Phthalocyaninato Quintuple-Decker Compl - American Chemical Society

Mar 23, 2015 - Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo-cho, ... eight states (I−J coupling).20 DyPc2 also exhibi...
0 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/IC

Effects of f−f Interactions on the Single-Molecule Magnet Properties of Terbium(III)−Phthalocyaninato Quintuple-Decker Complexes Yoji Horii,† Keiichi Katoh,*,†,‡ Nobuhiro Yasuda,§ Brian K. Breedlove,† and Masahiro Yamashita*,†,‡ †

Department of Chemistry, Graduate School of Science, Tohoku University, 6-3, Aramaki-Aza-Aoba, Aoba-ku, Sendai, Miyagi 980-8578, Japan ‡ CREST, JST, 4-1-8, Honcho, Kawaguchi, Saitama 332-0012, Japan § Japan Synchrotron Radiation Research Institute, SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan S Supporting Information *

ABSTRACT: Single-molecule magnet (SMM) properties of terbium(III)−phthalocyaninato quintuple-decker complex TbCdCdTb were studied and were compared with those of other multiple-decker complexes (triple-decker: TbTb, quadruple-decker: TbCdTb) to elucidate the relationship between magnetic dipole interactions and SMM properties. From Xray crystallography performed with synchrotron radiation, the TbIII−TbIII distance in TbCdCdTb was determined to be 9.883 Å. From alternating current magnetic studies on TbCdCdTb, the activation energy for spin reversal (Δ) increased with an increase in the direct current magnetic field (Hdc). This behavior is similar to that of TbCdTb, although the increase in Δ for TbCdTb is smaller. On the other hand, for TbTb, which has shortest TbIII−TbIII distance, Δ did not depend on Hdc, indicating that there is a correlation between SMM properties and the strength of the TbIII−TbIII interactions. By comparing the Zeeman diagrams for multiple-decker complexes, we found that the TbIII−TbIII interactions affected the magnetic field regions where quantum tunnelling of the magnetization was active. The results obtained from Zeeman diagrams are consistent with the results obtained from the magnetic studies.

■

levels with angular momenta (expressed as |JZ>) of |0>, |±1>, | ±2>, |±3>, |±4>, |±5>, and |±6> due to ligand-field splitting from phthalocyaninato ligand.14 As a consequence, there is an energy gap between the |±5> and |±6> levels of ∼400 cm−1, which is attributed to Δ (Supporting Information, Figure S2). The total angular momentum of |±6> couples with the nuclear spins of the TbIII ions (I = 3/2), and the ground state splits into eight states (I−J coupling).20 DyPc2 also exhibits SMM properties, but the electronic structure is more complicated than that of TbPc2 due to complicated ligand-field splitting, a half integer I, and the existence of nuclear isomers.14,20 In addition to the ligand-field splitting and I−J coupling, magnetic dipole (MD) interactions between TbPc2 units affect the SMM properties.31−44 Multiple-decker complexes, which have two TbPc2 units connected by diamagnetic phthalocyaninato

INTRODUCTION Single-molecule magnets (SMMs) exhibit several interesting properties, such as slow magnetic relaxation1,2 and quantum tunnelling of the magnetization (QTM).3−5 Moreover, SMMs show magnetic hysteresis loops at low temperature even if they are isolated molecules and, thus, are good candidates for use as bits in ultrahigh density information storage devices. 6 Lanthanide(III) (LnIII) complexes have been extensively studied because they are relatively easy to design7−10 and synthesize, and a considerable number of LnIII SMMs exhibit high activation energies for spin reversal (Δ)11−13 compared to transition metal SMMs. In addition, TbIII−bisphthalocyaninato double-decker complexes (TbPc2) have been studied for use in spintronic devices because they show excellent chemical stability and high Δ values.23−30 The electronic structure of TbPc2 reported by Ishikawa and co-workers14−22 is shown in Supporting Information, Figure S1. In TbPc2 SMMs, the ground multiplet of the TbIII ions, 7F6, splits into seven energy © XXXX American Chemical Society

Received: December 10, 2014

A

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ligands or CdII ions,45−48 can be used to elucidate the effects of the MD interactions because these complexes have different intramolecular Tb III−Tb III distances, meaning that the magnitudes of the MD interactions are different (Figure S1). Previously, we reported the magnetic properties of triple, quadruple, and quintuple-decker complexes having TbIII or DyIII ions and showed that changes in the MD interactions affect the magnetic properties.39−43 Fukuda and co-workers38 and Sakaue and co-workers36 suggest that the effects of the MD interactions can be explained by considering the Zeeman diagrams for the complexes. Fukuda and co-workers38 have reported the magnetic properties of a mono-TbIII quadrupledecker complex, which contains diamagnetic YIII and magnetic TbIII ions, and a di-TbIII quadruple-decker complex and have explained the dynamic magnetic properties of these complexes on the basis of their Zeeman diagrams, which had previously only been used for explaining magnetic hysteresis loops for SMMs.20,49 Herein we use Zeeman diagrams to discuss the magnetic properties of TbIII−Pc complexes with CdII ions. In this study, by comparing the magnetic properties of TbIII quintuple-decker (TbCdCdTb), Tb III quadruple-decker (TbCdTb), and triple-decker (TbTb) complexes, we show how the magnitude of the MD interactions affects the SMM properties. Although we previously reported that TbCdCdTb and TbCdTb show similar magnetic properties,40 there are slight differences due to the differences in the intramolecular TbIII−TbIII interactions (i.e., MD interactions). Every complex in this article contains the butoxy-substituted phthalocyaninato ligand 2,3,9,10,16,17,23,24-octabutoxy-phthalocyaninato (obPc) to improve the solubility and, therefore, the crystallinity of the complexes,39 which made it possible to determine the crystal structure of TbCdCdTb for the first time (see Figure 1).

