A novel one-dimensional coordination polymer of cadmium(II)/triazine extending by di-chloro and di-iodo bridges

Abstract

A new triazine ligand salt, 2,2′,2″-(1,3,5-triazine-2,4,6-triyl)tris(hydrazin-1-ium) chloride [THT·(HCl)₃], and the one-dimensional coordination polymer of cadmium, [Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O), are prepared and characterized by elemental analysis, Fourier-transform infrared spectroscopy, 1H NMR spectroscopy, and single-crystal X-ray diffraction (for the cadmium polymer). X-ray analysis revealed that the polymeric backbone is extended alternatively by di-iodo and di-chloro bridges; this type of bridge is not observed previously between any metal atoms. There are two types of cadmium atoms in the polymer: cadmium of the polymeric chain and terminal cadmiums. The geometry around the first (CdN₂Cl₂I₂) is octahedral, while the latter (CdN₃I₂) has an incline to square-pyramidal geometry. The triazine ligand of this structure also bridges two cadmium atoms and acts as an N₃ donor toward the terminal cadmium atoms and as an N₂ donor toward the cadmium atoms of the chain. In the crystal network of the cadmium polymer, the hydrogen bonds of N–H···X (X: O, N, I) form different hydrogen bond motifs, including $R_{2}^{2}$ (8), $R_{4}^{2}$ (10), $R_{4}^{3}$ (14), $R_{4}^{4}$ (18), $R_{6}^{4}$ (20), $R_{6}^{6}$ (24), $R_{6}^{6}$ (28), and $R_{6}^{6}$ (32).

Keywords

cadmium(II)chloro bridge coordination polymer iodo bridge triazine

Introduction

1,3,5-Triazine (s-triazine) has been widely used in organic reactions^1–6 and offers access to a multitude of useful molecules^7,8 due to its specific structure and electronic properties. This scaffold has provided the basis for the design of biologically active molecules with diverse therapeutic profiles,^9–13 herbicides,^14,15 and polymer photostabilizers.¹⁶ Some trisubstituted-1,3,5-triazines are also used as liposomes.¹⁷

Cyanuric chloride is an essential organic intermediate in which three chlorines can be replaced by –NH₂, –OH, –SH, or –NHR, step by step, with high yields.^18,19 Cyanuric chloride derivatives have been studied for decades, especially the amino derivatives. It is generally accepted that the first chlorine of cyanuric chloride can be easily substituted by an NH₂ group at 0–5°C, the second one at 30–50°C, and the third one typically above 80°C, which depends on the activity of amine nucleophiles.²⁰

The last decade has witnessed an upsurge of research efforts toward the development of coordination polymers (CPs). Due to their novel and innovative methodologies with diversified and tunable structural frameworks, significant interest has been created in coordination chemistry, focusing mainly on applications such as catalysis,²¹ drug delivery,²² gas storage,²³ luminescence,²⁴ separation,²⁵ adsorption,²⁶ photo-physical studies, antimicrobial properties, materials chemistry,²⁷ and pollutant sequestration.²⁸

To extend the chemistry of CPs containing 1,3,5-triazines, in this work, the preparation and characterization (elemental analysis, Fourier-transform infrared spectroscopy (FTIR), and 1H NMR spectroscopy) of a new triazine chloride salt, 2,2′,2″-(1,3,5-triazine-2,4,6-triyl)tris(hydrazin-1-ium) chloride [THT·(HCl)₃] (Figure 1), and a one-dimensional (1D) cadmium(II) CP, [Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O) (1), is presented along with structural analysis of 1 and crystallographic studies.

Figure 1.

Structure of 2,2′,2″-(1,3,5-triazine-2,4,6-triyl)tris(hydrazin-1-ium) chloride (THT·(HCl)₃).

Results and discussion

The reaction between THT·(HCl)₃ and CdI₂ in a branched tube provided the 1D CP of 1. The polymeric complex is air stable and soluble in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO).

