Substituted group directed assembly of zinc coordination compounds based on bifunctional ligands, from mono, di to tristetrazole–carboxylate

Abstract

Four different tetrazole–carboxylate ligands, monotetrazole–carboxylate Hatza (Hatza = 5-aminotetrazole-1-acetic acid), Hpytza (Hpytza = 5-(4-pyridyl)tetrazole-2-acetic acid), ditetrazole–carboxylate H₂datza (H₂datza = N,N-di(tetrazol-5-yl)amine-N2,N2′-diacetic acid) and tristetrazole–carboxylate, H₃tzpha (H₃tzpha = 1,3,5-tris(tetrazol-5-yl)benzene-N2,N2′,N2′′-trisacetic acid) have been chosen to be reacted with zinc salts, resulting in the formation of four novel compounds, [Zn(atza)₂(H₂O)₄] (1), [Zn(pytza)₂] (2), [Zn(datza)(H₂O)₂]·3H₂O (3) and [Zn₃(tzpha)₂(H₂O)₁₂]·MeOH·EtOH·4H₂O (4), whose structures are controlled by not only the number and different coordination modes of the tetrazole–carboxylate but also the complementary hydrogen bonds. These compounds have been characterized by elemental analysis, IR and single crystal X-ray diffraction. Compound 1 displays a simple zero dimensional mononuclear structure, 2 shows a classic 3D 8-connect (3⁶, 4¹⁶, 5⁶) tsi network topology, 3 features a 1D ladder-like chain while 4 is a 1D beaded chain. Furthermore, the luminescence properties investigated at room temperature in the solid state show excellent ligand-centered luminescence.

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Introduction

The past decade has witnessed the prosperity of coordination compounds based on tetrazole ligands, due to not only the diversity in their structures,¹ but also their tunable applications in the fields of magnetism,² ion exchange,³ catalysis,⁴ ferroelectrics,⁵ absorption⁶ and so on. On the one hand, tetrazole–carboxylates, with abundant nitrogen and oxygen atoms of the tetrazole ring and the carboxylate group, tend to display a variety of coordination modes and the –CH₂– spacer may improve the flexibility of these ligands, allowing the formation of multiple framework structures. On the other hand, to the best of our knowledge, it is universally acknowledged that coordination compounds based on d¹⁰ metals are outstanding candidates for potential application as luminescence materials.⁷ In our previous work, some relevant compounds based on tetrazole–carboxylate ligands were reported, such as the ones derived from tetrazole-5-carboxylate, 5-(2-pyridyl)tetrazole-2-acetic acid, 5-(3-pyridyl)tetrazole-2-acetic acid, 5-(2-pyrimidyl)tetrazole-2-acetic acid and 5-(2-pyrazinyl)tetrazole-2-acetic acid.⁸ Taking all of the observations into consideration, in this paper, Hatza (Hatza = 5-aminotetrazole-1-acetic acid), Hpytza (Hpytza = 5-(4-pyridyl)tetrazole-2-acetic acid), H₂datza (H₂datza = N,N′-di(tetrazol-5-yl)anime-N2,N2′-diacetic acid), H₃tzpha (H₃tzpha = 1,3,5-tris(tetrazol-5-yl)benzene-N2,N2′,N2′′-trisacetic acid) (Scheme 1) have been chosen to be reacted with zinc salts; [Zn(atza)₂(H₂O)₄] (1), [Zn(pytza)₂] (2), [Zn(datza)(H₂O)₂]·3H₂O (3) and [Zn₃(tzpha)₂(H₂O)₁₂]·MeOH·EtOH·4H₂O (4) have been obtained. We anticipate that the four ligands can display various coordination modes and that the crystal structures will vary from each other as it is difficult to predict the effects on the structures when the 5-substituted group of the tetrazole ring is replaced by an amino group, a pyridyl ring or even other tetrazole–carboxylate groups. In this paper, we will describe their syntheses, crystal structures and the influence of the 5-substituted group of the tetrazole ring as well as the luminescence properties of compounds 1 to 4.

