Temperature identification on two 3D Mn(II) metal–organic frameworks: syntheses, adsorption and magnetism

Abstract

Two new 3-D NaCl-type frameworks of [AmineH⁺][Mn(HCOO)₃] (AmineH⁺ = N(CH₃)₄⁺ for 1 and AmineH⁺ = NH₄⁺ for 2) have been synthesized at different temperatures. The N(CH₃)₄⁺ cation was generated in situ by the decomposition of a large number of DMF molecules. The potential porosity of the coordination framework of 1 has been estimated using a computational method based on Connolly's algorithm, indicating that compound 1 presents a kinetic radius greater than 1.61 Å. Because unremovable guest cations are clogged in the channels, 1 presents no significant adsorption for CO₂ gas upon desolvation by long-duration thermal activation. In addition, the magnetic behavior of the two compounds was explored.

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Introduction

The design and synthesis of porous metal–organic frameworks (MOFs) are now of great interest due to the significance of discovering new topology and molecular magnetic materials.^1–3 In this field, a significant body of work on the construction of coordination polymers based on the formate ligand has already been reported. A good example is the MOFs with a general formula cation@M(HCOO)₃ (cation = alkylammonium), where M = divalent metal ion. Those MOFs undergo a ferroelectric (or antiferroelectric) phase transition in the temperature range of 160–185 K, depending on the M(II) involved. They also exhibit a canted antiferromagnetic ordering at low temperatures (T_c = 8–36 K).^4–10 Thus, this cation@M(HCOO)₃ family represents a new class of multiferroics. However, all the alkylammonium cations are deliberately introduced and taken as templates in the construction of materials. Sometimes these effects may restrain the building blocks to form the desired assemblies.¹¹

Very recently, it has been found that the anionic formates and dimethylammonium cations are the hydrolysis products of DMF, which is used as one of the co-solvents. Dimethylformamide is not stable in the presence of strong bases or strong acids and is easily hydrolyzed affording formic acid and dimethylamine, particularly at elevated temperatures.^12–15 Moreover, Su et al. pointed out that dimethylamines were often found as a by-product of reactions realized in DMF solvent that contain a small amount of water.^12a To investigate how the presence of alkylammonium influences the course of the reaction, we pre-defined the temperature effect, the ratio of coligand, and the organic solvents to explore the system. Herein, we report two new 3-D NaCl-type frameworks of [AmineH⁺][Mn(HCOO)₃] (AmineH⁺ = N(CH₃)₄⁺ for 1 and AmineH⁺ = NH₄⁺ for 2). Our study shows that the change in temperature could influence the subtle variables that lead to the resulting structures; particularly, a temperature-driven C–N bond cleavage of DMF molecules can control the assembly of the resulting metal–organic coordination polymers. In addition, the magnetism and absorption behaviors are explored.

Experimental

Materials and method

All reagents were purchased from commercial sources and used as received. IR spectra were recorded with a Perkin-Elmer Spectrum One spectrometer in the region 4000–400 cm⁻¹ using KBr pellets. TGA were carried out with a Metter-Toledo TA 50 under dry dinitrogen flux (60 mL min⁻¹) at a heating rate of 5 °C min⁻¹. X-ray powder diffraction (XRPD) data were recorded on a Rigaku RU200 diffractometer at 60 kV, 300 mA for Cu Kα radiation (λ = 1.5406 Å), with a scan speed of 2 °C min⁻¹ and a step size of 0.013° in 2θ. Magnetic susceptibility data of the powder sample restrained in parafilm were measured on an Oxford Maglab 2000 magnetic measurement system in the temperature range 300–1.8 K and at a field of 1 kOe. The gas sorption isotherm was measured with ASAP 2020 M adsorption equipment.