Spectroscopic characterization

In the FTIR spectrum of THT·(HCl)₃, the presence of ν (N–H) stretching vibrations confirms the successful condensation of hydrazine with 2,4,6-trichloro-1,3,5-triazine. These vibrations shifted by −24 and +67 cm⁻¹, respectively, after complexation to cadmium(II) iodide. In the FTIR spectra of ligand and 1, three peaks at ca. 1550, 1420, and 800 cm⁻¹ were assigned to the vibrations of the 1,3,5-triazine unit due to quadrant stretching, semicircle stretching, and out-of-plane ring bending, respectively.²⁹ The quadrant stretching vibration of THT is split after coordination to the cadmium(II) ion, which can be attributed to the asymmetric coordination of the triazine unit.

The presence of a water molecule in 1 affects its IR spectrum in three regions,^30,31 including 3500–3550 cm⁻¹ for asymmetric and symmetric OH stretches, 1635 cm⁻¹ for H₂O bending, and 200–600 cm⁻¹ for librational modes. These modes are due to rotational oscillations of the water molecules restricted by interactions with neighboring atoms, and they are classified into three types (wagging (ρ_w), twisting (ρ_t), and rocking (ρ_r)) depending upon the direction of the principal axis of rotation.^31,32 In the spectrum of the complex, the bending stretch of the amine moiety is overlapped with the sharp peak of the triazine unit.

In the 1H NMR spectrum of the ligand salt, all the protons of the protonated hydrazine arms were observed as a broad peak with a maximum at 4.02 ppm. This peak is shifted to lower magnetic field after the complexation process and is split into two singlets. The protonated salt ligand loses three protons after binding to the cadmium(II) ion, which leads to an increase in the electron density on this ligand, and thus, all protons are shifted to lower magnetic field in 1.

Crystal structure of [Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O)

X-ray structural analysis of 1 (Figure 2) reveals a 1D CP extending in the crystallographic ac plane. In this structure, there are two types of bridges between cadmium atoms: di-chloro bridges and di-iodo bridges. Searching the Cambridge Structural Database (CSD)³³ revealed that there are no examples containing such bridges between two metal atoms or ions. The two chloro and iodo bridges are asymmetric (bond length difference: 0.268 and 0.097 Å, for chloro and iodo, respectively), with the iodo bridge being more symmetric than the chloro bridge.

Figure 2.

The ORTEP-3 diagram of the molecular structure of 1. Ellipsoids are drawn at the 50% probability level. The polymeric chain is shown in orange color.

A search of the CSD for all structures containing di-chloro or di-iodo bridges revealed the results shown in Figure 3. This comparison shows that the bond lengths and angles of Cd₂(μ-X)₂ (X: Cl, I) units are comparable with those of CSD analogues, except for one of the Cd–Cl bonds which is 0.232 Å longer than the CSD average (Figure 3). The Cd–I^bridge bond length among the CSD examples containing a Cd₂(μ-X)₂ moiety is 0.331 Å longer than the Cd–Cl^bridge example. The Cd–Cl–Cd bond angle is larger than the Cl–Cd–Cl angle while replacing the chloride ions (Figure 3(b)) with iodide (Figure 3(c)) leads to the order of the angles changing to Cd–I–Cd < I–Cd–I owing to the larger size of the iodide ion. In another comparison, deviation of the atoms from a plane through the atoms of the Cd₂(μ-X)₂ (X: Cl, I) units was calculated. The CSD average for all deviations was calculated to be 0.043 and 0.015 Å, which means that the two units are planar and have a rectangular shape. No deviation has been observed for similar units of 1.

Figure 3.

Coordinated bond lengths and average angles for (a) Complex 1: the Cd₂(μ-Cl)₂ unit in 1, (b) CSD average: the Cd₂(μ-Cl)₂ unit in the CSD, (c) CSD average: the Cd₂(μ-I)₂ unit in the CSD and (d) Complex 1: the Cd₂(μ-I)₂ unit in 1.

