Changing Single-Molecule Magnet Properties of a Windmill-Like

Oct 13, 2017 - (1-3) SMMs exhibit several features, such as slow magnetic relaxation and quantum tunneling of the magnetization (QTM), which are not o...
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Changing Single-Molecule Magnet Properties of a Windmill-Like Distorted Terbium(III) α‑Butoxy-Substituted Phthalocyaninato Double-Decker Complex by Protonation/Deprotonation Yoji Horii,† Yusuke Horie,† Keiichi Katoh,*,† Brian K. Breedlove,† and Masahiro Yamashita*,†,‡,§ †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan ‡ WPI Research Center, Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan § School of Materials Science and Engineering, Nankai University, Tianjin 300350, China S Supporting Information *

ABSTRACT: Synthesis, structures, and magnetic properties of αbutoxy-substituted phthalocyaninato double-decker complexes Tb(α-obPc)2 (1−) (α-obPc: dianion of 1,4,8,11,15,18,22,25octa(n-butoxy)phthalocyaninato) with protonated (1H), deprotonated (1[HDBU]), and diprotonated forms (1H2+) are discussed. X-ray analysis was used to confirm the position of the proton in 1H, and it was revealed that the protonation induced asymmetric distortion in 1H. In contrast, 1[HDBU] was distorted in a highly symmetric windmill-like fashion. 1H is arranged in a slipped column array in the crystal packing, whereas 1[HDBU] is arranged in a one-dimensional fashion, in which the magnetic easy axes of 1[HDBU] lie along the same line. From direct-current (dc) magnetic measurements, ferromagnetic Tb−Tb interactions occur in both 1H and 1[HDBU], and magnetic hysteresis was observed. However, the area of the magnetic hysteresis in 1[HDBU] is larger than that in 1H, meaning that magnetic relaxation time (τ) is longer in 1[HDBU]. In addition, the results of alternating-current magnetic measurements in a zero dc magnetic field indicate that τ of 1[HDBU] is longer as compared to 1H. In other words, protonation/deprotonation affects not only the molecular structures and crystal packing but also the single-molecule magnet properties.

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INTRODUCTION Single-molecule magnets (SMMs), discovered in 1993, have potential applications as ultrahigh density molecular memories and quantum bits (qubits).1−3 SMMs exhibit several features, such as slow magnetic relaxation and quantum tunneling of the magnetization (QTM), which are not observed for classical bulk magnets.2,4−6 The Tb(III)−phthalocyaninato doubledecker complex TbPc2 shows SMM properties with a large activation energy for spin reversal (ca. 400 cm−1) and high chemical stability.7−9 Because of these characteristics, TbPc2 has been used for the research in spintronic devices.10−19 Moreover, TbPc2 analogues show good SMM properties. Tanaka et al. have reported proton-induced switching of SMM properties of a Tb(III)-porphyrinato double-decker complex Tb(HTPP)(TPP) (TPP = 5,10,15,20-tetraphenylporphyrin) by using a reversible protonation−deprotonation reaction.20,21 The proton in Tb(HTPP)(TPP) is connected to the pyrrole-N atom of TPP, which makes the coordination geometry around the Tb(III) ion a distorted square antiprismatic (SAP) one. On the one hand, because of the low coordination symmetry in Tb(HTPP)(TPP), this complex does not show SMM proper© 2017 American Chemical Society

ties. On the other hand, the deprotonated Tb(TPP)2 acts as an SMM due to the highly symmetric SAP coordination geometry. In contrast to Tb(HTPP)(TPP), protonated Tb(III)− phthalocyaninato−porphyrinato heteroleptic double-decker complex Tb(TPP)(HPc) has its proton on the meso-N atom of the phthalocyaninato ligand.22 Since the acidic protons did not disturb the SAP coordination geometry around the Tb(III) ions, Tb(TPP)(HPc) still exhibits SMM properties. In 2010, Jiang et al. reported the synthesis and crystal structures of [M(Hα-obPc)(α-obPc)] (α-obPc = dianion of 1,4,8,11,15,18,22,25-octa(n-butoxy)phthalocyaninato, M = Eu(III), Y(III)) and the deprotonated complex [M(α-obPc)2]−.23 H2(α-obPc) has eight bulky groups on the alpha positions of phthalocyanine moiety, causing a saddle-shaped distortion of the π-plane. (Figure 1) This distortion makes the meso-N of H2(α-obPc) more basic than that in β-substituted phthalocyanine H2(β-obPc). In addition, complexes composed of α-obPc are known to make the network structures with Na+ or Pd(I) Received: August 19, 2017 Published: October 13, 2017 565

DOI: 10.1021/acs.inorgchem.7b02124 Inorg. Chem. 2018, 57, 565−574

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Inorganic Chemistry

Figure 1. Chemical structure of H2(α-obPc) and H2(β-obPc). H2(α-obPc) has saddlelike distortions due to large steric hindrance.

