Temperature-induced single-crystal-to-single-crystal transformation of a binuclear Mn(II) complex into a 1D chain polymer

Abstract

Dehydration of complex [Mn₂(4-Apha)₄(H₂O)₂]·2H₂O (1, 4-AphaH = 4-aminophenylhydroxamic acid) upon heating at 393 K led to the removal of coordinated water molecules to give anhydrous [Mn(4-Apha)₂] (2). The crystal structure transformation has been characterized by variable temperature PXRD and TGA-DSC analysis. Single crystal X-ray diffraction analysis reveals that complex 1 exhibits a binuclear structure and complex 2 shows a 1D chain structure. Both of them show antiferromagnetic exchange interactions between the metal centers.

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Introduction

Recently, single crystal to single crystal transformations (SCSC) have received considerable attention in solid state reactions and crystal engineering,¹ due to their potential applications in molecular switches and sensors, as well as their inherent structural interest.^2–4 The SCSC transformation can be induced by external stimuli such as temperature,^5,6 solvent,^7–11 light,¹² concentration,¹³ and anions.¹⁴ The crystal structure transformation provides a unique source for synthesizing products that are otherwise not possible through normal solution chemistry.¹⁵ Coordination complexes have been proved to be an important class of compounds that exhibit single crystal to single crystal transformations. This may result in changes in the coordination number and geometry of the coordination complexes or even dimensionality.¹⁶

To design coordination polymers with interesting structures and excellent performance, the choice of metal centers with suitable ligands and the synergistic effects of the ligands are essential.¹⁷ Hydroxamic acids (HAc) and theirs derivatives, with multiple coordination atoms and powerful metal-chelating ability, have been employed to construct a diverse host of fascinating coordination compounds including classical homometallic di- and trinuclear complexes,^18,19 high-nuclear clusters,^20,21 and transition- and lanthanide-transition- metallacrowns.^22–24 Water molecules, as a common solvent used in the synthesis of coordination complexes, play an important role in the construction of coordination complexes. They can act not only as ligands bonded to metal centers but also as lattice molecules intercalated into the hole of frameworks via hydrogen-bonding interactions.^17,25

Two new complexes, namely [Mn₂(4-Apha)₄(H₂O)₂]·2H₂O (1) and [Mn(4-Apha)₂] (2) (4-AphaH = 4-aminophenylhydroxamic acid), were successfully synthesized in this paper. Interestingly, complex 1 can transform into complex 2 with the increase of temperature. This article mainly focuses on the roles of water molecules, no matter whether they are coordinated or solvated. The single-crystal-to-single-crystal transformation via thermal dissociation of coordinated water molecules and removal of solvated water molecules was studied. And also, the properties of two complexes were investigated. Both of them show antiferromagnetic exchange interactions between the metal centers.

Experimental section

Materials and physical measurements

The ligand 4-APhaH was prepared by the literature method.²⁶ The reagents and solvents were purchased from commercial suppliers and used without further purification. The C, H and N microanalyses were carried out with a Carlo-Erba EA1110 CHNO-S elemental analyzer. FT-IR spectra were recorded from KBr pellets in the range of 4000–400 cm⁻¹ on a Nicolet MagNa-IR 500 spectrometer. Powder X-ray diffraction (PXRD) was recorded on a Rigaku D/Max-2500 diffractometer at 40 kV and 100 Ma with a Cu-target tube and a graphite monochromator. Thermal gravimetric analysis (TGA) was conducted on a SDT Q600 instrument in flowing N₂ with a heating rate of 5 °C min. Magnetic susceptibility measurements were performed in the temperature range 2–300 K, using a Quantum Design MPMS XL-7 SQUID magnetometer.

