Diverse mixed-ligand metal complexes with in situ generated 5-(pyrazinyl)tetrazolate chelating-bridging ligand: in situ synthesis, crystal structures, magnetic and luminescent properties

Abstract

The purpose of this work is to systematically investigate the influence of the functional co-ligands and metal ions on the self-assembly and properties of the in situ generated 5-(pyrazinyl)tetrazolate (ptz⁻)-based metal complexes. A series of nine metal complexes, {[Co₃(H₂O)₄(ptz)₂(btec)]·5H₂O}_n (1), [Cu₂(H₂O)₄(ptz)₂(H₂btec)]_n (2), [Hg₂(ptz)₂Cl₂]_n (3), [Hg₃(H₂O)₂(ptz)₄(μ_1,1-N₃)₂]·2H₂O (4), Hg(H₂O)₂(ptz)₂ (5), [Cd₂(H₂O)₂(ptz)₂(ox)]_n (6), [Cd(H₂O)(ptz)I]_n (7), {[Cd(H₂O)(ptz)X]·H₂O}_n (X = Br for 8 and Cl for 9) (btec = 1,2,4,5–benzenetetracarboxylate and ox = oxalate), was obtained by a subtle variation of co-ligands and metal ions, in which the Hg^II ion is firstly observed as an efficient Lewis acid to catalyze the in situ reaction. Significantly, resulted from the cooperative and/or competitive binding between the mixed ligands and the metal ions with different coordination geometry, complexes 1–9 feature interesting structural subunits (mono-, bi- and tri-metallic core) and various dimensionalities ranged from polymeric three-dimensional (3D) frameworks to discrete zero-dimensional (0D) entities. Three high-dimensional complexes with a binodal (3,8)-connected tfz-d microporous framework for 1, a trinodal (3,4)-connected unusual (6·8²)₂(6·8³·10²) topology for 3, and a six-connected α-polonium-type network for 6 are observed, respectively. Complexes 2 and 7–9 are bi-metallic node-based 2D layers (7) and 1D chains (2, 8, and 9). In contrast, the rarely linear tri-metallic core and commonly observed mononuclear entities are generated for 4 and 5, respectively. Additionally, Cu^II-based polymer 2 displays weak intra-dimeric antiferromagnetic coupling interaction. Complexes 1 and 3–9 with considerable metal ion-dependent thermal stability exhibit ptz⁻-based photoluminescence with variable intensity.

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Introduction

Self-assembled frameworks containing metal ions and mixed organic bridging ligands are of great current interest due to their unexpected coordination architectures and unique functions.¹ Up to now, the exactly structural predictions for the complexes bearing polydentate N- and O-containing ligands and metal ions with variable coordination geometry are extremely difficult and challenging, because the overall structures of the target complexes highly depend on the nature of the metal ions and/or the ligands, the synthetic conditions as well as the preparation methods. In this regard, polydentate 5-substituted-1H-tetrazolate-based ligands generated by in situ reactions, such as pyrazinyl-,^2–5 pyrimidyl-,^6–7 position isomeric pyridyl-,^8–10 phenyl-,^11–13 amino-,¹⁴ have been extensively employed as excellent chelating and/or bridging linkers to construct novel metal complexes with interesting optical properties, due to their diverse connectivity modes and high structural stability. For example, a series of metal-directed (Zn^II,^2, 4 Cd^II,² Cu^II,^2–3 Ag^I,² Co^II,³ and Mn^II 4) binary supramolecular architectures bearing the in situ generated 5-(pyrazinyl)tetrazolate (ptz⁻) ligand have been recently reported, indicating that the different metal ions upon the self assembly process help to build diverse coordination geometry, while the hydrogen bonding and other supramolecular interactions are responsible for the ordered extension of the architecture dimensionality. More recently, Bu and co-workers¹⁵ have explored the assembly behaviour of group IIB metal ions with the pre-synthesized ptz ligand as well as the emission properties of the resulting complexes, finding the luminescence and structural dependence on the metal ions and preparing conditions. Herein, as the continuing investigations on the influence of the co-ligand and metal ions on the structures and properties of polyazolate-based metal complexes,^16–21 we report the synthesis and optical/magnetic properties of a series of nine co-ligand-assisted ternary ptz⁻-based metal complexes. It is well known that the presence of functional co-ligand can significantly produce multiple effects on the core ligand in addition to the metal coordination sphere, which ultimately govern the overall structures of the mixed-ligand complexes. As a result, a series of nine co-ligand-regulated ternary ptz⁻-based metal complexes with interesting subunits (mono-, bi- and tri-metallic core) and various dimensions (0D, 1D, 2D and 3D) was generated, which includes three high-dimensional frameworks with a (3,8)-connected tfz-d net for 1, a trinodal (3,4)-connected unusual (6·8²)₂(6·8³·10²) topology for 3, and a six-connected α-polonium-type network for 6, one bimetallic node-based 2D layer (7), three bimetallic node-based 1D chains (2, 8, and 9), one unusual linear trimetallic core for 4, and commonly observed mononuclear entities for 5, respectively. Undoubtedly, such interesting structural diversity is significantly due to the spontaneous combinations of the variable coordination modes between the core ptz⁻ ligand and the specific co-ligands with either bridging or terminal function. Besides, we observed for the first time that Hg^II ion can be used as effective Lewis acid catalyst to construct the in situ-generated coordination framework. Thus, the Lewis acid catalysts available in in situ reaction cover Zn^II, Cu^I/II, Ag^I, Co^II, Cd^II, Ni^II, Mn^II, Fe^II and Hg^II salts. On the other hand, resulted from the nature of the metal ion and ptz⁻ ligands, the resulting complexes display favorable emissions for 1 and 3–9 and weak antiferromagnetic coupling interaction for 2.

Experimental

Materials and methods

All reagents were commercially purchased (2-cyanopyrazine was from TCI and other analytical-grade reagents were from Tianjin Chemical Reagent Factory) and used without further purification. Doubly deionized water was used for the conventional synthesis. Elemental analyses for C, H and N were carried out with a CE-440 (Leeman Labs) analyzer. Fourier transform (FT) IR spectra (KBr pellets) were taken on an Avatar-370 (Nicolet) spectrometer in the range of 4000–400 cm⁻¹. Thermogravimetric analysis (TGA) experiments were carried out on Shimadzu simultaneous DTG-60A compositional analysis instrument from room temperature to 800 °C under N₂ atmosphere at a heating rate of 5 °C min⁻¹. Fluorescence spectra of the polycrystalline powder samples were performed on a Cary Eclipse fluorescence spectrophotometer (Varian) equipped with a xenon lamp and quartz carrier at room temperature. Variable-temperature susceptibility measurement of 2 was carried out at an applied DC field of 1000 Oe from 4 to 300 K temperature range on a Quantum Design (SQUID) magnetometer MPMS-XL-7.

Preparation of complexes 1–9

{[Co₃(H₂O)₄(ptz)₂(btec)]·5H₂O}_n (1). CoCl₂·6H₂O (47.6 mg, 0.2 mmol), H₄btec (25.4 mg, 0.1 mmol), 2-cyanopyrazine (10.5 mg, 0.1 mmol), NaN₃ (13.0 mg, 0.2 mmol) and water (10 mL) were sealed in a parr Teflon-lined stainless steel vessel (23 mL) and heated to 160 °C for 96 h under autogenous pressure. Orange block-shaped crystals suitable for X-ray analysis were obtained directly, washed with ethanol, and dried in air. Yield: 72% based on 2-cyanopyrazine. Anal. calcd for C₂₀H₂₆Co₃N₁₂O₁₇: C 27.20, H 2.97, N 19.03%. Found: C 27.14, H 3.02, N 19.15%. IR (KBr, ν/cm⁻¹): 3406 (br), 3102 (w), 1600 (s), 1572 (vs), 1485 (m), 1415 (s), 1370 (vs), 1318 (w), 1280 (w), 1170 (w), 1149 (w), 1074 (w), 1045 (m), 938 (w), 870 (m), 810 (w), 770 (w), 580 (m), 541 (m), 433 (m).

[Cu₂(H₂O)₄(ptz)₂(H₂btec)]_n (2). Hydrothermal reaction of CuCl₂·2H₂O (34.1 mg, 0.2 mmol), H₄btec (25.4 mg, 0.1 mmol), 2-cyanopyrazine (10.5 mg, 0.1 mmol), NaN₃ (13.0 mg, 0.2 mmol) and water (10 mL) at 100 °C for two days generated blue block-shaped crystals of 2. Yield: 62% based on 2-cyanopyrazine. Anal. calcd for C₁₀H₉N₆CuO₆: C 32.22, H 2.43, N 22.55%. Found: C 32.45, H 2.24, N 22.43%. IR (KBr, ν/cm⁻¹): 3425 (br), 3097 (w), 1712 (ms), 1631 (s), 1564 (s), 1502 (w), 1425 (s), 1382 (ms), 1346 (w), 1295 (w), 1222 (ms), 1171 (m), 1116 (ms), 1073 (m), 1046 (m), 945 (w), 862 (w), 829 (w), 592 (w), 530 (w), 445 (w).

[Hg₂(ptz)₂Cl₂]_n (3). To an aqueous solution (6 mL) containing NaN₃ (13.0 mg, 0.2 mmol) and HgCl₂·0.5H₂O (54.3 mg, 0.2 mmol) was slowly added an ethanol solution (6 mL) of 2-cyanopyrazine (10.5 mg, 0.1 mmol) and H₄btec (12.7 mg, 0.05 mmol) with constant stirring. After further stirring for one hour, the resulting mixture was filtered. Colourless strip-shaped crystals suitable for X-ray analysis were obtained by evaporation of the filtrate in one week, washed with ethanol, and dried in air. Yield: 43% based on 2-cyanopyrazine. Anal. calcd for C₁₀H₆Cl₂Hg₂N₁₂: C 15.67, H 0.79, N 21.93%. Found: C 15.70, H 0.84, N 21.77%. IR (KBr, ν/cm⁻¹): 3073 (w), 1421 (s), 1400 (ms), 1375 (w), 1294 (w), 1170 (ms), 1116 (w), 1095 (w), 1063 (w), 1025 (s), 862 (m), 762 (m), 577 (w), 514 (w).