Figure 2. Synthesis of 1H, 1[HDBU], and 1H2+.

ions through metal to meso-N bonds.24,25 Because of these unique properties of H2(α-obPc), α-substituted double-decker complexes can accommodate the proton on the meso-N positions. Our research group has reported the synthesis of protonated [Tb(α-obPcH)(α-obPc)] (1H), deprotonated [Tb(α-obPc)2]− (1[HDBU]), and diprotonated [Tb(αobPcH)2]+ (1H2+).26 Paramagnetic NMR spectroscopy and density functional theory (DFT) calculations showed that the protons in 1H and 1H2+ are on the meso-N atoms in α-obPc

and that protonation induces slight structural changes. It is known that even minute structural changes affect SMM properties.27,28 Therefore, slight structural changes induced by protonation/deprotonation should affect the SMM properties of 1−. In this article, synthesis, structures, and magnetic properties of 1− in various states are reported. Protonation of 1− induced vivid color changes, which were studied by using UV−vis−NIR absorption spectroscopy. From single-crystal X-ray diffraction 566

DOI: 10.1021/acs.inorgchem.7b02124 Inorg. Chem. 2018, 57, 565−574

Article

Inorganic Chemistry (SXRD) measurements, the proton in 1H was located on the meso-N position, which is consistent with a previous NMR study.26 The α-obPc ligands of 1[HDBU] were distorted in a windmill-like fashion. As compared to 1H, the molecular structure of 1[HDBU] has higher symmetry. From magnetic measurements on 1H and 1[HDBU], the magnetic relaxation time (τ) of 1[HDBU] in a zero direct-current (dc) magnetic field is longer than that of 1H. Our results show that slight changes in the molecular structures affect the SMM properties.

These results indicate that the position of the Q-band correlates with the electron density on the Pc ligands, and lowering the electron density causes a redshift in the Q-band. In case of the series of 1−, the electron density of the α-obPc ligands decreased in order of 1[HDBU], 1H, and 1H2+ due to the electron-withdrawing nature of proton, supporting the bathochromic shifts of the complexes through protonation. Crystal Structure. Crystals of 1H and 1[HDBU] suitable for SXRD were obtained by slow diffusion of toluene solution of the complexes into MeOH following literature methods.23 However, because of the low stability of 1H2+, we were unable to obtain a good structure. IR spectroscopy was performed on crystals of both 1H and 1[HDBU], and absorption peaks were observed in the range of 3200−3600 cm−1, which was ascribed to the H−N stretch. Upon protonation, the peaks shifted from 3245 cm−1 for 1H to 3598 cm−1 for 1[HDBU], as shown in Figure S1. These results suggest that the position of the proton changes from the meso-N of α-obPc to the basic N of DBU. Table 2 and Figure 4 show cell parameters and crystal

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RESULTS AND DISCUSSION Synthesis. Synthesis of 1H was conducted following a reported procedure (Figure 2).23,26 As previously reported by Jiang et al., dibenzo-18-crown-6 was used as the template. Without using a crown template, we could not detect 1H in the reaction product. After the synthesis of 1H, addition of 1,8diazabicyclo(5.4.0)undec-7-ene (DBU) into solution of 1H gave a light blue solution of 1[HDBU]. In the synthesis of 1H2+, an excess amount of acetic acid was added into the solution of 1H. Progression of the reaction was followed via the changes in UV−vis−NIR absorption spectra, as shown in Figure 3. Addition of DBU or acetic acid into pristine 1H in a

Table 2. Crystal Parameters of 1H and 1[HDBU] formula crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (K) ρ(g·cm−3) R1 wR2 GOF

Figure 3. UV−vis−NIR absorption spectra of 1− in various states. (inset) Color changes upon protonation/deprotonation.