Scheme 1 Schematic drawing for Hatza, Hpytza, H₂datza and H₃tzpha.

2. Experimental section

2.1 Materials and apparatus

The ligands were prepared according to the literature methods.⁹ Other chemicals were commercially available reagents of analytical grade and used without further purification. The elemental analyses for C, H and N were obtained on a Perkin-Elmer 2400 microanalyzer. The IR spectra were recorded (4000–400 cm⁻¹) on a NICOLET 380 spectrometer with pressed KBr pellets. The photoluminescence spectra were recorded on a Hitachi F4600 spectrofluorometer. The luminescence properties of compounds 1 to 4 were investigated at room temperature in the solid state.

2.2 Synthesis of [Zn(atza)₂(H₂O)₄] (1)

Hatza (0.0572 g, 0.4 mmol) was dissolved in 4 mL distilled water and the pH value was adjusted to 6 with KOH (0.2 M), then ZnSO₄·7H₂O (0.0576 g, 0.2 mmol) was added to the solution. The mixture was heated at 80 °C with stirring for 2 hours and then cooled to room temperature. Slow evaporation gave rise to the formation of colorless crystals of 1. For 1, yield: 35% based on Zn. Anal. calcd for C₆H₁₂N₁₀O₈Zn: C, 17.26; H, 2.90; N, 33.54%. Found: C, 17.43; H, 2.80; N, 33.76%. IR (KBr, cm⁻¹): 3320(s), 3189(s), 1638(s), 1538(s), 1498(m), 1427(m), 1398(s), 1316(m), 1280(m), 1138(w), 1116(w), 1071(w), 1005(w), 824(m), 761(w), 733(w), 697(m), 617(w).

2.3 Synthesis of [Zn(pytza)₂] (2)

Hpytza (0.0205 g, 0.1 mmol) was dissolved in 3.5 mL distilled water and the pH value was adjusted to 6 with KOH (0.2 M), then a mixture of ZnSO₄·7H₂O (0.1 mmol) and ethanol (3 mL) was added to the solution, which was sealed in a 25 mL telfon stainless steel container and heated at 120 °C for 48 h and then cooled to room temperature at a rate of 5 °C h⁻¹. Light yellow crystals of 2 were obtained. For 2, yield: 45% based on Zn. Anal. calcd for C₁₆H₁₂N₁₀O₄Zn: C, 40.56; H, 2.55; N, 29.57%. Found: C, 40.73; H, 2.50; N, 29.76%. IR (KBr, cm⁻¹): 1668(m), 1624(s), 1590(m), 1565(w), 1455(w), 1425(w), 1385(s), 1265(w), 1050(w), 727(m), 683(m).

2.4 Synthesis of [Zn(datza)(H₂O)₂]·3H₂O (3)

H₂datza (0.0538 g, 0.2 mmol) was dissolved in MeOH (5 mL) and water (2 mL), and the pH value was adjusted to 6 with KOH (0.2 M), then ZnCl₂·4H₂O (0.021 g, 0.1 mmol) was added. The mixture was stirred at 120 °C for 48 h and then cooled to room temperature at a rate of 5 °C h⁻¹, colorless block crystals of 3 were obtained. For 3, yield: 51% based on Zn. Anal. calcd for C₆H₁₅N₉O₉Zn: C, 17.05; H, 3.58; N, 29.83%. Found: C, 17.10; H, 3.50; N, 29.76%. IR (KBr, cm⁻¹): 3412(s), 1643(s), 1568(s), 1451(m), 1396(s), 1100(m), 849(m), 676(m).

2.5 Synthesis of [Zn₃(tzpha)₂(H₂O)₁₂]·MeOH·EtOH·4H₂O (4)

H₃tzpha (0.0456 g, 0.1 mmol) was dissolved in a mixture of 1 mL ethanol and 1 mL distilled water, then the pH value was adjusted to 6 with KOH (0.2 M). Then a mixture of 3 mL methanol and Zn(NO₃)₂·6H₂O (0.0634 g, 0.2 mmol) was added to the solution, which was heated at 60 °C with stirring. Then the solution was cooled to room temperature and slow evaporation gave rise to the formation of colorless crystals of 4. For 4, yield: 40% based on Zn. Anal. calcd for C₃₆H₇₀N₂₄O₃₂Zn₃: C, 27.94; H, 4.56; N, 21.73%. Found: C, 27.82; H, 4.50; N, 21.79%. IR (KBr, cm⁻¹): 3451(s), 1623(s), 1440(m), 1421(m), 1384(s), 1301(m), 1202(w), 1156(w), 1069(w), 901(w), 839(m), 749(w), 705(m), 681(m).

