Influence of porphyrin meso-attached substituent on the SMM behavior of dysprosium(III) double-deckers with mixed tetrapyrrole ligands

Abstract

Three sandwich-type neutral unprotonated mixed (phthalocyaninato)(porphyrinato) dysprosium(III) double-decker complexes Dy(Pc)(Por) [Por = TCPP, TPP, TBPP; Pc = unsubstituted phthalocyaninate, TCPP = 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinate, TPP = 5,10,15,20-tetrakis(phenyl)porphyrinate, TBPP = 5,10,15,20-tetrakis[(4-tert-butyl)phenyl]porphyrinate] (1–3) have been designed, prepared, and structurally studied. Systematic and comparative studies reveal the slow relaxation of magnetization under both zero and applied dc field for all three double-deckers, indicating their SMM nature. Stronger quantum tunneling of magnetization (QTM) observed for 3 in comparison with its two counterparts 1 and 2 with similar coordination geometry shows the influence of substituent at the meso-attached phenyl moieties on the magnetic properties.

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Introduction

Single molecular magnets (SMMs) with intriguing magnetic bistability have been intensively studied due to their potential applications in information storage, quantum computing, molecular spintronics and low-temperature magnetic coolers.^1,2 In comparison with the transition-metal involved SMMs, the lanthanide-based counterparts are of special importance because of their significant magnetic anisotropy arising from the large and unquenched orbital angular momentum.³ As a consequence, great efforts have been paid for the design and synthesis of lanthanide-based SMMs with continuously improved magnetic properties.⁴ Thus far investigations have clearly revealed the important effect of ligand field symmetry on generating anisotropic barriers.^3b,5 It is also well known that the relaxation process is easily affected by very subtle change in the coordination environment around the metal center.^3a,3b However, due to the lack of suitable systems containing a series of isostructural complexes with the same or at least similar coordination symmetry but changeable electronic structure for the lanthanide spin carrier, the exploration towards understanding such kind of effect on the magnetic properties of lanthanide SMMs was retarded. Fortunately, bis(tetrapyrrole) rare earth double-decker complexes provide a good chance to work for this target owing to the facility in modifying the tetrapyrrole ligands in the sandwich molecules.⁶

In the present paper, three neutral unprotonated mixed (phthalocyaninato)(porphyrinato) dysprosium(III) double-decker complexes Dy(Pc)(Por) {Por = TCPP, TPP, TBPP; Pc = unsubstituted phthalocyaninate, TCPP = 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinate, TPP = 5,10,15,20-tetrakis(phenyl)porphyrinate and TBPP = 5,10,15,20-tetrakis[(4-tert-butyl)phenyl]porphyrinate} (1–3) were designed, prepared, and structurally characterized, Scheme 1. Systematic and comparative studies over their magnetic properties reveal the slow relaxation of magnetization under both zero and applied dc field for all the three double-deckers, indicating their SMM nature. Stronger quantum tunneling of magnetization (QTM) observed for 3 in comparison with 1 and 2 indicates the influence of substituent at the meso-attached phenyl moieties on the magnetic properties.

Scheme 1 Schematic molecular structure of the sandwich-type mixed (phthalocyaninato)(porphyrinato) double-decker complexes 1–3 [R = CN, H, C(CH₃)₃].

