Self-assembly of two high-nuclearity manganese calixarene-phosphonate clusters: diamond-like Mn₁₆ and drum-like Mn₁₄

Abstract

Two novel high-nuclearity manganese clusters, [Mn₁₄(BSC4A)₃(tBuPO₃)₆(μ₄-OH)₃Cl(H₂O)(CH₃OH)]·1.5CH₃OH (1) and [Mn₁₆(BSC4A)₃(PhPO₃)₇(HPO₄)(μ₄-OH)₃Cl(H₂O)(CH₃OH)₄]·4.5CH₃OH (2) (H₄BSC4A = p-tert-butylsulfonylcalix[4]arene; tBuPO₃H₂ = tert-butylphosphonic acid; PhPO₃H₂ = phenylphosphonic acid) have been solvothermally obtained and structurally characterized. Crystal structural analyses reveal that the phosphonate ligands have a large influence on the structures of the polynuclear manganese clusters: complex 1 possesses a drum-like Mn^II₁₄ core, which is constructed from three Mn₄–BSC4A molecular building blocks (MBBs) and six tBuPO₃²⁻ ligands and capped by two Mn^II ions; while complex 2 has a diamond-like Mn^II₁₆ core, which also consists of three Mn₄–BSC4A MBBs, but connected by seven PhPO₃²⁻ linkers and a tetrahedral Mn₄ cluster housing a phosphate anion generated in situ. To the best of our knowledge, complex 2 gives the first calixarene-based cluster linked by both organic phosphonate and inorganic phosphate ligands, and also presents the highest nuclearity manganese complex with the H₄BSC4A ligand to date. Magnetic measurements suggest the presence of antiferromagnetic interactions between the adjacent Mn^II spin carriers for both complexes.

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Introduction

The exploration of polynuclear transition metal clusters is currently a fertile topic of research, not only because of their architectural beauty¹ but also because of their immense technological applications in molecular magnets,² catalysts,³ optics⁴ and so on.⁵ Numerous transition metal clusters with different nuclearities, distinct geometries and interesting properties have been obtained utilizing multidentate ligands through hydro(solvo)thermal conditions, which may lead to some unexpected architectures induced by in situ generated ligands.⁶ p-tert-Butylsulfonylcalix[4]arene (H₄BSC4A, Scheme 1), a calixarene derivative possessing four lower-rim phenolic oxygen atoms and eight sulfonyl oxygen atoms, could be an ideal choice for the construction of polymetallic compounds.⁷ Moreover, it has been found that one H₄BSC4A molecule tends to coordinate to three or four divalent transition metal (TM) ions to form a shuttlecock-like TM_3/4–BSC4A molecular building block (MBB), which can be further bridged by different linkers into polynuclear entities. For example, two novel barrel-shaped Co₁₆ containers built from four Co₄–BSC4A MBBs and eight dicarboxylate ligands,⁸ a novel nest-like C₄-symmetric Co₂₄ metallamacrocycle constructed by eight Co₃–BSC4A molecular building blocks (MBBs) and six 1,2,4-triazolate ligands,⁹ several octahedral M₂₄ nanocages (M = Co, Ni) with six M₄–BSC4A MBBs and twelve dicarboxylates or eight tricarboxylates have been reported in recent years.¹⁰ In addition, its analogs p-tert-butylcalix[4]arene (H₄BC4A) and p-tert-butylthiacalix[4]arene (H₄BTC4A) have also been widely utilized in the syntheses of polymetallic compounds.¹¹

Scheme 1 p-tert-Butylsulfonylcalix[4]arene (H₄BSC4A).

On the other hand, phosphonate ligands possess different anionic forms and can adopt various coordination modes to bind to different metal ions.¹² In the past few years, a number of groups have been utilizing phosphonate ligands with preformed cages/clusters or pyrazoles, 2-chloro-6-hydroxypyridine, antimonite and carboxylate as co-ligands to make new polymetallic complexes.¹³

