[Tb₄(OH)₄]-Cuboid Complex Dianion Stabilized with Six Carboxylate Bridges and Four Diketonate Caps

Abstract

A cubane-like complex dianion [Tb₄(OH)₄(tfa)₆(hfac)₄]²⁻ was synthesized, accompanied by two counter cations of [Ni(hfac)(2pyIN)₂]⁺, where Htfa, Hhfac, and 2pyIN stand for trifluoroacetic acid, 1,1,1,5,5,5-hexafluoropentane-2,4-dione, and 4,4,5,5-tetramethyl-2-pyridylimidazolin-1-oxyl, respectively. In the complex anion, each Tb ion was capped with hfac, and each Tb···Tb diagonal was bridged with tfa, thus forming an approximate Td symmetry, though the whole molecular formula was crystallographically independent in an orthorhombic Pbca space group. The ionic charge was −2 in total. The magnetic study revealed that the complex ions were magnetically isolated from each other. Practically no 4f–4f superexchange interaction was operative, while the 2p–3d ferromagnetic coupling seemed to be appreciable, as anticipated from the proposed magnetostructural relationship.

1. Introduction

Heterospin magnetic materials are fascinating to researchers because of the wide variety of symmetries and energy levels of the paramagnetic centers available from the viewpoint of frontier-orbital engineering [1,2,3,4,5,6,7,8,9,10,11,12]. Lanthanide ions, as a 4f spin source, have a large magnetic moment and strong magnetic anisotropy, and become indispensable in the development of conventional bulk magnets as well as recently exploited single-molecule magnets (SMMs) [4]. As a working hypothesis to construct exchange-coupled clusters and networks, plausible magneto-structure correlations have been proposed for 2p–3d [5,6,7], 3d–4f [8,9], and 2p–4f [10,11,12] heterospin systems. There are an increasing number of reports describing 2p–3d–4f triad systems [13,14,15,16,17]. Hopefully, these paramagnetic centers would be better strongly exchange-coupled, because the resultant large total spin quantum number is favorable for SMM performance.

Lanthanide ions usually behave as a hard acid compared with transition-metal ions, which exhibit a soft nature [18]. Upon such selectivity, a simple synthesis has widely been applied, where the 2p, 3d, and 4f spin sources are mixed in a one-pot manner [13,14,15,16,17]. During our development on 2p–3d–4f heterospin magnets, we have found a cubane-like complex dianion [Tb₄(OH)₄(tfa)₆(hfac)₄]²⁻, accompanied by two counter cations of [Ni(hfac)(2pyIN)₂]⁺ in a crystal lattice. Here, Htfa and Hhfac stand for trifluoroacetic acid and 1,1,1,5,5,5-hexafluoropentane-2,4-dione, respectively. Carbonyl compounds are often available as an antenna in luminescent materials [19,20]. The 2pyIN radical (4,4,5,5-tetramethyl-2-pyridylimidazolin-1-oxyl or 2-pyridyl iminonitroxide) [21] is popular in the metal-radical approach. We will describe the molecular/crystal structure and magnetic properties of the product. The formation of cubane-like clusters is not so astonishing since various oxo- and hydroxo-bridged metal clusters have been developed, but a highly symmetric architecture and dianionic valence is rare in the lanthanide cubane-like chemistry, to our knowledge.

2. Materials and Methods

2.1. Preparation

The complex formation was conducted according to the method for the known lanthanide–nitroxide complexes [22,23] with a slight modification. A heptane solution (20 mL) including [Tb(hfac)₃(H₂O)₂] [24,25] (40.3 mg; 0.049 mmol) was refluxed and concentrated to ca. 2 mL. Similarly, another heptane suspension (20 mL) including Ni(CF₃CO₂)₂·4H₂O (Wako, 19.3 mg; 0.054 mmol) was refluxed and concentrated to ca. 2 mL. The above solution and suspension were cooled to ca. 40 °C and poured into a dichloromethane solution (1.5 mL) including 2pyIN [19] (21.7 mg; 0.10 mmol). Immediately, the combined solution was filtered, and the filtrate was allowed to stand in a freezer (−22 °C). After 3 days, [Ni(hfac)(2pyIN)₂]₂[Tb₄(OH)₄(tfa)₆(hfac)₄](C₇H₁₆)_x (abbreviated as 1 hereafter) as orange polycrystalline precipitates were separated on a filter, washed with heptane, and air-dried (Scheme 1). The yield was 18.1 mg (40%). Mp 148–150 °C. Anal. (PerkinElmer 2400 II, PerkinElmer Inc., Waltham, MA, USA). Calcd. for C₉₇H₉₀F₅₄N₁₂Ni₂O₃₂Tb₄ (x = 1): C, 31.36%; H, 2.44%; N, 4.52%. Found: C, 31.42%; H, 1.25%; N, 4.72%. IR (neat, attenuated total reflection (Nicolet FT-IR 6700, Thermo Scientific, Waltham, MA, USA)) 3578, 1688, 1652, 1555, 1497, 1347, 1253, 1200, 1138, 1098, 794, 662, 583 cm⁻¹.