Results and discussion

Synthesis and spectroscopic characterization

Treatment of the half-sandwich complex Dy(Por)(acac), generated in situ from the reaction of [Dy(acac)₃]·nH₂O and corresponding porphyrin, with Li₂Pc in refluxing n-octanol for 4 h led to the isolation of the sandwich-type mixed (phthalocyaninato)(porphyrinato) double-decker complexes Dy(Pc)(Por) (Por = TCPP, TPP, TBPP) (1–3) in the yield of 16–22%. These compounds were soluble in common organic solvents such as CHCl₃, CH₂Cl₂, and toluene, and could be purified readily by column chromatography. Interestingly, unlike the case of Dy(Pc)(Por) (Por = TPP, TBPP) (2 and 3), during the preparation of Dy(Pc)(TCPP) (1), no triple-decker side products including both Dy₂(Pc)(TCPP)₂ and Dy₂(Pc)₂(TCPP) were obtained with the target double-decker complex isolated as the sole product most probably due to the electron-withdrawing 4-cyano groups introduced onto the meso-attached phenyl moieties of the porphyrin ligand. The newly prepared sandwich-type mixed (phthalocyaninato)(porphyrinato) double-decker compounds 1–3 gave satisfactory elemental analysis data, Table S1 (ESI†). The MALDI-TOF mass spectra of these compounds clearly show intense signals for the corresponding protonated molecular ion [M + H]⁺, which are in good agreement with corresponding calculated values, Table S1 (ESI†).

The electronic absorption spectra of 1–3 were recorded in CHCl₃ and the data are summarized in Table S2 (ESI†). As displayed in Fig. 1, the compounds show typical feature of the electronic absorption spectra of mixed ring double-deckers containing an unpaired electron localized on the phthalocyanine ring,⁶ exhibiting medium to strong phthalocyanine and porphyrin Soret bands at 328–332 and 402–404 nm, respectively, the Q bands at 466–476 and 726–732 nm, and the characteristic near-IR absorption in the region of 1184–1320 nm. Slight shift in these absorption bands reveals the slight different electronic structure of these compounds due to different substituent at the meso-attached phenyl moieties. This is also true for their IR spectra with the observation of an intense band at 1316–1319 cm⁻¹ as the marker phthalocyanine π-radical anion band,^6b,7 Fig. S1 (ESI†). In addition, the IR spectrum of 1 also shows a sharp band at 2227 cm⁻¹ due to the C≡N stretching vibration of the 4-cyanophenyl groups, Fig. S1 (ESI†).

Fig. 1 Electronic absorption spectra of double-deckers 1–3 in CHCl₃.

Structural studies

Single crystals of double-decker compounds 1 and 2 were obtained by slow diffusion of MeOH into the CHCl₃ solution of corresponding complexes. However, trials by employing the same method failed to provide single crystals of 3 suitable for X-ray diffraction analysis. Fortunately, slow diffusion of methanol into the o-dichlorobenzene solution of Dy(Pc)(TBPP) (3) and C₆₀ in the molar ratio of 1 : 3 led to the isolation of cocrystallates of Dy(Pc)(TBPP) 3C₆₀. The molecular structures of 1–3 were determined by X-ray diffraction analyses. As can be seen in Fig. 2 and S2 and S3 (ESI†), the central dysprosium(III) ion in 1–3 is eight-coordinated by four isoindole and four pyrrole nitrogen atoms from the phthalocyanine and porphyrin ligands, respectively, resulting in an approximately square-antiprismatic coordination polyhedron. The dysprosium metal center lies at 1.257–1.284 and 1.464–1.482 Å to the N₄ mean plane of Por and Pc, respectively, giving a ring-to-ring separation of 2.721–2.753 Å. The twist angle φ, which is defined as the rotation angle of one coordination square away from the eclipsed conformation of the twos, varies from 39.38 to 44.76° for these three complexes, all of which slightly deviate from the ideal square-antiprismatic symmetry (φ = 45°), indicating the little influence of the substituent at the meso-attached phenyl moieties of the porphyrin ligand on the coordination geometry of the center dysprosium(III) ion (Table 1).

