English

• 0.   Introduction

  Halocadmates(Ⅱ) have received a lot of attention because of their remarkable structural, thermal, catalyt- ic, and electrical properties^[1-4]. The flexibility of Cd(Ⅱ), due to the d¹⁰ configuration^[5], allows for a wide range of coordination numbers in part with unusual structural variations. They range from simple tetrahedral anions [CdX_4-n(H2O)_n]^x- (n=0-2, x=2-0) with halides and water as ligands to polymeric halidocadmate anions such as _∞[CdX_n]^- (n=1-3) forming 1D chains or 2D layers^[6]. The use of different organic ligands and their cations has previously allowed us to design and synthesize halido- cadmate polymeric compounds^[6-8]. The features of the organic cations on the packing interactions affect the expected crystal organization, hydrogen - bond interac- tions between the polar protonated amine or pyridine groups of the organic cations, and chloride of the inor- ganic anions, therefore affect also the specific proper- ties^[6-8]. In the frame of organic-inorganic hybrid perovskites, the impact of the organic cation on dimen- sionality has been broadly studied^[9-11]. In order to inves- tigate the effect of functional groups on pyridinium cations on the structure and physical properties of pyri- dinium cadmates, we have recently reported the 1D perovskite (2-amino-4-methylpyridinium)trichlorocad- mate(Ⅱ) (C₆H₉N₂)_∞¹ [CdCl₃] ^[12]. Herein, we report on our attempts to use 4 - (dimethylamino)pyridinium as a cation in the synthesis of a new pyridinium chlorocad- mate which has led to the compound bis(4-N, N-dimeth- ylaminopyridinium)tetrachlorocadmate(Ⅱ) hemihydrate (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O. We studied the crystal struc- ture using single- crystal X-ray diffraction, which was able to support the phase purity by powder X - ray diffraction (PXRD) and the hydrogen bonding network by Hirshfeld surface analysis, and investigated the IR and UV-Vis spectroscopic properties of the title compound.

  1.   Experimental

  1.1   Instrumentation

  The infrared spectrum was recorded using a KBr pellet on a Thermo Scientific FT-IR spectrometer (type Nicolet 6700), fitted with a DRIFTS-cell, in a range of 4 000-400 cm^-1. UV-Vis, emission, and excitation spec- tra were recorded on a Double Edinburgh Instruments FLSP920/FSP920 spectrometer setup in the solid state. Diffuse reflectance measurements were performed on a Perkin Elmer Lambda 900 UV - Vis - NIR spectropho- tometer in a range of 250-800 nm at room temperature. The PXRD pattern was recorded at room temperature using a Bruker D8 Advance diffractometer operating with Cu Kα radiation (λ =0.154 06 nm) in a wide 2θ range of 5°-80°. The working voltage was 40 kV, and the working current was 40 mA.

  1.2   Synthesis of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

  To a solution of 0.244 g (2 mmol) 4-(dimethylami- no)pyridine in 10 mL MeOH, 1 mL of 36% HCl were added. Then, 10 mL of a MeOH solution of CdCl₂·H₂O (0.200 g, 1 mmol) containing 1 mL of 36% HCl was added. After 2 h of stirring at room temperature, the mixture was filtered and left to evaporate. Slow evapo- ration produced colorless crystals after three weeks. Yield: 393 mg (0.77 mmol, 77% based on CdCl₂·H₂O). Anal. Calcd. for (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O(%): C, 33.06; H, 4.36; N, 11.02; Cd, 22.10; Cl, 27.88; Found (%): C, 33.02; H, 4.33; N, 11.07; Cd, 22.10; Cl, 27.86. The PXRD pattern of the title compound was in good agreement with the calculated (Fig. 1), confirming the phase purity of the synthesized product. In the experi- mental pattern, some reflections were very dominant over the others, e.g. those at around 2θ=12°, 17°, 22°, and 29°. We assume that this is due to a layered struc- ture.

  Figure 1

  

  Figure 1.  Experimental and calculated PXRD patterns of the compound

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  1.3   Single crystal X-ray diffraction

  Under a polarizing microscope, a good quality sin- gle crystal was selected with a parallelepiped shape of 0.35 mm×0.24 mm×0.17 mm, which was chosen for measurement on an automatic four-circle Enraf-Nonius diffractometer. A search was performed by means of 25 reflections using the CAD4 - Express program^[13], the radiation source used corresponds to the Kα radiation of molybdenum (λ=0.071 07 nm). The unit cell dimen- sions were measured and the data collection was per- formed. Absorption corrections were carried out using ψ-scans^[14].