Our group has been focused on preparing new calixarene-based polynuclear compounds with fascinating structures and interesting physical properties.¹⁴ In our recent work, we have isolated some calix[4]arene-based high-nuclearity complexes with interesting architectures and properties by the introduction of phosphonate/phosphate as co-ligands. For example, we have synthesized eight thiacalix[4]arene-based high-nuclearity TM_4n nanocages (TM = Co, Ni; n = 2–6) constructed by bridging TM₄–calix[4]arene MBBs with phosphate/phosphonate ligands,¹⁵ two large alkali-metal templated Na₂Co₂₄ and KCo₂₄ clusters constructed from six TM₄–thiacalix[4]arene MBBs and six phosphate linkers,¹⁶ and a tri-capped trigonal prismatic Co₉ clusters with spin-glass behavior based on calix[4]arene and phosphonate ligands.¹⁷ As an extension of our work, we have tried to expand the mixed calix[4]arene-phosphonate system using other transition metal salts. Fortunately, we have successfully obtained two new calix[4]arene-based manganese(II) clusters having the general molecular formula of [Mn^II₁₄(BSC4A)₃(tBuPO₃)₆(μ₄-OH)₃Cl(H₂O)(CH₃OH)]·1.5CH₃OH (1) and [Mn^II₁₆(BSC4A)₃(PhPO₃)₇(HPO₄)(μ₄-OH)₃Cl(H₂O)(CH₃OH)₄]·4.5CH₃OH (2) (tBuPO₃H₂ = tert-butylphosphonic acid; PhPO₃H₂ = phenylphosphonic acid). Single-crystal X-ray diffraction analyses reveal that the phosphonate ligands have a significant influence on the structure of the polynuclear manganese(II) compounds. Although both complexes contain three Mn₄–BSC4A molecular building blocks MBBs (Fig. 1), they are bridged by different numbers of manganese cations, different kinds and numbers of phosphonate ligands as well as other auxiliary ligands. In fact, the structure of 1 possesses a drum-like Mn^II₁₄ core, whereas 2 a diamond-like Mn^II₁₆ core. Bond distances, bond valence sum (BVS) calculations and charge balance authenticate that all manganese centers in both complexes are at +2 oxidation states.¹⁸ To the best of our knowledge, there are only three tetranuclear Mn₄ clusters based on H₄BSC4A in the literature, thus 2 presents the highest nuclearity manganese complex with H₄BSC4A ligand so far.¹⁹ In this work, the syntheses, crystal structures and magnetic properties of complexes 1 and 2 were studied.

Fig. 1 Shuttlecock-like MBB of Mn₄–BSC4A, whose base is capped by a μ₄-hydroxide anion.

Experimental section

General remarks

The H₄BSC4A ligand was synthesized as previously reported.²⁰ All manipulations were performed under aerobic conditions utilizing chemical materials purchased from commercial sources and without further purification. The (C, H, N) elemental analyses were performed using a German Elementary Varil EL III instrument. Thermogravimetric analysis (TGA) curves were carried out on a NETZSCH STA 449C thermal analyzer in the temperature range of 25–900 °C in a nitrogen stream with a heating rate of 10 °C min⁻¹. The powder X-ray diffraction (PXRD) data were collected by a RIGAKU-DMAX2500 X-ray diffractometer using Cu Kα radiation (λ = 0.154 nm) at room temperature. Magnetic susceptibility measurements were measured with a Quantum Design PPMS-9T and MPMS-XL magnetometer. The experimental magnetic data were corrected for the diamagnetism of the constituent atoms (Pascal's tables).

Synthesis of complex 1

A mixture of H₄BSC4A (0.1 mmol, 85 mg), MnCl₂·4H₂O (0.4 mmol, 80 mg), tBuPO₃H₂ (0.1 mmol, 14 mg) and 10 mL CH₃OH was sealed in a 25 mL Teflon-lined bomb at 130 °C for 72 h. Eventually, the mixture was cooled down slowly to room temperature for 24 h, after which colorless block-shaped crystals of 1 were isolated by filtration, washed with CH₃OH and air dried (58% yield based on H₄BSC4A ligand). Elemental analysis (%) for C₁₄₅H₁₇₇O₅₉P₆S₁₂ClMn₁₄: calcd: C, 41.08; H, 4.20; found (after dried in vacuum): C, 40.66; H, 4.13. The phase purity of the sample was determined by PXRD (Fig. S3, in the ESI†).

Synthesis of complex 2

Complex 2 was obtained in a procedure similar to that of 1, except that PhPO₃H₂ (0.1 mmol, 16 mg) was used instead of tBuPO₃H₂ (0.1 mmol, 14 mg). Yield 76% based on H₄BSC4A ligand. Elemental analysis (%) for C₁₆₆H₁₈₉O₆₉ P₈S₁₂ClMn₁₆: calcd: C, 41.23; H, 3.94; found (after dried in vacuum): C, 42.06; H, 3.87. The phase purity of the sample was determined by PXRD (Fig. S4, in the ESI†).

