Chiral and achiral copper(II) complexes: structure, bonding and biological activities

Abstract

Three novel layered inorganic–organic hybrids [S-(C₈H₁₂N)₂][CuCl₄] (1), [R-(C₈H₁₂N)₂][CuCl₄] (2) and an achiral phase obtained from the racemic solution, namely (C₈H₁₂N)₂[CuCl₄] (3) were synthesized under hydrothermal conditions and characterized by various physicochemical techniques. Compounds 1 and 2 were synthesized through the use of copper chloride as the inorganic motif and enantiomorphically pure sources of either (S)-α-methylbenzylamine or (R)-α-methylbenzylamine. They crystallize in the polar chiral space group C2 (no. 5). Compound 3 crystallizes in the polar achiral space group C2cb (no. 41). The supramolecular crystal structure of these materials is built from alternatively arranged inorganic and organic layers and stabilized by N–H⋯Cl hydrogen bonds between the inorganic and organic moieties and C–H⋯π interactions between the aromatic rings of the organic moieties themselves. All these non-covalent interactions, which bind the supramolecular aggregate, were illustrated using the non-covalent interaction (NCI) plot technique. The synthesized products were also screened for in vitro antioxidant and antimicrobial activities, while showing favorable antioxidant activities against DPPH as well as the discoloration of β-carotene. Moreover, it is worth noting that copper complexes exhibit high antihypertensive activity against the inhibition of angiotensin I-converting enzyme (ACE) by dietary anti-hypertensive agents.

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Introduction

The possibility of combining the rigidity and stability of inorganic frameworks and the diversity of organic molecules into a single structure makes organic–inorganic hybrids almost an infinite source of new materials.^1–3 These hybrid materials have been of considerable interest due to their structural diversity.^4–7 Especially, the use of chiral amines is a way to give non-centrosymmetric crystal structures producing nonlinear optical materials that are suitable for applications in optical telecommunication and data storage devices.^8–12 The reliable preparation of enantiomerically pure compounds is important in areas such as the pharmaceutical and food industries.¹³ The general importance of chiral amines is well recognized, and α-methylbenzylamine is well known as a simple, yet powerful chiral adjuvant.¹⁴ Enantiomerically pure α-methylbenzylamine and its derivatives have important applications as effective chiral adjuvants in the resolution of racemates, and as ligands in asymmetric (or dissymmetric) catalysts.¹⁵ Currently, this amine is being used as a chiral auxiliary and as a chiral base. In this paper we will not only take an interest in homochiral compounds, but also to bring out the formation and the importance of the noncentrosymmetric achiral crystals. Moreover, the crystallization of racemic mixtures of chiral components in noncentrosymmetric space groups continues to be a challenge for crystal engineering.¹⁶ Because the crystallization processes involving enantiopure and racemic ligands have fundamental differences owing to the differing symmetry requirements, it can be expected that the homochiral 3D frameworks prepared from enantiopure ligands usually have different topological features from those prepared from the corresponding racemate. This work, however, offers a rare example of homochiral and achiral solids that have nearly identical unit cells and crystal structures. Herein, we report two new homochiral materials and one achiral 3D material. [S-(C₈H₁₂N)₂][CuCl₄] (S(MBA)Cu, compound 1) and [R-(C₈H₁₂N)₂][CuCl₄] (R(MBA)Cu, compound 2) were synthesized using enantiomerically pure sources of (S)-α-methylbenzylamine and (R)-α-methylbenzylamine, respectively, while the achiral compound (C₈H₁₂N)₂[CuCl₄] (3) was synthesized using racemic α-methylbenzylamine. The thermal stability as well as the noncovalent interaction (NCI) index calculations and the biological activities of these compounds are discussed.

Experimental

Materials

Copper(II) chloride dihydrate (CuCl₂·2H₂O), hydrochloric acid (HCl; 37%), α-methylbenzylamine (MBA), (R)-(+)-α-methylbenzylamine (R-MBA) and (S)-(−)-α-methylbenzylamine (S-MBA) were purchased from Sigma-Aldrich and used without further purification. Angiotensin I-converting enzyme from rabbit lung, angiotensin I-converting enzyme (ACE) synthetic substrate hippuryl-L-histidyl-L-leucine (HHL), 1,1-diphenyl-2-picrylhydrazyl (DPPH), butylated hydroxytoluene (BHA), α-tocopherol, thiobarbituric acid (TBA) and β-carotene were purchased from Sigma Chemical Co. (St. Louis, MO, USA). All other chemicals, namely potassium ferricyanide, ferric chloride, acetic acid, hippuric acid (HA), ethanol and chloroform were of analytical grade.

Synthesis

All syntheses were carried out in home-built Teflon-lined stainless steel pressure bombs of 120 ml maximum capacity. 1 mmol of CuCl₂·2H₂O and 3 mmol of either (R)-α-methylbenzylamine, (S)-α-methylbenzylamine or α-methylbenzylamine were dissolved together in 10 ml of water and hydrochloric acid (pH ≈ 3). The mixture was placed in a Teflon-lined autoclave that was then sealed and heated to 120 °C for 4 days. It was then allowed to cool to room temperature in a cold water bath. Green plate crystals with suitable dimensions for crystallographic study were collected. The crystals were washed several times with distilled water and dried in open air. They are stable for months in normal conditions of temperature and humidity. The yield was almost quantitative.

Single-crystal data collection and structure determination

Suitable crystals were glued to a glass fiber on APEX II area detector 4-circle diffractometer. Intensity data sets were collected using Mo Kα radiation (λ = 0.71073 Å) through the Bruker AXS APEX2 Software Suite. The crystal structures were solved in the monoclinic symmetry, space group C2 (no. 5) for compounds 1 and 2, and in the orthorhombic symmetry, achiral space group C2cb for compound 3, according to the automated search for space group available in WinGX.¹⁷ Copper and chloride atoms were located using the direct methods with the program SIR97.¹⁸ C and N atoms from the amine were found from successive difference Fourier calculations using SHELXL-97.¹⁹ Their positions were validated from geometrical considerations as well as from the examination of possible hydrogen bonds. Flack parameters for compounds 1, 2 and 3 ranged −0.028(19) and 0.008(16), indicating good resolution in each refinement. H atoms were positioned geometrically and allowed to ride on their parent atoms, with C–H = 0.97 Å and N–H = 0.90 Å. The figures were made with Vesta v3.2.1 program.²⁰ Crystallographic data are given in Table 1.

