Complexes of Thioether-Based Schiff Base Ligands - ACS

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

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Two Types of Hexanuclear Partial Tetracubane [Ni4Ln2] (Ln = Dy, Tb, Ho) Complexes of Thioether-Based Schiff Base Ligands: Synthesis, Structure, and Comparison of Magnetic Properties Avik Bhanja,† Radovan Herchel,‡ Zdeněk Trávníček,§ and Debashis Ray*,† †

Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, India Department of Inorganic Chemistry, Faculty of Science, Palacký University, 17 Listopadu 12, 77146 Olomouc, Czech Republic § Division of Biologically Active Complexes and Molecular Magnets, Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University, Š lechtitelů 27, 783 71 Olomouc, Czech Republic

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‡

S Supporting Information *

ABSTRACT: New Schiff base ligand H2L containing N2O4S donor atoms has been explored for its ability to provide complexes of selected 3d and 4f metal ions. Room temperature reaction of H2L with NiCl2·6H2O and Ln(NO3)3·5H2O in the presence of Et3N in MeCN−MeOH (2:1) medium resulted in [Ni 4 Ln 2 (L) 2 (μ-Cl) 2 (μ 3 OH)4(H2O)6]Cl4·2H2O (where Ln = Dy3+ (1), Tb3+ (2), and Ho 3+ (3) and H 2 L = 2-((2-(2-(2-hydroxy-3methoxybenzylideneamino)ethylthio)ethylimino)methyl)-6methoxyphenol). Use of Ni(SCN)2·4H2O during synthesis provided SCN− ions for bridging and terminal coordination in [Ni4 Ln 2 (L) 2 (μ-NCS) 2 (μ3 -OH) 4 (NCS) 4 (H2 O) 2]·xMeOH· yH2O (where Ln = Dy3+ (4), x = 2, y = 4; Tb3+ (5) and Ho3+ (6), x = 0, y = 14.1). All six complexes possess a hexanuclear defective tetracubane topology having exchangeable bridging groups. The study of direct current magnetic susceptibility measurements revealed that the Ni(II) ions are engaged in ferromagnetic interaction with the DyIII, TbIII, and HoIII ions and have significant magnetic anisotropy in all six complexes. Alternating current susceptibility measurements confirmed that both of the two types of compounds qualify as zero-field singlemolecule magnets (SMMs), and the effective barrier for the reversal of the magnetic moment was found to be in the range Ueff = 23−31 K for 1−2 and 4−5, respectively. Detailed insight into the electronic structure and magnetic properties was calculated using DFT- and CASSCF-based analyses. The found isotropic exchange parameter (J) values are JNi−Ni = −4.7 cm−1 for 1 and JNi−Ni = +29.2 cm−1 for 4 and clearly indicate that the μ-NCS-bridge is a better candidate than μ-Cl for ferromagnetic exchange interactions. Out of the six complexes, only complex 5 displays TbIII centered emission peaks at 451 and 480 nm.

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INTRODUCTION Syntheses of novel multimetallic 3d−4f coordination complexes have attracted significant attention during the past two decades for synthetic challenges, unique structures, and importance in molecule-based magnetic behavior such as SMMs.1−7 Use of a symmetrical ligand system for multimetallic complexes of 4f and 3d−4f ions, showing an asymmetric coordination mode to the metal ions, has attracted a great deal of attention in recent times for their varied range of applications in high-density information storage, quantum computing, luminescence, and molecular spintronics.8−11 Most of the multimetallic complexes are synthesized with the aim to achieve high ground state spin (S) values so they can act as SMMs. Quite often the resulting complexes with large spin values do not behave as SMMs due to the absence of Ising-type magnetic anisotropy (D).12 The orientation of the ground electronic spin state (S) is stabilized by a large Ising or axial magnetic anisotropy (D), which leads to showing a slow © XXXX American Chemical Society

relaxation of the magnetization vector. Thus, to obtain a high anisotropy barrier (Ueff) to the reversal of the spin orientation, synthesizing a 3d−4f complex by incorporation of lanthanide metal ions is necessary as 4f metal ions are rich with significant single ion anisotropy that originated from the spin−orbit coupling.13 New examples of such compounds, supported by theoretical defenses, have shown that axially symmetric crystal-field environments around the lanthanide ions and strong exchange coupling between the metal ions could lead to the SMM behavior.14,15 The long relaxation times can be obtained from a large anisotropy barrier, but the process, like fast quantum tunneling of the magnetization (QTM), can invalidate the anisotropy barrier. A good SMM is supposed to preserve their magnetic state for a long time even after removal of external Received: May 23, 2019

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DOI: 10.1021/acs.inorgchem.9b01517 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry fields, which is only possible by suppression of ground state quantum tunneling of magnetization (QTM) and to do that it is very important to design a ligand system which can combine both 3d and 4f metal ions. Such a unique ligand environment can effectively govern the coordination geometry leading to influence of both effective energy barriers (Ueff) and magnetic blocking temperature (TB).16−20 In recent years, suppression of QTM efficiency has been reported in several 3d−4f complexes, where the 3d ion is incorporated to generate effective magnetic exchange interactions with the 4f ion, relative to the negligible 4f−4f interactions.21−23 For a binucleating ligand system, its two adjacent coordination pockets can be effectively utilized for binding of two 3d ions, which in the presence of external ancillary ligands undergo aggregation for a new type of 3d−4f complex. Incorporation of NiII ions within centrally located phenol-based ligands can give rise to ligand bound dinickel fragments and its spontaneous aggregation for higher order complex formation.24−26 The design strategy from the viewpoint of using phenol-based Schiff bases for the synthesis of a new family of 3d−4f-based aggregates is guided by the intention that the ligand should possess hard−soft acid−base (HSAB) category donor atoms and coordination pockets for selective coordination of 3d and 4f ions. Such a strategy was successful in providing many Ni-4f and Cu-4f complexes of varying nuclearity.27 Phenol-based Schiff bases are well-known to provide heterometallic coordination clusters from one-pot reactions involving a mixture of 3d and 4f metal ion salts and ligands.3,28−31 By examination of the ligand type, its structure, and the available donor atoms, it is impossible to apprehend the reaction products from the coordination of 3d and 4f ions, until and unless the experiment is performed. A thioether sulfur supported Schiff base having a {S(NO)2} inner cavity for holding more than one 3d ion and two outer bidentate {O2} parts to tie-up bigger 4f ions can thus be explored in this background. Herein, ligand 2-((2-(2-(2-hydroxy-3methoxybenzylideneamino)ethylthio)ethylimino)methyl)-6methoxyphenol (H2L) (Chart 1) was synthesized to identify its

literature.33−35 The heterometallic cluster of this type, assembled from the initially formed {Ni2L} fragments, is also unknown in the literature and thus prompted us to study the aggregation behavior and most pertinent physical properties in the solid state.