Table 1. Position of the Main Absorptions in UV−Vis−NIR Absorption Spectra λ1, nm (log ε)

λ2, nm (log ε)

λ3, nm (log ε)

702 (5.282) 728 (5.191) 769 (5.143)

399 (4.589) 441 (4.418) 494 (4.548)

323 (4.993) 322 (4.946) 322 (4.910)

1[HDBU]

C128H161.15N16O16Tb monoclinic P21/c 31.3066(5) 21.6280(4) 17.5715(4) 90 100.099(2) 90 11 713.3(4) 90 1.326 0.0547 0.1441 1.014

C138H179N18O17Tb monoclinic P21/c 18.088(3) 33.587(5) 21.957(3) 90 106.271(2) 90 12 805(3) 90 1.308 0.0966 0.2830 1.049

structures of 1H and 1[HDBU]. The stacking angle (φ) of the two α-obPc ligands in 1H and 1[HDBU] were determined to be 43.9° and 42.0°, respectively, indicating that Tb(III) ions have square-antiprismatic (SAP) coordination geometries. SAP geometry is a preferable structure for obtaining SMM properties.32−34 Tb−N distances in 1H (1[HDBU]) were determined to be in the range of 2.426 (2.437)−2.454 Å (2.454 Å). In other words, the Tb−N distances in 1H deviate more than those in 1[HDBU]. As reported by Tanaka et al., protonation of the ligand induces tiny distortions around the ligands.20−22 In the case of 1H, two of C−N (meso-N) distances (1.34−1.36 Å) are longer than the other six C−N distances (1.32−1.34 Å), as shown in Figure 4c. In addition, residual electronic density was observed on the two neighboring meso-N atoms, indicating that the proton in 1H was on them. Solution 1H NMR studies on 1H support the existence of proton on meso-N.26 There are two crystallographically inequivalent meso-N positions for protonation, and the proton is located on one of the two meso-N atoms (N1 or N2). In other words, the proton exhibits positional disorder. The restriction in which the sum of occupancies of proton in N1 and N2 to be unity did not afford the converged structure. In contrast, structural refinements without this restriction generated a steady result, and the

CHCl3 solution caused reversible vivid color changes through deprotonation/protonation. The main absorption bands and molar absorption coefficients are summarized in Table 1.

1[HDBU] 1H 1H2+

1H

Strong absorption bands from 700 to 800 nm were ascribed to the Q-band (λ1), which corresponds to the HOMO→LUMO transition of the phthalocyaninato (Pc) ligand. Protonation of 1− caused a bathochromic shift in the Q-band (λ1) and the band in the range of 400−500 nm (λ2). In contrast, the Soret band (λ3) did not shift upon protonation/deprotonation. Similar behavior has been observed in the UV−vis−NIR absorption spectra for TbPc2 in various oxidation states, where the Q-band shows bathochromic shift by oxidizing it from anionic TbPc2− to neutral TbPc2 to cationic TbPc2+.29−31 567

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Figure 4. Crystal structures of (a) 1H and (b) 1[HDBU]. The proton in 1H is located on a meso-N atom. On the one hand, 1H is unsymmetrical due to the acidic proton. On the other hand, 1[HDBU] adopts a windmill-like highly symmetric structure. Color coding: pink = Tb, orange = proton on meso-N, red = O, light blue = N, gray = C, and white = H. Conformations and bond lengths (Å) of (c) 1H and (d) 1[HDBU]. In 1H, the C−N bond lengths around the meso-N atom where proton is located are long, supporting the presence of a proton.

Figure 5. Crystal packing of (a) 1H and (b) 1[HDBU]. Red arrows indicate the expected magnetic easy axis of the double-decker units. In both cases, the angles between magnetic easy axes are less than 54.7°. Therefore, ferromagnetic intermolecular magnetic interactions should occur in both cases. Color coding: pink = Tb, red = O, light blue = N, and gray = C. n-Butoxy chains were omitted for clarity.

