Proton Control of the Lanthanoid Single-Ion Magnet Behavior of a

Article pubs.acs.org/IC

Proton Control of the Lanthanoid Single-Ion Magnet Behavior of a Double-Decker Complex with an Indolenine-Substituted Annulene Ligand Zhifu Liang,† Marko Damjanović,‡ Mritunjoy Kamila,† Goulven Cosquer,*,†,§ Brian K. Breedlove,† Markus Enders,*,‡ and Masahiro Yamashita*,†,§,∥,⊥ †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Sendai 980-8578, Japan Institute of Inorganic Chemistry, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany § CREST, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, 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: Two double-decker complexes with annulene ligands functionalized with indolenine groups were synthesized and characterized. The position of the proton acting as a counterion on one of the four indolenine nitrogen atoms was determined by using DFT calculations. Deprotonation and protonation of the complex induced by adding a base and an acid, respectively, were monitored by using NMR spectroscopy. Moreover, a correlation among the degree of protonation of the complex, the opening of the hysteresis, and the slow relaxation time is discussed.

■

INTRODUCTION Single-molecule magnets (SMMs) and single-ion magnets (SIMs) show slow magnetic relaxation properties, which have potential applications in high density data storage and molecular spintronics.1 Lanthanoid(III) ions are good candidates for SIMs because they often have large spin values, large magnetic anisotropies, and large unquenched orbital angular momenta as compared to first-row transition metal ions. In 2003, double-decker phthalocyaninato lanthanoid complexes were reported to be SMMs.2 A variety of double to quintuple-decker phthalocyaninato (Pc) and/or porphyrin (Por) complexes with a variety of metal ions in various oxidation states have been investigated.3 Due to the symmetry of the ligands, the complexes have C4 symmetry, which is the most favorable for avoiding mixing of the mJ levels responsible for the quantum tunneling of the magnetization (QTM).4 Besides Pc and Por, other tetra-aza macrocyclic ligands have scarcely been investigated as ligands for forming double-decker SMMs. Derivatives of dibenzotetraaza[14]annulenes, like Pc and Por ligands, have four nitrogen atoms in a plane, and two of those nitrogen atoms can be deprotonated to generate dianionic ligands, which easily coordinate to metal ions. However, unlike Pc and Por derivatives, which are delocalized aromatic (4n+2) compounds, dibenzotetraaza[14]annulenes are Hückel antiaromatic (4n) compounds. Moreover, Pc and © 2017 American Chemical Society

Por derivatives are rigid with a planar or concave structure, whereas dibenzotetraaza[14]annulenes are more flexible with planar and “saddle-shaped” structures. The saddle-shaped structure can prevent stacking of the complexes, reducing dipole interactions between metal ions, which play a role in the relaxation mechanism.5 The saddle-shaped structure can be tuned to control the magnetic behavior by chemical modification or application of external stimuli. A double-decker dysprosium complex with a dibenzotetraaza[14]annulene ligand has been reported to exhibit SMM behavior.6 Herein we report two double-decker lanthanoid complexes with an indolenine meso-substituted dibenzotetraaza[14]annulene ligand (L), [LnL2H] (Ln3+ = Dy3+ (1) and Tb3+ (2)). The position of the proton and its role in the geometry of the complexes were investigated by using NMR spectroscopy and DFT calculations. The magnetic properties of the complexes were measured in solid and solution states. We showed that adjusting the pH of the solution by adding an acid or base was an efficient way to tune the magnetic properties. DFT studies were used to determine the correlation between the protonated/deprotonated forms of the complexes and their magnetic properties. Received: March 9, 2017 Published: May 24, 2017 6512

DOI: 10.1021/acs.inorgchem.7b00626 Inorg. Chem. 2017, 56, 6512−6521

Article

Inorganic Chemistry

■

RESULTS AND DISCUSSION Synthesis and Characterization. Ligand H2L was synthesized by using a nontemplate method following a reported method.7 Complexes 1 and 2 were synthesized by reacting the neutral ligand H2L with Ln(acac)3·6H2O and purified by using column chromatography. Single crystals were obtained by evaporation of a mixed solution of CHCl3/ CH2Cl2/CH3OH. From single-crystal X-ray diffraction analyses performed on fresh crystals, described in more detail below, there were highly disordered solvent-accessible voids, filled with methanol, water, and CHCl3 or CH2Cl2. The solvent could be quickly and partially removed by keeping the crystals in air. This partial desolvation explains the difference between the chemical formulas for 1 and 2 obtained from crystal structure analyses and elemental analyses. Structural Description. Crystal structures of H2L and the doubly protonated form (H4L2+) are shown in Figures 1 and

