Dicyanamide Bridged Cu(II)36-Metallacrown-6 Complex with 1,4,7-Triisopropyl-1,4,7-Triazacyclononane and Binding Properties with DNA

A novel 36-metallacrown-6 complex [CuL(N(CN)2)(PF6)]6∙0.5H2O 1 was achieved using a tridendate ligand, 1,4,7-triisopropyl-1,4,7-triazacyclononane (L), and a flexible ligand, dicyanamide in MeOH. The μ1,5 bridging models of the dicyanamide ligand linked the macrocycle to form in a specific size with the chair conformation. The anion was important to form this 36-metallacrown-6 complex, as change was obtained with the larger anion BPh4−, binuclear copper compound 2. The magnetic property indicates that slightly ferromagnetic interactions resulted from a superexchange mechanism. DNA binding properties were also studied. UV and fluorescence spectra showed that complex 1 could bind with DNA.


Introduction
The control of ligand-field and metal-metal interactions in coordination chemistry has great effects for the development of novel magnetic and biologically active molecules. To optimize the properties, it is helpful to predict the relative arrangement of the metal ions, their geometry, and their stoichiometry within the molecular polynuclear complexes, in order to control the electronic and magnetic exchange interactions. The metallacrown strategy has led to advances in relevant areas, such as molecular magnetism [1][2][3][4][5] and luminescent complexes for biological imaging [6][7][8]. Metallacrowns have also been investigated for their multinuclear structures and interesting molecular architecture [9][10][11][12][13][14][15][16]. Many single-molecule-magnets (SMMs) are constructed by metallacrown complexes, such as DyX 4 M 12-Metallacrown-4 [17], Gd 2 Mn 4 [3], Mn 6 (Ishz) 6 [18], etc. The size of the metallamacrocycles can be determined by the nature of the bridging ligands and metal ions involved, such as the coordination geometries of the metal ions, the length of a bridging group between the metal centers, or the number of repeating units. Proper strategies are required to control the nuclear number of metallacrowns and obtain the desired compounds.
Reaction of LCuCl 2 species, NaN(CN) 2 as bridging ligand, and KPF 6 as anion in CH 3 OH solvent gave the hexanuclear complexes [22]. Single crystal X-ray analysis [23] of the compound illustrated that the compound is isomorphous and crystallized in the rhombohedral group R 3 . The crystal data and structure refinement are summarized in Table 1, and selected bond lengths (Å) and angles ( • ) are summarized in Table 2. Figure 1 gives a perspective view of compound 1. The structure presents a regular hexagon from the c axis ( Figure 1a). The copper atom was five coordinated with three N atoms from the L ligand and two N atoms from N(CN) 2 . Actually, the six copper atoms adopted a chair confirmation from the a axis (Figure 1b), where three Cu atoms were in the same plane and the other three Cu atoms were in another plane. N(CN) 2 − anions bridged LCu 2+ species, which formed a hexahydric Cu ring adopting the chair conformation. Cu···Cu···Cu angles and the Cu···Cu distances were the same, with 72.95 • and 7.70 Å, respectively. The angles of N-Cu-N in the ring were 79.17 • , while the C-N-C angle in the N(CN) 2 − anion was 121.8 (5) • . The packing diagram shows a regular array of hexahydric Cu rings and PF 6 − anions along the c axis, with a regular honeycomb-like structure ( Figure 2). PF 6 − looks like blooming flowers on the ipr 3 tacn rings, forming cycles with 12 PF 6 − anions.
These values for the angles of the geometry demonstrate that the formation of hexahydric rings in this reaction is strongly favored.     (7) Symmetry operation is P-1 for 1 and R-3 for 2.
Molecules 2018, 23, x FOR PEER REVIEW 3 of 8 (7) Symmetry operation is P-1 for 1 and R-3 for 2.  The distance from the center to the closest non-hydrogen atom of the ligand was 4.455 Å in 1, thus the inner cavity had an estimated volume of ca. 80 Å −3 ( Figure 3). Note that the anion was found to have a pronounced effect on the hexanuclear structure. When we changed the anion PF6 − to the larger anion BPh4 − with a similar nickel complex. A similar structure could not be obtained, but a binuclear Ni II complex 2 was obtained ( Figure 4) [24]. In complex 2, the Ni ions were five-coordinated with the N(1), N(2), N(3) from L ligands and N(4), N(6) from dicyanamide ligands.  (7) Symmetry operation is P-1 for 1 and R-3 for 2.  The distance from the center to the closest non-hydrogen atom of the ligand was 4.455 Å in 1, thus the inner cavity had an estimated volume of ca. 80 Å −3 (Figure 3). Note that the anion was found to have a pronounced effect on the hexanuclear structure. When we changed the anion PF6 − to the larger anion BPh4 − with a similar nickel complex. A similar structure could not be obtained, but a binuclear Ni II complex 2 was obtained ( Figure 4) [24]. In complex 2, the Ni ions were The distance from the center to the closest non-hydrogen atom of the ligand was 4.455 Å in 1, thus the inner cavity had an estimated volume of ca. 80 Å −3 (Figure 3). Note that the anion was found to have a pronounced effect on the hexanuclear structure. When we changed the anion PF 6 − to the larger anion BPh 4 − with a similar nickel complex. A similar structure could not be obtained, but a binuclear Ni II complex 2 was obtained ( Figure 4) [24]. In complex 2, the Ni ions were five-coordinated with the N(1), N(2), N(3) from L ligands and N(4), N(6) from dicyanamide ligands. Two Ni ions were bridged by dicyanamides, forming slightly distorted square-pyramidal geometry. The distances of Ni-N bonds were almost equal and amounted on average to 2.072 Å. The nearly planar dicyanamide utilized two of three possible N-donors to coordinate Ni ions, resulting in the formation of a twelve-membered, slightly puckered ring. The bond angle inside the ring, N(4)-Ni1-N(6), was 84.72(9) • . The intramolecular Ni···Ni separation was 7.386 Å.   Although we performed the reactions in absolute CH3OH, water molecules were observed in compound 1 probably due to non-removed or osmotic water from surroundings. We speculate that the cavity of the two compounds might show abilities to absorb and store H2O molecules. The powder X-ray diffraction technique was used. The compound was immersed in water for 24 h, and the XRD patterns of the bibulous solid showed the main reflections remaining nearly identical with the pristine samples, which supported the notion that the crystal lattice of the compound would remain intact after absorbing the water molecules ( Figure 5).   Although we performed the reactions in absolute CH3OH, water molecules were observed in compound 1 probably due to non-removed or osmotic water from surroundings. We speculate that the cavity of the two compounds might show abilities to absorb and store H2O molecules. The powder X-ray diffraction technique was used. The compound was immersed in water for 24 h, and the XRD patterns of the bibulous solid showed the main reflections remaining nearly identical with the pristine samples, which supported the notion that the crystal lattice of the compound would remain intact after absorbing the water molecules ( Figure 5). Although we performed the reactions in absolute CH 3 OH, water molecules were observed in compound 1 probably due to non-removed or osmotic water from surroundings. We speculate that the cavity of the two compounds might show abilities to absorb and store H 2 O molecules. The powder X-ray diffraction technique was used. The compound was immersed in water for 24 h, and the XRD patterns of the bibulous solid showed the main reflections remaining nearly identical with the pristine samples, which supported the notion that the crystal lattice of the compound would remain intact after absorbing the water molecules ( Figure 5). The magnetic susceptibilities were measured under a 1000 Oe applied magnetic field in the 2−300 K temperature range. As illustrated in Figure 6, the value of χmT increased with decreasing temperature from 300 K to 2 K, showing the presence of an antiferromagnetic interaction between the Cu(II) ions. The slightly ferromagnetic interactions resulted from a superexchange mechanism of adjacent Cu(II) ions. With decreasing temperature, the χmT increased slightly from 300 K to 24 K and then increased rapidly from 25 K to 2 K.

