Two Carboxylato-bridged Binuclear Cobalt(II) Complexes Pillared Coligands: Synth

XIN Ling-Yun LI Xio-Ling YIN Wei-Dong LIU Gung-Zhen②

a (College of Chemistry and Chemical Engineering, and Henan Key Laboratory of Function- oriented Porous Materials, Luoyang Normal University, Luoyang, Henan 471934, China)

b (School of Food and Drug, Luoyang Normal University, Luoyang, Henan 471934, China)

ABSTRACT By adopting mixed-ligand strategy, two Co(II) coordination polymers, [Co(phda)(itmb)(H2O)2]n (1) and [Co(phda)(Hpytz)]n (2) (H2phda = 1,2-phenylenediacetate, itmb = 1-(imidazo-1-ly)-4-(1,2,4-triazol-1-yl methyl) benzene and Hpytz = 3,5-di(4-pyridyl)-1,2,4-triazolate), were hydrothermally synthesized and then characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. The single-crystal X-ray diffraction reveals that two complexes feature carboxylato-bridged binuclear subunits extended further by the nitrogen-rich coligands to form different structures. Complex 1 containing phda-bridged binuclear cobalt is developed by itmb coligand to 1D double-stranded chain, whereas complex 2 exhibits 2D open bilayers, which result from the ribbon-like carboxylate chains pillared Hpytz coligands. In addition, the magnetic susceptibilities of both compounds were measured over the temperature range of 3～300 K, and the data were analyzed well using MagSaki software. The best fitting parameters were κ = 0.98, λ = -100 cm-1, Δ = 588 cm-1 and J = 0.6 cm-1 for complex 1 and κ = 0.86, λ = -110 cm-1, Δ = 756.8 cm-1 and J = -1.25 cm-1 for complex 2. The results indicate the existence of weaker ferromagnetic interaction for complex 1 and antiferromagnetic interaction for complex 2 between the metal centers in the dinuclear metal units.

Keywords: mixed-ligand, complexes, synthesis, structures, magnetic properties;

1 INTRODUCTION

Researches toward the exploration of coordination polymers (CPs) have rapidly progressed over the past decades because of the versatile utility of such materials in magnetic materials, energy storage, sensor, gas separation and storage, catalysis, and drug delivery[1-6]. Especially, the CPs based on di- or poly-nuclear metal units have attracted considerable attention in crystal engineering and molecule-based magnetic materials[7-10]. The cooperative interactions of closely coupled di- or polynuclear metal centers usually result in fascinating structures and promising magnetic properties that could not be achieved by single metal ions. To obtain such magnetic materials, we select the flexible benzene multi-carboxylates as building blocks because the flexible ligands can afford a good opportunity to enrich the structural and functional diversities of CPs[11-14], especially providing more opportunities for the formation of poly-nuclear CPs due to larger molecular size for this primary building unit. However, the carboxylate groups adopting a variety of coordination modes can result in different CPs and complex magnetic exchange among the metal centers. Thus, the design and synthesis of magnetic CPs based on well-characterized ligands are still a great challenge. Subsequently, a family of flexible benzene multi-carboxylates with long-spanning carboxyl groups has been used by us and others[15-18], such as 1,2-phenylenediacetate (H2phda) which has been proved to be a versatile building block for the construction of CPs through different coordination modes of its carboxyl groups, and some interesting structures have been successfully obtained[19-21].

On the other hand, the addition of a secondary N-con- taining coligand can provide much more complicated and fascinating structures in view of its cooperative coor- dination[22-25]. Among them, dipyridyl-type molecules have been used widely in the construction of CPs[26-28], while there are relatively few efforts to synthesize CPs derived from polydentate aromatic nitrogen heterocyclic ligands, such as pyrazoles, imidazoles, triazoles, etc. Here, we introduce the triazolate/imidazolate coligands into carboxylate systems to construct CPs, which are chosen as bridging ligands based on the following considerations: (i) They can act as multi-bridges between metal centers, thus supplying for more ligated modes. (ii) The prototropy and conjugation between aromatic nitrogen heterocyclic not only alter the electron density in different parts of the molecules, but also make the ligands more flexible[29].

