ZHANG Xin JIANG Xio-Feng CHEN Yun-Zhou CHEN Yun-Feng DING Li-Li② JIA Li-Hui,

a (Key Laboratory for Green Chemistry Process of Ministry of Education, School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, China)

b (Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Function Molecules, Hubei University, Wuhan 430062, China)

ABSTRACT A new dinuclear copper(II) compound [Cu2(CH3COO)2(L)2(H2O)2]·4H2O (1, HL = 4-pyridyl-NH- 1,2,3-triazole) has been synthesized and characterized. Single-crystal X-ray diffraction showed that complex 1 crystallizes in monoclinic space group P21/c with a = 0.8427(1), b = 1.0981(1), c = 1.3610(1) nm, β = 93.364(4)°, V = 1.2573(1) nm3, Mr = 643.56, Z = 2, Dc = 1.700 g/cm3, μ = 1.760 mm-1, R = 0.0376, wR = 0.0981, and possesses a planar Cu2N4 six-membered ring, in which both Cu(II) ions are bridged by two 1,2,3-triazole ligands. The neighboring dinuclear molecules are bound by strong intermolecular hydrogen bonds to create 1-D chains expanding along the b axis, which further form a 3-D supramolecular framework. Magnetic susceptibility measurement indicated the presence of weak antiferromagnetic interactions within the complex. The best fit using spin Hamiltonian yields the magnetic coupling constant J = -14.82 cm-1 above 30 K. The magnetic data are compared to those obtained for related double diazine bridged dinuclear copper(II) complexes and magneto- structural correlations are discussed.

Keywords: dinuclear Cu(II) complexes, 4-pyridyl-NH-1,2,3-triazole, magnetic properties, magneto-structural correlation; DOI: 10.14102/j.cnki.0254-5861.2011-2595

1 INTRODUCTION

Since the recent discovery of efficient copper catalyzed azide-alkyne cycloaddition (CuAAC) reactions, the 4-pyridyl- 1,2,3-triazole (Rpytri) “click” ligands have garnered particular interest as they can be viewed as readily functionalized analogues of the ubiquitous 2,2?-bipyridine (bipy) ligand systems and other nitrogen-containing bi-dentate chelators (Scheme 1)[1,2]. The ease construction, using “click” chemis- try, of functionalized variants of the Rpytri ligands has led to their burgeoning use in coordination chemistry[3,4].

Scheme 1. Coordination modes of HL and L- (Rpytri)

On the other hand, dinuclear copper(II) complexes con- taining such a diazine unit are especially interesting for investigation of their magneto-structural correlations. In this context, dinuclear copper(II) complexes with doubly azole-bridged metal ions have been previously observed with the anionic ligands 3-(2-pyridyl)-pyrazolate (A)[5,6], 3,5-di(2-pyridyl)-pyrazolate (B)[7,8], as well as with the 1,2,4-triazole-based ligands 3-(2-pyridyl)-1,2,4-triazole (C)[9]and 3,5-bis(pyridin-2-yl)-1,2,4-triazole (D)[10,11](Scheme 2). These diverse pyrazole and 1,2,4-triazole bridged dinuclear Cu(II) complexes feature a square pyramidal coordination geometry with thedx2-y2orbitals parallel to each other and pointing directly to the N atoms of the azole bridge. Furthermore, the magnetic interactions between both Cu(II) ions are overall antiferromagnetic, with the value ofJranging from -31～-368 cm-1.

Scheme 2. (a) Ligands forming dinuclear copper(II) complexes with doubly azole-bridged metal ions found in literature[5-11]; (b) The title ligand 4-pyridyl-1,2,3-triazole (HL)

Acting as a potential diazine ligand containing three sequent N donors in a coplanar five-membered heterocyclic ring, the Rpytri ligands provide bridging capability, which are useful tools in the synthesis of coordination complexes, exhibiting a great structural diversity and interesting properties. However, due to the difficulty in the synthesis of such Rpytri ligands, very few magnetically active 1,2,3- triazole bridged Cu(II) complexes have been synthesized and investigated. As far as we know, only two 1,2,3-triazole bridged dinuclear Cu(II) complexes have been reported until now[12,13]. These two new complexes represent few chelating-bridging dinuclear Cu(II) complexes containing the Rpytri ligand. In contrast to the antiferromagnetic interactions in most pyrazole and 1,2,4-triazole bridged dinuclear Cu(II) complexes, the magnetic exchange coupling within one of the two 1,2,3-triazole bridged Cu(II) complexes is ferromag- netic[11].