Preparation of complexes 1 and 2

[Mn₂(4-Apha)₄(H₂O)₂]·2H₂O (1). A mixture of 4-AphaH (0.0150 g, 0.1 mmol), Mn(CH₃COO)₂·4H₂O (0.0248 g, 0.1 mmol) and H₂O/ethanol (v/v = 1 : 1, 1 mL) was sealed in a 6 mL Pyrex-tube. The tube was heated at 50 °C for 3 days under autogenous pressure. Slow cooling of the resultant solution to room temperature gave brown rod crystals of complex 1. The yield of 1 is 0.0230 g (58%, based on Mn). Anal. calc. for C₂₈H₃₆Mn₂N₈O₁₂ (1, brown rod crystals): C, 42.76; H, 4.61; N, 14.25%. Found: C, 42.71; H, 4.50; N, 14.48%. Selected IR data (KBr, cm⁻¹): 3376(w), 3214(s), 1601(s), 1548(s), 1492(s), 1426(s), 1345(s), 1300(s), 1185(s), 1147(s), 1024(s), 903(s), 842(s), 746(s), 689(s), 625(s), 522(s).

[Mn(4-Apha)₂] (2). A mixture of 4-AphaH (0.0149 g, 0.1 mmol), Mn(CH₃COO)₂·4H₂O (0.0245 g, 0.1 mmol) and H₂O/ethanol (v/v = 1 : 1, 1 mL) was sealed in a 6 mL Pyrex-tube. The tube was heated at 120 °C for 2 hours under autogenous pressure. Slow cooling of the resultant solution to room temperature gave brown needle crystals of complex 2. The yield of 2 is 0.0189 g (53%, based on Mn). Complex 2 can also be prepared by directly heating the complex 1 to 120 °C in a mixed solvent of water and ethanol (H₂O/ethanol, v/v = 1 : 1). Anal. calc. for C₁₄H₁₄MnN₄O₄ (1, brown needle crystals): C, 47.07; H, 3.95; N, 15.68%. Found: C, 46.97; H, 3.73; N, 15.93%. Selected IR data (KBr, cm⁻¹): 3417(w), 1601(s), 1412(s), 1325(s), 1075(s), 923(s), 776(s), 691(s), 524(s).

X-ray crystallography

Data were collected at room temperature on Bruker Smart Apex II diffractometer equipped with a graphite monochromator utilising Mo Kα radiation (λ = 0.71073 Å); the ω and φ scan technique was applied. The structures were solved by direct methods using SHELXS-97 ²⁷ and refined on F² using full-matrix least-squares with SHELXL-97.²⁸ Hydrogen atoms of water molecules were localised from difference Fourier map and the positions of the rest of H atoms were located in geometrically calculated positions. Crystallographic data together with refinement details for 1 and 2 were summarised in Table 1. Selected bond lengths and angles were summarised in Table S1.† See CIF files for further details.

Table 1 Crystal data and refinement factors for 1 and 2

	1	2
Formula	C28H36Mn2N8O12	C14H14MnN4O4
Formula weight	786.53	357.23
Temperature/K	296(2)	296(2)
Crystal system	Monoclinic	Orthorhombic
Space group	P21/n	Iba2
a/Å	10.5089(11)	12.6392(14)
b/Å	7.7759(8)	17.209(3)
c/Å	20.040(2)	6.7442(8)
α/°	90.00	90.00
β/°	93.658(2)	90.00
γ/°	90.00	90.00
Volume/Å^3	1634.2(3)	1466.9(3)
Z	2	4
ρ mg mm^−3	1.598	1.618
μ/mm^−1	0.848	0.927
F(000)	812	732
2θ range/°	2.04–27.62	2.00–28.47
Measd/independent	9471/3740	4767/1776
Rint reflections	0.0172	0.0295
R1, wR2 [I > 2sigma(I)]	0.0295/0.0771	0.0268/0.0608
R1, wR2 (all data)	0.0356/0.0805	0.0340/0.0640
GOF on F^2	1.054	0.996
Largest diff. Peak and hole (e.A^−3)	0.557 and −0.429	0.218 and −0.150