Complexes 1 and 2 are isomorphic (Table 2). The asymmetric unit is composed of two independent doubledecker complexes (Figure 2) and several solvent molecules. The double-decker complexes were protonated on the nitrogen of an indolenine group, which kept the complexes neutral (see NMR study below for details). The Ln ions are coordinated by eight nitrogen atoms with bond distances in the range of 2.40− 2.45 Å (Table 3). The two complexes in the asymmetric unit are not strictly comparable. If the δ values are similar, i.e., ∼138°, ε is larger for one of the two complexes. Moreover, the angle between the LAPs of the two ligands of the double-decker complexes is close to 45° for one complex but close to 57° for the other, which induces a stronger deviation from ideal D4d square antiprism geometry for Ln ions (Table 4). This difference can be attributed to the presence of dichloromethane, or chloroform, interacting with one of the tetra-azacycles of one complex, but not the other one. The packing structure, controlled by hydrogen bonds (solvent/solvent or solvent/complex) and CH−π interactions, could not be described accurately due to the absence of any remarkable arrangement or motif. The distances among neighboring Ln ions was estimated to be over 11 Å, meaning that the Ln ions are somewhat isolated. All complexes have axial chirality and are the Δ enantiomers. However, we were not able to determine if the samples were enantiomerically pure. A priori, the Λ form could form, but it was not observed. Position of Acidic Proton. From single-point DFT calculations of the optimized complexes, the neutral complex with an acidic proton on the indolenine N-atom is 39 kJ/mol more stable than the case with the proton on a N-atom of the annulene moiety. Furthermore, it is apparent that, from the structure with the proton on the indolenine N-atom, this proton interacts with a neighboring indolenine N-atom on the other annulene ligand. The indolenine N to indolenine N distance was determined to be 3.21 Å, acidic H+ to other indolenine N-atom distance was 2.21 Å, and the (indolenine N)H···(indolenine N) angle was found to be 164.1°. The N−N distance is smaller than that obtained from the crystal structure data (5 and 6 Å). The difference is due to the presence of an H-bonded solvent molecule in the crystal. To economize the calculation time, the solvent was not included in the calculations. On the other hand, in the case of the nonprotonated end of the same molecule, the indolenine N to indolenine N distance was calculated to be 5.5 Å, which is considerably larger than the protonated end (Supporting Information). The theoretical results support the results from 1 H NMR spectroscopy discussed below. Furthermore, the theoretical structure accounts for the experimentally observed C2 symmetry axis of the neutral complex, which was assigned on the basis of the number of observed paramagnetic 1H NMR signals. Hyperfine 1H NMR Shifts. The contribution of the Ln ions to the Fermi contact can be calculated using the following equation:12

Figure 1. ORTEP representation of H2L. H-atoms other than those on N-atoms are omitted for clarity.

S1, respectively, and the crystal data are summarized in Table 2. Both forms are slightly distorted. To define this distortion and that in the complexes, we defined δ to be the angle between the N1−C8−C9−C7−N2 and N1*−C8*−C9*−C7*−N2* planes, ε as the angle between the N1−C8−C9−C7−N2 plane and the phenyl ring of the indolenine group at C9, and the ligand long axis plane (LAP) as the mean plane containing C10, C9, C9*, and C10*. ε in H4L2+ is larger than that in H2L (Table 1) due to the presence of a hydrogen bond between the Table 1. Selected Ligand Angles and Distances δ (deg) ε (deg) N1−N2 (Å) N1−N2* (Å) N2−N2* (Å) N1−N1* (Å)

H2L

H4L2+

0 2.48 2.731(4) 2.706(4) 3.724(5) 3.961(5)

0 9.87 2.728(6) 2.723(8) 3.784(7) 3.923(8)

protonated nitrogen atom of the indolenine group and the oxygen atom of the CF3SO3 counterion. In both forms, the four nitrogen atoms that form the donor set are arranged in a distorted square with a length along the diagonal of 3.8 Å, which is slightly shorter than that in Por (4.1 Å) and Pc ligands (3.9 Å).8,9 H2L are stacked via π−π interactions between the molecules (Figure S2). H4L2+ forms a similar columnar structure with a larger overlap between molecules (Figure S3).

δFC = ⟨Sz⟩

μB

2πA × 106 3kBTγI h

where ⟨Sz⟩ is the reduced value of the average spin polarization (31.500 for Gd, 31.853 for Tb),12 2πA/h is the hyperfine coupling constant (in rad s−1, obtained from DFT calculations), h is Planck’s constant, γI is the magnetogyric ratio of the observed nucleus, kB is the Boltzmann constant, μB is the Bohr 6513


Article

Inorganic Chemistry Table 2. Crystal Parameters for H2L, H4L2+, 1, and 2 chemical formula crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Z′ R1 (%) wR2 (%) a

H2L

H4L2+

1a

2b

C38H34N6 monoclinic P21/a 7.863(8) 18.239(18) 10.799(12) 90 110.374(14) 90 1452(3) 2 4 6.94 14.69

C40H36F6N6O6S2 orthorhombic Pbca 7.034(7) 19.64(2) 26.81(3) 90 90 90 3704(7) 4 8 9.92 26.27

C164.25H186Cl3Dy2N24O15.75 triclinic P1̅ 15.8977(10) 20.1016(15) 26.7760(20) 106.713(3) 101.080(2) 90.473(3) 8024.2(10) 4 2 5.96 14.87

C160H164Cl3N24O10.5Tb2 triclinic P1̅ 16.0264(17) 20.054(3) 26.773(4) 106.359(6) 101.809(3) 90.733(4) 8058.8(19) 4 2 6.77 19.05

1 = 2[DyL2H]·1.5CH2Cl2·10.75MeOH·5H2O. b2 = [TbL2H]·1CHCl3·7MeOH·3.5H2O.

Table 3. Select Angles and Distances in 1 and 2 1

2

Dy1

Dy2

Tb1

Tb2

LSP−LSP angle (deg) average Ln−N (Å) shortest Ln−Ln (Å)

45.00 2.420

57.63 2.423 11.036

46.16 2.426

57.35 2.432 11.015

N−N (Å)

5.01(1) 6.03(1)

5.16(1) 6.37(1)

5.04(1) 6.03(1)

5.09(1) 6.50(1)

δ (deg)

137.37 138.91

136.85 140.32

138.22 139.78

137.6 140.21

ε (deg)

7.92 8.63 16.30 18.11

2.69 18.40 25.65 28.07

7.89 8.42 15.72 18.42

2.91 15.27 25.14 26.46

Table 4. Deviationsa from the Ideal Geometry Obtained by Using SHAPE Software10,11 square antiprism triangular dodecahedron biaugmented trigonal prism biaugmented trigonal prism J50