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EXPERIMENTAL SECTION

Reagents and Starting Materials. Solvents and other chemicals used in this work were of reagent grade and used without further purification. The following chemicals were used as obtained: cysteamine hydrochloride, chloroethylamine hydrochloride, Dy(NO3)3·5H2O, Tb(NO3)3·5H2O, and Ho(NO3)3·5H2O (Alfa Aeser, UK); NiCl2·6H2O (SD Fine Chemicals, Mumbai); o-vanillin (Spectrochem Pvt. Ltd., Mumbai). In a gram scale synthetic procedure, Ni(SCN)2·4H2O was prepared following a reported procedure.36 Synthesis of 2-((2-(2-(2-Hydroxy-3methoxybenzylideneamino)ethylthio)ethylimino)methyl)-6methoxyphenol (H2L). H2L was prepared from a Schiff base condensation reaction. 2-(2-Aminoethylthio)ethanamine (1.20 g, 10 mmol) was dissolved in 30 mL of MeOH to get a clear solution. To this was added a methanolic solution (20 mL) of o-vanillin (3.04 g, 20 mmol) dropwise; the reaction mixture refluxed for 4 h (Scheme 2). On cooling to room temperature, a yellow crystalline precipitate appeared which was washed with n-hexane (2 × 5 mL) and dried over P4O10 under vacuum. Yield: 88.6%. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 2920 (w), 1631 (s), 1465 (s), 1336 (m), 1250 (s), 1080 (m), 1049 (m), 962 (m), 882 (w), 783(w), 739 (m). 1H NMR (400 MHz, CDCl3): 8.3 (s, 1H), 6.9 (t, 2H), 7.2−7.0 (d, 4H), 3.9 (s, 6H), 2.8 (d, 4H). Anal. Calcd for C20H24N2O4S: C, 61.81; H, 6.25; N, 7.23. Found: C, 62.18; H, 6.14; N, 7.11. General Synthetic Procedures for 1, 2, and 3. All the metal ion complexes (1−3) were obtained by following a general synthetic technique. H2L (0.038 g, 0.1 mmol) was dissolved in MeCN−MeOH (10 mL; 2:1), and triethylamine (0.02 g, 0.2 mmol) was added to get a clear solution. After 10 min of stirring, NiCl2·6H2O (0.036 g, 0.15 mmol) was added, and the resultant greenish reaction mixture was stirred at ambient temperature for 1 h. Addition of 10 mL of MeCN− MeOH solution of Ln(NO3)3·5H2O (0.1 mmol) (Ln = Dy3+ (1), Tb3+ (2), Ho3+ (3)) resulted in a greenish yellow reaction mixture which was stirred overnight and then filtered. A pale green single crystal suitable for single-crystal structure determination was obtained after a week by slow evaporation of the solvent at room temperature. Details for individual complexes are delineated below. [Dy2Ni4(L)2(μ-Cl)2(μ3-OH)4(H2O)6]Cl4·2H2O (1). The following reagents were used: H2L (0.038 g, 0.1 mmol), Dy(NO3)3·5H2O (0.044 g, 0.1 mmol), NiCl2·6H2O (0.036 g, 0.15 mmol), NEt3 (0.02g, 0.2 mmol). Yield: 0.037 g, 42.1% (based on Dy). Anal. Calcd for C40H64Cl6Dy2N4Ni4O20S2 (1757.50): C, 27.40; H, 3.45; N, 3.20. Found: C, 27.50; H, 3.86; N, 3.58. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 3357 (s), 2937 (w), 1636 (s), 1607 (w), 1473 (s), 1411 (w), 1305 (s), 1243(m), 1225 (s), 1169 (w), 1055 (m), 859 (w), 739 (m), 633 (w). UV−vis (solid): λmax (nm) = 422, 641, 894. [Tb2Ni4(L)2(μ-Cl)2(μ3-OH)4(H2O)6]Cl4·2H2O (2). The following reagents were used: H2L (0.038 g, 0.1 mmol), Tb(NO3)3·5H2O (0.044 g, 0.1 mmol), NiCl2·6H2O (0.036 g, 0.15 mmol), NEt3 (0.02 g, 0.2 mmol). Yield: 0.039 g, 44.6% (based on Tb). Anal. Calcd for C40H64Cl6Tb2N4Ni4O20S2 (1750.35): C, 27.51; H, 3.45; N, 3.21. Found: C, 27.75; H, 3.75; N, 3.55. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 3356 (s), 2936 (w), 1635 (s), 1606 (w), 1476 (s), 1410 (w), 1307 (s), 1248(m), 1229 (s), 1170 (w), 1059 (m), 861 (w), 738 (m), 635 (w). UV−vis (solid): λmax (nm) = 418, 645, 896. [Ho2Ni4(L)2(μ-Cl)2(μ3-OH)4(H2O)6]Cl4·2H2O (3). The following reagents were used: H2L (0.038 g, 0.1 mmol), Ho(NO3)3·5H2O (0.044 g, 0.1 mmol), NiCl2·6H2O (0.036 g, 0.15 mmol), NEt3 (0.02 g, 0.2 mmol). Yield: 0.033 g, 37.5% (based on Ho). Anal. Calcd for C40H64Cl6Ho2N4Ni4O20S2 (1762.43): C, 27.32; H, 3.44; N, 3.19.

Chart 1. Structure of H2L Presenting Two Types of Accessible Coordination Sites

coordination capacity to provide new examples of 3d−4f compounds. One earlier report showed that the reaction of a similar ligand, without the adjacent −OMe function, with nickel(II) salts resulted in only the NiL2·CH3OH·4H2O complex.32 Herein, we report the synthesis and characterization of a μ-chlorido-bridged family of three hexanuclear NiII4LnIII2 complexes [Ln2Ni4{L}2(μ-Cl)2(μ3-OH)4(H2O)6]Cl4·2H2O (Ln = Dy3+ (1), Tb3+ (2), Ho3+ (3)) and three μisothiocyanato-bridged congeners [Ln2Ni4{L}2(μ3-OH)4(μNCS)2(NCS)4(H2O)2]·2MeOH·4H2O (Ln = Dy3+ (4), Tb3+ (5), Ho3+ (6)). To date, no thioether sulfur-coordinated partial tetracubane-type NiII4LnIII2 cores are known in the B


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Inorganic Chemistry Scheme 1. Synthetic Routes for the Synthesis of 1−6 Complexes