Dibutoxyphthalonitrile was synthesized according to a reported method.50 Synthesis of H2(obPc)..51,52 3,4-Dibutoxyphthalonitrile (1 g, 3.7 mmol) and Li metal wire (200 mg), which was cut into small pieces, were placed in a 50 mL two-necked flask, and the flask was quickly purged with N2 to prevent the Li from being exposed to moisture. 2Dimethylaminoethanol (20 mL) was added to the flask, and the solution was refluxed with vigorous stirring for 8 h. After the mixture cooled to room temperature, acetic acid (30 mL) was slowly added dropwise to the reaction mixture, and then the solution was stirred for 30 min. The resulting green solution was extracted with CHCl3/ saturated (sat.) NaHCO3 (aq) and CHCl3/H2O. The CHCl3 extracts were dried over MgSO4, and the mixture was separated by using column chromatography over silica gel (Wakogel C-200/CHCl3). A deep green fraction was collected. Recrystallization from CHCl3/ MeOH gave a deep green powder of H2(obPc) (180 mg, 18%). Anal. Calcd (%) for C64H82N8O8: C 70.43, H 7.57, N 10.27. Found: C 70.43, H 7.48, N 10.31. Synthesis of M(obPc)2 (M = TbIII, Y III )..40,43,44,49 3,4Dibutoxyphthalonitrile (1 g, 3.7 mmol), M(CH3CO2)3·4H2O (190 mg, 0.46 mmol) and 1,8-diazabicyclo[5.4.0]undec-7-ene (80 μL) were refluxed in 1-hexanol (6 mL) for 20 h. The solvent was removed under reduced pressure. Column chromatography over silica gel with CHCl3 as the eluent afforded a green band and a deep blue band, which contained M(obPc)2 and a triple-decker complex (MM), respectively. The green band was further purified by using chromatography over Bio-Beads S-X1 with tetrahydrofuran (THF) as the eluent. Recrystallization from CHCl3/MeOH gave black crystals of M(obPc)2 (130 mg, 12%−21%). Tb(obPc) 2 : Anal. Calcd (%) for C128H160N16O16Tb: C 65.77, H 6.90, N 9.59. Found: C 65.83, H 6.76, N 9.57. Y(obPc)2: Anal. Calcd (%) for C128H160N16O16Y: C 67.80, H 7.11, N 9.88. Found: C 67.68, H 7.09, N 9.92. Synthesis of TbTb. TbTb was obtained as a byproduct in the preparation of Tb(obPc)2 and obtained as a blue band from a silica gel column. Purification of the blue band by using size exclusion chromatography over Bio-Beads S-X1 with THF as the eluent and recrystallizing from CHCl3/EtOH gave black crystals of TbTb (218 mg, 26%). Anal. Calcd (%) for C192H240N24O24Tb2: C 64.31, H 6.75, N 9.37. Found: C 64.39, H 6.67, N 9.43. Synthesis of TbCdTb..37,40,46 Tb(obPc)2 (100 mg, 0.042 mmol) and Cd(CH3CO2)2·2H2O (29 mg, 0.105 mmol) were refluxed in 1hexanol (11 mL) under an N2 atmosphere for 3 h. After the mixture cooled to room temperature, excess methanol (ca. 50 mL) was added to the reaction mixture. The resulting deep purple precipitate was collected by filtering over Celite and then was washed with methanol to remove the remaining 1-hexanol. The precipitate was extracted with CH2Cl2, affording a deep blue solution. After evaporating the CH2Cl2, the product was purified by using column chromatography over silica gel (Wakogel C-200) with CHCl3 as the eluent. After the green band containing Tb(obPc)2 eluted, the deep blue fraction containing TbCdTb was collected. Further purification by using gel permeation column chromatography (Bio-Beads S-X1/THF), followed by recrystallization from CH2Cl2/EtOH, afforded block-shaped black crystals of TbCdTb (65 mg, 63%). Anal. Calcd (%) for C256H320N32O32CdTb2: C 64.22, H 6.74, N 9.36. Found: C 63.97, H 6.80, N 9.34. Synthesis of MCdCdM..40,47 A mixture of M(obPc)2 (100 mg, 0.042 mmol), H2(obPc) (46 mg, 0.042 mmol), and Cd(CH3CO2)2· 2H2O (57 mg, 0.21 mmol) was refluxed in dry 1,2,4-trichlorobenzene (10 mL) under a N2 atmosphere for 3 h. After the reaction, the solvent was removed under reduced pressure. The obtained solid was purified by using column chromatography over silica gel (Wako gel C-200) with CHCl3 as the eluent, and the green fraction containing unreacted M(obPc)2 and H2(obPc) was removed. After further elution, a deep blue fraction containing MCdM, MCdCdM, and sextuple-decker (MCdCdCdM)48 was obtained. The solvent was evaporated from the deep blue fraction, and the residue was purified by using gelpermeation chromatography on a NEXT Recycling Preparative HPLC (LC-9210II/JAi) with a 99.5:0.5 (v/v) CHCl3/NEt3 solution as the eluent. After evaporation of the solvent, the complex was dissolved in a

Figure 1. Illustration of TbIII−obPc multiple-decker complexes.