Figure 2. (a) Top and (b) side views of double-decker complex 1. Hatoms are omitted for clarity.

a

magneton, T is the temperature, and the factor 106 is present for the conversion to ppm. There are two sign conventions used for ⟨Sz⟩, and the one not used in this report can be found for example in the book by Bertini et al.13 The molecular structures with optimized H-atom positions were used to obtain the geometric factors necessary for calculating the pseudocontact shifts (Supporting Information). The contact shift values from DFT calculations and the geometric factors were used to assign the peaks in the 1H NMR spectra of the studied paramagnetic complexes. Fermi Contact Shifts. Previously studied Pc double-decker complexes have H-atoms many bonds away from the paramagnetic Ln ions, resulting in a small contribution from the Fermi contact shift to the overall hyperfine shift.14 However, the aromatic H-atoms in the annulene ligands of the studied complexes are only a few bonds away from the

D4d D2d C2v C2v

Dy1

Dy2

Tb1

Tb2

1.041 2.601 2.862 3.471

2.164 2.364 4.074 4.674

1.200 2.704 3.090 3.635

2.170 2.471 4.048 4.674

Minimum value is better.

metal center, which means that a considerable Fermi contact shift is expected in the 1H NMR spectra. For this reason, DFT calculations were performed to obtain spin densities, and the resulting values were used to obtain the Fermi contact terms.14,15 The Fermi contact shift values are listed in Table S3. For calculating spin densities, the molecular structure of 2 was used, and only the positions of the H-atoms were optimized at the B3LYP level of theory with the def2svp16,17 basis set used for all light atoms. Moreover, the large-core quasi-relativistic effective core potential of a GdIII ion was used instead of that of a TbIII ion since the 8S electronic state of the GdIII ion alleviates the problem of spin−orbit coupling.18 Single-point calculations were performed on the optimized structures with a “superfine” integration grid with an 6514


Article

Inorganic Chemistry

Figure 3. 1H NMR spectra of (a) neutral 2, (b) after the addition of ca. 50 equiv of acetic acid, and (c) after addition of ca. 50 equiv of DBU. Spectra were recorded in CDCl3 at 235.0 K. Schematic representations of the neutral, cationic, and anionic complex are shown above each spectrum. Signal assignments are shown in Figure S8. * indicates diamagnetic impurities, residual solvent signals, and signals from either DBU or acetic acid. VT 1H NMR spectra of these compounds are shown in Figures S4−S7.

unrestricted TPSSh functional,14,19−21 the all-electron second order Douglass−Kroll−Hess method,22 and Pulay’s direct inversion in the iterative subspace (DIIS) extrapolation method.23 In cases where the DIIS method failed, the quadratically convergent SCF procedure was utilized.24 The EPR-II basis set25 was used on all light atoms, and the allelectron scalar relativistic basis set (SARC) was used for the TbIII ion.26 An SCF energy convergence criterion of 10−8 was used (maximum number of cycles was increased to 800 from the default value). NMR Spectra of Complexes and Their Interpretation. From the 1H NMR spectra (Figure 3), the neutral complex has low symmetry due to the presence of one acidic proton on one end of the molecule. Further protonation increases the symmetry of the complex, halving the number of observed signals. Deprotonation with an excess amount of DBU further reduces the number of observed 1H signals as the symmetry of the anionic complex is high when unperturbed by an acidic proton. The reversibility of the protonation or deprotonation of complex 2 was confirmed via the NMR measurements. Because of the fast paramagnetic relaxation of the 1H resonances close to the metal center, signals of the annulene core are broad. This prevented a more detailed signal assignment via 2D NMR spectroscopy. Thus, the assignment done in this work is not complete: the chemically equivalent Hatoms within a single ligand were identified. Most of the shifted signals correspond to the aromatic protons a and b. These protons are in the rigid part of the complex and have very small Fermi contact shift contributions (Supporting Information), meaning that their experimental chemical shifts can be used for calculating the axial component of the magnetic susceptibility anisotropy (Δχax) for 2 (Supporting Information). The Δχax values for 2 (neutral, anionic, and cationic forms) were found to be the same as those for the [Tb(obPc)2]− complex previously studied by using NMR within experimental error.14,27,28 The signals of the 1H resonances of the annulene core (signal group c) were broad and overlapped with signals of other resonances, and they were observed in the variable-

temperature (VT) spectra of the deprotonated complex (Figure S6). These protons are only three bonds away from the metal center and are expected to experience a significant contact shift. This was confirmed by relativistic single-point calculations (Table S3). A comparison of the ratios of the experimental pseudocontact shifts of the 1H resonances of the aromatic ring of the indolenine moiety with the ratios of the calculated geometric factors obtained from DFT optimizations of the monocationic and tricationic complexes with YIII ions led to the conclusion that, under the given acidic conditions (approximately 50 equiv acetic acid, CDCl3), 2 existed as a monocation (i.e., it is protonated twice). Further details are given in the Supporting Information. Magnetic Properties. The magnetic susceptibility (χ) of both complexes was measured in the range of 2−300 K (Figure 4). The χT values at room temperature were 14.25 and 11.36 cm3 K mol−1 for 1 and 2, respectively, and agree with the theoretical values for free Dy3+ ions (14.17 cm3 K mol−1) and Tb3+ ions (11.82 cm3 K mol−1), respectively.29 χT decreased with a decrease in T, reaching minimum values of 10.24 and 9.62 cm3 K mol−1 for 1 and 2, respectively, at 2.0 K. The decrease in the χT value was attributed to the depopulation of the mJ sublevels of the ground state.29 The magnetization of 1 became pseudosaturated from 1 T with a slope of 0.15 NβT−1 and a value of 4.16 Nβ at 5 T, whereas that of 2 saturated from 1 T with 3.78 Nβ. Moreover, only the magnetization of 1 exhibited butterfly-type hysteresis at 1.8 K (Figure 5). The χ for 1 and 2 showed frequency dependence without an external magnetic field up to 10 and 19 K, respectively (Figure S11). However, no peak for the out-of-phase signal was observed in the temperature and frequency ranges available, suggesting that quantum tunneling of the magnetization (QTM) occurs. Applying an external field of 1000 and 2000 Oe for 1 and 2, respectively, somewhat suppressed QTM (Figure S12). 6515


