Intermediate-Spin Iron(III) Complexes Having a Redox-Noninnocent

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Intermediate-Spin Iron(III) Complexes Having a Redox-Noninnocent Macrocyclic Tetraamido Ligand Takahiko Kojima,*,†,‡ Fumiya Ogishima,†,§ Takahisa Nishibu,†,§ Hiroaki Kotani,† Tomoya Ishizuka,† Toshihiro Okajima,⊥ Shunsuke Nozawa,∥ Yoshihito Shiota,¶ Kazunari Yoshizawa,‡,¶ Hiroyoshi Ohtsu,Δ Masaki Kawano,Δ Takuya Shiga,† and Hiroki Oshio† †

Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan ‡ CREST, Japan Science and Technology Agency, 4 Chome-1-8, Kawaguchi, Honcho, Saitama 332-0012, Japan ⊥ Kyushu Synchrotron Light Research Center, 8-7 Yayoigaoka, Tosu, Saga 841-0005, Japan ∥ Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan ¶ Institute for Materials Chemistry and Engineering, Kyushu University, Motooka, Nishi-Ku, Fukuoka 819-0395, Japan Δ Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan S Supporting Information *

ABSTRACT: An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM 4− ), (NEt4)2[FeIII(DTTM)Cl] (1), was synthesized and characterized by crystallographic, spectroscopic, and electrochemical methods. Complex 1 has a square-pyramidal structure in the S = 3/2 spin state. The complex exhibited two reversible redox waves at +0.36 and +0.68 V (vs SCE) in the cyclic voltammogram measured in CH2Cl2 at room temperature. The stepwise oxidation of 1 using chemical oxidants allowed us to observe clear and distinct spectral changes with distinct isosbestic points for each step, in which oxidation occurred at the phenylenediamido moiety rather than the iron center. One-electron oxidation of 1 by 1 equiv of [RuIII(bpy)3](ClO4)3 (bpy = 2,2′-bipyridine) in CH2Cl2 afforded square-pyramidal (NEt4)[Fe(DTTM)Cl] (2), which was in the S = 1 spin state involving a ligand radical and showed a slightly distorted square-pyramidal structure. Complex 2 showed an intervalence charge-transfer band at 900 nm, which was assigned on the basis of time-dependent density functional theory calculations, to indicate that the complex is in a class IIA mixed-valence ligand-radical regime with Hab = 884 cm−1. Two-electron oxidation of 1 by 2 equiv of [(4-Br-Ph)3N•+](SbCl6) in CH2Cl2 afforded two-electron-oxidized species of 1, [Fe(DTTM)Cl] (3), which was in the S = 1/2 spin state; complex 3 exhibited a distorted square-pyramidal structure. X-ray absorption near-edge structure spectra of 1−3 were measured in both CH3CN solutions and BN pellets to observe comparable rising-edge energies for the three complexes, and Mössbauer spectra of 1−3 showed almost identical isomer shifts and quadruple splitting parameters, indicating that the iron centers of the three complexes are intact to be in the intermediate-spin iron(III) state. Thus, in complexes 2 and 3, it is evident that antiferromagnetic coupling is operating between the unpaired electron(s) of the ligand radical(s) and those of the iron(III) center.

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INTRODUCTION Transition-metal complexes bearing redox-active organic ligands have been of great interest in coordination chemistry related to molecular magnetism as well as in bioinorganic chemistry related to the cooperativity of metal centers and © XXXX American Chemical Society

Special Issue: Applications of Metal Complexes with LigandCentered Radicals Received: January 5, 2018

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DOI: 10.1021/acs.inorgchem.8b00037 Inorg. Chem. XXXX, XXX, XXX−XXX

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RESULTS Synthesis of H4DTTM. The synthesis of H4DTTM was conducted through a one-pot template reaction between [Ni(1,2-diaminobenzene)3]2+ and diethylmalonyl dichloride in CH3CN in the presence of triethylamine under argon at 0 °C and just washing with water to remove nickel(II) ions (Scheme 1). The crude pale-yellow solid was washed with methanol,

ligands in oxidation reactions, as observed for metalloenzymes. Transition-metal complexes with radical ligands have been demonstrated to act as single-molecule magnets in the solid state.1 Metal-bound organic radical ligands can contribute not only to an increase in the total spins of the complexes but also to an increase in the magnitude of the magnetic exchange coupling, leading to higher blocking temperatures for singlemolecule magnets.1 In metalloenzymes, for example, galactose oxidase, in which a copper(II) center is ligated by a phenolate residue derived from tyrosine, undergoes one-electron oxidation to form a copper(II)-bound phenoxyl radical as the reactive species.2 Studies on metal phenoxyl radical complexes have been performed as mimics of the reactive intermediate of galactose oxidase and related enzymes to shed light on the characteristics of reactive species bearing radical character for ligands. 3 As for heme enzymes including horseradish peroxidase4a,b and cytochrome P450,4c,d dioxygen activation at the iron center affords iron(IV) oxo complexes, so-called “compound I”, in which the porphyrin ligand is one-electronoxidized as a “porphyrin π-radical cation”. Modeling studies on compound I have been widely made using synthetic iron porphyrin complexes to generate iron(IV) oxoporphyrin πradical-cation complexes.5 In addition to dianionic porphyrins, trianionic corroles that are porphyrinoids have also been reported to be one-electron-oxidized to form as anion-radical ligands.6 Wieghardt and co-workers have intensively investigated transition-metal complexes having redox-noninnocent ligands such as O,O′-coordinated o-benzosemiquinonate(1−), N,N′coordinated o-iminobenzosemiquinonate(1−), and S,S′-coordinated o-dithiobenzosemiquinonate(1−) π radicals.7 In those complexes, more than two radical moieties can bind to one metal center; when radical ligands coordinate to a paramagnetic metal ion such as iron(III) and chromium(III), the unpaired electrons of the radical ligands antiferromagnetically couple with those of the metal center.8 Polyanionic chelating ligands including deprotonated amide moieties have been utilized to stabilize high-valent transitionmetal complexes, and their redox properties have been studied to shed light on their reactivity in oxidation reactions. Margerum and co-workers have reported various linear diand tripeptide ligands to stabilize copper(III) and nickel(III) species, whose properties and reactivity have been studied in terms of the redox properties, self-decomposition, and electrontransfer reactions.9 On the other hand, Collins and co-workers have reported on the synthesis and characterization of highvalent transition-metal complexes having tetraamide macrocyclic ligands, including a phenylenediamidate moiety10 and their reactivity in peroxidase-mimicking H2O2 activation,11 water oxidation,12 and oxidation of organic compounds.13 However, the redox-active character of the phenylenediamidate-based polyanionic ligands has yet to be explored. Herein, we report the synthesis and characterization of iron complexes having tetraanionic 6,7,13,14-dibenzo-2,4,9,11-tetraoxo-3,3,10,10-tetraethyl-1,5,8,12-tetraazatetradecane (DTTM4−), which can be prepared by a one-pot template reaction using [Ni(1,2-diaminobenzene)3]2+ and diethylmalonyl dichloride, as a ligand. An iron(III) complex, (NEt4)2[FeIII(DTTM)Cl] (1), showed a reversible two-step oxidation, and one- and two-electron-oxidized species of 1 were also isolated and characterized by crystallographic and spectroscopic analyses to discuss the electronic structures.

