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Article

Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands

by
Kazuyuki Takahashi
*,
Takumi Noguchi
,
Keiji Ueda
,
Atsuhiro Miyawaki
and
Suguru Murata
Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan
*
Author to whom correspondence should be addressed.
Inorganics 2022, 10(6), 84; https://doi.org/10.3390/inorganics10060084
Submission received: 4 April 2022 / Revised: 9 June 2022 / Accepted: 14 June 2022 / Published: 16 June 2022
(This article belongs to the Special Issue Redox-Active Ligand Complexes)
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Abstract

:
To elucidate the oxidation behavior of the 2,2′-azobisphenolate (azp) ligand, a series of homoleptic 1:2 AlIII complexes of four azp derivatives (L1) with 5,5′-dichloro-, 5,5′-dimethyl-, 5,5′-di-t-butyl-, 3,3′,5,5′-tetramethyl-substituents and of one imino derivative (L2) were synthesized and obtained as TPP[Al(L)2]·solvent (TPP = tetraphenylphosphonium ion). The X-ray crystal structure analyses showed that the two ONO-tridentate ligands were meridionally coordinated to a central AlIII ion in an almost perpendicular manner to give a homoleptic octahedral coordination structure in all the AlIII complexes. The proton nuclear magnetic resonance spectra suggested that all the AlIII complexes retained the homoleptic coordination structure in solution. From the cyclic voltammetry measurements in dichloromethane solutions, all the AlIII complexes with the azp ligands showed two partially reversible oxidation waves, and an additional reversible or partially reversible reduction wave. The substitution effects on the first oxidation and reduction peak potentials were revealed in the AlIII complexes with the azp ligands. On the other hand, the imino complex showed a partially reversible oxidation wave accompanying a film deposition. The density functional theory (DFT) calculations indicated that the molecular orbital (MO) coefficients of the frontier MOs in the AlIII complexes were present on the ligands and were absent on the AlIII ion. These results confirmed that the azp ligands are susceptible to oxidation and can give a relatively stable oxidation species depending upon substituent effects.

Graphical Abstract

1. Introduction

Redox-active ligands are relatively easily oxidized or reduced molecules that can be coordinated with various metal ions. The electronic state of redox-active ligands can be switched by not only an electric potential but also by interactions with chemical substances. In particular, the interactions with a coordinated metal ion can create various exotic phenomena such as valence tautomerism in metal dioxolene complexes [1], electron-transfer-induced ferromagnetism in paddle-wheel ruthenium complexes with tetracyanoquinodimethane (TCNQ) derivatives [2], and extremely stable metallic state and various metal-insulator transitions in copper N,N′-dicyanoquinodiimines (DCNQIs) complexes [3]. These intriguing phenomena originate from either charge transfer, electron transfer, or orbital interaction between a metal ion and redox-active ligand. Because the electron transfer can induce spin-state switching and/or the change in magnetic exchange coupling and electric polarization involved in metal ions and redox-active ligands, these complexes can afford magnetic and dielectric switching materials.
2,2′-Azobisphenol (H2azp) is one of the azo dyes known as intense coloring and/or fluorometric reagents that can quantitatively analyze for specific ions such as magnesium [4,5,6,7], aluminum [4,8,9,10], and gallium [4,8,9]. The molecular structures of homoleptic 1:2 CrIII and CoIII complexes with azp derivatives were extensively studied by Schetty [11,12,13,14,15,16]. The meridional ONO-tridentate coordination to metal ions was confirmed by the crystal structure analysis for homoleptic 1:2 CrIII [17] and MnIV [18] complexes with a non-substituted azp ligand. TiIV [19,20], VIV [21,22,23], MnIII [24], CoIII [25], NiII [26,27], CuII [28], MoVI [29], PdII [26,27], and PtII [26,27] complexes with azp derivatives have been reported as well-characterized transition metal azp complexes. Recently, we discovered the homoleptic 1:2 FeIII [30,31] and heteroleptic FeIII [32,33] complexes from azp derivatives exhibited gradual and abrupt spin-crossover transitions. Moreover, single-crystal-to-single-crystal transformation [34] and spin-crossover-triggered linkage isomerization involved in a pedal-like motion of the azp ligand [35] were also found in the FeIII complexes with the azp derivatives. Very recently, double spin-crossover [36] and field-induced magnetic relaxation [37] were reported in homoleptic 1:2 FeIII complexes with an azp ligand.
The structural formula of the deprotonated azp molecule can be regarded as the two-electron reduction form of a neutral bis-orthoquinoid compound through a monoanionic semiquinone radical (Scheme 1). Therefore, it is expected that the azp derivatives are redox-active, which can be described by a two-electron oxidation (shown as sequential one-electron oxidation steps in Scheme 1). Although there have been several reports on the electrochemical reduction of azp derivatives and their metal complexes [18,24,29,38], the electrochemical oxidation has only been reported for the CoIII complex with the azp derivative [25]. Since the CoIV and CoV complexes are known [39,40], it has never been reported whether the observed oxidation wave was centered on the metal ion or azp ligand to date.
To understand the electrochemical oxidation behavior of the azp ligand, we focused on the homoleptic 1:2 AlIII complexes with azp derivatives. Although an AlIII complex could be reduced by potassium metal under very specific conditions [41], the AlIII ion will clearly be redox-innocent under oxidative conditions and we anticipate no contribution from the AlIII ion in our study. The reported 1:1 divalent and trivalent metal complexes with azp ligands are usually insoluble in common solvents, whereas the homoleptic 1:2 AlIII complexes were soluble in water and organic solvents [8,10] and retained an octahedral coordination structure in the solution state [42]. The coordination with an AlIII ion may stabilize the dianion of azp derivatives. Therefore, the electrochemical oxidation behavior of the azp ligand itself can be clarified using the homoleptic 1:2 AlIII complex. Moreover, the single crystals suitable for crystal structure analysis can be obtained for homoleptic 1:2 AlIII complexes as many crystal structures of the homoleptic 1:2 transition metal complexes with azp derivatives have been determined. To investigate both the substitution effect on redox behavior and the steric effect of 3-substitution on metal coordination, we prepared four H2azp derivatives (H2L1) with electron-donating (methyl- and t-butyl-) and electron-withdrawing (chloro-) substituents at 5- and 3,5-positions using an improved synthetic procedure. We also synthesized an imino derivative ligand (H2L2) to clarify the role of an azo group on redox properties. We report herein the syntheses, crystal structures, proton nuclear magnetic resonance (NMR) spectra, cyclic voltammograms, and density functional theory (DFT) calculations of the AlIII complexes 1 and 2 (Figure 1).

