A four-fold three-dimensional zinc(II) coordination polymer based on 4,4′-bis(2-methyl-imidazolyl)biphenyl and 5-sulfoisophthalate ligands: synthesis, structure and properties

2.1 Chemicals and reagents

All reagents and solvents were purchased from Jinan Henghua Sci. Tec. Co. Ltd. and were of analytical quality. They were used without further purification.

2.2 Physical measurements

Elemental analyses for C, H and N were conducted on a Perkin-Elmer 240C Elemental Analyzer. The IR absorption spectra were conducted on a Bruker VERTEX 80 spectrometer in the 4000–400 cm⁻¹ region. Powder X-ray diffraction (PXRD) patterns were recorded on a Shimadzu XRD-6000 X-ray diffractometer using CuKα (λ = 1.5418 Å) radiation at room temperature. Thermal analyses were performed on a NETZSCH STA 449 F5 Jupiter thermogravimetric analyzer under an N₂ atmosphere at a heating rate of 10 K min⁻¹. The luminescence spectra were recorded at room temperature on a Perkin-Elmer LS 55 spectrometer fluorescence spectrophotometer. The UV–vis spectra were obtained using a Perkin-Elmer Lambda 25 spectrophotometer.

2.3 Synthesis of [Zn₃(4,4′-BMIBP)₃(SIP)₂] _n (1)

A mixture of NaH₂sip (13.4 mg, 0.05 mmol), 4,4′-BMIBP (15.7 mg, 0.05 mmol), Zn(NO₃)₂·6H₂O (14.9 mg, 0.05 mmol) and H₂O (1.0 mL) in DMF (1.5 mL) was charged into a Teflon-lined stainless-steel vessel, heated from room temperature to 373 K in 2 h and held at this temperature for 3 days under autogenous pressure. The mixture was then cooled to 313 K in 24 h, and finally to room temperature. Yield: 74% based on Zn(NO₃)₂·6H₂O. – Elemental analysis for C₇₆H₆₀N₁₂O₁₄S₂Zn₃: calcd. C 56.15, H 3.72, N 10.34; found C 56.27, H 3.73, N 10.36%. – IR (KBr, cm⁻¹): 3436(w), 3127(w), 1618(s), 1505(s), 1429(m), 1348(s), 1261(m), 1153(s), 1103(m), 1009(s), 830(w), 780(w), 736(m), 679(w), 621(m), 549(w), 471(w).

2.4 X-ray structure determination of 1

Crystallographic data for complex 1 was collected on a Bruker Smart CCD diffractometer with MoKα radiation (λ = 0.71073 Å) using an ω–2θ scan mode at T = 293(2) K. An absorption correction was applied by using the multi-scan program Sadabs. The structure was solved by Direct Methods and refined by full-matrix least-squares technique based on F² using the Shelxl-2014 program package [13]. All non-hydrogen atoms were treated anisotropically. The hydrogen atoms of the organic ligands were geometrically generated using a riding model and isotropically refined. The atoms C26/C27/C29/C30 were disordered over two positions. The disorder was modelled by refining these two positions freely and setting the sum of their occupancies to be equal to 1; the final occupancy parameters were set as follows: 0.50:0.50 for C26/C27/C29/C30 and C26A/C27A/C29A/C30A. Some reflections were omitted automatically by Saint (Area Detector Control and Integration Software) because of overflow or beamstop. The crystallographic data are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 2175854 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Description of the structure

Complex 1 crystallizes in the monoclinic space group C2/c with Z = 4 trinuclear complexes. The asymmetric unit contains two crystallographically independent zinc(II) cations, Zn1, which resides on a crystallographic 2-fold rotation axis, and Zn2, one triply deprotonated sip³⁻ anion and one and one-half 4,4′-BMIBP ligands. As is illustrated in Figure 1, Zn1 is tetrahedrally coordinated in a {ZnO₂N₂} environment with two nitrogen donor atoms from two different 4,4′-BMIBP ligands and two carboxylate oxygen atoms of two different sip³⁻ anions. Zn2 possesses a {ZnN₂O₂} tetrahedral coordination sphere, with two nitrogen donor atoms from two different 4,4′-BMIBP ligands, one carboxylate oxygen atoms of one sip³⁻ anion and one sulfonate oxygen donor from another sip³⁻ anion. The distortion of the tetrahedron can be indicated by the calculated value of the parameter τ₄, which is 0.865 for Zn1 and 0.807 for Zn2. (Note that τ₄ is 0 for a perfect square-planar coordination and 1 for a perfect tetrahedral geometry [14]). Thus, the tetrahedral coordination in complex 1 is not perfect, but only slightly distorted.