By comparison of the bibliographic data for related compounds, several magnetically relevant factors, such as the torsion angle relative to the (N2)2moiety planar, the dihedral Cu-N-N-Cu angle and the Addison parameterτwere analyzed. Apparently, the magneto-structural correlations for different double diazine-bridged copper(II) systems are not that straightforward, and a more detailed analysis of the bridging geometry is necessary. Inspired by this goal, we focus our attention on the new rigid asymmetric 4-pyridyl- NH-1,2,3-triazole ligand, aiming at constructing magnetically active coordination complexes based on sole 1,2,3-triazole bridges between spin carriers. By introducing an additional pyridine ring to the NH-1,2,3-triazole, the ligand possesses both chelating and bridging capabilities. Recently, we have reported a Mn(II)-based 1D coordination structure [Mn(L2)]nconstructed from 4-pyridyl-NH-1,2,3-triazole ligand through hydrothermal reaction. The bridging-chelating coordination mode of the ligand and the magneto-structural correlation were discussed in detail[1]. Here we report the synthesis and structure of a new complex constructed from 4-(2-py- ridyl)-NH-1,2,3-triazole ligand through hydrothermal reaction, which is a doubly azole-bridged dinuclear copper(II) complex with similar Cu-(N=N)2-Cu units to the compounds mentioned above.

2 EXPERIMENTAL

2. 1 Materials and general methods

Commercially available solvents and copper(II) acetate were of analytical grade and used without further purification. The ligand 4-(2-pyridyl)-NH-1,2,3-triazole was synthesized according to literature[14].

The FT-IR spectra were recorded from KBr pellets in the range of 4000～400 cm-1on a Bruker Tensor II Spectrum FT-IR spectrometer. UV-Vis spectra for diluted samples (10～5 M in methanol) and powders were obtained on Perkin Elmer UV WinLab 6.0.4.0738/Lambda35 1.27.

PXRD data for complex 1 were collected in the range of 5～50° for 2θon crystalline samples using a Rigaku Dmax 2000 diffractometer with CuKαradiation (λ= 0.15418 nm, 40 kV, 40 mA) in flat-plate geometry at room temperature. The experimental powder X-ray diffraction pattern was compared to the calculated one from the single-crystal structure to identify the phase of the sample in Fig. S4 (Supporting information). Magnetic measurement was performed on a MPMS XL-5 SQUID (Superconductivity).

2. 2 Preparation of the title compound

A solution of Cu(OAc)2·H2O (0.25 mmol, 49.9 mg) in 5 mL distilled water was added to a solution of 4-(2-pyridyl)- NH-1,2,3-triazole (0.25 mmol, 36.5 mg) in 5 mL methanol. The mixture was then sealed in a Teflon-lined stainless-steel vessel and heated at 120 °C for 2 days. After cooling to room temperature, it was centrifuged and filtered. The filtrate was left to evaporate to afford blue block crystals in about two weeks. Yield: 55%. Selected IR bands (KBr): 3406 (s), 2364 (m), 1591 (s), 1398 (s), 1230 (w), 978 (w), 785 (m), 692 (m).