Table 1 Crystal data and structure refinement details for S(MBA)Cu, R(MBA)Cu and R/S(MBA)Cu

Compound	(1)	(2)	(3)
Empirical formula	C16H24N2CuCl4	C16H24N2CuCl4	C16H24N2CuCl4
Compound weight	224.86	224.86	224.86
Temperature (K)	293(2)	293(2)	293(2)
Crystal system	Monoclinic	Monoclinic	Orthorhombic
Space group	C2	C2	C2cb
a (Å)	10.5419(3)	10.546(3)	7.2117(10)
b (Å)	7.2444(2)	7.245(2)	10.3511(12)
c (Å)	13.9027(4)	13.915(4)	27.503(3)
β (°)	95.916(2)	95.843(10)	—
V (Å^3)	1056.09(5)	1057.7(5)	2053.1(4)
Z	4	4	8
ρcal (g cm^−3)	1.414	1.412	1.455
Crystal dimension, mm^3	0.31 × 0.25 × 0.21	0.28 × 0.26 × 0.2	0.37 × 0.35 × 0.12
Habit-colour	Plate, green	Plate, green	Plate, green
μ (mm^−1)	1.540	1.537	1.584
Index ranges	−12 ≤ h ≤ 13	−12 ≤ h ≤ 12	−9 ≤ h ≤ 8
	−9 ≤ k ≤ 9	−8 ≤ k ≤ 8	−10 ≤ k ≤ 13
	−18 ≤ l ≤ 18	−16 ≤ l ≤ 16	−28 ≤ l ≤ 33
Flack parameter	−0.028(19)	−0.015(16)	0.008(16)
Chirality	S	R	—
Unique data	2434	1937	3181
Observed data [I > 2σ(I)]	2067	1568	2505
F(000)	462	462	462
R1	0.0443	0.0342	0.038
wR2	0.1148	0.0605	0.081
GooF	0.945	0.947	1.026
No. param	106	106	106
Transmission factors	Tmin = 0.636; Tmax = 0.724	Tmin = 0.657; Tmax = 0.735	Tmin = 0.728; Tmax = 0.736

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Non-covalent interaction (NCI) index calculations

NCI analyses were performed using CRITIC 2 version 1.0 (ref. 21 and 22) to generate promolecular densities from the default numerical free-atom densities, using an approach similar described in previous reports.^23–25 The extraction of isosurfaces corresponding to particular interactions was performed using Mathematica 10.0.0.0, and visualized using Vesta v. 3.2.1.

Infrared spectroscopy

Infrared measurements were recorded using a Perkin Elmer 1650 FT-IR spectrophotometer. Samples were diluted with spectroscopic grade KBr and pressed into a pellet. Scans were collected over the range of 400–4000 cm⁻¹.

Thermal analyses

TGA-DTA measurements were performed on raw powders with a TGA/DTA ‘SETSYS Evolution’ (Pt crucibles, Al₂O₃ as a reference) under air flow (100 ml min⁻¹) of 1 and 3. The thermograms were collected on 11.5 mg samples in the RT-600 °C range (heating rate of 5 °C min⁻¹).

Determination of antioxidant activities in vitro of 1, 2 and 3

DPPH radical-scavenging. DPPH-scavenging activity was determined by the modified method of Bersuder, Hole, and Smith (1998).²⁶ For each compound, a volume of 500 μl of test sample was mixed with 500 μl of 99.5% ethanol and 125 μl of 99.5% ethanol containing 0.02% DPPH. These mixtures were shaken and then incubated for 60 min in the dark at room temperature, and the reduction DPPH radical was measured at 517 nm. The final assay concentration used was 2 mg ml⁻¹. A lower absorbance of the reaction mixture indicated a higher DPPH scavenging activity. DPPH radical-scavenging activity was calculated as follows:
where A control, A blank and A sample are, respectively, the absorbance of the control reaction (containing all reagents except the sample), the sample without DPPH solution and the sample with DPPH solution. The test was carried out in triplicate.

β-carotene bleaching assay. The ability of S(MBA)Cu, R(MBA)Cu and R/S(MBA)Cu to prevent β-carotene bleaching was assessed as described by Koleva et al. (2002).²⁷ A stock solution of β-carotene/linoleic acid mixture was prepared by dissolving 0.5 mg of β-carotene, 25 μl of linoleic acid and 200 μl of Tween 40 in 1 ml of chloroform. The chloroform was completely evaporated under vacuum in a rotatory evaporator at 40 °C, then 100 ml of bi-distilled water were added, and the resulting mixture was vigorously stirred. The emulsion obtained was freshly prepared before each experiment. Aliquots (2.5 ml) of the β-carotene/linoleic acid emulsion were transferred to test tubes containing different sample concentrations (0.05, 0.1, 0.2, 0.3, 0.4 and 0.5 mg ml⁻¹). After two hours incubation at 50 °C, the absorbance of each sample was measured at 470 nm. BHA was used as positive standard. The control tube contained no sample.

Antioxidant activity in β-carotene bleaching model in percentage was calculated with the following equation:

A% = [1 − (A₀ − A_t)/(A′₀ − A′_t)] × 100

where A₀ and A′₀ are the absorbances of the sample and the blank, respectively, measured at time zero, and A_t and A′_t are the absorbances of the sample and the blank, respectively, measured after 2 h. Tests were carried out in triplicate.

Ferric-reducing activity. The reducing power of the synthesized compounds was determined by the method of Yildirim et al. (2001).²⁸ Different concentrations of each sample (0.05 to 0.5 mg ml⁻¹) were mixed with 1.25 ml of 0.2 M phosphate buffer (pH 6.6) and 1.25 ml of 10 g l⁻¹ potassium ferricyanide solution. The mixtures were incubated for 30 min at 50 °C. After incubation, 1.25 ml of 100 g l⁻¹ TCA added and the reaction mixtures were centrifuged for 10 min at 3000 g. A 1.25 ml aliquot of the supernatant from each sample mixture was mixed with 1.25 ml of distilled water and 0.25 ml of 1.0 g l⁻¹ ferric chloride solution in a test tube. After a 10 min reaction time, the absorbance was measured at 700 nm. Higher absorbance of the reaction mixture indicated higher reducing power. The control was conducted in the same manner, except that distilled water was used instead of sample. The test was carried out in triplicate.