[Hg₃(H₂O)₂(ptz)₄(μ_1,1-N₃)₂]·2H₂O (4). A methanol solution (6 mL) containing 2-cyanopyrazine (10.5 mg, 0.1 mmol), H₄btec (12.7 mg, 0.05 mmol) and NaN₃ (13.0 mg, 0.2 mmol) was carefully layered onto a buffer layer of ethyl acetate (2 mL) in a straight glass tube, below which an aqueous solution of Hg(NO₃)₂·H₂O (33.0 mg, 0.1 mmol) was placed. Upon slow evaporation of the mixture at room temperature, colourless block-shaped crystals suitable for X-ray analysis were obtained on the wall of the tube within two weeks, washed with ethanol and water and dried in air. Yield: 26% based on 2-cyanopyrazine. Anal. calcd for C₁₀H₁₀Hg_1.5N₁₅O₂: C 17.84, H 1.50, N 31.21%. Found: C 17.70, H 1.74, N 31.07%. IR (KBr, ν/cm⁻¹): 3457 (br), 3063 (w), 2040 (s), 1624 (ms), 1529 (w), 1448 (w), 1398 (m), 1357 (w), 1169 (w), 1136 (m), 1057 (w), 1021 (m), 861 (w), 764 (w), 637 (w), 522 (w), 460 (w).

Hg(H₂O)₂(ptz)₂ (5). Complex 5 was prepared by adopting the same synthetic procedures as 3 only increasing the amount of NaN₃ to 0.3 mmol (19.5 mg). Colourless block-shaped crystals suitable for X-ray analysis were obtained by evaporation of the filtrate in one week, washed with ethanol, and dried in air. Yield: 22% based on 2-cyanopyrazine. Anal. calcd for C₅H₅Hg_0.5N₆O: C 22.63, H 1.90, N 31.66%. Found: C 22.41, H 1.87, N 31.75%. IR (KBr, ν/cm⁻¹): 3427 (br), 3059 (w), 1656 (w), 1562 (w), 1528 (w), 1398 (vs), 1356 (w), 1171 (m), 1138 (s), 1057 (ms), 1022 (s), 864 (m), 766 (w), 522 (w).

[Cd₂(H₂O)₂(ptz)₂(ox)]_n (6). Complex 6 was prepared by the same procedures as 1 except that CoCl₂·6H₂O and H₄btec were replaced by CdCl₂·2.5H₂O (45.7 mg, 0.2 mmol) and H₂ox·2H₂O (12.7 mg, 0.1 mmol), respectively. Colourless block-shaped crystals suitable for X-ray analysis were obtained directly, washed with ethanol, and dried in air. Yield: 64% based on 2-cyanopyrazine. Anal. calcd for C₆H₅CdN₆O₃: C 22.41, H 1.57, N 26.14%. Found: C 22.45, H 1.49, N 26.17%. IR (KBr, ν/cm⁻¹): 3322 (br), 3105 (w), 1619 (vs), 1465 (w), 1415 (s), 1377 (m), 1308 (s), 1171 (w), 1154 (m), 1133 (w), 1075 (w), 1047 (m), 1027 (w), 858 (w), 784 (m), 757 (w), 698 (w), 635 (w), 521 (m), 480 (w), 438 (w).

[Cd(H₂O)(ptz)I]_n (7). CdI₂·4H₂O (73.2 mg, 0.2 mmol), 2-cyanopyrazine (10.5 mg, 0.1 mmol), NaN₃ (26.0 mg, 0.4 mmol), and water (10 mL) were sealed in a parr Teflon-lined stainless steel vessel (23 mL) and heated to 160 °C for 72 h under autogenous pressure. Pale-yellow block-shaped crystals suitable for X-ray analysis were obtained directly, washed with ethanol, and dried in air. Yield: 39% based on 2-cyanopyrazine. Anal. calcd for C₅H₅CdIN₆O: C 14.85, H 1.25, N 20.78%. Found: C 14.74, H 1.30, N 20.41%. IR (KBr, ν/cm⁻¹): 3465 (br), 3087 (w), 1641 (m), 1536 (w), 1459 (w), 1408 (s), 1376 (m), 1287 (w), 1233 (w), 1165 (ms), 1073 (ms), 1041 (s), 852 (w), 762 (m), 603 (m), 523 (m), 417 (m).

{[Cd(H₂O)(ptz)Br]·H₂O}_n (8). Complex 8 was prepared by adopting the same procedures as 7 only using CdBr₂·4H₂O (68.8 mg, 0.2 mmol) instead of CdI₂·4H₂O. Colourless block-shaped crystals suitable for X-ray analysis were obtained directly, washed with ethanol, and dried in air. Yield: 47% based on 2-cyanopyrazine. Anal. calcd for C₅H₇BrCdN₆O₂: C 16.00, H 1.88, N 22.38%. Found: C 16.16, H 1.90, N 22.42%. IR (KBr, ν/cm⁻¹): 3201 (br), 3076 (w), 2074 (s), 1623 (m), 1559 (m), 1408 (s), 1375 (w), 1342 (w), 1280 (w), 1167 (m), 1147 (ms), 1070 (m), 1039 (ms), 867 (w), 761 (w), 663 (w), 510 (w), 479 (w), 422 (w).

{[Cd(H₂O)(ptz)Cl]·H₂O}_n (9). CdCl₂·2.5H₂O (45.7 mg, 0.2 mmol), 2-cyanopyrazine (10.5 mg, 0.1 mmol), NaN₃ (26.0 mg, 0.4 mmol) and water (10 mL) were sealed in a parr Teflon-lined stainless steel vessel (23 mL) and heated to 160 °C for 72 h under autogenous pressure. After cooling to room temperature, the mixture was filtered. Colourless block-shaped crystals suitable for X-ray analysis were obtained by evaporation of the filtrate within two weeks, washed with ethanol, and dried in air. Yield: 51% based on 2-cyanopyrazine. Anal. calcd for C₅H₇CdClN₆O₂: C 18.14, H 2.13, N 25.39%. Found: C 18.16, H 2.08, N 25.42%. IR (KBr, ν/cm⁻¹): 3218 (br), 3081 (w), 2076 (s), 1626 (m), 1559 (m), 1409 (s), 1376 (w), 1341 (w), 1282 (w), 1168 (m), 1147 (ms), 1071 (m), 1039 (ms), 869 (w), 760 (w), 668 (w), 510 (w), 479 (w), 422 (w).

X-Ray crystallography

Diffraction intensities for complexes 1–9 were collected using a Bruker APEX–II CCD diffractometer equipped with graphite-monochromated Mo-Kα radiation with radiation wavelength 0.71073 Å by using the φ–ω scan technique at 296(2) K. There was no evidence of crystal decay during data collection. Semiempirical absorption corrections were applied (SADABS), and the program SAINT was used for integration of the diffraction profiles.²² The structures were solved by direct methods and refined with the full–matrix least–squares technique using the SHELXS-97 and SHELXL-97 programs.²³ Anisotropic thermal parameters were assigned to all non-hydrogen atoms. Split occupancy of 0.5 was applied to the disordered lattice water molecule O(9) of 1. And one coordinated water molecule (O6) in 2 was found disordered over two positions and refined with the sum of the occupation factors restrained to one (0.765 for O(6) and 0.235 for O(6′), respectively). The organic hydrogen atoms were generated geometrically. The H atoms of the water molecules except for the splitting water were located from difference maps and refined with isotropic temperature factors. The crystallographic data and selected bond lengths and angles for 1–9 were listed in Tables 1–5, respectively.