In this structure, there are two cadmium(II) ions with different environments, including Cd(N^amine)₂(N^tz)(I^ter)₂ and Cd(N^amine)(N^tz)(µ-I)₂(µ-Cl)₂. The Cd1 atom with a coordination number of six has an octahedral geometry. A pentacoordinate geometry in Cd2 may adopt either a square-pyramidal or a trigonal bipyramidal structure. To determine the geometry for such complexes, the formula of Addison et al.³⁴ was applied. The angular structural parameter (τ) value for Cd2 was calculated to be 0.20, indicating an incline to square-pyramidal geometry.

The THT ligand acting as a bridging pentadentate (N^amine)₃(N^tz)₂-donor forms three five-membered chelate rings. A mean plane through the chelate rings revealed that these rings are almost planar (maximum deviation of atom from plane: 0.042 Å (C3), 0.023 Å (N2), and 0.076 Å (N4)). The dihedral angles between the mean planes through the triazine unit and each chelate rings are 4.23°, 4.01°, and 1.90°, confirming that these rings are almost co-planar and that the hydrazine arms of the ligand do not bend from the molecule plane. Except for the N1–C2 bond (1.28(4) Å), the other triazine unit bond lengths are comparable (1.34(4)−1.37(4) Å), which means that the bond delocalization at this unit is not perfect.

In the structural network of 1 (Figure 4), there are N–H···X (X: O, N, I) hydrogen bonds. Among the different atoms participating in intermolecular interactions, the nitrogen atom acts as a proton donor (amine units) and an acceptor (non-coordinated triazine nitrogen atom) at the same time. The N–H···I hydrogen bonds participate in the formation of one $R_{2}^{2}$ (8) hydrogen bond motif (two acceptors, two donors with a degree of 8)³² between two adjacent molecules on the side chains (Figure 5). In addition to these motifs, there are many other motifs, including $R_{4}^{2}$ (10), $R_{4}^{3}$ (14), $R_{4}^{4}$ (14), $R_{4}^{4}$ (18), $R_{6}^{4}$ (20), $R_{4}^{2}$ (10), $R_{6}^{5}$ (24), $R_{6}^{6}$ (24), $R_{6}^{6}$ (28), and $R_{6}^{6}$ (32).

Figure 4.

Packing of 1 along the ac plane, showing the participation of water molecules in hydrogen bonding and the octahedral and square-pyramidal geometries around the cadmium atoms.

Figure 5.

Packing of 1 along the ab plane, showing the $R_{2}^{2}$ (8) hydrogen bond motifs and 1D polymeric chain.

Conclusion

In this work, two new triazine compounds, 2,2′,2″-(1,3,5-triazine-2,4,6-triyl)tris(hydrazin-1-ium) chloride [THT·(HCl)₃] and 1D-[Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O) (1), have been synthesized, and their spectral properties were investigated. In polymer 1, there are two types of cadmium atoms with different geometries. The terminal cadmium atom has an incline to square-pyramidal geometry, while the geometry around the cadmium atom in the polymeric chain is octahedral. The polymeric structure is extended by a novel new method and involves double bridging iodo and double bridging chloro ligands. The THT ligand does not participate in formation of the polymeric backbone and connects the terminal cadmium to that of the chain. A study of the CSD examples containing a Cd₂(μ-X)₂ (X: Cl, I) unit revealed that this unit is planar and has a rectangular shape. In the crystal network of 1, the hydrogen bonds of N–H···X (X: O, N, I) form different types of hydrogen bond motifs, including $R_{2}^{2}$ (8), $R_{4}^{2}$ (10), $R_{4}^{3}$ (14), $R_{4}^{4}$ (14), $R_{4}^{4}$ (18), $R_{6}^{4}$ (20), $R_{4}^{2}$ (10), $R_{6}^{5}$ (24), $R_{6}^{6}$ (24), $R_{6}^{6}$ (28), and $R_{6}^{6}$ (32).

