Pentagonal-Bipyramid Ln(III) Complexes Exhibiting Single-Ion-Magnet

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Pentagonal-Bipyramid Ln(III) Complexes Exhibiting Single-IonMagnet Behavior: A Rational Synthetic Approach for a Rigid Equatorial Plane Arun Kumar Bar,†,‡ Pankaj Kalita,† Jean-Pascal Sutter,*,§,⊥ and Vadapalli Chandrasekhar*,∥,# †

School of Chemical Sciences, National Institute of Science Education and Research, Homi Bhabha National Institute, Bhubaneswar 752050, India § LCC CNRS, 205 Route de Narbonne, Toulouse F-31077, France ⊥ UPS, INPT, LCC, Université de Toulouse, Toulouse F-31007, France ∥ Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India # Tata Institute of Fundamental Research Hyderabad, Gopanpally, Hyderabad 500107, India S Supporting Information *

Ab initio calculations predict high Ueff values for the Dy(III) ion in a pentagonal-bipyramid (PBP) coordination geometry because of high axial CF symmetry.5f It is worth pointing out here that the equatorial CF enhances the QTM in the Ln ions that associate with the magnetic ground states possessing oblate electrostatic potential surfaces.3a,7 PBP complexes with a rigid equatorial plane and kinetically labile ligands at apical positions seem to be one of the promising building blocks toward the rational synthesis of multimetallic SMMs.9 In such building blocks, CF symmetry does not change upon association and the apical CF strength could be chemically tuned to tailor slow magnetization dynamics. However, unlike 3d transition-metal ions,9c the PBP geometry is less common in Ln-based coordination complexes.6a Remarkably, several Ln(III) complexes with PBP geometry are reported as SMMs10 displaying Ueff values as high as 828 cm−1.10a However, the coordination geometry of all of these complexes is rather serendipitous and is unlikely to persist upon chemical alteration within the coordination sphere. We were interested in developing a rational synthetic strategy that can result in deliberately PBP Ln(III)based magnetic building blocks with a rigid pentagonal-equatorial plane and with kinetically labile coordinating ligands at the apical sites. In this regard, we have employed a pentadentate chelating ligand that renders a rigid pentagonal-equatorial plane around the Ln(III) ions in heptacoordinated complexes where the apical sites are coordinated with Cl ions (Scheme 1). Herein, we report the syntheses, characterization, and magnetic properties of three novel heptacoordinated mononuclear complexes with the general formula (Et3NH)[(H2L)Ln I I I Cl 2 ] [where H 4 L = 2,6-diacetylpyridine bis(salicylhydrazone) and Ln = Tb (1), Dy (2), and Y0.94Dy0.06 (3); Scheme 1]. All of the complexes were synthesized upon treatment of the ligand H4L with 1 equiv of the corresponding hydrated lanthanide trichloride salt (LnCl3·xH2O), followed by treatment with 2 equiv of triethylamine (Et3N) under aerobic conditions in ethanol (EtOH; see the experimental section in the Supporting Information). The electrospray ionization mass

ABSTRACT: A pentadentate chelating ligand is employed for the facile synthesis of air-stable pentagonalbipyramid Ln(III) complexes with a rigid equatorial plane. The Dy(III) analogue exhibits single-ion-magnet behavior with Ueff/kB = 70 K under Hdc = 500 Oe.

M

olecule-based nanomagnets possess huge prospects in the next-generation modern technology.1 The discovery of single-ion-magnet (SIM) behavior in phthalocyanine-sandwiched Ln(III) mononuclear complexes by Ishikawa et al.2 has triggered tremendous research interest in the arena of moleculebased magnetism associated with Ln ions.3 Strong magnetic anisotropy and large spin ground states endow the Ln complexes, especially the Dy(III) analogues,3d,k with fascinating slow magnetization dynamics provided the Ln ions are in appropriate crystal-field (CF) environments.3a−k,4 Recent advances reveal that Ln-based complexes with low coordination numbers and high CF symmetry are expected to exhibit promising singlemolecule-magnet (SMM) behavior.5 However, it is worth mentioning that the Ln ions prefer large coordination numbers (8−10) and variable coordination geometry because of their large ionic size and highly shielded valence (4f) orbitals.4,6 Therefore, it is a challenge to synthetic chemists to control low coordination with the desired geometry in Ln-based complexes. Realization of the topologies (prolate/oblate) of the electrostatic potential surfaces corresponding to the magnetic ground states of the Ln(III) ions provides a straightforward rationale for Ln(III)-based SIMs.3a,7 For example, the highest magnetization states for the Dy(III) ion (MJ = ±15/2) incorporate an oblate-like electrostatic potential surface.3a,7 Therefore, the coordination environments providing very strong axial CFs and weak equatorial CFs render the magnetic ground states with large magnetization blockade barriers.5f,8 In addition to these, a high axial CF symmetry suppresses the quantum tunneling of magnetization (QTM) and, consequently, enhances the effective energy barrier for magnetization reversal (Ueff) and blocking temperature (TB).5f © XXXX American Chemical Society

