Synthesis,crystal structure determination,and spectroscopic characterization of [2,13-dibenzyl-5,16-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.0 7,12 )docosane] copper(II) dinitrate

Abstract

A novel metal complex, [Cu(L^b)](NO₃)₂ [L^b = 2,13-dibenzyl-5,16-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.0^7,12)docosane], was synthesized and characterized by single-crystal X-ray diffraction as well as other spectroscopic techniques. Structural analysis revealed that the copper(II) ion has an approximately square planar arrangement with stable trans-III configuration of the macrocyclic ligand. The Cu–N(tertiary) distance (2.0860 (14) Å) is greater than the Cu–N(secondary) distance (2.0229 (15) Å), which may be attributed to the steric effect of the benzyl group attached to the tertiary N atom. The nitrate anion and complex cation are linked by hydrogen bonds and weak Cu···O interactions. The metal complex showed strong blue luminescence when excited in the 355-391 nm region, and both blue as well as green emissions when excited in the 438–460 nm range.

Keywords

Crystal structure macrocyclic Cu(II) complex molecular spectroscopy

1 Introduction

The coordination chemistry and properties of most transition metal complexes with 14-membered macrocyclic ligands have already been studied. However, most of these ligands are tetraaza macrocyclic ligands, whereas studies on fully saturated macrocyclic frameworks containing six nitrogen atoms are relatively rare. Macrocycles with C-alkyl and N-alkyl groups on the polyaza macrocyclic ring and their transition metal complexes have attracted considerable attention because of their structural and chemical properties, which are quite different from those of the corresponding unalkylated macrocyclic systems. Cyclam-based compounds and their metal complexes have been reported to exhibit anti-HIV effects as well as to stimulate the activity of stem cells from bone marrow [1 –3]. The cyclam derivatives inhibit the entry of the HIV virus into white blood cells by binding with CXCR4, which is a chemokine receptor in the outer membrane. The strength of binding with the CXCR4 receptor is related to the anti-HIV activity. These 14-membered macrocyclic ligands have a moderately flexible structure, and can adopt both planar (trans) and folded (cis) configurations. These macrocycles have five configurational trans isomers that differ in terms of the chirality of the sec-NH centers [4, 5]. The trans-I, trans-II, and trans-V configurations can also fold to form cis-I, cis-II, and cis-V isomers, respectively [6]. The configuration of the macrocyclic ligand and the orientations of the N–H bonds are vital for CXCR4 chemokine receptor recognition [1 –3]. Therefore, understanding the coordination behavior and configuration of cyclam derivatives has become important in the design improvement and development of new highly effective anti-HIV drugs that specifically target alternative events in the HIV replicative cycle. The chemical properties and geometries of the copper(II)/nickel(II) complexes of macrocyclic ligands are influenced by structural characteristics, such as stereochemistry, chelate ring size, and type of substituents. To control the electronic properties of the coordinated metal centers, modification of the tetraazamacrocyclic ligands is necessary. Constrained ligands containing two cyclohexane rings and methyl or ethyl groups on the carbons have shown coordination behavior different from that of transition metal complexes with the parent cyclam [6, 7]. Various constrained cyclam ligands containing two 1,2-diaminocyclohexanediamine subunits and two methyl or ethyl groups at the carbon atoms are illustrated in Scheme 1.

Recently we reported the syntheses, crystal structures, and spectroscopic properties of copper(II)/nickel(II) complexes with 3,14-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.0^7,12)docosane (L^a) [8, 9]. As part of our current research on such compounds, herein, we describe the synthesis and chemical properties of a new copper(II) complex with an N-benzylated 14-membered macrocyclic ligand (Scheme 2).