[Dy2Ni4(L)2(μ3-OH)4(μ-NCS)2(NCS)4(H2O)2]·2MeOH·4H2O (4). The following reactants were used: H2L (0.038 g, 0.1 mmol), Dy(NO3)3· 5H2O (0.044 g, 0.1 mmol), Ni(SCN)2·4H2O (0.040 g, 0.15 mmol), NEt3 (0.02 g, 0.2 mmol). Yield: 0.041 g, 42.7% (based on Dy). Anal. Calcd for C48H68Dy2N10Ni4O20S8 (1919.37): C, 30.07; H, 3.36; N, 7.31. Found: C, 30.31; H, 3.26; N, 7.58. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 3422 (s), 2906 (w), 2104 (s), 2010 (s), 1635 (s), 1472 (s), 1301 (m), 1223 (m), 1079 (w), 956 (w), 740 (m), 464 (w). UV−vis (solid): λmax (nm) = 376, 605, 973. [Tb2Ni4(L)2(μ3-OH)4(μ-NCS)2(NCS)4(H2O)2]·2MeOH·4H2O (5). The following reactants were used: H2L (0.038 g, 0.1 mmol), Tb(NO3)3· 5H2O (0.043 g, 0.1 mmol), Ni(SCN)2·4H2O (0.040 g, 0.15 mmol), NEt3(0.02 g, 0.2 mmol). Yield: 0.039 g, 41.2% (based on Tb). Anal. Calcd for C48H68Tb2Ni4N10O20S8 (1909.80): C, 30.23; H, 3.38; N, 7.35. Found: C, 30.45; H, 3.28; N, 7.55. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 3425 (s), 2910 (w), 2101 (s), 2011 (s), 1636 (s), 1475 (s), 1305 (m), 1225 (m), 1079 (w), 957 (w), 745 (m), 466 (w). UV−vis (solid): λmax(nm) = 378, 601, 969. [Ho2Ni4(L)2(μ3-OH)4(μ-NCS)2(NCS)4(H2O)2]·14.1H2O (6). The following reactants were used: H2L (0.038 g, 0.1 mmol), Ho(NO3)3· 5H2O (0.045 g, 0.1 mmol), Ni(SCN)2·4H2O (0.040 g, 0.15 mmol), NEt3(0.02 g, 0.2 mmol). Yield: 0.035 g, 30.42% (based on Ho). Anal. Calcd for C51H111Ho2N10Ni4O38.5S8 (2301.80): C, 27.03; H, 3.95; N, 6.85. Found: C, 27.27; H, 4.18; N, 6.63. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 3426 (s), 2914 (w), 2108 (s), 2014 (s), 1636 (s), 1475 (s), 1305 (m), 1224 (m), 1078 (w), 953 (w), 738 (m), 461 (w). UV−vis (solid): λmax (nm) = 376, 603, 978. Physical Measurements. The absorption spectra were collected using a Shimadzu (model UV2450) spectrophotometer. Vis−NIR

Scheme 2. Synthetic Route to H2L

Found: C, 27.79; H, 3.63; N, 3.44. Selected IR data (KBr, cm−1, s = strong, m = medium, w = weak): 3353 (s), 2939 (w), 1634 (s), 1608 (w), 1479 (s), 1418 (w), 1306 (s), 1247(m), 1228 (s), 1170 (w), 1057 (m), 860 (w), 740 (m), 634 (w). UV−vis (solid): λmax (nm) = 423, 643, 897. General Synthetic Procedures for 4, 5, and 6. All three complexes were synthesized by following a common synthetic method. H2L (0.038 g, 0.1 mmol) was dissolved in 10 mL of MeCN−MeOH (2:1) solution. To this were subsequently added NEt3 (0.02 g, 0.2 mmol) and Ni(SCN)2·4H2O (0.040 g, 0.15 mmol); the reaction mixture stirred for 1 h at room temperature. Addition of 5 mL of (2:1) MeCN−MeOH solution of Ln(NO3)3·5H2O (0.1 mmol) to the previous solution resulted in a bright green solution, and the stirring was continued for another 2 h. After filtration, the filtrate was kept for crystallization through solvent evaporation at room temperature. Green, block shape crystals, suitable for singlecrystal X-ray analysis, were obtained after 4 days. Crystals were collected by filtration, washed with cold methanol, and air-dried. For the synthesis of complexes 4−6, the used reactants and characterization data for the products are given below. C


Article

Inorganic Chemistry

correction was applied using the SADABS.59 The positions of the heavier atoms Ni and Ln were determined easily, and the O, N, and C atoms were subsequently determined from the difference Fourier maps. The non-hydrogen atoms were refined with anisotropic displacement parameters, and the H atoms were incorporated at calculated positions and refined using the riding model. Some of the lattice solvent molecules of complex 6 cannot be modeled satisfactorily due to the disorders. So, the Olex-2 software with the mask program60 suite has been performed to discard those disordered solvent molecules and gave an electron density of 141.75. This value tentatively helps us to assign 14.1 H2O molecules for complex 6. Molecular structures of the complexes were visualized using DIAMOND software.61 The crystal data and refinements for compounds 1−6 are summarized in Table S1. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 1916961−1916963 and 1916965−1916966. These data can also be obtained free of cost at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre).

absorption spectra were recorded in a CARY-5000 UV−vis−NIR spectrophotometer. A PerkinElmer RX1 spectrometer was used to record the FTIR spectra of the synthesized complexes using KBr pellets. The phase purity of the compounds in the powder state was examined by powder X-ray diffraction patterns using a Bruker AXS Xray diffractometer (40 kV, 20 mA) using Cu Kα radiation (λ = 1.5418 Å) within a 2θ range of 5−70° and a fixed-time counting of 4 s at 25 °C. Elemental analysis was performed by a PerkinElmer model 240C elemental analyzer. MALDI-TOF mass spectral analysis was performed using a Bruker UltrafleXtreme instrument, and the matrix used is DHBH (2,5-dihydroxy benzoic acid). The steady-state fluorescence spectra were accrued with a Horiba JobinYvon spectrofluorometer (Fluorolog-3) equipped with a temperature controlled water cooled cuvette holder, and a 1 cm path length quartz cuvette was used to take the scan of the solutions. The temperature-dependent (T = 1.9−300 K) and field-dependent (B = 0−9 T, T = 2, 5, and 10 K) magnetization measurements were performed on a PPMS Dynacool with VSM option. Alternating current susceptibility data were measured on MPMS XL-7 SQUID magnetometer. The magnetic data were corrected for the diamagnetism of the constituent atoms and for the diamagnetism of the sample holder. Theoretical Calculations. ORCA 4.1 comprehensive quantum chemistry software was used for all the theoretical calculations.37,38 The DFT calculations were carried out with the B3LYP functional,39−41 and the isotropic exchange constant J was evaluated following Yamaguchi’s approach,42,43 by comparing the energies of HS (HS) and broken-symmetry (BS) spin states. The ZORA relativistic version of Ahlrich’s basis sets was utilized: ZORA-def2TZVP(-f) for Ni, O, N, Cl, and S atoms; ZORA-def2-SVP for C and H atoms;44 and SARC2-zora-QZV for Lu atoms.45 The calculations utilized the RI approximation with the AutoAux generation procedure46 and the chain-of-spheres (RIJCOSX) approximation to exact exchange47 as implemented in ORCA. Increased integration grids (Grid5 and Gridx5 in ORCA convention) and tight SCF convergence criteria were used in all calculations. The calculated spin density was visualized with the VESTA 3 program.48 The calculations of ligand-field terms and multiplets were done using state average complete active space self-consistent field (SA-CASSCF)49 wave functions. For calculations of DyIII ion properties, SARC2-ZORAQZVP for Dy atom, SARC2-ZORA-QZV for Lu atom, and ZORAdef2-SVP for the rest of the atoms were used. In the case of NiII ion properties calculations, ZORA-def2-TZVP was used for Ni atoms, SARC2-ZORA-QZV for Lu atoms, and ZORA-def2-SVP basis set for the rest of the atoms. In the state-averaged approach, all multiplets for a given electron configuration were equally weighted. The active space for the DyIII ion calculation was based on seven f-orbitals, and nine electrons in these orbitals resulted in 21 sextets, 224 quartets, and 490 doublets. The active space for NiII ions calculation was based on five d-orbitals, and eight electrons in these orbitals resulted in 10 triplets and 15 singlets. The ZFS parameters, based on dominant spin−orbit coupling contributions from excited states, were calculated through quasidegenerate perturbation theory (QDPT),50 in which an approximation to the Breit−Pauli form of the spin−orbit coupling operator (SOMF approximation)51 and the effective Hamiltonian theory52 were utilized. Increased integration grids (Grid6 and GridX6 in ORCA convention) and tight SCF convergence criteria were used in all calculations. Ab initio ligand-field theory (AILFT) was used to calculate energies of f- and d-orbitals.53,54 Crystal Structure Determination. Appropriate single crystals of 1−6 were chosen for data collection on a Bruker SMART APEX-III CCD X-ray diffractometer furnished with a graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation by the ω scan (width of 0.5° frame−1) method at 293 K with a scan rate of 6 s per frame. SAINT and XPREP software programs55 were used for data processing and space group determination. The structures were solved by the direct method technique from SHELXS-201456 and then refined by the fullmatrix least-squares technique using SHELXL-2014/757 programs within the WINGX protocol of version 1.80.05.58 Data were corrected for Lorentz and polarization effects; an empirical absorption