only eight signals were observed, meaning that the structure of 1[HDBU] has higher symmetry than that of 1H.26 No crystal solvent molecules were found in the crystal lattice of 1H, whereas that of 1[HDBU] contained one methanol molecule and one HDBU+ per 1− unit. 1H is arranged in a slipped column packing structure along the c-axis, and the intermolecular Tb(III)−Tb(III) distance was determined to be 10.672 Å. In the case of 1[HDBU], 1− and HDBU+ are arranged alternatingly along the c-axis and make a onedimensional (1D) columnar structure. Similar molecular arrangements have been reported for cocrystals of fullerenes and porphyrinato double-decker complexes.35 The magnetic easy axis of double-decker complex is perpendicular to the Pc plane. Therefore, the magnetic moments of 1[HDBU] are arranged in a straight line. In the case of lanthanoid complexes, magnetic dipole−dipole (MD) interactions afford the main

occupancies of proton in N1 and N2 were determined to be 0.71 and 0.43, respectively. The sum of occupancy is close to unity, indicating the validity of the assignment of proton. Total occupancies slightly higher than 1 are due to the experimental error, which originates from the tiny electronic density of disordered proton. As shown in Figure 3a,b, the structures of both 1H and 1[HDBU] are similar to each other. However, the conformations of the n-butoxy chains and π-planes are different. Figure 4c,d illustrate the difference in the conformation of the α-obPc ligands. In the case of 1H, two n-butoxy chains near the proton is on the same plane, and 1H has an unsymmetrical structure. In contrast, α-obPc in 1[HDBU] adopts a windmilllike structure, and its symmetry is higher than that in 1H. 1H NMR studies support these results. In the 1H NMR spectra of 1H, 56 signals were observed, whereas in those of 1[HDBU], 568

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decrease in the χMT values at low T is due to thermal depopulation of the Stark sublevels.34,40−43,46 MH curves for 1H and 1[HDBU] showed clear magnetic hysteresis at 1.82 K without opening at 0 Oe (Figure 7). Obviously, the area of the magnetic hysteresis in 1[HDBU] is wider than that in 1H. In the case of 1H*, the area of magnetic hysteresis increased slightly as compared to 1H. In addition, magnetic hysteresis opened slightly even in a zero dc field for 1H*. The same behavior was observed for 1[HDBU], but the enhancement of the magnetic hysteresis behavior through magnetic dilution (1[HDBU]*) is more drastic. Especially, the opening of the magnetic hysteresis at zero dc field was clearer in 1[HDBU]*. In addition, 1[HDBU]* showed clear magnetic hysteresis up to 10 K. According to these results, we thought that the reason for the wider magnetic hysteresis in 1[HDBU] as compared to 1H was due to the higher symmetry in 1[HDBU]. SMM properties are sensitive to the slight changes in coordination geometry around lanthanoid ions. From the crystal structural and 1H NMR spectral analyses, the symmetry of the molecular structure of 1[HDBU] is higher than that of 1H.26 The lower symmetry of the molecular structures induces off-diagonal terms in LF terms, which induce a tunnel gap as well as QTM.32 In addition, internal magnetic fields, which originate from intermolecular MD interactions, greatly affect the rate of QTM. It is known that a magnetic field parallel to magnetic easy axis acts as the exchange bias, which suppresses the QTM,47,48 whereas a transverse magnetic field induces QTM due to the enhancement of tunnel gap.49 Similarly, the QTM rate is dependent on the direction of the internal magnetic field. The 1D molecular arrangement like that of 1[HDBU] is one of the most effective packing for minimizing transverse internal magnetic fields as well as a rate of QTM, because the magnetic easy axes of 1− units align along the direction of 1D column. AC Magnetic Susceptibilities. To elucidate the dynamical magnetic properties, alternating-current (ac) magnetic susceptibility measurements were conducted in an ac field of 3 Oe. The in-phase (χM′) and out-of-phase (χM″) signals changed in different T ranges depending on the ac frequency (ν) in the range of 1−1488 Hz. As shown in Figure 8, the values of the ac magnetic susceptibilities for both 1H and 1[HDBU] were ν dependent, suggesting that 1H and 1[HDBU] are SMMs. In the absence of a dc magnetic field, comparison of the χM″T versus T plots for 1H and 1[HDBU] revealed that the peak positions for 1H at 1−5 Hz were in the lower-T regions than those for 1[HDBU]. In addition, Arrhenius plots for 1H in a zero dc field afford a T-independent regime in the low-T region, indicating fast QTM (Figure 9a). In contrast to 1H, no such T-independent region was observed in Arrhenius plots for 1[HDBU]. Therefore, we concluded that the QTM in 1[HDBU] was effectively suppressed. Moreover, the longer τ0 for 1H as compared to 1[HDBU] supports the mixing of Tindependent QTM contributions.50 Next, we measured the ac magnetic susceptibilities in the presence of a dc magnetic field to suppress the QTM in the ground states. Upon applying a 2000 Oe dc magnetic field, the peak shapes for 1H and 1[HDBU] became sharper. In addition, the T-independent regime in the Arrhenius plots for 1H disappears, and the plots become completely T-dependent over the entire T range. Fitting the Arrhenius plots for 1H and 1[HDBU] afforded activation energies for spin reversal (ΔE) and frequency factors (τ0), which are summarized in Table 3. In case of 1H, ΔE increased ca. 100 cm−1 in a dc magnetic field