Scheme 1. Template Synthesis of H4DTTM

followed by diethyl ether to obtain pure H4DTTM as a white solid in 51% yield. Although these types of template reactions have been reported to synthesize tetraamide macrocycles, this method was demonstrated to be effective for the synthesis of macrocyclic tetraamide compounds. As a byproduct, we isolated a 1:1 cyclic diamide compound, 2,3-benzo-5,7-dioxo6,6-diethyl-1,4-diazaheptane, from the filtrate obtained by washing the pale-yellow crude product, and the structure was determined by X-ray crystallography (Figure S1). Synthesis and Characterization of (NEt4)2[FeIII(DTTM)Cl] (1). Complex 1 was prepared as follows: DTTM4−, which was formed by the deprotonation of H4DTTM by Na[N(SiMe3)2], reacted with anhydrous FeCl2 in anhydrous dimethylformamide (DMF) under argon; the mixture was exposed to air to oxidize a precursor complex, and NEt4Cl in CH2Cl2 was added to the solution to supply countercations. The reaction mixture was extracted with CH2Cl2 to afford red solids of 1 after removal of the solvent. Recrystallization of the red solid sample from CH3CN with vapor deposition of hexane gave a single crystal suitable for an X-ray diffraction study. The crystal structure of the dianion moiety of 1 on the basis of the diffraction data obtained at 120 K is depicted in Figure 1a with 50% probability thermal ellipsoids. Selected bond lengths (Å) are listed in Table 1. Complex 1 crystallized in the monoclinic space group of Cm, a symmetrical plane including the Fe−Cl bond and crossing the centers of the C1−C1′ and C7−C7′ bonds in the figure. In the course of structure refinement, we found disorder of the iron(III) ion and chloro ligand; the population of the iron(III) ion was optimized to be 0.84 for Fe1−Cl1 and 0.16 for Fe2−Cl2, and the drawing shown in Figure 1a was provided on the basis of the major part of the iron(III) center. The geometry around the Fe1 center in 1 is square-pyramidal with τ5 = 0.14 The iron(III) ion was placed at 0.37 Å above the basal plane consisting of four amido nitrogen atoms toward the chloro ligand. The Fe1−Cl1 bond length was determined to be 2.362(3) Å, and the bond lengths of Fe1−N1 and Fe1−N2 were 1.900(5) and 1.954(4) Å, respectively. Collins and co-workers have reported a derivative of 1 having four methyl groups in place of the ethyl groups in 1.15 In the methyl derivative, the Fe−Cl bond length was reported to be 2.3592(6) Å, which was comparable to that of the Fe1 part of 1; the Fe−N bond lengths of the methyl derivative ranged from 1.9068(17) to 1.9387(17) Å with an average of 1.922 Å,15 which was shorter than that (1.927 Å) of 1. B


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Figure 2. X-band ESR spectra of 0.1 mM 1 (a), 1.0 mM 2 (b), and 0.1 mM 3 (c) in frozen CH2Cl2 under helium. Conditions: (a) frequency 9.399 GHz, microwave power 4 mW, modulation 100 kHz, 8.00 G; (b) frequency 9.681 GHz, microwave power 3 mW, modulation 100 kHz, 10.00 G; (c) frequency 9.683 GHz, microwave power 5 mW, modulation 100 kHz, 10.00 G. For 2, the complex was generated by adding 1 equiv of [RuIII(bpy)3](ClO4)3 to a 1 mM solution of 1 in CH2Cl2. The spectra were measured at 5.0 K (a) and 3.8 K (b and c).

Figure 1. ORTEP drawings of the dianionic moiety of 1 (a), the monoanionic moiety of 2 (b), and 3 (c) with 50% probability thermal ellipsoids. The structures shown were determined at 120 K. Hydrogen atoms are omitted for clarity.

Table 1. Selected Bond Lengths (Å) of 1−3 at 120 K Fe1−Cl1 Fe1−N1 Fe1−N2 Fe1−N3 Fe1−N4 Fe2−Cl2 Fe2−N1 Fe2−N2 C1−N1 C6−N2 C10−N3 C15−N4

1a

2

3

2.362(3) 1.900(5) 1.954(4)

2.3260(7) 1.9057(17) 1.9071(17) 1.9035(17) 1.9026(17)

2.2409(4) 1.8917(10) 1.8867(11) 1.8842(10) 1.8958(10)

1.368(3) 1.373(3) 1.410(3) 1.421(3)

1.3652(16) 1.3649(16) 1.3648(16) 1.3658(16)

2.376(19) 2.051(7) 1.906(7) 1.426(7) 1.411(7)

a

The Fe1−Cl1 part is major (84%), and the Fe2−Cl2 part is minor (16%). Figure 3. Mössbauer spectra of 1 (a), 2 (b), and 3 (c) measured at 20 K.

