DING Nan LIU Zhong-Yi CHEN Rin-Min BO Yi-Lin WANG Xiu-Guang ZHAO Xiao-Jun YANG En-Cui

(College of Chemistry,Tianjin Key Laboratory of Structure and Performance for Functional Molecules,MOE Key Laboratory of Inorganic-organic Hybrid Functional Material Chemistry,Tianjin Normal University,Tianjin 300387,China)

ABSTRACT One new coordination polymer, [Co4(H2O)4(μ3-OH)2(atz)2(L)]n (1), was solvothermally prepared by the reaction of 3-amino-1,2,4-triazole (Hatz), 4,8-disulfonyl-2,6-naphthalenedicarboxylic acid (H4L) and CoII salt. It crystallizes in monoclinic system, space group P21/c with a = 10.4454(7), b = 12.2214(8), c = 10.0818(7) ?, β = 102.284(7), V =1257.56(15)?3,Dc=2.325 g/cm3,Mr=880.24,Z=2,F(000)=880,μ=2.859 mm-1,the final R= 0.0426 and wR = 0.0822 for 5526 observed reflections with I ＞2σ(I). Structural analyses indicate that 1 exhibits a three-dimensional (3D) pillared-layer framework with triazolate extended CoII4 layers supported by rigid L4-connectors. Magnetically, complex 1 displays strong spin-frustrated antiferromagnetism due to the triangular magnetic lattice and cooperative antiferromagnetic couplings mediated by quadruple heterobridges.

Keywords:pillared-layer framework,1,2,4-triazole,spin-frustration;

1 INTRODUCTION

Design and synthesis of molecule-based magnetic materials (MMMs) are currently attracting considerable attention due to their impressive structural diversities, potential applications in quantum computation and information storage, as well as their significance in the interpretation of the fundamental magneto-structural correlation[1-3].Undoubtedly, a synthetic challenge involves developing simple and efficient methods to aggregate two or more spin carriers in a small, single molecular entity or in a high-dimensional framework by a short extended mediator[4-6].In this area,a subtle choice of five-membered heterocyclic 1,2,4-triazole has been proved to be an effective strategy for designing MMMs. In addition to adopting diverse bridging modes that aggregate spin carriers into a magnetic motif and mediating different magnetic couplings by -NN-/-NCNmoieties,1,2,4-triazole can also create antisymmetric magnetic exchange,as well as spin-competition in metal-triazolate magnetic lattices, which can produce significant spin-canted antiferromagnetism,metamagnetism and spin frustration[7-10].Previous investigations have found that aromatic multicarboxylate co-ligands can influence the structure of metal-triazolate lattices and regulate magnetic behaviors by changing the number, positon and deprotonation of the carboxylate groups, as well as replace non-carboxylate substituents for aromatic multicarboxylates[11-16]. Among these different carboxylate co-ligands, aromatic multicarboxylates with sulfo group can significantly disrupt the metal-triazolate magnetic lattices due to its different bridging groups with flexible connection modes[15-17]. Moreover, the weak coordination bond between metal ion and sulfo group may induce single-crystal to single-crystal transformation[17].Up to date, several CuII/CoII-trz-based complexes with 5-sulfoisophthalate or 2-sulfoterephthalate have been successively reported[17-19], exhibiting ferrimagnetism, spin competition and magnetic switch from weak ferromagnetism to antiferromagnetism.More interesting,some of them even exhibit diverse structures and magnetism but contain the same components[19]. Encouraged by those interesting results, 4,8-disulfonyl-2,6-na- phthalenedicarboxylic acid (H4L) was selected as a co-ligand to react with 3-amino-1,2,4-triazole (Hatz) and CoIIsalts. As well as we know, it is the first time that aromatic multicarboxylate with two sulfo groups was introduced in metal-triazolate system. A novel three-dimensional (3D) pillared-layer framework,[Co4(H2O)4(μ3-OH)2(atz)2(L)]n, con- taining triazole extended CoII4layers was isolated under solvothermal reaction. Magnetically, complex 1 displays strong spin-frustrated antiferromagnetism due to the triangular magnetic lattice and cooperative antiferromagnetic couplings mediated by quadruple heterobridges.

