Drastic Redox Shift and Electronic Structural Changes of a

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Drastic Redox Shift and Electronic Structural Changes of a Manganese(III)-Salen Oxidation Catalyst upon Reaction with Hydroxide and Cyanide Ion Takuya Kurahashi* Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi 444-8787, Japan S Supporting Information *

ABSTRACT: Flexible redox properties of a metal complex are important for redox catalysis. The present study shows that the reaction of a manganese(III) salen complex, which is a wellknown oxidation catalyst, with hydroxide ion gives a transient manganese(III) species with drastically lowered redox potential, where the redox difference is −1.21 V. The reaction with cyanide ion gives a stable manganese(III) species with almost the same spectroscopic and redox properties, which was characterized as an anionic [MnIII(salen)(CN)2]− of low-spin S = 1 state, in contrast to the starting MnIII(salen)(OTf) having usual high-spin S = 2 manganese(III). The present study has thus clarified that the drastic redox shift comes from an anionic six-coordinate [MnIII(salen)(X)2]− species where X is either OH− or CN−. Resonance Raman measurements show that the stretching band of the imino group shifts from 1620 to 1597 cm−1 upon conversion from MnIII(salen)(OTf) to [MnIII(salen)(CN)2]−, indicative of lowered CN double bond character for [MnIII(salen)(CN)2]−. The observed deformation of a salen ligand is a clear indication of an increased electron population on the imino π*-orbital upon formation of low-spin manganese(III). It was proposed that the electronic structure of [MnIII(salen)(CN)2]− may contain only limited contribution from valence tautomeric [MnIV(salen− •)(CN)2]−, in which the imino group of a salen ligand is reduced by one-electron via intramolecular electron transfer from low-spin manganese(III). The present study has clarified an unexpected new finding that a salen ligand works as a reservoir for negative charge to stabilize lowspin manganese(III).

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INTRODUCTION

To achieve facile activation of a manganese(III) salen catalyst, I have investigated the reaction of a manganese(III) salen complex with the hydroxide ion.5 One of the expectations is that a manganese(III)-hydroxide species, which is generated upon reaction with hydroxide ion, is a direct precursor for highvalent manganese-oxo active oxidant in the case of electrochemical and photochemical activation. The other point is that the reaction with hydroxide ion expectedly lowers the redox of catalysts, leading to an energetically favorable activation. It has long been known that the reaction with hydroxide ion induces autoreduction of porphyrin complexes of iron(III)6 and manganese(III)7 to give manganese(II) and iron(II) complexes. Because hydroxide ion is definitely not a reductant,8 it is strongly suggested that the reaction of these metal complexes with hydroxide ion gives some unidentified species with high reducing ability. More recently, it was reported that the reaction of a rhodium(III) porphyrin with hydroxide ion gives a rhodium(II) porphyrin.9 Autoreduction of a metal complex is not necessarily limited to the porphyrin system, but was also reported for a bis(benzene-1,2-dithiolato) copper(III) com-

A manganese(III) salen complex, in which salen is a diiminediphenolate ligand, is a well-known oxidation catalyst.1,2 Typically, an oxidation reaction of a substrate is carried out using a catalytic amount of a manganese(III) catalyst and a stoichiometric amount of a terminal oxidant. The first event of a catalytic reaction is an oxidation of manganese(III) ion with a terminal oxidant. The resulting high-valent transient species, often referenced as manganese(V)-oxo, is primarily responsible for substrate oxidation. One of the challenges in oxidation chemistry is the use of mild reaction conditions to prevent undesirable side reactions. However, the catalytic reactions using a manganese(III) salen catalyst usually requires a strong oxidant such as iodosylbenzene, sodium hypochlorite, and m-chloroperoxybenzoic acid.1,2 An electrochemical method has also been employed to generate an active species via one-electron oxidation of a manganese(III) salen complex as an electrocatalyst.3 A photoredox catalysis is a promising future route to a high-valent metal-oxo active oxidant, and very recently, the one-electron oxidation of a manganese(III) salen complex using a photosensitizer was reported.4 © XXXX American Chemical Society

Received: September 26, 2017

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DOI: 10.1021/acs.inorgchem.7b02474 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry plex.10 Another interesting example is an antimony(V) porphyrin bearing hydroxide axial ligands, which is reduced upon photoirradiation.11,12 I also reported that the reaction of a manganese(III) salen complex with aqueous alkaline solution yields a manganese(II)−manganese(III) dimer.5 Autoreduction of metal complexes was generally believed to originate solely from metal ions, and the role of the organic ligand has not been considered. However, it is important to note that autoreduction is also known for organic compounds such as quinones, viologens, and C60, even without metal ions.8 Fukuzumi, Kadish, and co-workers investigated in detail the reaction of p-benzoquinone and C60 with alkoxide ion.13,14 They revealed that the reaction with alkoxide ion generates the alkoxide adduct of p-benzoquinone and C60, which functions as an electron donor to reduce p-benzoquinone and C60. The present study investigated the reaction of a manganese(III) salen complex with hydroxide ion at low temperature, which shows the formation of a transient manganese(III) species. Notably, one-electron oxidation of the new manganese(III) species occurs at much lower redox potential (−0.40 V vs Fc−Fc+), as compared to the starting MnIII(salen)(OTf) (+0.81 V vs Fc−Fc+), resulting in a drastic redox change of −1.21 V. While the manganese(III) complex generated from the reaction with hydroxide ion is an unstable transient species, the reaction with cyanide ion gives a stable manganese(III) species with almost the same spectroscopic and redox properties, which was characterized as an anionic six-coordinate [MnIII(salen)(CN)2]− of low-spin S = 1 state, in contrast to the starting MnIII(salen)(OTf) having the usual high-spin S = 2 manganese(III). It is thus clearly shown that the drastic redox shift comes from the formation of [MnIII(salen)(X)2]−, where X is either OH− or CN−. In order to investigate the role of a salen ligand in the formation of anionic [MnIII(salen)(CN)2]− species, further spectroscopic studies were carried out using absorption, resonance Raman, and 1H NMR spectroscopy. Remarkably, the imino group of the salen ligand in [MnIII(salen)(CN)2]− shows lowered double bond character, compared with the starting MnIII(salen)(OTf), indicative of an increased electron population on the imino π*-orbital. This is a clear indication that a salen ligand plays a role in accommodating negative charge from low-spin manganese(III), hence stabilizing the anionic [MnIII(salen)(CN)2]− species. Although the function of a salen ligand as a reservoir for positive charge is wellknown,15−17 the present study reveals an unexpected new function as a reservoir for negative charge.

193 K. Absorption spectral changes were observed without isosbestic points until 2.5 equiv of Bu4NOH was added. As shown in Figure 1, the resulting species shows characteristic

Figure 1. Absorption spectral changes of MnIII(L-t-Bu)(OTf) (black line) upon the addition of Bu4NOH (0.5, 1.0, 1.5, 2.0, 2.5 equiv) in CH3CH2CN at 193 K (0.5 mM, 0.1 cm cell).

absorptions in the visible region at 465 and 556 nm with shoulders around 510 and 580 nm. The solution has a reddish color at 193 K, which changes to pale brown upon warming to room temperature, indicating that the product species are only stable at lower temperature. Then, the product species was investigated by measuring 2H NMR spectra at 193 K in CH3CH2CN for manganese complexes with selectively deuterated salen ligands, L-t-Bu-d2 and L-t-Bu-d4 shown in Chart 2. As shown in Figure 2, the Chart 2. Selectively Deuterated Salen Ligands and Their Abbreviationsa

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RESULTS Reaction of MnIII(salen)(OTf) with Hydroxide Ion. The MnIII(L-t-Bu)(OTf) complex, which is a manganese(III) salen complex having OTf− as the counterion (see Chart 1 for the Lt-Bu structure), was reacted with Bu4NOH in CH3CH2CN at

a In L-t-Bu-d4, 2H atoms are selectively incorporated into the phenolate rings (80% D) and the tert-butyl groups (7% D). In L-t-Bu-d2, 2H atoms are selectively incorporated into the imino group (99.5% D).