Complex 1 exhibited an anisotropic electron-spin resonance (ESR) signal at g = 4.14 and 2.05 in CH2Cl2 at 5.0 K, as shown in Figure 2a, which was typical for the S = 3/2 spin state.16 The Mössbauer spectrum of 57Fe-labeled 1 measured at 20 K exhibited a doublet at δ = 0.236 mm s−1 (relative to iron steel) with a quadruple splitting of ΔEQ = 3.93 mm s−1, as shown in Figure 3a, supporting the intermediate-spin state.17,18 Collins and co-workers have reported an iron(III) complex with a macrocyclic tetraamido ligand with a 3,4-dimethoxy-phenylenediamido moiety to demonstrate the spin states of the

iron(III) centers to be in S = 3/2;18 for the complex, the Mössbauer parameters have been reported to be δ = 0.23 mm s−1 and ΔEQ = 3.72 mm s−1,18 which are almost identical with those of 1. Magnetic susceptibility measurements on a powdered sample of 1 were performed in the temperature range 1.8−300 K (Figure S2). The χmT value at 300 K of 1.863 emu mol−1 K is slightly smaller than the spin-only value expected for one iron(III) ion with an intermediate spin of S = C


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Inorganic Chemistry /2 (1.875 emu mol−1 K; g = 2.0). Lowering the temperature caused a decrease of the χmT value, gradually in the range of 20−300 K and then more abruptly at around 20 K, before reaching a value of 0.950 emu mol−1 K. The decrease above 20 K should be derived from temperature-independent paramagnetism, and the rapid decrease below 20 K can be understood to be due to intermolecular antiferromagnetic interactions. The iron valence state can be directly proven by the X-ray absorption near-edge structure (XANES) spectrum at Fe Kedge (1s → 4p). Both in the solid state and in a CH3CN solution, the rising edges of the main Fe 4p feature of 1 corresponded to the energy position of an iron(III) standard, [Fe(TMP)Cl] (TMP = tetramesitylporphyrinato), as shown in Figure S3. Redox Behavior of 1. The cyclic voltammogram (CV) of 1 was obtained in CH2Cl2 in the presence of 0.1 M [(nbutyl)4N]PF6 as an electrolyte at room temperature (Figure 4). 3

solids of 2. The crystal structure of 2 was determined by X-ray crystallography at 120 and 296 K. The crystal structure of 2 was determined in the monoclinic space group of C2/c at 120 K and C2 at 296 K. At 120 K, the molecular motions in the crystal were suppressed; as a result, a new mirror plane, passing through the nitrogen atom of one of the tetraethylammonium ions, emerged to double the unit cell volume. However, the intrinsic crystal structure and packing is intact between those measured at 120 and 296 K. An ORTEP drawing of the monoanionic moiety of 2 (120 K) is depicted in Figure 1b. Selected bond lengths (Å) determined at 120 K are listed in Table 1. The geometry of the iron center was square-pyramidal, and the τ5 value was 0.011.14 The iron center was located at 0.358 Å above the basal plane defined by the four amido nitrogen atoms toward the chloro ligand. The bond length of the Fe−Cl bond is 2.3260(7) Å, which is shorter than that in 1 [2.362(3) Å]. The Fe−N bond lengths were determined to be 1.9057(17) Å for Fe−N1, 1.9071(17) Å for Fe−N2, 1.9035(17) Å for Fe−N3, and 1.9026(17) Å for Fe−N4. The Fe−N bond lengths were significantly shortened for one of the phenylenediamido moieties. In the structure determined at 296 K, the bond lengths were not as changed, as can be seen in Figures S4a and S5. In order to elucidate the valence of the iron center in 2, we measured the XANES spectrum of 2 in the solid state and in CH3CN at room temperature (Figure S3). The rising-edge energy of the Fe 4p feature of 2 both in the solid state and in solution was essentially the same as that observed for 1, indicating that the iron center of 2 should be in the iron(III) oxidation state. The Mössbauer spectrum of 2 also showed a doublet with an isomer shift of 0.174 mm s−1 and a quadrupole splitting (ΔEQ) of 3.14 mm s−1, which were also essentially the same as those of 1, as shown in Figure 3. We thus conclude that the oxidation occurs at the ligand rather than at the iron center, i.e., [FeIII(DTTM4−)(Cl−)]2+/[FeIII(DTTM•3−)(Cl−)]+, in the first oxidation step. The X-band ESR spectrum of 2 in CH2Cl2 showed no significant signals, as depicted in Figure 2b, which was due to the rapid spin−spin relaxation between Fe3+ and anion-radical species. Note that a small signal at g = 4.25 was derived from a negligible amount of high-spin iron(III) (S = 5/2) impurity. In order to clarify the spin state of 2, we conducted the Evans method using 1H NMR spectroscopy in CDCl3/o-dichlorobenzene-d4 (10:3, v/v) at room temperature. The signal of tetramethylsilane in the mixed solvent was used to estimate the magnetic susceptibility of 2 as 2.78 μB (0.97 emu mol−1 K; Figure S6), which was close to the value expected for the spinonly value of the S = 1 state [2.83 μB (1.00 emu mol−1 K)].19 Thus, the spin state of 2 was assigned to S = 1, which should be derived from the antiferromagnetic coupling between unpaired electrons of the intermediate-spin iron(III) center (S = 3/2) and those of the monoanion radical (S = 1/2).20 The existence of the ligand radical was supported by the observation of an intervalence charge transfer (IVCT) band and density functional theory (DFT) calculations, as described below. Preparation and Characterization of the Two-Electron-Oxidized Species of 1 [[Fe(DTTM)Cl] (3)]. Twoelectron-oxidized species 3 was obtained by adding 1 equiv of [(4-Br-Ph)3N•+](SbCl6) to the CH2Cl2 solution of 2 at room temperature, as shown in Scheme 2. In the course of the oxidation of 2, we could observe a clear spectral change to exhibit a characteristic intense absorption band at 563 nm, together with a weak absorption band at 1080 nm,

Figure 4. CV of 1 in CH2Cl2 containing 0.1 M [(n-butyl)4N]PF6 as an electrolyte at room temperature.