2.2. Structural Analysis

X-Ray diffraction data of a single crystal of 1 were collected on a Saturn 70 hybrid pixel array detector (Rigaku, Tokyo, Japan) with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The structure was solved by a heavy-atom method and expanded using Fourier techniques in the Olex2 program [26]. The parameters were refined on Shelxl [27]. The thermal displacement parameters of non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located at calculated positions and treated as “riding”. A disorder model was applied to four trifluoromethyl groups. Considerable residual electron densities were found in a void space, and solvent heptane molecules could not be placed, possibly owing to severe disorder. A “squeeze” treatment [28] was applied to 183 residual electrons in a void space of 1016 ų per asymmetric cell, corresponding to three heptane molecules as a solvent mask. Selected data are as follows: C₁₁₁H₁₂₂F₅₄N₁₂Ni₂O₃₂Tb₄, orthorhombic Pbca, a = 32.1908(6), b = 29.0734(5), c = 32.2901(6) Å, V = 30220.1(9) ų, Z = 8, d = 1.721 g cm⁻³, µ(MoKα) = 2.228 mm⁻¹, R(F) (I > 2σ(I)) = 0.0526, wR(F²) (all reflections) = 0.1185, goodness-of-fit parameter = 0.976, T = 96 K for 41279 reflections. For experimental details and geometrical parameter tables, see CCDC reference number 2153576.

2.3. Magnetic Measurements

Direct and alternating current (dc and ac, respectively) magnetic susceptibilities of 1 were measured on MPMS-XL7 and MPMS3 SQUID magnetometers (Quantum Design Inc., San Diego, CA, USA). For the dc measurements, the magnetic responses were corrected with diamagnetic blank data of the sample holder measured separately, and further diamagnetic contribution was estimated from Pascal’s constants [29].

3. Results and Discussion

Compound 1 was prepared from the starting materials [Tb(hfac)₃(H₂O)₂], Ni(CF₃CO₂)₂, and 2pyIN in a one-pot manner. The product was stable under ambient conditions, but the crystal solvent molecules incorporated escaped easily from the crystal lattice. The composition was confirmed to be [Ni(hfac)(2pyIN)₂]₂[Tb₄(OH)₄(tfa)₆(hfac)₄] by means of spectroscopic, analytic, and X-ray diffraction analyses. The single-crystal X-ray diffraction study clarified that the space group was orthorhombic Pbca with a whole formula unit crystallographically independent (Figure 1a). Considerable residual electron densities were found in a void space in a lattice, being ascribable to heptane. The complex cations and anions were discrete and accordingly seem to be magnetically isolated from each other. On the other hand, the Ni²⁺ ion and 2pyIN ligands must be magnetically correlated because of the direct metal–radical coordination bonds (see below).

As Figure 1b shows, the anionic portion has a tetranuclear Tb³⁺ cage cluster in 1. Each Tb³⁺ ion is coordinated with three OH⁻ ions, and each OH^– ion plays the role of a tripodal bridge, forming a cube-like structure. The intra-cuboid Tb–O bond lengths varied from 2.329(3) to 2.429(3) Å, the Tb–O–Tb angles from 105.06(11) to 108.67(11)°, and the O–Tb–O angles from 69.04(10) to 71.47(10)°. Thus, the cubane-like cluster was considerably distorted with the wider Tb–O–Tb angles and narrower O–Tb–O angles, compared to 90°. The peripheral ligands were characterized as follows: each Tb³⁺ ion was capped with an hfac anion, and every Tb···Tb diagonal was bridged with a tfa anion. Several tfa-bridged lanthanide complexes are known in the literature [30]. Thus, the tetra(µ₃-hydroxo)- and hexa(µ-η²(O,O’)-tfa)-bridged structure in 1 can be regarded as an approximate Td symmetry. The Tb³⁺ ions are eight-coordinate, and the SHAPE analysis [31] indicates that the TbO₈ coordination spheres are best described as a triangular dodecahedron (TDD-8). Hydrogen bonds were found between the anion and cation moieties; the O···O distances were 2.991(5) and 2.918(4) Å for O2–H2···O25 and O4–H4···O29, respectively. Such interactions brought about an L-shaped cation–dianion–cation arrangement, and the resultant bulkiness seems to afford a void space in a crystal lattice, where the crystal solvent molecules were incorporated.