X-ray crystallography

Data collection for 1 was performed on a SuperNova diffractometer at 100 K with graphite-monochromated Mo-Kα (λ = 0.71073 Å), while data of 2 was collected using a Rigaku Saturn 70 CCD diffractometer at 293 K with graphite-monochromated Mo-Kα (λ = 0.71073 Å). Crystallographic data and structure refinement for compounds 1 and 2 were summarized in Table 1. Data were corrected for absorption using the CrystalClear program.²¹ Structure solution by direct methods and full-matrix least-squares refinement on F² were solved by using SHELXTL-97 program package.²² All the non-hydrogen atoms were refined anisotropically except some badly disordered carbon atoms of tert-butyl groups. Organic hydrogen atoms of the ligands were placed in calculated positions and refined isotropically with fixed thermal factors. Moreover, diffuse electron density together with highly disordered MeOH molecules of crystallization could not be generated and were not included for both structures, thus the crystal structures were treated by the “SQUEEZE” method, a part of the PLATON package of crystallographic software.²³ This had dramatically improved the agreement indices. Moreover, the high R₁ and wR₂ factor for both title complexes might be due to the weak crystal diffractions and the disorder of the tert-butyl groups. Crystallographic details for both compounds are available in the ESI† in CIF format.

Table 1 Crystallographic data and structure refinement for complexes 1 and 2

Complex	1	2
a R1 = ∑‖Fo| − |Fc‖/∑|Fo|.b wR2 = {∑[w(Fo^2 − Fc^2)^2]/∑[w(Fo^2)^2]}^1/2.
Formula	C145H177O59P6	C166H189O69P8
	S12ClMn14	S12ClMn16
M/g mol^−1	1.4225	1.4528
T/K	100	293
Crystal system	Monoclinic	Orthorhombic
Space group	P21/c	Pnma
a/Å	18.7857(3)	34.378(4)
b/Å	39.0027(6)	28.140(4)
c/Å	27.3636(4)	22.849(3)
α (°)	90.00	90
β (°)	99.2080(10)	90
γ (°)	90.00	90
V/Å^3	19790.8(5)	22 104(5)
Z	4	4
μ (mm^−1)	1.122	1.139
Data measured	101 744	144 413
Ind. reflns	34 809	20 856
Parameters	2183	1313
GOF on F^2	1.050	1.086
R1^a [I > 2σ(I)]	0.0561	0.0709
wR2^b	0.1335	0.1987
CCDC number	1046923	1046924

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Results and discussion

Crystal structures

Solvothermal treatment of H₄BSC4A with tBuPO₃H in methanol solution affords colorless single crystals of 1. X-ray data diffraction analysis reveals that complex 1 crystallizes in the monoclinic system with space group P2₁/c and has a tetradecanuclear Mn^II₁₄ cluster. As can be seen from Fig. 2, there are three fully deprotonated BSC4A⁴⁻ ligands, each of which coordinates to four Mn^II ions by its lower-rim phenoxy oxygen and bridge sulfur oxygen atoms forming a shuttlecock-like Mn₄–BSC4A MBB, whose bottom is capped by one μ₄-hydroxide anion. The bottoms of the Mn₄–BSC4A MBBs are linked together through the oxygen atoms from six different tBuPO₃²⁻ ligands to form the body of a drum, whose “upper” and “lower” faces are each further capped by a Mn^II ion (Fig. 3). These 14 crystallographically independent Mn^II ions can be divided into three groups according to their coordination environments: (i) 12 Mn ions (Mn1–Mn12) from the abovementioned three Mn₄−BSC4A MBBs are coordinated by two phenoxo oxygen atoms along with one sulfonyl oxygen atom originating from one BSC4A⁴⁻ ligands, one μ₄-hydroxide anion, and two oxygen atoms from two independent tBuPO₃²⁻ ligands to form a six-coordinated distort octahedral geometries; (ii) Mn13 is bounded by five oxygen atoms deriving from three different tBuPO₃²⁻ ligands, one water oxygen and one chlorine to form a seven-coordinated pentagonal bipyramid geometry; (iii) while the rest Mn14 ion is coordinated by four oxygen atoms from three independent tBuPO₃²⁻ ligands and one ligated MeOH molecule to form a five-coordinated square-pyramid geometry. Impressively, six tBuPO₃²⁻ ligands coordinate to the Mn ions in two different chelating modes: three with [5.2₁₂2₃₄2₄₅] coordination mode, and the rest three with [5.221] fashion according to Harris notation (Scheme 2).²⁴ To the best of our knowledge, there are two compounds supported by H₄BSC4A ligands with tetradecanuclear structures. One is constructed from two Co₄–BSC4A building blocks, two Co₃–BSC4A subunits and 12 bridging tetrazolate ligands,^7d and the other one is a cationic cluster composed of three BSC4A⁴⁻ ligands and 14 Co^II ions and formed by two superposed planar, bodycentered, hexagonal cores by seven Co^II ions.^7e However, they are different from complex 1 which possesses a drum-shaped Mn^II₁₄ core.