2. Results and Discussion

2.1. Synthesis

2.1.1. Synthesis of Substituted H2azp Ligands (H2L1) and Imino Derivative Ligand (H2L2)

Symmetrically alkyl-substituted H2azp derivatives were usually synthesized by the homocoupling reaction of corresponding 2-hydroxyphenyl diazonium salts in the presence of a Cu(I) complex catalyst [43]. However, this synthetic procedure usually gave relatively low yields and was unsuitable for a large-scale preparation because the reaction accompanies vigorous foaming. Because the direct azo-coupling reaction of 2-hydroxyphenyl diazonium salt sometimes failed to obtain the product, we utilized the two-step synthetic procedure for the benzene-fused azp derivative [31]. This synthetic procedure has the advantage that the symmetrical and unsymmetrical H2azp derivatives with a position-selective substitution and electron-withdrawing substituents can be synthesized. The synthetic scheme of the ligands is shown in Scheme 2. The substituted 2-methoxyanilines 3 were converted into the diazonium salts and then reacted with an aqueous NaOH solution of 4- or 2,4-substituted phenols 4. Deprotection of methyl group was achieved using AlCl3-pyridine in chloroform, to give H2azp-5,5′-Cl2 (H2L1Cl2) [44], H2azp-5,5′-Me2 (H2L1Me2) [43], H2azp-5,5′-tBu2 (H2L1Bu2) [43], and H2azp-3,3′,5,5′-Me4 (H2L1Me4) in moderate yields. Dehydration condensation between 6 and 7 afforded the imino derivative ligand (H2L2Me2).

2.1.2. Synthesis of Homoleptic 1:2 AlIII Complexes

The homoleptic 1:2 AlIII complexes were prepared by the reaction of AlIII(NO3)3·9H2O with 2 equivalents of anionic L1 or L2 ligands generated by the reaction of NaOCH3 in methanol, followed by the addition of tetraphenylphosphonium (TPP) bromide (Scheme 3). Recrystallization in appropriate solvents gave the homoleptic AlIII complexes 1 and 2 as dark red needles and orange needles, respectively. The compositions of the AlIII complexes were confirmed by elemental analysis and crystal structure analysis described below.

2.2. Crystal Structures of the AlIII Complexes 1 and 2

Crystallographic data of the AlIII complexes 1 and 2 are listed in Table S1. The crystals except for 1-Bu2 contained some solvent molecules. The AlIII complex crystals are hereafter designated as the corresponding compound number without indicating the solvent molecules. The crystal structures for 1-Me2, 1-Bu2, and 2-Me2 belonged to the triclinic system with P 1 ¯ . The crystal structures of 1-Cl2 and 1-Me4 belonged to the monoclinic system with Pna21 and C2/c, respectively. The asymmetric units for 1-Me2, 1-Bu2, and 2-Me2 were one TPP cation and one [AlIII(L)2] anion, whereas that for 1-Cl2 was two TPP cations and two [AlIII(L1Cl2)2] anions. The asymmetric unit for 1-Me4 was one TPP cation and two halves of [AlIII(L1Me4)2] anions. All the ligands in 1-Bu2, 1-Me4, and 2-Me2 showed an orientational disorder, whereas the orientations of one ligand in 1-Me2 and all the ligands in 1-Cl2 were ordered. The molecular structures of the [AlIII(L1)2] anions in 1-Cl2 and 1-Me2 are shown in Figure 2. The two ligands were meridionally coordinated to a central AlIII ion in an almost perpendicular manner to give a homoleptic octahedral coordination structure. The substitution of the 3-position on the azp ligand with the methyl group was found to have no significant effect on metal coordination. The structural parameters of the orientationally ordered ligands in 1-Cl2 and 1-Me2 are listed in Table 1. The coordinate Al-O bond lengths involved in the five-membered and six-membered chelates in 1-Cl2 and 1-Me2 were 1.870(4)–1.890(2) and 1.843(2)–1.856(3) Å, respectively, whereas the Al-N bond lengths were 2.008(2)–2.030(4) Å. The coordinate bond lengths to the oxygen atoms were similar to those in the CoIII [25] complexes and shorter than those in the FeIII low-spin and high-spin complexes [30] and CuII [28] complex. This trend can be explained by the relative effective cation radii of the metal ions [45]. On the other hand, the trend of the coordinate bond lengths to the nitrogen atoms is the opposite. The bond lengths of the azp ligand in 1-Cl2 and 1-Me2 were almost the same as those in the CoIII [25] and CuII [28] complexes.

