CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 589-591
Communications Syntheses, Structures, and Magnetic Properties of Two Novel Three-Dimensional Frameworks Built from M4O4 Cubanes and Dicyanamide Bridges Zuodong Lin,† Zhifeng Li,‡ and Hongjie Zhang*,† Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, and Graduate School of the Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and School of Material & Chemical Engineering, Jiangxi UniVersity of Science and Technology, Ganzhou 341000, P. R. China ReceiVed July 19, 2006; ReVised Manuscript ReceiVed March 5, 2007
ABSTRACT: Two novel coordination polymers Ni4(CH3O)4(CH3OH)4(dca)4 (1) and Co4(CH3O)4(CH3OH)4(dca)4 (2) have been synthesized by solvethermal reaction. X-ray single-crystal analysis reveals that the two complexes are isostrutural and possess 3D frameworks that are built from the M4O4(M) Ni (1) and Co (2)) cubanelike building blocks linked by dicyanamide (dca) bridges. The temperature dependence of the magnetic susceptibility was measured and the DC experiment data were fitted using the Heisenberg spin Hamiltonian. The discovery of single-molecule magnets (SMMs) is one of the most exciting developments of magnetism.1 With a large ground state total spin quantum number S, uniaxial anisotropy, and an energy barrier ∆ ) |DS|S2 (|DS|(S2 - 1/4) for half-integer spins), the SMMs might be applicable candidates for realizing ultrahigh density information storage and future quantum computer.2 Attracted by those potential applications, many efforts has been devoted to synthesizing new SMMs and investigating the essence of magnetic properties of those molecules.3 Moreover, the bottom-up methods provide a new strategy for constructing high-dimensional frameworks using building blocks that have special characters to realize the special functions. So the SMMs can be functional building blocks because of their unique magnetic properties. This became more realizable especially after the interaction between the SMMs was reported. The pioneer work of Wensdorfer and co-workers4 demonstrated an interesting phenomenon about an antiferromagnetic coupling between two single molecular magnets and opened two different directions of the SMM research field. First of all, realizing the importance of intermolecular magnetic interaction for quantum tunneling of magnetization (QTM), some research groups investigated the origin of this problem by modifying chemical compositions of special SMMs.5 Secondly, it provides a probability to finetune magnetic properties through the interaction of SMMs. This was followed by using SMMs as the building blocks to create novel two- and three-dimensional networks and investigating the underlying mechanism of interactions between SMM and SMM. However, few successful examples have been reported till now.6 Herein, we report two novel three-dimensional frameworks that are constructed from cubanelike M4O4 cubanes and dicyanamide (dca) bridges. There are two significant points of our crystals: (1) the M4O4 cubanelike clusters were our building blocks, which have been extensively investigated for their specific magnetic properties.7 In * To whom correspondence should be addressed. Tel: 86-431-5262127. Fax: 86-431-5698041. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Jiangxi University of Science and Technology.
these M4O4 cubane, the M-O-M angles are close to 90°, which favors to the orthogonality of the magnetic orbits and ferromagnetic exchange. Especially, it was reported that the cubanelike M4O4 complexes can behave as SMMs. 5a,5c,7c,7d (2) The dca anion has been chosen as the bridge and is capable of coordinating transitional metal ions in various modes. Remarkably, the long-distance magnetic ordering was observed in some binary M(dca)2 (M ) Cr, Mn, Fe, Co, Ni) compounds,8 thus the dca anion has been widely used to synthesize the molecular-based magnetic materials. Solvothermal reaction of NaN(CN)2 and NiCl2‚6H2O or CoCl2‚ 6H2O in 10 mL of methanol at 75 °C gave polyhedron crystals of Ni4(CH3O)4(CH3OH)4(dca)4 (1) or Co4(CH3O)4(CH3OH)4(dca)4 (2), which are isomorphous in terms of structure. These coordination polymers were built up from cubane M4O4 building blocks and dca bridges. For the tetranuclear cubane M4O4 core, nickel or cobalt atoms occupy the four alternative corners of the cube, whereas the oxygen atoms of methoxide groups occupy the rest of the four corners. The octahedral coordination sphere of the metal atom has been fulfilled by three µ3 oxygen atoms in the cubane, one oxygen atom from bonded OMe and two nitrogen atoms from dicyanamide ligand. The geometry around Ni is slightly distorted. The longest Ni-O distance (2.116 Å) corresponds to the oxygen atom of the O-Me, and the other three Ni-O distances are relatively shorter (from 2.051 to 2.058 Å). The distances of Ni-N are approximately equal to these of Ni-O, which belong to cubane (2.057 and 2.089 Å). The cubane is also distorted because of the smaller angles of O-Ni-O within the cubane, which are equal to 81.73, 81.17, and 81.90° (the structural details of 2 can be found in the Supporting Information). Finally, to form a three-dimensional framework, we connected each M4O4 building block to other four M4O4 cores by eight dca bridges that exhibited the end-to-end (µ-1,5) bridging mode through the two nitrile nitrogen atoms. The temperature dependence of the DC and AC magnetic susceptibility were measured on powder samples of 1 and 2 under a 1000Oe applied field in the temperature rang from 2 to 300 K. For 1, above 60 K, the 1/χ product obeyed the Curie-Weiss law with C ) 5.10 emu mol-1 and θ ) 19.49 K. The positive θ
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590 Crystal Growth & Design, Vol. 7, No. 4, 2007
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Figure 3. The hysteresis loop at 2 K (black), 4 K (red), and 10 K (green) for 1.
