An Mn(II) cluster–based coordination framework derived from a C 3 symmetric ligand: Synthesis,structure,and magnetic properties

Abstract

A new coordination framework {[Mn₇(L)₄(OH)₂(H₂O)₄(DMA)₄]·8H₂O·6DMA}_n (H₃ L = 3,3′,3″-[1,3,5-benzenetriyltris(carbonylimino)]tris(benzoate); DMA = N,N-dimethylacetamide) is synthesized under mixed solvothermal conditions. Structural analysis reveals that this complex possesses a three-dimensional framework with (3,10)-c topology based on different [Mn₆] clusters. Moreover, magnetic investigations suggest that intra-cluster Mn(II) ions exhibit antiferromagnetic coupling.

Introduction

Studies of coordination frameworks, especially of metal-organic frameworks (MOFs) or porous coordination polymers (PCPs), have focused on their structural and chemical diversities as well as their potential structure-related applications, ¹ for instance, gas storage and separation,^2–4 catalysis,^5–7 magnetism,^8,9and chemical sensing.^10–12 Importantly, magnetic coordination frameworks have shown great promise due to their interesting magnetism and significant potential applications in high-density information storage, quantum computing, and magnetic refrigeration.^13–18 Consequently, many strategies have been developed to control the self-assembly process of magnetic coordination frameworks. ¹⁹ To our knowledge, the metal center (or secondary building units, SBUs) plays a significant role during the synthesis of products with targeted structural features and predesigned functions.^20–22 Specifically, the SBUs of metal clusters of different sizes and connectivity can form novel high-connected topological nets. Generally, coordination frameworks with metal clusters as SBUs are helpful for the control of pores in the resulting structures, and various functional coordination frameworks can be obtained. ²³ As a result, it is a powerful synthetic strategy to construct high-connected coordination frameworks in the self-assembly process by using metal cluster entities with various sizes and coordinated sites as SBUs.

In addition, multicarboxylate ligands combined with functional groups are usually selected to synthesize functional complexes with diverse structures and topologies.^24–28 Meanwhile, the multicarboxylate ligands with acid-based systems are significant because of their special nature, having the ability to compensate charge balance, coordination deficiency, repulsive vacuum, and weak interactions all at the same time. ²⁹ Numerous investigations have mainly focused on entirely rigid or flexible multicarboxylate coordination systems.^30–33 As a triangular amide-containing ligand, 3,3′,3′′-[1,3,5-benzenetriyltris(carbonylimino)]tris(benzoate) (H₃ L) possesses more coordination sites (–C=O) and a potential hydrogen-bonding donor group (–NH). These features of H₃ L could enrich its coordination modes and form high-dimensional supramolecular architectures in the self-assembly processes. ³⁴

Taking the above mentioned points into consideration, we selected H₃ L as the primary ligand to construct a metal cluster–based coordination framework, and a new Mn(II) complex {[Mn₇(L)₄(OH)₂(H₂O)₄(DMA)₄]·8H₂O·6DMA}_n (1) based on distinct [Mn₆] clusters was obtained.

Results and discussion

Description of the crystal structure

Single-crystal X-ray analysis indicated that complex 1 crystallizes in the space group Pī (see Table 1). The selected bond lengths and angles are summarized in Table S1 (Supporting Information, SI). The IR spectrum of 1 was shown in Figure S1 (SI). Complex 1 reveals a three-dimensional (3D) structure based on different Mn(II) clusters. In complex 1, there are seven crystallographically independent Mn(II) ions, assuming different coordination geometries. As shown in Figure 1(a), the Mn(1) ion is hexa-coordinated to five oxygen atoms from the carboxylate groups of five unique L³⁻ anions and one oxygen atom from μ₃-OH. The Mn(2) ion also assumes a hexa-coordinated mode and is surrounded by four oxygen atoms from the L³⁻ ligand, one oxygen atom from μ₃-OH and one oxygen atom from one water molecule. Of note, the Mn(3) ion sits in a penta-coordinated environment by coordination to four oxygen atoms from the L³⁻ ligand and one oxygen atom from μ₃-OH. The Mn–O bond distances are in the range of 2.099(4)–2.302(4) Å. Finally, the Mn(1), Mn(2), Mn(3) ions and their symmetry-related Mn(II) ions form a [Mn₆] cluster unit through four carboxylate groups of the L³⁻ ligands, and is named as [Mn₆]-I. In addition, the Mn(4) ion is penta-coordinated by three oxygen atoms from three carboxylate groups, one oxygen atom from μ₃-OH and one oxygen atom from an N,N-dimethylacetamide (DMA) molecule [Mn(4)–O 2.076(5)–2.177(6) Å]. The Mn(5) ion sits in a hexa-coordinated environment supplied by four oxygen atoms from the L³⁻ ligands, one oxygen atom from μ₃-OH and one oxygen atom from a DMA molecule [Mn(5)–O 2.128(5)–2.448(5) Å]. It is worth noting that the coordination environment of Mn(3) and Mn(6) is identical [Mn(6)–O 2.087(5)–2.186(5) Å]. As a result, the Mn(4), Mn(5) and Mn(6) ions and their three symmetry Mn(II) ions are connected by four carboxylate groups to form another [Mn₆] cluster unit, which is named [Mn₆]-II (Figure 1(b)). The Mn(7) ion is coordinated by one oxygen atom from the L³⁻ ligand, three oxygen atoms from three terminal water molecules, and two oxygen atoms of two coordinated DMA molecules, and shows pseudo-octahedral geometry [Mn(7)–O 2.163(6)–2.221(6) Å] (Figure 1(c)).