X-ray crystallography

Single crystal X-ray diffraction analyses of the two compounds were carried out on a Bruker SMART APEX II CCD diffractometer equipped with a graphite monochromated MoKα radiation (λ = 0.71073 Å) by using the ϕ/ω scan technique at room temperature. The intensities were corrected for Lorentz and polarization effects, as well as for empirical absorption based on multi-scan techniques; all structures were solved by direct methods and refined by full-matrix least-squares fitting on F² by SHELX-97.¹⁶ Absorption corrections were applied by using the multi-scan program SADABS.¹⁷ The hydrogen atoms of the organic ligands were placed in the calculated positions and refined using a riding model on the attached atoms with isotropic thermal parameters 1.2 times those of their carrier atoms. Ammonium H atoms were located in a difference Fourier map, and their positions and isotropic displacement parameters were refined. Table 1 shows the crystallographic data of 1–2. Selected bond distances and bond angles are listed in Table 2.

Table 1 The crystallographic data of 1–2

Complex	1	2
Empirical formula	C7H15MnNO6	C3H7MnNO6
Formula mass	264.14	208.04
Crystal system	Orthorhombic	cubic
Space group	Pnma	Im3̄
a [Å]	8.9042(11)	12.331
b [Å]	12.7757(16)	12.331
c [Å]	9.2004(11)	12.331
α [°]	90	90
β [°]	90	90
γ [°]	90	90
V [Å^3]	1046.61(11)	1874.93(11)
Z	4	8
dcalcd [g cm^−3]	1.676	1.474
M [mm^−1]	1.271	1.397
F(000)	548	840
Reflections collected	5963	1659
R(int)	0.0193	0.0105
R1, wR2 [I > 2σ(I)]	0.0217, 0.0655	0.0556, 0.1743
R1, wR2 (all data)	0.0234, 0.0668	0.0606, 0.2017

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Table 2 Selected bond distances (Å) and angles (°) in 1–2

1
Mn1–O1#1	2.2178(8)	Mn1–O2#1	2.1992(7)
Mn1–O3#1	2.2074(7)	O1–Mn1–O1	180
O1–Mn1–O2	90.15(3)	O2–Mn1–O2	180
O2–Mn1–O3	91.91(3)	O3–Mn1–O3	180
Symmetric code: (i) −x + 1, −y, −z + 1

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2
Mn1–O1#1	2.190(3)	O1–Mn1–O1	180
Symmetric code: (i) −x + 1/2, −y + 1/2, −z + 1/2

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Synthesis of these complexes

[N(CH₃)₄][Mn(HCOO)₃] (1). A mixture of Mn(OAc)·4H₂O (0.5 mmol), 4-iodobenzoic acid (0.75 mmol), triethylamine (0.04 mL), DMF (10 mL), and CH₃OH (10 mL) was stirred for 30 min in air. The resulting solution was kept at 160 °C for 72 h in an oven then cooled to 25 °C. The resulting crystals were filtered off, washed with water and dried in air. C₇H₁₅MnNO₆ (M_w = 264.14 g mol⁻¹). Calcd: C, 31.83%; H, 5.72%; N, 5.30%. Found C, 31.61%; H, 5.64%; N, 5.33%. IR (KBr, cm⁻¹): 3020(m); 2843(m); 1604(vs); 1480(m); 1395(vs); 1355(s); 950(s); 759(m).

[NH₄][Mn(HCOO)₃] (2). A mixture of Mn(OAc)·4H₂O (1.5 mmol), NH₃·H₂O (0.04 mL), DMF (10 mL), CH₃OH (10 mL) was stirred for 30 min in air. The resulting solution was kept at 140 °C for 72 h in an oven, and then cooled down to 25 °C. The resulting crystals were filtered off, washed with water and dried in air. C₃H₇MnNO₆ (M_w = 208.04 g mol⁻¹). Calcd: C, 17.32%; H, 3.39%; N, 6.73%. Found C, 17.02%; H, 3.22%; N, 6.81%. IR (KBr, cm⁻¹): 3014(m); 2830(m); 1602(m); 1389(s); 951(m); 766(m).

Results and discussion

Synthesis of 1 and 2

In a previous report,^12a it has been proven that the DMF molecule is easily decomposed and that this decomposition is significantly related to the reaction temperature. In the present reaction system of 1, the anionic formates and tetramethylammonium cations are of the hydrolysis products of the DMF used as one of the co-solvents. We propose the possible mechanism for this reactive system (see Scheme 1). From that mechanism, CH₃OH may play an important role in generating the tetramethylammonium cation.