In the CV, two reversible redox waves were observed at +0.36 and +0.68 V versus SCE. The peak separations of the redox waves were 135 mV for the first wave and 150 mV for the second. Although the electrochemical reversibility may be referred to as “quasi-reversible” for the two processes, the peak currents were constant during multiple scanning, and thus no electrochemical−chemical process occurs in the course of the two-step oxidation of 1. For each oxidation process observed in the CV of 1, we performed spectroscopic redox titration with the use of [RuIII(bpy)3](ClO4)3 for the first step and [(4-Br-Ph)3N•+](SbCl6) for the second step as oxidants. Complex 1 exhibited an absorption spectrum (thick black trace in Figure 5a) with an absorption maximum at 504 nm. In the first oxidation process, the absorption spectrum of 1 changed to show a broad absorption band at 900 nm, accompanying the rise at 456 nm because of the formation of [RuII(bpy)3]2+, shown in Figure 5a as the green trace. Further one-electron oxidation of the oneelectron-oxidized species by [(4-Br-Ph)3N•+](SbCl6) allowed us to observe a strong and characteristic absorption band at 563 nm and a decrease of the absorption at 900 nm together with a rise at 1080 nm, showing isosbestic points at 402, 504, 807, and 1020 nm, depicted in Figure 5b as the blue trace. Thus, the two-step oxidation of 1 affords two different oxidized species in the CH2Cl2 solution. Preparation and Characterization of the One-Electron-Oxidized Species of 1 [(NEt4)[Fe(DTTM)Cl] (2)]. The one-electron-oxidized species of 1 was prepared by the oxidation of 1 with 1 equiv of [RuIII(bpy)3](ClO4)3 in CH2Cl2 at room temperature (Scheme 2). Recrystallization of the crude product from chloroform with the vapor diffusion of hexane afforded pale-purple crystalline D


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Figure 5. Spectral change of 1 (7.68 × 10−5 M) in the first oxidation step (a) and that in the second oxidation step (b) at room temperature: solvent, CH2Cl2; oxidant, [RuIII(bpy)3](ClO4)3 for part a and [(4-Br-Ph)3N•+](SbCl6) for part b.

Scheme 2. Oxidation Reactions of 1a

a

Countercations (NEt4+) are omitted for 1 and 2.

at 0.414 Å above the basal mean plane consisting of the four amido nitrogen atoms toward the chloro ligand. The Fe−Cl bond length is 2.2409(4) Å, which is shorter than those of 1 [2.362(3) Å] and 2 [2.3260(7) Å], probably because of the stronger Lewis acidic character of the iron center caused by two-electron oxidation of the DTTM2− ligand to reduce the electron donation from the two phenylenediamido moieties as described below. The Fe−N bond lengths are also shortened to 1.8917(10) Å for Fe−N1, 1.8867(11) Å for Fe−N2, 1.8842(10) Å for Fe−N3, and 1.8958(10) Å for Fe−N4. The structure determined at 296 K was almost the same as that at

accompanying four isosbestic points at 402, 504, 807, and 1020 nm, as shown in Figure 5b. Complex 3 was isolated by recrystallization of a crude product, which was obtained by the reaction of 1 with 2 equiv of [(4-Br-Ph)3N•+](SbCl6) in CH2Cl2, from diethyl ether in 26% yield as black-purple crystals. The crystal structure of 3 was determined by X-ray crystallography at both 120 and 296 K. An ORTEP drawing of 3 (120 K) is displayed in Figure 1c. Selected bond lengths (Å) are listed in Table 1. Complex 3 was crystallized in the monoclinic space group P21/n and showed a distorted squarepyramidal structure with τ5 = 0.34.14 The iron center is located E


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Figure 6. Summary of the bond lengths (Å) for 1 (black), 2 (green), and 3 (blue) at 120 K; see Table 1. Note that the bond lengths of 1 were described for the major part (86%) of the two disordered structures.

DFT calculations on 2 and 3 were conducted to reveal the spin-density distribution in each complex. As depicted in Figure 7a, one o-phenylenediamido moiety has a large spin density

120 K without a change of the space group (Figures S4b and S8). In the ESR spectrum of 3 in CH2Cl2 at 3.8 K, an anisotropic spectrum was observed with g = 2.160, 2.069, and 2.026, as shown in Figure 2c, suggesting that the spin state of 3 should be S = 1/2. The Mössbauer spectrum of 3 at 20 K showed a doublet with δ = 0.121 mm s−1 and ΔEQ = 2.89 mm s−1, which are also essentially the same as those of 1 and 2, as depicted in Figure 3c. In the XANES spectra of 3 (Figure S3), the energy of the Fe 4p rising edges both in the solid state and in CH3CN solution also corresponded to that of the iron(III) standard, indicating that the oxidation state should be the same as those of 1 and 2, that is, trivalent iron(III). Thus, we concluded that the iron center of 3 should remain in the intermediate-spin iron(III) state both in the solid state and in solution, even after the two-electron oxidation of 1. Thus, we can conclude that the second oxidation also occurs on the ligand. Considering the spin state of 3 as S = 1/2, the two unpaired electrons of the twoelectron-oxidized DTTM2− ligand coupled with the intermediate-spin iron(III) center. A similar situation can be found for a square-pyramidal iron(III) bis(o-iminobenzosemiquionato) complex with an iodide ion as the axial ligand.21 Although the magnetic susceptibility measurement of 3 was performed, estimation of the J values was unsuccessful because of complicated intermolecular magnetic interactions. It is, however, noted that fairly strong π−π stacking of the radical ligands leads to strong intermolecular antiferromagnetic interactions (see Figure S9). Electronic Structures of 1−3. As described above, the iron centers of 1−3 should be in the intermediate-spin iron(III) state, and oxidation occurs at the ligand, i.e., at the two ophenylenediamido moieties. A close look at the crystal structures of 1−3 allowed us to find that the bond lengths of the phenylenediamido moieties change in accordance with the oxidation reactions. The bond lengths of the phenylenediamido moieties of 1−3 are summarized in Figure 6. As the ligand oxidation proceeded, the bond lengths of the C−N bonds of the o-phenylenediamido moieties were shortened. In Figure 6, the first oxidation afforded a shortening of the C−N bond on the left side, but those on the right side were not affected; the second oxidation resulted in a shortening of the C−N bonds on the right side, and all four C−N bonds were shortened to 1.365 Å. Such a shortening of the C−N bond should be due to the contribution of an o-diiminosemiquinonato resonance structure, as observed for o-diamidobenzene ligands in the course of oxidation of the nickel(II), palladium(II), and platinum(II) complexes22 and cobalt(III) complexes23 reported by Wieghardt and co-workers.