2 Materials and method

2.1 Synthesis of ligand and metal complex

The ligand 3,14-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.0^7,12)docosane (L^a) was synthesized using a previously reported method [7 –9]. The ligand 2,13-dibenzyl-5,16-diethyl-6,13,17-tetraazatricyclo(16.4.0.0^7,12)docosane (L^b) was synthesized according to the following procedure: 0.376 g of ligand L^a in 10 mL of ethanol, and 0.34 g of benzyl bromide and 0.25 g of Na₂CO₃ in 4 mL of water were mixed in a 50 mL round-bottomed flask. The solution was refluxed for 24 h and cooled to room temperature. The resultant white solid was filtered off and washed with cold water, and the crude compound was recrystallized from THF. The new Cu(II) complex was synthesized as follows: 0.060 g of Cu(NO₃)₂·3H₂O was dissolved in 10 mL of a methanol:THF (1 : 1) mixed solvent. Then, 0.015 g of ligand L^b was added, whereupon the color of the reaction mixture changed to deep blue. The mixture was then heated to 65 °C and heating was continued for 3 h. Subsequently, the mixture was cooled to room temperature and filtered, and the clear solution was kept for recrystallization. After a few days, pale purple color crystals suitable for X-ray analysis were obtained. Yield: 58%. Elemental Analysis Calculated for CuC₃₆H₅₆N₆O₆: C, 59.04; H, 7.71; N, 11.47%. Found: C, 59.43; H, 7.84; N, 11.20%. UV-vis data in acetonitrile [λ_max in nm (ɛ in M^–1 cm^–1)]: 290 (8106), 338 (4727), 495 (267). IR spectrum (KBr, cm^–1): 3175 (vs) and 3147 (vs) (ν NH), 3085 (m) and 3054 m (aromatic ν CH), 1603 m and 1497 m (aromatic ν C = C), 1454 s and 1153 m (δ CH₂), 1387 vs (ν_as NO₂), 1325 m (ν_s NO₂), 1016 w (ν NO), 924 w (ρ CH₃), 864 m (γ NH), 790 s (ρ CH₂), 522 w, 492 w, 422 m.

2.2 Physical measurements

The UV-Visible absorption spectrum was recorded on a Cary 5000 spectrophotometer. The mid-infrared spectrum was obtained using a KBr pellet on a JASCO 460 plus series FT-IR spectrometer. Emission and excitation spectra were measured by a Shimadzu RF-5301 spectrofluorophotometer. Analyses for C, H, and N were performed on a Carlo Erba 1108 Elemental Vario EL analyzer.

2.3 Crystal structure analysis

A plate-shaped pale purple crystal of the title compound with approximate dimensions 0.06×0.06×0.01 mm³ was coated with paratone-N oil to prevent crystallinity losses upon exposure to air. The diffraction data were measured on a 2D SMC ADSC Quantum-210 detector with a silicon (111) double-crystal monochromator (0.72000 Å) at the Pohang Accelerator Laboratory, Korea using synchrotron radiation and a nitrogen cold stream (95 K). The ADSC Quantum-210 ADX program [10] was used for data collection, and HKL3000sm(v703r) [11] was used for cell refinement, reduction, and absorption correction. The structures were solved by direct methods and refined by full-matrix least-squares calculations using the SHELXTL-PLUS (Ver. 6.14) software package [12]. Molecular graphics were produced using DIAMOND-3 [13]. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were first located in a difference map; then, N–H hydrogen atoms were refined with distance restraints and C–H hydrogen atoms were constrained to ride on the parent carbon atom, with C–H = 0.98 Å and U_iso(H) = 1.5U_eq(C) for methyl groups and C–H = 0.99 Å and U_iso(H) = 1.2U_eq(C) for methylene groups. The crystallographic experimental data and refinement parameters are summarized in Table 1.

3 Results and discussions

3.1 Crystallography

A perspective view of the complex [Cu(L^b)](NO₃)₂ with the atom numbering scheme is shown in Fig. 1. Displacement ellipsoids are drawn at the 50% probability level.