Chart 1. Salen Ligands and Their Abbreviations

solution of MnIII(L-t-Bu-d4)(OTf) and 3 equiv of Bu4NOH shows two 2H NMR signals at 52.1 and 26.8 ppm, which arise from the phenolate 2H atoms. The solution of MnIII(L-t-Bud2)(OTf) and 3 equiv of Bu4NOH shows only one signal at −61.7 ppm, which arises from the imino 2H atoms. 2H NMR spectra indicate that the solution of MnIII(L-t-Bu)(OTf) and 3 equiv of Bu4NOH contains only one species. The NMR shifts B


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Inorganic Chemistry

Figure 2. 2H NMR spectra of MnIII(L-t-Bu-d4)(OTf) + 3 equiv of Bu4NOH (red line) and MnIII(L-t-Bu-d2)(OTf) + 3 equiv of Bu4NOH (blue line) in CH3CH2CN at 193 K.

of the product species are significantly different from those of the starting MnIII(L-t-Bu)(OTf), which shows phenolate 2H signals at −52.4 and 5.1 ppm18 and an imino 2H signal at −695 ppm19 at 193 K. This indicates that the electronic structure of a MnIII(L-t-Bu) complex is altered upon reaction with hydroxide ion. The observation of two phenolate signals and one imino signal for the present species is in clear contrast to Mn(L-t-Bu) dimers, such as a di-μ-oxo dimanganese(IV) complex20 and a μhydroxo dimanganese(III) complex, 5 which show four phenolate signals and two imino signals, because of a different environment for the left and right halves of a L-t-Bu ligand. Thus, the formation of a dimeric species could be rule out. A six-coordinate [MnIII(L-t-Bu)(OH)2]− is the most probable structure according to the 2H NMR result. The reaction of MnIII(L-t-Bu)(OTf) with 2.5 equiv of Bu4NOH was investigated with electron paramagnetic resonance (EPR) spectroscopy. The starting manganese(III) complex shows a parallel-mode EPR signal at g = 8.01,21 which disappears upon reaction with hydroxide ion (Figure 3a). EPR measurements in a usual perpendicular mode (Figure 3b) also indicated the disappearance of the manganese(III)-derived signal at g ≈ 8.0, and detected a small signal at g ≈ 2.0 for the solution of MnIII(L-t-Bu)(OTf) and 2.5 equiv of Bu4NOH. The signal at g = 2 suggests the formation of a manganese(II) species as a consequence of autoreduction, but the yield of manganese(II) is very small, as judged from the signal intensity. The main product from the reaction with Bu4NOH is virtually EPR-silent, which excludes the formation of d3 manganese(IV) and d5 manganese(II). The formation of dimeric species, which are EPR-silent, is ruled out from 2H NMR results. The EPR data may be compatible with a monomeric manganese(III) complex, because the intensity of EPR signals from manganese(III) of an integer spin system is dependent on the coordination environment.22 Redox properties of the product species were compared with those of the starting MnIII(L-t-Bu)(OTf) complex. According to cyclic voltammetry measurements in CH3CH2CN at 193 K (Figure 4), the starting MnIII(L-t-Bu)(OTf) complex shows a reversible redox wave at 0.81 V vs a ferrocene-ferrocenium couple (Fc-Fc+) (blue line), which was previously assigned as arising from the one-electron oxidation of MnIII(L-t-Bu)(OTf).19 The one-electron reduction of MnIII(L-t-Bu)(OTf) shows an irreversible redox wave at −1.23 V, which is less negative than the irreversible redox wave at −1.52 V for the reduction of O2 under exactly the same conditions (see Figure S11 in the Supporting Information).

Figure 3. X-band EPR spectra of frozen CH3CH2CN solution (0.5 mM) of MnIII(L-t-Bu)(OTf) (blue line) and MnIII(L-t-Bu)(OTf) + 3 equiv of Bu4NOH (red line), measured at 4 K: (a) parallel mode and (b) perpendicular mode.

In contrast, the product species generated from MnIII(L-tBu)(OTf) and 3 equiv of Bu4NOH shows a reversible redox wave at −0.40 V (red line), which is strikingly lower than the corresponding redox wave at 0.81 V for the starting MnIII(L-tBu)(OTf). The change of the redox potential is −1.21 V. However, the one-electron reduction of the product species occurs at a redox potential of −1.41 V, which is only slightly altered from the redox potential of MnIII(L-t-Bu)(OTf) at −1.23 V. To assign the redox wave at −0.40 V, the product species from the reaction with Bu4NOH was oxidized by a thin-layer electrochemical oxidation (Figure 5). Absorption spectral changes with clear isosbestic points were observed, even at 0.1 V vs Fc−Fc+. The controlled-potential electrochemical oxidation at this voltage generates a different species with an absorption at 595 nm. It was also confirmed that the same oxidation reaction could be carried out using ferrocenium triflate. Then, the reversible redox wave at −0.40 V is unambiguously assigned as the one-electron oxidation of the product species from MnIII(L-t-Bu)(OTf) and 3 equiv of Bu4NOH. The absorption spectrum, as a result of the oneelectron oxidation, is very similar to those of MnIV(L-t-Bu)(X)2 complexes (see Figure S12 in the Supporting Information), in which X (= CF3CH2O, N3, and Cl) coordinates to manganese(IV) as external axial ligands.23 It is then suggested that the reaction of MnIII(L-t-Bu)(OTf) with 2.5 equiv of Bu4NOH generates an anionic [MnIII(L-t-Bu)(OH)2]− complex, which is oxidized to MnIV(L-t-Bu)(OH)2. EPR measurements for the electrochemically generated product are also consistent with C