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RESULTS AND DISCUSSION

Synthetic Procedures. Heptadentate ligand H2L was obtained following a standard Schiff base condensation reaction of 2-(2-aminoethylthio)ethylamine and o-vanillin. Reportedly, 2-(2-aminoethylthio)ethylamine on condensation with salicylaldehyde provides a pentadentate ligand H2L′ which produces a Ni2L′2 unit through bridging from two terminal phenolate groups, and the other five positions of octahedral NiII ions are occupied by ligand donor atoms.35 We were also motivated to use the pentadentate part of our H2L ligand to bind two NiII ions and adjacent methoxy functions to trap the lanthanide ions. The stepwise reactions of H2L with NiCl2·6H2O followed by addition of NEt3 and Ln(NO3)3·5H2O (Ln = Dy3+, Tb3+, and Ho3+) in a 1:1.5:2:1 molar ratio in MeCN−MeOH (2:1 v/v) medium provided [Ln2Ni4L2(μ-Cl)2(μ3-OH)4(H2O)6]Cl4· 2H2O (Ln = Dy3+(1), Tb3+(2), Ho3+(3); Scheme 1). All the complexes were isolated as single-crystalline materials, and chosen single crystals were used for X-ray structure determinations. The chemical reaction involved during the generation of complexes 1−3 is summarized in eq 1. 2H 2L + 2Ln(NO3)3 ·6H 2O + 8NEt3 + 4NiCl 2· 6H 2O → [Ln2Ni4L 2(μ‐Cl)2 (μ3 ‐OH)4 (H 2O)6 ]Cl4 ·2H 2O + 6NHEt3NO3 + 2NHEt3Cl + 24H 2O

(1)

The crucial chlorido-bridges have appeared from the anion of the nickel(II) salt. External addition of NaCl in the presence of Ni(ClO4)2·6H2O failed to result in the products, and thus, this indicates the uniqueness of the used metal ion salts. Next, we successfully used Ni(SCN)2·4H2O in lieu of NiCl2·6H2O for ancillary bridge substitution. The isothiocyanato-bridged aggregate was obtained in a similar manner in place of the chlorido-bridges. A new family of 4:2-type (incorporating four 3d and two 4f ions) hexanuclear Ni-4f coordination aggregates [Ln2Ni4L2(μ3-OH)4(μ-NCS)2(NCS)4(H2O)2]·xMeOH·yH2O (Ln = Dy3+ (4), x = 4, y = 2; Tb3+ (5) and Ho3+ (6), x = 0, y = 14.1; Scheme 1) was obtained as given in eq 2. D


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Inorganic Chemistry

The hexanuclear aggregate is formed by the simultaneous coordination of two doubly charged and functionally heptadentate L2− units, around two different types of metal ions with distinctly different coordination demands. The solid and crystalline products obtained from the above-mentioned reactions were characterized by recording their FTIR and DRS (diffuse reflectance spectroscopy) spectral signatures. Detailed discussions and Figures S2 and S3 were included in Supporting Information. Description of the Crystal Structures of Complexes 1−3. Suitable single crystals of 1−3 were obtained after recrystallization from a MeCN−MeOH (2:1, v/v) solvent mixture. All three tetracationic complexes are found to be isostructural, and interestingly, all of them crystallize in same monoclinic space group, P21/n. In all three cases the asymmetric unit contains half of the whole molecule, viz., [Ni2IILnIIIL(μ3-OH)(μ2-OH)(μ-Cl)(H2O)2]Cl2·H2O (Ln = Dy (1), Tb (2), Ho (3)) (Figure S6 in the Supporting Information). As all the complexes are structurally similar, a detailed description of the structure of complex 1 (Ln = Dy3+) has therefore been considered as a representative one for the entire series. Selected bond distances and bond angles are presented in Tables S4−S9 in the Supporting Information. The perspective view of the structure for 1 is shown in Figure 1. The analysis of the X-ray structure revealed that each L2− unit binds two NiII centers in pocket I and pocket II of the ligand, making this part of the ligand asymmetric for coordination to two 3d ions (Chart 1). The bidentate O,O donor parts of pockets III and IV were used to trap the bigger and oxophilic DyIII ions. Thus, 1 can be described as a hexanuclear and

2H 2L + 2Ln(NO3)3 ·6H 2O + 8NEt3 + 4Ni(SCN)2 ·4H 2O + 2MeOH → [Ln2Ni4{L}2 (μ3 ‐OH)4 (μ‐NCS)2 (NCS)4 (H 2O)2 ] ·2MeOH· 4H 2O + 6NHEt3NO3 + 2NHEt3SCN + 18H 2O (2)

Figure 1. Molecular structure of 1. Hydrogen atoms, solvent molecules, and counteranions are omitted for clarity.

Figure 2. (a) Distorted octahedral geometry around both the NiII centers in different coordination environments. (b) Distorted triangular dodecahedron geometry around the DyIII center. E


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Inorganic Chemistry

bridged by two μ3-O atoms from two hydroxido-bridges to form a Dy2O2 rhombus with angles of 108.19° and 71.81° for Dy1−O8−Dy1* and O8−Dy1−O8*, respectively. Formation of this Dy2O2 rhombus is crucial during the condensation of two ligand anion capped Ni2DyO3Cl subunits. The Dy1···Dy1* separation within this rhombus is 3.804 Å (Figure 3). The presence of four types of donor O atoms around the Dy centers is responsible for a variation in Dy−O distances within the 2.292−2.629 Å range. The ligand phenoxido O donor gave the shortest Dy−O separations, while the −OMe donors provided the longest ones. Within the Ni2Dy partial cubane units, all together four types of Ni−O− Dy bond angles are observed within range 99.96−106.2°, resulting in two Ni···Dy separations of 3.413 and 3.438 Å (Figure 3). Angles above 90° indicate distortion toward the planar structure of these partial cubic parts, preventing any kind of complete cube formation. The Ni−Cl−Ni angle is 82.78°, and the Ni···Ni separation is 3.225 Å. During the aggregation of two Ni2Dy partial cube units, two new Ni···Dy separations are accomplished at 3.452 Å. Mean plane analysis for the coplanarity of six metal ion centers revealed that all four NiII centers are in the same plane whereas both the DyIII centers are placed 0.53 Å above and below with respect to this plane. Crystal Structures of Complexes 4−6. Isostructural isothiocyanato-bridged complexes [Ln2Ni4L2(μ3-OH)4(μNCS)2(NCS)4(H2O)2]·xMeOH·yH2O (Ln = Dy3+ (4), x = 4, y = 2; Tb3+ (5) and Ho3+ (6), x = 0, y = 14.1) were obtained as X-ray diffraction quality single crystals after 4 days by slow evaporation from a MeCN−MeOH (2:1, v/v) mixture (Figure 4). Compound 5 consistently gave very poor quality crystals