2.3. 1H NMR Spectra of the Ligands and AlIII Complexes

1H NMR spectra for the ligands H2L1, H2L2, and AlIII complexes 1 and 2 were measured in chloroform-d and DMSO-d6. All the AlIII complexes gave well-resolved spectra assigned to the single set of signals attributed to the corresponding deprotonated ligands and TPP cations. No other signals were observed in the aromatic region. Compared with the corresponding protonated ligands (H2L), the signals for the ligands coordinated to an AlIII ion were shifted to higher fields (Figure 3). Lyčka et al., demonstrated an octahedral coordination for the homoleptic 1:2 AlIII complex with an azp derivative ligand by both the solid-state and solution-state NMR spectroscopy and also observed a similar higher-field shift [42]. The present observations suggest that all the AlIII complexes 1 and 2 retained the homoleptic octahedral coordination structure in solution.

2.4. Cyclic Voltammetry (CV)

To investigate the redox properties of the homoleptic 1:2 AlIII complexes 1 and 2, the cyclic voltammograms were recorded at a scan speed of 100 mV s−1 in a 0.1 M Bu4NPF6 solution of dichloromethane. The voltammograms for all the complexes are shown in Figure 4, Figures S1 and S2. The redox potentials are summarized in Table 2.
All the azp complexes 1 showed two partially reversible oxidation waves (Figure 4a,b). Although two or three additional oxidation waves were observed in the further anodic scan (Figure S1), the reduction waves coupled to the first and second oxidation waves for 1-Cl2 and 1-Me2 disappeared after the anodic scan potentials exceeded the third oxidation waves. A comparison of the first oxidation peak potentials of the AlIII complexes 1 in Table 2 clearly shows the presence of the substitution effect. On the other hand, the reversibility of the oxidative processes varies from one complex to the others. The reduction wave coupled to the first oxidation wave was almost absent in 1-Me2, whereas the peak currents of the reduction wave were larger on increasing either the size or the number of substituents in the alkyl-substituted complexes. These observations suggest that the irreversibility of the wave is associated with the reactivity of the oxidation species rather than from the dissociation of coordination bonds.
In the cathodic scan, only 1-Cl2 showed a reversible and an additional partially reversible reduction (Epa = −1.81 (shoulder peak), Epc = −1.95 V vs. Fc/Fc+) waves, whereas the other azp complexes showed a partially reversible reduction wave in the measurable range (Figure 4c and Figure S1). The substitution effect on the first reduction peak was also observed in 1.
The imino complex 2-Me2 exhibited a partially reversible oxidation wave, which accompanied the deposition of an orange film on the working electrode (Figure 4a,b). Although two or three additional oxidation waves were observed in the further anodic scan (Figure S1), these three oxidation waves disappeared in the second and subsequent anodic scans. On the other hand, there was no reduction wave in the cathodic scan in the measurable range (Figure 4c). The anodic shift of the first oxidation peak from 2-Me2 to 1-Me2 by 0.20 V suggests that the electronegativity of an azo group was higher than that of an imino group. On the other hand, the oxidation species of the azp ligand complexes 1 were more stable than those of the imino derivative complex 2.
Although the cyclic voltammograms were recorded in the presence of an equivalent mole of ferrocene (Figure S2), the number of electrons for the oxidation and reduction waves cannot be determined.

2.5. Density Functional Theory Calculations

To give an insight into the origin of the redox waves and the difference in redox behavior between the AlIII complexes 1 and 2, the density functional theory (DFT) calculations in the presence of CH2Cl2 were carried out for [AlIII(L)2] anions except for the [AlIII(L1Bu2)2] anion, which is expected to give a similar result to the [AlIII(L1Me2)2] anion. All the AlIII complexes anions could be fully optimized using the polarizable continuum model (PCM) [46]. The energy levels of frontier molecular orbitals (MOs) are listed in Table 3. The frontier molecular orbitals for the [AlIII(L1Me2)2] and [AlIII(L2Me2)2] anions are shown in Figure 5.
The distribution of MO coefficients for the frontier orbitals was similar in all the AlIII anions. The frontier MOs had no MO coefficients on the aluminum atoms as anticipated. In all calculations, the HOMO had larger coefficients on the phenyl rings, whereas the LUMO had larger coefficients on the azo or imino group. The energy levels of HOMO and LUMO were nearly degenerate with those of the HOMO−1 and LUMO+1, respectively. The difference between HOMO and HOMO−1 and between the LUMO and LUMO+1 was that the MO coefficients of one ligand were inverted (Figure 5). The correlation plots between the first oxidation peak potential and the HOMO energy and between the first reduction peak potential and the LUMO energy are shown in Figure 6. These two plots each show an almost linear correlation between the orbital energy and peak potential, which indicates that the first oxidation and first reduction arise from the HOMO and LUMO, respectively.
As the neutral one-electron oxidative derivatives were optimized under vacuum using the DFT method, the coefficient distributions of the SOMOs for 1-Me2 and 2-Me2 were almost the same as those of the corresponding HOMOs (Figure S3). Moreover, the SOMO distributions of two phenolate moieties of the azp ligand were also similar to those of a semiquinone radical reported in the literature [47]. These imply that the oxidation of the azp ligands may form a semiquinone-like radical.
The difference in the distribution of MO coefficients between [AlIII(L1Me2)2] and [AlIII(L2Me2)2] anions is small, but the MO coefficients on the phenyl ring bonded with the nitrogen on the imino group were larger than those bonded with the carbon atom on the imino group in [AlIII(L2Me2)2] anion. This may result in the difference in the stability against oxidation between [AlIII(L1Me2)2] and [AlIII(L2Me2)2] anions.

3. Materials and Methods

3.1. Synthesis

All the chemicals were purchased and used without further purifications. 2-Methoxyanilines 3c and 3d were prepared by methylation followed by reduction reactions of corresponding nitrophenols.

