Synthesis , structures , and properties of two new binuclear complexes based on carboxyl-substituted nitronyl nitroxide : [ M 2 ( NITpBA ) 4 ( H 2 O ) 2 ] ( M = Zn and Cu )

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-4,5dihydro-1H-imidazol-1-oxyl-3-oxide], were stylized and characterized. Magnetic susceptibility measurements revealed antiferromagnetic exchange interactions in the two complexes.


INTRODUCTION
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 [1].Carboxylate bridges mediate exchange interactions between coordinated metal centres [2,3].Metal complexes with bridging carboxylates as well as stable organic radical ligands are of considerable interest to the field of molecular magnetism [4].In an effort to bring together the two areas of research, carboxyl-substituted nitronyl nitroxide, as an important anionic ligand, was deemed favorable for the realization in reactions with metal ions.To the best of our knowledge, to prepare transition metal complexes with nitronyl nitroxide-substituted carboxylate ligands, many investigations on the scope have been recently performed, but the related reports are very scarce [5,6].The only structurally characterized transition metal compounds are 1D-chain systems in which two nitronyl nitroxides NITpBA − are coordinated with two metal centers via one NO-group and a monodentate carboxylate group [7,8].Miller's group even reported on copper (II) compounds with NITpBA − assumed to be in a bidentatebridging mode, but did not provide structural information on the related radical-metal compounds [9].Rentschle's group examined the reaction of NITpBAH with different metal salts, bases, and terminal capping coligands following with various methods but could not isolate any dimeric compounds.Ligand exchange reaction of NITpBAH with copper (II) acetate hydrate, however, leads to the desired system.Recrystallization from hot dimethylsulfoxide/methanol (2:1) leads to formation of small green-blue crystals Cu 2 (μ-NITpBA) 4 /H 2 O/dmso [10].
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 [M 2 (μ-NITpBA) 4 (H 2 O) 2 ] (M = Zn 1 and Cu 2).

General
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-o xide was prepared by the literature method [11].Elemental analyses (C, H, and N) were carried out with a Perkin Elmer 240 C elemental analyzer.IR spectra were recorded from 400 to 4000 cm −1 on an Avatar-360 spectrophotometer using KBr pellets.Variable-temperature magnetic susceptibilities were measured with a MPMS-7SQUID magnetometer at a magnetic field of 2000 G. Diamagnetic corrections were made with Pascal's con-stants for all constituent atoms.[12].The structures were solved by direct methods with SHELXS-97 program [13] and refined with SHELXS-97 [14] by full matrix least-squares technique on F 2 .All the non-hydrogen atoms were refined with anisotropic temperature para-meters.Hydrogen atoms of organic ligands were fixed in ideal positions.The relevant parameters of the crystal structures for complex 1 and 2 are listed in Table 1, and the selected bond lengths and ond angles are given in Tables 2 and 3, respectively.b

Description of Crystal Structures
Crystal analysis show that compounds 1 and 2 are iso structural.They crystallize in the monoclinic system, P2 1 /c (No.14) space group, which are different from that lecture [10] (triclinic system, space group P 1 ).The asymmetric unit consists of one-half of a dimeric metal ion bridged by O-carboxyl from four nitronyl nitroxide.
The two halves are related to each other through a center of inversion located in the middle of the M-M axis.The coordination geometry of Zn (II) and Cu (II) lies at the center of the distorted octahedral geometry formed by five oxygen atoms, which come from four equivalent nitroxide ligand, one water molecule, and the M-M distances of the neighboring metal atoms is [Zn1-Zn1(A) 2.9672 Å and Cu1-Cu1(A) 2.6457Å], which is rather short and may be regarded as the normal M-M bonding range [15,16].Their molecular structures are depicted in Figures 1 and 2, respectively.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 [11]  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.
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-O water 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

Magnetic Properties
We have then examined the temperature dependences of OPEN ACCESS χ M T and χ M 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 χ M T 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 χ M T 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 [21].The result indicates that a weak antiferromagnetic exchange interaction is predominant, which is agreement with literatures [22,23].For complex 2, the χ M T 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].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.
. The C-C bond lengths of the phenyl rings show no alternation and are located in the aromatic region ranging from 1.361 to 1.394 Å.The Zn-O water distance of 1.964 Å is shorter than that of Zn-O whose oxygen atoms are from Ph-COO − .Four oxygen atoms from Ph-COO − of the nitroxide ligand compose the equatorial plane.The O5-Zn1-O6A angle in the equatorial plane is 158.99˚, and the O1-Zn1-O2A angle is equal to 158.59˚.The O1W-Zn1-Zn1A angle in the axial position is 171.23˚.The Zn (II) center is displaced by 0.324 Å from the basal O4-plane toward the axial oxygen atom from H 2 O.The fragment O4-N2-C8-N1-O3 is non-planar, and it forms a dihedral angle of 65.1˚ with the plane of the phenyl ring (C6-C5-C4).Conversely, the fragment O8-N4-C22-N3-O7 is nearly planar and forms a dihedral angle of 35.1˚ with the plane of the phenyl ring (C19-C18-C20).The IR spectrum shows the N-O stretching vibration of the NITpBA at 1355 cm −1 as well as the antisymmetric as and the symmetric s COO v  COO v  stretching mode of the caroxyl group at 1612 and 1413 cm −1 , respectively, in accordance with a bidentatebridging mode [10,17,18].

Table 1 .
Crystal data and structure refinement for the two complexes 1 and 2.