Article

Inorganic Chemistry

For 2, a single relaxation time was observed and attributed to a combination of direct, Orbach, and QTM processes. A 2000 Oe field did not fully suppress the QTM relaxation process. This could explain why no hysteresis was observed for the magnetization of 2. On the bases of DFT calculations, the protonation of the indolenine N-atom plays a role in the geometry of the ligand due to its mobility through the hydrogen bond. Addition or removal of the proton from the complex affects the magnetic properties of the complex by changing the coordination sphere. Thus, the magnetization curve and the frequency dependence of χ were measured in the presence of acid or base. For technical reasons (cf. experimental section), the χT versus T data could not be acquired, and the measurement was focused on 1 because the magnetization showed hysteresis. The measurements were performed three times in a CH2Cl2 solution without any acid or base (1n), with an excess of acetic acid (1c), and with an excess of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (1a). Compounds 1 and 1n have the same protonation, i.e., one Hatom for four indolenine groups, and are neutral. 1 is in the solid state, whereas 1n is in solution. Hysteresis was observed for both, although the opening for 1n was smaller (Figure 5). In studies on the frequency dependence of χ for 1n, the value significantly shifted to a higher frequency as compared to 1. In addition, the out-of-phase signal was broad (Figure S14). These results were attributed to a high degree of freedom of the complex in solution, as compared to the solid state, and/or less hydrogen-bonding between the complex and crystal solvent molecules. Since the ground state configuration of the dysprosium ion is highly sensitive to the crystal field, changes in the coordination sphere affect the magnetic properties.32

Figure 4. χT vs T at 1000 Oe, with magnetization curves at 1.8 K in the inset.

When an external field was applied, clear frequency and temperature dependences of χ were observed for 1 (Figure 6). Two relaxation times were extracted by using the Havriliak− Negami model.30,31 Although the origin of the two relaxation times remains unclear, the faster relaxation was attributed to an Orbach process with an energy barrier of 23.6 cm−1 (Table S10). The mechanism for the slower relaxation time was less clear, and three models could be used to describe it: (i) a direct, a Raman, and an Orbach process; (ii) a direct and two Orbach processes; (iii) a Raman and two Orbach processes (Figure S13).

Figure 5. Hysteresis loop obtained for 1 in solid and solution states (1n) with base (1a), acid (1c) at 1.8 K, with their derivatives in the bottom part of each graph. 6516


Article

Inorganic Chemistry

hysteresis was observed, and it underwent very slow relaxation of the magnetization. For 1c, no hysteresis was observed, and it underwent much faster relaxation of the magnetization. The optimized structures of the three forms were used to explain this behavior (Table 5). Protonation of the Nindolenine-atom induces intramolecular H-bonds between the neighboring Nindolenine-atoms. Therefore, the bonded indolenine groups are more twisted (ε = 4−5° for the nonbonded indolenine, and ε = 22−25° for the bonded indolenine). Moreover, the distance between the Nindolenine-atoms was shorter due to the H-bond (from 6 to 3 Å), and the ligand was more planar (δ = 134° in 1a, 151° in 1n, and 162° in 1c) (Figure S10). This means that the lone pair of the Nannulene-atoms are more in the plane of the ligand for 1c than for 1a. In other words, when the ligand is bent, the lone pairs of the Nannulene-atoms point to the dysprosium center, and the relaxation is slow. In addition, hysteresis becomes more open. This is a clear example of the effective point charge effect33 due to the C2 symmetry and the antiaromatic nature of the ligand. In other double-decker complexes containing Por or Pc ligands, the ligands have aromatic C4 symmetry and cannot bend so easily. By extrapolation, decreasing the planarity will enhance the magnetic properties. From DFT calculations, the tricationic form was more bent than the anionic form was. Efforts are underway to obtain the tricationic form to confirm the results of the calculations.

■

CONCLUSIONS Two lanthanoid double-decker complexes containing a dibenzotetraaza[14]annulene derivative were synthesized and characterized by using single-crystal X-ray diffraction and paramagnetic NMR spectroscopy. The position of the proton on the indolenine N-atom was determined. Addition and removal of this proton and addition of another proton were followed by using NMR spectroscopy, and the effects of protonation on the geometry of the complex were studied by using DFT calculations. The terbium complex 2 underwent slow relaxation in an external field, but hysteresis was not observed. On the other hand, the dysprosium complex 1 showed butterfly-type hysteresis and underwent slow relaxation of the magnetization in an applied field. Moreover, protonation and deprotonation of 1 in a dichloromethane solution affected the hysteresis opening, the relaxation rate, and the relaxation pathway. A correlation between the magnetic properties (hysteresis opening and the relaxation rate) and the degree of protonation of 1 was observed. Protonation of 1 caused the ligand to become more planar, the hysteresis to close, and the relaxation to become faster. This behavior is due to the electrostatic effect of the lone pair of the annulene N-atoms on the crystal field around the dysprosium ion. Decreasing the planarity of the ligand increased the effective point charge. Several studies are in progress to investigate this correlation, especially studies on the anionic and cationic forms in the solid state. Specific attention is being paid to the possibility of a magneto-chiral effect in this complex.

