Rod-packing motif: a new metal–organic polymer showing unusual rod-packing architecture

Abstract

A new metal–organic polymer, namely {Co(2,2′,4,4′-H₂bptc)}_n (1), 2,2′,4,4′-H₄bptc = biphenyl-2,2′,4,4′-tetracarboxylic acid) obtained viahydrothermal reaction, compound 1 shows an unusual hex-type rod-packing architecture. Moreover, the magnetic studies of 1 disclose a weak intra-chain ferromagnetic interaction.

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Recently, metal–organic frameworks (MOFs) with well-regulated network structures have received remarkable attention, not only due to its potential applications in many fields such as magnetism, molecular separation, catalysis, luminescence, and sorption, but also its intriguing topology matrix.^1–5 Up to now, a large number of MOFs have been reported with various topology structures such as 3-, 4-, 5-, 6-, 7-, 8-, 12-connected uninodal nets, (3,4)-, (3,5)-, (4,6)-, (3,8)-, (4,8)-, (4,12)-, (3,24)-connected binodal nets.^6–10 On the other hand, based on the recent review reported by O’Keeffe and co-workers, more and more MOFs were reported to bear four-connecting pcu-tpye, and limited cases are defined as six-connecting hex-type, three-connecting bnn-type, or (4,6)-connecting rod-packing structures.^11,12 To construct such specials, symmetrical dicarboxylate-like ligands such as 1,4-benzenedicarboxylate, 4,4′-biphenyldicarboxylate, 2,7-pyrenedicarboxylate, 2,6-naphthalenedicarboxylate, 2,5-dihydroxybenzenedicarboxylate, 2,5-thiophenedicarboxylate, 2,7-tetrahydropyrenedicarboxylate, and symmetrical multi-carboxylate ligands such as 1,3,5-benzenetricarboxylate, 2,7-tetrahydropyrenedicarboxylate are usually employed, where the combination between metal ions and carboxyl groups generates rod-like arrangement and benzene, or biphenyl, or other fragments acts as spacers to link these rod-like substructures together, resulting in the overall side-equal rod-packing matrix. Taking the nature of symmetrical multi-carboxylate ligand leading to overall side-equal rod-packing matrix into account, it is possible to yield overall side-unequal rod-packing matrix or uncommon rod-packing arrangement by means of introduction of asymmetrical multi-carboxylate ligand. Herein, we select a less developed multi-carboxylate ligand of biphenyl-2,2′,4,4′-tetracarboxylic acid to target this object.¹³ As a multi-carboxylate ligand, biphenyl-2,2′,4,4′-tetracarboxylic acid has the potential to provide four carboxyl groups on 2,2′,4, and 4′ sites, where the two carboxyl groups on 4 and 4′ sites are almost symmetrical, and the two carboxyl groups on 2 and 2′ sites are asymmetrical, due to steric hindrance. Viahydrothermal reaction of Co(NO₃)₂ with 2,2′,4,4′-H₄bptc, polymer 1 is obtained. The structure analysis gives the double-edged, side-unequal hex-type rod-packing framework. The synthesis, structure, and magnetic properties in detail are listed below.‡

A mixture of Co(NO₃)₂·6H₂O (0.06 g, 0.2 mmol) and 2,2′,4,4′-H₄bptc (0.03 g, 0.1 mmol) was placed in a 25 mL Teflon-lined stainless-steel vessel, and then the pH was adjusted to 5 by addition of 1 mol L⁻¹NaOH. The mixture was sealed and heated at 160 °C for 4 d, and then the reaction system was cooled to room temperature. Purple crystals were obtained in yield (based on 2,2′,4,4′-H₂bptc²⁻): 48%. Elemental analysis (%): calcd C 49.64, H 2.08. Found: C 49.76, H 2.01. A single-crystal X-ray diffraction analysis gives that polymer crystallizes in the orthorhombic, Pbcn space group and the asymmetrical unit contains one Co(II) ion and one 2,2′,4,4′-H₂bptc²⁻ ligand. In 1, the Co(II) ion with the position occupation of 0.5 is six-coordinated by six 2,2′,4,4′-H₂bptc²⁻oxygen atoms, resulting in the axis-elongated octahedral geometry, where the Co–O bond lengths of 2.1717(18) Å located at the axis site is longer than that of 2.0675(16)–2.0757(15) Å located at the basic plane about 0.1 Å, but these Co–O bond lengths are comparable with the literature value.¹⁴(Fig. 1) For the organic multi-carboxylate ligand of 2,2′,4,4′-H₂bptc²⁻, the carboxylic acids on 2 and 2′ sites are deprotonated with the C–O bond lengths of C7–O1/1.246(3) Å, C7–O4/1.285(3) Å and display the bidentate coordination mode, whereas the carboxylic acid on 4 and 4′ sites are protonated with the C–O bond lengths of C8–O2/1.231(3) Å, C8–O3/1.300(3) Å and the C=O group displays the monodentate coordination mode. Due to steric hindrance, the biphenyl plane is largely distorted with the dihedral angel of ca. 75.2° for the two phenyl planes, resulting in the overall C₁ point group for the 2,2′,4,4′-H₂bptc²⁻ ligand (Fig. 2).