2.6 X-ray crystallography

Single crystal X-ray crystal data were collected on a Rigaku SCX mini CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.071073 Å). The intensity data were collected by the ω scan technique and were reduced using the Crystal-Clear program,¹⁰ and an absorption correction (multi-scan) was applied. The reflection data were also corrected for Lorentz and polarization effects. The structures were solved by direct methods and refined on F² by full matrix least squares using SHELXTL.¹¹ All the non-hydrogen atoms were located from the Fourier maps, and were refined anisotropically. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non-H atom to which they were bonded. Selected bond lengths and angles are listed in Table 1. Crystallographic data are summarized in Table S1.† Hydrogen-bonding parameters are given in Table S2.† Compounds 1–4 are deposited in the CCDC with numbers 1047132–1047135.

Table 1 Selected bond distances (Å) and angles (°) for 1–4^a

a Symmetry code: for 1: A: −x, 2 − y, 1 − z; for 2: A: 0.5 − x, 0.5 − y, −z; B: −0.5 + x, 0.5 − y, −0.5 + z; C: −x, 1 − y, z; D: x, 1 − y, −0.5 + z; E: −x, y, −0.5 − z; for 3: A: −x + 1, −y, −z; B: −x, −y, −z; for 4: A: −x, 2 − y, −z.
[Zn(atza)2(H2O)4] (1)
Zn(1)–O(1A)	2.074(2)	Zn(1)–O(1)	2.074(2)
Zn(1)–O(4)	2.128(2)	Zn(1)–O(4A)	2.128(2)
Zn(1)–O(3A)	2.138(2)	Zn(1)–O(3)	2.138(2)
O(1A)–Zn(1)–O(1A)	180.00(13)	O(1A)–Zn(1)–O(4)	92.70(10)
O(1)–Zn(1)–O(4)	87.30(10)	O(1A)–Zn(1)–O(4A)	87.30(10)
O(1)–Zn(1)–O(4A)	92.70(10)	O(4)–Zn(1)–O(4A)	180.0
O(1A)–Zn(1)–O(3A)	92.67(10)	O(1)–Zn(1)–O(3A)	87.33(10)
O(4)–Zn(1)–O(3A)	88.02(10)	O(4A)–Zn(1)–O(3A)	91.98(10)
O(1A)–Zn(1)–O(3)	87.33(10)	O(1)–Zn(1)–O(3)	92.67(10)
O(4)–Zn(1)–O(3)	91.98(10)	O(4A)–Zn(1)–O(3)	88.02(10)
O(3A)–Zn(1)–O(3)	180.000(1)
[Zn(pytza)2] (2)
Zn(1)–O(1)	2.099(3)	Zn(1)–O(2B)	2.131(3)
Zn(1)–N(5A)	2.177(3)
O(1E)–Zn(1)–O(1)	88.31(15)	O(1E)–Zn(1)–O(2D)	178.82(11)