Figure 6. T dependence of the out-of-phase signal of χ for (a) 1 in a field of 1000 Oe and (b) 2 in a field of 2000 Oe. The insets are fitted Arrhenius plots for each compound.

The cationic compound 1c, obtained by adding an excess of acetic acid to a CH2Cl2 solution of 1, did not show clear hysteresis, whereas the anionic compound 1a, obtained by adding excess DBU to a CH2Cl2 solution of 1, showed butterfly-type hysteresis, like that observed for 1 in the solid state. A similar response was observed for the frequency dependence of χ. 1c underwent significantly faster relaxation in a 1000 Oe field than 1n did, whereas 1a underwent significantly slower relaxation (Figure 7). Moreover, only 1a underwent relaxation without an external magnetic field (Figure S15). However, in solution, an asymmetric frequency-dependent signal with a tail in the low frequency region was observed for each complex. Thus, it is impossible to extract the relaxation time by fitting the experimental data with the Cole−Cole or the Havriliak−Negami model with one or several relaxation times (Figure S16). An Arrhenius plot (Figure 7b) was made using the frequency corresponding to the maximum of the out-ofphase signal. The relaxation mechanism for 1c was determined to be a combination of direct, Raman, and Orbach processes with an energy barrier of 49 cm−1 (Table S11). For 1n, the mechanism was determined to be a combination of direct and Raman processes, whereas 1a relaxed through QTM and Orbach processes (Δ = 24 cm−1) in a 0 Oe field and Raman and Orbach processes (Δ = 39 cm−1) in a 1000 Oe field. A clear correlation was found between the degree of protonation and the magnetic properties. For 1a, a clear

■

EXPERIMENTAL SECTION

The organic compounds were purchased from Wako or TCI, and the lanthanoid precursors were purchased from Strem. The deuterated solvents (o-C6D4Cl2, CDCl3, Sigma-Aldrich) were dried (over CaH2), stored over molecular sieves, and thoroughly degassed before use. 1,8Diazabicyclo[5.4.0]undec-7-ene (DBU) and glacial acetic acid were used as received (Sigma-Aldrich, 99% or higher purity). Deprotonation 6517


Article

Inorganic Chemistry

Figure 7. Temperature dependence of the out-of-phase signal in a field of 1000 Oe for (a) 1a, (c) 1n, and (d) 1c. (b) Arrhenius plots for each form with best-fit lines. in vacuo. Yield 549 mg, 86%. 1H NMR (CDCl3, 500 MHz): δ14.619 (t, 2H, NH); 8.989 (d, 4H, CHN); 7.560 (d, 2H); 7.42 (d, 2H, Ar-H); 7.40 (d, 2H, Ar-H); 7.35 (d, 2H, Ar-H); 7.32 (d, 2H, Ar-H); 7.19− 7.22 (m, 6H, Ar-H); 1.62 (s, 12H, CH3). A red crystal of H2L suitable for SXRD measurements was obtained by slow evaporation of a CH2Cl2 solution. A crystal of H4L2+ was prepared by evaporation of an acetonitrile (5 mL) solution of H2L (59 mg, 0.1 mmol) and trifluoromethanesulfonic acid (30 mg, 0.2 mmol) in a 20 mL flask for several days. Synthesis of H[DyL2]·xCH2Cl2·yCH3OH·zH2O (1). H2L (118 mg, 0.2 mmol) and Dy(acac)3 6H2O (46 mg, 0.1 mmol) were mixed in 15 mL of 1,2,4-trichlorobenzene. The resulting mixture was heated and stirred at 220 °C for 24 h. The solution was then cooled to room temperature, and 1 was purified by using column chromatography on SiO2 with CHCl3/CH3OH (v/v = 100:1) as the eluent. Deep red block-shaped crystals, suitable for SXRD measurements, were obtained by evaporation of a mixed solution of CHCl3/CH2Cl2/CH3OH (v/v = 50:2:1). Yield: 31 mg, 24.8%. Anal. Calcd for H[DyL2]·4CH3OH· 2H2O, C80H85N12O6Dy (%): C, 65.27; H, 5.75; N, 11.42. Found: C, 65.64; H, 5.26; N, 11.99. Synthesis of H[TbL2]·xCHCl3·yCH3OH·zH2O (2). 2 was synthesized like 1 but with Tb(acac)3·6H2O (46 mg, 0.1 mmol) instead of Dy(acac)3·6H2O. Yield: 27 mg, 20.8%. Anal. Calcd for H[TbL2]· 3CH3OH·3H2O, C81H91N12O8Tb (%): C, 64.02; H, 6.04; N, 11.06. Found: C, 64.36; H, 5.45; N, 10.95. Characterization Methods. Elemental analyses for C, N, and H were performed on a PerkinElmer 240C elemental analyzer (PerkinElmer, Waltham, MA, USA) at the Research and Analytical Centre for Giant Molecules, Tohoku University. The magnetic measurements were performed on a crystalline powder by using a Quantum Design MPMS-SQUID-5T magnetometer. The diamagnetic

Table 5. Selected Angles and Distances Obtained from DFT Calculations [YIIIL2]− [YIIIL(LH)]0 LSP−LSP angle (deg) average Ln−N (Å)

[YIII(LH)2]+ [YIII(LH2)2]3+

50.53

41.94

47.71

59.91

2.470

2.465

2.457

2.490

N−N (Å)

6.173 6.173

3.213 5.517

3.191 3.191

7.320 7.320

δ (deg)