centrosymmetric NiII4LnIII2 complex formed from four fused defective open cubes, where the decisive bridging is ensured by four exogenous μ3-OH and four endogenous μ-OPh groups. Four six-coordinate NiII centers are either in a NO4Cl or NO3SCl distorted octahedral coordination environment that originated from the asymmetric binding of the pentadentate part of the ligand (pockets I and II). The bidentate N,O half from imine nitrogen and phenoxido oxygen of two ligands binds the Ni1 and Ni1* centers with ancillary bridge supports from two μ3-hydroxido and one μ-chlorido groups (Figure S7 in the Supporting Information). The tridentate O,N,S donors of the other halves of the two ligand anions bind Ni2 and Ni2* in a meridional fashion whereas the bridging phenoxido oxygen donors connect the Dy1/Dy1* centers. The sixth coordination site is occupied by a terminal H2O molecule (Figure 2a). Following the coordination to two nickel(II) ions, the available phenoxido group and the adjacent −OMe function made the bidentate O,O chelation available to 4f ions only. Solvent water derived μ3-hydroxido and metal ion salt derived μ-chlorido groups act as the main ancillary connectors. The μ-chloridobridge is useful for the incorporation of two adjacent nickel(II) ions within the ligand frame. Four solvent derived μ3hydroxido species not only augment the dinickel entity but also bring the dysprosium centers to this. Around Ni1, the O2−Ni1−Cl1 axis is the longest (4.422 Å) from the coexistence of two long Ni1−O2 and Ni1−Cl1 bonds of magnitude 2.015 and 2.418 Å, respectively. For Ni2, the situation is similar to the longest O5−Ni2−Cl1 axis of 4.573 Å, formed from long Ni2−O5 (2.126 Å) and Ni2−Cl1 (2.460 Å) bonds. The tridentate O,N,S donor half provided a shorter Ni2−N2 bond at 2.013 Å compared to the binding from the N,O half giving a Ni1−N1 separation of 2.052 Å. The Ni−O bond distances within the 2.011−2.139 Å range show variation due to the presence of three types of O donor atoms (Table S4). The Ni2−S1 distances are the longest ones at 2.424 Å. Each L2− unit while trapping two NiII centers provided a Ni1··· Ni2 separation of 3.225 Å (Figure 3). The Ni1···S1 separation beyond average binding distances is 3.900 Å, where the S1 is bound to Ni2.

Figure 4. Molecular structure of 4 with partial atom numbering scheme. Solvent molecules and hydrogen atoms are omitted for clarity.

Figure 3. Defective tetracubane core view of crystal 1 with Ni−Ni, Ni−Dy, and Dy−Dy distances in Å. H atoms are omitted for clarity. Color code: O, red; Cl, light green.

from analogous crystallization attempts many times. As a result, structures could not be refined up to the requisite standard. However, their unit cell parameters and a comparison of the powder XRD (Figure S4 in the Supporting Information) pattern indicate that it is an isomorph with complex 4. Finally, elemental analysis confirmed the formulation. All three compounds are electroneutral as terminal H2O molecules bound to the NiII centers are now

The eight-coordinate coordination environments of Dy1 and Dy1* are accomplished by two bidentate O,O parts from two ligand anions, three hydroxido-bridges, and one terminal H2O ligand. Eight donor atoms around the DyIII centers revealed a distorted trigonal dodecahedron coordination environment as verified by SHAPE 2.1 (vide inf ra in the Theoretical Calculations section, Figure 2b). Dy1 and Dy1* are doubleF


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Inorganic Chemistry

Figure 5. (a) Distorted octahedral geometry around the two NiII centers. (b) Distorted trigonal dodecahedral geometry around the DyIII centers.

replaced by NCS− ions. Complex 4 and complex 5 crystallize in triclinic space group P1̅ whereas complex 6 crystallizes in the monoclinic C2/c space group. However, the X-ray structure analysis revealed that the core structure of complex 6 is isostructural with complex 4 and differs only by lattice solvent molecules. So, a detailed structural report of complex 4 (Ln = Dy3+) has been considered as an illustrative one for this group. Magnitudes of selected bond lengths and angles are summarized in Tables S10 and S11 in the Supporting Information. The overall arrangements of the atoms within the aggregates are similar as found in the cases of complexes 1−3. Thus, only the unique structural features of the complex 4 are described here. Replacement of chlorido-bridges by isothiocyanato-bridges resulted in four NiII centers in six-coordinate distorted octahedral N3O3 and N3O2S environments (Figure 5a). This time, along with μ3-hydroxido groups, μ-isothiocyanto groups were utilized for complex formation. Around Ni2, three N donors, one from imine N atoms and two from isothiocyanato groups, showed meridional binding, whereas around Ni1 all three are in facial coordination. Around Ni2, these three different N donors give three different Ni1−N distances in the 2.028−2.203 Å range. The N5 donor from the isothiocyanate ion stays in the apical position around Ni1 but remains in the basal position around Ni2, considering the long axis as the unique one. The dihedral angle between the two basal planes bearing two NiII ions is 79.6° (Figure S12 in the Supporting Information). The O2−Ni1−N5 axis around Ni1 is the longest at 4.163 Å due to the presence of two long Ni1−O2 and Ni1− N5 bonds of 2.027 and 2.156 Å, respectively. Around Ni2, the longest axis is O3−Ni2−S1 of 4.447 Å, formed from long Ni2−O3 (2.012 Å) and Ni2−S1 (2.438 Å) bonds. The tridentate O,N,S donor half provided a shorter Ni2−N2 bond

at 2.028 Å compared to the binding from the O,N half giving a Ni1−N1 separation of 2.094 Å. The asymmetric unit of complex 4 consists of the [Ni2IIDyIIIL(μ2-OH)(μ-NCS)(NCS)2] unit (Figure S11 in the Supporting Information). The Ni−N distances for bridging isothiocyanato groups are shorter at 2.204 and 2.156 Å compared to the Ni−Cl separations from the bridging chlorido functions in 1. This resulted in more compaction by L2− units at a Ni1···Ni2 separation of 3.150 Å. Within this part, the angular disposition of Ni1−O5−Ni2 is 100.4° compared to 92.5° for the Ni1− N5−Ni2 part. The geometry around each DyIII centers is a distorted trigonal dodecahedron as supported by SHAPE 2.1 (vide infra in the Theoretical Calculations section) (Figure 5b). The Dy1···Dy1* separation within 4 is 3.764 Å. The variation in Dy−O distances is within 2.288−2.565 Å from the coordination of three different types of O donors. This time both the two MeO− functions around each DyIII ions register the longest bonds (2.544−2.565 Å). Mass Spectroscopic Identification of the Fragments. To identify the stability and structural integrity of the 3d−4f aggregates in solution, we have used MALDI-TOF MS analysis using 2,5-dihydroxybenzoic acid (DHBH) as the matrix. The chlorido-bridged aggregates (1−3) are insoluble in common organic solvents, whereas isothiocyanato clipped ones (4−6) are soluble in DMSO. The characteristic mass spectra of complexes 4−6 are similar in nature. Thus, we provide the detailed description of the fragments available only from 4. All three complexes (4−6) gave a characteristic spectrum having a base peak at m/z = 311.08 for the mononuclear NiII complex of partially hydrolyzed ligand L1− (C12H17NiO2S, calcd 311.03) and a peak at m/z = 445.59 for a mononuclear [Ni(HL)]+ fragment from the tridentate coordination of three coordination sites remaining vacant (Figure 6a). Another peak G


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Inorganic Chemistry

Figure 6. Experimental MALDI-TOF MS spectrum and the simulated pattern for isotopic distribution for the molecular ion peak at (a) m/z = 445.59 for [NiHL]+ and at (b) m/z = 655.70 for [Ni2(L)(H2O)2(SCN)2 + H+]+.

