A unusual two-dimensional azido-Cu(II) network with benzoate derivative as a co-ligand exhibiting ferromagnetic order and slow magnetic relaxation

Abstract

A 2D layer-like azido-copper system was constructed from linear tetranuclear metal motif with mixed-ligand bridges of carboxylate, azido and hydroxyl groups. The strong ferromagnetic coupling between Cu(II) ions was due to the counter complementarity of the multiple super-exchange pathways, leading to a dramatic magnetic order and slow relaxation.

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Molecule-based magnets have achieved considerable attention in recent years because of their several interesting and potentially useful magnetic behaviors,¹ including single-molecule magnets (SMMs),² single-chain magnets (SCMs),³ photomagnetisms,⁴ spin crossover⁵ and spin glass.⁶ Among the transition metal-based systems, a popular branch for the construction of molecular magnets is to combine various spin carriers via short linkages of one to three atoms in length. The azido ligand is an ideal candidate for a short bridging coupler because it offers abundant structural and magnetic diversity, owing to its ease of coordination with transition metal nodes in various bridging structures,⁷ such as μ-1,1 (end-on, EO), μ-1,3 (end-to-end, EE), μ-1,1,3 and other modes, and its efficient magnetic coupling (Scheme 1). Magnetic exchange can be modulated by an azido bridge exhibiting ferromagnetism (FM) or antiferromagnetism (AF) depending on bridging mode, bonding geometries, and the category of metal ions.⁸ In principle, EE-azido supports antiferromagnetic interactions, whereas an EO bridge allows ferromagnetic interactions. Despite several exceptions,⁹ as well as other mixed azido bridging modes that are also responsible for interesting magnetic properties, the competitive action between FM and AF occurs when the two bridge models emerge concurrently.¹⁰ In the past decades, several dinuclear and polynuclear one-dimensional or multidimensional azido-metal compounds were obtained under different reaction conditions.¹¹ Particularly, azido-Cu(II) systems have attracted great attention in the field of molecule-based magnetic materials,¹² resulting from the superiority of understanding the fundamental nature of magnetic interactions and magneto-structural relationships in these compounds on the molecular level.¹³

Scheme 1

For azido-Cu(II) compounds, it has been suggested that the strong ferromagnetic coupling in the EO-azido linker occurs at a Cu–N–Cu bond angle close to 108°; an antiferromagnetic exchange would be observed for larger Cu–N–Cu bond angles.^8–11 Consequently, in order to tune the structure and dimensionality of azido-metal compounds with fascinating magnetic properties, particularly in azido-Cu(II) systems, an optimized strategy is to introduce various co-ligands into the systems. The most commonly used auxiliary ligands are carboxylate-containing compounds, which would bridge Cu(II) ions to yield multitudinous systems and employ different bridging motifs to transmit various magnetic interactions.¹⁴ For example, μ-1,1-azido and syn–syn-carboxylate ligands generally bridge the Cu(II) centers to produce a tetra-coordinated quadrangular geometry.¹⁵ Especially, molecular orbit calculations systematically expound that the counter complementarity effect, resulting from mixed azido and carboxylate bridges, weakens the impact of the antiferromagnetic interaction to the point where predominantly ferromagnetic behavior is awakened.¹⁶ Therefore, it would be worthwhile and meaningful to fine-tune the intra- and intermolecular magnetic interactions relying on the coefficient of the azido and carboxylate spacers in the field of molecular magnetism. A sequence of azido/carboxylate-Cu(II) compounds were recently reported with isolated 1D ferromagnetic chains involving μ-1,1-azido and syn–syn-carboxylate ligands.¹⁷ Although strong ferromagnetic coupling between Cu(II) ions is observed in most of these compounds, there are still a few cases featuring dazzling architecture and long-range magnetic ordering as well as slow magnetic dynamics.¹⁸

Inspired by the aforesaid facts, a benzoate ligand with trifluoromethyl substituent (4-trifluoromethyl benzoic acid, 4-Htfmba) was adopted as co-ligand in the formation of the azido-Cu(II) compounds. Fortunately, a novel azido-based Cu(II) compound, [Cu₄(4-tfmba)₄(N₃)₃(OH)]_n (1), has been prepared by the following general procedure: reaction of a solution containing copper(II) salt, sodium azide and the 4-tfmba ligand under hydrothermal conditions (ESI†). Details of the crystallographic data are given in Table S1.† The present case acts as an attractive two-dimensional (2D) ferromagnetic network composed of an unprecedented tetranuclear azido-copper structure with anfractuous pathways of exchange coupling, directing strong ferromagnetic coupling between adjacent Cu(II) ions, and even magnetic order and slow relaxation of the magnetization, which are rarely found in the azido/carboxylate-Cu(II) systems reported previously.