In this work, we selected the flexible phenylenediacetate (H2phda) as a bridging ligand, and further investigate the effect of 1-(imidazo-1-ly)-4-(1,2,4-triazol-1-yl methyl) ben- zene and 3,5-di(4-pyridyl)-1,2,4-triazolate as coligand on the complex structures, producing two different binuclear cobalt coordination complexes. One is a 1D double-stranded chain featuring the carboxylate binuclear pillared 1-(imidazo-1-ly)- 4-(1,2,4-triazol-1-yl methyl) benzene ligand, and the other is a 2D open bilayer interconnected by phenylenediacetate ligand and 3,5-di(4-pyridyl)-1,2,4-triazolate coligand. Furthermore, magnetic properties of complexes 1 and 2 are also syste- matically investigated.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All reagents used in these syntheses were of analytical grade and obtained from commercial sources without further purification. Elemental analyses (C, H and N) were performed on a Vario EL III elemental analyzer. Infrared spectra were recorded on a VECTOR-22 spectrophotometer within 400～4000 cm-1using the samples prepared as pellets with KBr. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker AXS D8 Advance diffractometer using graphite-monochromatized CuKα(λ= 1.542 Å) radiation at room temperature. Thermo-gravimetric analyses (TGA) were performed on a STA449C integration thermal analyzer in flowing N2at a heating rate of 10 °C/min. Variable-tempera- ture magnetic susceptibilities were measured by using a MPMS-7 SQUID magnetometer. Diamagnetic corrections were made with Pascal's constants for all constituent atoms. The effective magnetic moments were calculated from the equationµeff= 2.828(χMT)1/2, whereχMis the molar magnetic susceptibility. All the magnetic calculations were made using the MagSaki[30]magnetic software program.

2. 2 Synthesis of [Co(phda)(itmb)(H2O)2]n (1)

Typically, the mixture containing Co(OAc)2·4H2O (0.20 mmol, 24.9 mg), H2phda (0.10 mmol, 19.4 mg), itmb (0.10 mmol, 22.3 mg) and 6 mL of water was placed in a 23 mL Teflon-lined stainless-steel reactor. The vessel was heated to 120 °C for 4 days, then cooled slowly to room temperature. Pink blocked crystals were collected by filtration after washing with deionized water, ethanol and acetone, and allowed to dry in air. Yield 46% (based on H2phda ligand). Elemental analysis calcd. (%) for C22H23CoN5O6: C, 51.57; H, 4.52; N, 13.67. Found (%): C, 51.36; H, 4.65; N, 13.58.

2. 3 Synthesis of [Co(phda)(Hpytz)]n (2)

Compound 2 was prepared by a similar way as that descri- bed for 1, except that itmb was replaced by Hpytz (0.10 mmol, 22.3 mg). Pink blocked crystals were obtained. Yield 43%. Elemental analysis calcd. (%) for C22H17CoN5O4: C, 55.71; H, 3.61; N, 14.76. Found (%): C, 55.32; H, 3.85; N, 14.48.

2. 4 Crystal structure determination

Suitable single crystals of 1 and 2 were mounted on a Bruker Smart APEX II CCD diffractometer equipped with graphite-monochromated MoKaradiation(λ= 0.071073 nm) by using aΦ-ωscan technique at room temperature. Semi-empirical absorption corrections were applied usingSADABS[31,32]. The structures were solved by direct methods and refined by full-matrix least-squares onF2. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located from difference Fourier map, and then added geometrically. All calculations were performed using the SHELX-97 program package[33,34]. Crystal data and experimental details for compounds 1 and 2 are contained in Table 1. Selected bond distances/angles and hydrogen bonds are listed in Tables 2 and 3, respectively. Coordination modes of H2phda ligand observed in complexes 1 and 2 are shown in Scheme 1.

Table 1. Crystal and Structure Refinement Data

Table 2. Selected Bond Lengths (Å) and Bond Angles (°)

Table 3. Hydrogen Bond Distances (Å) and Angles (°) for Compounds I and II*

Scheme 1. Coordination modes of H2phda observed in complexes 1 and 2

3 RESULTS AND DISCUSSION

3. 1 Crystal structure description of [Co(phda)(itmb)(H2O)2]n (1)

Single-crystal X-ray analysis shows the asymmetric unit of compound 1 contains one crystallographically separate Co(II) cation, one phda dianion, one itmb ligand, and two coordinated water molecules, as shown in Fig. 1a. Each Co atom is octahedrally coordinated by two carboxylate O donors of two symmetry-related phda anions and two coordinated water molecules in the equatorial plane with two N donors of two symmetry-related itmb ligands in the axial positions. The Co-O bond lengths are in the range of 2.0882(17)～2.1397(18) Å and the Co-N bond lengths are 2.129(2) and 2.1454(19) Å, respectively.