O(1)–Zn(1)–O(2D)	90.69(15)	O(1)–Zn(1)–O(2C)	90.32(15)
O(1E)–Zn(1)–N(5A)	92.75(11)	O(1)–Zn(1)–N(5B)	83.81(11)
O(2D)–Zn(1)–N(5A)	83.81(11)	O(2C)–Zn(1)–N(5A)	87.77(11)
O(2C)–Zn(1)–N(5A)	95.61(11)	O(1E)–Zn(1)–N(5A)	180.00(9)
N(5A)–Zn(1)–N(5B)	175.22(11)
[Zn(datza)(H2O)2]·3H2O (3)
Zn(1)–O(2)	2.052(3)	Zn(1)–O(4)	2.080(3)
Zn(1)–O(5A)	2.089(5)	Zn(1)–O(3)	2.138(3)
Zn(1)–N(9B)	2.145(6)	Zn(1)–N(1B)	2.177(5)
O(2)–Zn(1)–O(4)	91.61(12)	O(2)–Zn(1)–O(5A)	91.58(17)
O(4)–Zn(1)–O(5A)	89.39(16)	O(2)–Zn(1)–O(3)	177.15(16)
O(4)–Zn(1)–O(3)	89.55(11)	O(5A)–Zn(1)–O(3)	85.83(17)
O(2)–Zn(1)–N(9B)	90.07(18)	O(4)–Zn(1)–N(9B)	174.86(18)
O(5A)–Zn(1)–N(9B)	95.4(2)	O(3)–Zn(1)–N(9B)	88.99(18)
O(2)–Zn(1)–N(1B)	94.73(17)	O(4)–Zn(1)–N(1B)	94.83(16)
O(5A)–Zn(1)–N(1B)	172.3(2)	O(3)–Zn(1)–N(1B)	87.76(17)
N(9B)–Zn(1)–N(1B)	80.2(2)
[Zn3(tzpha)2(H2O)12]·MeOH·EtOH·4H2O (4)
Zn(1)–O(8)	2.068(8)	Zn(1)–O(7)	2.071(8)
Zn(1)–O(1)	2.085(6)	Zn(1)–O(10)	2.108(7)
Zn(1)–O(9)	2.117(7)	Zn(1)–O(4A)	2.120(6)
Zn(2)–O(11A)	2.080(6)	Zn(2)–O(11)	2.080(6)
Zn(2)–O(12)	2.079(7)	Zn(2)–O(12A)	2.079(7)
Zn(2)–O(2)	2.097(6)	Zn(2)–O(2A)	2.097(6)
O(8)–Zn(1)–O(7)	87.9(4)	O(8)–Zn(1)–O(1)	89.7(3)
O(7)–Zn(1)–O(1)	96.4(3)	O(8)–Zn(1)–O(10)	175.5(3)
O(7)–Zn(1)–O(10)	87.6(3)	O(1)–Zn(1)–O(10)	91.1(3)
O(8)–Zn(1)–O(9)	96.2(3)	O(7)–Zn(1)–O(9)	175.6(3)
O(1)–Zn(1)–O(9)	85.5(3)	O(1O)–Zn(1)–N(9)	88.4(3)
O(8)–Zn(1)–O(4A)	89.3(3)	O(7)–Zn(1)–O(4A)	88.7(3)
O(1)–Zn(1)–O(4A)	174.8(3)	O(10)–Zn(1)–O(4A)	90.4(3)
O(9)–Zn(1)–O(4A)	89.5(3)	O(11A)–Zn(2)–O(11)	180.0(2)
O(11A)–Zn(2)–O(12)	87.7(3)	O(11)–Zn(2)–O(12)	92.3(3)
O(11A)–Zn(2)–O(12A)	92.3(3)	O(11)–Zn(2)–O(12A)	87.7(3)
O(12A)–Zn(2)–O(12A)	180.000(1)	O(11A)–Zn(2)–O(2)	90.6(2)
O(11)–Zn(2)–O(2)	89.4(2)	O(12)–Zn(2)–O(2)	91.5(3)
O(12A)–Zn(2)–O(2)	88.5(3)	O(11A)–Zn(2)–O(2A)	89.4(2)
O(11)–Zn(2)–O(2A)	90.6(2)	O(12)–Zn(2)–O(2A)	88.5(3)
O(12)–Zn(2)–O(2A)	91.5(3)	O(2)–Zn(2)–O(2A)	180.000(1)