Determination of the angiotensin I-converting enzyme (ACE) inhibitor activity

ACE-inhibitory activity was assessed as described by Nakamura et al. (1995).²⁹ A volume of 80 μl containing different concentrations of each compound was added to 200 μl of 5 mmol l⁻¹ HHL and preincubated at 37 °C for 3 min. Samples and HHL were prepared in 100 mmol l⁻¹ borate buffer (pH 8.3) containing 300 mmol l⁻¹ NaCl. The reaction was then initiated by adding 20 μl of 0.1 U ml⁻¹ ACE from rabbit lung prepared in the same buffer. After incubation at 37 °C for 30 min the enzymatic reaction was stopped by adding 250 μl of 0.05 mol l⁻¹ HCl. The liberated hippuric acid (HA) was extracted with ethyl acetate (1.7 ml) and then evaporated at 95 °C for 10 min. The residue was dissolved in 1 ml of distilled water and the absorbance of the extract at 228 nm was determined using a UV-visible spectrophotometer. ACE-inhibitory activity was calculated using the equation:
where A is the absorbance of each sample generated in the presence of ACE inhibitor component, B is the absorbance of each sample generated without ACE inhibitors and C is the absorbance of each sample generated without ACE (corresponding to HHL autolysis in the course of enzymatic assay). The IC₅₀ value was defined as the concentration of inhibitor required to reduce the hippuric acid peak by 50% (indicating 50% inhibition of ACE).

Antimicrobial activity

The synthesized compounds were screened in vitro for their antibacterial activities against six strains of bacteria: Salmonella enterica ATCC 43972, Escherichia coli ATCC 25922, Micrococcus luteus ATCC 4698, Klebsiella pneumoniae ATCC 13883, Listeria monocytogenes ATCC 43251 and Salmonella typhinirium ATCC 19430. The antimicrobial activities were determined by the method of Berghe and Vlietinck (1991).³⁰ A culture suspension (200 μl) of the tested microorganisms (10⁶ colony forming units (cfu) per ml of bacteria cells) was spread on a Mueller–Hinton broth. Each sample (25 and 50 mg ml⁻¹) was prepared under stirring in 0.1% acetic acid. Then, bores were made using a sterile borer and loaded with 60 μl of sample. The Petri dishes were incubated in a humidified close container for 4 hours at 4 °C. At the end of incubation time (24 h at 37 °C), antimicrobial activities were measured as the diameter of the clear zone of growth inhibition and compared to a negative control (0.1% acetic acid) dissolved in Petri plates.

Statistical analysis

All experiments were carried out in triplicate, and average values with standard deviation errors are reported. Mean separation and significance were analyzed using the SPSS software package (SPSS, Chicago, IL). Correlation and regression analysis was carried out using Excel.

Results and discussion

Infrared spectra

Table 2 represents the major selected absorptions in the IR spectra of S(MBA)Cu (Fig. 1a) and the racemic phase R/S(MBA)Cu (Fig. 1b), with their respective assignments. The presence of α-methylbenzylammonium³¹ was evidenced by the appearance of the typical absorptions bands for bending of the NH₃⁺ group at ∼1500 cm⁻¹, deformation of CH₃ at 1396 cm⁻¹, deformation of C–H at 700 cm⁻¹ and stretching of the C–C ring at 1461 cm⁻¹.

Table 2 Assignments of the bands of the infrared absorption spectra for compounds 1 and 3

(1)	(3)	Vibrational mode assignment
699	698	C–H out of plane deformation mode
	767
926	920	CH2 rocking mode
970	970	C–C stretching mode
1060	1058	In plane C–H deformation mode
1080	1096
1165	1160
1229	1222
1292	1292	CH3 deformation mode
1318	1324	NH3^+ rocking mode
1380	1387	CH3 symmetric deformation mode
1455	1494	C–C ring stretching
1501
1601	1601
1690	1665	NH3^+ symmetric bending mode
2950	2941	Antisymmetric CH2 stretch
3055	3061	Aromatic CH
3490	3500

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Fig. 1 The infrared absorption spectra of compounds 1 (a) and 3 (b), dispersed in a KBr pellet.

Crystal structure

Crystal structures of [S-(C₈H₁₂N)₂][CuCl₄] (1) and [R-(C₈H₁₂N)₂][CuCl₄] (2). The existence of a chiral centre in an organic ligand is very important for the construction of noncentrosymmetric or chiral coordination polymers that exhibit desirable physical properties such as ferroelectricity³² and nonlinear optical second harmonic generation.³³ Chiral (R)-α-methylbenzylamine or (S)-α-methylbenzylamine has a chiral centre which have shown tremendous scope in the synthesis of hybrid complexes.^34,35 Strong similarities in both structures are observed in 1 and 2. These two enantiomorphically pure compounds adopt a layered hybrid-motif and crystallize in the noncentrosymmetric polar chiral space group C2 (no. 5), for which the only symmetry elements are 2-fold rotation axes. Compounds 1 and 2, are enantiomorphic and only differ in the stereochemistry of the methylbenzylammoniium cation (Fig. 2).

Fig. 2 A view of the asymmetric unit cell of 1–2, showing the atom-numbering scheme and displacement ellipsoids drawn at the 50% probability level. [Symmetry operation: (i) 2 − x, y, −z, (ii) 1 − x, y, −z].