Figure 1:

A view of the local coordination of the Zn(II) cations in complex 1. Displacement ellipsoids are drawn at the 50% probability level (symmetry codes: (i) 1 − x, y, 1/2 − z; (ii) 1 − x, −1 − y, −z; (iii) x, 1 − y, −1/2 + z; (iv) −x, −1 − y, −z).

In complex 1, the H₂sip⁻ anion of the reagent is completely deprotonated and links three Zn(II) ions via two monodentate carboxylate groups and one monodentate sulfonate group to give a double-chain [Zn₃(SIP₃)₂] _n in which there is a [Zn₂(sip)₂] 16-membered metallomacrocyclic subunit with a Zn⋯Zn distance of 7.957(3) Å (Figure 2). More interesting, there is a two-fold axis passing through the macrocycle in Figure 2. Moreover, these adjacent double-chain lines are further supported by the bridging 4,4′-BMIBP ligands as pillars to produce a 3D framework (Figure 3). In order to simplify the structure of complex 1, a topological analysis was carried out. From the topological view, each sip³⁻ anion can be defined as a 3-connected node, Zn1 and Zn2 ions are 4-connected nodes and the 4,4′-BMIBP ligand acts as a linker between two 4-connected nodes. The structure of complex 1 therefore can be described as a binodal (3,4)-connected net with a Schläfli symbol of {4·6²·8²·10}₂{4·6²}₂{6⁴·10²} [15]. The large cavity of complex 1 enables three further independent equivalent nets to interpenetrate, thus giving a 4-fold interpenetrating 3D architecture (Figure 4). Complex 1 and two previously reported Cd-centered CPs such as [Cd(MIP)(4,4′-BMIBP)] _n (H₂MIP: 5-methylisophthalic acid; 4,4′-BMIBP: 4,4′-bis(2-methyl-imidazolyl)biphenyl) (2) [12], and [Cd(5-SO₃-1,3-HBDC)(BIMB)] _n (5-HSO₃-1,3-H₂BDC: 5-sulfo-1,3-benzenedicarboxylate acid: BIMB 4,4′-bis(1-imidazolyl)bibenzene) (3) [16], nicely demonstrate how the coordination modes of the ligands and the central metal ions greatly influence the crystal structures of the resultant CPs. Complex 2 crystallizes in the orthorhombic space group P2₁2₁2₁ and exhibits a novel fivefold interpenetrating 3D diamondoid framework. Complex 3 crystallizes in the monoclinic space group C2/c. For the 5-SO₃-1,3-HBDC ligand in complex 3, one COO⁻ group adopts a chelating coordination mode, while the SO₃⁻ group coordinates in a bridging bidentate mode. The ligand 5-SO₃-1,3-HBDC links two Cd(II) ions to form a 2D 3-connected (4·8²) network. Each BIMB ligand links the adjacent 2D layers to afford a 3D framework. Complex 3 has a 3D (3,5)-connected topological structure with a (4·6·8) (4·6⁵·8³·10) Schläfli symbol.

Figure 2:

View of the double chain [Zn₃(SIP₃)₂] _n of complex 1.

Figure 3:

A perspective view of the 3D structure of complex 1.

Figure 4:

Topological representation of the 4-fold interpenetrating nets in complex 1.

3.2 Thermal analysis

The thermal stability of complex 1 was investigated by TGA under an N₂ atmosphere with a heating rate of 10 K min⁻¹. As depicted in Figure 5, the network decomposed quickly at a temperature as high as 753 K, suggesting that complex 1 is thermally very stable.