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3. Results and discussions

3.1 Synthesis consideration and general characterization of compounds 1–4

Compound 1 can be prepared under either ambient or high temperature. Compound 2, however, can only be obtained under hydrothermal conditions. No compounds formed when Zn(NO₃)₂·6H₂O/Zn(ClO₄)₂·6H₂O/ZnCl₂·6H₂O/Zn(OAc)₂·6H₂O were substituted for ZnSO₄·7H₂O, showing that the temperature and counter anion are crucial to the formation of this compound. Compounds 3 and 4 can also be prepared when we change the counter anions. Compounds 1 to 4 are both air stable. The elemental analysis shows that the components of these complexes are well in accordance with the results of the structural analysis. The peaks at 3189–3451 cm⁻¹ for compounds 1, 3 and 4 can be attributed to the O–H stretching vibration of the free or coordinated water molecules while compound 2 does not show a similar peak, implying that it contains neither guest water nor coordinated water in the molecule. The characteristic bands of the carboxylate groups appeared in the usual region at 1623–1643 cm⁻¹.¹² These are correspondent to the results of the X-ray diffraction analysis.

3.2 Crystal structure of [Zn(atza)₂(H₂O)₄] (1)

The X-ray analysis reveals that compound 1 crystallizes in the triclinic lattice space group P1̄ and the asymmetric unit contains one Zn(II) ion, one atza⁻ anion, and two water molecules. As is shown in Fig. 1, each Zn(II) center is six-coordinated by four oxygen atoms from four water molecules (O3, O3A, O4, O4A) and two oxygen atoms from two carboxylate groups (O1, O1A), forming a distorted octahedral coordination arrangement. Each atza acts as a monodentate ligand to connect to one Zn(II) center, whereas the nitrogen atoms are uncoordinated, thereby giving a mononuclear structure. The Zn–O distances range from 2.074–2.138 Å, which are similar to those of other zinc compounds.¹² Adjacent mononuclear units are further held together by various hydrogen bonds to generate a 3D supramolecular network (Table S2, Fig. S1†).

Fig. 1 The mononuclear structure in compound 1, showing the coordination environment of Zn(II).

3.3 Crystal structure of [Zn(pytza)₂] (2)

Compound 2 consists of one Zn(II) ion and two pytza⁻ anions with the monoclinic lattice space group C2/c. Each Zn(II) center is in a distorted octahedral coordination arrangement surrounded by four oxygen atoms from the two carboxylate groups of the two pytza ligands (O1, O1C, O2, O2D) and two nitrogen atoms from the two pyridyl rings (N5A, N5B) of the two pytza ligands (Fig. 2). Each pytza ligand acts as a tridentate ligand to coordinate to three independent Zn(II) centers via the nitrogen atom and the carboxylate group in a μ_1,3-COO syn–syn bridging mode, forming a three dimensional network. To understand the 3D topology, the ligands are viewed as green balls (Fig. 3). The structure can be further simplified as an 8-connect (3⁶, 4¹⁶, 5⁶) tsi network topology (Fig. 4). The Zn–O bond length range from 2.099 to 2.131 Å is in good accordance with the previously reported zinc compounds,^8c,12 and so are the Zn–N distances.¹³ Compared to that of [Zn(2-pytza)₂(H₂O)₂] or [Zn(3-pytza)₂(H₂O)₂]·2H₂O where 2-pytza = 5-(2-pyridyl)tetrazole-2-acetato and 3-pytza = 5-(3-pyridyl)tetrazole-2-acetato, the structure of compound 2 is substantially distinct since 5-(2-pyridyl)tetrazole-acetato usually adopts the classic N,N′ chelating mode and 5-(3-pyridyl)tetrazole-2-acetato only acts as a bidentate ligand via the pyridine-C and one of the carboxylate-O atoms. Therefore, compound 2 is a three-dimensional network rather than a mononuclear one or a 1D chain. In contrast with compound 1, when the amino group is replaced by the pyridine ring which can act as a bridging ligand, the coordination site of pytza increases and the structure transfers from a mononuclear one to a three dimensional one. It is worthwhile to point out that compound 2 contains neither coordinated molecules nor guest solvent ones, which differs from the other three compounds in this article. Non-classic hydrogen bonds exist between the C–H group of the pyridyl group and the oxygen atom of the carboxylate group [C(6)–H(6)⋯O(1) 2.48 Å/124°, C(7)–H(7)⋯O(2) 2.40 Å/123°] to stabilize the supramolecular assembly (Table S2†).