The [Tb₄(OH)₄]⁸⁺ core in 1 was similar to that of the known lanthanide cuboids [32,33,34,35]. A possible formation mechanism has already been discussed elsewhere [35]. In comparison with the known compounds, it should be noted that the present complex had peripheral ligands located in a highly symmetrical manner. Furthermore, a dianionic valence of the cuboidal complex ion is rather rare.

As for the cation moiety of 1 (Figure 1c,d), we can find two 2pyIN chelate rings around the Ni²⁺ (3d⁸) ion. The C_2py-C_2im–N–Ni torsion angle, ϕ, is proposed to be a convenient metric to assess magnetostructural relationship. The torsion angles were 1.1(6), 1.7(6), −2.5(6), and 0.7(5)° around the C40–C33–N1–Ni1, C52–C45–N4–Ni1, C69–C62–N7–Ni2, and C74–C81–N12–Ni2 bonds, respectively. These small torsions guarantee the absence of d_z2/d_x2_–y2–π* orbital overlap. A closely related 2p/3d = 1/1 complex [Ni(hfac)₂(2pyIN)] was reported to display orthogonal d–π* orbital arrangement and ferromagnetic coupling [36]. According to the magnetostructural relationship, the critical |ϕ_C|, where the exchange coupling alters from ferro- to antiferromagnetic, was reported to be 12.8(8)° for the Cu²⁺ and Ni²⁺ compounds [7] and 21(1) or 26(3)° for the Ni²⁺ compounds [37]. We can safely predict that the [Ni(hfac)(2pyIN)₂]⁺ portions in 1 should behave as a ground quintet species.

Figure 2a displays the dc magnetic susceptibility result on 1. A relatively temperature-insensitive character appeared, indicating that each complex ion was magnetically isolated from each other. This finding also implies that the Tb–O–Tb superexchange coupling was negligible. At 300 K, the χ_mT value was 50.5 cm³ K mol⁻¹, in good agreement with the theoretical value 50.7 cm³ K mol⁻¹ expected from the sum of four Tb³⁺ ions (11.81 cm³ K mol⁻¹ each with the angular moment J = L + S = 6 and the Landé factor g_J = 3/2), two Ni²⁺ ions (1.00 cm³ K mol⁻¹ each), and four radicals (0.375 cm³ K mol⁻¹ each). On cooling, the χ_mT value gradually decreased and reached 45.6 cm³ K mol⁻¹ at 1.8 K, and this profile can partly be explained in terms of depopulation effect regarding the J^z sublevels in the Tb³⁺ ion. Such a lanthanide single-ion property sometimes disturbs exchange coupling analysis. In the present case, the 2p–3d interaction in the cation portions could not be evaluated.

The M–H curve (Figure 2a, inset) displays a positive slope even at 7 T, and the magnetization (34.4 N_Aµ_B at 7 T and 1.8 K when the sample was unfixed) was lower than the theoretical maximum value 44 N_Aµ_B, mainly because of the strong magnetic anisotropy of the Tb³⁺ moment.

Figure 2b shows the in-phase (χ’) and out-of-phase parts (χ”) of the molar ac magnetic susceptibilities for 1, measured in no dc bias field. In further experiments with applied fields of 1000 and 2000 Oe, no appreciable χ” was recorded either. There have been several reports on Dy-based SMMs including Dy₄(OH)₄ cuboids [33]. The Kramers theorem suggests that Dy³⁺ ion has a double-well potential surface suitable for SMMs but Tb³⁺ ion does not [38,39]. After combining the results of the Tb₄- and Dy₄-based cuboids, the single-ion magnetic property of Tb³⁺ and Dy³⁺ seems to regulate the whole molecular magnetism. This conclusion is compatible with the negligible 4f–4f coupling in the cuboid.

4. Conclusions

A dianionic cuboidal complex ion [Tb₄(OH)₄(tfa)₆(hfac)₄]²⁻ was synthesized, accompanied by two counter cations of [Ni(hfac)(2pyIN)₂]⁺ in a crystal lattice. In the complex anion, each Tb ion is capped with hfac and bridged with tfa in a highly symmetrical manner. The magnetic study revealed that the complex ions were magnetically isolated and that the Tb–O–Tb superexchange coupling was negligible, although the 2p–3d–4f heterospin material was realized Building upon the present architectural study, the use of a wide diversity of carboxylates and diketonates will be of interest to supramolecular chemistry involving a lanthanide cuboid building block.