■

EXPERIMENTAL SECTION

The purchased reagents and solvents were used without further purification. Terbium(III) acetate tetrahydrate Tb(CH3CO2)3·4H2O, yttrium(III) tetrahydrate Y(CH3CO2)3·4H2O, 1,8-diazabicyclo[5.4.0]undec-7-ene, 2-dimethylaminoethanol, 1-hexanol, and cadmium acetate dihydrate Cd(CH3CO2)2·2H2O were purchased from Wako chemicals. Metal lithium wire (diameter is 3 mm) and dry 1,2,4trichlorobenzene were purchased from Aldrich. The solvents used for column chromatography were special grade from Wako chemicals. 3,4B

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. (a) Side view and (b) top view of the crystal structure of TbCdCdTb. Butoxy chains are highly disordered, and only the chains with highest occupancy are shown for clarity. The crystal packing of TbCdCdTb is shown as (c) a two-dimensional array extended in the ab plane and (d) a slipped column along the c-axis (pink; Tb, orange; Cd, blue, N, red; O, gray; C). The values in the figure are the intermolecular TbIII−TbIII distances. minimum amount of CH2Cl2, to which EtOH was diffused to afford black block crystals (21%−26%). TbCdCdTb: electrospray ionization mass spectrometry (ESI-MS): m/z (%): 2994.86 [M]2+. Anal. Calcd (%) for C320H400N40O40Cd2Tb2: C 64.17, H 6.73, N 9.35. Found: C 63.92, H 6.78, N 9.33. YCdCdY containing two EtOH and two H2O molecules in the crystal lattice: ESI-MS: m/z (%): 1965.22 [M+H +2Na]3+. Anal. Calcd (%) for C324H413N40O44Cd2Y2: C 65.13, H 6.97, N 9.38. Found: C 65.12, H 6.88, N 9.47. Preparation of Magnetically Diluted Sample (TbCdCdTbd). TbCdCdTb (5.37 mg, 0.896 μmol) and YCdCdY (27.63 mg, 4.723 μmol) were placed together in a sample vial and dissolved in 5 mL of CH2Cl2. After the solution was mixed for 5 min, the CH2Cl2 was evaporated, and the remaining solid was used without further purification (29.24 mg). Physical Property Measurements. UV−vis spectra for the complexes in CHCl3 and toluene solutions (2.5 × 10−6 M) were acquired on a SHIMADZU UV-3100PC in a quartz cell with a path length of 1 cm at 298 K (Supporting Information, Figure S5). IR spectroscopy was performed on KBr pellets on a Jasco FT/IR-4200 spectrometer at 298 K (Supporting Information, Figure S6). Cyclic voltammetry and differential pulse voltammetry measurements were performed using a 620-D electrochemical analyzer (ALS). A glassy carbon electrode was used as the working electrode, and a Pt wire was used as the counter electrode. A Ag+/Ag electrode was used as the reference electrode. However, the values were adjusted to the E1/2 value for ferrocenium/ferrocene (Fc+/Fc). A solution containing 100 mM tetrabutylammonium hexafluorophosphate ((TBA)PF6), which was used as the supporting electrolyte, and 0.5 mM sample in superdehydrated CH2Cl2 was purged with N2 gas. The scan rate was 100 mV s−1. ESI-MS (Supporting Information, Figures S3 and S4) and elemental analyses were performed at the Research and Analytical Centre for Giant Molecules, Tohoku University. Magnetic measurements were performed on Quantum Design SQUID magnetometers MPMS-XL and MPMS-5S. Samples were placed in gel capsules and fixed with n-eicosane to prevent them from moving during measurements. The data were corrected for the diamagnetic contributions from the n-eicosane, and the samples themselves were calculated by using Pascal’s law. X-ray Crystal Structure Analysis. Crystallographic data for TbCdCdTb were collected on a large cylindrical IP/CCD camera53 installed in the SPring-8 beamline BL02B1 with a Rigaku Mercury2 CCD camera. The synchrotron radiation was monochromated by using a Si(311) double crystal monochromator (λ = 0.686 80 Å). The measurements were performed at a temperature (T) of 90(2) K.

Crystals were mounted on a MicroMount (MiTeGen) in the mother liquor and immediately cooled to prevent solvent loss. The initial structure was solved by using SIR2004, and structural refinements were performed by using SHELXL-2014.54 The non-hydrogen atoms except for the solvent molecules were refined anisotropically using weighted full-matrix least-squares on F2, and the hydrogen atoms were generated at positions calculated using a riding model. Crystal parameters for TbCdCdTb: C320H400N40O40Tb2Cd2, 2.13 (H2O) 6.405 (C2H6O), Mr = 6322.91, monoclinic, C2/c, a = 24.213(2), b = 38.234(4), c = 35.147(3) Å, β = 99.449(7)°, V = 32 096(5) Å3, Z = 4, ρcalcd = 1.309 g cm−3, R1 = 0.1050, wR2(all) = 0.3616. The crystal data for TbCdCdTb have been deposited at the Cambridge Structural Database (CCDC-913090). Powder X-ray diffraction (PXRD) measurements were performed on crushed polycrystalline samples loaded into the capillary (diameter: 0.8 mm, length: 80 mm, Hilgenderg) with mother liquor by using an AFC-7R/ LW (Rigaku). PXRD patterns were simulated from the single-crystal data by using Mercury 3.0.55

■

RESULTS AND DISCUSSIONS Structural Analysis. TbCdCdTb crystallized in the monoclinic space group C2/c. TbCdCdTb has two Tb(obPc)2 units connected by one obPc and two CdII ions. The Tb(obPc)2 units are crystallographically equivalent, and the intramolecular TbIII−TbIII distance was determined to be 9.883 Å. The stacking angle between obPc ligands coordinated to the TbIII ions was determined to be 21.8°, and the TbIII ion is located closer to obPc(1) than it is to obPc(2) due to electroreplusion between the TbIII and CdII ions coordinated by obPc(2) (Figures 2a and 3 and Table 1). Similar behavior can be seen for TbCdTb and TbTb but not for mononuclear Tb(obPc)2.40 In other words, the coordination environments around the TbIII ions in TbCdCdTb, TbCdTb, and TbTb are distorted square antiprisms, and the overall symmetry of the complex is low in comparison to that of Tb(obPc)2 (nearly square antiprism). The intramolecular TbIII−TbIII distance in TbCdCdTb is clearly longer than those in the other multipledecker complexes. The stacking angles involving the TbIII ions (θ) become narrower with an increase in the number of stacks. The coordination environment around the TbIII ions of TbCdCdTb resembles that of TbCdTb40 due to the similarities in θ and the obPc−TbIII distances (A, B, and C in Figure 3 and C

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 3. Schematic illustration of the square antiprism formed by the TbIII−obPc complexes. A, B, and C represent the obPc(1)−obPc(2), obPc(1)−Tb, and obPc(2)−Tb distances, respectively. Stacking angle between obPc(1) and obPc(2) is expressed as θ. The raw data for the TbIII−obPc complexes are listed in Table 1.