Received: January 11, 2018

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DOI: 10.1021/acs.inorgchem.8b00059 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

measure analyses using the SHAPE program11 reveal a distorted PBP geometry with D5h (pseudo) CF symmetry around the Ln ions (Table S4). All of the complexes crystallize out without any cocrystallized solvent. Solid-state packing diagrams show that the shortest intermolecular Ln−Ln distance is ∼7.6 Å (Figure S6). Interestingly, there is essentially no short contact between these two closest molecules, which could provide a magnetic superexchange pathway.9b The purity of the bulk samples of all of the complexes was confirmed by elemental (CHN) analyses (see the experimental section in the Supporting Information). The solid-state phase purity of complex 3 was confirmed by powder X-ray diffraction studies (Figure S7). The temperature-dependent magnetic susceptibilities of 1 and 2 were studied in the temperature range 2−300 K and are presented as χMT versus T plots in Figure 2. The measured room

Scheme 1. Schematic Representation for the Syntheses of Complexes 1−3a

a

(i) Reaction of the ligand H4L with the respective Ln(III) salt was carried out in EtOH, and the product was not isolated; (ii) the followup treatment with 2 equiv of Et3N was performed in situ.

spectrometry (ESI-MS) spectrum of 2 displayed one of the most intense peaks, appearing around m/z 663, which can be attributed to the molecular-ion peak corresponding to [(H2L)LnIIICl2]− (Figures 1 and S3). The isotopic distribution analysis (Figure 1)

Figure 2. Temperature-dependent magnetic susceptibilities (open circles) of 1 (red) and 2 (blue). Inset: Field-dependent magnetizations (open circles) of 1 (red), 2 (blue), and 3 (green) at 2 K within the field range of 0−5 T. The solid lines are only guides for the eyes.

temperature (300 K) χMT values [12.2 cm3 mol−1 K (1) and 14.5 (2) cm3 mol−1 K] are in good agreement with the expected values [11.82 (with S = 3 and gJ = 3/2 for TbIII in 1) and 14.17 cm3 mol−1 K (with S = 5/2 and gJ = 4/3 for DyIII in 2)] for one magnetically isolated LnIII ion. Upon cooling, the χMT values remain more or less the same up to around 150 K, followed by a rapid decrease upon further cooling (Figure 2). Such a decrease can be ascribed to depopulation of the Mj levels (CF effect) of the anisotropic Ln(III) ions. The field-dependent magnetizations for 1−3 have been recorded in the field range of 0−5 T (Figures 2 and S8). At low temperature (2 K) and high field (5 T), the magnetization values are observed to be 5 μB (1) and 5.6 μB (2 and 3), which agree well with the generally observed values for magnetically exchange-free Ln(III) ions [Tb(III) in 1 and Dy(III) in 2 and 3]. Notably, the magnetization values rise steeply with increasing field at lower field regions and start to attenuate within the field range of 0.1−0.2 T in the temperature range 2−5 K (Figure S8). To probe the slow relaxation of magnetization, dynamic (alternating-current, ac) magnetic susceptibility studies were carried out in the temperature range 2−25 K with and without applied static fields. No out-of-phase ac susceptibility (χM′′) was detected for the Tb(III) analogue (1; Figure S9). On the other hand, a distinct maximum in the χM′′ versus T plot was observed at around 14 K for the Dy(III) analogue (2) in zero field (Figure S9) with, however, an additional prominent feature found at lower temperatures. This latter contribution was drastically reduced upon application of a direct-current (dc) field, and a detailed ac magnetic susceptibility study carried out for 2 with an

Figure 1. Top: Isotopic distribution of the ESI-MS peak corresponding to the molecular-ion peak of 2. Bottom: Single-crystal X-ray structure of 2. The Et3NH countercation is omitted for clarity.

also supports this assignment. Such observations imply stability of the complexes in solution as well as under high-energy electron spray. Single-crystal X-ray diffraction analysis unambiguously confirmed formation of the complexes and their coordination geometry (Figures 1 and S4). All of the complexes crystallize in an orthorhombic crystal system with the Cmc2(1) space group. Crystallographic data and refinement parameters of the complexes are given in Table S2. The selected bond lengths and bond angles of the complexes are summarized in Table S3. Figure 1 depicts the crystal structure of the anionic Dy complex in 2 as the representative example, while the molecular structures of 1 and 3 are portrayed in Figure S4. One pyridyl N, two imino N, and two carboxy O atoms of the ligand chelate with the Ln(III) ion, generating a coplanar equatorial coordination environment (Figure S5) with a pseudopentagonal geometry (∠O−Dy−Nim = 65.63°; ∠Npy− Dy−Nim = 65.02°, and ∠O−Dy−O = 98.69°). The equatorial Dy−O/N bond distances lie in the range of 2.26−2.45 Å, confirming pentacoordination. Completing the heptacoordination environment around the Dy(III) ion, two Cl atoms are coordinated at the apical sites with a Cl−Dy bond distance of 2.64 Å and a Cl−Dy−Cl bond angle of 166.32°. Continuous-shapeB