Fig. 2 The coordination environment of Zn(II) in compound 2. Hydrogen atoms are omitted for clarity.

Fig. 3 The pyridine and tetrazole rings are simplified as green balls.

Fig. 4 The 3D 8-connect (3⁶, 4¹⁶, 5⁶) tsi network of compound 2.

3.4 Crystal structure of [Zn(datza)(H₂O)₂]·3H₂O (3)

Compound 3 consists of one Zn(II) anion, one datza²⁻ ligand, two coordinated water molecules and three uncoordinated water molecules in a crystallographically independent asymmetric unit, with the triclinic space group P1̄. Each Zn(II) center is in a distorted octahedral coordination arrangement whose coordination sites are occupied by two tetrazole nitrogen atoms (N1B, N9B), two carboxylate oxygen atoms (O2, O5A) and two water molecules (O3, O4) (Fig. 5). The Zn–N bond distances are 2.144–2.174 Å and Zn–O distances fall in the range of 2.053–2.138 Å, which are similar to those of reported zinc complexes.¹³ Each datza²⁻ acts as a tetradentate ligand via two nitrogen atoms of the two tetrazolyl rings in a chelating mode (N1, N9) and two oxygen atoms of the carboxylate group in a bridging mode (O1, O5). Two such ligands set up a double bridge between neighboring Zn(II) ions, forming a 1D extension along the a axis with a Zn⋯Zn distance of 5.945(2) Å. When the amino group is replaced by the tetrazole–carboxylate group, the datza adopts a N(tetrazole),N′(tetrazole) chelating mode, and bridges two further Zn(II) centers via the carboxylates to form a ladder-like chain. Adjacent 1D chains are further connected by O–H⋯O, C–H⋯N and N–H⋯O hydrogen bonds to generate a 3D network (Table S2, Fig. S2†). The 3D net topology contains rhomboidal grids with the diagonal lengths of the rhomboidal grids being 13.5346(37) Å.

Fig. 5 The coordination environment of Zn(II) in compound 3. Hydrogen atoms are omitted for clarity.

3.5 Crystal structure of [Zn₃(tzpha)₂(H₂O)₁₂]·MeOH·EtOH·4H₂O (4)

The X-ray analysis reveals that compound 4 crystallizes in the triclinic lattice space group P1̄. As shown in Fig. 6, each Zn(II) ion is six-coordinated by two oxygen atoms from carboxylate groups from two independent tzpha³⁻ ligands and four oxygen atoms from four water molecules. The Zn–O distances range from 2.068 to 2.120 Å,¹³ so the coordination arrangement can be described as a slightly distorted octahedron. Each tzpha³⁻ acts as a tridentate ligand to connect to three Zn(II) centers via the two O atoms of one carboxylate in a μ_1,3-COO syn–syn bridging mode and via one carboxylate-O atom from another carboxylate group in a monodentate mode, whereas the third carboxylate group is uncoordinated, therefore, compound 4 displays a one dimensional chain extending along the c axis (Fig. 7a). When the amino group is replaced by the bitetrazole–carboxylate benzene, only the carboxylate-O atoms are coordinated to the Zn(II) center, whereas the tetrazole rings are uncoordinated. The structure can be simplified as a beaded chain and the diagonal lengths of the rhomboidal grids are 15.6425 and 15.685 Å, respectively (Fig. 7b). Adjacent 1D chains are further held together by various hydrogen bonds to generate a three dimensional supramolecular network (Table S2, Fig. S3†).

Fig. 6 The coordination environment of Zn(II) in compound 4. Hydrogen atoms are omitted for clarity.

Fig. 7 The 1D chain structure of compound 4 extending along the c axis. Hydrogen atoms are omitted for clarity.