Single-crystal X-ray diffraction analysis reveals that compound 1 is characterized by an intricate 2D layer-like structural network. The compound is composed of unusual tetranuclear Cu(II) clusters in which the asymmetric unit contains four Cu(II) ions, one μ₃-hydroxyl anion, four 4-tfmba ligands with μ₂-O,O′ and μ₃-O,O,O′ coordination patterns and three azido anions with μ-1,1 and μ-1,1,3 bridging modes. All Cu(II) centers exhibit uniformly penta-coordinated geometries in variously distorted square-based pyramids (Fig. 1a). The coordination environment of Cu1 (CuO₄N) is one nitrogen atom from a μ-1,1,3-azido anion [Cu1–N4 = 1.959(8) Å], and four oxygen atoms, one from a μ₃-hydroxyl anion and the remaining three from μ₂-O,O′ and μ₃-O,O,O′-carboxylates, of which one oxygen atom is located in the apical position [Cu1–O9 = 1.923(7) Å, Cu1–O5 = 1.909(7) Å, Cu1–O1 = 1.915(7) Å, Cu1–O1B = 2.551(8) Å]. Cu2 consists of a CuN₂O₃ chromophore in which two nitrogen atoms (one falls into the apex) are derived from two different μ-1,1,3-azido ligands [Cu2–N1 = 1.958(9) Å, Cu2–N6A = 2.645(7) Å], one oxygen atom belonging to a μ₃-hydroxyl anion and two oxygen atoms from μ₂-O,O′-carboxylates [Cu2–O9 = 1.916(7) Å, Cu2–O3 = 1.906(7) Å, Cu2–O6 = 1.947(8) Å]. The coordination sphere of Cu3 is CuN₃O₂, in which the three N atoms originate from one μ-1,1-azido anion and two μ-1,1,3-azido anions, one of which is situated in the apical position [Cu3–N7 = 1.938(9) Å, Cu3–N1 = 1.964(9) Å, Cu3–N3A = 2.623(7) Å] as well as two oxygen atoms from two different μ₂-O,O′-carboxylates [Cu3–O4 = 1.899(8) Å, Cu3–O7 = 1.900(8) Å]. The last copper atom, Cu4, is five-coordinated to a N₂O₃ unit which includes two N atoms from μ-1,1- and μ-1,1,3-azido ligands [Cu4–N7 = 2.021(9) Å, Cu4–N4B = 2.041(9) Å], three O atoms from one μ₃-hydroxyl anion, one μ₂-O,O′-carboxylate and one μ₃-O,O,O′-carboxylate (one occupies the top position) [Cu4–O9 = 2.364(7) Å, Cu4–O8 = 1.931(7) Å, Cu4–O2A = 1.937(7) Å]. The average Cu–N and Cu–O bond lengths are in the range of 1.938–2.645 Å and 1.899–2.557 Å, respectively, which are in agreement with those of other carboxylate/azido mixed-bridge Cu(II) compounds.^{12f,g,13d–f} Notably, the azido groups are quasi linear with N–N–N angles in the range of 177.14–178.61°, and display unsymmetrical N–N bond lengths. The difference in N–N bond lengths is ascribed to the diverse coordination numbers of the two terminal N atoms. The N–N bonds in μ-1,1,3-azido ligands involved in bidentate coordination (N(1)–N(2) = 1.209 Å, N(4)–N(5) = 1.211 Å) are relatively longer than the bonds with monodentate N atoms (N(2)–N(3) = 1.161 Å, N(5)–N(6) = 1.129 Å), whereas the N–N bond in μ-1,1-azido ligands containing the donor atoms (N(7)–N(8) = 1.275 Å) are larger than N(8)–N(9) = 1.189 Å. Selected bond lengths and angles are listed in Table S2.† The single copper chain is made of the tetranuclear units combined by EO-azido, syn–syn carboxylate and hydroxyl bridges. The intrachain Cu–Cu separations fall in the range of 3.335–3.507 Å, whereas the Cu–N–Cu angles range from 116.5–122.5°. It is noteworthy that two adjacent chains are connected by μ₃-hydroxyl anions and μ₃-O,O,O′-carboxylates, producing a secondary building unit (SBU) of duplex Cu(II) chains (Fig. 1b); the distance between Cu(II) ions linked by hydroxyl is 3.670 Å, whereas those linked by carboxylate are 3.292 Å apart. The SBUs are further integrated by μ-1,1,3-azido ligands to form the 2D layer framework (Fig. 1c), and the shortest Cu–Cu separation (5.095 Å) between two adjacent SBUs is in the normal range of EE-azido bridging mode.¹⁹ The structural network of compound 1 can be described as well-isolated layers of penta-coordinated Cu(II) ions, leading to magnetic independence for these layers with long Cu–Cu distances of 16.5 Å.