  The structure was solved using direct methods and refined by full-matrix least-squares refinement on F² using the WinGX software package^[15]. Crystal data, data collection, and structure refinement details are summarized in Table 1. The water molecule was mod- eled by an anisotropic ADP. Thus, the hydrogen atoms on H₂O were not located. The composition and the den- sity in the cif file were corrected for the two missing H atoms. All remaining H atoms were positioned geomet- rically and refined using the riding model (N—H(ring) was set to 0.086 nm using U_iso(H) =1.2 U_eq(N), C—H (aromatic) was set to 0.093 nm with U_iso(H)=1.2 U_eq(C) and C—H (methyl) was set to 0.096 nm with U_iso(H) = 1.5 U_eq(C)).

  Table 1

  

  Table 1.  Crystal data and structure refinement parameters for (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  Parameter	(C7H11N2)2[CdCl4]·0.5H2O		Parameter	(C7H11N2)2[CdCl4]·0.5H2O
  Empirical formula	C28H44Cd2Cl8N8O		V/nm3	1.056 5(7)
  Formula weight	509.57		Dc/(g·cm-3)	1.6
  Temperature/K	293(2)		μ/mm-1	1.55
  Crystal system	Triclinic		F(000)	508
  Space group	P1		θ range for data collection/(°)	2.5-28
  a/nm	0.796 9(3)		Limiting indices	-10 ≤ h ≤ 6, -10 ≤ k ≤ 10, -22 ≤ l ≤ 22
  b/nm	0.804 4(3)		Reflection collected, unique, parameter	6 686, 5 062, 221
  c/nm	1.695 6(5)		GOF on F2	1.03
  α/(°)	95.71(3)		Final R indices [I > 2σ(I)]	R1=0.046
  β/(°)	100.82(4)		R indices (all data)	wR2=0.135
  γ/(°)	94.79(3)		Tmax, Tmin	0.953, 1.000
  Z	1		(Δρ)max and (Δρ)min/(e·nm-3)	721 and -572

  CCDC: 2136319.

  2.   Results and discussion

  2.1   Single crystal X-ray analysis

  The title compound (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O crystallizes in the triclinic P1 space group. The asym- metric unit includes two 4 -(dimethylamino)pyridinium cations, an isolated [CdCl₄]^2- anion and half a water molecule (Fig. 2).

  Figure 2

  

  Figure 2.  Asymmetric unit of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O with a numbering scheme

  Displacement ellipsoids are drawn at a 50% probability

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  The coordination geometry of the four chlorides around Cd(Ⅱ) is distorted tetrahedral. The Cd—Cl bonds are in a range of 0.243 92(18)-0.247 61(16) nm. Most of the Cl—Cd—Cl angles deviate only marginally from the ideal 109°, (Cl2—Cd—Cl4 110.75(6)°; Cl3— Cd—Cl1 110.47(6)°; Cl3—Cd—Cl4 110.38(6)°; Cl3— Cd—Cl2 110.36(7)°; Cl4—Cd—Cl1 109.94(6)°) while Cl2—Cd—Cl1 with 104.82(6)° is slightly more acute. The calculated average values of the different distances and angles distortion indices (DIs) are 0.004 8 for Cd— Cl and 0.001 4 for Cl—Cd—Cl. These values are com- parable to those previously reported^{[5, 12, 16-18]}. The C1—N1—C5 (120.7(6)°) and C8—N2—C12 (119.9(5)°) bond angles in the pyridinium cations are wider than those in the neutral pyridine (114.15(5)°)^[19], which is in line with the protonation on the pyridine N atom.

  The crystal structure of the title compound (Fig. 3) shows alternating organic and inorganic layers parallel to the (001) plane and located at x=n+1/2 (n ∈ Z) ^[18]. Overlapping rows of coplanar pyridinium rings held together by π-stacking interactions with the centroid- centroid distances of 0.368 5(17), 0.375 0(14), and 0.376 6(17) nm (Fig. 4), are complemented by rows of tetrachloridocadmate anions and water molecules. These layers form a 3D architecture through N—H… Cl, N—H…O, C—H…Cl, and C—H…O hydrogen bonds (Fig. 3, Table 2).