134.48 134.48

150.77 151.36

162.05 162.06

120.32 120.32

ε (deg)

4.95 4.95 4.95 4.96

4.07 22.24 5.18 22.47

24.37 23.70 23.73 24.38

10.04 10.04 10.04 10.04

and protonation of complexes were achieved by adding excess amounts (ca. 50 equiv) of DBU and acetic acid, respectively, to their solutions.26 The macrocyclic H2L ligand was synthesized following a previously reported procedure.7 Synthesis of the complexes was performed using an adapted procedure for lanthanoid porphyrin double-decker complexes.34 Synthesis of Indolenine-Substituted Annulene Ligands (H2L and H4L2+). An ethanol solution of 2-diformylmethylidene-3,3dimethylindole (430 mg, 2 mmol), 1,2-diaminebenzene (216 mg, 2 mmol), and acetic acid (0.5 mL) was refluxed for 1 h. The orange precipitate was collected by filtration, washed with ethanol, and dried 6518


Article

Inorganic Chemistry corrections were calculated by using Pascal’s constants.35 The magnetic susceptibility data were recorded in the temperature range of 1.8−300 K in an external field of 1000 Oe. The dynamic magnetic susceptibility data were obtained in the frequency range of 0.1−1000 Hz in an oscillating field of 3 Oe. For the solution-state magnetic measurements, since χpara of the ion decreases with an increase in the temperature, the ratio χpara/χdia and the measured signal decrease. To minimize this problem, a saturated solution was used. The solutions were prepared by dissolving 1 in a minimum amount of CHCl3. After filtration, ca. 1 cm (0.2 mL) of solution was carefully added to an NMR tube. The masses of the sample and the solvent were measured after the measurement and confirmed with the values from the MH curve in solid and solution states. Increasing the temperature induced thawing of the solution and precipitation of the complex. Thus, the data are not for the solution state but for the solid state, which is not desired. For this reason, χT values were not measured, and the sample was held in the magnetometer at 10 K to freeze the solution and avoid precipitation. NMR Measurements. NMR spectra were collected in a magnetic field of 7.9 T (400 MHz) on a Bruker Avance II instrument with a BBFO probe and in a field of 14.1 T (600 MHz) with a Bruker Avance III instrument with a QNP Cryoprobe (inner coil tuned to 13C, cold preamplifier). Prior to VT measurements, temperature calibration was performed via the method of Berger et al. by using temperature dependence of the residual nondeuterated methanol signal in deuterated methanol.36 Solvent resonances were used as references for all spectra.37 Samples for NMR spectroscopy were prepared and stored in an inert gas atmosphere in NMR tubes with J. Young valves (Teflon plugs). Due to the low solubility of the studied complexes, only 1H NMR experiments were performed. Because of larger hyperfine shifts and smaller line widths, 2 was used for the NMR analysis. DFT Calculations. To elucidate the position of the acidic proton in the neutral complexes, the approach utilized in our previously reported work was applied.26 DFT structure optimizations were performed by starting from the molecular structure of the neutral complex and by varying the position of the acidic proton (either on the coordinating N-atom or indolenine N-atom of the annulene ligand). These optimizations were performed using the diamagnetic YIII ion. This was done to save computing time since the ionic radii of YIII, TbIII, and DyIII ions are nearly the same (115.9 pm for YIII ions, 116.7 pm for TbIII ion, and 118.0 pm for DyIII ion in 8-coordinated complexes).38 DFT calculations were performed using the software package Gaussian 09 (revision D.01).39 Geometry optimizations were done using the restricted B3LYP functional.40,41 The def2svp15,16 basis set was used for all light atoms, and the Stuttgart ECP basis set was used for the YIII ion.42−44 We performed optimizations both with and without the inclusion of dispersion interactions (Grimme DFT-D3 with BeckeJohnson damping).13,45−47 The structures obtained without the dispersion correction were in better agreement with the molecular structures of the complexes (Supporting Information) and, thus, were used in subsequent single-point calculations. The quadratically convergent self-consistent field (SCF)23 procedure was utilized when convergence could not be achieved using the default SCF procedure, and a “superfine” integration grid was applied. No symmetry restrictions were imposed during the optimizations (keyword “nosymm”). The absence of imaginary frequencies was used in identifying stationary points on the potential energy surface. Singlepoint calculations of the optimized geometries were done with the same approach as the optimizations and with the def2tzvp15,16 basis set used on all atoms. Energies from single-point calculations and xyz coordinates of optimized structures are given in the Supporting Information. X-ray Crystallography. The crystal structures measurements were performed at 296 K (1) and 110 K (2) on a Rigaku Saturn70 CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 nm) produced by a VariMax microfocus X-ray rotating anode source. The crystal structures were solved via SIR-201148 and refined by using full-matrix least-squares based on F2 in SHELX.49 Except for solvent and H-atoms, anisotropic refinement was performed. Non-

hydrogen atoms of the solvent molecules were refined anisotropically as much as possible. Hydrogen atoms were added as a rigid model. The H-atoms of the water molecules were determined by using the HYDROGEN50 subroutine of WinGX. The crystallographic data are deposited as CCDC Nos. 1546794−1546797.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00626. Crystallographic data, VT 1H NMR spectra of complexes, results of DFT calculations, magnetic data, and additional details, including Figures S1−S16 and Tables S1−S11 (PDF) Accession Codes

CCDC 1546794−1546797 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.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Goulven Cosquer: 0000-0003-2692-1230 Markus Enders: 0000-0003-0415-1992 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We appreciate the support from the IGPAS program of Tohoku University and the Japanese Government (Monbukagakusho; scholarship for Z.L.) and from the Beilstein-Institut zur Förderung der Chemischen Wissenschaften and the GermanJapanese University Consortium (HeKKSaGOn; scholarship and travel support for M.D.). The authors acknowledge support for computational resources by the state of Baden-Württemberg (Germany) through bwHPC and the German Research Foundation (DFG) through grant no. INST 40/467-1 FUGG.