The stereochemistry of the copper(II) complex may be considered square planar or tetragonally octahedral, depending on whether or not the out of plane O atoms of NO₃^- anions are considered to be bonded to the copper atom. Crystallographic analysis of [Cu(L^b)](NO₃)₂ showed that the copper(II) ion has a square planar coordination environment, with four N atoms from the macrocycle [14]. The macrocyclic backbone in this copper(II) complex adopts the stable trans-III configuration, in which two benzyl as well as two ethyl substituents lie on opposite sides of the square plane of the macrocyclic nitrogen donors. Important bond lengths and angles are given in Table 2.

The Cu–N distances in [Cu(L^b)](NO₃)₂ (2.025 and 2.086 Å) are within the expected range, and are comparable to those observed in related complexes, e.g., [Cu(L¹)](NO₃)₂·3H₂O (2.021(2)–2.029(2) Å) [15], [Cu(L¹)](ClO₄)₂ (2.005(2)–2.048(2) Å) [16] and [Cu(L¹)(bip)₂] [bip = 3-(2-benzimidazole)propionato] (2.015(2)–2.037(2) Å) [17]. The longer distance for Cu–N(tertiary) (2.0860 (14) Å), compared to for Cu–N(secondary) (2.0229 (15) Å), may be attributed to the steric effect of the benzyl group attached to the tertiary N atom. The nitrate ions are located above and below the coordination planes, and each ion is linked to the cation via an N-H···O hydrogen bond and a weak Cu···O interaction. The Cu1···O1S distance of 2.936 (2) Å is much longer than the axial bond lengths in [Cu(cyclam)(H₂O)₂]F₂·4H₂O (2.484(6) Å), [Cu(cyclam)(DMF)₂](PF₆)₂ (2.3985(17) Å) and [Cu(L¹)(H₂O)₂]Cl₂ (2.649(2) Å [18 –20]. The nitrate groups have a slightly distorted trigonal planar geometry due to hydrogen bonding interactions and the weak interaction with the copper(II) ion. The Cu1···O1S bond is not perpendicular to the CuN₄ plane, the actual angles O1S-Cu1-N1 and O1S-Cu1-N2 being 83.29(2)° and 79.28(2)°, respectively. The observed chromophore system is comparable to those found in [Cu(L¹)](NO₃)₂·3H₂O [15] and [Cu(L¹)](ClO₄)₂ [16]. As is usually observed, the five-membered chelate rings adopt a gauche conformation, whereas the six-membered rings are in chair conformations. The bond angles of the five- and six-membered chelate rings around copper(II) are 85.30(6)° and 94.70(6)°, respectively. The ethyl groups are attached axially as substituents to the chair six-membered rings, whereas the five-membered rings have methylene carbon atoms of the fused cyclohexane rings as substituents in equatorial positions; the cyclohexane rings are also in a chair conformation, with nitrogen atoms in equatorial positions. The mean C–N and C–C distances, along with C–N–C and C–C–N angles in the macrocyclic ligand, are typical for macrocyclic tetramine complexes [15, 16]. However, the N–C distances of the coordinated nitrogen atoms are slightly longer than the corresponding N–C distances of the free ligand [21]. In complex [Cu(L^b)](NO₃)₂, the hydrogen atom on N2 of the macrocycle is involved in a N–H···O interaction with NO₃^- (Fig. 2). Oxygen atoms of NO₃^- are also involved in weak hydrogen bonds through C–H···O interaction networks. Further distances and symmetry codes are given in Table 3.