The detailed crystal structure of 1 will be given and deviations and similarities between the two structures commented on. The asymmetric unit of 1 consists of one methylbenzylammonium cation (HMBA) and one half-anion, bisected by a twofold rotation axis of C2 space group through the metal center: Cu (Wyckoff site: 4e) (Fig. 2). Electronic subshell d9 of Cu(II) is responsible for distortions of symmetry of the coordination polyhedron. This deals with the Jahn–Teller effect. The shape of the four-coordinated tetrahalocuprate(II) ions changes from square planar³⁶ to distorted tetrahedral³⁷ and the geometry of [CuX₄]²⁻ species is influenced by the crystal-packing forces resulted from the size and the form of counter cations,³⁸ hydrogen bonding to cations,^39–41 and halide–halide interactions in solid.⁴² To define the degree of distortion of [CuCl₄]²⁻ coordination polyhedra, we propose a very simple geometry index for four-coordinate complexes, τ₄ , which was introduced by Addison, Reedijk and coworkers in 1984.⁴³ The formula for our four-coordinate τ₄ index is simply the sum of angles α and β (the two largest θ angles in the four-coordinate species) subtracted from 360°, all divided by 141°. The values of τ₄ will range from 1.00 for a perfect tetrahedral geometry, to zero for a perfect square planar geometry. The calculated τ₄ parameter for 1 is 0.79 (Table 4), which demonstrates that the [CuCl₄]²⁻ anion exhibits a distorted tetrahedral geometry.

The bidimensional arrangement is shown in Fig. 3, in which a double layer of (S)-methylbenzylammoniium cations is embedded between two consecutive inorganic [CuCl₄]²⁻ sheets forming an alternated inorganic-organic layered structure. Because of the steric hindrance of the organic component, the distance between the mean planes of two adjacent inorganic layers is remarkably high. Indeed, it corresponds to the value of the c unit cell parameter, i.e. 13.91 Å (see Table 1). The (S)-methylbenzylammoniium cations in 1 sit on general positions. The benzene rings are ordered within the layers and are perpendicular to the chiral functional groups. The (R)-methylbenzylammoniium cations in 2 are on general positions and have similar conformational features as to the (S)-methylbenzylammoniium cation in 1. The bond lengths and angles within the organic cation are close to the usual values observed in others homologous derivates (Table 3).^34,35 In supramolecular chemistry, weak interactions such as hydrogen bonding and C–H⋯π interactions contribute significantly to the self-assembly and molecular recognition processes. Indeed, the organic sheet is formed by the protonated amines which are linked together through the aromatic–aromatic interactions between the benzene rings in a perpendicular arrangement to form a T-shaped configuration.⁴⁴ In 1, the centroid to centroid distance is of 5.68(2) Å which is close enough to enable a C(6)–H(6)⋯Cg interaction to occur with a distance of 2.97(4) Å (Table 4), (Fig. 4). A second weaker interaction in complex 1 is the intermolecular hydrogen bonding interactions involving amide N–H groups and the neighboring [CuCl₄]²⁻ anions (Table 5). These hydrogen-bonding interactions ensure the stabilization of the crystal structure in the solid state (Fig. 4).

Fig. 3 Packing diagram of 1, showing the lamellar character and the stacking along the c axis. Hydrogen atoms are omitted for clarity.

Table 3 Selected bond distances (Å) and angles (°) for 1 and 3^a

Within the mineral moiety		Within the organic moiety
a Symmetry codes for 1: (i) −x + 1, y, −z; symmetry codes for 3: (i) x, −y, 1 − z.
S(MBA)Cu (1)
Cu1–Cl2	2.2355 (11)	C2–C1	1.519 (6)
Cu1–Cl2i	2.2355 (11)	C2–C3	1.498 (6)
Cu1–Cl1i	2.2641 (9)	N1–C2	1.500 (5)
Cu1–Cl1	2.2641 (9)	C3–C8	1.396 (6)
Cl2–Cu1–Cl2i	96.05 (8)	C4–C5	1.382 (7)
Cl2–Cu1–Cl1i	152.22 (5)	C6–C5	1.372 (10)
Cl2i–Cu1–Cl1i	92.76 (4)	C8–C7	1.384 (9)
Cl2–Cu1–Cl1	92.76 (4)	C3–C2–N1	109.3 (3)
Cl2i–Cu1–Cl1	152.22 (5)	C3–C2–C1	114.3 (4)
Cl1i–Cu1–Cl1	91.56 (5)	N1–C2–C1	109.1 (4)
		C4–C3–C8	118.3 (4)
		C4–C3–C2	121.6 (4)
		C8–C3–C2	120.1 (4)
		C3–C4–C5	120.7 (5)
		C7–C8–C3	120.2 (5)
		C5–C6–C7	119.8 (5)
		C6–C7–C8	120.6 (6)
		C6–C5–C4	120.4 (6)
R/S(MBA)Cu (3)
Cu–Cl1	2.2373 (12)	C2–C1	1.538 (6)
Cu–Cl1i	2.2373 (12)	C2–C3	1.503 (6)
Cu–Cl2	2.2634 (11)	C4–C5	1.404 (7)
Cu–Cl2i	2.2634 (11)	N–C2	1.505 (5)
Cl1–Cu–Cl1i	96.27 (7)	C3–C4	1.393 (6)
Cl1–Cu–Cl2	92.58 (4)	C3–C8	1.397 (6)
Cl1i–Cu–Cl2	152.22 (4)	C8–C7	1.374 (7)
Cl1–Cu–Cl2i	152.22 (4)	C7–C6	1.383 (7)
Cl1i–Cu–Cl2i	92.58 (4)	C6–C5	1.355 (7)
Cl2–Cu–Cl2i	91.69 (6)	C3–C4–C5	120.9 (5)
		C3–C2–C1	113.9 (3)
		N–C2–C1	108.9 (3)
		C3–C2–N	109.5 (3)
		C4–C3–C8	118.6 (4)
		C4–C3–C2	120.3 (4)
		C5–C6–C7	120.4 (5)
		C7–C8–C3	119.6 (4)
		C8–C7–C6	121.2 (5)

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Table 4 Details of C–H⋯Cg interactions for 1–3^a

D–H⋯Cg	D–H (Å)	H⋯Cg (Å)	Cg⋯Cg (Å)	∠D–H⋯Cg (°)
a Symmetry codes: (i) 1 − x, y, 1 − z; (ii) −1/2 + x, −1/2 + y, −z and (iii) −1/2 + x, −1 + y, 1/2 − z.
(1)
C6–H6⋯Cg1i	0.93	2.965	5.68	146.61
(2)
C6–H6⋯Cg1ii	0.93	2.978	5.078	146.74
(3)
C6–H6⋯Cg2iii	0.93	2.886	5.23	148.77

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Fig. 4 Ball and stick representation of the crystal packing arrangement of 1. Blue isosurfaces showing the face-to-edge C–H⋯π interaction between the cations and the hydrogen bonds interactions between the inorganic chain and the organic cations.