Experimental

Materials and instrumentation

All starting chemicals and solvents were of reagent or analytical grade and were used as received. The infrared spectra of the samples as KBr disks in the range 400–4000 cm⁻¹ were recorded with an FTIR TENSOR 27 spectrometer. 1H NMR spectra were recorded on a Bruker Aspect 3000 instrument. The carbon, hydrogen, and nitrogen contents were determined using a Thermo Finnigan Flash Elemental Analyzer 1112 EA. The melting points were determined with a Barnstead Electrothermal 9200 electrically heated apparatus.

Preparation of THT·(HCl)₃

A mixture of 2,4,6-trichloro-1,3,5-triazine (4.0 g, 21.69 mmol) and an excess amount of hydrazine hydrate (30 mL) were reacted under reflux conditions (without solvent) for 24 h by keeping the temperature at 80°C. The unreacted hydrazine hydrate was removed using a rotary evaporator to give a white powder. The product was washed twice with ethanol (20 mL). Yield 5.23 g, 66%. m.p. 197°C. Anal. calcd for C₃H₁₂Cl₃N₉ (280.54): C, 12.84; H, 4.31; N, 44.94; found: C, 13.01; H, 4.38; N 44.52. IR (KBr): 3275 m and 3150 m (ν N–H), 1579 s (ν triazine ring quadrant stretching and or δ NH₂), 1420 s (ν triazine ring semicircle stretching), 1175 s (ν C–N), 1093 s (ν N–N), 797 m (γ triazine) cm⁻¹. 1H NMR (300 MHz, DMSO-d₆): δ = 4.02 (br s, 12H, NH, NH₂⁺).

Preparation of [Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O)

THT·(HCl)₃ (0.14 g, 0.50 mmol), and CdI₂ (0.32 g, 0.87 mmol) were placed in the large arms of a branched tube with a total capacity of 15 mL. A mixture of ethanol/H₂O (1:1 volume) was carefully added to fill both arms. The tube was then sealed, and the ligand-containing arm was immersed in an oil bath at 60°C, while the other arm was maintained at ambient temperature.^35–37 After 5 days, crystals deposited in the cooler arm and were filtered off and dried in air. Yield 0.07 g, 37%. m.p. 240°C. Anal. calcd for C₃H₁₃Cd₂ClI₃N₉O₂ (848.17): C, 4.25; H, 1.54; N, 14.86; found: C, 4.31; H, 1.55; N, 14.95. IR (KBr): 3546 m (ν_as O–H), 3495 m (ν_s O–H), 3251 m (ν_as NH₂), 3194 m (ν_s NH₂), 3134 m (ν N–H), 1635 s (δ H₂O), 1581 and 1541 s (ν triazine ring quadrant stretching and or δ NH₂), 1419 s (ν triazine ring semicircle stretching), 1170 m (ν C–N), 1088 s (ν N–N), 794 m (γ triazine), 582 w (ρ_r H₂O), 485 m (ρ_w H₂O) cm⁻¹. 1H NMR (300 MHz, DMSO-d₆): δ = 2.95 (s, 3H, NH), 2.15 (s, 6H, NH₂).

Crystal structure determination and refinement

X-ray diffraction data for 1 were collected at T = 173 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with an XtaLAB P100 diffractometer. CuKα radiation (λ = 1.54184 Å) was used, and intensity data were collected using ω and ϕ steps accumulating area detector images spanning at least a hemisphere of reciprocal space. All data were corrected for Lorentz polarization effects. A multiscan absorption correction was applied by using CrysAlis Pro.³⁸ The structure was solved using the intrinsic phasing method (SHELXT³⁹) and refined by full-matrix least-squares against F² (SHELXL-2013⁴⁰). Non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were refined geometrically using a riding model, except for those on the waters of solvation, which were not modelled. All calculations were performed using Olex 2.⁴¹ Selected crystallographic data are presented in Table 1. Diagrams of the molecular structure were created using SHELXP³⁹ and Diamond.⁴² Selected bond lengths are displayed in Table 2 and hydrogen bond geometries in Table 3.

Table 1.

Crystal structure data and structure refinement of complex 1.