3.2 Infrared spectroscopy

The FT-IR spectrum (KBr) of the newly synthesized copper(II) complex exhibited bands at 3680–3300 cm^–1 attributable to the ν(O–H) mode of moisture from the KBr disc surface. The IR spectrum showed peaks at 3175 and 3147 cm^–1 (N—H), 3085 and 3054 cm^–1 (aromatic C—H), 3000-2860 cm^–1 sp³ or aliphatic C-H), and 1603 and 1497 cm^–1 (aromatic C=C) [22 –24]. The lowering of ν(N–H) compared to that of a free ligand can be explained by coordination and hydrogen-bonding of the secondary amine. Nitrate can coordinate to a metal as a monodentate, chelating bidentate or bridging ligand [9]. The IR absorptions at 1016, 1325, and 1387 cm^–1 may correspond to the ν(NO) of nitrate. Peaks observed at 1454 and 1153 cm^–1 may be attributable to ν(CH₂), while those at 1294, 1263, 1200, 1142, 1074 and 924 cm^–1 may also be assigned to ν(CH₃) [9]. IR spectroscopy is often useful in assigning the cis and trans isomers of transition metal complexes with cyclam derivatives [5 , 25]. The trans isomer [Cu(L^b)](NO₃)₂ showed two groups of bands, viz. overlapping peaks near 893 cm^–1 arising from the secondary amine vibration and one band near 800 cm^–1 due to the methylene vibration. The IR spectrum also exhibits two bands at 864 and 790 cm^–1 in the CH₂ rocking wavenumber region. The peaks at 522, 492, and 461 cm^–1 may be assigned to the Cu-N and Cu-O stretching modes.

3.3 Electronic absorption spectroscopy

The UV-Visible spectrum of [Cu(L^b)](NO₃)₂ was measured in acetonitrile. Absorption bands were observed at 200–220, 290, and 338 nm (shoulder) and an asymmetric broad band with a λ_max at 495 nm was observed in the 430–750 nm region (Fig. 3). The absorption bands observed at 200–220, 290, and 338 nm (shoulder) may be assigned to n→π*, π → π*, and CT transitions, respectively. The weak bands observed in the visible region (430–800 nm) may be considered an envelope of the three possible d–d transitions (xz, yz → x²-y²; z² → x²-y²; xy → x²-y²) in the copper(II) tetramine chromophore. UV/Vis data correspond to a type-III square planar structure in solution [26 –28].

3.4 Photoemission and excitation Spectroscopy

Emission spectra of [Cu(L^b)](NO₃)₂ were measured in acetone at room temperature and emission peaks were observed in the visible region. The complex showed very strong blue luminescence when excited at 372 and 391 nm and the luminescence spectra consisted of three peaks at 405, 429, and 452 nm. Excitation at 355 nm also led to three peaks at 405, 429, and 452 nm with relatively lower intensities (Fig. 4). Excitation at 438 and 460 nm led to blue emission at 476 nm and green emissions at 506 and 543 nm, as shown in Fig. 4. These observed emission spectra of the [Cu(L^b)](NO₃)₂ of copper(II) complex with square planar structure in solution is different from that observed for the octahedral Ni(II) complex [9]. An interesting feature of the emission spectra of the title compound is the observation of a very strong blue luminescence corresponding to 372 and 391 nm excitation by a Xe lamp. The excitation spectrum of [Cu(L^b)](NO₃)₂ was also recorded under the same experimental conditions, showing peaks at 372, 391, and 438 nm (Fig. 5).

4 Conclusion

A novel macrocyclic metal complex [2,13-dibenzyl-5,16-diethyl-2,6,13,17-tetraazatricyclo(16.4.0.0^7,1²)docosane]copper(II) dinitrate, [Cu(L^b)](NO₃)₂, was synthesized and characterized by single-crystal X-ray diffraction and other spectroscopic techniques. The results revealed that the copper(II) complex adopts a slightly distorted square planar CuN₄ (trans-III configuration) arrangement with two NO₃^- groups occupying symmetric sites above and below the square coordination plane. The hydrogen atoms on N or C positions of the macrocycle are involved in N–H···O or C–H···O interactions with NO₃^-, and the complex is stabilized by hydrogen bonds. The metal complex showed strong blue luminescence when excited in the 355–391 nm region, and both blue as well as green emissions when excited in the 438–460 nm range in acetone.

Footnotes

Acknowledgments

This work was supported by a grant from 2016 Research Funds of Andong National University. The X-ray crystallography experiment at PLS-II BL2D-SMC beamline was supported in part by MSIP and POSTECH.

Appendix

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