3.1.1. General Synthetic Procedure of (2′-Methoxyphenylazo)-2-Hydroxybenzene Derivatives 5

A solution of 2-methoxyanilines 3a–d (36.5 mmol) in 50 mL of water and 9.4 mL of conc. HCl was stirred and cooled to 0 °C. A solution of NaNO2 (2.77 g, 40 mmol) in 7 mL of water was added dropwise to the solution. The mixture solution turned to be yellowish-orange and kept below 5 °C. After confirming the existence of HNO2 using a KI-starch paper, the solution was quickly transferred to a solution of phenols 4a–d (35.5 mmol) in 25 mL of an aqueous solution of NaOH (2.92 g, 72.9 mmol). The resulting suspension was stirred for 1 h below 5 °C and then warmed to room temperature. To this suspension was added 2 mL of 3.5% HCl and stirred. The precipitate was filtered and dried in vacuo.
5,5′-Dichloro-2-hydroxy-2′-methoxyazobenzene (5a): 3.0 g of 3a and 2.0 mL of 4a were used. Recrystallization from ethyl acetate gave 5a (2.3 g, 41%) as dark red needles. 1H NMR (400 MHz, CDCl3) δ 13.19 (s, 1H), 7.94 (d, J = 2.6 Hz, 1H), 7.84 (d, J = 2.6 Hz, 1H), 7.43 (dd, J = 8.6, 2.6 Hz, 1H), 7.29 (dd, J = 8.6, 2.6 Hz, 1H), 7.04 (d, J = 8.9 Hz, 1H), 6.99 (d, J = 8.8 Hz, 1H), 4.01 (s, 3H) ppm.
5,5′-Dimethyl-2-hydroxy-2′-methoxyazobenzene (5b): 5.0 g of 3b (36.5 mmol) and 3.8 g of 4b (35.5 mmol) were used. Recrystallization from ethyl acetate gave 5b (4.8 g, 51%) as orange crystals. 1H NMR (400 MHz, CDCl3) δ 13.46 (s, 1H), 7.74 (d, J = 1.6 Hz, 1H), 7.66 (d, J = 1.7 Hz, 1H), 7.26 (dd, J = 8.4, 2.3 Hz, 1H), 7.13 (dd, J = 8.4, 2.2 Hz, 1H), 6.98 (d, J = 8.5 Hz, 1H), 6.93 (d, J = 8.4 Hz, 1H), 3.97 (s, 3H), 2.38 (s, 3H), 2.36 (s, 3H) ppm.
5,5′-Di-t-butyl-2-hydroxy-2′-methoxyazobenzene (5c): 0.55 g of 3c (3.1 mmol) and 0.44 g of 4c (3.1 mmol) were used. Recrystallization from ethyl acetate gave 5c (0.42 g, 41%) as orange crystals. 1H NMR (400 MHz, CDCl3) δ 13.58 (s, 1H), 7.97 (d, J = 2.5 Hz, 1H), 7.90 (d, J = 2.5 Hz, 1H), 7.50 (dd, J = 8.7, 2.5 Hz, 1H), 7.37 (dd, J = 8.7, 2.5 Hz, 1H), 7.02 (d, J = 8.7 Hz, 1H), 6.97 (d, J = 8.7 Hz, 1H), 3.99 (s, 3H), 1.39 (s, 9H), 1.37 (s, 9H) ppm.
3,3′,5,5′-tetramethyl-2-hydroxy-2′-methoxyazobenzene (5d): 0.92 g of 3d (6.1 mmol) and 0.73 mL of 4d (6.1 mmol) were used. Recrystallization from ethyl acetate gave 5d (1.3 g, 75%) as orange crystals. 1H NMR (400 MHz, CDCl3) δ 13.54 (s, 1H), 7.59 (m, 1H), 7.48 (m, 1H), 7.12 (m, 1H), 7.06 (m, 1H), 3.91 (s, 3H), 2.35 (s, 6H), 2.34 (s, 3H), 2.29 (s, 3H) ppm.

3.1.2. General Synthetic Procedure of 2,2′-Dihydroxyazobenzene (H2azp) Derivatives H2L1

A finely powdered AlCl3 (73.4 mmol) was added portionwise to a stirred solution of 5 (18.3 mmol) in 75 mL of chloroform. The resulting red suspension was warmed to 30 °C and then 38 mL of pyridine was added dropwise to this suspension. The mixture was heated to reflux for 2 h and then cooled to room temperature. The mixture was poured into 150 mL of ice water and then acidified by adding 30 mL of 36% HCl. After evaporating chloroform, the mixture was heated at 80 °C for two hours. The resulting precipitate was filtered and dried in vacuo.
5,5′-Dichloro-2,2′-dihydroxyazobenzene (H2L1Cl2): H2L1Cl2 was synthesized using 2.0 g of 5a. Recrystallization from ethyl acetate gave H2L1Cl2 as orange crystals (1.1 g, 56%). 1H NMR (400 MHz, CDCl3) δ 11.90 (s, 2H), 7.73 (d, J = 2.6 Hz, 2H), 7.34 (dd, J = 2.6, 8.9 Hz, 2H), 7.02 (d, J = 8.9 Hz, 2H), ppm.
5,5′-Dimethyl-2,2′-dihydroxyazobenzene (H2L1Me2): H2L1Me2 was synthesized using 4.7 g of 5b. Recrystallization from ethyl acetate gave H2L1Me2 as orange crystals (3.3 g, 74%). 1H NMR (400 MHz, CDCl3) δ 12.14 (s, 2H), 7.51 (d, J = 1.4 Hz, 2H), 7.16 (dd, J = 8.4, 2.1 Hz, 2H), 6.94 (d, J = 8.4 Hz, 2H), 2.38 (s, 6H) ppm.
5,5′-Di-t-butyl-2,2′-dihydroxyazobenzene (H2L1Bu2): H2L1Bu2 was synthesized using 0.42 g of 5c. Recrystallization from ethyl acetate gave H2L1Bu2 as orange crystals (0.28 g, 70%). 1H NMR (400 MHz, CDCl3) δ 12.28 (s, 2H), 7.67 (d, J = 2.4 Hz, 2H), 7.41 (dd, J = 8.7, 2.5 Hz, 2H), 7.00 (d, J = 8.7 Hz, 2H), 1.36 (s, 18H) ppm.
3,3′,5,5′-Tetramethyl-2,2′-dihydroxyazobenzene (H2L1Me4): H2L1Me4 was synthesized using 1.3 g of 5d. Recrystallization from ethyl acetate gave H2L1Me4 as orange crystals (0.78 g, 66%). 1H NMR (400 MHz, CDCl3) δ 12.55 (s, 2H), 7.34 (s, 2H), 7.05 (s, 2H), 2.33 (s, 6H), 2.28 (s, 6H) ppm. Anal. Calcd. for C16H18N2O2: C, 71.09; H, 6.71; N, 10.36%. Found: C, 71.03; H, 6.72; N, 10.32%.