DNA Binding
In order to investigate whether DNA was the biological target of the compound, its interactions with Calf-thymus DNA (CT-DNA) were studied by UV-Vis and fluorescence spectroscopy. CT-DNA was prepared with Tris-HCl/NaCl buffer with pH of 7.5. The absorption spectra of compound 1 with absence and presence CT-DNA at various concentrations are shown in Figure 7a. The potential CT-DNA binding ability of complexes was studied by UV spectroscopy by following the intensity changes of the intraligand π-π* transition band at 232 nm and 298 nm. Upon the addition of an increasing amount of CT-DNA (from 10 −5 to 10 −4 M) to the complexes (10 −5 M), 20% hypochromism and slight red shift (7-12 nm) were observed, indicating that interactions between the DNA phosphate groups and Cu cations, or Cu coordination to guanine bases, might have happened. The magnetic susceptibilities were measured under a 1000 Oe applied magnetic field in the 2−300 K temperature range. As illustrated in Figure 6, the value of χ m T increased with decreasing temperature from 300 K to 2 K, showing the presence of an antiferromagnetic interaction between the Cu(II) ions. The slightly ferromagnetic interactions resulted from a superexchange mechanism of adjacent Cu(II) ions. With decreasing temperature, the χ m T increased slightly from 300 K to 24 K and then increased rapidly from 25 K to 2 K. The magnetic susceptibilities were measured under a 1000 Oe applied magnetic field in the 2−300 K temperature range. As illustrated in Figure 6, the value of χmT increased with decreasing temperature from 300 K to 2 K, showing the presence of an antiferromagnetic interaction between the Cu(II) ions. The slightly ferromagnetic interactions resulted from a superexchange mechanism of adjacent Cu(II) ions. With decreasing temperature, the χmT increased slightly from 300 K to 24 K and then increased rapidly from 25 K to 2 K.