Scheme 1 Possible mechanism of DMF decomposition in this work.

In order to investigate the effect of the molar ratio of Mn(OAc)₂ to 4-iodobenzoic acid in the formation of 1, the reaction mixtures in four different molar ratios, 1 : 1, 1.5 : 1, 1 : 1.5, and 2 : 1 (metal-to-ligand), were heated in pure DMF at three different temperatures (120, 140, and 160 °C) for three days. The title complex 1 was obtained under the molar ratios of 1 : 1 and 1.5 : 1 at 160 °C. 1 cannot be prepared under other conditions, and only colorless powders were obtained. They are all characterized by XRPD (Fig. S1†). It should be pointed out that triethylamine has no influence on the resulting product. We can get the same product if the triethylamine is absent from the reaction system.

Keeping similar conditions except the use of DMSO, THF, and CH₃CN as solvents in the place of DMF or CH₃OH, the title compound cannot be obtained. From Scheme 1, CH₃OH is used as a medium and has a critical role in the formation of the N(CH₃)₄⁺ cation. Thus, CH₃OH is useful and is a co-solvent in the syntheses of the title compounds. These results indicate that the use of THF or CH₃CN as a solvent is not an appropriate choice for the synthesis of 1. Therefore, under similar reaction conditions, solvents such as ethanol, acetonitrile, and DMSO are not easily decomposed.

In order to investigate the role of 4-iodobenzoic acid in the decomposition process of DMF, manganese acetate was dissolved in a mixture of DMF (10 mL), CH₃OH (10 mL), and triethylamine (0.04 mL) and heated at 160 °C resulting in the formation of solid material 1. This result indicates that the ligand 4-iodobenzoic acid does not play an important role in the decomposition of DMF at the elevated temperatures and pressures.

Keeping all other conditions unchanged, using 140 °C as the temperature and introducing NH₃·H₂O into the system, a new compound 2 was obtained. NH₄⁺ was generated from the reactive process. Herein, we explored the M(II)–HCOOH system and constructed some new products, as concluded in Chart 1.

Chart 1 Assembly of compounds controlled by the temperature and solvents.

The IR spectra of the two compounds are simple and similar to each other, including the characteristic IR bands for HCOO– groups. All of the bands are coincident with those previously reported.¹⁸ The frequencies of the symmetric and antisymmetric carboxylate modes of the formate ion in the two compounds are not affected by the changes of the frameworks for the different cations. Therefore, it indicates that these local modes depend solely on the local geometry of the OCO unit and not by its surroundings.

Structural description of 1 and 2

Complexes 1 and 2 have similar frameworks; the minor difference is the disparate cations (the alkylammonium cation for 1 and the ammonium cation for 2) that are occupied at the channels of the compounds. Thus, the main structure of 1 will be described in detail herein (Fig. S2†). The XPS spectrum results (638.5 eV Mn(II) character) indicate that Mn(II) could be present in the coordination polymer of 1 (Fig. S3†). The structural feature for 1 is an anionic NaCl-framework of [Mn^II(HCOO)₃], which is very similar to a reported MOF.¹² The structure is similar to MnO, Prussian Blue, AM^II(dca)₃, and the recently reported Mn (formate)₃,¹² but it is very different from the common non-zeotype topological nets (such as sql, kag, nbo, lvt, cds, qtz, dia, lon, and pts).^1e For the two compounds, the range of the size of the charge-balancing cation is mainly due to the apparent breathing nature of the framework brought by the change of the Mn–O–C angles, increasing from averaged values of 135.5° in 1 and 128.3° in 2, and accompanied by a considerably smaller change in the O–C–O angles. As a result, the volume per two [AmineH][Mn(HCOO)₃] (the unit cell for 1 and 2 at room temperature) decreases with the increasing size of the cation, viz.: 1046.6(2) ų for 1 ((CH₃)₄N⁺) and 1874.93(11) ų for 2 (NH₄⁺), with a volume change up to 79%. This phenomenon is very different from the reported examples.¹² The cavity volumes, calculated by PLATON, is about 43.5% for 1 and 38.9% for 2. The range of Mn⋯Mn distances is 6.2–6.4 Å, spanned by the HCOO bridge (Fig. 1).