contribution to the intermolecular magnetic interactions.36−38 The angle (θ) between the magnetic moments defines whether the interactions are ferromagnetic or antiferromagnetic. If the angle is narrower than 54.7°, ferromagnetic interaction are expected.38,39 In case of 1H and 1[HDBU], θ was narrower than 54.7°, and there were intermolecular ferromagnetic interactions. However, the magnitude of the intermolecular ferromagnetic interactions in 1[HDBU] is stronger than those in 1H because of the head-to-tail arrangement of the magnetic moments in 1[HDBU]. DC Magnetic Susceptibilities. The dc magnetic susceptibility (χM) of 1H and 1[HDBU] is acquired by using SQUID magnetometer in the temperature (T) range of 1.8−300 K. χMT values of 1H and 1[HDBU] at 300 K were 11.60 an 11.40 cm3 K mol−1, respectively. The values are consistent with the expected value for 1 mol of free Tb(III) ions (11.81 cm3 K mol−1). The χMT values decreased slightly with a decrease in T to 20 K due to the thermal depopulation of the excited Stark sublevels and/or ligand field (LF).34,40−43 In the case of 1[HDBU], the χMT values clearly increased below 20 K, and the increase was ascribed to intermolecular ferromagnetic interactions.44,45 Similar behavior was observed for 1H, but the magnitude of growth in the χMT value at low T was small. In both cases, the crystal packing of the double-decker molecules supports the existence of ferromagnetic interactions. Although the intermolecular Tb−Tb distance in 1[HDBU] is longer than that in 1H, the magnitude of intermolecular Tb−Tb interactions in 1[HDBU] are stronger than those in 1H because of the head-to-tail arrangement of the magnetic moments in 1[HDBU]. The increase in the χMT values for 1[HDBU] is larger than that for 1H, indicating stronger Tb− Tb interactions in the former complex as expected from the crystal packing (Figure 5). To remove the effects of the intermolecular interactions, magnetically diluted samples, which were prepared by mixing 5 equiv of diamagnetic Y(III) complexes (2H or 2[HDBU]) with 1 equiv of the Tb(III) analogues, were prepared. χMT versus T plots for 1H* and 1[HDBU]* are shown in the inset of Figure 6. In contrast to the pure samples, magnetically diluted 1H* and 1[HDBU]* showed no increase in the χMT values in the low-T region due to the absence of intermolecular Tb−Tb interactions. A

Figure 6. χMT vs T plots for 1H and 1[HDBU]. (inset) χMT vs T plots for magnetically diluted 1H* and 1[HDBU]*. The increase in the χMT values disappeared when the complexes were magnetically diluted due to quenching of the Tb−Tb interactions. 569

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Figure 7. M/Msat vs H plots for (a) 1H and 1[HDBU] and (b) magnetically diluted 1H* and 1[HDBU]*. Area of the magnetic hysteresis for 1[HDBU] is larger than that for 1H.

Figure 8. χM″T vs T plots for (a) 1H and (b) 1[HDBU] in 0 and 2000 Oe dc magnetic fields. The ac magnetic susceptibility of 1[HDBU] is less field-dependent as compared to 1H. This behavior implies that QTM in 1[HDBU] is more suppressed than that in 1H.

due to the suppression of QTM.50 On the one hand, ΔE of 1H and 1[HDBU] at 2000 Oe are similar to each other. On the other hand, the τ0 value for 1H is 1 order of magnitude longer than that for 1[HDBU]. τ0 is related to the phonon mode of the compound.8 Therefore, the difference in the τ0 value originates from the structural differences between 1H and 1[HDBU]. To obtain detailed information about the dynamic magnetic relaxations, χM′ (χM″) versus ν (ac frequency) measurements were conducted. For both 1H and 1[HDBU], Argand plots