Fig. 2 Molecular structure of complex 1. The hydrogen atoms and isolated solvent molecules are omitted for clarity.

Fig. 3 Core structure of complex 1. Calix[4]arene ligands and tBu groups are omitted for clarity.

Scheme 2 Observed bonding modes of the phosphonate and phosphate ligands.

The phosphonate ligands have a pivotal influence on the final structure of title complexes. By replacing the tBuPO₃H ligand to PhPO₃H ligand in the similar reaction, complex 2 was obtained. It crystallizes in an orthorhombic cell with the space group Pnma, and contains an unprecedented diamond-like Mn^II₁₆ core. As shown in Fig. 4a, complex 2 is also constructed from three Mn₄−BSC4A MBBs to compare with 1, but linked by seven PhPO₃²⁻ ligands and one phosphate-based tetrahedral Mn₄ cluster. The “upper” part of the core is built by three Mn₄–BSC4A MBBs and linked together through four different PhPO₃²⁻ ligands, while the “lower” part is a tetrahedral Mn₄ cluster, which are bridged by three different PhPO₃²⁻ ligands and one in situ generated HPO₄²⁻ anion (Fig. 5). Then the “upper” part and “lower” part are further linked by three different PhPO₃²⁻ ligands to form a diamond-like Mn^II₁₆ core. Unlike 1, complex 2 has a crystallographic symmetry plane, so its asymmetric unit includes half of the formula and there are nine crystallographically independent manganese atoms (Mn1–Mn9) in this structure. Six Mn ions (Mn1–Mn6) from the Mn₄−BSC4A MBBs have the same coordination environments to compare with those of 1. The remaining three Mn ions are all six-coordinated and exhibit octahedral geometries, but they have different coordination environments. Specially, Mn7 is in O₆ environment with four oxygen atoms from two different PhPO₃²⁻ ligands, one oxygen from HPO₄²⁻ anion and one water oxygen; Mn8 is in ClO₅ environment with two oxygen atoms from two different PhPO₃²⁻ ligands, one oxygen from HPO₄²⁻ anion, one chlorine anion and two MeOH molecules; Mn9 is in O₆ environment with four oxygen atoms from three different PhPO₃²⁻ ligands, one oxygen from HPO₄²⁻ anion and one MeOH molecules (Fig. 4b). Notably, seven PhPO₃²⁻ anions adopt four different binding modes: (i) three ligands coordinate in a [6.222] manner; (ii) two adopt the [5.221] modes; (ii) one binds in a [5.2₁₂2₃₄2₄₅] fashion; (iv) while the remaining one coordinates in a [4.211] mode. It should be note that the difference between the final structures of title complexes might arise from the unexpected in situ generated HPO₄²⁻ anion. The HPO₄²⁻ anion in this structure is generated from in situ reactions of PhPO₃H₂ molecule during the solvothermal conditions, which coordinates to four Mn centers in a [4.1111] coordination mode (Scheme 2) to form a tetrahedral Mn₄ cluster and further to generate unexpected cluster structure. We note that this phosphate anion carries a hydrogen atom, because one of the distances between the phosphorus and oxygen atoms are considerably longer than the other three, which is in agreement with the previously reported phosphate-based compounds.²⁵ To the best of our knowledge, complex 2 gives the first calixarene-based cluster linked by both organic phosphonate and inorganic phosphate ligands. This sheds some light into the design and construction of other calixarene-based high-nuclearity compounds by utilizing both phosphonate and phosphate linkers. Although there have been several reports on calix[4]arene-based M₁₆ (M = Co, Ni) clusters, those clusters are squares constructed by four Co₄–calix[4]arene MBBs with eight in situ generated 5-methyltetrazolates or rigid 1,3-benzenedicarboxylates, this unusual diamond-like Mn₁₆ architecture has not been reported heretofore.¹⁰

Fig. 4 (a) Molecular structure of complex 2. (b) X-ray asymmetric unit of 2. The hydrogen atoms and isolated solvent molecules are omitted for clarity.