Figure 7. Spin-density distribution in 2 (a) and 3 (b), calculated at the B3LYP/6-311+G** level of theory. Red and purple surfaces represent the α and β spin densities, respectively.

distribution for the one-electron-oxidized species 2. As for the two-electron-oxidized species 3, both of the phenylenediamido moieties possess a large spin density, as shown in Figure 7b. DFT-optimized structures of 2 and 3 were scrutinized in comparison with the crystal structures. The changes of the bond lengths in the course of the oxidation reactions showed a similar tendency. As depicted in Figure S10, the optimized structure of 2 exhibited shortened C−N bonds for the left-hand side compared to those of the right-hand side, as found in the crystal structure of 2; the optimized structure of 3 showed shortened C−N bonds in both sides, as observed in the crystal structure of 3. The calculation results are fully consistent with the observations in the crystal structure analysis as mentioned above. The spin density at the iron(III) center was calculated to be 2.81 for 1, 2.45 for 2, and 2.65 for 3, indicating that all of the iron(III) centers should be in the S = 3/2 spin state. Considering the spin states of 1 (S = 3/2), 2 (S = 1), and 3 (S = 1/2), the unpaired electron delocalized on the o-phenylenediamido moiety should be antiferromagnetically coupled with those of the iron(III) center in the S = 3/2 spin state. We conducted SQUID experiments on 2 and 3 to evaluate the magnetic interactions between the iron(III) center and ligand radical(s); however, the magnetic susceptibility data did not allow us to elucidate the characteristics of the complexes including magnetic coupling constants, which might be due to complicated intermolecular magnetic interactions in the solid state, as shown in Figures S7 and S9. In light of all of the aforementioned considerations, we conclude that the stepwise oxidation reactions for 1 occur, as shown in Scheme 3. F


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Scheme 3. Schematic Description of Electronic Structure Change in the Course of Stepwise Oxidation of the DTTM4− Ligand

Figure 8. TD-DFT calculations for the assignments of absorptions observed for 2 (a) and 3 (b) at the CAM-B3LYP level of theory. Charge transfer occurs from the brown to purple regions.

moiety to a π* orbital of the one-electron-oxidized ophenylenediamido moiety, as depicted as no. 4 in Figure 8a: This transition can be ascribed to an IVCT. On the other hand, the absorption observed at 560 nm for 3 was attributed to an overlapped absorption including two LMCT bands; one is due to a transition from the Cl− ligand to the dz2 orbital (no. 16),

The absorptions observed for 2 and 3 (Figure 5) were analyzed using time-dependent density functional theory (TDDFT) calculations at the CAM-B3LYP level of theory. For 2, a broad absorption band was observed at 900 nm. The absorption was assigned to a transition from π-bonding orbitals of the amido parts of a nonoxidized o-phenylenediamido G


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(III) complexes bearing one-electron-oxidized salen ligands in solution; in the complexes, charge transfer has been proposed to occur from a phenolate moiety to a semiquinonato moiety intramolecularly.27 Nonplanarity of the Amide Moieties. The nonplanarity of the organic amido moieties of 1−3 was analyzed on the basis of the concept proposed by Dunitz and Winkler.29 Dunitz’s amide nonplanarity parameters are defined as shown in Chart 2,

and the other is due to a transition from the singly occupied molecular orbital of the one-electron-oxidized o-phenylenediamido moieties (no. 19) to a dπ orbital, as demonstrated in Figure 8b. In addition, the TD-DFT-optimized structures of 1− 3 showed a similar tendency in the change of the bond lengths, as depicted in Figure S12. Analysis of the Mixed-Valence State of 2. In the case of 2, the IVCT band due to the ligand radical was observed at 900 nm, as shown in Figure 5a, with an absorption coefficient of 1960 M−1 cm−1. The IVCT transition is elucidated to occur in the manner shown in Chart 1 on the basis of the results of the

Chart 2. Definition of Dunitz’s Amide Nonplanarity Parametersa

Chart 1. IVCT Transition in 2

a

TD-DFT calculations depicted in Figure 8a. The comproportionation constant (KC) of the ligand-centered mixed-valence state was determined to be 2.6 × 105 at 298 K on the basis of the cyclic coltammetry data presented in Figure 4, using the equation KC = 10ΔE/59,24 where ΔE (mV) is the difference between the first and second oxidation potentials, 320 mV for 1. The stability of the mixed-valence state of 2 was evaluated in light of the ΔG (= −RT ln KC) value as −30.9 kJ mol−1 at 298 K. Marcus−Hush analysis of the IVCT band for a class II mixed-valence system was made on the basis of eq 1:25−28 0 Δν1/2

1/2

= [16RT (ln 2)νmax ]

Δν01/2

and those of 1−3 are summarized in Table 2.30 The τ value represents the degree of twisting of out-of-plane deformation, and the χC and χN values represent the degree of bending of an amide moiety.29b Table 2. Dunitz’s Amide Nonplanarity Parameters (Chart 2) of 1−3 τ = (ω1 + ω2)/2 N1−C4−O1 N1′−C4′−O1′ N2′−C6′−O2′ N2−C6−O2

1/2

= (2310λ)

= (2310νmax )1/2

(1)

−1

where (cm ) is the bandwidth at half-height, R is the gas constant, T is the temperature (K), and νmax (cm−1) is the absorption maximum. The observed νmax was 1.1 × 104 cm−1 to afford the calculated Δν01/2 value as 1.0 × 103 cm−1. Because the observed Δν1/2 value is estimated to be 1.1 × 103 cm−1, which is almost consistent with the calculated value, the mixed-valence state of 2 is assigned to the partially delocalized class II mixedvalence state. In light of the Mulliken−Hush equation (eq 2) 0 1/2 Hab = 0.0206(εmax νmax Δν1/2 ) /rab

The circle represents an iron-bound amide nitrogen atom.