■

REFERENCES

(1) Bogani, L.; Wernsdorfer, W. Molecular Spintronics Using SingleMolecule Magnets. Nat. Mater. 2008, 7 (3), 179−186. (2) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. Lanthanide Double-Decker Complexes Functioning as Magnets at the Single-Molecular Level. J. Am. Chem. Soc. 2003, 125 (29), 8694−8695. (3) Wang, H.; Wang, B.-W.; Bian, Y.; Gao, S.; Jiang, J. SingleMolecule Magnetism of Tetrapyrrole Lanthanide Compounds with Sandwich Multiple-Decker Structures. Coord. Chem. Rev. 2016, 306 (P1), 195−216. (4) Tang, J.; Zhang, P. Lanthanide Single Molecule Magnets; Springer: Berlin, 2015. (5) Horii, Y.; Katoh, K.; Yasuda, N.; Breedlove, B. K.; Yamashita, M. Effects of f-f Interactions on the Single-Molecule Magnet Properties of Terbium(iii)-Phthalocyaninato Quintuple-Decker Complexes. Inorg. Chem. 2015, 54 (7), 3297−3305. (6) Williams, U. J.; Mahoney, B. D.; DeGregorio, P. T.; Carroll, P. J.; Nakamaru-Ogiso, E.; Kikkawa, J. M.; Schelter, E. J. A Comparison of

6519


Article

Inorganic Chemistry the Effects of Symmetry and Magnetoanisotropy on Paramagnetic Relaxation in Related Dysprosium Single Ion Magnets. Chem. Commun. 2012, 48 (45), 5593−5595. (7) Khaledi, H.; Olmstead, M. M.; Ali, H. M.; Thomas, N. F. Indolenine Meso-Substituted Dibenzotetraaza[14]annulene and Its Coordination Chemistry toward the Transition Metal Ions MnIII, FeIII, CoII, NiII, CuII, and PdII. Inorg. Chem. 2013, 52, 1926−1941. (8) Saltsman, I.; Goldberg, I.; Balazs, Y.; Gross, Z. CCDC 609953. Exp. Cryst. Struct. Determ. 2007, DOI: 10.5517/ccngpxp. (9) Bissell, E. C. CCDC 115712. Exp. Cryst. Struct. Determ. 1970, DOI: 10.5517/cc3wdn0. (10) Ruiz-Martínez, A.; Casanova, D.; Alvarez, S. Polyhedral Structures with an Odd Number of Vertices: Nine-Coordinate Metal Compounds. Chem. - Eur. J. 2008, 14 (4), 1291−1303. (11) Alvarez, S.; Alemany, P.; Casanova, D.; Cirera, J.; Llunell, M.; Avnir, D. Shape Maps and Polyhedral Interconversion Paths in Transition Metal Chemistry. Coord. Chem. Rev. 2005, 249, 1693− 1708. (12) Peters, J. A.; Huskens, J.; Raber, D. J. Lanthanide Induced Shifts and Relaxation Rate Enhancements. Prog. Nucl. Magn. Reson. Spectrosc. 1996, 28 (3−4), 283−350. (13) Bertini, I.; Luchinat, C.; Parigi, G. Solution NMR of Paramagnetic Molecules - Applications to Metallobiomolecules and Models; Elsevier: Amsterdam, 2001. (14) Damjanović, M.; Morita, T.; Katoh, K.; Yamashita, M.; Enders, M. Ligand Pi-Radical Interaction with F-Shell Unpaired Electrons in Phthalocyaninato-Lanthanoid Single-Molecule Magnets: A Solution NMR Spectroscopic and DFT Study. Chem. - Eur. J. 2015, 21 (41), 14421−14432. (15) Rodríguez-Rodríguez, A.; Esteban-Gómez, D.; De Blas, A.; Rodríguez-Blas, T.; Botta, M.; Tripier, R.; Platas-Iglesias, C. Solution Structure of Ln(III) Complexes with Macrocyclic Ligands through Theoretical Evaluation of 1H NMR Contact Shifts. Inorg. Chem. 2012, 51 (24), 13419−13429. (16) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7 (18), 3297−3305. (17) Weigend, F. Accurate Coulomb-Fitting Basis Sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8 (9), 1057−1065. (18) Dolg, M.; Stoll, H.; Savin, A.; Preuss, H. Energy-Adjusted Pseudopotentials for the Rare Earth Elements. Theor. Chim. Acta 1989, 75 (3), 173−194. (19) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Climbing the Density Functional Ladder: Non-Empirical Meta-Generalized Gradient Approximation Designed for Molecules and Solids. Phys. Rev. Lett. 2003, 91 (14), 146401. (20) Roca-Sabio, A.; Regueiro-Figueroa, M.; Esteban-Gómez, D.; de Blas, A.; Rodríguez-Blas, T.; Platas-Iglesias, C. Density Functional Dependence of Molecular Geometries in lanthanide(III) Complexes Relevant to Bioanalytical and Biomedical Applications. Comput. Theor. Chem. 2012, 999, 93−104. (21) Esteban-Gómez, D.; de Blas, A.; Rodríguez-Blas, T.; Helm, L.; Platas-Iglesias, C. Hyperfine Coupling Constants on Inner-Sphere Water Molecules of Gd III -Based MRI Contrast Agents. ChemPhysChem 2012, 13 (16), 3640−3650. (22) Barysz, M.; Sadlej, A. J. Two-Component Methods of Relativistic Quantum Chemistry: From the Douglas-Kroll Approximation to the Exact Two-Component Formalism. J. Mol. Struct.: THEOCHEM 2001, 573, 181−200. (23) Pulay, P. Improved SCF Convergence Acceleration. J. Comput. Chem. 1982, 3 (4), 556−560. (24) Bacskay, G. B. A Quadratically Convergent Hartree-Fock (QCSCF) Method. Application to Closed Shell Systems. Chem. Phys. 1981, 61 (3), 385−404. (25) Barone, V.; Adamo, C.; Russo, N. Density Functional Approach to the Structures and EPR Parameters of Open Shell Systems. The Case of Fluorovinyl Radicals. Chem. Phys. Lett. 1993, 212 (1−2), 5− 11.