Table 5 Hydrogen-bonding details (Å, °) for 1–3^a

D–H⋯A	D–H (Å)	H⋯A (Å)	D⋯A (Å)	∠D–H⋯A (°)
a Symmetry codes for 1: (i) 1/2 − x, −1/2 + y, −z; (ii) −1/2 + x, 1/2 + y, z. Symmetry codes for 2: (i) 3/2 − x, 1/2 + y, −z; (ii) −1/2 + x, −1/2 + y, z. Symmetry codes for 3: (i) 1 + x, −y, 1 − z.
(1)
N1–H1B⋯Cl1	0.89	2.56	3.22	132.3
N1–H1B⋯Cl2	0.89	2.69	3.47	148.2
N1–H1A⋯Cl1i	0.89	2.42	3.23	152.6
N1–H1C⋯Cl2ii	0.89	2.34	3.18	158.3
(2)
N1–H1C⋯Cl1	0.89	2.56	3.23	132.4
N1–H1C⋯Cl2	0.89	2.70	3.48	148.3
N1–H1A⋯Cl1i	0.89	2.41	3.22	152.0
N1–H1B⋯Cl2ii	0.89	2.33	3.17	158.2
(3)
N–HC⋯Cl2	0.89	2.56	3.20	130.5
N–HA⋯Cl2i	0.89	2.41	3.21	150.6

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Crystal structure of the racemic phase (3). The 230 space groups may be divided into several categories.^45,46 The most common, of course, is centrosymmetric space groups. In the context of the chemistry presented here, the inversion symmetry in these space groups reflects that both enantiomers are related to one another through the centers of inversion. Noncentrosymmetric space groups do not contain inversion centers. Within the noncentrosymmetric space groups, enantiomorphic space groups (crystal classes 1, 2, 3, 4, 6, 222, 23, 32, 422, 432 and 622) contain only rotation and screw axes, and cannot result in one chiral component being related to the other. Racemic structures in enantiomorphic space groups are called kryptoracemates.⁴⁷ Some polar space groups contain mirror or glide planes that allow for chiral components to be related to one another while preserving noncentrosymmetry (crystal classes m, mm2, 3m, 4mm, 6mm). Structures in these space groups are polar, achiral and noncentrosymmetric. Rotoinversions can also relate chiral components to one another in nonpolar achiral crystal classes (4̄ and 6̄). Compounds 1 and 2 crystallize in the space group C2 (crystal class 2), which is polar and chiral. Compound 3, however, crystallizes in C2cb (crystal class mm2) which is polar and achiral, this compound crystallizes spontaneously from a racemic solution. Since, to our knowledge, it is the first supramolecular hybrid organic-inorganic compound, using a racemic mixture of 1-phenylethanamine, crystallizes in an noncentrosymmetric achiral form.^48–50 Compounds 1 and 2 were prepared from enantiomerically pure sources of (S)-α-methylbenzylamine and (R)-α-methylbenzylamine, respectively and crystallized in the chiral and polar space group C2, whereas 3 was prepared from racemic α-methylbenzylamine and crystallized in the achiral and polar space group C2cb.

The crystal structures of 1 and 3 are closely related. Indeed, the asymmetric unit of 3 consists of one methylbenzylammoniium cation and one half-anion, bisected by a twofold rotation axis of C2cb space group through the metal center: Cu (Wyckoff site: 4e) (Fig. 5).

Fig. 5 A view of the asymmetric unit cell of the racemic phase (3), showing the atom-numbering scheme and displacement ellipsoids drawn at the 50% probability level. [Symmetry operation: (iii) x, −y, 1 − z].

Such as 1 and 2, compound 3 is a layered hybrid material and the most significant difference between the racemic phase and the pure enantiomers in 1 and 2 is the conformation of the layers. In 3, the inorganic sheets are parallel to the (ac) plane and the unit cell has two layers at z = 0 and z = 1/2. The hybrids with the S and R enantiomers, on the other hand, have only one layer at z = 0, in the unit cell. Thus, to accommodate the extra layer, the unit cell is doubled in 3 to that of 1 and 2. Consequently, the interlayer distance measured between two means planes of two adjacent inorganic layers is equal to half of the c parameter which is greater than 13.75 Å. Because of the crystal structure similarity between the three complexes, Fig. 6 shows that the individual organic cations are linked to the inorganic layer by N–H⋯Cl hydrogen bonds (Table 5) and to adjacent cations through C–H⋯π interactions (Table 4), to provide the cohesion and the stability of the three-dimensional supramolecular architecture.

Fig. 6 Packing diagram of 3, showing the lamellar character and the stacking along the c axis. Hydrogen atoms are omitted for clarity.

NCI analysis

For overall complexes, the hydrogen-bonding interactions and aromatic–aromatic interactions were identified and visualized using non-covalent interaction (NCI) index calculations. The NCI similar fingerprints plots for 1 and 2, shown in Fig. 7, contain several distinct regions. Each individual peak corresponds to a single non-covalent interaction, either attractive or repulsive. Attractive interactions have negative sign (λ₂)ρ values, while repulsions appear at positive sign (λ₂)ρ values. The low-value isosurfaces of reduced density gradient (RDG) are able to rank the strength of various interactions and to distinguish their nature. Thus, the nature of all NCI between −0.03 and 0 in sign (λ₂)ρ were identified by generating isosurface plots. These designations are shown in Fig. 8. Three principal interaction types were identified: hydrogen bonds (N–H⋯Cl and C–H⋯Cl), aromatic–aromatic attractions and van der Waals attractions. The color electron density of the peaks in these plots gives an idea of the interaction strength with the N–H⋯Cl hydrogen bonds (shown in purple) being particularly relevant.

Fig. 7 NCI analysis of (a) S(MBA)Cu and (b) R(MBA)Cu.

Fig. 8 NCI analysis of the moderate-and low density attractions in S(MBA)Cu.