3.6 Discussion of coordination modes and structures

In this work, we have selected four new tetrazole–carboxylate ligands to react with zinc salts in order to investigate the influence of the different 5-substituted groups of the tetrazole ring on the crystal structures of the resulting complexes. Four new coordination compounds based on Hatza, Hpytza, H₂datza, H₃tzpha have been successfully constructed. For compound 1, atza acts only as a monodentate ligand via one of the carboxylate oxygen atoms, the structure is a simple zero dimensional mononuclear one. However, the amino group can form various strong N–H⋯O hydrogen bonds to stabilize the supramolecular assembly. When the amino group is replaced by the pyridine ring, the coordination sites increase in that both the pyridine-N and carboxylate-O atoms participate in the coordination, therefore, the structure changes to an 8-connect (3⁶, 4¹⁶, 5⁶) tsi network topology while the number of hydrogen bonds decreases sharply simultaneously. When the number of tetrazolyl rings and carboxylate groups increases from one to two, another coordination mode of datza has been found in compound 3, where datza²⁻ not only adopts a classic N(tetrazole),N′(tetrazole) chelating mode but also acts as a bridging ligand, and the number of coordination sites adds up to 4. Compound 3, however, becomes a one dimensional ladder-like chain since only one oxygen atom of the carboxylate group is coordinated to the Zn(II) center, the others being uncoordinated. Theoretically speaking, H₃tzpha with three tetrazole rings and carboxylate groups, should have the most coordination sites, but it adopts a relatively simple coordination mode. This may be explained by stereo effects. Therefore, compound 4 displays a 1D beaded chain which is different from that of compound 3 (a 1D ladder-like chain) (Scheme 2).

Scheme 2 The influence of different substituted group of the tetrazole ring on the structure of compounds 1–4.

4. Luminescence properties

The fluorescence properties of inorganic–organic hybrid coordination polymers, especially those based on d¹⁰ metal centers, have been investigated for potential applications as fluorescence-emitting materials.¹⁴ The luminescence of compounds 1–4 and the free ligands were investigated at room temperature in the solid state. 5-aminotetrazole-1-acetic acid, 5-(4-pyridyl)tetrazole-2-acetato potassium salt, N,N-di(tetrazol-5-yl)amine-N2,N2′-diacetate and 1,3,5-tris(tetrazol-5-yl)benzene-N2,N2′,N2′′-trisacetic acid exhibit photoluminescence with maximum intensity at 445, 429, 401 and 422 nm upon excitation at 308, 370, 307 and 372 nm, respectively. Compounds 1–4 show maximum emission at 469, 442, 423 and 417 nm upon excitation at 360, 370, 313 and 380 nm, respectively (Fig. 8). Compared to the corresponding ligands, a red shift of 24 nm for 1, 13 nm for 2, 22 nm for 3, and a blue shift of 5 nm for 4 have been observed, respectively, which are well in accordance with those of the previously reported zinc complexes.¹² Therefore, these emissions can be ascribed to ligand centered luminescence. Generally, the intraligand fluorescence emission wavelength is determined by the energy gap between the π and π* molecular orbitals of the free ligand, which is related to the extent of π conjugation.¹² Compounds 1–4 show even stronger emission because the coordination of Zn(II) further enlarges the conjugated system to enhance the luminescence.

Fig. 8 The emission spectra of Hatza, Hpytza, H₂datza, H₃tzpha and compounds 1 to 4 at room temperature in the solid state.

5. Conclusions

In summary, we are the first to report the four novel zinc coordination compounds based on Hatza, Hpytza, H₂datza and H₃tzpha. The structures are controlled not only by the number and different coordination modes of the tetrazole–carboxylates but also by the complementary hydrogen bonds. These compounds show outstanding luminescence properties, compared to the ligands. Our research results indicate that as promising ligands, Hatza, Hpytza, H₂datza and H₃tzpha have great potentials in the field of coordination compounds and further explorations are underway in our work group.

Acknowledgements

The authors acknowledge financial support from the Natural Science Foundation of Jiangsu Province (Grant no. BK2012210), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant no. 10KJB430001) and the Opening Fund of Jiangsu Key Laboratory of Advanced Functional Materials (Grant no. 12KFJJ010).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1047132–1047135. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra03848k

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