Table 1 Structural data for the mixed double-deckers 1–3

	1	2	3
a The average dihedral angle of the individual pyrrole or isoindole ring with respect to the corresponding N4 mean plane.b Defined as the rotation angle of one macrocycle away from the eclipsed conformation of the two macrocycles.
Average M–N(pyrrole) bond distance [Å]	2.430	2.420	2.420
Average M–N(isoindole) bond distance [Å]	2.462	2.463	2.462
M–N4(Por) plane distance [Å]	1.284	1.270	1.257
M–N4(Pc) plane distance [Å]	1.469	1.482	1.464
Interplanar distance [Å]	2.753	2.752	2.721
Dihedral angle between the two N4 planes [°]	0.34	0.33	0.46
Average dihedral angle ϕ for the Por ring [°]^a	11.29	11.63	11.82
Average dihedral angle ϕ for the Pc ring [°]^a	14.64	13.82	18.74
Average twist angle [°]^b	43.29	39.38	44.76
The nearest Dy…Dy distance	9.105	12.792	16.654

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Fig. 2 Molecular structure of Dy(Pc)(TCPP) (1) in top and side views with the hydrogen atoms omitted for clarity [Dy(III) green, C grey, N blue].

The crystal packing diagrams of the neutral mixed ring double-deckers 1 and 2 are shown in Fig. S4 and S5 (ESI†). The adjacent dysprosium(III) double-decker molecules in the single crystal of 1 form a dimer via the π–π interaction of phthalocyanine ligands with the nearest Dy…Dy distance of 9.105 Å. Different from 1, due to the effect of solvated CHCl₃ molecules, the double-decker molecules in the single crystal of 2 are further separated with a larger Dy…Dy distance of 12.792 Å in comparison with 1. As for 3 in the present case, the double-decker molecules in the C₆₀-cocrystallates are separated by fullerene molecules, giving the nearest Dy…Dy distance of 16.654 Å, which obviously should be different from that in the solid state of pure 3. However, inspection over the structural data of mixed (phthalocyaninato)(porphyrinato) dysprosium(III) double-decker complexes reported thus far including 1 and 2 in the present work suggests the Dy…Dy distance larger than 8 Å in the solid state of pure 3,^8e therefore excluding the effective intermolecular interaction on the magnetic properties of this double-decker compound.^4f

Electrochemical studies

The redox behavior of 1–3 was studied by cyclic voltammetry (CV) in CH₂Cl₂. Four reversible one-electron processes are revealed for these double-decker complexes which can be attributed to the successive removal or addition of electrons from or to the ligand-based orbitals as the dysprosium(III) center cannot be oxidized or reduced under these conditions. As shown in Table S2 (ESI†), the half-wave potentials of the two oxidation (E_Oxd1 and E_Oxd2) and two reduction (E_Red1 and E_Red2) processes decrease slightly from 1 to 3 due to the change from electron-withdrawing cyano substituent for 1 to electron-donating tert-butyl substituent for 3 via proton for 2 attached at the meso-attached phenyl moieties of the porphyrin ligand in the double-decker. The potential difference between E_Oxd1 and E_Oxd2 (ΔE_1/2⁰), which represents the gap between the semi-occupied orbital and second HOMO, decreases from 0.75 to 0.72 V for 1–3. This trend is in accord with the red-shift of the near IR absorption band at 1184–1320 nm, confirming the difference in the electronic structure of these compounds due to different substitutent at the meso-attached phenyl moieties.