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Inorganic Chemistry

samples were also measured, but I could not find an infrared band that is sensitive for both 18O and 2H exchanges, as expected for M−OH stretching24 (see Figure S14 in the Supporting Information). Then, the [MnIII(L-t-Bu)(OH)2]− formulation is only a proposal. A magnetic susceptibility measurement using the Evans method was done for the proposed Bu4N+[MnIII(L-t-Bu)(OH)2]− formulation. The [MnIII(L-t-Bu)(OH)2]− complex shows the μeff value of 2.99 μB, indicative of the S = 1 state (μeff = 2.83μB), in contrast to the μeff value of 4.92μB for the starting MnIII(L-t-Bu)(OTf) complex, which is consistent with S = 2 (μeff = 4.90μB) for highspin d4 manganese(III). This result indicates that the reaction of S = 2 MnIII(L-t-Bu)(OTf) with 2.5 equiv of Bu4NOH induces the spin-state change from S = 2 to S = 1 manganese(III). The [MnIII(L-t-Bu)(OH)2]− complex was further reacted with 3 equiv of acetic acid at low temperature. The resulting solution shows the μeff value of 4.46μB, indicating that the S = 1 [MnIII(L-t-Bu)(OH)2]− complex is converted back to the S = 2 MnIII(L-t-Bu)(OAc) complex. According to the recovery of the μeff value, it is estimated that ca. 10% of MnIII(L-t-Bu)(OTf) was decomposed during the reaction with Bu4NOH. However, a maximum of 10% error is well within the range for the reliable assignment of the S = 1 state for [MnIII(Lt-Bu)(OH)2]−. Stable [MnIII(salen)(CN)2]− upon Reaction with Cyanide Ion. The reaction of MnIII(L-t-Bu)(OTf) with Bu4NOH gives a species that is distinct from the starting MnIII(L-tBu)(OTf), but the electronic structure of the resulting species could not be fully investigated, because of low stability. Then, the reactions with cyanide, methoxide, and azide ion were carried out in CH3CH2CN at 193 K (Figure S15 in the Supporting Information). The reactions with Bu4NCN and Bu4NOMe induce exactly the same absorption spectral changes as the reaction with Bu4NOH shown in Figure 1. But the equivalents of these ion, required for the complete conversion, are different: only 2.0 equiv of Bu4NCN but 9.0 equiv of Bu4NOMe, compared with 2.5 equiv of Bu4NOH. In contrast, Bu4NN3 was not effective for the formation of the present species, even by the use of excess Bu4NN3 (20 equiv). These observations clearly indicate that the role of added ion is to bind strongly to manganese(III). The product from the reaction with cyanide ion, which is stable at room temperature, was investigated in more detail. Trials to obtain crystals suitable for X-ray diffraction from the reddish solution of MnIII(salen)(OTf) + excess of Bu4NCN gave crystalline material of brown color after repeated crystallization, when a nonsubstituted salen, bis(salicylidene)ethylenediamine, was utilized. Elemental analysis of the crystalline material shows that the composition of the product is MnIII(salen)(CN). Although the MnIII(salen)(OTf) with 2 equiv of Bu 4NCN is readily soluble in CH3 CN, the MnIII(salen)(CN) crystal is totally insoluble in common organic solvents, including CH3CN. However, the addition of excess Bu4NCN to the suspension of MnIII(salen)(CN) in CH3CN gave the clear reddish solution again, suggesting that the isolated MnIII(salen)(CN) is not the species of the present interest. A survey of previous literature shows that the MnIII(salen)(CN) complex obtained here is exactly the same compound reported by Matsumoto, Tuchagues, and coworkers.25 This compound is a cyanide-bridged one-dimensional coordination polymer containing alternating [MnIII(salen)(CN)2]− and [MnIII(salen)]+ units. Solid-state studies revealed that the manganese(III) center in the

Figure 4. Cyclic voltammograms of MnIII(L-t-Bu)(OTf) (blue line) and MnIII(L-t-Bu)(OTf) + 3.0 equiv of Bu4NOH (red line) in CH3CH2CN containing 0.1 M of Bu4NOTf at 193 K under an argon atmosphere. The black circles indicate the stating point of scans and the arrows indicate the direction of scans.

Figure 5. Absorption spectral changes of MnIII(L-t-Bu)(OTf) + 2.5 equiv of Bu4NOH (0.5 mM, 0.05 cm cell) upon controlled-potential electrochemical oxidation at 0.1 V vs Fc−Fc+ in CH3CH2CN containing 0.1 M of Bu4NOTf at 193 K.

the formation of manganese(IV) (see Figure S13 in the Supporting Information). To obtain evidence for the coordination of OH− in the product species from the reaction of MnIII(L-t-Bu)(OTf) with 2.5 equiv of Bu4NOH, I attempted to measure resonance Raman spectra for the solution as well as the solid sample that was prepared by evaporation of CH3CH2CN under vacuum at low temperature. However, the solution and the solid sample exhibited fluorescence upon laser irradiation, which hampered resonance Raman measurements. Infrared spectra for solid D


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Inorganic Chemistry [MnIII(salen)(CN)2]− unit (NC-MnIII−CN) adopts a low-spin S = 1 state, in contrast to the usual high-spin S = 2 for the [MnIII(salen)]+ unit (CN-MnIII−NC). A t-butyl derivative, MnIII(L-t-Bu)(CN), is also only slightly soluble in CH3CN, and the absorption spectrum measured in this solvent indicated that the MnIII(L-t-Bu)(CN) complex is a usual high-spin manganese(III) complex (see the black line in Figure 6). 1H NMR spectrum of MnIII(L-t-Bu)(CN) is

[MnIII(L-t-Bu)(CN)2]− complex is stable under ambient conditions in the presence of air and moisture. The [MnIII(L-t-Bu)(CN)2]− complex was investigated with 1 H and 2H NMR spectroscopy. As shown in Figure 7 (blue

Figure 7. 1H NMR spectrum (black line) of [MnIII(L-t-Bu)(CN)2]− in CD3CN at 243 K (5 mM). 2H NMR spectra of [MnIII(L-t-Bud4)(CN)2]− (red line) and [MnIII(L-t-Bu-d2)(CN)2]− (blue line) in CH3CN at 243 K (5 mM). The signals denoted with an asterisk (*) comes from residual CDH2CN.

Figure 6. Absorption spectral changes of MnIII(L-t-Bu)(CN) upon the addition of Bu4NCN (0.2, 0.4, 0.6, 0.8, 1.0 equiv) in CH3CN at 243 K (0.5 mM, 0.1 cm cell). Inset: Plots of the absorbance of the 465 nm band against the equivalents of Bu4NCN.

line), the [MnIII(L-t-Bu-d2)(CN)2]− complex shows one imino signal at −49.1 ppm. The [MnIII(L-t-Bu-d4)(CN)2]− complex shows two phenolate signals at 43.4 and 22.9 ppm, along with two t-butyl signals within a diamagnetic region (shown as a red line in Figure 7). Plots of chemical shifts versus 1/T are linear below 263 K, but deviate from linearity above 273 K (see Figure S18 in the Supporting Information), consistent with the distribution of MnIII(L-t-Bu)(CN) in the equilibrium with [Mn III(L-t-Bu)(CN) 2]− above 273 K. Estimated signal positions at 193 K by extrapolations of the linear lines are 26.8 and 53.2 ppm for the phenolate signals and −62.7 ppm for the imino signal, which are almost the same as those observed in the reaction with Bu4NOH in CH3CH2CN at 193 K (Figure 2). In the case of MnIII(L-t-Bu)(CN), the phenolate signals from the left and right halves of the chiral L-t-Bu ligand are resolved, consistent with the symmetry expected from the fivecoordinate structure (Figure S16). However, the phenolate signals from [MnIII(L-t-Bu-d4)(CN)2]− are not resolved, consistent with a more-symmetric six-coordinate structure. Infrared (IR) spectroscopy shows that the ν(CN) stretching mode at 2139 cm−1 for MnIII(L-t-Bu)(CN) is shifted to a lower energy at 2106 cm−1 for [MnIII(L-t-Bu)(CN)2]− (Figure S19 in the Supporting Information), indicative of larger π-back-donation from manganese to CN in [MnIII(L-t-Bu)(CN)2]−. The binding of CN− to MnIII(L-t-Bu)(CN) would decrease the overall σ-donation from CN to manganese, which also contributes to the shift of the CN stretching to a lower energy. The [MnIII(L-t-Bu)(CN)2]− complex is virtually EPR-