2 EXPERIMENTAL

2.1 Reagents and instruments

Hatz, cobalt acetate tetrahydrate and trimethylamine were commercially purchased and used as received without further purification. H4L was prepared according to the literature[20]. Doubly deionized water was used for the conventional synthesis. Elemental analyses for C, H and N were carried out with a CE-440 (Leeman-Labs) analyzer.FT-IR spectrum (KBr pellet) was taken on an Avatar-370 spectrometer (Nicolet) in the range of 4000～400 cm-1. Powder X-ray diffraction (PXRD)patterns were obtained from a Bruker D8 ADVANCE diffractometer at 40 kV and 40 mA for CuKαradiation (λ= 1.5406 ?), with a scan speed of 0.1 sec/step and a step size of 0.01o in the 2θof 2 ～50°. TG experiment was carried out on a Shimadzu simultaneous DTG-60A compositional analysis instrument from room temperature to 800 ℃under a N2atmosphere at a heating rate of 10 ℃·min-1.Variable-temperature magnetic susceptibility measurement of 1 was carried out at an applied DC field of 1 kOe from 2 to 300 K on a Quantum Design (SQUID) magnetometer MPMSXL-7. Diamagnetic corrections were calculated by using Pascal ′ s constants, and an experimental correction for the sample holder was applied.

2.2 Synthesis of[Co4(H2O)4(μ3-OH)2(atz)2(L)]n(1)

Co(OAc)2·4H2O (49.8 mg, 0.1 mmol), H4L(42.4 mg, 0.1 mmol) and Hatz (16.8 mg, 0.2 mmol) were dissolved in mixed CH3OH-H2O medium (10.0 mL,V:V=6:4)and the initial pH was adjusted to 6.0 by trimethylamine. The resulting mixture was then transferred into a Teflon-lined stainless-steel vessel(23.0 mL) and heated to 170 ℃for 96 h under autogenous pressure. After the mixture was cooled to room temperature at a rate of 2.1 ℃·h-1, red block-shaped crystals of 1 were obtained in 66%yield (based on CoIIsalt). Elemental analysis (%)calcd. for C16H20Co4N8O16S2: C, 21.83; H, 2.29; N,12.73. Found (%): C, 21.89; H, 2.34; N, 12.81.FT-IR (cm-1): 3436(s), 3349(s), 1620(s), 1557(s),1394(s), 1359(m), 1279(w), 1205(s), 1173(s),1110(w), 1033(ms), 936(w), 872(w), 812(w),768(w),664(m),615(m),535(w),473(w).

2.3 Structure determination

A suitable single crystal with dimensions of 0.25mm × 0.22mm × 0.20mm was carefully selected and glued on a thin glass fiber. X-ray single-crystal diffraction data for 1 were collected on an Agilent SuperNova,Dual,Cu at zero,AtlasS2 diffractometer equipped with mirror-monochromated Cu-Kαradiation (λ= 1.54184 ?) at 150 K.There was no evidence of crystal decay during data collection. Semi-empirical multiscan absorption corrections were applied bySCALE3 ABSPACKand the programsCrysAlisProwere used for the integration of diffraction profiles[21,22]. The structure was solved by direct methods and refined with the full-matrix least-squares technique using theShelXTandShelXLprograms[23,24]. Anisotropic thermal parameters were assigned to all non-H atoms. The organic hydrogen atoms were geometrically generated. H atoms attached to water molecules were located from difference Fourier maps and refined with isotropic temperature factors.These data can be obtained, upon request, from the Director, Cambridge Crystallographic Data Centre,12 Union Road, Cambridge CB21EZ, U.K. For 1, a total of 5526 reflections with 2208 unique ones(Rint= 0.0434) were collected in the range of 2.60≤θ≤25.01o,of which 2208 were observed withI＞2σ(I). The finalR= 0.0385 andwR= 0.0874 (w=1/[σ2(Fo2)+(0.0579P)2+14.2637P],whereP=(Fo2+ 2Fc2)/3),S= 1.064, (Δρ)max= 0.87 and (Δρ)min=-0.67 e·?-3.