Figure 4. PXRD patterns for TbCdCdTb and YCdCdY at 293 K. The black pattern was simulated from the single-crystal X-ray data for TbCdCdTb.

Table 1). TbCdCdTb complexes in the ab plane are wellseparated from each other because the butoxy chains of obPc extend out in the ab plane, preventing the molecules from approaching closely (Figure 2c). On the other hand, along the c axis, TbCdCdTb molecules are arranged in slipped columns due to the π−π interactions between obPc(1), and the nearest TbIII−TbIII distance was determined to be 11.994 Å (Figure 2d). PXRD patterns for crystals of YCdCdY were similar to those for TbCdCdTb (Figure 4), indicating that crystal structure of YCdCdY is similar to that of TbCdCdTb. Electrochemical Analysis. To elucidate the redox properties of TbCdCdTb, cyclic voltammograms (CVs) and differential pulse voltammograms (DPVs) were acquired (Supporting Information, Figure S7 and Table S1). In addition, CVs and DPVs were acquired for Tb(obPc)2, TbTb, and TbCdTb under the same experimental conditions for comparison. CVs for TbCdCdTb showed four oxidation waves at 0.335, 0.154, −0.226, −0.429 V and four reduction waves at −1.582, −1.760, −2.053, and −2.316 V (Figure 5, red line). These values are similar to those for the quintuple-decker complexes (NdCdCdNd, SmCdCdSm, EuCdCdEu) reported by H. Wang and co-workers.47 In DPVs for TbCdCdTb, five peaks for the oxidation processes and four peaks for the reduction process were observed (Supporting Information, Figure S7). The E1/2 values, except for 0.677 V, are consistent with those obtained from the CVs. E1/2 values from the DPVs for the TbIII−obPc complexes are shown in Figure 6. The gap between each E1/2 value became narrower with an increase in the number of stacks, indicating that there is a decrease in the onsite Coulomb repulsion due to the longitudinal extension of the π-conjugated system. It is thought that holes or electrons generated during the redox processes are delocalized over the obPc columns in multiple-decker complexes.45,56 The same behavior was observed in the CV measurements (Supporting Information, Table S2). Magnetic Measurements. Previously, we reported the magnetic properties of TbIII−obPc multiple-decker complexes.40 In direct current (dc) magnetic susceptibility

Figure 5. CVs for the TbIII−obPc complexes at 293 K. Measurements were performed using CH2Cl2 solutions containing 0.5 mM of the analyte and 100 mM (TBA)PF6 as the electrolyte. The scan rate was 100 mV s−1. The values were referenced to the E1/2 value for the Fc+/ Fc couple. Glassy carbon was used as the working electrode, and a Pt wire was used as the counter electrode.

measurements, χMT versus T plots for TbTb, TbCdTb, and TbCdCdTb showed an increase in χMT in the low T region, as shown in Supporting Information, Figure S8, due to ferromagnetic interactions between the TbIII ions. The maximum value of χMT increases in the order of TbCdCdTb < TbCdTb < TbTb because the MD interactions become stronger with a decrease in the intramolecular TbIII−TbIII distance. In addition, the χMT values of the diluted sample of TbCdCdTb (TbCdCdTbd), which was a 1:5 mixture of TbCdCdTb and YCdCdY, indicate that intramolecular interactions between the TbIII ions should occur (Supporting Information, Figure S9). To determine the relationship between the magnetic relaxation pathway and the magnitude of the TbIII interactions,

Table 1. Structural Parameters for the TbIII−obPc Complexes

obPc(1)−obPc(2) distance (A), Å obPc(1)−TbIII distance (B), Å obPc(2)−TbIII distance (C), Å stacking angle (θ), degree intramolecular TbIII−TbIII distance, Å

TbCdCdTb

TbCdTb

TbTb

Tb(obPc)2

3.028 1.343 1.686 21.8 9.883

3.193 1.357 1.686 23.0 6.629

3.047 1.290 1.758 31.8 3.517

2.834 1.435 1.398 44.7

D

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

tion, Figure S16). The ac χ value for each complex depended on the frequency (ν), indicating that TbTb, TbCdTb, and TbCdCdTb are SMMs.40 Alternating Current Magnetic Susceptibility of TbCdCdTb. Figure 7 shows ac χM″T versus T plots for TbCdCdTb. As Hdc increased, the χM″T peaks became sharper and shifted to the higher T region due to suppression of QTM upon applying an Hdc. Suppression of QTM induces Zeeman splitting, and thus, an energy gap between the up and down spin ground states occurs (Supporting Information, Figure S2 and S18). The same behavior was observed for TbCdCdTbd (Supporting Information, Figure S10). Figure 8 shows Arrhenius plots of the peak top T values from the χM″T versus T plots and the ac ν. Magnetic relaxation time (τ) is expressed as τ = 1/(2πν). In the high T region, τ for TbCdCdTb strongly depended on T because an Orbach process (Supporting Information, Figure S2), where the spin reversal occurs through first excited Stark sublevels, is dominant. In the low T region, τ was weakly dependent on T. When Hdc was zero or weak, the weak dependence of τ at low T is due to mixing of the Orbach process and QTM. In an