(χM″ vs χM′ plots) had semicircular shapes, indicating that the complexes undergo a single relaxation process (Figure 10). Fitting the Argand plots by using a generalized Debye model (eq S1) afforded the magnetic relaxation times (τ). The τ value is consistent with that obtained from χM″T versus T plots. As seen in the dc magnetic results, there are intermolecular magnetic interactions in the crystal packings of pure 1H and 1[HDBU]. To elucidate the effects of the magnetic interactions, the τ values for pure (1H and 1[HDBU]) and magnetically diluted (1H* and 1[HDBU]*) samples were 570

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Figure 9. Arrhenius plots of 1H and 1[HDBU] at (a) 0 and (b) 2000 Oe dc magnetic fields. Solid lines were fitted by using the Arrhenius equation.

Table 3. Fitting Parameters Obtained from Arrhenius Fits in Figure 9 ΔE (cm−1) 1H@0 Oe 1[HDBU]@0 Oe 1H@2000 Oe 1[HDBU]@2000 Oe

330 419 435 433

τ0 (s) 5.72 1.48 2.68 7.71

× × × ×

Figure 11. Comparison of the τ values for (a) pure 1H and 1H* and (b) magnetically diluted 1[HDBU] and 1[HDBU]* with and without a dc magnetic field. The τ values were obtained by fitting the Argand plots using eq S1. Solid lines are guides only.

10−9 10−12 10−12 10−13

the low-T regions, which was ascribed to QTM. In the case of 1H*, the τ values in the low-T region became longer. These results indicate that QTM in 1H is suppressed through the magnetic dilution. In contrast, Arrhenius plots for 1[HDBU] were scarcely affected by magnetic dilution, meaning that QTM is already suppressed in 1[HDBU]. Because of low QTM rate

compared. τ values, shown in Figure 11, were obtained from fitting the Argand plots by using generalized Debye model equations (eq S1). Changes in the Arrhenius plots upon magnetic dilution are clearly seen in case of 1H in a zero dc magnetic field. The τ values for 1H were less T-dependent in

Figure 10. Argand (χM″ vs χM′) plots for (a) 1H and (b) 1[HDBU] in 0 and 2000 Oe dc magnetic fields. Solid lines were fitted by using a generalized Debye model (eq S1). 571

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prepared by slow diffusion of a toluene solution of the complex into MeOH. X-ray diffraction data for 1H and 1[HDBU] were collected at 90 K on Rigaku Saturn 724+ CCD and Bruker APEX-II CCD diffractometers, respectively, with graphite monochromated Mo Kα radiation (λ = 0.7107 Å). The crystal was mounted on a Micromount with mineral oil. In case of 1H, diffraction peaks were integrated by using CrysAlis pro (Rigaku). The initial structures of both 1H and 1[HDBU] were solved by using SHELXT2014.52 Refinement by a full-matrix least-squares method on F2 by using SHELXL2014 was performed. All non-hydrogen atoms were refined anisotropically using a least-square method, and hydrogen atoms were fixed at calculated positions. Powder X-ray diffraction (PXRD) measurements were performed on crushed polycrystalline samples loaded into a capillary tube (diameter, 0.5 mm; length, 80 mm; Hilgenderg) with the mother liquor by using an AFC-7R/LW instrument (Rigaku). Magnetic measurements were performed on a Quantum Design SQUID magnetometer MPMS XL. Magnetic fields of 500 and 3 Oe were used for dc and ac magnetic measurements, respectively. Samples for magnetic measurements were fixed in n-eicosane. All data were corrected for the sample holder, n-eicosane, and diamagnetic contributions from the molecules by using Pascal’s constants.

in 1[HDBU], magnetic hysteresis for 1[HDBU] is wider than that for 1H, as shown in Figure 7.