3 RESULTS AND DISCUSSION

3.1 Crystal structure

Complex 1 crystallizes in the monoclinicP21/cspace group withZ= 2 in each asymmetric unit(Table 1), exhibiting a 3D pillared-layer framework with atz-extended[Co4(μ3-OH)2]6+layers supported by rigid L4-ligands.There are two independent CoIIions, two coordinated water molecules, one atzanion, half a fully deprotonated L4-ligand, and oneμ3-OH-group in the asymmetric unit. As shown in Fig. 1, both unique CoIIions are six-coordinated in octahedral coordination geometries with different distortion. Co(1) is coordinated by four O atoms from one coordinated water molecule, oneμ3-OHgroup, one carboxylate group of L4-, one sulfo group of L4-as well as two N atoms from two separate atz-ligands. Co(2) is surrounded by five O donors from one coordinated water molecule, twoμ3-OH-groups, one carboxylate group of L4-, one sulfo group of L4-as well as one N atom form one atz-ligand. The bond lengths of Co-O and Co-N fall in the range of 2.018(3)～2.302(3)? (Table 1),compared to the previously reported CoII-based complexes with mixed carboxylate and polyazole ligands[6-10]. The centrosymmetric L4-ligand in 1 adopts an octahedral dentate coordination mode with twoμ2-k2:O3,O4-COO-and twoμ2-k2: O6,O8-SO3-, in which the carboxylate group exhibitssyn,syn-configuration.

Table 1. Selected Bond Lengths(?)and Bond Angles(°)for 1

Fig.1. Local coordination environments of CoII ions in 1(Hydrogen atoms are omitted for clarity;symmetry codes:A=x,1/2-y,-1/2+z;B:1-x,1-y,1-z;C:-x,1-y,1-z;D:-1+x,y,z)

As shown in Fig. 2, a pair of symmetry-relatedμ3-OH-groups hold four separate CoIIions together,generating a centrosymmetric tetranuclear[Co4(μ3-OH)2]6+cluster. The CoII···CoIIseparations and Co-O-Co bond angles by twoμ3-OH-groups range from 3.1687(2)to 3.5614(2)? and 100.345(3)to 115.268(3)°. As shown in Fig. 2, adjacent CoII4units are extended byμ3-N1,N2,N4-trz-, generating a wavy layer with the inter-subunit distance of 6.1787(3) ?. Furthermore, the CoII4cluster-based layers are periodically supported by L4-ligands through coordination bonds between CoIIions andsyn,syn-COO-/μ2-SO3-, generating a 3D pillaredlayer framework with the adjacent interlayer CoII···CoIIseparation of 10.4454(7) ? (Fig. 3). In addition, 3D framework of 1 is consolidated through abundant hydrogen-bonding interactions betweenμ3-OH-group/coordinated water molecules/amino group of atz-anion and carboxylate/sulfo groups(Table S1).

Fig.2. 2D wavy layer with CoII4 units extended by triazole bridges and its triangular magnetic pathways(Green lines represent the magnetic coupling pathways)

Fig.3. 3D Pillared-layer framework of 1

From the viewpoint of magneto-structural correlations, the magnetic interactions are significantly mediated by the quadruple heterobridges (μ3-OH-,μ3-atz-,syn,syn-COO-andμ2-SO3-) within the 2D layer, because the nearest CoII···CoIIseparations across the L4-linker are far from those between adjacent 2D layers. More interestingly, such 2D magnetic layer exhibits vertex- and edge-sharing triangle lattice(Fig. 2)when the magnetic pathways are treated as linkers. Obviously, this type of spin arrangement can geometrically induce a spin-frustration, when all magnetic pathways transfer antiferromagnetic couplings[7,25].

3.2 PXRD,TGA and FT-IR spectra

The structural consistency and phase purity of the bulk products of 1 have been evidenced by comparing the experimental and computer-simulated PXRD patterns (Fig. S1). TGA analysis was carried out to explore the thermal stability of 1. The first weight-loss process appeared at 250 °C and ended at 330°C(Fig. S2),which should be ascribed to the removal of coordinated water molecules (calcd.8.2%, obs. 8.3%). Then the mixed ligands are rapidly removed when the temperature is above 330 °C. No complete decomposition was observed until 800°C.