Magnetic properties

The temperature dependence of the magnetic susceptibility data for 1–3 was collected in the temperature range of 2–300 K under an applied field of 2 kOe. As can be seen in Fig. S7 (ESI†), the curve of the magnetic susceptibility χ_MT for 1–3 shows the temperature dependence character. The χ_MT values of 14.43, 14.56, and 14.44 cm³ K mol⁻¹ for 1–3 at 300 K are consistent with the value of 14.55 cm³ K mol⁻¹ expected from combination of one single-electron radical (S = 1/2 and g = 2.0) and one Dy(III) ion [⁶H_15/2, S = 5/2, L = 5, g = 4/3].^5a,8 When the temperature is lowered, the χ_MT values of these three compounds decrease slowly until about 50 K, then rapidly decrease to a minimum value of 10.03, 9.11, and 9.82 cm³ K mol⁻¹ for 1, 2, and 3 at 2 K, respectively. The overall magnetic behavior of all these three compounds can be attributed mainly to the crystal-field effects such as thermal depopulation of the dysprosium(III) Stark sublevels and intramolecular magnetic interaction between the dysprosium and one-electron radical.^8a,9 In addition, as shown in Fig. S7–S9 (ESI†), the three non-superposition curves for 1–3 display a rapid increase at low field and eventually achieve the maximum value of 6.47–6.95μ_B at 5 T without reaching the theoretically magnetization saturation [10.00μ_B for even one Dy(III) ion (⁶H_15/2, g = 4/3)], revealing the crystal-field effect on the dysprosium ion.^5a,8d–f,9 As also can be seen in these figures, the non-superposition field dependence magnetization curves obtained at 2.0, 3.0, and 5.0 K for the mono-dysprosium involved double-decker compounds 1–3 indicate the presence of crystal field effect, thus leading to magnetic anisotropy for the Dy(III) ion in these double-decker complexes.

The dynamics of magnetization was studied on multicrystalline powder sample of 1–3 in a 3.0 Oe ac field oscillating at 10–997 Hz. Fig. 3 shows the plots of χ′ vs. T and χ′′ vs. T in a zero dc magnetic field for all the three compounds. As can be seen, both 1 and 2 exhibit the frequency-dependent character in the in-phase signal (χ′) and out-of-phase signal (χ′′), indicating the slow relaxation of magnetization and revealing the SMM nature for these two complexes. However, in the case of 3 only inconspicuous frequency-dependent character in the out-of-phase signal (χ′′) can be observed. Unfortunately, further understanding corresponding relaxation process for these three compounds failed since no complex shows χ′′ peaks due to the fast relaxation associated with the quantum tunneling of magnetization (QTM) at zero dc magnetic field, which can be ascribed to the degeneracy of the two ground Kramers states of each single dysprosium ion according to previous report.^5a,8d–f,9 Moreover, graphical representation of χ′′ versus χ′ (Cole–Cole plot) at 3.0 and 4.0 K for 1–3 give one semicircle, suggesting the existence of one magnetic relaxation processes, Fig. S10 (ESI†). Fitting of the experimental data according to the modified Debye function equation^4e gives the following sets of parameters with α = 0.16–0.24 for 1, α = 0.09–0.22 for 2 and α = 0.08–0.22 for 3.

Fig. 3 Temperature depenpence of the in-phase (χ′) and out-of-phase (χ′′) ac susceptibility of 1 (A and B), 2 (C and D), and 3 (E and F), respectively, under zero applied dc field.

For the purpose of better understanding the magnetic behavior of these mono-dysprosium involved sandwich bis(tetrapyrrole) complexes, an external direct current (dc) magnetic field is used to suppress the quantum tunneling of magnetization. Under an applied 2000 Oe magnetic field, the ac susceptibility data for 1–3 show an overall reduction in height due to saturation effects that depress the susceptibility¹⁰ and the observation of the clear χ′′ peaks for 1 and 2 indicates an effective suppression of QTM. On the basis of a thermally activated mechanism, τ = τ₀ exp(U_eff/kT) and τ = 1/(2πν), the Arrhenius law fitting for the data under 2000 Oe dc magnetic field was carried out. As shown in Fig. S11 and S12 (ESI†), a linear relationship exists between ln(τ) and 1/T for both 1 and 2, which in turn results in a barrier U_eff = 25.2 cm⁻¹ (36.3 K) and τ₀ = 2.7 × 10⁻⁷ s for 1 with R = 0.998 together with U_eff = 23.3 cm⁻¹ (33.6 K) and τ₀ = 5.1 × 10⁻⁸ s for 2 with R = 0.999, suggesting the presence of one thermally activated relaxation process under an external applied magnetic field for both compounds. It is also worth mentioning that even under an applied magnetic field, only frequency-dependent character can be observed for 3, Fig. 4, suggesting stronger tunneling effect of this compound in comparison with 1 and 2.