2. 3 X-ray crystallographic study of [Cu2(CH3COO)2(L)2(H2O)2]·4H2O

The X-ray crystallographic data for the single crystal of [Cu2(CH3COO)2(L)2(H2O)2]·4H2O were collected on an X taLAB Mini (Rigaku OD, 2015) employing graphite-mono- chromated MoKαradiation (λ= 0.71073 ?) at room temperature by using anωscan mode (3.00<θ<26.00°). Empirical absorption correction used spherical harmonics implemented in SCALE3 ABSPACK scaling algorithm. Complex 1 was solved by direct methods using the Olex2 program with the SHELXS package and refined with SHELXL[15]. A total of 6459 reflections were measured, of which 2472 independent reflections (Rint= 0.016) were used in calculation. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in ideal positions and refined using a riding model. The finalR= 0.0376,wR= 0.0981 (w= 1/[σ2(Fo2) + (0.066P)2+ 0.125P], whereP= (Fo2+ 2Fc2)/3), (Δ/σ)max=0.003, andS= 1.114 for 2196 observed reflections withI> 2σ(I). The maximum and minimum peaks in the final difference Fourier map are 0.499 and -1.343 e·?-3, respectively. The selected bond lengths and bond angles are given in Table S1.

3 RESULTS AND DISCUSSION

3. 1 Crystal structure

The molecular structure with atomic numbering of the dinuclear copper(II) unit are depicted in Fig. 1a, whereas the bond lengths and bond angles relevant to the copper coordination sphere are given in Table S1. Complex 1 crystallizes in monoclinic space groupP21/c. X-ray crystal structure analysis of complex 1 reveals that its asymmetric unit composes of one Cu(II) ion, one 4-pyridyl-NH-1,2,3- triazole, one acetate ion, one coordinating water molecule and two lattice water molecules. Each Cu(II) center is penta- coordinated to three nitrogen atoms (one pyridine N and two triazole N) of two distinct L-ligands (Cu-N lengths being 1.9839(17)～2.0412(24) ?), one oxygen atom of acetate anion (Cu-O = 1.9849(15) ?), and one oxygen atom from the coordinating water molecule (Cu-O = 2.3286(17) ?) to form a square-pyramidal geometry. The geometry around the central copper atom can be viewed in terms of a distorted square pyramid with the Addison parameterτ= 0.132 for Cu(II) (for a square pyramidτ= 0 and for a trigonal bipyramidτ= 1; structure parameter,τ=(β-α)/60; whereαandβare the two largest angles around the central atom)[16]. The CuIIcenter deviates from the mean equatorial plane defined by three coordinated nitrogen atoms and one oxygen atom from acetate anion toward the apical O(1w) by 0.140 ?. In complex 1, two L-ligands bridge two CuIIions to form an approximately planar [Cu-(L)]2binuclear structure containing a six-membered ring, (Cu-N-N-)2, in which the Cu···Cu distance is 4.0822(3) ?, a common structural feature of those complexes. The coordinating water molecules and acetate anions are displaced alternatively above and below the (Cu-N-N-)2plane, respectively.

Table 1. Hydrogen Bonds for the Dinuclear Cu(II) Complex [Cu2(CH3COO)2(L)2(H2O)2]·4H2O (?, °)

Each Cu(II) ion is five-coordinated with a distorted square pyramidal geometry. Three nitrogen atoms from the ligands and a carbonyl oxygen from acetate define the square, whereas a coordinating water molecule occupies the apical position. In the presence of acetate ions and water molecules, there are abundant hydrogen bonds (Table 1) that contribute to the formation of the 3D framework. As shown in Fig. 1b, O-H···O and N-H···O lead to forming the 1D chain: (a) O(2W)-H(2WA)···O(1) (dH(2WA)···O(1) 2.01(4) ?, O(2W)-H(2WA)···O(1) 161.1(18)o); (b) O(2W)- H(2WB)···N(4) (dH(2WB)···N(4) 1.969(10) ?, O(2W)- H(2WB)···N(4) 162(2)°). Moreover, the O-H···O bonds are involved in the construction of 3D network owing to the fact that water molecules are stronger acceptors (Fig. S1, Table 1). It is no doubt that these strong hydrogen bonds contribute significantly to the alignment of the molecules of complex 1 in the crystalline state. In addition, as shown in Fig. 1c, there is one offset intermolecularπ-πstacking in the structure, further assembling the above-mentioned hydrogen-bonded chains to form a 3-D supramolecular framework. The interplanar centroid-to-centroid distance between the neighboring parallel pyridine and triazole aromatic rings of L-1is 3.6917(14) ?, indicating the presence of slipped face-to-faceπ-πstacking interactions that further stabilize the crystal structure[17].