Figure 6. Energy level diagrams of the E1/2 values of the TbIII−obPc complexes obtained from DPVs for 0.5 mM sample solutions in 100 mM (TBA)PF6 CH2CH2 solutions. Pulse width = 0.2 s, sampling width = 0.02 s, pulse period = 0.5 s (Supporting Information, Figure S7 and Table S1).

alternating current (ac) magnetic studies (χM′ (in-phase)T and χM″ (out-of-phase)T vs T plots) in various dc fields (Hdc) were conducted on TbCdCdTb (Figure 7), TbCdTb (Supporting Information, Figure S12a,b), and TbTb (Supporting Informa-

Figure 7. χM″T vs T plots for TbCdCdTb in several Hdc. χM″T peaks become sharper with an increase in Hdc from 0 to 500 Oe. Hdc dependence was weak in the Hdc range of 1000−3000 Oe. E

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Hdc is applied because the energy gap induced by Zeeman effects is a few wavenumbers at most. In other words, the large changes in Δ indicate the suppression of QTM. When Hdc < 1000 Oe, QTM of TbCdCdTb was partially suppressed, which was shown by Δ < 200 cm−1. When Hdc > 1000 Oe, Δ saturated at ∼200 cm−1 because QTM was completely suppressed, and Δ did not increase above Hdc = 1000 Oe, again showing that QTM for TbCdCdTb was completely suppressed when Hdc > 1000 Oe. Comparison of the Alternating Current Magnetic Behaviors of TbCdCdTb and TbCdTb. The same magnetic measurements were conducted on TbCdTb. TbCdTb showed magnetic properties close to those of TbCdCdTb, but there were differences in the Hdc dependence of Δ. In the case of TbCdTb, Δ reached 200 cm−1 at 1500 Oe (Figure 9, blue

Figure 9. Δ vs Hdc plots for TbCdCdTb, TbCdTb, and TbTb.

plots), whereas that for TbCdCdTb reached 200 cm−1 at 1000 Oe, meaning that TbCdTb requires a stronger Hdc to suppress QTM completely than TbCdCdTb does. As shown in Figure 8, τ for TbCdCdTb (τ(TbCdCdTb)) was shorter than that for TbCdTb (τ(TbCdTb)) in a zero Hdc, and they approached each other with an increase in Hdc. At 500 Oe, the τ values were basically the same. Above 500 Oe, τ(TbCdCdTb) became longer than τ(TbCdTb). In other words, the ac magnetic behavior of TbCdCdTb is more dependent on Hdc than that of TbCdTb is. Zeeman Diagrams. To understand the differences in the ac χMT values of TbCdTb and TbCdCdTb, we must consider the Zeeman diagrams for both complexes. We previously reported the magnetic properties of TbTb and the magnetic heat capacity (Cm) in an Hdc and showed that the ground states of TbTb in a zero Hdc are composed of stable ferromagnetic | ±6, ±6> and unstable antiferromagnetic |±6, ∓6> states.39 The dependence of Cm of TbTb on Hdc applied along the magnetic easy axis could be reproduced theoretically. The degenerate Jz = |−6, −6>, |+6, +6> is split into Jz = |−6, −6> and Jz = |+6, +6> in an Hdc (Zeeman splitting), and the crossing at ca. ±3500 Oe is due to level crossing of the Jz = |−6, +6>, |+6, −6>, and Jz = |−6, −6> states. Furthermore, in the micro-SQUID experiments, TbTb clearly exhibited a butterfly-shaped hysteresis loop with an Hdc applied along the easy magnetization axis. In the upsweep, the magnetization jump at ca. ±3500 Oe is due to level crossing of the doublet ground state.39 Fukuda and coworkers have explained the ac magnetic properties of quadruple-decker complexes on the basis of Zeeman diagrams,36,38 and the differences in the ac magnetic properties of TbCdCdTb and TbCdTb can be explained in a similar manner.

Figure 8. Arrhenius plots for TbCdCdTb and TbCdTb in several Hdc. Solid straight lines were fitted using the Arrhenius equation. In the Hdc range of 0−250 Oe, τ for TbCdCdTb was faster, and in Hdc of 500 Oe, τ values for both complexes were the same. In the Hdc range of 1000− 3000 Oe, τ for TbCdCdTb was longer.

Hdc strong enough to completely suppress QTM, the weak dependence of τ was attributed to a direct relaxation process (Figure S2), where τ depends on T−1.17,38,57,58 The τ value obtained from the ν dependence data for TbCdCdTb in an Hdc of 2000 Oe (Supporting Information, Figure S11) at low T were proportional to T−1, further indicating that a direct process is occurring (Supporting Information, Figure S15). Figure 8 shows the activation energy for spin reversal (Δ) obtained from fitting the Arrhenius equation (eq 1),17 where kB is the Boltzmann constant and τ0 is frequency factor, using five points from the higher T region in Figure 8. ln τ =

Δ + ln τ0 kBT

(1)

The Δ value increased linearly with an increase in Hdc and reached ∼200 cm−1 at 1000 Oe. This value is similar to that for other TbIII−phthalocyaninato complexes40 and is attributed to the energy gap between the ground and the first excited Stark sublevels. When Hdc = 1000 Oe, Δ slightly fluctuated near 200 cm−1. Δ should not change drastically (50−200 cm−1) when an F

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 10. (a) Zeeman diagrams for the TbIII−obPc multiple-decker complexes. Because of the selection rules for QTM, QTM occurs in the red region in the diagrams. The QTM area for TbCdCdTb is narrower than that for TbCdTb, and therefore, a weaker field is needed for complete suppression of QTM. TbTb showed no QTM around Hdc = 0.