DOI: 10.1021/acs.inorgchem.8b00059 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry applied dc field of 1.5 kOe yielded well-defined χM′′ maxima spanning over 7−16 K within a frequency domain of 25−1500 Hz (Figure S10). Analysis of the Cole−Cole plots between 7.5 and 16 K revealed a narrow width of the distribution of the relaxation time, suggesting a single relaxation process operative for 2 within this temperature range (see Figure S11 and Table S5 for α values). However, toward lower temperatures, the α values rapidly increased. Such low-temperature behavior is indicative of additional fast relaxation mechanisms that are likely to result from dipolar interactions between the paramagnetic centers and hyperfine interactions.9b To reduce such contributions and confirm the molecular origin of the slow magnetization dynamics of the Dy(III) ion in this PBP coordination environment, we have considered the Y(III) analogue with 6% Dy(III) site populations, (Et3NH)[(H2L)Y0.94Dy0.06Cl2] (3), in which the Y(III) and Dy(III) centers have the same coordination environments as that for 2. A fielddependent magnetization study for 3 at 2 K (Figure 2) confirmed the relative Dy atom population and revealed a sharper increase of the magnetization for low fields compared to 2. ac magnetic susceptibility studies were carried out under Hdc = 500 Oe to suppress a small tail in χM′′ appearing below 3 K. The χM′′ versus T plots for different frequencies (1−1500 Hz) led to well-defined maxima in the temperature range 4−17 K (Figure S12). Analysis of the Cole−Cole plots for 3 yielded small α values between 4.5 and 15.5 K (Figure S13 and Table S6), in agreement with a single relaxation mechanism within this temperature window. Relaxation times between 4 and 16 K were obtained by modeling the respective frequency dependence of χM′′ with the extended Debye model; the results are plotted as τ versus 1/T in Figure 3, together with the relaxation times obtained for 2 (with

In conclusion, Ln(III) complexes with PBP coordination geometry are readily obtained using the pentadentate 2,6diacetylpyridine bis(salicylhydrazone) ligand, thus making controlled access to such heptacoordinated species straightforward. For the pentadentate ligand used here, the noncoordinating phenol groups stabilize the molecular complexes through intramolecular hydrogen-bonding interactions. Moreover, because of the presence of these bulky peripheral groups, the Ln(III) centers are mutually far apart in the solid state. Consequently, the observed slow magnetization dynamics are purely of molecular origin. With simple chloride ligands in the apical positions, the Dy(III) analogue exhibits SIM behavior. The lower effective energy barrier for magnetization reversal compared to a few reported PBP Dy(III)-based SIMs9 most certainly stems from a weaker axial CF and a stronger equatorial CF in 2. Interestingly, mass spectrometric analysis indicates stability of the equatorial coordination environments and lability of the axial coordination sites. Therefore, the axial ligand fields could be chemically tuned without changing the CF symmetry around the Ln(III) ions. This also provides an excellent opportunity to use these complexes, especially the Dy(III) analogues, as robust magnetic building blocks toward the construction of multimetallic high-performance SMMs. Further research in this direction is underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00059. Syntheses and analytical data for 1−3 (PDF) Accession Codes

CCDC 1582493−1582495 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.

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AUTHOR INFORMATION

Corresponding Authors

Figure 3. Left: Frequency dependence of the out-of-phase magnetic susceptibility (χM′′) for 3 at different temperatures between 4 and 17 K under Hdc = 500 Oe (the solid lines are only guides for the eyes). Right: Semilogarithmic plot of the relaxation time as a function of the inverse temperature for 2 (dots) and 3 (open circles). The red line is the best fit of the exponential equation to the linear variation found between 10 and 16 K.

*E-mail: [email protected] (J.P.S.). *E-mail: [email protected] or [email protected] (V.C.). ORCID

Vadapalli Chandrasekhar: 0000-0003-1968-2980 Present Address ‡

A.K.B.: School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.

an applied field). Both sets of data match well with each other, confirming the molecular origin of the relaxation behavior. They exhibit a linear variation above 10 K likely to result from a thermally activated relaxation process. Analysis of the linear variation for 3 with the Arrhenius law, τ = τ0 exp(Ueff/kBT), gave Ueff/kB = 70 K with τ0 = 1.9 × 10−6 s. Attempts to simulate τ = f(T) throughout the 2−17 K region considering concomitant contributions of an Orbach and a Raman process or Raman and direct processes lead to poor fitting for the low-temperature part even when possible relaxation by QTM was considered (Figure S14). However, the contribution of such an alternative process cannot be discarded. These results clearly support the occurrence of a slow relaxation of magnetization for the Dy(III) derivative.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.C. is thankful to the Department of Science and Technology, New Delhi, India, for a National J. C. Bose Fellowship. A.K.B. and P.K. are thankful to the National Institute of Science Education and Research, Bhubaneswar, India, for postdoctoral and doctoral fellowships, respectively.

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DEDICATION Dedicated to Prof. R. N. Mukherjee on the occasion of his 65th birthday. C

DOI: 10.1021/acs.inorgchem.8b00059 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00059 Inorg. Chem. XXXX, XXX, XXX−XXX