DNA Binding
In order to investigate whether DNA was the biological target of the compound, its interactions with Calf-thymus DNA (CT-DNA) were studied by UV-Vis and fluorescence spectroscopy. CT-DNA was prepared with Tris-HCl/NaCl buffer with pH of 7.5. The absorption spectra of compound 1 with absence and presence CT-DNA at various concentrations are shown in Figure 7a. The potential CT-DNA binding ability of complexes was studied by UV spectroscopy by following the intensity changes of the intraligand π-π* transition band at 232 nm and 298 nm. Upon the addition of an increasing amount of CT-DNA (from 10 −5 to 10 −4 M) to the complexes (10 −5 M), 20% hypochromism and slight red shift (7-12 nm) were observed, indicating that interactions between the DNA phosphate groups and Cu cations, or Cu coordination to guanine bases, might have happened.

DNA Binding
In order to investigate whether DNA was the biological target of the compound, its interactions with Calf-thymus DNA (CT-DNA) were studied by UV-Vis and fluorescence spectroscopy. CT-DNA was prepared with Tris-HCl/NaCl buffer with pH of 7.5. The absorption spectra of compound 1 with absence and presence CT-DNA at various concentrations are shown in Figure 7a. The potential CT-DNA binding ability of complexes was studied by UV spectroscopy by following the intensity changes of the intraligand π-π* transition band at 232 nm and 298 nm. Upon the addition of an increasing amount of CT-DNA (from 10 −5 to 10 −4 M) to the complexes (10 −5 M), 20% hypochromism and slight red shift (7-12 nm) were observed, indicating that interactions between the DNA phosphate groups and Cu cations, or Cu coordination to guanine bases, might have happened. As a fluorescence spectral method, the relative binding of the complex to CT-DNA was studied though an ethidium bromide (EB)-bound CT-DNA solution with Tris-HCl/NaCl buffer (pH = 7.5). Fluorescence intensities at 610 nm were measured with various complex concentrations. Fluorescence titration spectra are shown in Figure 7b. The emission intensity showed a reduction with increasing concentration of complex 1, suggesting that the compound can replace EB from CT-DNA and intercalate into the DNA double helix [25]. Ethidium bromide is an intercalator that gives a significant increase in fluorescence emission [26].

Conclusions
In summary, using ipr3tacn copper complex with dicyanamide as bridge ligand, we successfully synthesized and characterized a novel Cu(II)36-metallacrown-6 complex which contained regular hexacyclic metal rings adopting the chair conformation. Anion choice in preparation of the title compounds might play a significant role in the assembly of this metallacrown. The results in this article may provide an incremental advancement to the field of metallacrown chemistry.  As a fluorescence spectral method, the relative binding of the complex to CT-DNA was studied though an ethidium bromide (EB)-bound CT-DNA solution with Tris-HCl/NaCl buffer (pH = 7.5). Fluorescence intensities at 610 nm were measured with various complex concentrations. Fluorescence titration spectra are shown in Figure 7b. The emission intensity showed a reduction with increasing concentration of complex 1, suggesting that the compound can replace EB from CT-DNA and intercalate into the DNA double helix [25]. Ethidium bromide is an intercalator that gives a significant increase in fluorescence emission [26].

Conclusions
In summary, using ipr 3 tacn copper complex with dicyanamide as bridge ligand, we successfully synthesized and characterized a novel Cu(II)36-metallacrown-6 complex which contained regular hexacyclic metal rings adopting the chair conformation. Anion choice in preparation of the title compounds might play a significant role in the assembly of this metallacrown. The results in this article may provide an incremental advancement to the field of metallacrown chemistry.