Table 1 Crystal and structure refinement data for 1–9^a

	1	2	3	4	5	6	7	8	9
a R 1 = Σ‖Fo| − |Fc‖/|Fo|. b wR2 = [Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)^2]^1/2.
Empirical formula	C20H26Co3N12O17	C10H9CuN6O6	C10H6Cl2Hg2N12	C10H10Hg1.5N15O2	C5H5Hg0.5N6O	C6H5CdN6O3	C5H5CdIN6O	C5H7BrCdN6O2	C5H7CdClN6O2
FW/g mol^−1	883.32	372.77	766.35	673.22	265.45	321.56	404.45	375.48	331.02
Crystal system	Triclinic	Triclinic	Orthorhombic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P1̄	P1̄	Pbca	P21/c	P21/c	P21/n	P21/c	P21/n	P21/c
Crystal size/mm	0.24 × 0.23 × 0.22	0.18 × 0.14 × 0.13	0.20 × 0.16 × 0.15	0.22 × 0.20 × 0.18	0.20 × 0.18 × 0.15	0.26 × 0.23 × 0.22	0.22 × 0.20 × 0.17	0.22 × 0.20 × 0.18	0.23 × 0.22 × 0.20
a/Å	8.2152(4)	7.8365(10)	16.1407(6)	7.2819(6)	5.5940(9)	5.8768(5)	7.276(2)	7.4195(11)	7.3629(3)
b/Å	8.9387(4)	8.2326(11)	6.9435(2)	17.4904(16)	12.643(2)	13.1226(12)	10.159(3)	13.9422(19)	13.9028(6)
c/Å	12.3961(5)	10.7944(15)	30.3299(11)	14.8161(10)	11.2431(16)	11.5664(11)	16.055(5)	9.9089(14)	11.8372(4)
α/°	109.8460(10)	111.154(2)	90.00	90.00	90	90	90	90	90
β/°	92.6510(10)	95.728(2)	90.00	115.224(3)	106.662(7)	94.7880(10)	112.545(11)	95.093(2)	123.425(2)
γ/°	106.7250(10)	94.674(2)	90.00	90.00	90	90	90	90	90
V/Å^3	809.46(6)	641.03(15)	3399.2(2)	1707.1(2)	761.8(2)	888.88(14)	1096.0(6)	1021.0(3)	1011.31(7)
Z	1	2	8	4	4	4	4	4	4
θ range/°	1.77–25.01	2.04 – 25.00	1.34–25.01	1.91–25.01	2.48–25.01	2.35–25.00	2.43–25.00	2.92–25.01	3.14–25.00
μ/mm^−1	1.614	1.751	18.383	13.539	10.140	2.460	4.788	6.043	2.414
F(000)	447	376	2752	1244	500	620	744	712	640
Limiting indices	−9 ≤ h ≤ 9	−6 ≤ h ≤ 9	−19 ≤ h ≤ 10	−8 ≤ h ≤ 8	−6 ≤ h ≤ 6	−6 ≤ h ≤ 6	8 ≤ h ≤ 8	−8 ≤ h ≤ 8	−8 ≤ h ≤ 8
	−10 ≤ k ≤ 10	−9 ≤ k ≤ 9	−8 ≤ k ≤ 7	−20 ≤ k ≤ 20	−15 ≤ k ≤ 13	−8 ≤ k ≤ 15	12 ≤ k ≤ 6	−13 ≤ k ≤ 16	−16 ≤ k ≤ 12
	−14 ≤ l ≤ 1	−12 ≤ l ≤ 12	−31 ≤ l ≤ 36	−17 ≤ l ≤ 13	−13 ≤ l ≤ 9	−13 ≤ l ≤ 13	19 ≤ l ≤ 19	−11 ≤ l ≤ 9	−11 ≤ l ≤ 14
Data/restraints/parameters	2823/0/241	2235/0/213	2958/0/235	2965/0/259	1333/0/115	1556/0/145	1935/0/128	1760/0/136	1757/0/136
GOF	1.023	1.036	1.050	1.060	1.052	1.063	1.053	1.055	1.076
R int	0.0107	0.0111	0.0552	0.0554	0.0340	0.0142	0.0268	0.0163	0.0213
R 1 ^a, wR2^b [I > 2σ(I)]	0.0280/0.0725	0.0305/0.0722	0.0336/0.0859	0.0282/0.0710	0.0326/0.0870	0.01570.0396	0.02020.0525	0.04110.1052	0.03550.0775
R 1, wR2 [all data]	0.0310/0.0743	0.0390/0.0774	0.0373/0.0881	0.0327/0.0734	0.0389/0.0931	0.0164/0.0400	0.0221/0.0533	0.0430/0.1063	0.0361/0.0778
Residuals/e Å^−3	0.447 and −0.534	0.447 and −0.497	1.995 and −1.740	1.114 and −1.840	0.712 and −1.999	0.264 and −0.525	0.603 and −0.440	1.112 and −1.292	0.926 and −0.778

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Results and discussion

Syntheses and IR spectra

In order to explore the influence of co-ligands and metal ions on the in situ generated metal complexes, in situ reactions of 2-cyanopyrazine and sodium azide in the presence of Cu²⁺, Co²⁺, Ni²⁺,²⁴ Hg²⁺, Cd²⁺ and aromatic/aliphatic acid (H₄btec and H₂ox) as well as halide anions (Cl⁻, Br⁻, and I⁻) were performed under suitable reaction conditions (see Scheme 1) and preparation methods (hydrothermal, evaporation and diffusion). As a result, complexes 1 and 2 with the same mixed ligands but different metal centers were obtained by changing the hydrothermal temperature (160 °C vs. 100 °C). Unexpectedly, the different reaction temperature essentially led to two different deprotonated forms of the H₄btec co-ligand in the two complexes. The three Hg^II-ptz⁻-based complexes, 3–5, were firstly generated by in situ reactions under mild reaction conditions.¹⁵ Thus, the effective Lewis acid catalysts in the in situ reaction are extensively extended from Zn^II to Cd^II, Mn^II, Cu^I/II, Ni^II, Co^II, Ag^I, Fe^II and Hg^II ions. Notably, the stoichiometric H₄btec species in preparation of 3–5 prefers to adjust the pH value of the medium, rather than acting as a co-ligand. Additionally, the molar ratio of NaN₃ to Hg^II salts was observed to significantly tune the final structure of the complexes. For example, high-dimensional framework of 3 can be easily transformed into the discrete mononuclear 5 by increasing the molar ratio from 1 : 1 (NaN₃ : Hg^II) to 1.5 : 1. And upon further increment of the molar ratio up to 2 : 1, complex 4 with unusual trinuclear motif can be obtained, although the preparation method of the three complexes is slightly different from each other (evaporation for 3 and 5vs. diffusion for 4). On the other hand, hydrothermal reactions at 160 °C afforded four Cd^II-based mixed-ligand complexes with short aliphatic carboxylate for 6 and comparable halide anions (I⁻, Br⁻ and Cl⁻) for 7–9, respectively. Therefore, the suitable temperature in the ptz⁻−based in situ reaction plays much more important than other possible factors that influenced the formation of the target complexes.

Scheme 1 Preparation details for complexes 1–9.

In the FT-IR spectra, the strong bands appeared above 3200 cm⁻¹ for all the complexes but 3 should be ascribed to the stretching vibrations of O–H, suggesting the presence of free and/or coordinated water molecule.²⁵ The characteristic bands for weak aromatic C–H vibrations were located at ca. 3100 cm⁻¹ and the skeletal vibrations of the aromatic ring were observed in the 1400–1626 cm⁻¹ region for 1–9. Additionally, the absence of the bands at 2200 cm⁻¹ for stretching vibrations of cyano group, together with the new bands emerged at 1400–1500 cm⁻¹ for the stretching vibrations of a tetrazole group, jointly suggested the formation of the ptz⁻ ligand.² For the three complexes with the organic acid co-ligand, the absence of characteristic band at ca. 1700 cm⁻¹ in both 1 and 6 indicates the complete deprotonation of the carboxylic acid; while the appearance of the peak at 1712 cm⁻¹ in 2 suggested the existence of protonated –COOH. Correspondingly, the asymmetric (ν_as) and symmetric vibrations (ν_s) of the carboxylate group were observed at 1600, 1572 and 1415, 1370 cm⁻¹ for 1, 1564, 1631, 1564 and 1425, 1382 cm⁻¹for 2, 1619 and 1415 cm⁻¹ for 6, respectively. Their differences (Δv) between v_as(COO⁻) and v_s(COO⁻) suggested the potentially variable binding modes of the carboxylate groups.²⁵ For 4, the strong band at 2040 cm⁻¹ for the asymmetric stretching of N₃⁻, together with the medium bands locating at 1169 and 1357 cm⁻¹for the v_s(N₃⁻) stretching confirmed the existence of the end-to-on bridging mode of azide anion.^26–28 Thus, the IR results were in agreement with the single-crystal X-ray diffractions.

Crystal structures for 1–9

{[Co₃(H₂O)₄(ptz)₂(btec)]·5H₂O}_n (1). Complex 1 crystallizes in the triclinic P1̄ space group, displays a robust 3D tfz-d framework built from unusual linear trinuclear [Co₃(H₂O)₄(btec)]²⁺ cations and a pair of ptz⁻ linkers. As shown in Fig. 1a, the trinuclear [Co₃(H₂O)₄(btec)]²⁺ subunit is centrosymmetric, consisting of three separate Co^II atoms with Co1 locating at the crystallographic inversion center, one fully deprotonated btec⁴⁻ anion with another inversion center, and two pairs of coordinated water molecules. Both crystallographically independent Co^II atoms have distorted octahedral geometry (see ESI, Fig. S1),† in which Co1 is equatorially surrounded by four carboxylate O atoms from two different btec⁴⁻ ligands and axially occupied by two pyrazinyl N atoms from two separate ptz⁻ anions. Instead, Co2 is coordinated by three N atoms from two ptz⁻ ligands, one carboxylate O donor of btec⁴⁻ anion and two coordinated water molecules, respectively. The bond distances of Co–O are generally shorter than those of Co–N (see Table 2), although they are comparable to those previously reported values.²⁹ Co1, symmetry-related Co2 and Co2A (symmetry code: A = −x, 1 − y, 1 − z) are doubly held together by two carboxylate groups of btec⁴⁻ in bidentate bridging and monodentate modes to generate a linear trinuclear [Co₃(H₂O)₄(btec)]²⁺ subunit bearing four terminal water molecules (see Fig. 1a). The Co⋯Co separation within the trinuclear subunit is 5.0432(2) Å, and the bond angle of Co2–Co1–Co2A is 180°.

Fig. 1 (a) The trinuclear Co^II subunit of 1 (the centrosymmetric btec⁴⁻ ligands are shaded differently, and H atoms are omitted for clarity; Symmetry codes: A = −x, 1 − y, 1 − z; B = 1 − x, −y, 1 − z). (b) 2D layer of 1. (c) 3D framework of 1 and its binodal (3,8)-connected tfz-d topological network.