Table 1.

Crystal data and structure refinement parameters for complex 1.

Complex	1
Formula	C160H184N22O60Mn7
Fw	3759.87
Space group	Pī
a (Å)	18.591(6)
b (Å)	20.664(7)
c (Å)	23.418(7)
a (°)	89.887(13)
β (°)	72.480(7)
γ (°)	88.020(14)
V ( Å3)	8574(5)
ρc(Mg m−3)	1.456
Z	2
Crystal size (mm)	0.27 × 0.26 × 0.23
θ range (°)	1.15–25.00
Reflections collected/unique	58472/28830
Completeness	95.5%
F(000)	3906
μ (mm−1)	0.594
R/wR [I > 2σ(I)] a	0.0728/0.1776
R/wR (all data)	0.0953/0.1900
GOF on F2	1.036

GOF: goodness of fit.

a

R = Σ(||F₀| − |F_C||)/Σ|F₀| wR2 = [Σw(|F₀|² − |F_C|²)²/(Σw|F₀|²)²]^1/2.

Figure 1.

The coordination environment of the Mn(II) in complex 1. Hydrogen atoms and disordered solvent molecules have been omitted for clarity. The pink ball represent Mn atom, the red ball represent O atom. A is symmetry code: -x+1,-y+1,-z+1.

In complex 1, we find that two types of [Mn₆] cluster units are in different connecting environments. To be specific, there are four [Mn₆]-I clusters (gold balls), six [Mn₆]-II clusters (pink balls), and four discrete Mn(7) ions surrounding each [Mn₆]-I cluster via the linkage of 12 L³⁻ ligands (Figure 2(a)). However, each [Mn₆]-II cluster is connected by 10 L³⁻ ligands to six [Mn₆]-I clusters and four [Mn₆]-II clusters (Figure 2(b)). As a result, a complicated 3D framework was obtained through the combination of [Mn₆]-I and [Mn₆]-II clusters by L³⁻ ligands (Figure 3). To provide further insight into the structure of 1, it is necessary to simplify the node and linker reference nets from the topological point of view. Based on the above considerations, each L³⁻ ligand acts as a 3-connected node, meanwhile each [Mn₆] cluster can be considered as a 10-connected node. Thus, the overall 3D network of 1 can be rationalized as a (3,10)-c net with point symbol {4¹⁰·5⁸·6¹⁶·7²·8⁹}{4¹²·5⁶.6¹⁴·7¹⁰·8³}{4²·5}₂{4³}₄ (Figure 4).

Figure 2.

View of the connecting environment of [Mn₆] clusters in complex 1. (a) Type I [Mn₆] cluster (gold balls). (b) Type II [Mn₆] cluster (pink balls).

Figure 3.

The 3D structure of 1.

Figure 4.

The 3,10-c topology structure of 1.

Thermogravimetric analysis

To explore the structure stability, thermogravimetric analysis (TGA) of complex 1 was performed. The results show that complex 1 exists three steps weight losses, which were corresponding to the loss of solvent molecules and organic ligands. As shown in Figure S2 (SI), the TGA curve shows the first stage weight loss of 5.77% in the temperature range of 73–147 °C corresponding to the release of water molecules (calcd. 5.74%). The second loss of 23.43% completed at 370 °C corresponding to the combustion of DMA molecules (calcd. 23.14%). Then, a rapid mass loss began at 390 °C, which corresponded to the decomposition of organic ligands.