3.1.3. Synthetic Procedure of N-(2-Hydroxy-5-Methylphenyl)-5-Methylsalicylaldimine (H2L2Me2)

2-amino-4-methylphenol (1.0 g, 8.1 mmol) and 2-hydroxy-5-methylbenzaldehyde (1.1 g, 8.1 mmol) were dissolved in 25 mL of methanol. The solution was heated to reflux and stirred for 2 h. The solution was cooled to room temperature. The orange precipitate was filtered and dried. (2.4 g, 51%). 1H NMR (400 MHz, CDCl3) δ 12.08 (s, 2H), 8.63 (s, 1H), 7.23–7.21 (m, 2H), 7.01 (d, J = 8.2, 1.5 Hz, 1H), 6.95–6.93 (m, 2H), 6.91 (d, J = 8.2 Hz, 1H), 2.335 (s, 3H), 2.327 (s, 3H) ppm.

3.1.4. General Synthetic Procedure of the [AlIII(L1)2] (1) and [AlIII(L2)2] (2) Complexes

A 28% methanol solution of sodium methoxide (4.0 mmol) was added dropwise to a suspension of H2L1 or H2L2 (0.74 mmol) in 10 mL of methanol. The suspension was heated to 60 °C and stirred for 20 min to become a dark solution. A solution of Al(NO3)3·9H2O (0.37 mmol) in 9 mL of methanol was added to the solution and then heated to reflux for 2 h. Tetraphenylphosphonium bromide salts (0.93 mmol) in 15 mL of methanol were added to the solution and then stirred for 30 min. After evaporating the solution to a few mL, water was added. The precipitate was filtered and washed with water and dried in vacuo.
TPP[AlIII(L1Cl2)2]∙H2O (1-Cl2∙H2O): 1-Cl2 was synthesized using 200 mg of H2L1Cl2. Recrystallization of the crude salt from dichloromethane-hexane gave 238 mg (73%) as dark purple needles. The elemental analysis was performed using an air-dried sample. 1H NMR (400 MHz, CDCl3) δ 7.83–7.79 (m, 4H), 7.68–7.63 (m, 8H), 7.56 (d, J = 2.7 Hz, 2H), 7.51–7.46 (m, 8H), 7.44 (d, J = 2.8 Hz, 2H), 6.89 (dd, J = 8.9, 2.7 Hz, 2H), 6.85 (dd, J = 9.1, 2.9 Hz, 2H), 6.41 (d, J = 8.9 Hz, 2H), 6.34 (d, J = 9.0 Hz, 2H) ppm. Anal. Calcd. for C48H34AlCl4N4O5P: C, 60.91; H, 3.62; N, 5.92%. Found: C, 61.01; H, 3.60; N, 5.96%.
TPP[AlIII(L1Me2)2]∙CH3OH·H2O (1-Me2∙CH3OH·H2O): 1-Me2 was synthesized using 100 mg of H2L1Me2. In total, 170 mg of crude salt was obtained (46%). Recrystallization of the crude salt from methanol-diethyl ether gave dark purple thin rectangles. 1H NMR (400 MHz, CDCl3) δ 7.76–7.72 (m, 4H), 7.63–7.59 (m, 8H), 7.45–7.41 (m, 12H), 6.68 (dd, J = 8.6, 2.3 Hz, 2H), 6.64 (dd, J = 8.7, 2.6 Hz, 2H), 6.29 (d, J = 8.4 Hz, 2H), 6.21 (d, J = 8.6 Hz, 2H), 2.14 (s, 6H), 2.12 (s, 6H) ppm. Anal. Calcd. For C53H50AlN4O6P: C, 70.97; H, 5.62; N, 6.25%. Found: C, 70.69; H, 5.55; N, 6.16%.
TPP[AlIII(L1Bu2)2] (1-Bu2): 1-Bu2 was synthesized using 200 mg of H2L1Bu2. Recrystallization of the crude TPP salt from acetonitrile-diethyl ether gave 255 mg (82%) as dark red platelets. 1H NMR (400 MHz, DMSO-d6) δ 7.99–7.94 (m, 4H), 7.84–7.79 (m, 8H), 7.77–7.71 (m, 8H), 7.65 (d, J = 2.5 Hz, 2H), 7.52 (d, J = 2.7 Hz, 2H), 7.21 (dd, J = 8.8 Hz, 2.6 Hz, 2H), 7.18 (dd, J = 8.9 Hz, 2.7 Hz, 2H), 6.35 (d, J = 8.8 Hz, 4H), 1.29 (s, 18H), 1.28 (s, 18H) ppm. 1H NMR (400 MHz, CDCl3) δ 7.74–7.71 (m, 4H), 7.73 (d, J = 2.5 Hz, 2H), 7.62–7.57 (m, 8H), 7.54 (d, J = 2.7 Hz, 2H), 7.48–7.43 (m, 8H), 6.93 (dd, J = 8.9 Hz, 2.6 Hz, 2H), 6.90 (dd, J = 8.9 Hz, 2.8 Hz, 2H), 6.33 (d, J = 8.7 Hz, 2H), 6.23 (d, J = 8.8 Hz, 2H), 1.23 (s, 18H), 1.22 (s, 18H) ppm. Anal. Calcd. for C64H68AlN4O4P: C, 75.72; H, 6.75; N, 5.52%. Found: C, 75.34; H, 6.75; N, 5.88%.
TPP[AlIII(L1Me4)2]·1.5H2O (1-Me4·1.5H2O): 1-Me4 was synthesized using 200 mg of H2L1Me4. Recrystallization of the crude salt from acetonitrile-diethyl ether gave 231 mg (69%) as dark red platelets. 1H NMR (400 MHz, DMSO-d6) δ 7.99–7.94 (m, 4H), 7.84–7.79 (m,8H), 7.77–7.71 (m, 8H), 7.27 (d, J = 2.1 Hz, 2H), 7.20 (d, J = 2.3 Hz, 2H), 6.81 (d, J = 2.0 Hz, 2H), 6.79 (d, J = 2.2 Hz, 2H), 2.18 (s, 6H), 2.17 (s, 6H), 1.81 (s, 6H), 1.58 (s, 6H) ppm. 1H NMR (400 MHz, CDCl3) δ 7.74–7.70 (m, 4H), 7.63–7.58 (m, 8H), 7.45–7.40 (m, 8H), 7.26 (the signal overlapped with that of CHCl3, 2H), 7.13 (d, J = 2.1 Hz, 2H), 6.62 (d, J = 1.7 Hz, 4H), 2.15 (s, 6H), 2.13 (s, 6H), 1.65 (s, 6H), 1.50 (s, 6H) ppm. Anal. Calcd. for C56H55AlN4O5.5P: C, 72.32; H, 5.96; N, 6.02%. Found: C, 72.14; H, 5.94; N, 6.21%.
TPP[AlIII(L2Me2)2] (2-Me2): 2-Me2 was synthesized using 200 mg of H2L2Me2. Recrystallization of the crude salt from acetonitrile-diethyl ether gave 204 mg (58%) as orange needles. The elemental analysis was performed using an air-dried sample. 1H NMR (400 MHz, CDCl3) δ 8.37 (s, 2H), 7.77–7.74 (m, 4H), 7.66–7.61 (m, 8H), 7.48–7.43 (m, 8H), 7.00 (d, J = 1.6 Hz, 2H), 6.85 (d, J = 2.1 Hz, 2H), 6.72 (dd, J = 8.5, 2.2 Hz, 2H), 6.47 (dd, J = 8.3, 1.7 Hz, 2H), 6.26 (d, J = 8.5 Hz, 2H), 6.22 (d, J = 8.2 Hz, 2H), 2.11 (s, 6H), 2.11(s, 6H) ppm. Anal. Calcd. For C58H52AlN4O4P: C, 76.76; H, 5.49; N, 3.32%. Found: C, 76.79; H, 5.34; N, 3.37%.