Fig. 1. (a) Molecular structure of complex 1. (b) Infinite quasi 1-D chain formed through H-bonds in complex 1. (c) View of the face-to-face π-π stacking interactions in complex 1

3. 2 Magnetic properties

The magnetic susceptibility of complex 1 was measured at 1 kOe in the temperature range of 2～300 K, and the data are plotted in Fig. 2. As shown in Fig. 2, a strong anti- ferromagnetic exchange coupling between the Cu2+ions was observed. TheχMTvalue at room temperature for complex 1 is 0.79 cm3·K·mol-1, slightly larger than the spin-only value for the sum of two uncoupled electrons (χMT= 0.75 cm3·K·mol-1forg= 2.0). The temperature dependence ofχM-1obeys the Curie-Weiss lawχM=C/(T-θ) with the Curie constantC= 0.84 cm3·K·mol-1and the negative Weiss constantθ= -21.68 K, which indicates the presence of antiferromagnetic interaction between Cu2+ions (Fig. 2a). The magnetic susceptibilitiesχMfor compound 1 in a field of 1 kOe show a sharp peak upon cooling and reach a maximum value of 0.01456 cm3·mol-1at 28.07 K, suggesting the presence of possible antiferromagnetic interaction in complex 1 (Fig. 2b). The field dependence of the magnetization of complex 1 measured at 2 K increases linearly (Fig. 2c). At 2 K, the magnetization at 60 kOe is only 0.0439 Nβ, far from the saturation value of 1 Nβexpected for one isolated spinS= 1/2. This is another evidence supporting antiferromagnetic inter- action in complex 1. To estimate a magnitude of the magnetic exchange constantJbetween the copper(II) ions, the magnetic data for complex 1 have been fitted to the well-known Bleaney-Bowers[18]equation (1). According to the isotopic Heisenberg exchange HamiltionianH=2JS1·S2, theS1=S2=1/2 for Cu(II) ions. In formulas,N,g,β,kandThave their usual meanings.

Fig. 2. (a) Temperature dependent χMT products (black circles) and χ-1 (blue squares) for complex 1 in an applied dc field of 1000 Oe. The magenta and red solid lines correspond to Curie-Weiss fitting above 30 K and Bleaney-Bowers fitting above 2 K, respectively. (b) Plots of temperature dependence of χM of complex 1 measured at a 1000 Oe field. (c) Field dependence of magnetization at 2 K for complex 1

A good fit to the experimental data was obtained using the parametersJ= -15.09 cm-1andg= 2.09[19]. In conclusion, seen from magnetic data of complex 1, a rather strong antiferromagnetic interaction is observed within the complex.

3. 3 Magneto-structural correlations

In order to understand the magnetic exchange mechanism through 1,2,3-triazole bridging ligands and establish their magneto-structural relationship in magnetically active dinuclear Cu(II) complexes, we make comparisons among the isotropic exchange transmit capacities of various azole ligand systems, especially pyrazole, 1,2,4-triazole and 1,2,3-triazole ligand systems (Table 2) based on the literatures on dinuclear bis(diazine) copper(II) compounds

Table 2. Magnetic and Structural Parameters for Dinuclear Doubly Bridged Triazole-N1,N2-Copper(II) Compounds

From Table 2, we can draw some interesting conclusions:

1) The symmetry of the ligand bridging mode is the most important factor influencing the magnetic coupling strength. Compared the exchange parameterJfor symmetric compounds 4～6 to those of the asymmetric dinuclear doubly diazine bridged copper(II) compounds 1～3, it can be seen that in the former cases the absolute value ofJis significantly larger. In general, for dinuclear bis(diazine) copper(II) compounds, the unpaired electron of the copper(II) ion is in a magnetic orbital of d(x2-y2) symmetry, which is situated in the plane of the equatorial coordination sphere around the copper(II) ions and partially delocalized on the equatorial ligands. The above mentioned azole-bridged complexes usually show antiferromagnetic coupling, the strength depending on the geometry of the Cu-(N=N)2-Cu bridging unit. Therefore, by a symmetric diazine bridging mode (in the equatorial plane) the largest possible absolute value forJis expected to be in the order of 240～300 cm-1. A more asymmetric bridging mode will result in less effective overlap between the d(x2-y2) orbitals, and the isotropic magnetic exchange will decrease.