Magnetic properties

To confirm the phase purity of complex 1, the powder X-ray diffraction (PXRD) pattern was characterized. As shown in Figure S3 (SI), the experimental PXRD pattern of 1 is in good agreement with its corresponding simulated pattern, indicating the fine phase purity of the sample. Primary magnetic studies were performed on a polycrystalline sample of 1 under an applied field of 1 kOe in the temperature range of 2–300 K, and the results are shown in Figure 5. At 300 K, the experimental χ_MT value (32.69 emu mol⁻¹ K) is a little larger than the theoretical value (30.63 emu mol⁻¹ K) of seven magnetically isolated Mn(II) ions (S = 5/2, g = 2.0). As the temperature lowers, the χ_MT constantly drops to 2.05 emu mol⁻¹ K (the minimum) at 2 K. The best fitting of the molar magnetic susceptibility in the range of 45–300 K using the Curie–Weiss law gives C = 40.39 emu mol⁻¹ K and θ = −72.73 K for complex 1. The negative θ value, along with the shape of χ_MT, suggests strong antiferromagnetic (AF) coupling among Mn(II) ions. ³⁵ In addition, the M versus H curve at 2 K also confirms this conclusion (Figure S4, SI). As M increases slowly, H reaches 5.03 Nβ at 50 kOe, which is much smaller than the theoretical saturation value (35 Nβ). It is noteworthy that the L³⁻ ligand only provides a negligible AF interaction among clusters.

Figure 5.

Temperature dependence of magnetic susceptibilities in the form of χ_mT (Ο) using an applied field of 1 kOe (red solid line for the least-square fitting at 2–300 K) and the χ_m⁻¹ versus T plot (□) for 1 (red solid line for the Curie–Weiss fitting).

Conclusion

Complex 1 shows a new 3D coordination framework with a (3,10)-c topology based on different [Mn₆] clusters. A larger Mn–O–Mn angle and carboxylic groups with syn-syn-μ₂-η¹:η¹ mode caused a strong AF interaction among Mn(II) ions. Experimental data show that AF interactions exist in [Mn₆] clusters, while there is a negligible AF effect among Mn(II) ions. In short, complex 1 presents AF behavior.

Experimental

Materials and measurements

All solvents and reagents were obtained from commercial sources without further purification. H₃ L was synthesized according to the literature. ³⁶ Elemental analyses were performed on a Perkin-Elmer 240C analyzer. Infrared (IR) spectra were measured on a WQF-510A FTIR spectrometer as KBr pellets. PXRD was conducted on a Rigaku Miniflex 600. Magnetic data were collected by using the crystal sample on a Quantum Design MPMS XL7 SQUID magnetometer. TGA was carried out on a Rigaku standard TG-DTA analyzer with a heating rate of 10 °C min⁻¹ from ambient temperature to 800 °C, and an empty Al₂O₃ crucible was used as reference. The data were corrected using Pascal’s constants to calculate the diamagnetic susceptibility, and experimental correction for the sample holder was applied

Preparation of the complex

{[Mn₇(L)₄(OH)₂(H₂O)₄(DMA)₄]·8H₂O·6DMA}_n (1)

A mixture of Mn(CH₃COO)₂·4H₂O (0.0245 g, 0.1 mmol) and H₃ L (0.029 g, 0.05 mmol) in 8 mL of the component solvents (V_DMA:Vdeionized water = 5:3) was sealed in a 25-mL Teflon-lined autoclave and heated at 90 °C for 3 days. After the autoclave had cooled to room temperature at 10 °C h⁻¹, light yellow block crystals were obtained. Yield 17% based on Mn(CH₃COO)₂·4H₂O. Anal. Calcd. for C₁₆₀H₁₈₈N₂₂O₆₀Mn₇: C, 51.06; H, 5.03; N, 8.19(%). Found: C, 51.11; H, 4.95; N, 8.13(%). IR (KBr, cm⁻¹): 3375(w), 3266(w), 3066(w), 2539(w), 1666(m), 1548(m), 1527(s), 1411(s), 1321(s), 1254(m), 1181(s), 957(m), 866(m), 782(m), 721(w).

X-ray crystallography

X-ray single-crystal diffraction data of 1 were collected on a Rigaku SCX-mini diffractometer with graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). The program CrystalClear was used for integration of the diffraction profiles. All structures were solved by direct methods using the SHELXS program SHELXTL package ³⁷ and refined by full-matrix least-squares methods with SHELXL.^38,39 Metal atoms were located from the E-maps, and other non-hydrogen atoms excluded in counterions were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F². The H atoms were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. Further details of the crystal data and the structure refinement for 1 are summarized in Table 1.

Supplemental Material

Revised_Supporting_Information_CHL-20-0050_1 – Supplemental material for An Mn(II) cluster–based coordination framework derived from a C3 symmetric ligand: Synthesis, structure, and magnetic properties

Supplemental material, Revised_Supporting_Information_CHL-20-0050_1 for An Mn(II) cluster–based coordination framework derived from a C3 symmetric ligand: Synthesis, structure, and magnetic properties by Yong-Jun Bian, Yuan Tian, Ai-Hua Zhang and Yong-Qiang Chen in Journal of Chemical Research