and the bond variance σ2, which is related to the Debye−Waller factor resulting from thermal motion, and static disorder of the absorbing and scattering atoms. The nonstructural parameter ΔE0 (E0 is the energy at which k = 0) was also allowed to vary but was restricted to a common value for every component in a given fit. Coordination numbers were systematically varied in the course of the fit but were fixed within a given fit. The CO groups were modeled by fixing the distances and σ2 of the multiple scattering paths with the corresponding single scattering path (see Tables S4 and S5). X-ray Emission Spectroscopy. Mn Kβ XES measurements were performed at beamline 6-2 of the SSRL synchrotron. The source of Xrays at beamline 6-2 is a 56-pole, 0.9 T wiggler, which delivers about 1013 photons/s at the sample position. A Rh-coated Si mirror is used to vertically collimate the beam, and a cylindrically bent Si mirror, also Rh-coated, is used to focus, resulting in a beam spot at the sample of about 0.1 × 0.5 mm2 (vertical × horizontal). The incident energy was set to 7500 eV using a liquid nitrogen cooled DCM equipped with Si(111) crystals. The XES spectra were recorded using the multicrystal spectrometer installed at the 6-2 beamline operating in Johansson geometry. Five spherically bent Ge(620) crystal analyzers (100 mm diameter, 1 m radius of curvature) aligned on intersecting Rowland circles were used to select the Mn Kβ emission energy, which was detected by a Vortex SDD. The samples were maintained at ∼10 K using an Oxford CF1208 continuous-flow liquid helium cryostat. Signal attenuation in the air path sample−analyzers−detector was minimized by placing a helium-filled bag in this path. The Mn XES data were recorded in the range from 6480 eV up to 6555 eV to cover the Kβ mainline (∼6450−6500 eV) and the V2C region (6505 to 6555 eV). The energy stepping and integration time were varied in these two regions to increase the data quality and optimize data collection time. Around the Kβ mainline an energy step size of 0.25 eV was used, and in the V2C region the step size was 0.15 eV. A limit of X-ray exposure time before radiation-induced changes were observed was systematically established for each sample via observation of the Kβ mainline emission signal intensity over time. The duration of these time scans was equal to the time required for one full scan. This protocol was repeated with increasing filter levels

in order to determine the smallest filter which showed negligible change in signal intensity over the course of the time scan. This filter level was used for all subsequent scans on the sample. The data were collected such that each illuminated spot was used during one scan, after which the sample was moved and a fresh spot was chosen. At least 10 successive scans for each compound were averaged in order to improve the data quality. The emitted spectra were calibrated by using the spectrum of KMnO4 as a reference with the maximum of the Kβ at 6543.2 eV. Theoretical Details. Gradient corrected, (mGGA) spin unrestricted density functional theory (DFT) calculations were performed using ORCA 3.03.55 The TPSS66,67local functional and the following basis sets were employed for the calculations: Ahlrich’s all electron triple-ζ basis set def2-TZVPP68,69 with three polarization functions on Mn, def2-TZVP on all other atoms. Calculations were performed in a dielectric continuum using the conductor like screening model (COSMO)70 with the dielectric properties of acetonitrile. The choice of TPSS was made based on comparison to experimental data, including redox potentials, XRD structures, IR stretching frequencies, and XANES pre-edge features. Comparison of the experimental and calculated data was in good agreement using TPSS. For completeness, the following functionals were also tested: TPSSh, M06-L, M06, B3LYP, and BP86. M06-L and B3LYP. TPSS was selected due to the accuracy with which it reproduced both structural and thermochemical experimental data. Starting guess geometries were obtained from the respective crystal structures and modified appropriately where needed. Time-dependent (TD)-DFT calculations were performed with the electronic structure program ORCA to calculate the energies and intensities of the Mn K pre-edges.43 TD-DFT calculations were performed using the quadruple-ζ basis set def2-QZVPP71 on all atoms. The tight convergence criterion was imposed on all calculations. The calculated energies and intensities are derived pseudo-Voigt line profiles (Gaussian contribution of 0.5), generated using the MultiWFN code, broadened with half-widths of 1.0 eV for solution state spectra (3 and 6) and 0.6 eV for solid state spectra (1, 2, 4, and 5) to account for core-hole lifetime and instrument resolution. The calculated preedge energy positions were linearly scaled to match the experimental spectra. This is generally the case with core level TD-DFT calculations since DFT does not describe core potentials accurately, resulting in the core levels being too high in energy relative to the valence levels. The TD-DFT results were compared with experimental data to judge the efficacy of the DFT calculation (Figure S37). The molecular contour plots were generated in ChemCraft,72 and the individual atomic orbital contributions to the molecular orbitals were obtained using MOAnalyzer.73

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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.9b00652. Tabulated X-ray data, and molecular structures obtained from single crystal X-ray diffraction for 3 and 6. 1H NMR spectra of 2, 3, 5, and 6. 13C NMR of 3 and 6. Additional information detailing the Mn K-edge EXAFS analysis and DFT and TD-DFT calculation details on the various complexes investigated in this study (PDF) Accession Codes

CCDC 1900421−1900422 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 7462

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AUTHOR INFORMATION

Corresponding Authors

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

Benjamin D. Matson: 0000-0001-5733-0893 Elizabeth A. McLoughlin: 0000-0003-1481-266X Robert M. Waymouth: 0000-0001-9862-9509 Ritimukta Sarangi: 0000-0002-2764-2279 Author Contributions #

B.D.M. and E.A.M. contributed equally.

Funding

This material is based on work supported by the National Science Foundation (CHE-1565947, R.M.W.), the Precourt Institute for Energy (2017-4-Waymouth), NIHP41GM103393, DOE-BES (SSRL operations), DOE-BER. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. The SSRL Structural Molecular Biology Program is supported by the DOE Office of Biological and Environmental Research, and by the National Institutes of Health, National Institute of General Medical Sciences (including P41GM103393). This material is based on work supported by the National Science Foundation (CHE1213403). The contents of this publication are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS, NCRR, or NIH. E.A.M. is grateful for a Center for Molecular Analysis and Design (CMAD) Fellowship. B.D.M. was supported by the Precourt Institute for Energy (2017-4-Waymouth).

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