Despite the crystallographic change of 3, Fig. 9 clearly underlines that all complexes show similar and nearly identical (NCI) plots. The strong similarities in NCI fingerprints for compounds 1–3 suggest a consistent mode of stabilization for the supramolecular assemblies through hydrogen-bonding.

Fig. 9 NCI analysis of R/S(MBA)Cu (3), showing (a) both moderate-and low-density attractions and (b) an expanded view of the moderate-and low-density attractions.

Thermal decomposition

The simultaneous (TG-DTA) curves of compound S(MBA)Cu, carried out with a heating rate of 5 °C min⁻¹ from 25 to 450 °C, are depicted in Fig. 10. According to the TG curve, compound 1 seems to be stable up to about 180 °C, however, the TDA curve depicts a strong endothermic peak at 151 °C which may be denoting a structural phase transition. The only weight loss observed between 190 and 225 °C is due to the loss of the organic moiety and Cl₂ molecule, (observed weight loss, 69.01%, theoretical, 70.16%). This decomposition process is accompanied by two successive endothermic peaks on the DTA curve at 249 and 253 °C.

Fig. 10 Simultaneous TG-DTA curves for the decomposition of 1, under flowing nitrogen (5 °C min⁻¹ from 25 to 600 °C).

Fig. 11 shows the simultaneous (TG-DTA) curves carried out with a heating rate of 5 °C min⁻¹ from 25 to 600 °C of the racemic phase. Similarly to compound 1, the transformation at 150 °C may be indicating a structural phase transition, which is manifested as one intense endothermic peak. The TG curve shows a weight loss occurring between 190 and 325 °C which corresponds to the loss of the organic moiety and one Cl₂ molecule. While the experimental weight loss as observed in the TG curve is 68.56%, the theoretical weight loss as the suggested pattern is 70.16%. This decomposition process is revealed by a strong endothermic peak on the DTA curve, with the maximum being at 270 °C. By interest in phenomena relating to the phase transition, we shall be concentrated to focus other percolation techniques to obtain specific information about the thermal behavior of these complexes.

Fig. 11 Simultaneous TG-DTA curves for the decomposition of 3, under flowing nitrogen (5 °C min⁻¹ from 25 to 600 °C).

Biological activities

Antioxidant activities. In this study, various antioxidant assays, including 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging, reducing power and β-carotene bleaching assays were employed to evaluate the antioxidant activity of compounds 1, 2 and 3.

DPPH free radical-scavenging activity. DPPH (1,1-diphenyl-2-picrylhydrazyl) is a free radical compound frequently used to determine the free radical-scavenging activities of compounds.⁵¹ When DPPH radicals encounter a proton-donating substrate such as an antioxidant, the radicals are scavenged and the absorbance at 517 nm is reduced.⁵² The decrease in absorbance is taken as a measure for radical scavenging activity. DPPH radical scavenging activities of 1, 2 and 3 and the positive control BHA are presented in Fig. 12. Copper complexes showed good activities as a radical scavenger compared with BHA as standard. In fact, the DPPH radical scavenging activity of 1, 2 and 3 reached 91.5%, 70.1% and, 78.3% respectively, at 5 mg ml⁻¹. All compounds exhibited high activities as a radical scavenger, but the DPPH scavenging activity of S(MBA)Cu is significantly higher than that of R(MBA)Cu and of the racemic phase R/S(MBA)Cu. For all complexes, the scavenging activity increased steadily at the concentration range of 0.05–0.5 mg ml⁻¹, while the scavenging reached a maximum plateau from 0.05 to 0.3 mg ml⁻¹ for BHA, which indicated that the scavenging activity of copper complexes against DPPH radical was less than that of BHA. These results were in agreement with previous metallic complexes studies, where the ligand has an antioxidant potential and it is expected that the metal moiety will increase its activity.⁵³

Fig. 12 DPPH radical scavenging activities of 1, 2 and 3 at different concentrations. BHA was used as positive control.

β-carotene–linoleate assay system. The β-carotene–linoleic bleaching inhibition assay simulates membrane lipid oxidation and can be considered a good model for membrane based lipid peroxidation. In this assay, the oxidation of linoleic acid produces hydroperoxyl radicals evolving towards lipid hydroperoxides, conjugated dienes, and volatile by-products, which simultaneously attack the highly unsaturated β-carotene resulting in bleaching of the reaction emulsion.⁵⁴

As shown in Fig. 13, the water-soluble copper complexes as well as the positive control BHA inhibited the oxidation of β-carotene to different degrees. It showed significantly (p < 0.05) the highest ability to prevent the bleaching of β-carotene (58.33 ± 1.4%, 62.92 ± 1.6% and 64.81 ± 1.5%, for 1, 2 and 3, respectively) at the concentration of 5 mg ml⁻¹. These results demonstrated that copper complexes based on α-methylbenzylamine have strong effects against the discoloration of β-carotene.

Fig. 13 Determination of antioxidant activity using the β-carotene bleaching method of 1, 2 and 3 at different concentrations. BHA was used as positive control.

Reducing power. The reducing power assay is often used to evaluate the ability of an antioxidant to donate an electron or hydrogen.²⁸ Different studies have indicated that there is a direct correlation between antioxidant activities and reducing power antioxidant activity.⁵⁵ The presence of reducers (i.e. antioxidants) causes the reduction of the Fe³⁺/ferric cyanide complex to the ferrous form. Therefore the Fe²⁺ complex can be monitored by measuring the formation of Perl's Prussian blue at 700 nm.

Fig. 14 shows the reductive capabilities of copper complexes compared to BHA. The reducing power of all samples is very weak. Indeed, the highest rate of activity reachs 0.187, 0.257 and 0.212, for 1, 2 and 3, respectively, at 0.5 mg ml⁻¹.

Fig. 14 Reducing power activity of 1, 2 and 3 at different concentrations. BHA was used as positive control.

