Two new binuclear complexes, [M2(μ-NITpBA)4(H2O)2] (M = Zn 1 and Cu 2) [where NIpBA = 2-(4-carboxyl-phenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide], were stylized and characterized. Magnetic susceptibility measurements revealed antiferromagnetic exchange interactions in two complexes.
The past decade has triggered fast-growing interests in nitronyl nitroxide radicals (NITR) as building blocks in the engineering of molecular-based magnet. The reasons are mainly that they are the most stable paramagnetic ligands, even in combination with metal ions where they can also act as bridging ligands [
With the purpose of obtaining materials with unusual molecular high nuclearity spin clusters, we were interested in the preparation of carboxylate-bridged metal complexes with pendant organic radical substituents. In this paper, we present the simple synthesis and properties of two new metal–radical binuclear complexes [M2(μ- NITpBA)4(H2O)2] (M = Zn 1 and Cu 2).
All reagents used in the synthesis were of analytical grade without further purification. 2-(4-carboxyl-phenyl)- 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl-3-oxide was prepared by the literature method [
NITpBAH (0.4 mmol, 0.1076 g) was dissolved in 4 mL 0.2 M NaOH. The complex 1 was synthesized by the addition of NITpBA− and ZnCl2 (0.2 mmol, 0.0275 g) to 30 mL of methanol-water. The mixture was stirred for 2 h and then filtered. The resulting blue filtrate was kept at room temperature for slow evaporation. After a few days, dark blue crystals of compound 1 suitable for X-ray analysis were obtained. Anal. Found: C, 53.12; H, 5.30; N, 8.61% Cacl. for C56 H68 N8 O18 Zn2 (1): C, 52.83; H, 5.39; N, 8.81%. IR (KBr disk): 1612 cm−1 (), 1413 cm−1 (), 1355 cm−1 (nNO).
The complex 2 was prepared in a procedure similar to complex 1 except that ZnCl2 was replaced by CuCl2 Dark blue crystals were obtained expectedly. Anal. Found: C52.53; H, 5.16; N, 8.74% Cacl. for C56 H68 N8 O18 Cu2 (2): C, 52.98; H, 5.40; N, 8.83%. IR (KBr disk): 1610 cm−1 (), 1407 cm−1 (), 1350 cm−1 (nNO).
X-ray diffraction intensities were collected on a Bruker Smart CCD diffractometer equipped with a graphitemonochromated Mo Κα radiation (= 0.71073 Å) by using a ω - φ scan technique at room temperature. Absor ption correction were applied using SADABS program [
Symmetry codes: #1: x + 1, y, z + 2.
Symmetry codes: #1: x, y + 2, z.
Crystal analysis show that compounds 1 and 2 are iso structural. They crystallize in the monoclinic system, P21/c (No.14) space group, which are different from that lecture [
In complex 1, the C1-O1 and C1-O2 bonds from the same carboxylate group are equal to 1.246 and 1.249 Å, respectively, whereas the distance of C15-O5 and C15- O6 are of 1.244 and 1.254 Å, respectively. The Ph-COO bonds of 1.5093 Å are close to that reported in the literature [
In complex 2, the Cu1-Cu1A distance is 2.6457 Å, which is a typical value for dimeric copper (II) carboxylate adducts [19,20]. The Cu-Owater distance of 2.121 Å is longer than that of Cu-O whose oxygen atoms come from Ph-COO−. The Cu (II) core is displaced by 0.205 Å from the basal O4-plane toward the axial oxygen atom of the solvent. Mean deviation from base O4-plan
is 0.0009 Å. The fragment O4-N2-C8-N1-O3 is nonplanar, where the mean deviation from the plane is 0.0676 Å, and forms a dihedral angle of 86.1˚ with the plane of the phenyl ring (C4-C5-C6). The fragment O8-N4-C22-N3-O7 is nearly planar, and the plane for ms a dihedral angle of 36.4˚ with the phenyl ring (C19-C18-C20). The product shows IR absorptions at 13 50, 1407 and 1610 cm−1, respectively.
We have then examined the temperature dependences of
χMT and χM for complex 1 and 2 in order to analyze the exchange coupling in this six-spin system. Temperature dependences of the molar magnetic susceptibility for two complexes were measured in a temperature range of 2 K to 300 K at a magnetic field of 2000 G. The results are given in Figures 3 and 4, respectively.
For complex 1, the χMT at room temperature has a value of 1.535 emu∙K∙mol−1, which is close to the expected value for uncoupled spins of S = 4/2 (1.5 emu∙ K∙mol−1) for two zinc centers and four nitronyl nitroxide radical-ligands. It was regarded as a tetra-radical system to evaluate the exchange coupling constants. The χMT value decreases slowly and reaches 1.43 emu∙K∙mol−1 at 100 K as the temperature is lowered, and subsequently decreases rapidly during further lowering of temperature. Based on the structural results, it undergoes a major magnetic interaction in the present system, in which the exchange interaction of NITpBA radicals through Zn (II) core exists [
For complex 2, the χMT value at room temperature is 2.14 emu∙K∙mol−1, which is significantly lower than that of the expected value for six uncoupled spins of S = 1/2 (2.25 emu∙K∙mol−1) for two copper centers and four nitronyl nitroxide. The result indicates that a strong antiferromagnetic exchange interaction is predominant. The result is in accord with those reported [10,24].
Two new binuclear complexes formulae of [M2(μ- NITpBA)4(H2O)2] (M = Zn 1 and Cu 2) [where NITpBA =2-(4-carboxyl-phenyl)-4,4,5,5-tetramethyl-4,5-dihydro- 1H-imidazol-1-oxyl-3-oxide)] were synthesized and structurally characterized. The simple procedure of the NITpBAH with metal chloride yielded dimeric complexes 1 and 2. The temperature dependences of the magnetic susceptibility show dominant itram-olecular antiferromagnetic exchange interaction in complexes 1 and 2. According to the result of the complex 1, it demonstrates that the magnitude of the intramolecular magnetic exchange has little relationship with the crystal system and space group.
This work was supported by the National Natural Science Foundation of China (No. 21071006), the Natural Science Foundation of Henan Province (No. 102102 210457) and the Natural Science Foundation of the Henan Higher Education Institutions of China (No. 2010 B150001).
Detailed crystallographic data in CIF format for the title complex are available from Cambridge Crystallographic Data Center (CCDC ID: 876210 and 876209). CIF deposits may now be made using our new online service: https://www.ccdc.cam.ac.uk/services/structure_deposit.