Figure 5:

Thermogravimetric curve of complex 1.

3.3 Solid-state luminescence properties

It is well known that CPs constructed with d¹⁰ Zn(II) ions and conjugated ligands are potential photoactive materials. So, the solid-state fluorescence spectrum of the reagents NaH₂sip and 4,4′-BMIBP and of complex 1 were measured at room temperature. The emission peaks of the former are at 357 nm (λ_ex = 245 nm) and 366 nm (λ_ex = 252 nm), which can be attributed to the π* → n or π* → π transitions. As shown in Figure 6, complex 1 exhibited an emission peak at 365 nm when excited at 286 nm. Because the Zn(II) ion is difficult to oxidize or reduce due to its d¹⁰ configuration, the emission of complex 1 must be attributed to ligand-centered electronic transitions [17, 18].

Figure 6:

The solid-state emission spectrum of complex 1 recorded at room temperature.

3.4 Photocatalytic properties

In order to evaluate the catalytic performance of 1, methylene blue (MB) as a model dye contaminant was selected for evaluation. As shown in Figure 7, the absorption peak intensities of MB significantly decrease during the degradation reaction under UV irradiation in the presence of complex 1 and approximately 88% of MB are decomposed within 50 min. The percentage degradation is only 3% under the same condition without adding complex 1 (Figure 8). Further, the photocatalytic properties of complex 1 were found to be better than that of the previously reported CPs [19], [20], [21]. Two Zn(II) CPs [Zn(L)(bib)] and [Zn(L)(phen)] (H₂L: 1,3-bis(3-carboxylphenoxy)benzene acid; bib:1,4-bis(1-imidazolyl)benzene; phen: 1,10-phenanthroline) degraded 43.4 and 41.2% MB within 100 min [19]. A 2D Cu(II)-based CP [Cu₂(HL)₂(bib)₂] (H₃L: 5-(4′-carboxylphenoxy)isophthalic acid; bib: 1,1′-(1,4-butanediyl)bis(imidazole)) photodegraded MB up to 63.0% within 100 min [20]. A 3D Mn-based CP [Mn₄(NDC)₂(bb)₄] (H₂NDC: 1,4-naphthalenedicarboxylic acid; bb: 4,4′- bis(imidazolyl)biphenyl) exhibited 29.4% decomposition of MB within 100 min [21]. The recycling ability of complex 1 was tested because of its possible importance for industrial applications. As is shown in Figure 9, complex 1 can be reused for at least five cycles without a significant decrease of the photocatalytic efficiency. Also, the PXRD patterns of complex 1 after five catalytic cycles were consistent with those of the as-prepared samples which is manifesting that the title complex possesses good framework stability and integrity which remain unaffected after five catalytic cycles (Figure 10) [22, 23].

Figure 7:

UV–Vis spectra monitoring the photocatalytic degradation of MB of complex 1.

Figure 8:

Photocatalytic decomposition of MB in aqueous solution under UV irradiation with the use of complex 1 and the control experiment without any catalyst.

Figure 9:

The photocatalytic MB degradation during five consecutive runs over complex 1.

Figure 10:

X-ray powder diffraction patterns of complex 1 before and after catalytic experiments and the XRPD pattern simulated from the single-crystal structure determination.

To explore the mechanism of photocatalysis, radical trapping experiments were executed. Isopropyl alcohol, benzoquinone and ammonium oxalate served as scavengers for hydroxyl radicals (˙OH), superoxide ions (˙O₂⁻) and holes (h⁺), respectively. They were added separately to the system under the photocatalytic process. As Figure 11 shows, the scavenging experiments manifested that addition of isopropyl alcohol, benzoquinone and ammonium oxalate resulted in significant change in the efficacy of the MB degradation by complex 1 from 88% to 45%, 52% and 39%, respectively. This result makes clear that ˙OH, ˙O₂⁻ and h⁺ are the main active species involved during photocatalytic degradation [24, 25].

Figure 11:

Trapping of the active species during the photocatalytic reaction of complex 1 (IPA, isopropyl alcohol; BQ, benzoquinone; AO, ammonium oxalate).