Fig. 1 (a) Linkage of the tetranuclear Cu centers in 1. (b) Elementary unit of duplex Cu(II) chains. Nitrogen atoms are omitted for clarity. (c) The 2D layer extended based on duplex chains unit linked by EE-azido (hydrogen atoms are omitted for clarity).

Magnetic measurements have been carried out on crystalline samples of compound 1, which was pure-phase, as confirmed by PXRD (Fig. S1†). Magnetic susceptibility data of 1 was collected in the temperature range of 2–300 K. The χ_MT versus T curve is shown in Fig. 2. The χ_MT value at 300 K is 2.50 cm³ K mol⁻¹, which is larger than the anticipated value for four isolated Cu(II) ions (ca. 1.5 cm³ K mol⁻¹). Upon cooling, the χ_MT product increases to 64 cm³ K mol⁻¹ at 5 K, signifying ferromagnetic interaction between Cu(II) cations, and finally decreases to 28 cm³ K mol⁻¹ at 2 K. These features clearly suggest an intensely ferromagnetic coupled system accompanied by intermolecular antiferromagnetic interactions coexisting in the compound; note that this only applies at low-temperature range.²⁰ Furthermore, the large χ_MT value of 64 cm³ K mol⁻¹ at 6 K is inappropriate for a tetranuclear Cu(II) compound unless it is ferromagnetically ordered in a 2D network.²¹ The susceptibility follows the Curie–Weiss law at temperatures above 35 K (Fig. 2), with C = 2.314 cm³ K mol⁻¹ and a Weiss constant of θ = 24.37 K. The positive θ value supports the occurrence of strong ferromagnetic coupling between the Cu(II) centers in the compound. Structurally, the 2D layer-like network provides diverse pathways of magnetic transmission. The overall ferromagnetic interaction corresponding to the exchange depends on the orbital counter complementarity effect¹⁶ of the intrachain syn–syn carboxylate and EO-azido bridges, with the Cu–Cu distances and Cu–N–Cu angles in the normal range compared to the azido/carboxylate mixed-bridged Cu(II) compounds reported previously. On the other hand, the antiferromagnetic interaction may be driven by weak interchain magnetic exchange (linked by EE-azido and syn–anti carboxylate oxygen) owing to its nonplanarity. Moreover, the μ₃-hydroxyl anion, which links three Cu atoms as a triangle, could be ferro- or antiferromagnetic. If this pathway is antiferromagnetic, spin frustration could be a significant feature of the entity.²² These pathways will synergistically determine the coupling constant J value. In addition, the antiferromagnetic interactions are very weak if the layers are well-separated.

Fig. 2 χ_MT and 1/χ_M vs. T plots for 1; the red solid line is the best fit to the Curie–Weiss law.

Regarding attempts to fit the χ_MT versus T curve, however, it is unfortunate that the appropriate fitting model was not discovered for the present 2D layer compound assembled from linear tetranuclear Cu(II) units with its promiscuously magnetic transmission channel. Notwithstanding, we have tentatively analyzed the magnetic data for the tetranuclear unit (Fig. S2†) using the MAGPACK³¹ package based on the isotropic Hamiltonian given in eqn (1)

Ĥ = −2J₁Ŝ₁Ŝ₂ − 2J₂Ŝ₂Ŝ₃ − 2J₃Ŝ₃Ŝ₄ (1)

The data (10–300 K) led to the best-fitting parameters: g = 2.21, J₁ = 17.22 cm⁻¹, J₂ = 46.11 cm⁻¹ and J₃ = 24.35 cm⁻¹. The FM nature of the J values driven by the counter complementarity of the multiple super-exchange is consistent with that of known azido-Cu(II) compounds.^12–14

Field dependence of magnetization for 1 measured at 2 K (Fig. 3) rises sharply at low field, and approaches saturation at 50 kOe, characterizing the dominating ferromagnetic interaction. In principle, the sudden increase of the magnetization at low field represents long-rang ferromagnetic ordering or very large J values. As the Brillouin calculation for an S = 1/2 state shows in Fig. 3, the M–H curve goes above the Brillouin model from lower magnetic fields to higher fields, which further demonstrates the ferromagnetic behavior in the system.