  Figure 3

  

  Figure 3.  View of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O on the bc plane

  Dotted lines show hydrogen bonds: C—H…Cl (orange), C—H…O (green), N—H…Cl (red), and N—H…O (blue)

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  Figure 4

  

  Figure 4.  π-stacking interactions between the neighboring aromatic organic cations in (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

  Centroids of the pyridinium cation are marked in red

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  Table 2

  

  Table 2.  Hydrogen bonds in the structure of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  D—H…A	d(D—H)/nm	d(H…A)/nm	d(D…A)/nm	∠D—H…A/(°)
  N1—H1N…O1Ai	0.087(6)	0.236(5)	0.294 6(12)	125(5)
  N1—H1N…Cl12	0.087(6)	0.253(6)	0.294 6(12)	142(5)
  N2—H2N…Cl1	0.086(9)	0.242(9)	0.319 1(7)	150(7)
  C1—H1…Cl2ii	0.093	0.280	0.363 7(6)	150
  C9—H9…O1A	0.093	0.243	0.333 0(12)	162
  C12—H12…Cl4	0.093	0.280	0.353 4(7)	137
  Symmetry codes: i -x+1, -y, -z+2; ii x, y, z-1.

  2.2   Hirshfeld surface analysis

  CrystalExplorer 3.1 software^[20] was employed to generate the Hirshfeld surface thus assessing the contributions of the different intermolecular interac- tions in the structure. The Hirshfeld surface volume and surface area are calculated to be 0.518 96 nm³ and 0.463 72 nm². The d_norm mapping of Hirshfeld (Fig. 5) are shown in blue, red, and white color schemes. The red region (d_norm is negative) is due to intermolecular N—H…Cl, C—H…Cl, N—H…O, and C—H…O hydrogen bonds the white colored regions represent the contacts which are equal to the van der Waals radii (d_norm=0), and the blue spots (d_norm is positive) show the contacts longer than the sum of van der Waals radii^[21]. The 2D fingerprint of the Hirshfeld surface (Fig. 6) shows the relative contribution to various intermolecu- lar contacts. The H…Cl/Cl…H interaction contributes 36.9% of the total Hirshfeld surface, the H…H interac- tions have a significant contribution amount of 32.3%, and the contribution of C…H/H…C interaction to 7.5%. From this we conclude that these van der Waals forces exert an important influence on the stabilization of the packing in the crystal structure, other visible spots in the surfaces refer basically to O…H/H…O, N…H/H…N, Cl…O/O…Cl, C…C.

  Figure 5

  

  Figure 5.  Hirshfeld surface of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O mapped with d_norm

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  Figure 6

  

  Figure 6.  Two-dimensional fingerprint plots of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O: (a) all contacts; (b) H…Cl/H…Cl, (c) H…H, (d) C…H/H…C, (e) H…O/O…H and (f) N…H/H…N contacts showing the percentage of total Hirshfeld surface area

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  2.3   IR spectroscopy

  The IR spectrum of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O (Fig. 7) was assigned based on comparison with the spectra of 4-(dimethylamino)pyridinium in dimercurate (C₇H₁₁N₂)₂[Hg₂Cl₆] (C₇H₁₁N₂=4-(dimethylamino)pyridini- um) ^[22], the mixed cadmate/mercurate (C₆H₉N₂)₂ (Hg0_.75Cd_0.25)Cl₄ (C₆H₉N₂=2 - amino - 4 - methylpyridini- um) ^[23], 3-hydroxy 2-nitropyridine^[24], and the Li salt [Li(4-dimethylaminopyridine)]Cl^[25].

  Figure 7

  

  Figure 7.  Infrared spectrum of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  The band at 3 258 cm^-1 can be assigned to the ν(N—H) stretching vibration of the pyridinium cations in the title compound. The bands at 3 127-3 100 cm^-1 are ascribed to asymmetric and symmetric stretching of the NCH₃ groups (ν_as and ν_s) ^[22]. Overtones of deforma- tion modes were observed between 2 000 and 1 980 cm^-1. The N—H bending observed at 1 650 cm^-1, and the bands at 1 565 and 1 445 cm^-1 are attributed to the C=C and C=N stretching modes of the pyridine ring^[23]. The sharp peak at 1 394 cm^-1 is assigned to the symmetric C=C (aromatic) stretching^[24]. The absorp- tion vibration bands corresponding to the C—N group are identified at about 1 217 cm^-1. These bands observed in the wavelength range of 1 071 - 750 cm^-1 come from C—N, C—C stretching vibrations and sym- metrical C—N stretching vibrations^[25].