substantially the same as that of the starting MnIII(L-tBu)(OTf) (Figure S16 in the Supporting Information), indicating that the electronic structure is not altered upon the binding of one cyanide ion. Similar to the nonsubstituted derivative, the MnIII(L-t-Bu)(CN) is almost insoluble in CH2Cl2, CH3OH, and CH3CH2CN, but the addition of excess Bu4NCN readily dissolved MnIII(L-t-Bu)(CN) in these solvents. The reaction of MnIII(L-t-Bu)(CN) with Bu4NCN was investigated with absorption spectroscopy in CH3CN at 243 K (see Figure 6). The absorption spectrum of MnIII(L-t-Bu)(CN) (shown in Figure 6 as a black line) was altered upon the addition of Bu4NCN to give the spectrum shown in Figure 6 as a red line, which is substantially the same spectrum from the reaction of MnIII(L-t-Bu)(OTf) with 2.5 equiv of Bu4NOH (Figure 1). Importantly, the reaction of MnIII(L-t-Bu)(CN) with Bu4NCN shows clear isosbestic points, and only 1 equiv of Bu4NCN is required for complete conversion. This is indicative of a simple process from one species to the other, that is, from MnIII(L-t-Bu)(CN) to [MnIII(L-t-Bu)(CN)2]−. The same reaction could be carried out at room temperature, but more than 2 equiv of Bu4NCN is required (Figure S17 in the Supporting Information). This indicates that [MnIII(L-t-Bu)(CN)2]− is a product of the equilibrium with the starting MnIII(L-t-Bu)(CN), in which the formation of the [MnIII(L-tBu)(CN)2]− product is favored at lower temperature. The E


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Inorganic Chemistry

a six-coordinate structure in solution, as observed for the MnIII(salen)(CN) coordination polymer in the solid state. Here, the electronic structure of [MnIII(salen)(CN)2]− in solution was investigated in detail. Magnetic susceptibility measurements were carried out in CD3CN at 243 K using the Evans method. The MnIII(L-tBu)(OTf) complex gives μeff = 4.92μB, which is consistent with S = 2 (μeff = 4.90μB) for high-spin d4 manganese(III). In contrast, the [MnIII(L-t-Bu)(CN)2]− complex shows μeff = 3.08μB, which is assigned as arising from S = 1 (μeff = 2.83μB). This result indicates that the binding of two cyanide ions to MnIII(L-t-Bu)(OTf) induces the spin-state change from S = 2 to S = 1. Magnetic susceptibility measurements were also performed for the solid samples using a superconducting quantum interference device (SQUID) susceptometer. As shown in Figure 8, the MnIII(L-t-Bu)(CN) complex shows the μeff value

silent (Figure S20 in the Supporting Information) as is the case with the reaction of MnIII(L-t-Bu)(OTf) with Bu4NOH. According to the spectroscopic similarities including absorption, 2H NMR, and EPR spectra, the reaction of MnIII(L-tBu)(OTf) with Bu4NOH or Bu4NCN produces a sixcoordinate [MnIII(L-t-Bu)(OH)2]− or [MnIII(L-t-Bu)(CN)2]− complex and the electronic structure of [MnIII(L-t-Bu)(OH)2]− and [MnIII(L-t-Bu)(CN)2]− is exactly the same, irrespective of the difference of the external axial ligand. Note that the reaction with OH− in CH3CH2CN may liberate CN−, leading to the formation of [MnIII(L-t-Bu)(CN)2]− in both cases. But [MnIII(L-t-Bu)(OH)2]− and [MnIII(L-t-Bu)(CN)2]− are undoubtedly different species, because [MnIII(L-t-Bu)(OH)2]− does not show the ν(CN) IR band that arises from [Mn III(L-t-Bu)(CN) 2]− (Figure S21 in the Supporting Information). Redox properties of [MnIII(L-t-Bu)(CN)2]− were investigated with cyclic voltammetry in CH3CN at 243 K (Figure S22 in the Supporting Information). In contrast to [MnIII(L-tBu)(OH)2]−, the [MnIII(L-t-Bu)(CN)2]− complex shows an irreversible redox wave. An anodic oxidation peak of [MnIII(L-tBu)(CN)2]− was observed at −0.29 V vs Fc−Fc+, which is comparable to the corresponding oxidation peak of [MnIII(L-tBu)(OH)2]− at −0.36 V (Figure 4). To further investigate redox properties, one of the substituents of the salen ligand was varied (Chart 1), and cyclic voltammetry measurements were carried out for [MnIII(L-OMe)(CN)2]−, [MnIII(L-Cl)(CN)2]− and [MnIII(L-NO2)(CN)2]−. The redox waves of these species are also irreversible (see Figure S22). Remarkably, the anodic oxidation peak potentials (Ea) are altered, depending on the substituent, as summarized in Table 1. The Ea value increases in Table 1. Anodic Oxidation Peak Potentials (Ea) for OneElectron Oxidation of [MnIII(salen)(CN)2]−a Ea vs Fc−Fc+ (V) III

−

[Mn (L-OMe)(CN)2] [MnIII(L-t-Bu)(CN)2]− [MnIII(L-Cl)(CN)2]− [MnIII(L-NO2)(CN)2]−

−0.31 −0.29 −0.15 0.00

a

The Ea values are determined from cyclic voltammograms that are measured in CH3CN at 243 K (Figure S22).

Figure 8. Temperature dependence of the magnetic moment (μeff) of polycrystalline samples of MnIII(L-t-Bu)(OTf), MnIII(L-t-Bu)(CN), and Bu4N+ [MnIII(L-t-Bu)(CN)2]− in an applied field of 1 kOe.

the order of OMe < t-Bu < Cl < NO2, indicating that redox properties of [MnIII(salen)(CN)2]− could be modulated by the ligand structure. Previously, redox potentials of the manganese(III), cobalt(II), and nickel(II) complexes with L-OMe, L-t-Bu, and L-Cl were determined.19,26 In every case, the Ea value increases, depending on the substituent in the order of OMe < t-Bu < Cl. Spectroscopic and Magnetic Details of [MnIII(salen)(CN)2]−. According to the previous report by Matsumoto, Tuchagues, and co-workers,25 the six-coordinate [MnIII(salen)(CN)2]− unit in the coordination polymer adopts a low-spin S = 1 manganese(III) state in the solid state. However, the solution magnetic and spectroscopic properties of [MnIII(salen)(CN)2]− were not investigated, because the dissolution of the coordination polymer results in comproportionation of [MnIII(salen)(CN)2]− and [MnIII(salen)]+ units to give a five-coordinate MnIII(salen)(CN) having a usual high-spin S = 2 manganese(III) state. The present [MnIII(L-t-Bu)(CN)2]− complex generated by the reaction of MnIII(t-Bu)(CN) with 1 equiv of Bu4NCN is expected to have

of 4.00 μB at room temperature (blue circle), which is different from the μeff value of 4.90μB for S = 2 MnIII(L-t-Bu)(OTf) (black circle). The μeff value of MnIII(L-t-Bu)(CN) drastically increases below 50 K and reaches a maximum value of 13.0μB at 4 K. Characteristic magnetic properties of MnIII(L-t-Bu)(CN) are due to the formation of a cyanide-bridged one-dimensional coordination polymer.25 The solid sample of Bu4N+ [MnIII(L-t-Bu)(CN)2]− was obtained as a monohydrate by evaporation of the solvent from the CH3CN solution of MnIII(L-t-Bu)(CN) and 1 equiv of Bu4NCN. Purification was not carried out, because the starting MnIII(L-t-Bu)(CN) was preferentially precipitated. The Bu4N+ [MnIII(L-t-Bu)(CN)2]− complex shows the μeff value of 3.00μB in the temperature range from 50−340 K (red circle), which clearly indicates that the Bu4N+[MnIII(L-t-Bu)(CN)2]− complex is a low-spin S = 1 compound in the solid state as well as in solution. Interestingly, the magnetic data of MnIII(L-t-Bu)(CN) and Bu4N+ [MnIII(L-t-Bu)(CN)2]− show a spike increase at 40 K, in F