Figure 7. Experimental and simulated isotopic distribution patterns for the molecular ion peaks at m/z = 990.9 for [Ni2DyL(μ3OH)(H2O)4(SCN)3 + 2MeOH]+.

at m/z = 655.70 can be assigned to the species [Ni2L(H2O)2(SCN)2 + H+]+ (C22H27N4Ni2O6S3, calcd 655.08) expected from the attachment of two NiII centers to L2− (Figure 6b). The more intense peak at m/z = 990.90 can be assigned for the trinuclear species [Ni 2 DyL(μ 3 -OH)(H 2 O) 4 (SCN) 3 + 2MeOH]+ (C25H37N5Ni2DyO11S4, calcd 990.90) (Figure 7). The MALDI-TOF spectra of 5 and 6 are given in the Supporting Information (Figures S15 and S16). All the abovementioned mass spectroscopic results suggest that mono- to trinuclear fragments do exist in DMSO solution. The trinuclear fragment next self-assemble to form the hexanuclear aggregate during crystallization. Magnetic Properties. The temperature- and field-dependent DC magnetic data were acquired on polycrystalline samples of 1−6 as depicted in Figure 8. The ground spin state for pseudo-octahedral NiII is S = 1, and the ground states of LnIII ions are 6H15/2 for DyIII, 7F6 for TbIII, and 5I8 for HoIII. Therefore, the theoretical values of the effective magnetic moment calculated for Ni4Ln2 are as follows: μeff/μB = 16.1 for Ni4Dy2, μeff/μB = 14.9 for Ni4Tb2, and μeff/μB = 16.0 for Ni4Ho2, considering gNi = 2.0; J = 15/2 for DyIII and gDy = 1.33; J = 6 for TbIII and gTb = 1.50; J = 8 for HoIII and gHo = 1.25. The higher room temperature μeff/μB values, 15.9 for 1, 14.9 for 2, 15.9 for 3, 16.1 for 4, 14.1 for 5, and 14.7 for 6, are common for NiII containing complexes due to second order spin−orbit coupling.62 The increase of the effective magnetic moment below 50 K is observed for all compounds 1−6 (Figure 8), which reveals the prevailing ferromagnetic exchange interactions among paramagnetic metal ions within the Ni4Ln2 cluster. The isothermal magnetization data at T = 2, 5, and 10 K were

also acquired and are shown in Figure 10 as is the reduced isothermal magnetization for clarity. The experimental values of the molar magnetization at the lowest temperature (2 K) and highest magnetic field (9 T) are summarized in Table 1 and are much lower than the theoretical saturation values of the isothermal magnetizations calculated as Mmol/NAμB = 28.0 for Ni4Dy2, Mmol/NAμB = 26.0 for Ni4Tb2, and Mmol/NAμB = 28.0 for Ni4Ho2. However, in all six complexes, a sharp increase in magnetization was observed, implying that the ground state was primarily populated, and from the nonoverlapping nature of the curve at a different temperature, it can be assumed that the complexes are dominated with notable magnetic anisotropy induced by the ligand-field and the exchange phenomena.63,64 Both the large magnetic anisotropy and the ferromagnetic exchange suggest that the studied compounds are good candidates for the assessment of singlemolecule magnet behavior, and therefore, also, AC susceptibility measurements were performed. Interestingly, the AC susceptibility measurements revealed a nonzero out-of-phase signal of AC susceptibility at zero static magnetic field for all six compounds, confirming the presence of the slow relaxation of magnetization typical for SMMs. Therefore, the temperature dependence of AC susceptibility was measured for frequencies 1−1500 Hz as shown in Figures 9 and 10 for 1 and 4, and in Figures S17−S20 for 2−3 and 5− 6. In the cases of both Ni4Dy2 (1 and 4) and Ni4Tb2 (2 and 5) compounds, the out-of-phase component of the AC susceptibility (χ″) shows a frequency-dependent maxima. So, complexes 1, 2 and 3, 4 behave as single-molecule magnets (SMMs) at zero static field which is rare in other reported H


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Inorganic Chemistry

Figure 9. Alternating current susceptibility data for 1. Top: in-phase χ′ and out-of-phase χ″ molar susceptibilities at zero-magnetic field BDC = 0.0 T (full lines are only guides for eyes). Middle: frequency dependence of in-phase χ′ and out-of-phase χ″ molar susceptibilities fitted with one-component Debye’s model using eq 3 (solid lines). Bottom: the Argand (Cole−Cole) plot with solid lines fitted with eq 3; on the right, the fit of resulting relaxation times τ with the Arrhenius law (red line).

Figure 8. Direct current magnetic data for complexes 1−6. Temperature dependence of the effective magnetic moment (left) and the reduced isothermal molar magnetizations measured at 2, 5, and 10 K (right).

diamagnetic analogues Lu and Zn enabled us to use the spin Hamiltonian formalism and the broken-symmetry DFT approach for the calculation of the isotropic exchange between neighboring Ni atoms Ni1 and Ni2. The isotropic exchange parameter J was calculated using the following form of the spin Hamiltonian

ferromagnetic Ni−Ln complexes.65 Therefore, the temperature-dependent experimental AC data were analyzed with the one-component Debye model χ (ϖ ) = χS +

(χT − χS ) 1 + (iϖτ )1 − α

(3)

Ĥ = −J(S1·S2)

which led to isothermal (χT) and adiabatic (χS) susceptibilities, relaxation times (τ), and distribution parameters (α) and finally to construction of the Argand (Cole−Cole) plot. Then, we can apply the Arrhenius equation, and the characteristic information about the SMMs was extracted as τ0 = 4.52 × 10−9 s, Ueff = 23.0 K for 1; τ0 = 1.07 × 10−9 s, Ueff = 26.3 K for 2; τ0 = 1.56 × 10−8 s, Ueff = 26.0 K for 4; τ0 = 5.03 × 10−9 s, Ueff = 30.6 K for 5. Theoretical Calculations. To elucidate the impact of different bridging motifs, μ-Cl vs μ-NCS, on electronic structure and magnetic properties, the theoretical calculations were performed for complexes 1 and 4. First, the experimental X-ray data were used to extract molecular geometries of [Dy 2 Ni 4 (μ-L)(μ-Cl) 2 (μ 3 -OH) 4 (H 2 O) 6 ] 4 + of 1 and [Dy2Ni4L2(μ3-OH)4(μ-NCS)2(NCS)4(H2O)2] of 4. The replacement of two Dy atoms and two Ni atoms by their