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Results and discussion

Synthesis

Complexes 1 and 2 were synthesized by hydrothermal reactions. Temperature plays a vital role in the crystal growth process. When the mixture (4-AphaH, Mn(CH₃COO)₂·4H₂O and H₂O/ethanol,v/v = 1 : 1, 1 mL) is heated at 50 °C for three days, brown rod crystals of 1 were obtained. The color of the resultant solution is light brown. However, when the mixture is heated at 120 °C for 2 hours, brown needle crystals of 2 were obtained. The color of the resultant solution is dark brown. Interestingly, complex 1 can transform into complex 2 either in solution or in solid form. We have tried to heat complex 1 in a mixed-solvent of H₂O/ethanol (v/v = 1 : 1, 1 mL) at 120 °C for 2 hours, and single crystals of complex 2 were obtained. The single crystals were characterized by X-ray diffraction. The results showed that complex 1 had been transformed into complex 2 (Scheme 1). To study the transform condition, the single crystals of 1 were separated and heated at 120 °C for 2 hours, and then power X-ray diffraction was used to analysis the resultant solid. The results were in good agreement with complex 2.

Scheme 1 Synthesis of complexes 1 and 2.

Description of structures

Structure of [Mn₂(4-Apha)₄(H₂O)₂]·2H₂O (1). The crystal structure determination reveals that complex 1 displays a binuclear structure, and crystallizes in monoclinic space group P2₁/n. It consists of two Mn^II ions, four 4-Apha⁻ anions, two coordinated water molecules and six guest water molecules. In the crystal structure, 4-Apha⁻ ligand exhibits μ₁–η¹:η¹ and μ₂–η²:η¹ coordination modes (Scheme 2). As shown in Fig. 1a, the Mn^II ion is coordinated with two 4-Apha⁻ ligands (one exhibits μ₁–η¹:η¹ coordination mode and the other displays μ₂–η²:η¹ coordination mode) through four O atoms, and one O atom (μ₂–O4A) of neighboring 4-Apha⁻ ligand and one water molecule. The distance of Mn–O ranges from 2.1360(12) to 2.2220(12) Å, which are in good agreement with the references.^23,29 The coordination geometry of Mn^II atom can be best described as a distorted octahedron. The Mn^II ions connect to each other through O4 and O4A atoms to form a binuclear structure. The distance of Mn^II⋯Mn^II is 3.3181(4) Å. The angle of Mn1–O4–Mn1A is 97.45(5)°.

Scheme 2 Coordination modes of 4-Apha⁻ anion in 1 and 2.

Fig. 1 (a) The ORTEP structure of 1 (with 50% probability). Symmetry code: A: −x + 1,−y + 1,−z. (b) The hydrogen bonds packing diagram of 1 viewed along b-axis. Other hydrogen atoms were omitted for clarity. Color codes: violet, Mn; red, O; blue, N; gray 80%, C; light green, H.

In addition, complex 1 features intermolecular hydrogen bonds (N–H⋯O) between NH₂ groups of 4-Apha⁻ as hydrogen donors and O atoms of coordinated or guest water molecules as hydrogen acceptors (N2–H2A⋯O5, N2–H2B⋯O6 and N4–H4B⋯O6). Intermolecular hydrogen bonds (N–H⋯O) also existed between NH groups of 4-Apha⁻ as hydrogen donor and NH₂ groups of 4-Apha⁻ as hydrogen acceptors (N1–H1⋯N2). In addition to N–H⋯O and N–H⋯N hydrogen bonds, intermolecular hydrogen bonds O–H⋯O were existed between coordinated water or guest water molecules as hydrogen donors and O atoms of HN–O(1-) in 4-Apha⁻ ligands as hydrogen acceptors. These hydrogen bond interactions connect the binuclear structure to generate a 3-D supramolecular network structure (Fig. 1b). All hydrogen bonds interactions were shown in Fig. 1b and hydrogen bonds details were listed in Table S2.†

Fig. 2 (a) The ORTEP structure of 2 (with 50% probability). Symmetry codes: A: −x,−y + 1,z; B: −x + 0,y + 0,z−1/2; C: x + 0,−y + 1,z−1/2. (b) The hydrogen bonds packing diagram of 2 viewed along c-axis. Other hydrogen atoms were omitted for clarity. Color codes: violet, Mn; red, O; blue, N; gray 80%, C; light green, H.