The symmetry-related two cobalt atoms are connected by a pair of phda anions adoptingamonodentate (Scheme 1a) coordination mode to form a binuclear cobalt unit with the Co···Co distance of 6.3325(9) Å (Fig. 1b). Extension of the structure through two pairs of bisymmetric itmb molecules results in a one-dimensional double-stranded chain running along theaaxis, as shown in Fig. 1c. Notably, the presence of coordinated water molecule leads to the formation of H-bonding interactions (Table 3). Besides intramolecular O(5W)-H(2W)···O(4) and O(6W)-H(3W)···O(2) hydrogen bonds, intermolecular O(6W)-H(4W)···O(2) interactions further extend the adjacent double-stranded chains to produce a 2D double layer (Fig. 1d). The detailed hydrogen bond parameters are listed in Table 3. The adjacent 2D layers are stacked in a parallel fashion and cohered together by the weak effect of van der Waals force into the entire 3D supramolecular network of complex 1 (Fig. 1e).

Fig. 1. (a) ORTEP plot of the asymmetric unit of Co(II) in 1, with the symmetry related part drawn as open circle. Symmetry codes: a = 2-x, 1-y, 1-z; b = 1+x, -1+y, z. (b) Dinuclear cobalt connected by phda ligands (c) View of 1D double-stranded chain connected by phda and itmb ligands. (d) View of the cobalt 2D thick layer in H-bonding interactions. (e) View of the 3D supramolecular networks in van der Waals force

3. 2 Crystal structure description of [Co(phda)(Hpytz)]n (2)

The structure of complex 2 is a 2D bilayer with binuclear cobalt nodes. The asymmetric unit contains one Co(II) cation, one phda dianion and one Hpytz ligand. As shown in Fig. 2(a), each Co(II) ion is five coordinated by three oxygen atoms from three carboxylate groups belonging to different phda molecules (Co-O: 1.9805(17)～2.0304(18) Å) and two nitrogen atoms from Hpytz ligands (Co-N: 2.1548(19) and 2.1564(19) Å), displaying a highly distorted trigonal bipyramidal coordination environment.

Each phda anion acts as a tridentate ligand through monodentate and bidentate bridging modes (Scheme 2b) connected to the adjacent Co(II) ions to form a 1D ladder-like chain along theadirection (Fig. 2b), which contains an alternating arrangement of 8- and 18-membered rings with Co⋅⋅⋅Co separations being 3.8959(6) and 6.1856(7) Å, respectively. Furthermore, these ribbonlike chains are further linked by Hpytz ligands to generate a 2D open bilayer (Fig. 2c). Two adjacent bilayers are bridged by H-bonds between coordination H2O and triazole N atom of Hpytz ligand (N(4)-H(4N)···O(4),d(N···O) = 2.724(3) Å,∠(H-N···O) = 168.1°) to produce its 3D supramolecular network (Fig. 2d).

Fig. 2. (a) ORTEP plot of the asymmetric unit of Co(II) in 2, with the symmetry related part drawn as open circle. Symmetry codes: a = -x, 1-y, -z; b = 1+x, y, z; c = -1+x, -1+y, -1+z. (b) View of a 1D chain featuring phda-bridged. (c) 2D bilayered block developed by Hpytz ligand. (d) View of 3D supermolecular networks cohered by H-bonds

3. 3 IR and TGA

Both compounds are further characterized by IR spectra, which were consistent with their structures as determined by single-crystal X-ray diffraction. IR spectra of both complexes exhibit several characteristic bands. Broad bands observed in the region of ～3100 cm-1in 1 represent O-H stretching modes within ligated water molecules. The sharp characteristic bands of dicarboxylate groups in the usual region are at 1610～1630 and 1480～1500 cm-1for the asymmetric stretching and the 1360～1395 cm-1for the symmetric stretching.

Thermogravimetric analyses (TGA) for complexes 1 and 2 were performed on crystalline samples from room tempera- ture to 800 °C under a N2atmosphere to investigate their dehydration and degradation behaviors, as shown in Fig. 3. The TGA curves of complex 1 show that its decomposition proceeds in three obvious steps. The first one undergoes a 7.23% mass loss between 91 and 172 °C, corresponding to the loss of two ligated water molecules per formula unit (calculated 7.03%). The weight loss of the final two steps in complex 1 takes place over 182 up to 455 °C, corresponding to the pyrolysis of the two organic ligands (phda carboxylate ligand and itmb coligand). The final residue is attributed to CoO component (observed 14.60%, calculated 14.62%). Different from complex 1, the framework of complex 2 is stable up to 236 °C. After that, a series of consecutive weight losses until 560 °C corresponds to decomposition of organic ligands, and the remnant is 14.34%.

3. 4 X-ray powder diffraction studies

In order to check the phase purity of these compounds, the X-ray powder diffraction (XRPD) patterns of both complexes were checked at room temperature. As shown in Fig. 4(a) and 4(b), the peak positions of the experimental XRPD patterns of both complexes are in agreement with the simulated ones of both compounds, demonstrating the single phase purity of the products. The difference in intensity may be due to the preferred orientation of the microcrystalline powder samples.