N1−C18−O1 N2−C7−O2 N3−C9−O3 N4−C16−O4 N1−C18−O1 N2−C7−O2 N3−C9−O3 N4−C16−O4

(2)

where Hab, εmax, and rab stand for the electronic coupling parameter, absorption coefficient (λ = 900 nm and ε = 1960 M−1 cm−1) of the IVCT band, and distance (Å) between the redox-active centers, respectively, and Δν01/2 can be defined by eq 2. On the basis of the crystal structure of 2, rab is estimated to be 7.69 Å as the distance between the centroids of the two benzene rings. Thus, the Hab value is calculated to be 884 cm−1 using eqs 1 and 2. On the basis of the results mentioned above, the 2Hab/λ value was calculated to be 0.16, which fell in the range of 0 < 2Hab/λ < 1 − Δν01/2/λ, where Δν01/2/λ = 0.46 for 2.28 The result indicates that complex 2 should be classified in class IIA. Kurahashi and Fujii have reported on mixed-valence ligand radicals categorized in the class II regime for manganese-

Complex 1 −143.61 143.61 153.13 −153.13 Complex 2 −137.39 138.20 156.50 −160.05 Complex 3 142.37 143.98 142.78 146.28

χN

χC

28.01 −28.01 −13.01 13.01

5.51 −5.51 −13.11 13.11

16.77 −16.18 −11.18 12.18

3.22 −3.17 −6.61 4.45

−12.11 −19.01 −5.67 −17.39

−2.32 −0.55 −4.67 2.50

The amide linkages in 1−3 showed nonplanarity, as represented by the averaged |τ| values: The order of the outof-plane deformation was 3 (143.85°) > 2 (148.03°) > 1 (148.37°). Thus, the nonplanar deformation is enlarged by oxidation of the o-phenylenediamidate moiety. In the course of the one-electron oxidation of 1, the out-of-plane deformation of the amide moiety is larger in 2 than in 1: The |τ| values of N1− C4−O1 and N1′−C4′−O1′ (143.61°) became smaller for N1− C18−O1 (137.39°) and N2−C7−O2 (138.20°), indicating that the amide linkage of the oxidized o-phenylenediamidate moiety shows larger deformation. This observation is consistent with the fact that oxidation occurs at the π-bonding orbitals, the highest occupied molecular orbital (HOMO) and HOMO−1, H


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Inorganic Chemistry

pale-orange solid quantitatively. The 57Fe-labeled sample of 1 was prepared using 57FeCl2 obtained in place of 56FeCl2. Synthesis of 6,7,13,14-Dibenzo-2,4,9,11-tetraoxo-3,3,10,10-tetraethyl-1,5,8,12-tetraazatetradecane (H4DTTM). Diethylmalonyl dichloride (7.5 mL, 4.7 × 10−2 mol) was added to the solution of nickel(II) tris(o-phenylenediamine)perchlorate (8.68 g, 1.6 × 10−2 mol) in acetonitrile (200 mL) containing triethylamine (7.8 mL, 5.6 × 10−2 mol) under argon. The solution was stirred for 12 h at 0 °C with an ice bath to form a blue-white precipitate, and then the precipitate was filtered and washed with water, ethanol, and diethyl ether. The solid was dried in vacuo to give a white powder of the compound (3.70 g, 7.96 × 10−3 mol) in 51% yield. 270 MHz 1H NMR (CDCl3): δ 0.94 (t, J = 7.4 Hz, 12H, CH3, H1), 1.84 (q, J = 7.3 Hz, 8H, CH2, H2), 6.98 (m, 4H, Ph, H3), 7.14 (m, 4H, Ph, H4), 8.01 (s, 4H, NH, H5). ESITOF-MS (MeOH, negative): m/z 463.14 (calcd for [H3DTTM]−: m/ z 463.23), 499.11 (calcd for [H4DTTM + Cl]−: m/z 499.21). Anal. Calcd for C26H32N4O4·0.25H2O: C, 66.58; H, 6.98; N, 11.94. Found: C, 66.61; H, 6.91; N, 12.13. Synthesis of (Et4N)2[FeIII(DTTM)Cl] (1). H4DTTM (581 mg, 8.2 × 10−4 mol) was dissolved in DMF (superdehydrated grade, 60 mL), and the solution was degassed by three freeze−pump−thaw cycles. To the solution was added sodium bis(trimethylsilyl)amide (920 mg, 5.02 × 10−3 mol) at room temperature under argon. The mixed solution was stirred for 10 min under argon. Then, FeIICl2 anhydrate (152 mg, 1.2 × 10−3 mol) was dissolved in dehydrated DMF (2.5 mL), and the solution was degassed by three freeze−pump−thaw cycles. The FeIICl2 solution was added to the mixed solution slowly under argon. The solution was stirred for 16 h at room temperature under argon. The solution was stirred for a further 24 h at room temperature with bubbling dried air. The solvent was removed in vacuo at 60 °C, and then a solution of tetraethylammonium chloride (303 mg, 1.83 × 10−3 mol) dissolved in dichloromethane (60 mL) was added. The solvent was extracted with dichloromethane to obtain a red solution. After removal of the solvent, the residue was dissolved in hot 1,2dichloroethane, and the solution was hot-filtered through filter paper. The filtrate was evaporated to dryness to give a red solid (179.7 mg, 2.2 × 10−4 mol) in 27% yield. ESI-TOF-MS (MeCN): m/z 516.11 (calcd for [FeIII(DTTM)]−: m/z 516.15). Anal. Calcd for C42H68O4N6ClFe·H2O: C, 60.75; H, 8.50; N, 10.12. Found: C, 60.83; H, 8.50; N, 10.28. Synthesis of the One-Electron-Oxidized Species of 1, [Fe(DTTM)(Cl)] (2). [RuIII(bpy)3](ClO4)3 (57 mg, 6.58 × 10−5 mol) was slowly added to a distilled dichloromethane solution of complex 1 (60.8 mg, 7.48 × 10−5 mol). The mixture was stirred for 30 min to a give blackyellow solution. After removal of the solvent, the residue was dissolved in chloroform and filtered. After evaporation of the solvent of the filtrate, the residue was recrystallized from dichloromethane/hexane to give a pale-purple solid (3.0 mg, 4.4 × 10−6 mol) of 2 in 6.7% yield. Anal. Calcd for C34H48O4N5ClFe·1/4H2O·1/4CH2Cl2: C, 58.12; H, 6.98; N, 9.89. Found: C, 58.05; H, 7.09; N, 9.88. Synthesis of the Two-Electron-Oxidized Species of 1, [Fe(DTTM)(Cl)] (3). Tris(4-bromophenyl)ammoniumyl hexachloroantimonate (205 mg, 2.51 × 10−4 mol) was slowly added to a distilled dichloromethane solution of complex 1 (101 mg, 1.24 × 10−4 mol). The mixture was stirred for 10 min to a give black-purple solution. After removal of the solvent, the residue was dissolved in diethyl ether and filtered. After evaporation of the solvent of the filtrate, the residue was recrystallized from benzene/hexane to obtain black-purple crystals (17.3 mg, 3.1 × 10−5 mol) of 3 in 26% yield. Anal. Calcd for C26H28O4N4ClFe: C, 56.45; H, 5.08; N, 10.03. Found: C, 56.59; H, 5.11; N, 10.15. X-ray Crystallography on 1−3. Recrystallization of 1 from an acetonitrile solution with vapor diffusion of hexane as a poor solvent gave single crystals of 1. Single crystals of 2 were obtained by recrystallization from its solution in acetonitrile with vapor diffusion of hexane as a poor solvent. The two-electron-oxidized complex 3 was recrystallized from its solution in benzene with vapor diffusion of hexane as a poor solvent. Single crystals of 1−3 were mounted on a mounting loop. All diffraction data were collected on a Bruker APEXII diffractometer equipped with a graphite-monochromated Mo Kα (λ =