Figure 1. (a) M4O4 cubane; (b) dca bridge between the cubanes. (c) View of the framework along the a-axis. The hydrogen atoms have been removed for clarity. Ni atoms are blue and O atoms are red.
Figure 2. Plot of χT vs T for 1. The solid line is the best fit from 15 to 300 K.
indicated an intracubane ferromagnetic interaction in 1. As shown in Figure 2, the χmT value of 1 was about 5.40 emu mol-1 K at room temperature, which is a little larger than the only spin value for four non-interacting nickel ions. As the temperature decreased, the χmT increased slowly until 55 K, subsequently began to suddenly increase, and finally reached the maximum value 11.10 emu mol-1 K at 10 K. This implied that ferromagnetic interactions occurred among the four Ni atoms of the cubane. The χmT decreased abruptly below 10 K, as a result of the zero-field splitting within the ground state and the presence of antiferromagnetic coupling interactions. The χmT vs T curve of 2 was similar to that of 1. The in-phase AC susceptibility of 1 and 2 did not exhibit the frequency dependence, and no signal was observed in the out-of-phase susceptibility at temperatures down to 2 K (the low-temperature limit of our experiment). But we expected that the signal could be found at temperatures even lower than 2 K. We used the Heisenberg spin Hamiltonian to analyze the DC data of two compounds. There are two different sets of M-O-M angles in the present complexes: (98.38°, 98.38°), corresponding to J1, and (97.64°, 97.43°), corresponding to J2, for 1; (98.43°, 98.43°) and (96.79°, 98.21°) for 2. Thus, two exchange parameters, J1 and J2, should be considered in the spin Hamiltonian.
H ˆ ) -2J1(S1S2 + S3S4) - 2J2(S1S3 + S1S4 + S2S3 + S2S4) ) -(J1 - J2)(S122 + S342) - J2ST
Where S12 ) S1 + S2, S34 ) S3 + S4, ST ) S12+ S34, and S1 ) S2 ) S3 ) S4 ) 1 for Ni (complex 1) and S1 ) S2 ) S3 ) S4 ) 3/2 for Co (complex 2). From 15 K, the experimental data of 1 were fitted well; J1, J2, and g values are -4.47 cm-1, 11.70 cm-1, and 2.18, respectively, and the agreement factor R ) 1.07 × 10-4. The J values obtained here are in good agreement with the rules of the previously reported [Ni4O4]4+ cubanes complexes.7a,9 That is, exchange coupling constant J is closely correlated with the Ni-O-Ni angles, 98° is the critical point, the ones smaller than that correspond to ferromagnetic exchange and larger than that correspond to antiferromagnetic exchange. For compound 2, the experimental data were fitted from 2 to 300 K; the J1, J2, and g values are 0.30 cm-1, 0.53 cm-1, and 2.68, respectively, and the agreement factor R ) 4.22 × 10-4. See the Supporting Information for more details. To further confirm the magnetic properties of compound 1, we performed magnetization measurements under the applied field from -5 to 5 T and at 2, 4, and 10 K. At 2 and 4 K, obvious butterflyshaped hysteresis loops could be observed. Shown in Figure 3, the hysteresis effect decreased, as expected when the temperature is increased, and vanished at 10 K. The magnetization measurement about compound 2 was also taken under the same conditions, but the magnetization did not reached saturation until 5 T (see the Supporting Information). Our original aim was to abain a suitable system to investigate the interactions between SMM and SMM. So the target will been achieved if the metal cubanes in the title compounds can behave as the SMMs. It is reported that the M4O4 cubanelike complexes can behave as SMMs.5a,5c,7c,7d But, unfortunately, there is no evidence to indicate that the cubanelike clusters in our crystals could be considered as the SMMs. However, the unique structure of our crystals might be a good example to study the magnetic interactions among the building blocks. In conclusion, two novel isostructural coordination polymers have been synthesized and characterized. The complexes possess the brief 3D frameworks that are built from M4O4 cubanes linked by dca bridges. The magnetic properties of 1 indicated a magnetostructural correlation that small differences in M-O-M angles in cubanelike complexes can result in large difference in J values. Finally, the synthesis of these two compounds shows a potential route for constructing new 3D frameworks by using various building blocks and some longer bridging ligands. Acknowledgment. The authors are grateful for financial aid from the National Natural Science Foundation of China (Grants 20372060, 20340420326, and 20490210) and the MOST of China (“973” Program, Grant 2006CB601103). We thank Professor Jianshi Zhou for valuable suggestions. Supporting Information Available: X-ray crystallographic data in CIF format; description of experimental procedures and some supplementary figures in PDF format. This material is available free of charge via the Internet at http://pubs.acs.org.
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