Table 2 Selected bond lengths (Å) and angles (°) for 1^a

a Symmetry codes: for 1: #1 −x + 1, −y + 1, −z + 1; #2 −x, −y + 1, −z + 1; #3 −x, −y + 2, −z + 2.
Co(1)–O(2)	2.0321(16)	Co(2)–O(5)	2.071(2)
Co(1)–O(3)	2.0594(17)	Co(2)–O(4)^#2	2.0762(17)
Co(1)–N(6)	2.207(2)	Co(2)–N(2)^#3	2.110(2)
Co(2)–N(5)	2.207(2)	Co(2)–N(1)	2.145(2)
Co(2)–O(6)	2.069(2)
O(2)–Co(1)–O(2)^#1	180.0	O(6)–Co(2)–N(2)^#3	92.02(9)
O(2)–Co(1)–O(3)	90.84(7)	O(5)–Co(2)–N(2)^#3	89.78(9)
O(2)^#1–Co(1)–O(3)	89.16(7)	O(4)^#2–Co(2)–N(2)^#3	91.96(8)
O(2)^#1–Co(1)–O(3)^#1	90.83(7)	O(6)–Co(2)–N(1)	87.73(8)
O(2)–Co(1)–N(6)	88.33(7)	O(5)–Co(2)–N(1)	86.36(8)
O(2)^#1–Co(1)–N(6)	91.67(7)	O(4)^#2–Co(2)–N(1)	171.17(8)
O(3)–Co(1)–N(6)	89.45(7)	N(2)^#3–Co(2)–N(1)	95.68(8)
O(3)^#1–Co(1)–N(6)	90.55(7)	O(6)–Co(2)–N(5)	89.81(9)
O(3)^#1–Co(1)–N(6)^#1	89.44(7)	O(5)–Co(2)–N(5)	87.66(9)
O(6)–Co(2)–O(5)	173.97(9)	N(2)^#3–Co(2)–N(5)	172.22(8)
O(6)–Co(2)–O(4)^#2	87.59(8)	N(1)–Co(2)–N(5)	76.83(8)
O(5)–Co(2)–O(4)^#2	98.11(8)

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Owing to the presence of centrosymmetric btec⁴⁻ anions, the trinuclear subunits are covalently extended to 1D ribbon along the crystallographic b-direction. A pair of head-to-tail arranged ptz⁻ ligands present their tetrazinyl N1, N2 and pyrazinyl N5 donors to chelate the Co2 within 1D ribbon and further bridge another Co2 from the parallel ribbons, resulting in a wave-like 2D layer (Fig. 1b). These layers are then covalently stacked together by the coordination bonds between the pyrazinyl N6 of ptz⁻ ligand and Co1 atom to produce a 3D robust MOF (Fig. 1c), which is isostructural to our previously reported [Ni₃(H₂O)₄(ptz)₂(btec)]·4H₂O}_n.²⁴ Thus, the ptz⁻ ligand in 1 presents a tetradentate μ₃-κ²N1,N5:κ¹N2:κ¹N6 chelating-bridging mode to contribute to the covalent 2D and 3D connectivity. Topologically, if the [Co₃(H₂O)₄(COO)₄]²⁺ core and ptz⁻ ligand are considered as two different nodes, 1 can be simplified as a bimodal (3,8)-connected tfz-d net with the (4³)₂(4⁶·6¹⁸·8⁴) topology symbol by OLEX.³⁰ On the other hand, the (3,8)-connected framework is microporous with 151.1 Å ³ cavity volume when removing the lattice water molecules.³¹ The resulting solvent-accessible cavities were estimated to be 18.7% of the unit cell volume.

[Cu₂(H₂O)₄(ptz)₂(H₂btec)]_n (2). Complex 2 exhibits an infinite 1D chain assembled from the centrosymmetric binuclear [Cu₂(H₂O)₄(ptz)₂]²⁺ nodes and doubly deprotonated H₂btec²⁻ connectors. As shown in Fig. 2a, the unique Cu^II atom has a slightly distorted octahedral structure surrounded by three N donors from two anionic ptz⁻ ligands, one carboxylate O atom from partial deprotonated H₂btec²⁻ co-ligand and two coordinated water molecules (see Table 3). Acting as a tridentate chelating-bridging ligand, a pair of ptz⁻ anions present a μ₂-κ²N1,N5:κ¹N2 binding mode to chelate Cu1 center by tetrazinyl N1 and pyrazinyl N5 donors and simultaneously bridge the centrosymmetric Cu1A by tetrazinyl N2 atom, generating a rigid bimetallic node with the Cu⋯Cu separation of being 4.0599(4) Å. Furthermore, the binuclear [Cu₂(H₂O)₄(ptz)₂]²⁺ nodes are infinitely extended by doubly deprotonated H₂btec²⁻ linkers to led to an infinite 1D covalent chain (see Fig. 2a), in which the nearest Cu⋯Cu distance across the H₂btec²⁻ co-ligand is 11.0437(10) Å.

Fig. 2 (a) View of the 1D chain of 2 (H-atoms were omitted for clarity, symmetry codes: A = −x + 1, −y + 2, −z + 2; B = −x + 2, −y + 2, −z + 1). (b) 2D supramolecular layer of 5 formed by intermolecular O–H⋯O and O–H⋯N hydrogen bonding interactions.

Table 3 Selected bond lengths (Å) and angles (°) for 2^a

a Symmetry Codes for 2: #1 −x + 1, −y + 2, −z + 2; #2 −x + 2, −y + 2, −z + 1.
Cu(1)–O(1)	1.932(2)	Cu(1)–N(1)	2.003(2)
Cu(1)–N(2)^#1	2.005(3)	Cu(1)–N(5)	2.058(3)
Cu(1)–O(5)	2.316(2)	Cu(1)–O(6)	2.645(4)
O(1)–Cu(1)–N(1)	173.06(10)	O(1)–Cu(1)–N(2)^#1	91.15(9)
N(1)–Cu(1)–N(2)^#1	94.11(10)	O(1)–Cu(1)–N(5)	94.70(9)
N(1)–Cu(1)–N(5)	80.44(10)	N(2)^#1–Cu(1)–N(5)	172.59(10)
O(1)–Cu(1)–O(5)	86.52(9)	N(1)–Cu(1)–O(5)	88.65(10)
N(2)^#1–Cu(1)–O(5)	93.33(10)	N(5)–Cu(1)–O(5)	91.56(9)
N(1)–Cu(1)–O(6)	85.36(12)	O(1)–Cu(1)–O(6)	98.61(11)
N(5)–Cu(1)–O(6)	78.76(17)	N(2)^#1–Cu(1)–O(6)	95.88(18)
O(5)–Cu(1)–O(6)	169.34(16)

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In the packing structure of 2, both the undeprotonated carboxylic group and terminal water molecule can act as hydrogen-bond donors to produce the interchain O–H⋯O and O–H⋯N noncovalent interactions with the deprotonated carboxylate group and pyrazinyl N6 acceptors of ptz⁻ ligand (see Table S1), which assemble the separate chains into a 2D supramolecular layer with the nearest interchain Cu⋯Cu separation of 8.2326(11) Å. Additionally, face-to-face π⋯π stacking interactions between interchain pyrazinyl and tetrazinyl rings are further reinforced these 2D layers (Fig. 2b). Furthermore, the adjacent 2D layers are further linked together to form a 3D network through the O5–H5B⋯O2 hydrogen-bonding interactions (see ESI, Fig. S2 and Table S1).†

Of the three ternary metal complexes (1, 2, and previously reported {[Ni₃(H₂O)₄(ptz)₂(btec)]·4H₂O}_n²⁴), the btec⁴⁻ co-ligand in both 1 and {[Ni₃(H₂O)₄(ptz)₂(btec)]·4H₂O}_n can act as a fully deprotonated hexadentate ligand to aggregate and extend two different trimetallic cores into two isostructural 3D polymers. In contrast, the incomplete deprotonated co-ligand in 2 just behaves as a connector to link dimeric Cu₂(ptz)₂ subunits into a 1D chain through bidentate bridging coordination mode.

[Hg₂(ptz)₂Cl₂]_n (3). Complex 3 is a trinodal (3,4)-connected 3D framework, showing unusual (6·8²)₂(6·8³·10²) topology.³⁰ The asymmetric unit of 3 consists of two crystallographically independent Hg^II ions, two ptz⁻ ligands, and two Cl⁻ atoms. As shown in Fig. 3a, both Hg^II atoms are five-coordinated, displaying the slightly distorted trigonal bipyramidal geometry. Hg1 atom is surrounded by one bridging Cl⁻ atom and four N donors (N1, N3C, N5, N12) from three separate ptz⁻ ligands. Instead, the N₃Cl₂ donor set of Hg2 is completed by three N atoms from two ptz⁻ ligands, one bridging Cl1, and one terminal Cl2 ligand, respectively. The bond lengths of Hg–Cl are generally longer than those of Hg–N (see Table 4), although they are comparable to those previously reported values.³² Two ptz⁻ ligands adopt μ₂-κ²N1,N5:κ¹N6 and μ₃-κ²N1,N5:κ¹N3:κ¹N6 modes to alternately link Hg1 and Hg2 into a zigzag-chain along the crystallographic c-axis and further extend the adjacent chains into a 2D covalent layer in bc-plane (Fig. 3b). The bridging Cl1 atom then connects the adjacent Hg1 and Hg2 atoms from the adjacent layers into a 3D robust MOF (Fig. 3c). Topologically, when the Hg1, Hg2, and tetradentate ptz⁻ are viewed as three different nodes (three-connected Hg1/Hg2 and four-connected ptz⁻), the 3D framework of 3 can be simplified into a trinodal (3,4)-connected network with the (6·8²)₂(6·8³·10²) topology (Fig. 3c).³⁰

Fig. 3 (a) Local coordination environments of Hg^II atoms in 3 and the coordination mode of the ptz⁻ ligand (H atoms were omitted for clarity; Symmetry codes: A = x, 0.5 − y, 0.5 + z; B = 1 − x, −0.5 + y, 0.5 − z; C = 0.5 −x, −0.5 + y, z; D = x, 0.5 − y, −0.5 + z, E = 0.5 − x, 0.5 + y, z; F = 1 − x, 0.5 + y, 0.5 − z). (b) 2D Hg^II-ptz layer of 3. (c) 3D framework of 3 and its trinodal (3,4)-connected (6·8²)₂(6·8³·10²) topology.