including the amide moieties (see Figure S10), and the spindensity distribution in the one-electron-oxidized form shown in Figure 7a. The bending of the amide moieties is larger for 1 than for 2 and 3, as represented by the χN values: The order of the amide bending is 1 (9.31°) > 2 (4.36°) > 3 (2.51°). Thus, the out-of-plane deformation of the DTTM4− ligand is controlled by the bending of the coordinated amido nitrogen atoms to suppress bending of the geometry around the nitrogen atoms.

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CONCLUSIONS We have synthesized and characterized an intermediate-spin iron(III) complex (1) having a macrocyclic tetraamido ligand, DTTM4−, which includes two redox-noninnocent o-phenylenediamido moieties. Complex 1 exhibited clear two-step reversible redox processes in the CV in CH2Cl2. Chemical oxidation of 1 in CH2Cl2 forms one-electron-oxidized (2) and two-electron-oxidized (3) complexes, in which one and two of the two o-phenylenediamido moieties are oxidized, respectively, and the iron centers are intact to be trivalent in the S = 3/2 spin state, as evidenced by the XANES and Mössbauer spectra. The spin states of 2 and 3 were analyzed by ESR spectroscopy and the Evans method using 1H NMR spectroscopy to be S = 1 and 1 /2, respectively, indicating that the unpaired electrons of the ligand radicals are antiferromagnetically coupled with those of the intermediate-spin iron(III) centers. The oxidized species exhibited characteristic charge-transfer absorptions in the visible region, and the origins of the absorptions are assigned on the basis of TD-DFT calculations. Complex 2 was revealed to be in the partially delocalized class IIA mixed-valence ligand-radical state in light of Marcus−Hush and Mulliken−Hush analysis. This work provides a strong basis for a deeper understanding of the redox behavior and electronic structures of iron(III) complexes having redox-noninnocent ligands, which are related to metalloenzymes that form metal complexes bearing radical ligands as reactive intermediates and also magnetic interactions between the metal centers and ligand radicals. The results described here also raise caution about elucidation of the redox behavior of metal complexes bearing redox-noninnocent ligands; electron-transfer oxidation does not always occur at a redox-active metal center, even though the ligand is highly negatively charged and strongly electron-donating.

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EXPERIMENTAL SECTION

General Procedures. Chemicals and solvents were purchased from commercial sources and used as received unless otherwise mentioned. Acetonitrile and CH2Cl2 were distilled over CaH2 under argon. UV−vis absorption spectra were measured on a Shimadzu UV3600 spectrometer at room temperature. Electrospray ionization timeof-flight mass spectrometry (ESI-TOF-MS) spectra were measured on a JEOL JMS-T100CS AccuTOF CS spectrometer. ESR spectra were measured on a Bruker BioSpin EMX-Plus 9.5/2.7 spectrometer with a liquid-nitrogen or a liquid-helium transfer system under nonsaturating microwave power conditions in CH2Cl2 at 5 and 100 K. 1H NMR measurements were performed on a Bruker AVANCE400 spectrometer. Electrochemical measurements were carried out on an ALS/CH Instruments electrochemical analyzer model 710D. [Ni(ophenylenediamine)3](ClO4)2,31 diethylmalonyl dichloride,32 and [RuIII(bpy)3](ClO4)333 were prepared in accordance with the reported procedure. 57 FeCl2. 57Fe foil (0.185 g, 4.79 × 10−3 mol) was stirred in a 11 M HCl aqueous solution (15 mL) for 47 h at 60 °C until the 57Fe foil was completely dissolved. The solvent and HCl gas were removed in vacuo at 80 °C and then dried up in vacuo for 24 h at 130 °C to obtain the I