3.2. Physical Measurements

Cyclic voltammograms were recorded using an Ivium Technologies CompactStat.h standard electrochemical interface. Platinum wires were used as working and counter electrodes. A BAS RE-7 nonaqueous reference electrode (Ag/Ag+) was used as a reference electrode. A few mg of the Al complexes were dissolved in a 0.1 M Bu4NPF6 solution of dichloromethane. A ferrocene was used as an internal reference and the potential axis was corrected by the half-wave potential of Fc/Fc+.

3.3. Crystal Structure Determinations

A crystal was mounted in a polyimide loop. A nitrogen gas flow temperature controller was used for the temperature variable measurements. All data were collected on a Bruker APEX II CCD area detector with monochromated Mo-Kα radiation generated by a Bruker Turbo X-ray Source coupled with Helios multilayer optics. All data collections and calculations were performed using the APEX2 crystallographic software package (Bruker AXS). The data were collected to a maximum 2θ value of 55.0°. A total of 720 oscillation images were collected. The APEX3 crystallographic software package (Bruker AXS) was used to determine the unit cell parameters and for data collection. Data were integrated using SAINT. Numerical absorption correction was applied using SADABS. The structures at all temperatures were solved by direct methods and refined by full-matrix least-squares methods based on F2 using the SHELXTL program. During the refinements, the azp ligands in all the Al complexes except 1-Cl2 were found to be orientationally disordered. The structures and temperature factors for the disordered azp ligands were restrained. During the refinements for 1-Cl2, the solvent molecules could not be refined properly and an inversion twin was detected. Thus, the SQUEESE procedure [48] was applied and the twin volume fractions were refined using TWIN and BASF commands. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were generated by calculation and refined using the riding model. CCDC 2156173–2156177 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge, UK).

3.4. DFT Calculations

The geometry optimized calculations for the AlIII complex anions and the neutral AlIII complexes were performed at the B3LYP level [49,50] using the Gaussian 16 program package [51]. The 6-311+G(d,p) basis set for H, C, O, N [52,53], and Al [54,55,56] atoms were used. The atomic coordinates for the [AlIII(L1Me2)2] anion were taken from the main component of the disordered ligands in the crystal structure of 1-Me2. The atomic coordinates for the other aluminum anions were generated from the corresponding substitutions of the optimized [AlIII(L1Me2)2] anion. After the geometry optimization under vacuum, the geometry optimization calculations in the presence of CH2Cl2 were performed using the polarizable continuum model (PCM) [47]. No imaginary frequencies were found in the optimized structures. Cartesian coordinates of all AlIII complex anions calculated by the B3LYP level of theory are summarized in Tables S1–S4 (Supplementary Materials). The geometry optimizations for the neutral one-electron oxidative AlIII complex species under vacuum were performed using the atomic coordinates of the optimized [AlIII(L1Me2)2] and [AlIII(L2Me2)2] anions under vacuum. No imaginary frequencies were found in the optimized structures. Cartesian coordinates of the neutral AlIII complexes [AlIII(L1Me2)2] and [AlIII(L2Me2)2] calculated by the B3LYP level of theory are summarized in Tables S5 and S6.