Fig. 1 As shown here, there are five 2,2′,4,4′-H₂bptc²⁻ ligands around Co(II) site, two 2,2′,4,4′-H₂bptc²⁻ ligands coordinating to Co(II) with one carboxyl group on 4 or 4′ site, two 2,2′,4,4′-H₂bptc²⁻ ligands (upper) linking to Co(II) by means of one carboxyl group on 2 or 2′ site, and one 2,2′,4,4′-H₂bptc²⁻ ligands (lower) connecting to Co(II) through two carboxyl groups both on 2 and 2′ sites.

Fig. 2 The coordination mode of 2,2′,4,4′-H₂bptc²⁻ ligand in 1. The hydrogen atoms are omitted for clarity.

As shown in Fig. 3, along c direction the Co(II) ions are in-turn associated together by double 2,2′,4,4′-H₂bptc²⁻ carboxylate bridges on 2 and 2′ sites, resulting in the 1D rod-like arrangement of {Co(CO₂)₂}_n with the closest Co–Co distance of ca. 4.8 Å. As shown in Fig. 4 and 5, through 2,2′,4,4′-H₄bptc spacers each {Co(CO₂)₂}_n rod connects to six neighbouring, identical {Co(CO₂)₂}_n rods, resulting in the hex-type rod-packing architecture.

Fig. 3 View of the 1D rod-like arrangement of {Co(CO₂)₂}_n, along the c direction.

Fig. 4 View of the hex-type rod packing.

Fig. 5 View of the hex-type rod packing framework.

In the literature, MOFs belonging to hex-type rod packing frameworks are rare and the sides of hex-type rod packing in the reported cases are equal. The length of side is equal to the length of symmetrical organic ligand, see Fig. 6(left). By contrast, in this work, asymmetrical organic ligand of 2,2′,4,4′-H₂bptc²⁻ is employed, resulting in the somewhat irregular hex-like rod packing, where one {Co(CO₂)₂}_n rod connects to four identical {Co(CO₂)₂}_n rods by means of phenyl units and links two other identical {Co(CO₂)₂}_n rods viabiphenyl units. This distinct connectivity among rods leads to the construction of somewhat irregular hex-like rod packing with unequal sides, see Fig. 6 (right). It is worth noting that each {Co(CO₂)₂}_n rod connects six identical {Co(CO₂)₂}_n rods via double organic spacers.

Fig. 6 The comparison between the hex rod packing and irregular hex rod packing.

Fig. 7 View of experimental (black) and simulated (red) χ_MTvs.T plots, and inserted, experimental (black) and simulated (red) 1/χ_Mvs.T plots.

The investigation of magnetic susceptibility was carried out between 2 and 300 K in an applied field of 0.2 T. The plot of temperature dependence of χ_MT is shown in Fig. 7. The χ_MT value of 3.79 cm³ mol⁻¹ K at room temperature is bigger than the expected χ_MT value (1.875 cm³ mol⁻¹ K) of one magnetism-isolated Co(II) ions with S = 3/2 and g = 2.0, due to spin-orbit coupling.¹⁵ The decrease at 17.7–300 K and the onset of increase at low temperature suggests that both orbital contribution and spin canting effects could be operating.¹⁵ The Curie–Weiss fit at T >17.7 K affords C = 4.34 cm³ mol⁻¹ K and θ = −45.2 K, indicating the presence of quenched orbital contribution and/or antiferromagnetic behaviour.

In this work, we report a new metal–organic polymer and its synthesis, structure, magnetic properties in detail. The outstanding structure feature of 1 is the irregular hex rod packing with unequal sides. To some extent, the present research results has proved that the asymmetrical multi-carboxylate ligand of 2,2′,4,4′-H₄bptc has the potential to construct irregular rod packing matrix. Moreover, the weak intra-chain ferromagnetic interaction found in polymer 1 is also notable.

This work was supported by the National Natural Science Foundation of China (50872057).

Notes and references

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Footnotes

† CCDC reference number 772399. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c0ce00066c

‡ Crystal data for 1: C₁₆H₈CoO₈, M_r = 387.15, orthorhombic, space groupPbcn, a = 15.745(3), b = 9.1922(18), c = 9.4397(19) Å, β = 90°, V = 1366.25 ų, Z = 4, T = 296(2), D_c = 1.882 mg m⁻³, R_int = 0.0859, reflections collected, R₁(wR₂) = 0.0565(0.1276) and S = 1.089 for 1622 reflections with I > 2σ(I).

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