(4)

and the energy difference between high-spin (HS) and brokensymmetry (BS) spin states, Δ = EBS − EHS, was employed in the J-parameter calculation by Yamaguchi’s approach as J=

2Δ (⟨S2⟩HS − ⟨S2⟩BS )

(5) −1

The respective calculations yielded JNi−Ni = −4.7 cm for 1 and JNi−Ni = +29.2 cm−1 for 4, suggesting that the μ-NCS ligand is important for mediating the ferromagnetic exchange in contrast to the μ-Cl ligand where weak antiferromagnetic exchange was calculated. The spin densities of the HS state of 4 are plotted in Figure 11. The Ni−Ni magnetic interaction is known to depend on the Ni−O−Ni angle. When the angle is shorter than 98° it shows ferromagnetism, and greater values show antiferromagnetism.66,67 In our case, complexes 1 and 4 I


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Inorganic Chemistry

Figure 11. Calculated spin density distribution in [Lu2Zn2Ni2L2(μ3OH)4(μ-NCS)(NCS)4(H2O)2] of 4 for a high-spin system from single point energy calculations using the B3LYP functional. The spin density is shown with violet surfaces (cutoff set to 0.005 e bohr−3).

parameter does not correlate with bridging angle or with Ni−Ni separation (Table S14). In the case of μ-NCS and μ-O double-bridged known complexes, the Ni−NCS−Ni angle greater than 92° results in dominant ferromagnetic exchange coupling which results in a ferromagnetic Ni−Ni interaction even though the Ni−O−Ni angle is >98° (Table S15, refs S9−S10). An earlier report showed an antiferromagnetic exchange coupling with J = −43.6 cm−1 for a complex having a Ni−NCS−Ni angle of 88.9° (Table S15, ref S11). Herein, the Ni−NCS−Ni angle is 92.54° for 4 with J = +29.3 cm−1. These observations thus indicate that, for μ-NCS and μ-O double-bridged complexes, the crossover angle for the ferro- to antiferromagnetic exchange interaction lies between 88.9° and 92°, respectively. However, more examples of μ-NCS/O double-bridged Ni2 complexes with definite theoretical calculations are needed to establish this observation. Next, the CASSCF method was used to estimate the electronic structure and the magnetic anisotropy of NiII and DyIII ions in 1 and 4. The calculations were done for both crystallographically independent Ni atoms, Ni1 and Ni2, and also for the Dy1 atom by replacing all other paramagnetic metal atoms with their diamagnetic analogues, Zn and Lu. The computational results for Ni atoms are visualized in Figure S21, where the splitting of d-orbitals is shown together with the ligand-field terms (triplets and singlets). Evidently, the heteroleptic ligand sphere together with significant deviation from ideal Oh symmetry is responsible for removing the degeneracy of the t2g- and eg-orbitals. Also, these factors affected the splitting of the lowest ligand-field multiplet originating from the 3Γ ground state ligand-field term as shown in Figure S21 (right). The SHAPE program was used to assess the geometrical deviation from an ideal octahedron by a Continuous Shape Measures calculation (Table S2), and the largest deviation was found for the Ni2 atom of 1 (OC-6 = 1.256), for which also the largest energy splitting of the S = 1 ground state is observed. The further analysis of the CASSCF calculations based on the effective Hamiltonian theory was done to derive the zerofield splitting parameters of Ni atoms, which are listed in Table 2. In the case of 1, the positive D parameters were found for NiII ions, but in the case of Ni1 the large rhombicity (E/D → 1/3) means that the easy axis type of the magnetic anisotropy

Figure 10. Alternating current susceptibility data for 4. Top: in-phase χ′ and out-of-phase χ″ molar susceptibilities at zero-magnetic field BDC = 0.0 T (full lines are only guides for eyes). Middle: frequency dependence of in-phase χ′ and out-of-phase χ″ molar susceptibilities fitted with one-component Debye’s model using eq 3 (solid lines). Bottom: the Argand (Cole−Cole) plot with solid lines fitted with eq 3; on the right, the fit of resulting relaxation times τ with the Arrhenius law (red line).

Table 1. Selected Magnetic Properties of 1−6 compd

μeff/μB (2 K)

μeff/μB (300 K)

μeff/μB (theor)a

Mmol/NAμB (2 K, 9 T)

Mmol/NAμB (theor)a

1 2 3 4 5 6

12.9 12.5 13.4 20.2 20.5 17.7

15.9 14.9 15.9 16.1 14.1 14.7

16.1 14.9 16.0 16.1 14.9 16.0

18.6 17.0 17.3 18.8 17.7 17.0

28.0 26.0 28.0 28.0 26.0 28.0

a

The values obtained from DFT calculations.

show Ni−O−Ni angles of 104.41° and 100.4°, respectively, leading to an antiferromagnetic interaction only. However, surprisingly we got a ferromagnetic interaction with a positive magnitude of J for 4. Thus, the Ni−X−Ni (X = Cl− in 1 and NCS− in 4) angle is crucial to control the Ni−Ni interaction, and we can justify this observation by comparing the calculated JNi−Ni values of Ni(II) complexes with the μ-Cl and μ-O double-bridged motifs and with the μ-O and μ-NCS doublebridged motifs reported so far. In the case of the μ-Cl and μ-O double-bridged complexes, both antiferromagnetic exchange and ferromagnetic exchange were found, but the possible magnetostructural correlation is elusive, because the JJ


Article

Inorganic Chemistry Table 2. CASSCF Calculated Parameters for Ni atoms of 1 and 4 chromophore D (cm−1) E/D gx gy gz

Ni1 (1)

Ni2 (1)

Ni1 (4)

Ni2 (4)

{NiClNO4} 5.94 0.330 2.321 2.346 2.294

{NiClNO3S} 14.5 0.091 2.354 2.370 2.256

{NiN3O3} −7.14 0.237 2.294 2.269 2.331

{NiN3O2S} −5.65 0.138 2.298 2.288 2.334

is present.68,69 On the contrary, both NiII ions in 4 possess negative D-parameters resulting in the easy-axis-type of magnetic anisotropy. The absolute values of |D| are in the range expected for pseudo-octahedral complexes.70,71 Next, the CASSCF calculations were done for DyIII ions in 1 and 4, and the splitting of f-orbitals together with the energy levels of the ligand-field multiplet originating from 6H15/2 is depicted in Figure 12.

effective spin Hamiltonian with S = 1/2, and respective gfactors were derived as listed in Table 3. The ground state Kramers doublets have large axial magnetic anisotropy with very small gx and gy components (Table 3), which hinders the possibility for the quantum tunnelling of the magnetization, thus rationalizing the zero-field SMM behavior of these compounds. However, already the first excited Kramers doublet for Dy of 1 and 4 has significantly larger gx and gy components, and the angle between the gz component of the Kramers pairs between the first and the second ones is quite large, which suggests that the magnetic relaxation is limited to the first excited state. Furthermore, the knowledge of the ground state Kramer’s doublet gz-vector orientation enabled us to judge the nature of the dipole−dipole interaction between two Dy atoms in 1 and 4 using the following relationship for the energy of the dipole−dipole interaction.72 Edipolar = −

μ0 μi μj 4π r 3

(3 cos2 θ − 1)

(6)

The angle θ is defined by the orientation of the magnetic moments (μi and μj) with respect to the Dy···Dy connecting line, and for θ < 54.7° the ferromagnetic dipolar interaction is expected, whereas for θ > 54.7° the antiferromagnetic dipolar interaction is presumed. The situation for 1 and 4 is depicted in Figure S22, and the respective angles are θ = 35.3° for 1 and θ = 54.0° for 4. These values propose the presence of the ferromagnetic dipolar exchange between DyIII ions. Steady-State Emission Behavior. From the viewpoint of the characteristic optical properties of these coordination aggregates, it is expected that they can show emission behavior for luminescence applications. The solid-state luminescence spectra of all the complexes are measured at room temperature. Upon exiting the H2L at 295 nm, it shows two broad emission bands at 399 and 466 nm. Systematically the spectra of all the complexes show similar ligand centered fluorescence behavior with enhanced emission intensity. The complexes with an isothiocyanide bridging unit show a higher magnitude of fluorescence intensity compared to the chloride-bridged one.