Fig. 4 Temperature dependence of the in-phase (χ′) and out-of-phase (χ′′) ac susceptibility of 1 (A and B), 2 (C and D), and 3 (E and F), respectively, under 2000 Oe applied dc field.

As detailed above, slow relaxation of magnetization as revealed at least by the frequency dependent out-of-phase ac susceptibility signals under zero applied dc field and 2000 Oe applied dc field for all the three neutral unprotonated mixed ring dysprosium(III) double-decker complexes indicates their SMM nature. However, along with the change in the substituent attached at the meso-attached phenyl moieties of the porphyrin ligand in the double-decker molecule from electron-withdrawing cyano group for 1 to electron-donating tert-butyl one for 3 via proton for 2, much stronger quantum tunneling of magnetization (QTM) was revealed for 3 in comparison with 1 and even 2 despite the similar slightly deviated square-antiprismatic coordination polyhedron for the dysprosium ion in the three sandwich-type double-decker molecular structures, suggesting the larger perturbation of the ligand field strength to the two orthogonal ground state sublevels of Dy(III) for the former one compound than for the latter two complexes.^5a,8d–f,9 The slightly higher U_eff of 1 (36.3 K) observed under an applied 2000 Oe magnetic field in comparison with 2 (33.6 K) gives additional support for this point.

Conclusions

Briefly summarizing above, three sandwich-type mixed (phthalocyaninato)(porphyrinato) dysprosium complexes with similar coordination geometry were designed and prepared. Single crystal X-ray diffraction analysis discloses the quite similar coordination geometry for the dysprosium ion sandwiched between the two tetrapyrrole ligands in these three double-decker compounds. Electronic absorption spectroscopic and electrochemical results indicate the slight difference in the electronic structure of these double-decker compounds due to different electron-donating or withdrawing nature of substituent at the meso-attached phenyl moieties of the porphyrin ligand. Slow relaxation of magnetization under both zero and applied dc field revealed for all the three double-deckers reveals their SMM nature. However, stronger quantum tunneling of magnetization (QTM) observed for 3 in comparison with 1 and 2 discloses the influence of substituent at the meso-attached phenyl moieties of the porphyrin ligand on the magnetic properties. Further studies towards clearly clarifying the coordination strength effect on the magnetic properties of lanthanide-involved SMMs are underway.

Experimental section

General

n-Octanol was freshly distilled from sodium under nitrogen. Column chromatography was carried out on silica gel columns (Merck, Kieselgel 60, 70–230 mesh) with the indicated eluents. Dichloromethane for voltammetric studies was freshly distilled from CaH₂ under nitrogen. The electrolyte [Bu₄N][ClO₄] was recrystallized from tetrahydrofuran. The compounds [Dy(acac)₃]·nH₂O,¹¹ H₂(TCPP),¹² H₂(TPP),¹³ H₂(TBPP),¹³ and Li₂(Pc)¹⁴ were prepared according to literature procedure. All other reagents and solvents were used as received. Dy^III(Pc)(Por) (Por = TCPP, TPP, TBPP) (1–3), were prepared according to the published procedure.¹⁵