Table 4 Selected bond lengths (Å) and angles (°) for 3–5^a

a Symmetry codes for 3: #1 x, −y + 1/2, z + 1/2; #2 −x + 1, y − 1/2, −z + 1/2; #3 −x + 1/2, y − 1/2, z. for 4: #1−x, −y + 2, − z. for 5: #1 −x + 1, −y + 1, −z + 1.
3
Hg(1)–N(1)	2.126(6)	Hg(1)–Cl(1)	2.3274(1)
Hg(1)–N(12)	2.507(7)	Hg(1)–N(3)^#3	2.7743(1)
Hg(1)–N(5)	2.7294(1)	Hg(2)–Cl(1)	3.0603(1)
Hg(2)–N(7)	2.111(6)	Hg(2)–Cl(2)	2.2901(2)
Hg(2)–N(6)^#1	2.568(7)	Hg(2)–N(11)	2.7873(1)
N(1)–Hg(1)–Cl(1)	162.78(17)	N(1)–Hg(1)–N(12)	101.4(2)
Cl(1)–Hg(1)–N(12)	95.47(16)	N(3)^#3–Hg(1)–Cl(1)	106.955(1)
N(3)^#3–Hg(1)–N(5)	85.801(1)	N(3)^#3–Hg(1)–N(12)	87.025(1)
N(5)–Hg(1)–Cl(1)	94.620(1)	N(5)–Hg(1)–N(12)	168.981(1)
N(5)–Hg(1)–N(1)	68.911(1)	N(3)^#3–Hg(1)–N(1)	77.747(1)
Cl(1)^#2–Hg(2)–N(6)^#1	78.791(1)	Cl(1)^#2–Hg(2)–N(7)	84.394(1)
N(7)–Hg(2)–Cl(2)	164.2(2)	N(7)–Hg(2)–N(6)^#1	94.3(2)
Cl(2)–Hg(2)–N(6)^#1	101.23(17)	Cl(2)–Hg(2)–N(11)	97.599(1)
N(11)–Hg(2)–N(7)	67.990(1)	N(11)–Hg(2)–N(6)	142.879(2)
Cl(1)^#2–Hg(2)–Cl(2)	100.811(1)	Cl(1)^#2–Hg(2)–N(11)	128.383(1)
4
Hg(1)–N(7)	2.057(5)	Hg(1)–O(1)	2.657(6)
Hg(1)–N(11)	2.759(7)	Hg(1)–N(5)	2.742(1)
Hg(1)–N(1)	2.090(5)	Hg(2)–N(2)	2.8428(2)
Hg(1)–N(13)	2.642(5)	Hg(2)–N(13)	2.026(5)
N(7)–Hg(1)–N(1)	175.47(19)	N(13)–Hg(1)–O(1)	75.41(19)
N(7)–Hg(1)–N(13)	93.79(18)	N(13)^#1–Hg(2)–N(13)	179.999(2)
N(1)–Hg(1)–N(13)	85.74(17)	N(2)^#1–Hg(2)–N(2)	180.000
N(7)–Hg(1)–O(1)	91.1(2)	N(2)^#1–Hg(2)–N(13)	94.079(3)
N(1)–Hg(1)–O(1)	84.4(2)	N(13)–Hg(2)–N(2)	85.921(2)
5
Hg(1)–N(1)^#1	2.080(4)	Hg(1)–O(1)	2.539(4)
Hg(1)–N(5)	2.7082(4)
N(1)^#1–Hg(1)–N(1)	179.999(1)	N(1)^#1–Hg(1)–O(1)^#1	89.49(17)
O(1)^#1–Hg(1)–O(1)	180.000(1)	N(1)^#1–Hg(1)–O(1)	90.51(17)
N(1)–Hg(1)–N(5)	69.493(3)	N(1)–Hg(1)–O(1)	89.49(17)
N(5)–Hg(1)–O(1)	101.250(3)	N(5)–Hg(1)–O(1)^#1	78.750(3)
N(5)^#1–Hg(1)–N(5)	180	N(1)^#1–Hg(1)–N(5)	110.507(4)
N(1)–Hg(1)–O(1)^#1	90.505(3)

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[Hg₃(H₂O)₂(ptz)₄(μ_1,1-N₃)₂]·2H₂O (4). Rather than being polymeric frameworks of 1–3, complex 4 is a discrete trinuclear entity bridged by pairs of N₃⁻ and ptz⁻ anions. The fundamental unit of 4 consists of three separate Hg^II atoms with Hg2 locating at the crystallographic inversion center, two terminal water molecules, a pair of N₃⁻ anions, four separate ptz⁻ anions acting as bridging and terminal ligands respectively, and two lattice water molecules. As shown in Fig. 4a, Hg1 is in a distorted octahedron formed by one terminal water molecule and five N atoms from one N₃⁻ and two different ptz⁻ ligands. Instead, Hg2 is in a square-planar arrangement to four N atoms from pairs of ptz⁻ and N₃⁻ anions (see Table 4).

Fig. 4 (a) The discrete trinuclear structure of 4 (H atoms were omitted for clarity; symmetry code: A = −x, −y + 2, −z). (b) 2D supramolecular layer of 4 formed by O–H⋯N hydrogen bonding interactions.

Hg2 and Hg1 as well as Hg2 and Hg1A are doubly strengthened by the azide anion and one of the crystallographically independent ptz⁻ ligand containing N1∼N6 to generate linear trinuclear structure with the bond angle of Hg1–Hg2–Hg1A of 180°. The azide anion in trimetallic core adopts an end-on bridging mode and the ptz⁻ ligand acts in a tridentate μ₂-κ²N1,N5:κ¹N2 fashion. The Hg⋯Hg separation within the trinuclear unit is 3.9459(3) Å. By contrast, the other ptz⁻ anion defined by N7∼N12 only presents its N7 and N11 donors to chelate Hg1 atom in a bidentate μ₁-κ²N1,N7 mode. Thus, the two ptz⁻ anions in 4 play different roles (bridging and terminal roles) for the formation of the unusual trinuclear structure. As shown in Fig. 4b, intermolecular O–H⋯N hydrogen-bonding interactions between the coordinated water and ptz⁻ ligands assemble the discrete trinuclear units into a 2D supramolecular layer.

Hg(H₂O)₂(ptz)₂ (5). Complex 5 without any organic co-ligand exhibits a centrosymmetric mononuclear structure. The unique Hg^II ion lies on the inversion center and coordinated by four N atoms from a pair of chelating ptz⁻ ligands in an equatorial plane and two water molecules in the axial positions (Fig. 5a), displaying the deformed octahedral coordination geometry (see Table 4). The discrete mononuclear structure is isostructural to various metal complexes with the ptz⁻ ligand, [M(ptz)₂(H₂O)₂] (M = Cd^II,⁵ Zn^II,^2,33 Mn^II,^33,34 Fe^II,³⁵ Co^II,^3,36 Cu^II,³ Ni^II,³⁷). Each centrosymmetric mononuclear structure is H-bonded with its six neighbours through two pairs of O–H⋯N interactions produced by coordinated water and ptz⁻ ligand, giving rise to a regular 3D hydrogen-bonded network (see Fig. 5b and ESI, Table S1).†

Fig. 5 (a) Mononuclear structure of 5 (H atoms were omitted for clarity; symmetry code: A = −x + 1, −y + 1, −z + 1). (b) 3D packing diagram of 5 formed by O–H⋯N hydrogen-bonding interactions.

[Cd₂(H₂O)₂(ptz)₂(ox)]_n (6). Complex 6 exhibits a 6-connected 3D α-P_o-type framework with cationic Cd^II-oxalate⁻ strands interconnected by bidentate bridging ptz⁻ ligands. The asymmetric unit of 6 consists of one Cd^II cation, one ptz⁻ anion, half an oxalate dianion with an inversion center, and one coordinated water molecule. As shown in Fig. 6a, the crystallographic independent Cd1 atom is in a distorted octahedron completed by three carboxylate O atoms from two different oxalate dianions and one water molecule in an equatorial plane and two pyrazinyl N atoms from two symmetry-related ptz⁻ anions locating at the axial positions (see Table 5). Each centrosymmetric oxalate co-ligands adopts a μ₄-κ²O1′,O2:κ¹O2;κ²O1,O2′:κ¹O2′ chelating-bridging mode to coordinate with four crystallographically equivalent Cd^II atoms. As a result, an infinite Cd^II-oxalate strand is generated along the crystallographic a-axis with the nearest Cd⋯Cd distance of 3.8709(3) Å (see Fig. 6b). Furthermore, the ptz⁻ ligand serves as a bridging μ₂-κ¹N1:κ¹N6 connector to link the antiparallel Cd^II-oxalate strands into a 3D architecture (see Fig. 6c). This is the first time that the μ₂-κ¹N1:κ¹N6 coordination mode of the ligand is observed in ptz⁻−based metal complexes. Topologically, if the bimetallic [Cd₂(H₂O)₂]⁴⁺ cation bridged by carboxylate O2 of oxalate co-ligand is considered as a node and the bridged oxalate and ptz⁻ ligands as the linkers, the 3D framework of 6 can be simplified into a 6-connected α-P_o-type structure with the (4¹²·6³) topology symbol (see Fig. 6c).³⁰

Fig. 6 (a) Local environment of Cd^II atom in 6 (H-atoms were omitted for clarity, symmetry codes: A = x − 1, y, z; B = −x + 0.5, y + 0.5, −z + 1.5; C = 2 − x, 1 − y, 2 − z). (b) The linkage of the oxalate-Cd^II ribbon. (c) 3D framework of 6 and its 6-connected α-P_o-type topological structure.