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Inorganic Chemistry contrast to the magnetic data of MnIII(L-t-Bu)(OTf). The spike increase at 40 K was ascribed to dioxygen absorbed on the surface of the sample. According to the previous report, a SQUID measurement on a polyethylene straw, commonly used as a sample holder, gave a similar large peak from dioxygen at ∼48 K, when the evacuation before the measurement is insufficient.27 In the present SQUID measurements, the samples are evacuated sufficiently, and no such peak was discernible for the MnIII(L-t-Bu)(OTf) solid under exactly the same measurement conditions. It is thus indicated that the MnIII(L-t-Bu)(CN) and Bu4N+ [MnIII(L-t-Bu)(CN)2]− solid has affinity for dioxygen. One of the most prominent features of the S = 1 [MnIII(L-tBu)(CN)2]− complex is characteristic visible absorptions in the range of 450−650 nm (Figure 6). In order to gain understanding of the electronic structure of [MnIII(L-tBu)(CN)2]−, substituent effects on visible absorptions were investigated using L-OMe, L-t-Bu, L-Cl, and L-NO2 (Chart 1). As shown in Figure 9a, the absorption at 465 nm in [MnIII(L-tBu)(CN)2]− (denoted by a black line in Figure 9a) is not altered, changing the substituent from t-Bu to Cl in [MnIII(LCl)(CN)2]− (denoted by the blue line in Figure 9a), but is shifted to longer wavelength at 493 nm changing the substituent from t-Bu to MeO in [MnIII(L-OMe)(CN)2]−

(denoted by the red line in Figure 9a). The [MnIII(LNO2)(CN)2]− complex (magenta line) shows an absorption of three-times-higher intensity at 445 nm. Interestingly, the absorption at 365 nm in [MnIII(L-t-Bu)(CN)2]− also shows exactly the same trend of absorption shifts upon the exchange of the substituent. But the absorption features at ∼600 nm show a different trend, indicating the origin of the absorption at ∼600 nm is different from the absorptions at 365 and 465 nm. Figure 9b shows the absorption spectra of the metal-free salen ligands. The L-t-Bu ligand (denoted by the black line in Figure 9b) shows an absorption at 328 nm, which was previously assigned as arising from the imino groups of a L-t-Bu ligand. The absorption at 328 nm in L-t-Bu is only slightly altered to 332 nm upon changing the substituent from t-Bu to Cl in L-Cl (denoted by the blue line in Figure 9b), but is substantially shifted to longer wavelength at 348 nm upon changing the substituent from t-Bu to OMe in L-OMe (represented by the red line in Figure 9b). The L-NO2 ligand (shown as the magenta line in Figure 9b) shows an absorption of three-times-higher intensity at 330 nm, along with the shoulder peak at 410 nm. It is quite interesting to note that the absorption shifts of [MnIII(L-t-Bu)(CN)2]− at 365 and 465 nm upon the exchange of the substituent are exactly the same as the shifts of the imino transition at 328 nm for L-t-Bu. This observation implies that the absorptions at 365 and 465 nm for [MnIII(L-t-Bu)(CN)2]− both comes from the imino group of the L-t-Bu ligand. It is reasonable to assign one of the absorptions at 365 nm in [MnIII(L-t-Bu)(CN)2]− as arising from the L-t-Bu ligand, because the L-t-Bu ligand without metal ion shows an absorption at a similar wavelength (328 nm). However, the other absorption at 465 nm in [MnIII(L-t-Bu)(CN)2]−, which is also suggested as arising from the imino group, is significantly shifted. The absorption shift to lower energy observed for one of the imino moieties suggests that the electronic structure of the imino group is altered upon the formation of [MnIII(L-tBu)(CN)2]−. To further confirm the assignment, solvent effects were investigated for [MnIII(L-Cl)(CN)2]− (see Figure S23 in the Supporting Information). Upon changing the solvent from CH3CH2CN to noncoordinating CH2Cl2 or protic CH3OH, the absorptions at 365 and 465 nm are not shifted at all, while the absorption feature at ∼600 nm is varied, depending on the solvent. This result is consistent with the above assignment that the absorptions at 365 and 465 nm are ligand-centered transitions, not charge-transfer transitions. A resonance Raman spectrum of [MnIII(L-t-Bu)(CN)2]− was measured at the excitation wavelength of 488 nm (Figure 10), which selectively excites one of two ligand-centered transitions of [MnIII(L-t-Bu)(CN)2]− observed at 465 nm. As shown by the red line in Figure 10, the [MnIII(L-t-Bu)(CN)2]− complex shows resonance Raman bands at 1597 and 1524 nm, which were unambiguously assigned as arising from the imino and phenolate groups, respectively, by the 2H-labeling experiments (see the Supporting Information for details). The starting MnIII(L-t-Bu)(OTf) complex shows resonance Raman bands (blue line) from the imino and phenolate groups at 1620 and 1536 nm, respectively, which were also assigned by the 2Hlabeling experiments. Importantly, the resonance Raman band from the imino group shows the most remarkable shift from 1620 nm to 1597 nm. The shift to lower wavelength is indicative of lower double bond character for the imino group in [MnIII(L-t-Bu)(CN)2]−, compared to the starting MnIII(L-t-

Figure 9. (a) Absorption spectra of [MnIII(salen)(CN)2]− in CH3CH2CH2CN at 173 K (0.5 mM, 0.1 cm cell). The [MnIII(salen)(CN)2]− complexes were prepared by the reaction of MnIII(salen)(OTf) with 50 equiv of Bu4NCN. (b) Absorption spectra of salen ligands in CH3CN at 293 K (0.02 mM, 1 cm cell). G


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Inorganic Chemistry

Chart 3. MnIII(salen)(OTf) Complexes for the Assignment of the H and Me 1H NMR Signals

Figure 10. line) and wavelength The bands solvent.