Table 2. τ(TbCdCdTb) and τ(TbCdTb) at 7.3 K in Several Hdc Estimated from the Arrhenius Plots Shown in Figure 8

When two Tb(obPc)2 are in close proximity to each other, ferromagnetic (|±6, ±6>) and antiferromagnetic states (|±6, ∓6>) form, and these states are energetically separated by the MD interactions between the Tb III ions (Supporting Information, Figure S1). In the cases of TbCdCdTb and TbCdTb, the Tb(obPc)2 units lie along the easy axis of magnetization, and the ferromagnetic |±6, ±6> state is more stable than antiferromagnetic (|±6, ∓6>) state is, which is similar to the case of TbTb.39 This explanation is consistent with the fact that these complexes show ferromagnetic interactions in the low T region of the χMT versus T plots (Supporting Information, Figure S8). The energy gap between the |±6, ±6> and |±6, ∓6> states in a zero Hdc becomes larger with an increase in the magnitude of TbIII−TbIII interactions because the strong TbIII−TbIII interactions stabilize |±6, ±6> and destabilize |±6, ∓6>. Thus, the energy gap for TbCdCdTb is smaller than that for TbCdTb. In other words, the Zeeman diagrams shown in Figure 10 are consistent with the experiment data. The widths of the lines in the diagrams are because the coupled states are finely split by I−J coupling. The |±6, ±6> and |±6, ∓6> states of TbCdCdTb and TbCdTb mix in a zero Hdc, whereas those of TbTb do not because the energy gap between these states is large compared to those of TbCdCdTb and TbCdTb.39 Note that QTM between |+6, +6> and |−6, −6> or between |+6, −6> and |−6, +6> is forbidden because it requires simultaneous inversion of two TbIII spins, whose transition probabilities are very low.36,38,50 Possible Mechanism for the Difference in the Alternating Current Magnetic Properties of TbCdCdTb and TbCdTb. In this section, we discuss the difference in the ac magnetic properties of TbCdCdTb and TbCdTb (Figures 8 and 9) by using the Zeeman diagrams in Figure 10. (1). Hdc = 0−500 Oe. From Hdc = 0 to 500 Oe, there is overlap between the |±6, ±6> and |±6, ∓6> states of both TbCdCdTb and TbCdTb in the Zeeman diagrams, indicating that QTM is allowed for both complexes. In this region, the relaxation occurs via an Orbach process and QTM (Supporting Information, Figure S18). The main difference between the complexes is the size of the area where QTM is allowed. When Hdc < 500 Oe, the QTM area (red region in Figure 10) for TbCdCdTb is larger than that for TbCdTb, which means that the rate of QTM for TbCdCdTb becomes faster than that for TbCdTb. Therefore, τ for TbCdCdTb is smaller than that for TbCdTb (Figure 8: 0 and 250 Oe). In Table 2, τ(TbCdCdTb) and τ(TbCdTb) values at 7.3 K in various Hdc are listed. When Hdc = 0

Hdc, Oe 0 250 500 1000 1500 2000 2500 3000

τ(TbCdCdTb), s 6.6 1.4 5.2 6.0 6.3 7.9 7.4 7.1

× × × × × × × ×

10−3 10−2 10−2 10−2 10−2 10−2 10−2 10−2

τ(TbCdTb), s 1.0 2.2 5.3 4.7 5.5 5.8 6.6 6.1

× × × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

and 250 Oe, τ(TbCdCdTb) is shorter than τ(TbCdTb) is, which is consistent with the above explanation. (2). Hdc = 500 Oe. When Hdc = 500 Oe, both the QTM areas and the τ values for TbCdCdTb and TbCdTb are similar. τ(TbCdCdTb) and τ(TbCdTb) at 500 Oe at 7.5 K were determined to be 5.2 × 10−2 and 5.3 × 10−2 s, respectively. (3). Hdc = 1000 Oe. When Hdc = 1000 Oe, the QTM area for TbCdCdTb is smaller than that for TbCdTb, and τ(TbCdCdTb) at 7.5 K (6.0 × 10−2 s) is greater than τ(TbCdTb) (4.7 × 10−2 s) is. Moreover, QTM in TbCdCdTb is strongly suppressed, and magnetic relaxation mainly occurs via an Orbach process and a direct process (Figure S18). The relaxation process for TbCdTb is composed of an Orbach process, QTM, and a direct process. (4). Hdc = 1500−3000 Oe. When Hdc > 1500 Oe, the QTM processes of both TbCdCdTb and TbCdTb should be suppressed, meaning that τ(TbCdCdTb) and τ(TbCdTb) should be the same. However, τ(TbCdCdTb) in the low T region was slightly longer than that of TbCdTb (Figure 8 and Table 2) because the relaxation process changed from QTM to a direct process with an increase in Hdc, as shown in Figure S18.17,38,57,58 This is consistent with the fact that τ at 2000 Oe is proportional to T−1 (Supporting Information, Figure S15). The transition rate for the direct process involving TbCdCdTb might be smaller than that involving TbCdTb, and thus, τ at low T is longer in a strong Hdc. Comparison of the Alternating Current Magnetic Properties of TbTb with Those of TbCdCdTb and TbCdTb. In comparison to the Δ values for TbCdCdTb and TbCdTb, that for TbTb did not show strong Hdc dependence (Figure 9 and Supporting Information, Figures S16 and S17). In the case of TbTb, QTM in an Hdc of 0 Oe is forbidden because of the absence of mixing between the |±6, ∓6> and | ±6, ±6> states.36 Therefore, there is no crossover between the QTM and Orbach processes. In other words, the relaxation G

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry process at high T for TbTb is a pure Orbach process in the Hdc range of 0−2000 Oe (Supporting Information, Figure S18), and τ is not affected by Hdc. Therefore, Δ of TbTb was not dependent on Hdc, and this behavior is similar to those for TbCdCdTb and TbCdTb. At 3000 Oe, although QTM is possible, the direct process is a possible relaxation pathway in such a strong Hdc at low T.17,38,57,58 We have reported that TbTb shows pure QTM in an Hdc of 3000 Oe,39 which is consistent with the Zeeman diagrams.

ogy, Japan. The synchrotron radiation experiments were performed in the BL02B1 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal No. 2012B1109).