Fig. 5 Core structure of complex 2. Calix[4]arene ligands and phenyl groups are omitted for clarity. Symmetry code: A x, 3/2-y, z.

Magnetic studies

The variable-temperature dependence of solid state direct current (dc) magnetic susceptibilities were collected on the polycrystalline samples of 1 and 2 in the temperature range of 2–300 K under a field of 1 kOe. These results are plotted as the χ_mT products (χ_m is molar magnetic susceptibility) versus temperature in Fig. 6 and 7. The χ_mT values at 300 K are 48.92 and 54.81 cm³ K mol⁻¹ for 1 and 2, which are smaller than the expected values of 61.25 and 70 cm³ K mol⁻¹ for 14 and 16 non-interacting high-spin Mn^II spin carrier (S = 5/2, g = 2), respectively.²⁶ As the temperature is decreased, the χ_mT values continuously decrease to 2.07 and 5.06 cm³ K mol⁻¹ until the lowest temperature 2 K for 1 and 2, respectively. The χ_m⁻¹ data above 50 K obey the Curie–Weiss Law with Curie constants (C) = 62.38 and 70.37 and Weiss constants (θ) = −83.72 and −85.96 cm³ K mol⁻¹ for 1 and 2, respectively. The negative Weiss constants, together with the trend of χ_mT vs. T plots, reveal the presence of strong antiferromagnetic interaction between Mn^II ions. However, the coupling parameters (J) of both title clusters cannot be carried out owing to the complexities of the structures.

Fig. 6 Temperature dependence of χ_mT measured in a 1 kOe field. The solid lines are the best fitting to the Curie–Weiss Law. Magnetization curves (inset graphs) measured at 2−7 K for 1.

Fig. 7 Temperature dependence of χ_mT measured in a 1 kOe field. The solid lines are the best fitting to the Curie–Weiss Law. Magnetization curves (inset graphs) measured at 2−7 K for 2.

The field dependence of magnetization (M) versus field (H) data for 1 and 2 from 2 to 7 K are investigated with the applied magnetic field H in the range 0–80 kOe (inset of Fig. 6 and 7, respectively). The magnetization of 1 and 2 continuously increases almost linearly and the magnetization value at the highest field (80 kOe and 2 K) is 12.24 and 12.95 Nβ for complexes 1 and 2 respectively, which are far from the saturation sum values of 14 and 16 Mn^II ions. The temperature-dependent alternating current (ac) susceptibility studies of 2 are also carried out between 20 and 2 K with and without applied dc field, but no peaks for the out-of-phase component is observed (Fig. S1, in the ESI†). Moreover, no obvious hysteresis loop is observed for 2 at 2 K (Fig. S2, in the ESI†). All these results reveal neither long-range ordering nor single-molecule magnet behavior above 2 K for 2.

Conclusion

In conclusion, we have synthesized two novel polynuclear manganese clusters based on H₄BSC4A and phosphonate ligands. We found that the phosphonate ligands have an important influence on the final structures of title complexes. When tBuPO₃H₂ was utilized, a tetradecanuclear drum-like Mn^II₁₄ cluster (1) was obtained. By variation of the phosphonic acid from tBuPO₃H₂ to PhPO₃H₂, a hexadecanuclear diamond-like Mn^II₁₆ cluster (2) was synthesized. To the best of our knowledge, complex 2 presents the highest nuclearity manganese complex supported by H₄BSC4A ligand and also gives the first calixarene-based cluster linked by both organic phosphonate and inorganic phosphate ligands. Magnetic measurements suggest the presence of antiferromagnetic interactions between the adjacent Mn^II centers. Our future studies will focus on polynuclear lanthanide compounds of mixed calixarene-phosphonate ligands.

Acknowledgements

We thank 973 Program (2011CB932504), National Natural Foundation of China (21131006) and the Natural Science Foundation of Fujian Province.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data in CIF format, field dependence of magnetization (M) versus field (H) data, the M–H plots, TGA analyses and PXRD patterns for complexes 1 and 2. CCDC 1046923 and 1046924. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra02530c

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