Resonance Raman spectra of MnIII(L-t-Bu)(OTf) (blue [MnIII(L-t-Bu)(CN)2]− (red line) at the excitation of 488 nm in CH3CN at room temperature (5 mM). denoted with an asterisk (*) come from the CH3CN

Bu)(OTf). The observed deformation of a L-t-Bu ligand is a clear indication of an increased electron population on the imino π*-orbital. Then, the L-t-Bu ligand accommodates negative charge from low-spin manganese(III) to stabilize [MnIII(L-t-Bu)(CN)2]−. The [MnIII(L-t-Bu)(CN) 2]− complex shows a single resonance Raman band at 1597 cm−1, while the resonance Raman band at 1620 nm from the starting MnIII(L-t-Bu)(OTf) complex disappears. In contrast, an IR spectrum of [MnIII(L-tBu)(CN)2]− shows two IR bands at 1595 and 1608 cm−1 and the IR band at 1608 cm−1 comes from the starting MnIII(L-tBu)(OTf) complex (Figure S24). According to these observations, selective excitation of the 465 nm absorption in [MnIII(L-t-Bu)(CN)2]− results in a single resonance Raman band at 1597 cm−1. 1 H NMR spectroscopy was utilized to investigate the structural change of the imino group upon the formation of [MnIII(L-t-Bu)(CN)2]−. For this purpose, 1H NMR shifts of all the protons attached to the phenolate ring and the imino carbon were unambiguously determined (see the Supporting Information for details). To elucidate a paramagnetic shift pathway, a methyl substituent was incorporated to the phenolate or imino carbon, and their 1H NMR shifts were also determined (see Chart 3). Figure 11 shows 1H NMR shifts of proton and methyl groups in MnIII(salen)(OTf) and [MnIII(salen)(CN)2]−. In the case of MnIII(salen)(OTf), 1H NMR shifts of a proton always show an opposite sign to the 1H NMR shifts of a methyl group attached to the same position. The same is true for 1H NMR shifts of a proton and a methyl group attached to the phenolate ring (3,3′, 4,4′, 5,5′, and 6,6′ positions) in [MnIII(salen)(CN)2]−. Such an alternating shift pattern for a proton and a methyl group attached to the aromatic carbon is commonly observed, and this is known as spin polarization. However, only the proton and the methyl group attached to the imino carbon (8,8′ position) in [MnIII(salen)(CN)2]− show a negative sign in both cases: −49.1 ppm for the proton and −40.8 ppm for the methyl group. 1H NMR measurements indicate the anomaly at the imino group of a salen ligand.

Figure 11. 1H or 2H NMR shifts (243 K) of H and Me groups attached to the phenolate (positions 3−6) and imino carbons (position 8) in MnIII(salen)(OTf) and [MnIII(salen)(CN)2]−.

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DISCUSSION One of the points to be discussed is the origin of the drastic redox shift of −1.2 V for MnIII(salen)(OTf) upon reaction with hydroxide or cyanide ion. It was previously reported that the coordination of hydroxide ion to manganese(III) indeed lowers the redox of manganese(III), resulting in facile oxidation to H


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Inorganic Chemistry manganese(IV).28,29 According to the study reported by Busch and co-workers, the Mn(III)/Mn(IV) redox shows a moderate change (−0.34 V) upon displacement of Cl to N3 as external ligands, but shows a significant change (−0.84 V) upon displacement of Cl to OH.28 Anxolabéhère-Mallart et al. reported a similar observation, where the displacement of Cl to OH or MeO as an external ligand to manganese(III) induces the same redox change (−0.30 V).29 Borovik and co-workers reported the only example of a manganese(III)-oxo complex, a deprotonated product of a manganese(III)-hydroxide,30,31 which shows a surprisingly lowered Mn(III)/Mn(IV) redox at −1.0 V vs Fc−Fc+.32 The present study shows that both hydroxide and cyanide ions induce a similar redox shift, indicative of the importance of the σ-donation ability. An electronic configuration of d4 manganese(III) ion may also be a factor to alter the redox of manganese(III). The manganese(III) ion almost always adopts a high-spin S = 2 configuration in solution. Examples of a low-spin S = 1 manganese(III) in solution are limited. A [MnIII(CN)6]3− ion is a classic example.33 A macrocyclic porphyrin ligand having a planar N4 coordination site is another example to accommodate low-spin S = 1 manganese(III) upon binding of two cyanide or imidazolate anions as external axial ligands.34,35 The latest example is manganese(III)−poly(pyrazolyl)borate complexes, in which two tridentate poly(pyrazoly)borate monoanionic ligands bind to manganese(III), as reported by Schultz and coworkers.36,37 Schultz, Baik, and co-workers investigated the role of a low-spin S = 1 configuration in a stepwise Mn(II)/ Mn(III)/Mn(IV) electron transfer.37 But the Mn(III)/Mn(IV) redox potentials are seemingly not significantly altered for their low-spin S = 1 manganese(III) complexes (0.96−1.30 V vs Fc− Fc+). According to their study, the spin-state change would not be a dominant factor for the redox of manganese(III). Then, it is proposed that the present drastic redox shift comes from the coordination of strong σ-donor ligands to manganese(III) as well as the anionic nature of the [MnIII(salen)(X)2]− complex. The other important point to be discussed is the structural change of a salen ligand upon formation of low-spin manganese(III). Resonance Raman spectroscopy revealed lowered double bond character for the imino group upon the formation of [MnIII(salen)(CN)2]−, which is a clear indication of a transfer of electron population from low-spin manganese(III) to the imino π* orbital. The relevant system that has been studied most extensively is bipyridine complexes. One of classic examples is a molybdenum(II) bipyridine complex, in which the bipyridine ligand shows a significant deformation as clarified by X-ray diffraction.38 The deformation of the bipyridine was interpreted as the π-back-donation from molybdenum(II) to the bipyridine, which is now found to be incorrect. Recent extensive studies using cutting-edge experimental and theoretical techniques have clearly shown that bipyridines work as a single-electron acceptor to generate bipyridine anion radicals, not as a π-acceptor to accommodate electrons from low-valent metal ion by means of π-back-donation.39−52 One of the key observations is that metric structural data of the bipyridine ligand in highly reduced metal complexes show very similar deformation, irrespective of metal ions, including alkali metal and transition metal ions of a different d n electron configuration, indicative of a distinct one-electron reduced state for the bipyridine ligand, as independently shown by Goicoechea39−41,46 and Wieghardt.42−45,47−52 Another point discussed by Scarborough and Wieghardt is that bipyridines are only weak π-acceptors, because the LUMO of bipyridines is

much higher in energy than the highest-filled metal d-orbital.43 Then, the π-back-donation from the metal to the bipyridine ligand cannot play a significant role in metal complexes with bipyridines. The π-accepting property of the imino group in a salen ligand should be even weaker than that of bipyridines. It was also shown that the one-electron reduction of the imino group in salen complexes requires highly reducing conditions.53−55 In addition, the deformation of bipyridine ligands was previously induced by the interaction with low-valent metal ions such as manganese(I).46 Thus, it is rather unexpected to see the increased electron population on the imino π*-orbital by the interaction with manganese(III) ion. Then, the pathway for the transfer of electron population is discussed on the basis of the present spectroscopic data. The binding of two σ-donor ligands to the high-spin S = 2 manganese(III) of a (dxy)1(dxz)1(dyz)1(dz2)1 configuration in MnIII(salen)(OTf) destabilizes the dz2 orbital, giving a low-spin S = 1 manganese(III) of a (dxy)2(dxz)1(dyz)1 configuration. Similar to the previous discussion on the valence tautomerism from NiII(salen+ •) to NiIII(salen) upon binding of two pyridines,56 the binding of two CN− or OH− ligands to manganese(III) partly triggers the electron transfer from manganese(III) to the imino π*-orbital, which accompanies a characteristic visible absorption that is assigned as arising from the ligand-centered transition. This could be interpreted as either π-back-donation or electron transfer, from low-spin manganese(III) to the imino group. An interesting experimental observation regarding π-backdonation or electron transfer is the 1H NMR shifts of the proton and methyl groups attached to the sp2 carbon of MnIII(salen)(OTf) and [MnIII(salen)(CN)2]− (Figure 11). An anomaly was found for the imino carbon in [MnIII(salen)(CN)2]−, where the proton and methyl signals do not show an alternating shift pattern, in contrast to all of the other positions. This is indicative of a distribution of an unpaired electron on the imino carbon as a new paramagnetic center upon formation of [MnIII(salen)(CN)2]−. The methyl group acquires a larger paramagnetic shift from the unpaired electron on the imino carbon, because of a hyperconjugative interaction between the pπ orbital of the imino carbon and the C−H bond of the methyl group. A hyperconjugative interaction is not possible for the proton attached to the imino carbon. Then, the unpaired electron on the imino carbon induces a different shift pattern that is specific at this position, in contrast to the other positions that are separated by more than one carbon from the unpaired electron. The distribution of a distinct unpaired electron on the imino carbon might be more consistent with the electron transfer mechanism, as shown in Scheme 1. The electron population on the imino π*-orbital as a consequence of π-backScheme 1. Proposed Electronic Structure