Fig. 1 View of 3D framework showing different channels in 1 and 2.

Each Mn(II) ion, located at an inversion center, is connected to its six nearest neighbors by six bridging formate ligands in an anti–anti mode (Fig. 2); thus, the Mn(II) ion has an ideal octahedral coordination geometry with Mn–O distances in the range of 2.1987(8)–2.2071(8) Å. From a topological point of view, this 3D framework can be considered as a classical NaCl-type network (Fig. 3). The protonated (CH₃)₄N⁺/NH₄⁺ cations are located in the cavities of the NaCl-type frameworks. Their N/C–H groups form both convenient N–H⋯O and weak hydrogen C–H⋯O bonds to the oxygen atoms of the frameworks. Similar molecular geometries of the framework and the charge counter-balanced cations are shown in Fig. 4.

Fig. 2 Framework perspective of 1 showing the 3D connection.

Fig. 3 Framework perspective of 1 showing NaCl-type topology.

Fig. 4 View of different motifs containing diverse cation in this work and documented report.

The structure of 1 is unchanged, except for a slight contraction of approximately 0.52%, when the temperature is lowered from 375 to 100 K. In each case, there is only one crystallographically independent Mn site (see ESI† for cif).

Thermogravimetric and IR analyses

The IR spectra of the two polymers are similar, with characteristic IR bands for protonated amine cations and HCOO– groups, these bands being coincident with those previously reported. As shown in Fig. S4,† the frequencies of the symmetric and antisymmetric carboxylate vibration modes of the formate ion in the present two compounds are not affected by the changes of the framework for the different cations.

The TG curves of 1 and 2 showed a weight loss of 20.9% and 7.9% from 35 to 195 °C corresponding to the release of the cation guest molecules (calculated 28.1% and 8.6%), respectively (Fig. S5†). The removal of the cation guest molecules leads to the protonation of the framework, thus becoming unstable. Consequently, another abrupt weight loss is followed by the departure of formate ligands up to 370 °C.

Gas sorption property

From Fig. 5a, it is strange that the CO₂ sorption amount rises gradually from P/P₀ = 0 to 1.0. This is not a typical Type I isotherm behavior for the microporous materials but similar to Type II or Type III isotherms as defined by the IUPAC classification. Usually, this strange sorption feature can be attributed to two reasons: first, 1 is an anionic framework and those channels may be blocked by the large, unremovable N(CH₃)₄⁺ cations; second, the interaction between CO₂ molecules and the anionic framework is very weak.

Fig. 5 (a) Gas sorption isotherms at 195 K in 1 (left) and (b) porosity profile. The yellow sphere symbolizes the biggest guest molecule that can be hosted (1.61 Å) (right).

The computational method, based on Connolly's algorithm has already been described and successfully used elsewhere.¹⁹ It allows the porosity profile of a material to be designed on the basis of its crystal structure.¹⁹ This compound crystallizes in the orthorhombic system and presents considerably large square cross section channels that spread along the b, a + c, and a − c directions. As can be seen in Fig. 5b, these channels cannot host guest molecules that present a kinetic radius greater than 1.61 Å. The kinetic radius of CO₂ is a slightly larger (1.65 Å), and that could explain why CO₂ adsorption is so difficult to realize. The potential porosities have been calculated for He and H₂ (kinetic radii of 1.30 Å and 1.45 Å, respectively)^19c to be 3200 m² g⁻¹ and 3190 m² g⁻¹, respectively.¹⁹ Further studies are needed to completely understand the sorption behavior of the anionic framework.