Fig. 3. TGA curves of complexes 1 and 2

Fig. 4. XRPD patterns of complexes 1 (a) and 2 (b) in contrast to the simulate pattern

3. 5 Magnetic properties of cobalt compounds

Considering the relatively close distance among the Co(II) centers in both compounds, their magnetic properties were investigated. For both compounds, they can be considered as Co(II) binuclear from the viewpoint of magnetism, in which two Co(II) centers are linked by double ndca bridges with the Co⋅⋅⋅Co distances of 6.3325(9) Å for compound 1 and 3.8959(6) Å for compound 2, since the coupling through other ligands can almost be negligible due to the long Co⋅⋅⋅Co separation (14.3289 Å for 1 and 14.7156 Å for 2). At same time, Co(II) for compound 1 has a distorted octahedral geometry with O(1), O(3), O(5W), O(6W), N(1) and N(5). Although the distance, 3.3454 Å, between Co(II) and O(1) for compound 2 is relatively far, coordination geometry around Co(II) for 2 can also be considered as a distorted octahedron with O(1), O(2), O(3), O(3A), N(1) and N(5).

The temperature-dependent magnetic susceptibilities of both compounds have been measured under a 2000 Oe applied field in the temperature range of 3～300 K. The temperature dependencies ofχMandµeffper Co(II) for 1 and 2 are shown in Fig. 5a and 5b. Theµeffvalues per Co(II) of complexes at room temperature are 4.75µBfor 1 and 4.53µBfor 2, respectively, which are larger than two (3.87µB;µso= [4S(S + 1)]1/2; S = 3/2) uncoupled Co(II) cations. This indicates that an important contribution of the orbital angular momentum typical for the4T1gground term is involved[5,35]. The magnetic moments decrease with decreasing the temperature, and this can be elucidated considering three factors: (1) the contribution of the orbital angular momentum; (2) an intramolecular magnetic coupling between two Co(II) ions; (3) an intermolecular antiferromagnetic coupling[36,37]. All the magnetic calculations were made using MagSaki magnetic software program. Considering the axially distorted octahedral field, a calculation mode “Co(II) (dinuclear, axial oct)” is selected, producing three parameters for analyzing the cryomagnetic data in an axially distorted octahedral field. At this stage the Hamiltonian is

and the magnetic susceptibility equation is as follows[36,38]:

Fig. 5. Temperature dependencies of χM (O) and µeff (Δ) of complexes 1 (a) and 2 (b). Solid red curves are drawn with the parameters κ = 0.98, λ = -100 cm-1, Δ = 588 cm-1 and J = 0.6 cm-1 for complex 1 (a) and κ = 0.86, λ = -110 cm-1, Δ = 756.8 cm-1 and J = -1.25 cm-1 for complex 2 (b)

The data were well simulated in the temperature range of 3～300 K for both complexes, and the calculated curves are also shown in Fig. 5a and 5b, respectively. Fitting parameters wereκ= 0.98,λ= -100 cm-1,Δ= 588 cm-1andJ= 0.6 cm-1for complex 1 andκ= 0.86,λ= -110 cm-1,Δ= 756.8 cm-1andJ= -1.25 cm-1for complex 2, whereκis the orbital reduction factor andλis a spin-only coupling constant. The axial splitting parameterΔis defined as the splitting of the orbital degeneracy of4T1gterm by the asymmetric ligand component in the absence of any spin-orbit coupling, and is taken to be positive when the orbital singlet is the lowest. The other parameters including the axial zero-field splitting parameter D, the anisotropic g-factors, gzand gxcan be calculated using the fourκ,λ,ΔandJvariables[38,39], wheregz= 2.15,gx= 4.98 and D = 85.96 for 1 andgz= 2.04,gx= 4.75 and D = 63.37 for 2. TheR(χM) values, defined asR(χM) = ∑[(χM,calc-χM,obs)2]/(χM,obs)2, are 9.49×10-4for 1 and 1.59×10-3for 2. The small positiveJvalue indicates the existence of an weaker ferromagnetic interaction, whereas the negativeJvalue indicates the existence of an antiferromagnetic interaction[40].

4 CONCLUSION

We have presented the syntheses and structures of two Co(II) coordination polymers assembled from mixed-ligands of flexible H2phda and two nitrogen-rich coligands under hydrothermal conditions. Their structures show different binuclear developing networks from 1D double-stranded chain to 2D open bilayers, showing the remarked structure sensitivity to the N-donor coligands. Note that the magnetic suscepti- bilities of both compounds are fitted according to the dimeric modes, and the results of the best fit indicate the existence of weaker ferromagnetic exchange for complex 1 and antiferromagnetic exchange for complex 2 between the metal centers in the dinuclear metal units.