I


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donation has a d-electron nature and then would not alter 1H NMR shifts as an independent paramagnetic center. The formation of an anion radical on the imino group might be also consistent with the affinity for dioxygen, which was found for the six-coordinate Bu4N+ [MnIII(L-t-Bu)(CN)2]− solid by SQUID measurements (Figure 8). The magnetic susceptibility measurements for Bu4N+ [MnIII(L-t-Bu)(CN)2]− shows the μeff values of ∼3.00μB (S = 1) in a wide temperature range from 50 K to 340 K (Figure 8), which could be explained by an antiferromagnetic coupling between S = 3/2 manganese(IV) and a S = 1/2 anion radical spin on the ligand. According to the previous X-ray structure of the MnIII(salen)(CN) coordination polymer,25 the C−N bond distance of the imino group is almost the same for the [MnIII(salen)(CN)2]− unit (1.294 Å) and the [MnIII(salen)]+ unit (1.302 Å). But the Mn− O and Mn−N bonds of the [MnIII(salen)(CN)2]− unit (1.877 and 1.925 Å, respectively) are substantially shorter than those of the [MnIII(salen)]+ unit (1.897 and 1.958 Å, respectively). The shorter bond distances between the donor atoms of salen and manganese ion in the [MnIII(salen)(CN)2]− unit might be explained by an increased negative charge of the salen ligand as a consequence of the partial intramolecular electron transfer from manganese(III) to the salen ligand. One-electron donating properties of a salen ligand are well-known,15−17,57 and, in some cases, the degree of the resulting cation radical character was discussed theoretically58 and experimentally.26 However, the one-electron accepting properties of a salen ligand have been totally unknown. The process proposed in Scheme 1 is a new type of valence tautomerism, in which binding of OH−, CN−, and CH3O− to low-valency metal ion induces partial intramolecular electron transfer from metal ion to a ligand. It is expected that increasing the electron-accepting properties of a ligand may enhance valence tautomerism, leading to higher anion radical character of a ligand. Valence tautomerism has attracted renewed interest very recently. One example is manganese(V)-oxo species with a porphyrinoid ligand, which were shown to be converted to manganese(IV) complexes with a porphyrinoid cation radical upon the reaction with proton or Lewis acidic metal ion.59,60 Another example is a cytochrome P450 synthetic porphyrin model, which was reported to be a mixture of an iron(III)− thiolate and an iron(II)−thiyl radical.61 Both types of valence tautomerism have a significant impact on the reactivity of the metal complexes.

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EXPERIMENTAL SECTION

Instrumentation. Resonance Raman spectra were measured with an inVia Reflex laser Raman microscope (RENISHAW). Raman shifts were calibrated with single-crystal silicon (520.3 cm−1) for each measurement. Measurements were carried out for the CH3CN solution (5 mM) using the excitation wavelength of 488 nm and the laser power of 100% (ca. 2 mW at the sample). The sample solution in a glass tube (0.3 mL) was covered with a homemade quartz cap, to prevent evaporation of the solvent, and was placed on a microscope stage, using a homemade aluminum holder. A spectrum was obtained with an exposure time of 10 s. During one accumulation, the solution surface level was slightly lowered, because of evaporation during the laser irradiation. Then, an independent accumulation was repeated 10 times and the resulting 10 spectra were summed to give a final resonance Raman spectrum after subtraction of a baseline that was made using a decic polynomial function. IR spectra were measured under vacuum with an IFS66v/S FT-IR spectrometer (Bruker). IR spectra were obtained for KBr pellets at a resolution of 2 cm−1 as a sum of 32 scans. Typically, KBr pellets were prepared from a sample (1.50 mg) and KBr (150 mg), using a Tablet Master (Jasco). 1H and 2 H NMR spectra were measured in a borosilicate glass tube (5 mm OD) on a JNM-ECA600 600 MHz NMR spectrometer (JEOL). 1H and 2H NMR chemical shifts in CD3CN and CH3CN were referenced to CD2HCN and CDH2CN (1.93 ppm). Perpendicular- and parallelmode EPR spectra were recorded for 100 μL of the frozen CH3CH2CN solution (0.5 mM) in a quartz cell (5 mm OD) on an EMX Plus continuous wave X-band spectrometer (Bruker) with an ESR 910 helium-flow cryostat (Oxford Instruments) and a dual-mode cavity (Bruker). Measurement conditions are as follows: temperature, 4 K; microwave frequency, 9.675 GHz (perpendicular) or 9.479 GHz (parallel); microwave power, 20 mW; modulation amplitude, 10 G; modulation frequency, 100 kHz; time constant, 163.84 ms; and conversion time, 25.72 ms. Cyclic voltammograms were measured with a Model 2325 electrochemical analyzer (BAS) using an Ag/Ag+ reference electrode, a glassy-carbon working electrode and a platinumwire counter electrode. Measurements were carried out for the 0.5 mM solution in electrochemical-grade CH3CN containing 0.1 M Bu4NOTf at a scan rate of 50 mV s−1 at 243 K under an argon atmosphere, unless otherwise noted. The solution was deaerated by bubbling Ar gas at 243 K for 10 min. The E values were referenced to the E1/2 value of ferrocene, which was measured under identical conditions each time. Absorption spectra at low temperature were recorded in anhydrous CH3CN using a quartz cell (l = 0.1 cm) on an Agilent 8453 spectrometer (Agilent Technologies) that was equipped with an USP203 low-temperature chamber (UNISOKU). Absorption spectral measurements upon controlled-potential electrochemical oxidation were conducted at 193 K for the CH3CH2CN solution (0.5 mM) containing 0.1 M of Bu4NOTf in a quartz cell (l = 0.05 cm). To the thin-layer quartz cell, a gold-mesh working electrode, a platinum-wire counter electrode, and an Ag/AgCl reference electrode, which were connected to a potentiostat-galvanostat (Model 2325, BAS), were inserted. Gold mesh, silver rods, platinum wires, and sheets were obtained from Nilaco. Quartz cells (l = 0.1 and 0.05 cm), which are fitted for the low-temperature chamber, were custom-made by domestic glassware manufacturers (Eikosha and Agri). Elemental analyses were conducted on a Micro Corder JM10 (J-Science Lab). Solid-state magnetic susceptibility measurements were carried out using a MPMS-7 SQUID susceptometer (Quantum Design) operating in the temperature range of 2−340 K. Well-ground polycrystalline samples were wrapped in a plastic sheet and were loaded into the sample folder (a drinking straw). The susceptibility of the plastic sheet and the sample folder was measured in the same temperature range and field, to provide an accurate correction for its contribution to the total magnetic susceptibility. Prior to the measurement, the sample chamber was evacuated and refilled with helium 30 times. Diamagnetic corrections were estimated from Pascal constants.