Structure of [Mn(4-Apha)₂] (2). The crystal structure analysis reveals that complex 2 crystallizes in the orthorhombic crystal system of the Iba2 space group. The crystal structure of 2 (Fig. 2a) consists of one Mn^II ion and two singly deprotonated 4-Apha⁻ ligands. Each Mn^II ion is six coordinated by six O atoms (in coordination mode μ₂–η²:η¹) of four 4-Apha⁻ ligands to form a one-dimensional linear chain. The geometry of Mn^II can be described as a slightly distorted octahedron, which is similar with that of complex 1. The distance of Mn–O ranges from 2.1251(13) to 2.2518(17) Å. In addition, complex 2 features intermolecular hydrogen bonds interactions between NH₂ groups as hydrogen donors and O or N atoms of HN–O(1-) groups in Apha⁻ ligands as hydrogen acceptors. These hydrogen bonds interactions connect the 1D chain structure to give a 3D supramolecular network structure (Fig. 2b). Hydrogen bonds details were listed in Table S3.†

Single crystal to single crystal transformation studies

Heating complex 1 continuously in the solution at 120 °C for 2 hours, the single crystals of complex 1 can be transformed into the single crystals of complex 2. How do these structural transformations occur? We assume that the structural transition can be divided into two-step. As shown in Fig. 3, firstly, guest water molecules and coordinated water molecules can be lost with the increase of temperature. At this time, the coordination number of Mn^II ions will be changed from six to five. Secondly, the Mn^II ion coordinates with the neighboring O atoms of NH–O(1-) groups of 4-Apha⁻ ligands, to form a stable six coordinated fashion. Here, the coordination modes of 4-Apha⁻ ligands changed from μ₁–η¹:η¹ to μ₂–η²:η¹. With the change of the coordination modes, complex 2 was formed. Heating complex 1 in solid form at 120 °C for 2 hours, complex 1 shows a thermally color-changing behavior with the color of 1 shifting from brown to dark brown (Scheme 1). Notably, this crystal transformation can be further confirmed by in situ PXRD patterns shown in Fig. 4, and associated with a visible color change from red to green, with the increase of temperature.

Fig. 3 Structure transformation diagram.

Fig. 4 Variable temperature PXRD patterns of 1.

PXRD and TGA results

In order to check the phase purity of the complexes, the powder X-ray diffractions (PXRD) of 1 and 2 have been performed and compared with those simulated from the single-crystal structures. From Fig. S1,† it can be seen that, the measured powder X-ray diffraction (PXRD) pattern is in a good agreement with the simulated XRPD pattern from single crystal structure, indicating the phase purity of the samples (Fig. S1 and S2†).

The TGA-DSC analysis was performed under N₂ atmosphere in the temperature range of 25 to 700 °C. The results reveal that complex 1 possesses two obvious steps of weight loss (Fig. S3†). The first weight loss occurs in the range of 59 to 105 °C, with the weight loss of 9.9%, which corresponds to the loss of all coordinated and guest water molecules (calcd 9.2%). In this process, the exothermic peak on the DSC curve corresponds to the loss of weight and variation of crystal form. Above 161 °C, the frameworks begin to collapse, decompose to complicated oxides. This process corresponds to a sharp endothermic peak on the DSC curve at 182 °C, followed by a slow heat release. From the TGA-DSC curve of complex 2, it can be seen that complex 2 is stable blow 183 °C. Above 183 °C, the frameworks begin to collapse and decompose to complicated oxides (Fig. S4†), corresponds to the endothermic peak on the DSC curve at 218 °C, followed by a slow heat release.

Magnetic properties

Variable-temperature dc magnetic susceptibility data were recorded for polycrystalline samples of 1 and 2 at an applied magnetic field of 1000 Oe in the temperature range of 2–300 K.