Table 5 Selected bond lengths (Å) and angles (°) for 6–9^a

a Symmetry codes for 6: #1 x − 1, y, z; #2 −x + 1/2, y + 1/2, − z + 3/2; #3 −x + 2, −y + 1, −z + 2. For 7: #1 −x, −y, −z, #2 − x, y−1/2, − z + 1/2. For 8: #1 − x + 1, − y + 1, −z, #2 −x, −y + 1, −z. For 9: #1 −x + 1, −y, −z + 1; #2 −x + 2, −y, −z + 1.
6
Cd(1)–N(1)	2.283(2)	Cd(1)–O(2)^#1	2.3121(16)
Cd(1)–O(3)	2.3150(18)	Cd(1)–O(1)	2.3682(16)
Cd(1)–N(6)^#2	2.369(2)	Cd(1)–O(2)^#3	2.4007(16)
N(1)–Cd(1)–O(2)^#1	96.89(7)	N(1)–Cd(1)–O(3)	100.68(7)
O(2)^#1–Cd(1)–O(3)	143.14(6)	N(1)–Cd(1)–O(1)	83.21(6)
O(2)^#1–Cd(1)–O(1)	137.85(6)	O(3)–Cd(1)–O(1)	76.68(6)
N(1)–Cd(1)–N(6)^#2	177.86(7)	O(2)^#1–Cd(1)–N(6)^#2	81.18(6)
O(3)–Cd(1)–N(6)^#2	81.44(7)	O(1)–Cd(1)–N(6)^#2	97.59(7)
N(1)–Cd(1)–O(2)^#3	86.42(6)	O(2)^#1–Cd(1)–O(2)^#3	69.57(6)
O(3)–Cd(1)–O(2)^#3	143.26(6)	O(1)–Cd(1)–O(2)^#3	68.37(5)
N(6)^#2–Cd(1)–O(2)^#3	92.03(6)
7
Cd(1)–N(2)^#1	2.298(3)	Cd(1)–O(1)	2.359(3)
Cd(1)–N(6)^#2	2.412(3)	Cd(1)–N(1)	2.423(2)
Cd(1)–N(5)	2.427(3)	Cd(1)–I(1)	2.7871(8)
N(2)^#1–Cd(1)–O(1)	92.16(10)	N(2)^#1–Cd(1)–N(6)^#2	93.55(10)
O(1)–Cd(1)–N(6)^#2	162.42(9)	N(2)^#1–Cd(1)–N(1)	88.53(9)
O(1)–Cd(1)–N(1)	79.43(9)	N(6)^#2–Cd(1)–N(1)	84.11(9)
N(2)^#1–Cd(1)–N(5)	158.45(9)	O(1)–Cd(1)–N(5)	84.73(9)
N(6)^#2–Cd(1)–N(5)	83.86(10)	N(1)–Cd(1)–N(5)	69.93(9)
N(2)^#1–Cd(1)–I(1)	105.31(7)	O(1)–Cd(1)–I(1)	96.55(6)
N(6)^#2–Cd(1)–I(1)	97.94(6)	N(1)–Cd(1)–I(1)	165.79(6)
N(5)–Cd(1)–I(1)	96.23(6)
8
Cd(1)–Br(1)^#1	2.6992(11)	Br(1)–Cd(1)	2.7990(12)
Cd(1)–N(2)^#2	2.300(6)	Cd(1)–N(1)	2.356(5)
Cd(1)–O(1)	2.145(2)	Cd(1)–N(5)	2.396(6)
N(2)^#2–Cd(1)–N(1)	93.11(19)	N(2)^#2–Cd(1)–O(1)	89.6(2)
N(1)–Cd(1)–O(1)	87.44(19)	N(2)^#2–Cd(1)–N(5)	162.4(2)
N(1)–Cd(1)–N(5)	70.99(19)	O(1)–Cd(1)–N(5)	82.29(19)
N(2)^#2–Cd(1)–Br(1)^#1	98.72(14)	N(1)–Cd(1)–Br(1)^#1	167.68(14)
O(1)–Cd(1)–Br(1)^#1	89.21(13)	N(5)–Cd(1)–Br(1)^#1	96.82(14)
N(2)^#2–Cd(1)–Br(1)	90.19(15)	N(1)–Cd(1)–Br(1)	90.80(14)
O(1)–Cd(1)–Br(1)	178.21(13)	N(5)–Cd(1)–Br(1)	97.42(14)
Br(1)^#1–Cd(1)–Br(1)	92.58(3)
9
Cd(1)–N(2)^#1	2.302(4)	Cd(1)–N(5)	2.392(4)
Cd(1)–N(1)	2.361(4)	Cd(1)–Cl(1)	2.5812(18)
Cd(1)–O(1)	2.371(4)	Cd(1)–Cl(1)^#2	2.672(2)
N(2)^#1–Cd(1)–N(1)	92.98(14)	N(2)^#1–Cd(1)–O(1)	88.86(14)
N(1)–Cd(1)–O(1)	88.01(14)	N(2)^#1–Cd(1)–N(5)	161.85(14)
N(1)–Cd(1)–N(5)	71.10(14)	O(1)–Cd(1)–N(5)	82.10(13)
N(2)^#1–Cd(1)–Cl(1)	98.48(11)	N(1)–Cd(1)–Cl(1)	168.29(11)
O(1)–Cd(1)–Cl(1)	89.85(11)	N(5)–Cd(1)–Cl(1)	97.21(11)
N(2)^#1–Cd(1)–Cl(1)^#2	91.42(12)	N(1)–Cd(1)–Cl(1)^#2	91.64(11)
O(1)–Cd(1)–Cl(1)^#2	179.57(11)	N(5)–Cd(1)–Cl(1)^#2	97.54(11)
Cl(1)–Cd(1)–Cl(1)^#2	90.44(6)

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[Cd(H₂O)(ptz)I]_n (7). Complex 7 exhibits a 2D layered structure with the centrosymmetric binuclear [Cd₂(H₂O)₂I₂]²⁺ cations extended by pairs of anionic ptz⁻ linkers. As shown in Fig. 7a, the fundamental subunit is a centrosymmetric dimer constructed from pairs of Cd^II atoms, ptz⁻ anions, coordinated water molecules and I⁻ ligand. The Cd^II atom is in a slightly distorted octahedral geometry formed by four N atoms from three separate ptz⁻ ligands, one terminal I⁻ anion and one coordinated water molecule. Each ptz⁻ acts as a tetradentate μ₃-κ²N1,N5:κ¹N2:κ¹N6 chelating-bridging ligand to chelate one centrosymmetric Cd1 center and simultaneously bridge two other symmetry–related Cd1 atoms. The adjacent 2D layers are further assembled into a 3D network through the O1–H1B⋯I1 and O1–H1A⋯N4 hydrogen-bonding interactions between aqua and I⁻/ptz⁻ ligand (see ESI, Fig. S3).†

Fig. 7 (a) The centrosymmetric binuclear subunit and the local coordination environment of the Cd^II atom of 7 (H-atoms were omitted for clarity, symmetry codes: A = −x, −y, −z; B = −x, y − 0.5, −z + 0.5). (b) 2D covalent layer of 7.

{[Cd(H₂O)(ptz)Br]·H₂O}_n (8) and {[Cd(H₂O)(ptz)Cl]·H₂O}_n (9). By changing the co-ligand from I⁻ with larger atomic radius to comparatively small Br⁻ or Cl⁻ atom, the 2D layered structure of 7 is decreased to 1D chains with the binuclear [Cd₂(H₂O)₂(ptz)₂]⁺ cations alternately propagated by bridged Br⁻ anion for 8 or Cl⁻ anion for 9, respectively. The substructure of the bimetallic [Cd₂(H₂O)₂(ptz)₂]⁺ cation is also centrosymmetric dimer with the Cd^II atom in a slightly distorted octahedral geometry surrounded by three N donors from a pair of centrosymmetric ptz⁻ ligands, two bridging X atoms and one water molecule (see Fig. 8a, Tables 5). Different from the chelating-bridging role of the ptz⁻ ligand in 7, the core ligand in 8 and 9 only chelates two symmetry-related Cd1 and Cd1A to generate the binuclear subunit, rather than extending the high dimensional framework of the target complexes. Instead, pairs of halide anions doubly bridge the neighbouring binuclear subunits to lead to an infinite chain (see Fig. 8a). The Cd⋯Cd distance across the bridging halide anion is 3.8001(4) for 8 and 3.7011(2) Å for 9, which is 0.83 and 0.72 Å shorter than that chelated by ptz⁻ ligand. As shown in Fig. 8b, the chain-based structure is expanded into 3D supramolecular network through strong O–H⋯N hydrogen-bonding interactions between coordinated water and ptz⁻ ligand (see ESI, Table S1).†

Fig. 8 (a) 1D chain of 8 or 9 formed by bimetallic nodes and bridging halide linkers (H-atoms were omitted for clarity, symmetry codes: A = 1 − x, −y, 1 − z; B = 2 − x, −y, 1 − z). (b) 3D supramolecular network of 8 or 9 assembled from interchain O–H⋯N hydrogen bonding interactions.

Notably, although 8 and 9 possess the analogous covalent chain-based skeleton, the intermolecular weak interactions between lattice water and organic framework are different from each other. Lattice water can produce three-fold hydrogen-bonding interactions with the framework of 8 (two O–H⋯N and one O–H⋯O interactions, see ESI, Table S1).† In contrast, only one O–H⋯O hydrogen-bonding interaction can be observed in 9 (see ESI, Fig. S4 and Table S1).† On the other hand, comparisons of 7 bearing terminal I⁻ co-ligand with 8 and 9 bearing bridging Br⁻ or Cl⁻ auxiliary ligand reveal that the presence of the competing binding behaviour between the mixed-ligands due to the increase of halide radius.