  2.4   Optical band gap determination from diffuse reflectance spectra

  The diffuse reflectance spectrum (Fig. 8) shows two bands with maxima at 286 nm (4.33 eV) and 345 nm (3.59 eV) which are assigned to the π - π* excita- tions of 4 - (dimethylamino)pyridinium cations^{[22, 25-26]}. In EtOH solution, the absorptions of 4 - dimethylmamino- pyridinium at 213 nm (5.82 eV) and 278 nm (4.46 eV) were reported for the compound (C₇H₁₁N₂)[Cl₂Hg(μ-Cl₂) HgCl₂]^[22], thus the broad feature in the reflectance spec- trum peaking at 345 nm does not correspond isolated pyridinium cations.

  Figure 8

  

  Figure 8.  Diffuse reflectance spectrum of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  The optical band gap E_g was calculated using the Kubelka-Munk function^[27]:

  $ F\left( {{R_{\rm{d}}}} \right) = {\left( {1 - {R_{\rm{d}}}} \right)^2}/\left( {2{R_{\rm{d}}}} \right){\rm{ = }}\alpha /s $

  (1)

  where R_d is the diffuse reflectance. The absorption coef- ficient α is a function of the photon energy (hν)^[28-29]:

  $ \left( {\alpha h\nu } \right) = B{\left( {h\nu - {E_{\rm{g}}}} \right)^n} $

  (2)

  Where B is a constant and n is the index, which takes different values depending on the mechanism of inter- band transitions, with n=1/2 or n=2 corresponding to direct or indirect transitions, respectively^[29].

  The band gap energies were calculated from the plots of (αhν)^1/2 vs hν (Fig. 9, top) or (αhν)² vs hν (Fig. 9b, bottom). Extrapolation of the linear portion to the x-axis gave the E_g of 3.596 and 3.684 eV, respec- tively. In keeping with the lowest π - π* transition at 3.59 eV (Fig. 8), the character is thus that of a direct semiconductor. These values are quite high and the corresponding high gap between the HOMO and the LUMO indicates relatively high stability in terms of energy and a high chemical hardness^{[22, 30]}. The behavior as a semiconductor material, is in line with previous reports^{[22, 25, 31]} and we assume that the extensive π-stack- ing of the pyridinium cations is the underlying struc- ture in line with the observation of the 345 nm maxi- mum in the reflectance spectrum, while for isolated (dissolved) cations there is no such absorption.

  Figure 9

  

  Figure 9.  Plots of (αhν)^1/2 (top) and (αhν)² (bottom) vs hν for (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  Because of lacking suitable equipment, we were not able to experimentally study the semiconducting behavior of the title compound. We will do this in future studies. Quite generally, such UV semiconduc- tors (wide bandgap semiconductors) are interesting as photocatalysts^[29] or UV detectors and sensors^[32-34] in the form of nanoparticles, nanowires, or thin films^{[32, 34]}. Compared to the established binary and ternary metal oxides, nitrides, and carbides in this field, the hybrid organic - inorganic materials offer the benefit of easy synthesis, simple processability for the manufacturing of thin films, and high stability^{[9, 11, 35]}.

  2.5   Photoluminescence properties

  The solid - state photoluminescence spectrum at room temperature showed an emission maximum at 319 nm and upon excitation at 562 nm (Fig. 10). The spec- trum of the 4-(dimethylamino)pyridine molecule is very similar^[26] and we assign the emission to fluorescence originating from π - π* excited states of the aromatic cation. Remarkably, the maximum of the excitation spectrum at 319 nm (3.88 eV) coincides roughly with the minimum between the two maxima at 286 and 345 nm in the diffuse reflectance spectrum which lies at ca. 325 nm (3.81 eV) (Fig. 8).

  Figure 10

  

  Figure 10.  Emission and excitation spectra of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O: emission at λ_ex=319 nm (black), excitation at λ_em=562 nm (red)

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  3.   Conclusions

  In summary, the compound (C₇H₁₁N₂)₂[CdCl₄]· 0.5H₂O (C₇H₁₁N₂=4-(dimethylamino)pyridinium) was synthesized by slow evaporation at room temperature from HCl - containing MeOH solutions in a yield of 77%. The crystal structure was solved in the triclinic space group P1. The crystal structure shows multiple N—H…O, N—H…Cl, C—H…O, and C—H…Cl hydrogen bonds and π - π interactions. Hirshfeld sur- face analysis confirms the hydrogen bonding and π - stacking but also reveals that H…Cl/Cl…H, H…H, and C…H/H…C van der Waals interactions constitute about 77% of the total Hirshfeld surface. IR spectrosco- py confirms the presence of the different groups in the structure. UV - Vis diffuse reflectance measurement showed maxima at 286 nm (4.33 eV) and 345 nm (3.59 eV), the Tauc plot analysis gave a value of 3.596 eV for a direct semiconductor in line with the long-wavelength absorption of the material. Fluorescence at 562 nm upon excitation at 319 nm (3.89 eV) is assigned to excited π-π* states. Comparison of the reflectance da- ta and the luminescence behavior let us conclude that the photophysics is dominated by the 4-(dimethylami- no)pyridinium cations and a closer inspection shows that extensive π-stacking of these cations leads to pro- nounced layers in the structure, which are the underly- ing structural units for the semiconductor behavior. Thus, we have not obtained a semiconducting 1D or 2D material similar to the organic-inorganic hybrid perovskites in which the inorganic halidometalate largely determines the electronic properties, as we ini- tially hoped. Instead, we obtained a structure with iso- lated [CdCl₄]^2- anions separated by stacks of the pyri- dinium cations. These stacks are very probably the lead structures for the semiconducting properties. In future studies, we will investigate them experimentally in detail.