Empirical formula	C₃H₉Cd₂ClI₃N₉·2(H₂O)
Formula weight, g mol⁻¹	848.17
Crystal size, mm³	0.12 × 0.06 × 0.02
Temperature, K	173
Crystal system	Triclinic
Space group	P $\bar{1}$
Unit cell dimensions
a, Å	7.2444(2)
b, Å	7.3859(2)
c, Å	17.0237(9)
α, degrees	86.180(3)
β, degrees	82.881(3)
γ, degrees	83.601(2)
Volume, Å³	896.94(6)
Z	2
Calculated density, g cm⁻³	3.141
Absorption coefficient, mm⁻¹	61.05
F(000), e	764
θ range data collection, degrees	2.6–68.0
h, k, l ranges	−8 ⩽ h ⩽ 8, −8 ⩽ k ⩽ 8, −5 ⩽ l ⩽20
Reflections collected/independent	3191/3191
Data/ref. parameters	3191/182
R1/wR2 (I > 3 σ(I))	0.1057/0.3286
R1/wR2 (all data)	0.1098/0.3292
Goodness-of-fit on F²	1.167
Largest diff. peak/hole, e Å⁻³	3.04/−2.88

Table 2.

Selected bond lengths (Å) and angles (°) for complex 1 with estimated standard deviations in parentheses.

Bond length		Angle
Cd1–I1	2.872(3)	I1–Cd1–I1	91.6(1)
Cd1^a–I1	2.969(4)	I1–Cd1–Cl1	97.2(2)
Cd1–Cl1	2.612(9)	Cl1–Cd1–Cl1	86.9(2)
Cd1–Cl1^b	2.880(9)	Cl1–Cd1–N3	84.2(6)
Cd1–N3	2.35(2)	N3–Cd1–N5	71(1)
Cd1–N5	2.35(3)	N5–Cd1–I1	103.5(8)
Cd2–N2	2.27(3)	N9–Cd2–N2	68(1)
Cd2–N9	2.47(3)	N2–Cd2–N7	69(1)
Cd2–I2	2.742(4)	N7–Cd2–I3	105.1(8)
Cd2–I3	2.747(4)	I3–Cd2–I2	113.4(1)
		I2–Cd2–N9	108.7(7)

Symmetry operators: ^a−x, −y + 1, −z + 1; ^b−x + 1, −y + 1, −z + 1.

Table 3.

Hydrogen bond dimensions (Å and °) in complex 1.

D–H···A	d(D–H)	d(H···A)	d(D···A)	<(DHA)	Symmetry code on atom A
N5–H5A···O1	0.89	2.10	2.94(5)	157	x, y, z
N9–H9B···O1^d	0.89	2.29	3.04(5)	142	1 + x, −1 + y, z
N9–H9A···N1^c	0.89	2.20	3.07(4)	164	x, −1 + y, z
N6–H6···I2^a	0.86	2.74	3.51(3)	151	x, 1 + y, z
N7–H7A···I2^b	0.89	2.93	3.80(3)	165	2 − x, 1 − y, −z

Symmetry operators: ^ax, y + 1, z; ^b−x + 2, −y + 1, −z; ^cx, y − 1, z; ^dx + 1, y − 1, z.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Zahra Mardani

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A novel one-dimensional coordination polymer of cadmium(II)/triazine extending by di-chloro and di-iodo bridges

Abstract

Keywords

Introduction

Results and discussion

Spectroscopic characterization

Crystal structure of [Cd2(µ-THT)(µ-Cl)(µ-I)I2]·2(H2O)

Conclusion

Experimental

Materials and instrumentation

Preparation of THT·(HCl)3

Preparation of [Cd2(µ-THT)(µ-Cl)(µ-I)I2]·2(H2O)

Crystal structure determination and refinement

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References

Crystal structure of [Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O)

Preparation of THT·(HCl)₃

Preparation of [Cd₂(µ-THT)(µ-Cl)(µ-I)I₂]·2(H₂O)