Binding preference of the 5-(pyrazinyl)tetrazolate ligand in metal complexes

The 5-(pyrazinyl)tetrazolate ligand can potentially afford six different metal binding sites or their variable combinations, since it is structurally related to a tetrazolate ring connected with a pyrazinyl ring. Indeed, it has become a versatile ligand for bridging or chelating different metal ions. Up to date, eight different binding modes by the ligand (I: μ₁-κ²N1,N5, II: μ₂-κ¹N1:κ¹N6, III: μ₂-κ¹N2:κ¹N4, IV: μ₂-κ²N1,N5:κ¹N6, V: μ₂-κ²N1,N5:κ¹N2, VI: μ₃-κ²N1,N5:κ¹N2:κ¹N6, VII: μ₃-κ²N1,N5:κ¹N3:κ¹N6, VIII: μ₄-κ¹N1:κ¹N2:κ¹N3:κ¹N6)^{2–4,15,34,38} have been observed, which were summarized in Fig. 9. The ptz⁻ anion is repeatedly observed as a bidentate, tridentate and even tetradentate ligand to chelate and/or bridge different metal ions. Undoubtedly, such different coordination modes are significantly influenced by the presence of the different co-ligands and the metal centers. When a co-ligand was introduced in the ternary systems, cooperative binding can occur or bonding competition can be observed between the bridging, chelating and/or bridging-chelating ptz⁻ core-ligand and the terminal or bridging co-ligands. For example, ptz⁻ anion in complexes 2, 4, 5, 8, and 9 can only act as chelating/chelating-bridging ligand to complete the mono-, bi-, and tri-metallic subunit, respectively. By contrast, in 6 it just behaves as a bridging ligand to connect the adjacent Cd^II-oxalate ribbon. Interestingly, in complexes 1, 3, and 7, the ptz⁻ anion also serves as chelating-bridging ligand to extend the dimensionalities of the covalent frameworks. Additionally, the remained vacant N sites from the polydentate ptz⁻ ligand can serve as good hydrogen-bonding acceptors, which can significantly produce multiple noncovalent interactions to contribute to the architectural dimensionality. Such supramolecular interactions have been previously discussed in more detail.^2–3,15

Fig. 9 Summary of the binding modes for anionic ptz⁻ ligand in the metal complexes.

Thermogravimetric analysis

To explore the contributions of the mixed ligands and metal ions to the thermal stability of the polymeric/discrete structures as well as to confirm the content of disorder lattice water molecules in the complexes, the thermogravimetric (TG) analyses of the complexes 1–9 were measured from the room temperature to 800 °C under an inert atmosphere (see ESI, Fig. S5).† As a result, the 3D covalent framework of 1 loses separately its lattice and coordinated water molecules from room temperature to 68 °C (expt. 11.0%, calcd 10.2%) and from 86 °C to 170 °C (expt. 7.4%, calcd 8.2%). The mixed organic ligands, ptz⁻ and btec⁴⁻ anions, are rapidly removed between 350 °C and 385 °C, accompanying the collapse of the 3D framework of 1. The final residue is CoO (expt. 26.9%, calcd 25.5%). By contrast, low-dimensional 2 displays lower thermal stability than 1, although they are both constructed from the same mixed ligands. The removal of the coordination water in 2 leads to the first weight-loss of 9.0% between room temperature and 144 °C (calcd. 9.7%). And the next obvious weight-loss began at 234 °C and ended at 378 °C, ascribed to the synchronous decomposition of ptz⁻ and H₂btec²⁻ anions. The remaining residue of the polymer is CuO (expt. 21.5%, calcd 21.3%).

For the three Hg^II-based polymers, complex 3 without any lattice molecules is thermally stable up to 150 °C. Then an obvious weight-loss process is observed until 305 °C, corresponding to the decomposition of ptz⁻ ligand, vaporization of HgCl₂ and the formation of mercury (expt. 74.3% calcd 73.8%). Upon further heating, the residual mercury was slowly evaporated.³⁸ The TG curves of 4 and 5 are similar with each other. The first stage between 100 °C and 200 °C for 4 (expt. 5.7% calcd 5.4%) as well as between 77 °C and 128 °C for 5 (obs. 6.7%, calcd 6.8%) is due to the complete loss of water molecules. The second obvious weight-loss from 200 °C to 277 °C for 4 and from 237 °C to 267 °C for 5 is ascribed to the removal of organic ligands and mercury, respectively. Upon further heating to 800 °C, the residual mercury gradually evaporated.

The thermal decomposition profiles of 6–9 display consistently two-stage weight-loss processes for free and bound water molecules from 99 to 302 °C and the mixed-ligands from 317 to 633 °C, respectively. Surprisingly, the initial temperature (250 °C) of the first stage of 6 (250–302 °C; expt. 5.5%, calcd 5.6%) is much more stable by 140 °C than those of 7 (120 °C corresponding to the first stage between 120 and 154 °C, expt. 4.8%, calcd 4.5%), 8 (99 °C corresponding to the first stage between 99 and 180 °C, expt. 9.8%, calcd 9.6%) and 9 (106 °C corresponding to the first stage between 106 and 180 °C, expt. 9.2%, calcd 10.9%). The final product is CdO for 6 (expt. 40.0%, calcd 39.9%) and 9 (expt. 18.4%, calcd 19.4%), and Cd(CN)₂ for 7 (expt. 20.4%, calcd 20.3%) and 8 (expt. 22.6%, calcd 21.9%).

Luminescence spectra

Luminescence compounds are of great current interest because of their various applications in chemical sensors, photochemistry and structure electroluminescence (EL) displays and light-emitting diodes (LEDs).³⁹ Thus, the solid state emission spectra of all the resulting complexes except 2 were measured to explore their potential optical properties. As shown in Fig. 10, upon excitation at 375 nm for 1 and 3–4 or at 385 nm for four Cd^II-ptz-based complexes, only one emission at 422 nm for both 1 and 3, 425 nm for 4 and 427 nm for both 5 and 9, 428 nm for 6 and 432 nm for 8, was observed, which display slightly variable intensity. By contrast, complex 7 presents two moderate emissions at 423 and 434 nm, respectively. Considering that the isolated neutral ptz ligand can display a strong blue fluorescent emission band at 457 nm upon excitation at 357 nm,¹⁵ the emission located ca. 425 nm for all the complexes examined herein should be assigned to the intraligand transition of ptz⁻ ligand upon cation binding. The slight shift of the emission is probably due to the differences of anions and coordination environment around the central metal ions.^2,40 The low energy emission at 434 nm for 7 might come from interligand transfer between ptz⁻ and I⁻ anions.

Fig. 10 Solid emission spectra of complexes 1 and 3–9 at room temperature.

Magnetic behaviour of 2

As shown in Fig. 11, the χ_MT value for per Cu^II ion of 2 is 0.452 cm³ K mol⁻¹ at room temperature, which is slightly greater than that (0.42 cm³ K mol⁻¹) for a single Cu^II ion with S = 1/2 and g = 2.1. The value of χ_MT decreases to a minimum value 0.044 cm³ K mol⁻¹ as the temperature decreases to 7 K. Then it increases up to 0.060 cm³ K mol⁻¹ at 4 K. On the other hand, the χ_Mvs. T curve provides more information about the magnetic behaviour of 2: the χ_M value of 0.0015 cm³ mol⁻¹ at room temperature increases as the temperature decreases, arriving at a maximum value of 0.0105 cm³ mol⁻¹ at 22 K; χ_M then decreases practically to 0.0056 cm³ mol⁻¹ at 8 K before increasing a value of 0.0150 cm³ mol⁻¹ at 4 K. This behaviour clearly indicates a typical antiferromagnetic coupling and the presence of a small amount of paramagnetic impurities.⁴¹ Considering the magnetic exchange interaction occurs between the Cu^II ions within the ptz⁻−bridged dimer and between the two adjacent dimers across the bridged H₂btec²⁻ co-ligand, the best fits were obtained by using the eqn (1) for S = 1/2, where ρ is defined as the molar fraction of the paramagnetic impurity in 2, J is the coupling constant within the dimer, zJ′ is the coupling constant between two dimers, and the other symbols have the usual meanings.

Fig. 11 Plots of χ_M (○) and χ_MT (□) vs.T for 2. Solid lines correspond to the best fit indicated in the text.

The results of the best fit were g = 2.19, J = −13.13 cm⁻¹, zJ′ = −0.00018 cm⁻¹, ρ = 0.0335, with r = 2.6 × 10⁻⁴, where r is the agreement factor defined as Σ[(χ_MT)_obsd − (χ_MT)_calcd]²/Σ[(χ_MT)²_obsd. Obviously, the difference between J and zJ′ suggests the magnetic superexchange pathway by double ptz⁻ bridges is stronger than that by H₂btec²⁻ linker due to the relatively shorter intermetallic distance (4.0599(4) vs. 11.0437(10) Å). Additionally, the coupling constant of 2 is considerable smaller than those of binuclear Cu^II complexes linked by μ_1,2-1,2,4-triazole (−118 < J < −97), which are structurally related with the coordination behaviour of the Cu^II ions.⁴² In 2 the unpaired electron of the Cu^II occupies a magnetic orbital of d(x² − y²) symmetry, which is pointing towards the equatorially coordinating tetrazolate N atoms. Thus, the overlap between the magnetic orbital of the Cu^II ion and the σ-orbitals of the tetrazolate N atom significantly determine the strength of the magnetic coupling. The more these orbitals are directed towards each other the larger the overlap and therefore the antiferromagnetic interaction. Since the asymmetric bridging mode of ptz⁻ ligand in 2 led to the bond angles of Cu1–N1–N2 and Cu1A–N2–N1 of 140.7(2) and 124.38(19)°, which allowed a poor overlap than those with the symmetrical Cu^II–N–N–Cu^II structures⁴² and reasonably contributed a weak coupling constant of 2.