  Acknowledgments: We thank the director of XStruct Laboratory, Department of Chemistry, Ghent University, Krijgslaan 281 - S3, B - 9000 Ghent, Belgium, Prof. Dr. Kristof Van Hecke, and Dr. Marina Saab for the spectroscopic measurement.

  Disclosure statement: No potential conflict of interest was reported by the author(s).

  
  
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  Figure 1  Experimental and calculated PXRD patterns of the compound

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  Figure 2  Asymmetric unit of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O with a numbering scheme

  Displacement ellipsoids are drawn at a 50% probability

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  Figure 3  View of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O on the bc plane

  Dotted lines show hydrogen bonds: C—H…Cl (orange), C—H…O (green), N—H…Cl (red), and N—H…O (blue)

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  Figure 4  π-stacking interactions between the neighboring aromatic organic cations in (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

  Centroids of the pyridinium cation are marked in red

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  Figure 5  Hirshfeld surface of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O mapped with d_norm

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  Figure 6  Two-dimensional fingerprint plots of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O: (a) all contacts; (b) H…Cl/H…Cl, (c) H…H, (d) C…H/H…C, (e) H…O/O…H and (f) N…H/H…N contacts showing the percentage of total Hirshfeld surface area

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  Figure 7  Infrared spectrum of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  Figure 8  Diffuse reflectance spectrum of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  Figure 9  Plots of (αhν)^1/2 (top) and (αhν)² (bottom) vs hν for (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

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  Figure 10  Emission and excitation spectra of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O: emission at λ_ex=319 nm (black), excitation at λ_em=562 nm (red)

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  Table 1.  Crystal data and structure refinement parameters for (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

  Parameter	(C7H11N2)2[CdCl4]·0.5H2O		Parameter	(C7H11N2)2[CdCl4]·0.5H2O
  Empirical formula	C28H44Cd2Cl8N8O		V/nm3	1.056 5(7)
  Formula weight	509.57		Dc/(g·cm-3)	1.6
  Temperature/K	293(2)		μ/mm-1	1.55
  Crystal system	Triclinic		F(000)	508
  Space group	P1		θ range for data collection/(°)	2.5-28
  a/nm	0.796 9(3)		Limiting indices	-10 ≤ h ≤ 6, -10 ≤ k ≤ 10, -22 ≤ l ≤ 22
  b/nm	0.804 4(3)		Reflection collected, unique, parameter	6 686, 5 062, 221
  c/nm	1.695 6(5)		GOF on F2	1.03
  α/(°)	95.71(3)		Final R indices [I > 2σ(I)]	R1=0.046
  β/(°)	100.82(4)		R indices (all data)	wR2=0.135
  γ/(°)	94.79(3)		Tmax, Tmin	0.953, 1.000
  Z	1		(Δρ)max and (Δρ)min/(e·nm-3)	721 and -572

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  Table 2.  Hydrogen bonds in the structure of (C₇H₁₁N₂)₂[CdCl₄]·0.5H₂O

  D—H…A	d(D—H)/nm	d(H…A)/nm	d(D…A)/nm	∠D—H…A/(°)
  N1—H1N…O1Ai	0.087(6)	0.236(5)	0.294 6(12)	125(5)
  N1—H1N…Cl12	0.087(6)	0.253(6)	0.294 6(12)	142(5)
  N2—H2N…Cl1	0.086(9)	0.242(9)	0.319 1(7)	150(7)
  C1—H1…Cl2ii	0.093	0.280	0.363 7(6)	150
  C9—H9…O1A	0.093	0.243	0.333 0(12)	162
  C12—H12…Cl4	0.093	0.280	0.353 4(7)	137
  Symmetry codes: i -x+1, -y, -z+2; ii x, y, z-1.

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