The χ_MT value of 1 at 300 K is 8.75 cm³ K mol⁻¹ (Fig. 5). The χ_MT decreases gradually from 8.75 cm³ K mol⁻¹ to 8.38 cm³ K mol⁻¹ from 300 to 70 K and then at lower temperature decreases more steeply to reach 0.95 cm³ K mol⁻¹ at 2 K. Such behavior is characteristic of antiferromagnetic couplings between Mn^II centers. For antiferromagnetic systems, a simple Heisenberg Hamiltonian H = −JS₁ S₂ could be used to fit the experimental data. Assuming that two Mn(II) ions have the same isotropic g value, from the Van Vleck formula, the χ_MT expression for two S = 5/2 ions is in eqn (1):³⁰

Fig. 5 Temperature dependence of magnetic susceptibilities in the form of χ_MT vs. T for 1 at 1 kOe. Inset: temperature dependence of magnetic susceptibilities in the form of χ_M⁻¹ vs. T for 1 at 1 kOe. The solid line corresponds to the best fit from 300 K to 2 K.

The best fits were obtained with J = −0.60 cm⁻¹ and g = 2.02. The negative value of J is in full agreement with the hypothesis that the exchange interaction in Mn₂O₂ is antiferromagnetic. The reciprocal magnetic susceptibilities in 2–300 K follow the Curie–Weiss law of 1/χ_M = (T – θ)/C with Curie constant C = 8.89 cm³ K mol⁻¹ and Weiss constant θ = −5.49 K, which confirms the existence of the weak antiferromagnetic Mn^II⋯Mn^II interactions.

The value of χ_MT for 2 is 4.05 cm³ K mol⁻¹ at 300 K (Fig. 6). As temperature lowered, the χ_MT value first decreases smoothly and then decreases abruptly to the minimum value of 0.19 cm³ K mol⁻¹ at 2 K. Fitting the data at 10–300 K with the Curie–Weiss law gives C = 4.41 cm³ K mol⁻¹ and Weiss constant θ = −20.75 K. The negative value of θ indicates the weak antiferromagnetic couplings between Mn^II ions.

Fig. 6 Temperature dependence of magnetic susceptibilities in the form of χ_MT vs. T and χ_M vs. T for 2 at 1 kOe. Inset: temperature dependence of magnetic susceptibilities in the form of χ_M⁻¹ vs. T for 2 at 1 kOe. The solid line corresponds to the best fit from 300 K to 10 K.

Both of them show antiferromagnetic exchange interactions between the metal centers, which may be because they have a similar coupling effect. The Mn^II centers in complex 1 and 2 are six coordinated by oxygen atoms and connected to neighboring Mn^II ions by two bridging oxygen atoms. The coordination water molecules in complex 1 have no effect on the coupling between the Mn^II metal centers. The coupling effect between Mn^II ions is related to the bridging oxygen atoms. The distances of Mn⋯Mn in complexes 1 and 2 are very close, 3.32 Å and 3.37 Å, respectively, so their magnetic properties are similar, the coupling constant is different.

Conclusions

In summary, after heating for 2 hours at 120 °C, dihydrate complex [Mn₂(4-Apha)₄(H₂O)₂]·2H₂O (1, 4-AphaH = 4-aminophenylhydroxamic acid) can transform into anhydrous [Mn(4-Apha)₂] (2) in a single-crystal-to-single-crystal transformation. This structure transformation is temperature induced, and the coordination water molecules play a key role in the structure transformation. The crystal structure transformation is accompanied by the loss of coordination water molecules, which can be seen from the TGA-DSC diagram. The crystal transformation can also be confirmed by crystal color change and the temperature dependence PXRD of 1. Although the structure has been transformed, they have similar magnetic properties. Both of them show antiferromagnetic exchange interactions between the metal centers, which may be because they have a similar coupling effect.

Acknowledgements

This research was supported by the Natural Science Foundation of China (grant No. 21501061) and Doctoral Fund Project of Huanggang Normal University (grant No. 2015001803).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1482263 and 1482264 for complexes 1 and 2, respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra14364d

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