ACE inhibitory activity. The inhibition of angiotensin I-converting enzyme (ACE) by dietary anti-hypertensive agents is potentially an important strategy to manage hypertension. With this regard, it was demonstrated that the ACE inhibition is considered as a useful therapeutic approach in the treatment of high blood pressure. Metal complexes, especially those based on chiral amines, have a great importance in pharmaceutical and food industries. Therefore, copper complexes were tested for ACE inhibition activity. The results of the present study revealed that the ACE activity of all samples was concentration dependent. In fact, the values increased with increasing concentrations (Fig. 15). All compounds exhibited the highest inhibitory activity at 4 mg ml⁻¹, they possess an almost equal activity, but that of the racemate (97.07 ± 0.2%) was observed to be a little higher than the enantiomerically pure compounds (96.71 ± 0.3%, and 94.80 ± 0.4%, for 1 and 2, respectively).

Fig. 15 ACE-inhibitory activities of water-soluble copper complexes at different concentration. Values are given as mean ± SD from triplicate determinations. Identical letters above the bars indicate no significant differences by SPSS test (p ≤ 0.05).

Antimicrobial activity. Copper complexes have been evaluated in vitro against Gram positive and Gram negative bacteria. The antibacterial activity of all samples was assessed by evaluating the diameter of the clear zone of growth inhibition and the obtained results are given in Table 6. At 5 μg ml⁻¹ concentrations against the tested organisms, copper complexes showed weak antibacterial activity against Salmonella enterica with the diameters of zone inhibition ranging between 7 and 11 mm and moderate activities were noted by Salmonella typhinirium bacteria with a zone of inhibition ranging between 12 and 16 mm. Furthermore, the synthesized compounds showed absent antibacterial activity against the other tested organisms.

Table 6 Antibacterial activities of complexes against tested organisms at 5 μg ml⁻¹. Values represent averages ± standard deviations for triplicate experiments^a

Tested microorganisms	Inhibition zone diameter (mm) for complexes
	S(MBA)Cu	R(MBA)Cu	R/S(MBA)Cu
a (—): no inhibition of zone.
Salmonella enterica	6 ± 0.1	7 ± 0.1	9 ± 0.1
Escherichia coli	—	—	—
Listeria	—	—	—
Klebsiella pneumoniae	—	—	—
Micrococcus luteus	—	—	—
Salmonella typhinirium	—	12 ± 0.2	16 ± 0.2

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Conclusion

A series of chemically similar complex salts were prepared from methylbenzylamine and copper(II) chloride and their crystal structures were determined by single-crystal X-ray diffraction. Two structure types were distinguished and described: two enantiomorphically pure complexes adopt a similar layered hybrid-motif and crystallize in the noncentrosymmetric polar space group C2. Usually, the use of racemic methylbenzylamine in each system tends to direct crystallization toward centric space groups. However, as noted above, we have successfully prepared a racemic structure, which crystallizes in the achiral noncentrosymmetric and polar space group C2cb. Through several representative NCI plots, we have shown in this work how this procedure is useful in extracting information about the leading contributions determining the stability of a particular crystal packing. The preparation of high quality noncentrosymmetric single crystals makes them potential candidates for future practical applications, especially for the non-linear optic activity. Moreover, these complexes were also noted to display good antioxidant and anti-hypertensive activities. Despite the structural similarity observed among the studied complexes, the difference in their biological properties can be revealed. Then, it is well established that chirality/achirality of metal complexes play a decisive role in entire biological features. However, considering the examples presented in this paper and many more cases in literature, one cannot draw accurate correlations between chirality and biological activities of these materials. Overall, the results indicate that copper complexes based on α-methylbenzylamine have attractive chemical and biological properties that make them potential promising candidates for application as alternative additives in various food, cosmetic, and pharmaceutical preparations.

Acknowledgements

The authors would like to thank gratefully Mr Thierry Roisnel from the CDIFX (Centre de Diffractometrie X) – Sciences Chimiques de Rennes (UMR CNRS 6226), Groupe Matériaux Inorganiques: Chimie Douce et Réactivité, Université de Rennes I, France, for supplying single-crystal data collection. The authors are grateful, likewise, to Miss Malia B. Wenny for performing NCI calculation. A. J. N acknowledges the Henry Dreyfus Teacher-Scholar Program.