In the IR spectrum of 1, strong and sharp adsorptions at ca. 3436 and 3349 cm-1are the characteristic vibration of O-H and N-H,suggesting the presence of water molecule and amino group. The disappearance of a characteristic band at ca. 1710 cm-1is indicative of full deprotonation of the carboxylic group. And the asymmetric (νas) and symmetric vibrations (νs) of carboxylate group are found at 1620, 1557, 1394 and 1359 cm-1, respectively. The value of Δνbetweenνasandνs(Δν=νas-νs) is 163 and 261 cm-1, indicating the presence of bidentate bridging coordination modes of the carboxylate groups[26].The typical asymmetric or symmetric stretching peaks of -SO3-group are around 1205 and 1033 cm-1, respectively[20]. Thus, the IR results are in good agreement with the crystallographic data.

3.3 Magnetic properties

Variable-temperature (2 ～300 K) magnetic susceptibility was measured on the polycrystalline samples of 1 under an applied direct-current (dc)field of 1 kOe.As shown in Fig.4,theχMTvalue for each CoII4subunit of 1 is 11.50 cm3·K·mol-1at room temperature, which is comparable with the spin-only value (7.50 cm3·K·mol-1) of tetranuclear CoIIcluster withS=3/2 andg=2.0.Upon cooling,theχMTmonotonously decreases to 1.34 cm3·K·mol-1at 2.0 K. Above 40.0 K, the plot ofχM-1vs.Tobeys the Curie-Weiss law well withC=15.13 cm3·K·mol-1andθ= -95.2 K (Fig. 4 inset).Apparently, the large and negative Weiss constantθsuggests strong antiferromagnetic coupling between the neighboring CoIIions. According to the established magneto-structural relationships, theμ3-OH-group generally transfers antiferromagnetic couplings due to large CoII-O-CoIIbond angles (＞97°)[27]. Moreover,μ3-atz-,syn,syn-COO-andμ2-SO3-bridges also prefer to transmit antiferromagnetic couplings in CoIIsystem[10,27]. To quantitatively describe the magnetic couplings within the layer of 1, a 2D magnetic model with different magnetic bridges should be reasonably needed.However, considering the strong anisotropy of CoIIions, no appropriate model could be currently used to evaluate the magnetic couplings within/between the CoII4cluster.Thus,a model reported by Rueff et al. was used to roughly separate the spin-orbit coupling and antiferromagnetic exchange interactions[28]. The magnetic fitting was performed by using equation (χMT=Aexp(-E1/kT) +Bexp(-E2/kT)) above 10 K. Here,A+Bequals to the Curie constant, andE1andE2represent the activation energies corresponding to the spin-orbit coupling and the antiferromagnetic exchange interaction.The best-fit parameters areA+B=14.9 cm3·K·mol-1,E1= 91.68 K andE2= 7.76 K. The value forC=A+Bagrees with that obtained from the Curie-Weiss law in the high temperature range,and the value forE1/kis consistent with those given in the literature for both the effects of spin-orbit coupling and the site distortion (E1/kof the order of 100 K). The magnetic coupling constant between CoIIions mediating by the quadruple bridges is about -15.5 K based on the relationship ofχMT∝exp(J/2kT). The field dependence of magnetization was measured at 2 K (Fig. S3). At 70 kOe, the magnetization is 2.77Nβwhich is far from the saturation (2 ～3Nβfor one CoIIion). All these results confirm that 1 exhibits strong antiferromagnetic behavior.Notably,the absence of a peak in theχMvs.Tcurve indicates that the temperature of antiferromagnetic ordering(TN)is below 2 K,which is obviously due to the triangular alignment of spin carriers within the antiferromagnetic layer. Spin frustration parameterfis larger than 47.6 evaluated by the equationf= |θ|/TN[27], indicating strong spin frustration. Thus, the triangular spin arrangement and cooperative antiferromagnetic couplings mediated by the quadruple heterobridges are responsible for strong spin frustration of 1.

Fig.4. Plots of χMT and χM vs.T for 1(Solid lines correspond to the best least-square fits indicated in the text;Inset:plot of χM-1 vs.T)

4 CONCLUSION

In conclusion, a new pillared-layer framework with triazolate extended CoII4layers was solvothermally obtained, which displays strong spin-frustrated antiferromagnetism due to the triangular magnetic lattice and cooperative antiferromagnetic couplings.