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Inorganic Chemistry 0.71073 Å) radiation source by the ω−2θ scan. The diffraction data of 1 were measured at 120 K, whereas those of 2 and 3 were obtained both at 120 K and at 296 K. The structures were solved by direct methods using the Yadokari-XG program package,34 including SHELX2014 and SHELX-2017.35 In the structure refinements, contributions of the solvent molecules of crystallization were subtracted from the diffraction data by the SQUEEZE program.36 During the course of the refinement of the structure of 1, we found disorder of iron(III) and the chloro ligand. Therefore, we refined with a disorder model for the Fe− Cl part on both sides of the ligand plane and for one of the tetraethylammonium cation parts to avoid collision with a disordered chlorine atom. The refined occupancy is 0.84 for the Fe1−Cl1 part (major) and 0.16 for the Fe2−Cl2 part (minor). Although the DTTM ligand does not show crystallographic disorder, the disordered Fe1 and Fe2 locate above and below the DTTM ligand; each iron center holds one chloro ligand in the opposite direction. Crystallographic data for 1−3 are summarized in Table S1, and CCDC 1568561 for 1, 1568562 for 2 at 120 K, 1568563 for 2 at 296 K, 1568564 for 3 at 120 K, and 1568565 for 3 at 296 K contain the supplementary crystallographic data. Mö ssbauer Spectroscopy. Mössbauer experiments were carried out using a 57Co/Rh source in a constant-acceleration transmission spectrometer (Topologic Systems) equipped with an Iwatani HE05/ CW404 cryostat. The spectrometer was calibrated using a standard αiron foil. The spectra were recorded at 20 and 290 K for all complexes. The samples of 1 for Mössbauer experiments were obtained using 57 Fe-enriched starting materials (96.06%); those of 2 and 3 were prepared using nonenriched 1 as the starting material. ESR Measurements. ESR spectra were recorded on a Bruker BioSpin X-band spectrometer (EMXPlus) with a liquid-nitrogen transfer system (ER-4131VT) in a quartz tube (o.d. = 5 mm) or an ESR900 helium-flow cryostat (Oxford Instruments) in a quartz tube (o.d. = 4 mm). X-band (9.68 GHz) ESR spectra were obtained using a dual-mode cavity (Bruker ER-4116DM) at 3.8 K. The magnitude of the modulation was chosen to optimize the resolution and the signalto-noise (S/N) ratio of the observed spectrum under nonsaturating microwave power conditions (modulation amplitude, 8−10 G; modulation frequency, 100 kHz). ESR samples of 1 (1.2 × 10−3 M) and 3 (4.5 × 10−5 M) were prepared by dissolving isolated crystalline powders in CH2Cl2. An ESR sample of 2 (1.2 × 10−3 M) was prepared by mixing 1 (1.2 × 10−3 M) and 1.0 equiv of [RuIII(bpy)3](ClO4)3 in CH2Cl2. These prepared solutions were transferred in quartz tubes under a helium atmosphere. Electrochemical Measurements. Cyclic voltammetry was performed on an ALS 710D electrochemical analyzer using a platinum electrode as the working electrode, Ag/AgCl as the reference electrode, and a platinum wire as the auxiliary electrode at room temperature. Magnetic Susceptibility Measurements. Direct-current magnetic susceptibility measurements of polycrystalline samples of 1−3 were measured in the temperature range of 1.8−300 K with a Quantum Design MPMS-5XL SQUID magnetometer under an applied magnetic field of 2 T. Data were corrected for the diamagnetic contribution calculated from Pascal’s constants including the contribution of the sample holder. XANES Measurements. XANES spectra of solid samples of 1−3 in BN pellets at the Fe K-edge were collected on Beamline BL06 at the SAGA Light Source (Tosu, Japan)37 with a transmission mode. Those in CH3CN solutions at the Fe K-edge were measured at Beamline BL9A of Photon Factory (proposal 2014S2-006) in the High Energy Accelerator Research Organization (Tsukuba, Japan) with a fluorescent method using a 19-element germanium solid-state array detector (CANBERRA, USA). DFT and TD-DFT Calculations. Energy calculations for 1−3 were carried out using unrestricted density functional theory (UDFT), implemented in Gaussian 09.38 Geometry optimizations were performed with the B3LYP method.39,40 The stability of the optimized geometries was confirmed with vibrational analyses, and no imaginary frequency was found. To calculate UV−vis spectra, TD-DFT

calculations41 were carried out using the CAM-B3LYP functional.42 For the iron atom, the (14s9p5d)/[9s5p3d] primitive set of Wachters−Hay43,44 with one polarization f function (α = 1.05)45 was used, and for the hydrogen, carbon, nitrogen, oxygen, and chlorine atoms, the 6-311+G** basis set46 was used.

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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.8b00037. Summary of crystallographic data for 1−3 and 2,3-benzo5,7-dioxo-6,6-diethyl-1,4-diazaheptane, an ORTEP drawing of 2,3-benzo-5,7-dioxo-6,6-diethyl-1,4-diazaheptane, a magnetic susceptibility measurement plot for 1, XANES spectra of 1−3, a 1H NMR spectrum for the Evans method, ORTEP plots of 2 and 3 obtained at 296 K, HOMO and HOMO−1 of 1, comparisons of the crystal structures of 2 and 3 between 120 and 296 K, and Cartesian coordinates obtained by DFT calculations (PDF) Accession Codes

CCDC 1568560−1568565 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takahiko Kojima: 0000-0001-9941-8375 Hiroaki Kotani: 0000-0001-7737-026X Tomoya Ishizuka: 0000-0002-3897-026X Kazunari Yoshizawa: 0000-0002-6279-9722 Masaki Kawano: 0000-0001-9886-4226 Hiroki Oshio: 0000-0002-4682-4705 Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI (Grants 24245011, 26620038, and 17H03027 to T.K., Grant 15H00890 to S.N., Grant 25248014 to H.O., and Grant 26410065 to T.S.).

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REFERENCES

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