■

(1) Sessoli, R.; Gatteschi, D.; Novak, M. A. Nature 1993, 365, 141− 143. (2) Woodruff, D.; Winpenny, R. E.; Lavfield, R. A. Chem. Rev. 2013, 113, 5110−5148. (3) Thomas, L.; Lionti, F.; Ballou, R.; Gatteschi, D.; Sessoli, R.; Barbara, B. Nature 1996, 383, 145−147. (4) Friedman, J. R.; Sarachik, M. P.; Tejada, J.; Ziolo, R. Phys. Rev. Lett. 1996, 76, 3830−3833. (5) Gatteschi, D.; Sessoli, R. Angew. Chem., Int. Ed. 2003, 42, 268− 297. (6) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M. A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Nat. Mater. 2009, 8, 194−197. (7) Jiang, S. D.; Wang, B. W.; Sun, H. L.; Wang, Z. M.; Gao, S. J. Am. Chem. Soc. 2011, 133, 4730−4733. (8) Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078−2085. (9) Kajiwara, T.; Nakano, M.; Takahashi, K.; Takaishi, S.; Yamashita, M. Chem.Eur. J. 2011, 17, 196−205. (10) Zhang, P.; Zhang, Li.; Wang, C.; Xue, S.; Lin, S. Y.; Tang, J. J. Am. Chem. Soc. 2014, 136, 4484−4487. (11) Blagg, R. J.; Ungur, L.; Tuna, F.; Speak, J.; Comar, P.; Collison, D.; Wernsdorfer, W.; Mclnnes, E. J. L.; Chibotaru, L. F.; Winpenny, R. E. P. Nat. Chem. 2013, 5, 673−678. (12) Unger, L.; Roy, J. J. L.; Korobkov, I.; Murugesu, M.; Chibotaru, L. F. Angew. Chem., Int. Ed. 2014, 53, 4413−4417. (13) Rinehart, J. D.; Fang, M.; Evans, W. J.; Long, J. R. J. Am. Chem. Soc. 2011, 133, 14236−14239. (14) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. J. Am. Chem. Soc. 2003, 125, 8694−8695. (15) Ishikawa, N. J. Phys. Chem. A 2003, 107, 5831−5835. (16) Ishikawa, N. Polyhedron 2007, 26, 2147−2153. (17) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. J. Phys. Chem. B 2004, 108, 11265−11271. (18) Ishikawa, N.; Kaizu, Y. Coord. Chem. Rev. 2002, 226, 93−101. (19) Ishikawa, N.; Sugita, N.; Ishikawa, T.; Koshihara, S.; Kaizu, Y. Inorg. Chem. 2004, 43, 5498−5500. (20) Ishikawa, N.; Sugita, M.; Wernsdorfer, W. Angew. Chem., Int. Ed. 2005, 44, 2931−2935. (21) Ishikawa, N.; Sugita, M.; Wernsdorfer, W. J. Am. Chem. Soc. 2005, 127, 3650−3651. (22) Ishikawa, N.; Sugita, M.; Okubo, T.; Tanaka, N.; Iino, T.; Kaizu, Y. Inorg. Chem. 2003, 42, 2440−2446. (23) Katoh, K.; Yoshida, Y.; Yamashita, M.; Miyasaka, H.; Breedlove, B. K.; Kajiwara, T.; Takaishi, S.; Ishikawa, N.; Isshiki, H.; Zhang, Y. F.; Komeda, T.; Yamagishi, M.; Takeya, J. J. J. Am. Chem. Soc. 2009, 131, 9967−9976. (24) Komeda, T.; Isshiki, H.; Liu, J.; Zhang, Y. F.; Lorente, N.; Katoh, K.; Breedlove, B. K.; Yamashita, M. Nat. Commun. 2012, 2 (217), 1−7. (25) Urdampilleta, M.; Klyatskaya, S.; Cleuziou, J. P.; Ruben, M.; Wernsdorfer, W. Nat. Mater. 2011, 10, 502−506. (26) Schwöbel, J.; Fu, Y.; Brede, J.; Dilullo, A.; Hoffmann, G.; Klyatskaya, S.; Ruben, M.; Wiesendanger, R. Nat. Commun. 2012, 3 (953), 1−5. (27) Vincent, R.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Balestro, F. Nature 2012, 488, 357−360. (28) Komeda, T.; Isshiki, H.; Liu, J.; Katoh, K.; Shirakata, M.; Breedlove, B. K.; Yamashita, M. ASC Nano 2013, 7, 1092−1099. (29) Bogani, L.; Wernsdorfer, W. Nat. Mater. 2008, 7, 179−186. (30) Candini, A.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Affronte, M. Nano Lett. 2011, 11, 2634−2639.

■

CONCLUSIONS To elucidate the relationship between the MD interactions and the magnetic properties, we synthesized TbIII−obPc multipledecker complexes with and without CdII ions between the TbIII−obPc units. To the best of our knowledge, this is the first time that the crystal structure of the quintuple-decker complex TbCdCdTb has been obtained. The ac magnetic properties of TbCdCdTb and TbCdTb drastically changed when an Hdc was applied. On the other hand, those of TbTb showed little dependence on Hdc. The differences are due to the differences in the intramolecular TbIII−TbIII interactions (MD interactions) and can be explained on the basis of the Zeeman diagrams. In the case of TbCdCdTb, QTM is completely suppressed when Hdc > 1000 Oe because the QTM area for TbCdCdTb is located in the range of 0−1000 Oe. TbCdTb requires a stronger Hdc to suppress QTM because the QTM area is wider (0−1500 Oe) than that for TbCdCdTb. On the other hand, the Hdc dependence of the magnetic properties of TbTb is weaker than those of TbCdTb and TbCdCdTb due to absence of QTM near 0 Oe. These results clearly show that the SMM properties of TbIII−obPc multiple-decker complexes can be tuned by changing the strength of the MD interactions. The ability to fine-tune SMM properties will open new possibilities for the application of SMMs in molecular devices.