Electronic absorption spectra were recorded on a Lambda 750 spectrophotometer. MALDI-TOF mass spectra were taken on a Bruker BIFLEX III ultra-high resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer with α-cyano-4-hydroxycinnamic acid as the matrix. IR spectra were recorded in KBr pellets with 2 cm⁻¹ resolution using a Bruker Tensor 37 spectrometer. Elemental analyses were performed on an Elementar Vavio El III elemental analyzer. Electrochemical measurements were carried out with a BAS CV-50W voltammetric analyzer. The cell comprised inlets for a glassy carbon disk working electrode with a diameter of 2.0 mm and a silver wire counter electrode. The reference electrode was Ag/Ag⁺ (a solution of 0.01 M AgNO₃ and 0.1 M [Bu₄N][ClO₄] in acetonitrile), which was connected to the solution by a Luggin capillary whose tip was placed close to the working electrode. It was corrected for junction potentials by using ferrocenium/ferrocene (Fe⁺/Fe) couple [E_1/2 (Fe⁺/Fe) = 0.50 V vs. SCE] as an internal reference. Typically, a 0.1 M solution of [NBu₄][ClO₄] in CH₂Cl₂ containing 0.5 mM of the sample was purged with nitrogen for 10 min, and then the voltammograms were recorded at ambient temperature. The scan rate was 50 mV s⁻¹ for CV. Magnetic measurements were performed on a Quantum Design MPMS XL-5 SQUID magnetometer on polycrystalline samples. Data were corrected for the diamagnetism of the samples using Pascal constants and of the sample holder by measurement.

Preparation of Dy^III(Pc)(TCPP) (1)

A mixture of H₂TCPP (70.0 mg, 0.05 mmol) and Dy(acac)₃·nH₂O (26 mg, 0.05 mmol) in n-octanol (2 mL) was heated to reflux under a nitrogen atmosphere for about 4 h. After a brief cooling, Li₂Pc (26 mg, 0.05 mmol) was added and the resulting mixture was heated to reflux for a further 4 h under nitrogen. After cooling, the solvent was removed under reduced pressure and the residue was exposed to air for several days before further purification. The crude product was then purified by chromatography on a silica gel column with CHCl₃ as the eluent. A small amount of unreacted H₂TCPP was collected first followed by a brown band containing the targeting double-decker complex Dy^III(Pc)(TCPP) (1). Repeated chromatography followed by recrystallization from CHCl₃ and MeOH gave 1 as a dark brown powder (10 mg, 16%).

Preparation of Dy^III(Pc)(TPP) (2) and Dy^III(Pc)(TBPP) (3)

By employing the above procedure with H₂TPP (for 2) and H₂TBPP (for 3) instead of H₂TCPP as the starting material, double-decker compounds 2 and 3 were isolated in 20–22% yield.

X-ray crystallographic analysis of 1–3

Single crystals suitable for X-ray diffraction analysis were grown by diffusing MeOH into the CHCl₃ solution of compound 1 and 2. For 3, single crystals were obtained by diffusing MeOH into the 1,2-dichlorobenzene solution of 3 and fullerene in the mole ratio of 1 : 3. Crystal data and details of data collection and structure refinement are given in Table S4 and S5 (ESI†). Data were collected on an Oxford Diffraction Gemini E system with Cu_Kα radiation λ = 1.5418 Å at 150 K, using a ω scan mode with an increment of 1°. Preliminary unit cell parameters were obtained from 30 frames. Final unit cell parameters were obtained by global refinements of reflections obtained from integration of all the frame data. The collected frames were integrated using the preliminary cell-orientation matrix. The SMART software was used for data collecting and processing; ABSpack for absorption correction;¹⁶ and SHELXL for space group and structure determination, refinements, graphics, and structure reporting.¹⁷†

Acknowledgements

Financial support from the National Key Basic Research Program of China (Grant nos 2013CB933402 and 2012CB224801), Natural Science Foundation of China, Beijing Municipal Commission of Education, State Key Laboratory of Physical Chemistry of Solid Surfaces, Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, and University of Science and Technology Beijing is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR spectra in the region of 600–1800 and 2100–4000 cm⁻¹, molecular structure, molecular packing in single crystals, temperature dependence of χ_MT, M vs. H/T curves, analytical and mass spectroscopic data for the mixed double-deckers, electrochemical data and crystallographic data for the mixed double-deckers 1–3 (CIF). CCDC 951454–951456. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra15762a

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