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CONCLUSIONS We reported the synthesis and structural and magnetic analyses of protonated 1H, deprotonated 1[HDBU], and diprotonated 1H2+. Protonation/deprotonation of 1H caused vivid color changes due to changes in electronic structures, and this behavior was checked by using UV−vis−NIR spectroscopy. On the one hand, crystal structure analysis of 1H revealed that a proton was present on the meso-N position, and this proton reduced the symmetry of α-obPc, as shown in 1H NMR studies.26 On the other hand, deprotonated 1[HDBU] did not have a proton on the meso-N atom of the α-obPc ligands, causing a windmill-like structure. Compared to 1[HDBU], the α-obPc ligands in 1H are asymmetrically distorted, and the proton in 1H is located on one of the two asymmetrical mesoN positions. In other words, protonation lowers the symmetry of 1H. The molecules of 1H are packed in a slipped column structure, whereas those of 1[HDBU] are packed in a 1D structure. In the case of 1[HDBU], there are intermolecular ferromagnetic interactions due to the head-to-tail arrangement of magnetic moments in the crystal. Furthermore, the magnetic hysteresis for 1[HDBU] was wider than that for 1H. Especially, the magnetization of the magnetically diluted 1[HDBU]* showed clear magnetic hysteresis at 10 K. The ac magnetic measurements revealed that τ of 1[HDBU] without a dc magnetic field was longer than that of 1H. Our results show that deprotonation of 1H induces a higher symmetric molecular structure and enhances SMM properties.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02124. IR spectra, calculated and experimental PXRD patterns, generalized Debye model equations, plotted magnetic data, tabulated fitting parameters (PDF) Accession Codes

CCDC 1567299−1567300 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.

EXPERIMENTAL SECTION

The purchased reagents and solvents were used as provided by the supplier without further purification. DBU, 1-butanol, and 1-octanol were purchased from Wako chemicals. Metal lithium wire (3 mm in diameter) and dibenzo-18-crown-6 were purchased from SigmaAldrich. Tb(III) acetylacetonate trihydrate Tb(acac)3·3H2O was purchased from Strem Chemicals. The solvents used for column chromatography were special grade from Wako chemicals. 3,6Dibutoxyphthalonitrile was synthesized according to a reported method.51 Synthesis of 1H was reported in the previous article.26 Synthesis of [Tb(α-obPc)2]−[HDBU]+ (1[HDBU]). 1H was dissolved in toluene, and DBU was added drop by drop until there are no changes in UV−vis−NIR spectrum. Recrystallization from toluene/MeOH gave green crystalline solids of 1[HDBU]. MALDIMS: m/z 2338.15 (61%) [M]+, 2338.15 (100%) [M + H]+. Anal. Calcd (%) for C137H177N18O16Tb: C 66.06, H 7.16, N 10.12. Found: C 66.03, H 7.28, N 10.15 Synthesis of [Y(α-obPcH)(α-obPc)] (2H). For the preparation of magnetically diluted sample, we prepared diamagnetic Y(III) complex 2H. H2(α-obPc) (171 mg, 0.156 mmol), Y(acac)3·3H2O (36 mg, 0.0727 mmol), dibenzo-18-crown-6 (29 mg, 0.0804 mmol), and DBU (3 mL) were reacted in refluxing n-octanol (16 mL) for 10 h under N2 atmosphere. The solvent was evaporated under reduced pressure. The residue was chromatographed on a neutral alumina column (Al2O3, Merck) with CHCl3 as the eluent. A deep green fraction was collected. After the solvent was removed in vacuo, column chromatography was repeated over BioBeads S-X1 with tetrahydrofuran (THF). Recrystallization from toluene/MeOH gave dark green crystalline solids of 2H (26 mg, 28.8%). Anal. Calcd (%) for C128H161N16O16Y: C 65.73, H 6.93, N 9.58. Found: C 65.85, H 6.95, N 9.54. Physical Measurements. Elemental analysis and MS measurements were performed at the Research and Analytical Centre for Giant Molecules, Tohoku University. UV−vis−NIR spectra were collected on a SHIMADZU UV-3100PC with CHCl3 as a solvent. Single crystals of 1H and 1[HDBU] suitable for SXRD measurements were

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: (+81)22-795-6548. Phone: (+81)22-795-6548. (K.K.) *E-mail: [email protected]. (M.Y.) ORCID

Yoji Horii: 0000-0002-4789-6858 Author Contributions

These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by JSPS KAKENHI Grant Nos. JP20225003, JP15K05467, JP24750119, JP14J02656, Tohoku University Molecule & Material Synthesis Platform in Nanotechnology Platform Project, and CREST, JST (JPMJCR12L3), Japan.

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REFERENCES

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