■

ASSOCIATED CONTENT

S Supporting Information *

Differential pulse voltammograms, redox potentials, and dc magnetic susceptibilities for TbIII−obPc multiple-decker complexes, electronic structure of Tb III double-decker structures, typical relaxation processes, ESI-MS data, UV−vis and IR spectra, plots of χMT versus T, Δ and τ0 values, Debye model equation, χM′ versus ν and χM″ versus ν plots, τ versus T−1 plots, Arrhenius plots, illustrated possible relaxation processes for multiple-decker complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (K.K.) *E-mail: [email protected]. (M.Y.) Author Contributions

All of the authors contributed equally. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by a Grant-in-Aid for Scientific Research (S, Grant No. 20225003) and Grant-in-Aid for Young Scientists (B, Grant No. 24750119) from the Ministry of Education, Culture, Sports, Science, and TechnolH

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (31) Ishikawa, N.; Iino, T.; Kaizu, Y. J. Phys. Chem. A 2002, 106, 9543−9550. (32) Ishikawa, N.; Iino, T.; Kaizu, Y. J. Phys. Chem. A 2002, 107, 7879−7884. (33) Ishikawa, N.; Iino, T.; Kaizu, Y. J. Am. Chem. Soc. 2002, 124, 11440−11447. (34) Ishikawa, N.; Otsuka, S.; Kaizu, Y. Angew. Chem., Int. Ed. 2005, 44, 731−733. (35) Fukuda, T.; Kuroda, W.; Ishikawa, N. Chem. Commun. 2011, 47, 11686−11688. (36) Sakaue, S.; Fuyuhiro, A.; Fukuda, T.; Ishikawa, N. Chem. Commun. 2012, 48, 5337−5339. (37) Wang, H.; Liu, T.; Wang, K.; Duan, C.; Jiang, J. Chem.Eur. J. 2012, 18, 7691−7694. (38) Fukuda, T.; Matsumura, K.; Ishikawa, N. J. Phys. Chem. A 2013, 117, 10447−10454. (39) Katoh, K.; Kajiwara, T.; Nakano, M.; Nakazawa, Y.; Wernsdorfer, W.; Ishikawa, N.; Breedlove, B. K.; Yamashita, M. Chem.Eur. J. 2011, 17, 117−122. (40) Katoh, K.; Horii, Y.; Yasuda, N.; Wernsdorfer, W.; Toriumi, K.; Breedlove, B. K.; Yamashita, M. Dalton Trans. 2012, 41, 13582− 13600. (41) Katoh, K.; Isshiki, H.; Komeda, T.; Yamashita, M. Coord. Chem. Rev. 2011, 255, 2124−2148. (42) Katoh, K.; Yamamoto, K.; Kajiwara, T.; Takeya, J.; Breedlove, B. K.; Yamashita, M. J. Phys.: Conf. Ser. 2011, 303, 012035:1−10. (43) Katoh, K.; Umetsu, K.; Breedlove, B. K.; Yamashita, M. Sci. China: Chem. 2012, 55, 918−925. (44) Morita, T.; Katoh, K.; Breedlove, B. K.; Yamashita, M. Inorg. Chem. 2013, 52, 13555−13561. (45) Fukuda, T.; Biyajima, T.; Kobayashi, N. J. Am. Chem. Soc. 2010, 132, 6278−6279. (46) Wang, H.; Qian, K.; Bian, Y.; Jiang, J.; Gao, S. Chem. Commun. 2011, 47, 9624−9626. (47) Wang, H.; Kobayashi, N.; Jiang, J. Chem.Eur. J. 2012, 18, 1047−1049. (48) Wang, H.; Qi, D.; Xie, Z.; Cao, W.; Wang, K.; Shang, H.; Jiang, J. Chem. Commun. 2013, 49, 889−891. (49) Wernsdorfer, W.; Aliaga-Alcalde, N.; Hendrickson, D. N.; Christou, G. Nature 2002, 416, 406−409. (50) Binnemans, K.; Sleven, J.; Feyter, S. D.; Schryver, F. C. D.; Donnio, B.; Guillon, D. Chem. Mater. 2003, 15, 3930−3938. (51) Bai, Z.; Guo, Y.; Zhu, P.; Bian, Y.; Jiang, J. Inorg. Chem. 2010, 49, 9005−9011. (52) Kobayashi, N.; Ogata, H.; Nonaka, N.; Luk’yanets, E. A. Chem.Eur. J. 2003, 20, 5123−5134. (53) Sugimoto, K.; Ohsumi, H.; Aoyagi, S.; Nishibori, E.; Moriyoshi, C.; Kuroiwa, Y.; Sawa, H.; Takata, M. AIP Conf. Proc. 2010, 1234, 887−890. (54) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122. (55) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, R. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. J. Appl. Crystallogr. 2008, 41, 466−470. (56) Fukuda, T.; Hata, K.; Ishikawa, N. J. Am. Chem. Soc. 2012, 134, 14698−14701. (57) Guo, Y. N.; Xu, G. F.; Gamez, P.; Zhao, L.; Lin, S. Y.; Deng, R.; Tang, J.; Zhang, H. J. J. Am. Chem. Soc. 2010, 132, 8538−8539. (58) Fukuda, T.; Shigeyoshi, N.; Yamamura, T.; Ishikawa, N. Inorg. Chem. 2014, 53, 9080−9086.

I

DOI: 10.1021/ic502951t Inorg. Chem. XXXX, XXX, XXX−XXX