4. Conclusions

We report the improved procedure of obtaining the H2azp derivatives with electron-donating and electron-withdrawing substituents, and the preparations and structural characterizations of the homoleptic 1:2 AlIII complexes with the azp ligands and imino derivative. The ligand-centered oxidations in the homoleptic 1:2 AlIII complexes with the azp ligands in solution were confirmed by the 1H NMR spectroscopy, cyclic voltammetry, and DFT calculation. The present result revealed that a semiquinone-like radical can be generated by the oxidation reaction of an azp derivative and thus the azp derivatives are a redox-active ligand in either the reduction or oxidation direction. Since the azp ligands are more stable than the imino derivative ligand under oxidative conditions, the introduction of appropriate substituents to the azp ligand and the combination with an appropriate metal ion can lead to the possibility that the metal azp complex exhibits valence tautomerism or electron-transfer-induced magnetic transitions. Further investigation toward this goal is now in progress.

Supplementary Materials

The following are available online at www.mdpi.com/article/10.3390/inorganics10060084/s1, Figure S1: Cyclic voltammograms in the full potential range of the cathodic and anodic scans; Figure S2: Cyclic voltammograms in the presence of an equivalent mole of ferrocene; Figure S3: Molecular orbital surfaces of the SOMOs; Table S1: Crystallographic data for the AlIII complexes 1 and 2; Table S2: Cartesian coordinates of the [AlIII(L1Cl2)2] anion; Table S3: Cartesian coordinates of the [AlIII(L1Me2)2] anion; Table S4: Cartesian coordinates of the [AlIII(L1Me4)2] anion; Table S5: Cartesian coordinates of the [AlIII(L2Me2)2] anion; Table S6: Cartesian coordinates of the neutral [AlIII(L1Me2)2] complex; Table S7: Cartesian coordinates of the neutral [AlIII(L2Me2)2] complex.

Author Contributions

Conceptualization, K.T.; methodology, K.T.; validation, T.N., K.U., A.M. and S.M.; formal analysis, T.N., K.U., A.M. and S.M.; investigation, T.N., K.U., A.M. and S.M.; resources, K.T.; data curation, K.T.; writing—original draft preparation, K.T.; writing—review and editing, K.T.; visualization, K.T. and T.N.; supervision, K.T.; project administration, K.T.; funding acquisition, K.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by a Grant-in-Aid for Scientific Research (C) (No. 19K05402) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, a grant from the CASIO Science Promotion Foundation, and a grant from Hyogo Science and Technology Association.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The crystallographic data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: [email protected]).