Conclusions

In summary, by selectivly introducing functional auxiliary ligands with different size and binding modes and metal ions with different electron configuration, nine mixed-ligand metal complexes with in situ formed ptz⁻ core-ligand were presented, in which Hg^II ion became the first example that can be used as Lewis acid catalyst in situ reactions. Compared with the reported binary ptz⁻−based complexes, the favourably synergistic/competing binding ability contributed from the chelating-bridging ptz⁻ core-ligand and bridging/terminal co-ligands has essentially induced intriguing mono-, bi-, and tri-metallic-core-based coordination architecture ranged from separate 0D entities to infinite 3D frameworks. Furthermore, these target complexes exhibit weak antiferromagnetic coupling behaviour for Cu^II-based polymer as well as ptz⁻−based strong photoluminescence for the other complexes.

Acknowledgements

This present work was financially supported by the National Natural Science Foundation of China (20703030, 20871092 and 20973125), the Key Project of Chinese Ministry of Education (grant no. 209003), the Program for New Century Excellent Talents in University (NCET-08-0914), and the National Science Foundation of Tianjin (Grants 10JCZDJC21600 and 10JCYBJC04800) which are gratefully acknowledged.

References

1. (a) M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319; (b) S. G. Telfer and R. Kuroda, Coord. Chem. Rev., 2003, 242, 33; (c) S. A. Barnett and N. R. Champness, Coord. Chem. Rev., 2003, 246, 145; (d) B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629.
2. Z. Li, M. Li, X.-P. Zhou, T. Wu, D. Li and S. W. Ng, Cryst. Growth Des., 2007, 7, 1992.
3. M. A. M. Abu-Youssef, F. A. Mautner, A. A. Massoud and L. Öhrström, Polyhedron, 2007, 26, 1531.
4. J. Luo, X.-R. Zhang, L.-L. Cui, W.-Q. Dai and B. S. Liu, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2006, 62, m614.
5. W.-D. Song and D.-L. Xi, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2006, 62, m2841.
6. A. Rodríguez-Diéguez and E. Colacio, Chem. Commun., 2006, 4140.
7. J.-Y. Zhang, Q. Yue, Q.-X. Jia, A.-L. Cheng and E.-Q. Gao, CrystEngComm, 2008, 10, 1443.
8. (a) R.-G. Xiong, X. Xue, H. Zhao, X.-Z. You, B. F. Abrahams and Z.-L. Xue, Angew. Chem., Int. Ed., 2002, 41, 3800; (b) H. Zhao, Z.-R. Qu, H.-Y. Ye and R.-G. Xiong, Chem. Soc. Rev., 2008, 37, 84.
9. P. Lin, W. Clegg, R. W. Harrington and R. A. Henderson, Dalton Trans., 2005, 2388.
10. W. Ouellette, H.-X. Liu, C. J. O'Connor and J. Zubieta, Inorg. Chem., 2009, 48, 4655.
11. W.-C. Song, J.-R. Li, P.-C. Song, Y. Tao, Q. Yu, X.-L. Tong and X.-H. Bu, Inorg. Chem., 2009, 48, 3792.
12. Y. Chen, Z.-G. Ren, H.-X. Li, X.-Y. Tang, W.-H. Zhang, Y. Zhang and J.-P. Lang, J. Mol. Struct., 2008, 875, 339.
13. Z.-R. Qu, H. Zhao, X.-S. Wang, Y.-H. Li, Y.-M. Song, Y.-J. Liu, Q. Ye, R.-G. Xiong, B. F. Abrahams, Z.-L. Xue and X.-Z. You, Inorg. Chem., 2003, 42, 7710.
14. D.-S. Liu, G.-S. Huang, C.-C. Huang, X.-H. Huang, J.-Z. Chen and X.-Z. You, Cryst. Growth Des., 2009, 9, 5117.
15. Y. Tao, J.-R. Li, Q. Yu, W.-C. Song, X.-L. Tong and X.-H. Bu, CrystEngComm, 2008, 10, 699.
16. E.-C. Yang, H. K. Zhao, B. Ding, X. G. Wang and X. J. Zhao, Cryst. Growth Des., 2007, 7, 2009.
17. E.-C. Yang, H. K. Zhao, Y. Feng and X. J. Zhao, Inorg. Chem., 2009, 48, 3511.
18. E.-C. Yang, Y.-N. Chan, H. Liu, Z.-C. Wang and X.-J. Zhao, Cryst. Growth Des., 2009, 9, 4933.
19. E.-C. Yang, Q.-Q. Liang and P. Wang, Inorg. Chem. Commun., 2009, 12, 211.
20. E.-C. Yang, Z.-Y. Liu, X.-G. Wang, S. R. Batten and X.-J. Zhao, CrystEngComm, 2008, 10, 699.
21. E.-C. Yang, Q.-Q. Liang, X.-G. Wang and X.-J. Zhao, Aust. J. Chem., 2008, 61, 813.
22. SAINT; Bruker AXS: Madison, WI, 1998.
23. (a) G. M. Sheldrick. SHELXL–97Program for X-ray Crystal Structure Refinement; Göttingen University: Göttingen, Germany, 1997; (b) G. M. Sheldrick. SHELXS–97Program for X-ray Crystal Structure Solution; Göttingen University: Göttingen, Germany, 1997.
24. Y. Feng, E.-C. Yang, M. Fu and X.-J. Zhao, Z. Anorg. Allg. Chem., 2010, 636, 253.
25. K. Nakamoto, Infrared and Raman Spectra of Inorganicand Coordination Compounds, fourth ed., Wiley Press, 1986.
26. X.-Y. Wang, L. Wang, Z.-M. Wang and S. Gao, J. Am. Chem. Soc., 2006, 128, 674.
27. T.-F. Liu, D. Fu, S. Gao, Y.-Z. Zhang, H.-L. Sun, G. Su and Y.-J. Liu, J. Am. Chem. Soc., 2003, 125, 13976.
28. Y.-Z. Zhang, H.-Y. Wei, F. Pan, Z.-M. Wang, Z.-D. Chen and S. Gao, Angew. Chem., Int. Ed., 2005, 44, 5841.
29. (a) W. D. Junior, H. J. N. Ishley and R. R. Whittle, Inorg. Chem., 1982, 21, 3270; (b) J. Costamagna, F. Caruso, M. Rossi, M. Campos, J. Canales and J. Ramirez, J. Coord. Chem., 2001, 54, 247.
30. O. V. Dolomanov, A. J. Blake, N. R. Champness and M. J. Schröder, J. Appl. Crystallogr., 2003, 36, 1283.
31. A. L. Spek, PLATONa multipurpose crystallographic tool, Utrecht University, Utrecht, The Netherlands, 2001.
32. (a) A. Morsali and L. G. Zhu, Helv. Chim. Acta, 2006, 89, 81; (b) A. G. Orpen, L. Brammer, F. H. Allen, D. G. Watson and R. Taylor, J. Chem. Soc., Dalton Trans., 1989, S1.
33. J. Luo, X.-R. Zhang, L.-L. Cui, W.-Q. Dai and B.-S. Liu, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2006, 62, m614.
34. S.-W. Peng, Y.-L. Miao and W.-D. Song, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2006, 63, m253.
35. H. Deng, Y.-C. Qiu, R.-H. Zeng and F. Sun, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63, m450.
36. R.-H. Zeng, Y.-C. Qiu, Z.-H. Liu, Y.-H. Li and H. Deng, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63, m1591.
37. S.-D. Fan and J.-T. Liu, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63, m2034.
38. (a) S. Stagni, E. Orselli, A. Palazzi, L. D. Cola, S. Zacchini, C. Femoni, M. Marcaccio, F. Paolucci and S. Zanarini, Inorg. Chem., 2007, 46, 9126; (b) J.-T. Liu, S.-D. Fan and S. W. Ng, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2007, 63, m1652.
39. (a) Q. Wu, M. Esteghamatian, N. X. Hu, Z. D. Popovic, G. Enright, Y. Tao, M. D'Iorio and S. Wang, Chem. Mater., 2000, 12, 79; (b) J. E. McGarrah, Y. J. Kim, M. Hissler and R. Eisenberg, Inorg. Chem., 2001, 40, 4510; (c) G. De Santis, L. Fabbrizzi, M. Licchelli, A. Poggi and A. Taglietti, Angew. Chem., Int. Ed. Engl., 1996, 35, 202.
40. (a) V. W. W. Yam and K. K. W. Lo, Chem. Soc. Rev., 1999, 28, 323; (b) B.-D. Lourdes, N. R. David and C.-C. Mercedes, Chem. Soc. Rev., 2007, 36, 993.
41. A. Escuer, M. Font-Bardía, S. S. Massoud, F. A. Mautner, E. Peñalba, X. Solans and R. Vicente, New J. Chem., 2004, 28, 681.
42. (a) A. Bencini, D. Gatteschi, C. Zanchini, J. G. Haasnoot, R. Prins and J. Reedijk, J. Am. Chem. Soc., 1987, 109, 2926; (b) R. Prins, P. J. M. W. L. Birker, J. G. Haasnoot, G. C. Verschoor and J. Reedijk, Inorg. Chem., 1985, 24, 4128; (c) W. M. E. K. Oudenniel, R. A. G. De Graaff, J. G. Haasnoot, R. Prins and J. Reedijk, Inorg. Chem., 1989, 28, 1128; (d) P. M. Slangen, P. J. van Koningsbruggen, J. G. Haasnoot, J. Jansen, S. Gorter, J. Reedijk, H. Kooijman, W. J. J. Smeets and A. L. Spek, Inorg. Chim. Acta, 1993, 212, 289.

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Footnote

† Electronic supplementary information (ESI) available: X-ray data in CIF format, TG curves for 1–9 and additional figures and tables. CCDC reference numbers 756331–756339 for 1–9. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c000586j

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