Magnetic properties

The study of the magnetic properties reveals that 1 and 2 present antiferromagnetic interactions (Fig. 6). The χ_mT value for 1 was 3.03 cm³ K mol⁻¹ at 300 K, and this is smaller than the spin-only value (4.38 cm³ K mol⁻¹) for the S = 5/2 state, which indicates the presence of dominant ferromagnetic exchange interactions within 1 and suggests a very large ground-state spin (S) value.²⁰ When decreasing the temperature, χ_mT decreases gradually and reaches a minimum (1.49 cm³ K mol⁻¹) at 2 K. On analyzing the data above 5 K based on Lines' simple-cubic-lattice equation, the following values of the parameters were obtained: J = −0.14 cm⁻¹, g = 1.67 cm⁻¹, and TIP = 0 cm³ mol⁻¹. The obtained g value was too small, so assuming the local tilt (α°),²¹ the following values of the parameters were obtained: J = −0.14 cm⁻¹, g = 2.00 cm⁻¹, α = 34°, and TIP = 0 cm³ mol⁻¹.

Fig. 6 (a) χ_mT versus T for 1, and (b) χ_mT versus T for 2. Solid lines represent fits to the data.

Although it is difficult to separate the effect of antiferromagnetic interaction from the effect of zero-field splitting, the antiferromagnetic interaction is weak (−J < 1 cm⁻¹) at any rate in this system. The antiferromagnetic interaction of this complex appears to be smaller than that of the Mn complex.²² Three-dimensional lattice framework is somewhat distorted in this complex, and four types of orientations are found for manganese ions; the angles between the orientations are 8–40°. The lattice appears to be much distorted compared with the Mn–formate complex.²² Long-range ordering was not observed for this system, but was observed for the Mn complex.²¹

The χ_mT value was 3.13 cm³ K mol⁻¹ at 300 K in 2, and this is smaller than the spin-only value (4.38 cm³ K mol⁻¹) for the S = 5/2 state, which indicates the presence of dominant ferromagnetic exchange interactions within 2 and suggests a very large ground-state spin (S) value. When decreasing the temperature, χ_mT decreases gradually and reaches a minimum (2.53 cm³ K mol⁻¹) at 45 K, but χ_mT increases abruptly to show a maximum (2.73 cm³ K mol⁻¹) at 39 K, then decreases to a minimum (0.21 cm³ K mol⁻¹) at 2 K. On analyzing the data above 50 K based on Lines' simple-cubic-lattice equation, the following values of the parameters were obtained: J = −0.56 cm⁻¹, g = 1.73 cm⁻¹, and TIP = 0 cm³ mol⁻¹. The obtained g value was too small, so assuming the local tilt (α°), the following values of the parameters were obtained: J = −0.52 cm⁻¹, g = 1.99 cm⁻¹, α = 30°, and TIP = 0 cm³ mol⁻¹. In the crystal, the Mn–O axis tilts by 25.7° in the Mn₈ cubic lattice, and the angle is approximately in good agreement with the obtained α angle (30°).²¹

Up to now, several series of metal–formate frameworks containing alkylammonium and ammoniums have been reported, and their structures and magnetic properties are summarized in Table S1.† The crystallized materials usually display spin-canted AF or WF features. The 3D frameworks of the same type of metal are quite similar because of the similar M⋯M coupling strength through anti–anti formate linkages and the same neighbors around each metal ion, although the framework topologies and structural details are different.

Conclusions

In summary, we have successfully synthesized two new porous anionic frameworks constructed at different temperature conditions. The results indicate that the organoammonium ion does not have a dramatic influence on the frameworks. Both of them show magnetic behavior due to their similar M⋯M coupling strength through anti–anti formate linkages. We hope that the synthetic methods described herein will further facilitate the exploration of new types of multifunctional materials with interesting properties, including the combination of porosity and magnetism.

Acknowledgements

This work was partially supported by grants from the National Natural Science Foundation of China (21201044), Natural Science Foundation of Guangdong Province (S2012040007835), Training plan of Guangdong Province outstanding young teachers in Higher Education Institutions (Grant No. YQ2013084), Foundation for Distinguished Young Talents in Higher Education of Guangdong Province (LYM11069), Technologies R & D program of Zhanjiang (2011C3108013 and 2012C3106016).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 975365–975366. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra02609h

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