Acknowledgments

K.T. is grateful to T. Osakai and K. Eda at Kobe University for the discussion on electrochemistry and DFT calculation. K.T. is thankful to M. Lemaire at Brock University for reading the manuscript and suggesting the revision. K.T. thanks Y. Furuie at Kobe University for performing the elemental analysis.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 10 00084 sch001 550
Scheme 1. Possible redox scheme for the structural formula of 2,2′-azobisphenolate (azp) ligand.
Scheme 1. Possible redox scheme for the structural formula of 2,2′-azobisphenolate (azp) ligand.
Inorganics 10 00084 sch001
Inorganics 10 00084 g001 550
Figure 1. The molecular formula of the AlIII complexes 1 and 2 (TPP = tetraphenylphosphonium cation).
Figure 1. The molecular formula of the AlIII complexes 1 and 2 (TPP = tetraphenylphosphonium cation).
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Scheme 2. The synthesis scheme of ligands H2L1 and H2L2.
Scheme 2. The synthesis scheme of ligands H2L1 and H2L2.
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Scheme 3. The synthesis scheme of the AlIII complexes 1 and 2.
Scheme 3. The synthesis scheme of the AlIII complexes 1 and 2.
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Figure 2. ORTEP drawings with a 50% probability. (a) One of the [AlIII(L1Me2)2] anions in 1-Cl2; (b) The [AlIII(L1Me2)2] anion in 1-Me2. The atoms of a minor disordered ligand are shown without bond bars.
Figure 2. ORTEP drawings with a 50% probability. (a) One of the [AlIII(L1Me2)2] anions in 1-Cl2; (b) The [AlIII(L1Me2)2] anion in 1-Me2. The atoms of a minor disordered ligand are shown without bond bars.
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Figure 3. 1H NMR spectra in the range of 8–6 ppm for ligand H2L1Cl2 and AlIII complex 1-Cl2 in chloroform-d. * indicates spinning sidebands. # indicates signals from the TPP cation.
Figure 3. 1H NMR spectra in the range of 8–6 ppm for ligand H2L1Cl2 and AlIII complex 1-Cl2 in chloroform-d. * indicates spinning sidebands. # indicates signals from the TPP cation.
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Figure 4. Cyclic voltammograms of the AlIII complexes 1 and 2 in a 0.1 M Bu4NPF6 solution of dichloromethane. The scan speed was 100 mV s−1. (a) The anodic scans in the range of the first oxidation waves; (b) The anodic scans in the range of the first and second oxidation waves; (c) The cathodic scans in the range of the first reduction waves.
Figure 4. Cyclic voltammograms of the AlIII complexes 1 and 2 in a 0.1 M Bu4NPF6 solution of dichloromethane. The scan speed was 100 mV s−1. (a) The anodic scans in the range of the first oxidation waves; (b) The anodic scans in the range of the first and second oxidation waves; (c) The cathodic scans in the range of the first reduction waves.
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Figure 5. Molecular orbital surfaces. (a) The LUMO+1 of [AlIII(L1Me2)2]−; (b) The LUMO of [AlIII(L1Me2)2]−; (c) The HOMO of [AlIII(L1Me2)2]−; (d) The HOMO−1 of [AlIII(L1Me2)2]−; (e) The LUMO+1 of [AlIII(L2Me2)2]−; (f) The LUMO of [AlIII(L2Me2)2]−; (g) The HOMO of [AlIII(L2Me2)2]−; (h) The HOMO−1 of [AlIII(L2Me2)2]−.
Figure 5. Molecular orbital surfaces. (a) The LUMO+1 of [AlIII(L1Me2)2]−; (b) The LUMO of [AlIII(L1Me2)2]−; (c) The HOMO of [AlIII(L1Me2)2]−; (d) The HOMO−1 of [AlIII(L1Me2)2]−; (e) The LUMO+1 of [AlIII(L2Me2)2]−; (f) The LUMO of [AlIII(L2Me2)2]−; (g) The HOMO of [AlIII(L2Me2)2]−; (h) The HOMO−1 of [AlIII(L2Me2)2]−.
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Figure 6. The correlation plots between the HOMO and the first oxidation peak potential (a) and between the LUMO and the first reduction peak potential (b).
Figure 6. The correlation plots between the HOMO and the first oxidation peak potential (a) and between the LUMO and the first reduction peak potential (b).
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Table
Table 1. Coordination geometries and bonding parameters of the metal complexes with azp ligands.
Table 1. Coordination geometries and bonding parameters of the metal complexes with azp ligands.
Metal IonAlIIICoIIICuIIFeIII
Spin-state a—LS—LSHS
Geometry bOhOhSqOhOh
Reffc/Å0.5350.5450.5700.5500.645
Complex1-Cl21-Me2Co(L)Cu(L)TMA[Fe(azp)2]
Temp.90 K90 K233 K296 K90 K293 K
Bond Length/Å
M-O5m1.882(4)1.870(4)1.879(3)1.888(3)1.890(2)1.868(4)1.905(4)1.9217(14)1.9842(18)
M-O6m1.855(4)1.851(3)1.845(4)1.856(3)1.843(2)1.856(4)1.867(4)1.8898(14)1.9425(19)
M-N2.020(4)2.030(4)2.030(4)2.030(4)2.008(2)1.849(6)1.923(6)1.9220(15)2.1534(18)
C-O5m1.325(5)1.330(6)1.319(5)1.325(6)1.328(4)1.334(8)1.331(9)1.312(2)1.311(3)
C-N1.414(6)1.400(6)1.417(6)1.415(6)1.416(3)1.422(8)1.405(8)1.416(2)1.415(3)
C-O6m1.316(6)1.320(5)1.310(6)1.314(5)1.302(3)1.326(8)1.309(8)1.319(2)1.299(3)
C-N6m1.384(6)1.406(6)1.403(6)1.407(5)1.391(3)1.386(9)1.390(9)1.388(2)1.391(3)
N-N6m1.278(5)1.276(5)1.271(5)1.268(5)1.277(3)1.266(8)1.263(8)1.267(2)1.274(3)
referencethis workref. [25]ref. [28]ref. [30]
a LS: low-spin; HS: high-spin. b Oh: octahedral coordination; Sq: square-planar coordination. c Effective cation radii listed in Ref. [45]. 5m The atom involved in a five-membered chelate. 6m The atom involved in a six-membered chelate.
Table
Table 2. Redox potentials a (V vs. Fc/Fc+) for the AlIII complexes 1 and 2.
Table 2. Redox potentials a (V vs. Fc/Fc+) for the AlIII complexes 1 and 2.
Complex Ered1 Eox1 Eox2
EpaEpcE1/2bEpaEpcE1/2bEpaEpcE1/2b
1-Cl2−1.50−1.62−1.560.570.40 c0.490.90 d——
1-Me2−1.73 c−1.96—0.240.110.180.600.500.55
1-Bu2−1.86−1.98 d—0.240.120.180.610.490.55
1-Me4−1.82−2.07 d—0.09−0.030.030.460.350.41
2-Me2———0.04—————
a The scan speed was 100 mV s−1. b Emid = (Epa + Epc)/2. c Broad peak. d Shoulder peak.
Table
Table 3. Energy levels of frontier molecular orbitals of the AlIII complex anions (eV).
Table 3. Energy levels of frontier molecular orbitals of the AlIII complex anions (eV).
AnionLUMO+1LUMOHOMOHOMO−1
[AlIII(L1Cl2)2]−2.527−2.720−5.339−5.343
[AlIII(L1Me2)2]−2.132−2.324−4.946−4.953
[AlIII(L1Me4)2]−2.025−2.223−4.836−4.839
[AlIII(L2Me2)2]−1.562−1.742−4.762−4.763
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Takahashi, K.; Noguchi, T.; Ueda, K.; Miyawaki, A.; Murata, S. Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands. Inorganics 2022, 10, 84. https://doi.org/10.3390/inorganics10060084

AMA Style

Takahashi K, Noguchi T, Ueda K, Miyawaki A, Murata S. Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands. Inorganics. 2022; 10(6):84. https://doi.org/10.3390/inorganics10060084

Chicago/Turabian Style

Takahashi, Kazuyuki, Takumi Noguchi, Keiji Ueda, Atsuhiro Miyawaki, and Suguru Murata. 2022. "Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands" Inorganics 10, no. 6: 84. https://doi.org/10.3390/inorganics10060084

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