(26) Pantazis, D. A.; Neese, F. All-Electron Scalar Relativistic Basis Sets for the Lanthanides. J. Chem. Theory Comput. 2009, 5 (9), 2229− 2238. (27) Damjanović, M.; Horie, Y.; Morita, T.; Horii, Y.; Katoh, K.; Yamashita, M.; Enders, M. α-Substituted Bis(octabutoxyphthalocyaninato)Terbium(III) Double-Decker Complexes: Preparation and Study of Protonation by NMR and DFT. Inorg. Chem. 2015, 54 (24), 11986−11992. (28) Damjanović, M.; Katoh, K.; Yamashita, M.; Enders, M. Combined NMR Analysis of Huqe Residual Dipolar Couplings and Pseudocontact Shifts in Terbium(lll)-Phthalocyaninato Single Molecule Magnets. J. Am. Chem. Soc. 2013, 135, 14349−14358. (29) Kahn, O. Molecular Magnetism; Wiley-VCH: New York, 1993. (30) Kamila, M.; Cosquer, G.; Breedlove, B. K.; Yamashita, M. Packing Structure Effects on the Slow Magnetic Relaxation Pathways of Dysprosium (III) Complexes. Bull. Chem. Soc. Jpn. 2017, 90, 595. (31) Havriliak, S.; Negami, S. A Complex Plane Representation of Dielectric and Mechanical Relaxation Processes in Some Polymers. Polymer 1967, 8, 161−210. (32) Cosquer, G.; Pointillart, F.; Golhen, S.; Cador, O.; Ouahab, L. Slow Magnetic Relaxation in Condensed versus Dispersed dysprosium(III) Mononuclear Complexes. Chem. - Eur. J. 2013, 19 (24), 7895−7903. (33) Baldoví, J. J.; Borrás-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.; Gaita-Ariño, A. Modeling the Properties of Lanthanoid Single-Ion Magnets Using an Effective Point-Charge Approach. Dalt. Trans. 2012, 41 (44), 13705. (34) Zhang, Y.; Cao, W.; Wang, K.; Jiang, J. Constructing Bis(porphyrinato) Rare Earth Double-Decker Complexes Involving N-Confused Porphyrin. Dalt. Trans. 2014, 43, 9152−9157. (35) Bain, G. A.; Berry, J. F. Diamagnetic Corrections and Pascal’s Constants. J. Chem. Educ. 2008, 85 (4), 532−536. (36) Findeisen, M.; Brand, T.; Berger, S. A 1H-NMR Thermometer Suitable for Cryoprobes. Magn. Reson. Chem. 2007, 45 (2), 175−178. (37) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist. Organometallics 2010, 29 (9), 2176−2179. (38) Holleman, A.; Wiberg, E.; Wilberg, N. Lehrbuch Der Anorganischen Chemie; W. de Gruyter: Berlin, 1995; Vol. 101. (39) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian Inc.: Wallingford, CT, 2009. (40) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98 (7), 5648. (41) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (42) Bergner, A.; Dolg, M.; Küchle, W.; Stoll, H.; Preuß, H. Ab Initio Energy-Adjusted Pseudopotentials for Elements of Groups 13−17. Mol. Phys. 1993, 80 (6), 1431−1441. (43) Kaupp, M.; Schleyer, P. V. R.; Stoll, H.; Preuss, H. Pseudopotential Approaches to Ca, Sr, and Ba Hydrides. Why Are Some Alkaline Earth MX2 Compounds Bent? J. Chem. Phys. 1991, 94 (2), 1360−1366. 6520


Article

Inorganic Chemistry (44) Dolg, M.; Stoll, H.; Preuß, H.; Pitzer, R. M. Relativistic and Correlation Effects for Element 105 (Hahnium, Ha): A Comparative Study of M and MO (M = Nb, Ta, Ha) Using Energy-Adjusted Ab Initio Pseudopotentials. J. Phys. Chem. 1993, 97 (22), 5852−5859. (45) Kitagawa, Y.; Kawakami, T.; Yamanaka, S.; Okumura, M. DFT and DFT-D Studies on Molecular Structure of Double-Decker Phthalocyaninato-terbium(III) Complex. Mol. Phys. 2014, 112 (7), 995−1001. (46) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32 (7), 1456−1465. (47) Grimme, S. Dispersion Interaction and Chemical Bonding. In The Chemical Bond; Wiley-VCH: Weinheim, 2014. (48) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G.; Polidori, G.; Spagna, R. SIR97: A New Tool for Crystal Structure Determination and Refinement. J. Appl. Crystallogr. 1999, 32, 115. (49) Sheldrick, G. A Short History of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (50) Nardelli, M. Modeling Hydroxyl and Water H Atoms. J. Appl. Crystallogr. 1999, 32 (3), 563−571.

6521


Proton Control of the Lanthanoid Single-Ion Magnet Behavior of a

Recommend Documents