2) Furthermore, from the above mentioned systems of the five-membered ring diazine, it is apparent that pyrazolate has capacity to propagate the antiferromagnetic exchange more efficiently than 1,2,4-triazole and 1,2,3-triazole, which can be related to the presence of a third electronegative nitrogen atom in the triazole ring, possessing the ability to polarize spins within the ring and thus limiting the exchange. This is illustrated by Table 2.

3) Obviously, there are several main factors that modify the magnetic coupling parameterJin dinuclear bis(diazine) copper(II) compounds, such as the diazine plane, bond angles, diazine substituents R and so on. But different bridging ligands have different influencing factors. For doubly pyrazolate-bridged copper(II) dimers, extended Hiickel calculations have shown that the isotropic exchange interaction is rather insensitive to geometrical distortions such as deviations from the coplanarity of the metal ions and/or the pyrazoles. So, more subtle differences in bridging ligand geometry must be invoked to account for the differences in magnetic properties observed in copper(II) compounds with closely related ligands. For doubly 1,2,4-triazolate-bridged copper(II) dimers, the two d(x2-y2) magnetic orbitals overlap through the 1,2,4-triazole-N1,N2 bridges. Comparison of the difference in bridging 1,2,4-triazole ligand geometry for these compounds reveals a relationship between the Cu-N-N angles and the magnitude of the isotropic exchange constant, but it is difficult to say whether this relationship is linear with the Cu-N-N angle.

Compared to pyrazolate and 1,2,4-triazolate, the magnetic coupling parameterJin 1,2,3-triazolate-bridged copper(II) dimers is much weaker due to containing three sequent N donors in a coplanar five-membered heterocyclic ring. As far as the ferromagnetic exchange interaction was observed in the doubly 1,2,3-triazolate-bridged copper(II) dimers[12], it can be seen that the intramolecular Cu···Cu distance 4.44 ?, Addison parameter 0.36, and Cu-N=N-Cu torsion angle 112.68°are very different from other dinuclear bis(diazine) copper(II) compounds.

4 CONCLUSION

We have identified a new dinuclear copper(II) complex [Cu2(CH3COO)2(L)2(H2O)2]·4H2O with 1,2,3-triazoles showing a chelating-bridging mode. To the best of our knowledge, the structure and physical properties of coor- dination complexes containing 4-pyridyl-NH-1,2,3-triazole ligands have been seldom documented. Seen from the above coordination complexes we obtained[1,20-22], deprotonated 4-pyridyl-NH-1,2,3-triazole ligands not only serve as a chelator but bridge Mn(II) ions through the M-N-N-M connecting mode which offers antiferromagnetic coupling between spin carriers. The magneto-structural correlation is reasonably analyzed, indicating the magnetic coupling parameterJin 1,2,3-triazolate-bridged copper(II) dimers is much weaker due to containing three sequent N donors in a coplanar five-membered heterocyclic ring. The magnitude of the isotropic magnetic exchange constant depends on the two main factors: 1) the symmetry of the ligand bridging mode; 2) different ligands and their different R submitted groups attributed to the different electric cloud densities. For a further magnetic investigation on the coordination complexes constructed byμ-N2,N3-1,2,3-triazole bridges, the intro- duction of other transition metal ions and/or co-ligands is in progress.

SUPPLEMENTARY MATERIALS

Selected distances and angles (Table S1), hydrogen bonds

(Fig. S1),π-πstacking interactions (Table S2); infrared spectra for HL and complex 1 (Fig. S2), UV-Vis spectra (Fig. S3). The experimental and simulated PXRD patterns (Fig. S4) and TGA curve (Fig. S5) of the compound are available in the Supporting Information.