Fig. 3 Magnetization vs. H plot at 2 K for 1, the empty rhombus represents the Brillouin function of a system with S = 1/2.

In order to corroborate this hypothesis, the zero-field alternating-current (ac) susceptibility measurements were carried out under the oscillating field of 3.5 Oe in the range of 1.8–10 K and frequencies of 1, 10, 100, and 1000 Hz (Fig. 4). As observed, the AC measurements with frequency-dependence signals appear both in-phase and out-of-phase at 6 K, proving the performance of long-range ordering and slow magnetic relaxation in the system. However, there is no out-of-phase peak until the temperature drops to 2 K (Fig. 4). Under different applied fields of 200 Oe and 500 Oe, AC measurements at 1000 Hz have been executed (Fig. S3†). The obtained curves show no maxima and perform similar trends compared with that of zero applied field, illuminating that there is no possible quantum tunneling effect in the compound. An interesting phenomenon was observed concerning the frequency-dependent χ′′_M, which is reminiscent of that observed in the so-called super-paramagnets, including SMMs and SCMs.²³ However, the frequency dependence in this compound is very weak and the values for the activation energy (E_a) and the relaxation time (τ₀) are physically meaningless. This is commonly found in spin-glasses and is probably due to the occurrence of competitive ferromagnetic and antiferromagnetic interactions yielding a spin-frustrated system with a vitrification process.

Fig. 4 χ′_M and χ′′_M vs. T plots under the 0 Oe applied field for 1.

The DC magnetization of 1 is measured within ±7 kOe at 1.8 K (Fig. S4†); no evident hysteresis is aroused, but there is observable hysteresis in the field range 0–0.45 kOe and the hysteresis loop gives a narrow gap at 1.8 K (Fig. 5). The gap between the increasing and decreasing values is little noticed at low fields, which gain feeble remnant magnetization (M_r) of 0.157 μ_B and coercive field (H_c) of 11 Oe. This is comparable to the reported values (Table S3†).^13c–f Field-cooled (FC) and zero-field-cooled (ZFC) magnetization experiments were also executed. The FC and ZFC curves don't reach a maximum and diverge above 2 K at 10 Oe and 200 Oe (Fig. S5†), which is probably attributed to the lowered Néel temperature (T_N) or tiny coercivity, and is little affected under different fields.

Fig. 5 Hysteresis loop of compound 1 at 1.8 K with an enlargement of the low-field region.

Conclusions

In the present study, a novel azido-Cu(II) compound, [Cu₄(4-tfmba)₄(N₃)₃(OH)]_n (1), has been obtained with 4-trifluoromethyl-benzoic acid (4-Htfmba) as co-ligand under hydrothermal condition. Structural analyses reveal that compound 1 features a rare 2D structural architecture with intricately tetranuclear metal units supplying several types of pathways for magnetic transmission in the compound. Magnetic investigations illustrate that the compound exhibits dominating ferromagnetic interaction and magnetic ordering at low temperature range. More importantly, slow relaxation of the magnetization under zero static field is unambiguously observed in the title compound, which is caused by the strong intrachain ferromagnetic interactions as well as the counter complementarity effect imposed by the carboxylate bridge, which overcomes the antiferromagnetic effect of the EE-azido bridge resulting in an overall ferromagnetic interaction. The results further demonstrate the potential of combining azido and carboxylate ligands in the construction of magnetic coordination polymers. The possible advancement of this work to other metal ions with magnetic anisotropy is in progress, with special interest in the Co(II) or Ni(II)-containing compounds.

Acknowledgements

We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21463020, 21673180 and 21473135), the Nature Science Foundation of Ningxia Province (Grant No. NZ16035), Foundation for Fostering Outstanding Young Teachers of Ningxia Higher Education Institutions of China (Grant No. NGY2016063), and the Science Research Projects of Ningxia Higher Education Institutions of China (Grant No. NGY2015026).

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S5 and Tables S1–S3. CCDC 963677. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra20830d

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