Notes and references

1. D. B. Mitzi, J. Chem. Soc., Dalton Trans., 2001, 1–12.
2. G. Kickelbick, Hybrid Materials: Synthesis, Characterization and Applications, Wiley-VCH, Weinheim, 2007.
3. D. B. Mitzi, K. Liang and S. Wang, Inorg. Chem., 1998, 37, 321–327.
4. X. G. Meng, F. S. Mei and Z. R. Liao, Acta Crystallogr., 2005, 61, 3047–3049.
5. G. Y. Bai, C. F. Zhang, J. Simpson, Y. Chen and H. W. Peng, Acta Crystallogr., 2007, 63, 1095–1096.
6. J. Janczak and G. J. Perpétuo, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 2001, C57, 1431–1433.
7. Y. L. Fu, Z. W. Xu, J. L. Ren and S. W. Ng, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2005, 61, 774–775.
8. J. F. Nicoud and R. J. Twieg, in Nonlinear Optical Properties of Organic Molecules and Crystals, ed. D. S. Chemla and J. Zyss, Academic Press, Orlando, 1987, vol. 1, p. 227.
9. G. R. Meredith, F. Kajar, P. Prasad and D. Ulrich, ACS Symposium Series 233, in Nonlinear Optical of Organic Polymeric Materials, ed. D. J. Williams, Washington DC, 1983.
10. G. R. Meredith, F. Kajar, P. Prasad and D. Ulrich, Nonlinear Optical Effects in Organic Polymers, Kluwer, Dordrecht, 1989.
11. S. Chemla and J. Zyss, in Nonlinear Optical Properties of Organic Molecules and Crystals, Academic Press, New York, 1987.
12. H. S. Nawla and S. Miyata, in Nonlinear Optics of Organic Molecules and Polymers, CRC Press, New York, 1997.
13. J. Jacques, A. Collet and S. H. Wilen, in Enantiomers, Racemates, and Resolutions, Wiley-InterScience, 1981.
14. E. Juaristi, P. Murer and D. Seebach, Use of N,N₀-dimethylpropyleneurea (DMPU) as solvent in the efficient preparation of enantiomerically pure secondary amines, Synthesis, 1993, 1243–1246.
15. E. Juaristi, J. Escalante, J. L. León-Romo and A. Reyes, Tetrahedron: Asymmetry, 1998, 9, 715–740.
16. (a) D. Y. Curtin and I. C. Paul, Chem. Rev., 1981, 81, 525; (b) G. R. Desiraju, Crystal Engineering; The Design of Organic Solids, Elsevier, Amsterdam, 1989, pp. 225–259.
17. L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837–838.
18. A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori and R. J. Spagna, J. Appl. Crystallogr., 1999, 32, 115–119.
19. G. M. Sheldrick, SHELXL-97, Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997.
20. M. Koichi and F. J. Izumi, J. Appl. Crystallogr., 2008, 41, 653–658.
21. A. Otero-de-la-Roza, M. A. Blanco, A. M. Pendas and V. Luana, Comput. Phys. Commun., 2009, 180, 157–166.
22. A. Otero-de-la-Roza, E. R. Johnson and V. Luana, Comput. Phys. Commun., 2014, 185, 1007–1018.
23. A. Nourmahnad, M. D. Smith, M. Zeller, G. M. Ferrence, J. Schrier and A. J. Norquist, Inorg. Chem., 2015, 54, 694–703.
24. A. Nourmahnad, M. B. Wenny, M. Zeller, J. Schrier and A. J. Norquist, J. Solid State Chem., 2016, 236, 215–221.
25. P. D. F. Adler, R. Xu, J. H. Olshansky, M. D. Smith, K. C. Elbert, Y. Yang, G. M. Ferrence, M. Zeller, J. Schrier and A. J. Norquist, Polyhedron, 2016, 114, 184–193.
26. P. Bersuder, M. Hole and G. Smith, J. Am. Oil Chem. Soc., 1998, 75, 181–187.
27. I. I. Koleva, T. A. van Beek, J. P. H. Linssen and A. De Groot, Phytochem. Anal., 2002, 3, 8–17.
28. A. Yildirim, A. Mavi and A. A. Kara, J. Agric. Food Chem., 2001, 49, 4083–4089.
29. Y. Nakamura, N. Yamamoto, K. Sakai, A. Okubo, S. Yamazaki and T. J. Takano, J. Dairy Sci., 1995, 78, 777–783.
30. D. Vanden Berghe and A. J. Vlietinck, Antibacterial Activity and Sub-chronic Toxicity Studies of Vitellaria paradoxa Stem Bark Extract, Academic Press, 1991, pp. 47–69.
31. H. G. Brittain, Cryst. Growth Des., 2011, 11, 2500–2509.
32. D.-W. Fu, Y.-M. Song, G.-X. Wang, Q. Ye, R. G. Xiong, T. Akutagawa, T. Nakamura, P. W. H. Chan and S. D. Huang, J. Am. Chem. Soc., 2007, 129, 5346–5347.
33. Z. R. Qu, H. Zhao, X. S. Wang, Y. H. Li, Y. M. Song, Y. J. Lui, Q. Ye, R. G. Xiong, B. F. Abrahams, Z. L. Xue and X. Z. You, Inorg. Chem., 2003, 42, 7710–7712.
34. G. David Billing and A. Lemmerer, Acta Crystallogr., 2003, E59, 381–383.
35. G. David Billing and A. Lemmerer, CrystEngComm, 2006, 8, 686–695.
36. R. L. Harlow, W. J. Wells, G. W. Watt and S. H. Simonsen, Inorg. Chem., 1975, 14, 1768–1773.
37. I. Dgiaz, V. Fernandes, V. K. Belsky and J. L. Martinez, Z. Naturforsch., A: Phys. Sci., 1999, 54, 718–724.
38. A. R. Parent, C. P. Landee and M. M. Turnbull, Inorg. Chim. Acta, 2007, 360, 1943–1953.
39. A. R. Parent, C. P. Landee and M. M. Turnbull, Inorg. Chim. Acta, 2006, 359, 424–432.
40. A. Arzotto, D. A. Clemente, F. Benetello and G. Valle, Polyhedron, 2001, 20, 171–177.
41. S.-N. Choi, Y.-M. Lee, H.-W. Lee, S. K. Kang and Y.-I. Kim, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2002, 58, m583–m585.
42. F. F. Awwadi, R. D. Willet and B. Twamly, Cryst. Growth Des., 2007, 7, 624–632.
43. A. W. Addison and T. N. Rao, J. Chem. Soc., Dalton Trans., 1984, 7, 1349.
44. M. O. Sinnokrot, E. F. Valeev and C. D. Sherrill, J. Am. Chem. Soc., 2002, 124, 10887–10893.
45. P. S. Halasyamani and K. R. Poeppelmeier, Chem. Mater., 1998, 10, 2753–2769.
46. R. Gautier, A. J. Norquist and K. R. Poeppelmeier, Cryst. Growth Des., 2012, 12, 6267–6271.
47. L. Fabian and C. P. Brock, Acta Crystallogr., Sect. B: Struct. Sci., 2010, 66, 94–103.
48. O. Kammoun, T. Bataille, A. Lucas, V. Dorcet, I. Marlart, W. Rekik, H. Naïli and T. Mhiri, Inorg. Chem., 2014, 53, 2619–2627.
49. O. Kammoun, W. Rekik, T. Bataille, T. K. Mahmudov, N. K. Maximilian and H. Naïli, Organomet. Chem., 2013, 741–742, 136–140.
50. G. David Billing, Acta Crystallogr., Sect. E: Struct. Rep. Online, 2002, 58, m669–m671.
51. S. H. Baea and H. J. Suh, LWT--Food Sci. Technol., 2007, 40, 955–962.
52. K. Shimada, K. Fujikawa, K. Yahara and T. Nakamura, J. Agric. Food Chem., 1992, 40, 945–948.
53. A. A. H. Kadhum, A. B. Mohamad, A. A. Al-Amiery and M. S. Takriff, Molecules, 2011, 16(8), 6969.
54. E. N. Frankel, Lipid oxidation, Dundee: The Oil Press, 1998, pp. 23–41.
55. P. D. Duh, Y. Y. Tu and G. C. Yen, J. Am. Oil Chem. Soc., 1998, 75, 455–461.

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Footnote

† CCDC Crystallographic data for CCDC-1052900 (1), CCDC-1052899 (2) and CCDC-1052902 (3). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra09630a

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