Figure 12. Output of the CASSCF calculations with CAS (9,7) for the DyZn4Ln complexes of 1 and 4. Plot of the f-orbital splitting calculated by ab initio ligand-field theory (AILFT) (left) and the lowest ligand-field multiplets (right).

The utilization of the SHAPE program showed that both {DyO8} chromophores of 1 and 4 are closest to the triangular dodecahedron geometry (TDD-8 = 1.329 for 1 and 1.056 for 4) (Table S3). The larger deviation from ideal geometry is found for the Dy atom of 1, for which also the larger splitting of f-orbitals and ground state LF multiplets is calculated. There are eight Kramers doublets arising from 6H15/2 atomic multiplets, and each of them was characterized with the

Table 3. CASSCF Calculated Parameters for Dy Atoms of 1 and 4a Dy1 (1)

Dy1 (4)

E/hc (cm−1)

gx/gy/gz

α (deg)

E/hc (cm−1)

gx/gy/gz

α (deg)

0 132 284 366 438 533 712 788

0.004/0.006/19.227 0.056/0.066/16.318 0.514/0.823/13.765 2.501/2.776/10.531 3.978/5.544/8.797 1.090/1.560/14.222 0.067/0.305/15.953 0.074/0.268/18.135

0 174 154 36.0 86.8 99.6 74.2 106.8

0 97 165 261 299 369 519 628

0.006/0.013/18.784 0.247/0.402/14.571 0.664/0.994/12.763 3.479/5.432/9.297 0.256/2.410/10.444 1.406/2.450/12.885 0.254/0.474/17.216 0.047/0.120/19.092

0 167 20.8 99.8 111 91.6 94.1 119.3

α is the angle between the gz component of the first Kramers doublet and the excited ones.

a

K


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Inorganic Chemistry

bidentate bites to bind oxophilic LnIII centers. Luminescence study on complexes 4−6 in DMSO solvent showed that only complex 5 exhibits Tb(III) centered emission, and complexes 4 and 6 only showed ligand centered emission. The DC magnetic measurements revealed ferromagnetic exchange and significant magnetic anisotropy in all the compounds. The AC susceptibility confirmed the slow relaxation of the magnetization in zero static magnetic field, establishing the reported compounds as zero-field SMMs. DFT- and CASSCF-based calculations for Ni4Dy2 complexes (1 and 4) were analyzed to see the impact of the variation of chlorido and isothiocyanato bridging ligands on the electronic structure and magnetic properties. Calculations found significant axial anisotropy of the ground state Kramers doublet of DyIII atoms in 1 and 4, with the energy of the first excited Kramers doublet slightly larger in 1. The magnitude of the D parameter of NiII ions was found to be negative for Ni1 and Ni2 in 4 in contrast to 1 having a positive value. To summarize, the alternation of the bridging ligands (halogenido vs pseudohalogenido) has important consequences both for magnetic exchange and the magnetic anisotropy of reported compounds, and especially the observation of zerofield SMM behavior renders these molecular systems promising for further chemical modifications aiming for improved magnetic properties.

Herein, we report only the spectra of the Tb analogue (complex 2 and complex 5) in the Supporting Information (Figure S23a). The room temperature liquid state photoluminescence behavior of H2L was measured in DMSO solution showing a broad emission band centered at 401 nm and a strong luminescence band at 494 nm from excitation at λexc= 295 nm. An extensively conjugated π system of H2L is responsible for the broad luminescence band. The photoluminescence characteristics of complexes 4−6 have also been measured in DMSO solutions, and the results are given in Figure 13. Both complexes 4 and 6 show strong ligand-based

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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.9b01517. NMR, FTIR, UV−vis, and crystal data; PXRD curves; selected bond lengths and angles; AC susceptibility; Arrhenius plots; and CASSCF calculated parameters (PDF)

Figure 13. (a) Luminescence spectra of complexes 4, 5, and 6 in DMSO solvent at λexc= 295 nm.

broad luminescence centered at 451 and 480 nm, respectively, and the emission peaks are blue-shifted. Under a similar condition, complex 5 shows weak ligand-based emission and exhibits a typical emission pattern of the TbIII center at 489 and 545 nm corresponding to 5D4 → 7F6 and 5D4 → 7F5 transitions, respectively.73 So, the hexanuclear complex in the solid state does not show any metal centered emission, but when the complex dissolves into DMSO, it shows weak metal centered sharp emission bands. This feature suggests the MALDI-TOF identified solution stable fragment [Ni2TbL(μ3-OH)(H2O)4(SCN)3 + 2MeOH]+ is responsible for these weak emission bands for complex 5. The o-vanillin containing Schiff base ligand backbone coordinating 3d−4f metal ions are well-known for exhibiting interesting photophysical properties.74,75 In this fragment the chelating ligand acts as “antenna” to transfer energy from the ligand to the TbIII center of that fragment.76 With the incremental addition of complex 5 (0−0.8 mol equiv), a decrease in the luminescence intensity was observed for the ligand centered broad emission band as well as the metal centered sharp emission band (Figure S23b).

Accession Codes

CCDC 1916961−1916963 and 1916965−1916966 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]. uk, 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 Author

*E-mail: [email protected]. ORCID

Radovan Herchel: 0000-0001-8262-4666 Zdeněk Trávníček: 0000-0002-5890-7874 Debashis Ray: 0000-0002-4174-6445 Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Synergistic coordination driven aggregation reactions of H2L with NiCl2·6H2O and nitrate salts of three Ln(III) ions resulted in self-assembled 3d−4f clusters. This is the first report of such aggregates from the use of any soft thioether bearing ligand system. The L2− form of the coordinating ligand is appropriate to trap two NiII ions from the bridging action of Cl− and NCS− ions, leaving behind available O,−OMe-based

ACKNOWLEDGMENTS A.B. is thankful to DST-INSPIRE for a research fellowship. We gratefully acknowledge DST-FIST, New Delhi, for providing the single-crystal X-ray diffractometer facility at the Department of Chemistry, IIT Kharagpur. The authors (R.H. and Z.T.) gratefully thank the Ministry of Education, Youth and Sports of the Czech Republic (a project no. LO1305) for L


Article

Inorganic Chemistry financial support. R.H. also acknowledges the financial support received from the Department of Inorganic Chemistry, Palacký University, in Olomouc, Czech Republic.

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