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CONCLUSIONS The present study has shown that the reaction of a manganese(III) salen catalyst with hydroxide or cyanide ion gives a new manganese(III) species, which was characterized as a six-coordinate anionic low-spin [MnIII(salen)(X)2]− complex, where X is either OH− or CN−. The redox potential of a manganese(III) salen catalyst is drastically lowered by −1.2 V upon the formation of anionic [MnIII(salen)(OH)2]−, which is a valuable finding for future endeavors to investigate an energetically favorable route to high-valency manganese-oxo active oxidant. Most importantly, the present study reveals that the imino groups in a salen complex functions as a reservoir for negative charge from low-spin manganese(III) to stabilize anionic [MnIII(salen)(X)2]−. Modulation of a ligand structure is now underway to use the present findings for the design of a new redox catalyst. J


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dithiolato) Copper Dianionic Complex and Redox Switch by O2/ HO(−). Inorg. Chem. 2014, 53, 12799−12808. (11) Knör, G.; Vogler, A.; Roffia, S.; Paolucci, F.; Balzani, V. Switchable Photoreduction Pathways of Antimony(V) Tetraphenylporphyrin. A Potential Multielectron Transfer Photosensitizer. Chem. Commun. 1996, 1643−1644. (12) Ertl, M.; Wöß, E.; Knör, G. Antimony Porphyrins as Red-Light Powered Photocatalysts for Solar Fuel Production from Halide Solutions in the Presence of Air. Photochem. Photobiol. Sci. 2015, 14, 1826−1830. (13) Fukuzumi, S.; Yorisue, T. Quinone/Hydroxide Ion Induced Oxygenation of p-Benzoquinone to Rhodizonate Dianion (C6O62−) Accompanied by One-Electron Reduction to Semiquinone Radical Anion. J. Am. Chem. Soc. 1991, 113, 7764−7765. (14) Fukuzumi, S.; Nakanishi, I.; Maruta, J.; Yorisue, T.; Suenobu, T.; Itoh, S.; Arakawa, R.; Kadish, K. M. Formation of Radical Anions in the Reaction of p-Benzoquinone and C60 with Alkoxide Ions. J. Am. Chem. Soc. 1998, 120, 6673−6680. (15) Lyons, C. T.; Stack, T. D. P. Recent Advances in Phenoxyl Radical Complexes of Salen-Type Ligands as Mixed-Valent Galactose Oxidase Models. Coord. Chem. Rev. 2013, 257, 528−540. (16) Thomas, F. Ligand-centred oxidative chemistry in sterically hindered salen complexes: An interesting case with nickel. Dalton Trans. 2016, 45, 10866−10877. (17) Clarke, R. M.; Herasymchuk, K.; Storr, T. Electronic structure elucidation in oxidized metal−salen complexes. Coord. Chem. Rev. 2017, 352, 67−82. (18) Kurahashi, T.; Fujii, H. Comparative Spectroscopic Studies of Iron(III) and Manganese(III) Salen Complexes Having a Weakly Coordinating Triflate Axial Ligand. Bull. Chem. Soc. Jpn. 2012, 85, 940−947. (19) Kurahashi, T.; Fujii, H. One-Electron Oxidation of Electronically Diverse Manganese(III) and Nickel(II) Salen Complexes: Transition from Localized to Delocalized Mixed-Valence Ligand Radicals. J. Am. Chem. Soc. 2011, 133, 8307−8316. (20) Kurahashi, T.; Hada, M.; Fujii, H. Di-mu-oxo Dimetal Core of Mn(IV) and Ti(IV) as a Linker Between Two Chiral Salen Complexes Leading to the Stereoselective Formation of Different M- and PHelical Structures. Inorg. Chem. 2014, 53, 1070−1079. (21) Campbell, K. A.; Lashley, M. R.; Wyatt, J. K.; Nantz, M. H.; Britt, R. D. Dual-Mode EPR Study of Mn(III) Salen and the Mn(III) Salen-Catalyzed Epoxidation of cis-β-Methylstyrene. J. Am. Chem. Soc. 2001, 123, 5710−5719. (22) Campbell, K. A.; Yikilmaz, E.; Grant, C. V.; Gregor, W.; Miller, A.-F.; Britt, R. D. Parallel Polarization EPR Characterization of the Mn(III) Center of Oxidized Manganese Superoxide Dismutase. J. Am. Chem. Soc. 1999, 121, 4714−4715. (23) Kurahashi, T.; Hada, M.; Fujii, H. Critical Role of External Axial Ligands in Chirality Amplification of trans-Cyclohexane-1,2-diamine in Salen Complexes. J. Am. Chem. Soc. 2009, 131, 12394−12405. (24) Yin, G.; McCormick, J. M.; Buchalova, M.; Danby, A. M.; Rodgers, K.; Day, V. W.; Smith, K.; Perkins, C. M.; Kitko, D.; Carter, J. D.; Scheper, W. M.; Busch, D. H. Synthesis, characterization, and solution properties of a novel cross-bridged cyclam manganese(IV) complex having two terminal hydroxo ligands. Inorg. Chem. 2006, 45, 8052−8061. (25) Matsumoto, N.; Sunatsuki, Y.; Miyasaka, H.; Hashimoto, Y.; Luneau, D.; Tuchagues, J.-P. [{Mn(salen)CN}n]: The First OneDimensional Chain with Alternating High-Spin and Low-Spin MnIII Centers Exhibits Metamagnetism. Angew. Chem., Int. Ed. 1999, 38, 171−173. (26) Kurahashi, T.; Fujii, H. Unique Ligand-Radical Character of an Activated Cobalt Salen Catalyst That Is Generated by Aerobic Oxidation of a Cobalt(II) Salen Complex. Inorg. Chem. 2013, 52, 3908−3919. (27) Dubroca, T.; Hack, J.; Hummel, R. Comment on “Unusual Magnetic Transitions and Nature of Magnetic Resonance Spectra in Oxide Glasses Containing Gadolinium”. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 026403.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02474. Materials, preparations of manganese(III) salen complexes, preparations of [MnIII(salen)(X)2]− (X = CN−, OH−, CH3O−), assignments of resonance Raman bands from MnIII(salen)(OTf) and [MnIII(salen)(CN)2]−, assignments of 1H NMR signals from MnIII(salen)(OTf) and [MnIII(salen)(CN)2]−, magnetic susceptibility measurements by Evans method (Figures S10−S24) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected]. ORCID

Takuya Kurahashi: 0000-0002-9289-2397 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grant Nos. 23550086 and 15K05462). The financial support from the Naito Foundation is gratefully acknowledged. Spectroscopic and magnetic data in this paper were obtained using Instrument Center in Institute for Molecular Science, Okazaki, Japan.

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REFERENCES

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