Unprecedented 3D Host-Framework Based on a Ni5 Cluster with

Cu(ii), Co(ii), Mn(ii) and Ni(ii) ions; synthesis, structure, and magnetic properties. Ranjan Patra , Hatem M. Titi , Israel Goldberg. CrystEngCom...
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Unprecedented 3D Host-Framework Based on a Ni5 Cluster with Helical Chains as Guest: Synthesis, Structure, and Magnetic Property Fang-Hua Zhao, Yun-Xia Che, and Ji-Min Zheng* Department of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: The hydrothermal reaction of NiCl2·6H2O, azelaic acid (H2aze), and 1,4-bis(1-imidazolyl)benzene (L) has afforded a new 3D polynuclear nickel complex, {[Ni5(aze)3(H2aze)0.5(μ3-OH)2(L)4][Ni(aze)(L)(H2O)2]2Cl2}·4H2O (1). The cationic [Ni5(aze)3(H2aze)0.5(μ3-OH)2(L)4]2+ forms the 3D hostframework, representing as an α-Po net, and neutral [Ni(aze)(L)(H2O)2] displays as left- and right-handed chiral helical chains and acts as guest, penetrating the 1D channels of the host-framework alternately along the b direction. Magnetic studies reveal spin-canting behavior for complex 1.

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(aze)(L)(H2O)2] and free chlorine anion with free water molecules. The cationic pentanuclear Ni5 clusters display rectangular symmetry and form the 3D host-framework; the neutral portion forms left- and right-handed chiral helical chains penetrating the 1D channels of the host-framework alternately along the b direction; magnetic studies reveal spin-canting behavior for complex 1. Hydrothermal reaction of NiCl2·6H2O, H2aze, and L in a 1:1:1 molar ratio at 180 °C generated the green block-shaped crystals of 1. Complex 1 crystallizes in the monoclinic P21/c space group, which consists of pentanuclear cationic cluster [Ni5(aze)3(H2aze)0.5(μ3-OH)2(L)4]2+, neutral mononuclear [Ni(aze)(L)(H2O)2], free chlorine anion, and free water molecules. In the pentanuclear cluster motif (Figure 1a), three crystallographically independent Ni(II) centers are connected by six carboxyl and two μ3-OH groups to form a pentanuclear [Ni5(μ3-OH)2(COO)6]2+ core, in which the five Ni(II) centers are all coplanar with a rectangular arrangement. The Ni3 atom lies on the inversion center and is coordinated by six oxygen atoms from two μ3-OH groups and four individual aze ligands. Ni2 and Ni4 have the same coordination polyhedra, which are coordinated by two nitrogen atoms of a L ligand and four oxygen atoms from one μ3-OH group and three carboxylate oxygen atoms of two distinct aze ligands. The Ni− O/N bond lengths are in the range of 2.000(3)−2.181(3) Å (Table S1 of the Supporting Information). The distances from the central Ni3 ion to the peripheral Ni2 and Ni4 ions are 3.0373(10) and 3.0631(7) Å, respectively. The short edge Ni2···Ni4 of the rectangle is 3.6073(10) Å, and the long edge Ni2···Ni4A of the rectangle is 4.9195(12) Å. The Ni−O−Ni

ecently, the assembly of cluster-based coordination complexes has attracted considerable attention, owing to their intriguing topology architectures and potential applications.1 The polynuclear metal clusters have been demonstrated to be effective to construct high-connected networks because of their higher and variable coordination numbers and flexible coordination environments.2 And the construction of metal clusters as secondary building units (SBUs) has also been demonstrated to be a powerful synthetic strategy to produce highly porous frameworks.3 On the other hand, polynuclear metal clusters as good candidates to construct molecular magnets have also attracted great interest in magnetochemistry for their diverse and interesting magnetic properties.4 Therefore, much work has focused on the rational design of such complexes by controlling the favored geometry of ligands and metals. However, the design and syntheses of polynuclear metal complexes with predictable structures and properties are still a challenge in coordination chemistry. It has been suggested5 that Ni(II) clusters often show ferromagnetic exchange between metal centers, and Ni(II) ion is known to have large single-ion zero-field splitting.6 Therefore, it should be an ideal candidate to construct polynuclear clusters with various magnetic properties. To date, many polynuclear Ni(II) complexes ranging from Ni4 to Ni 32 species7 have been synthesized and magnetically characterized. Among them, Ni4 clusters with cubane structure have been extensively reported, while pentanuclear Ni(II) complexes with rectangular symmetry have been less documented.8 In this work, we selected the flexible azelaic acid and rigid 1,4-bis(1-imidazolyl)benzene (L) to construct the polynuclear nickel complex, {[Ni5(aze)3(H2aze)0.5(μ3-OH)2(L)4][Ni(aze)(L)(H2O)2]2Cl2}·4H2O (1), which consists of three components of [Ni5(aze)3(H2aze)0.5(μ3-OH)2(L)4]2+, neutral [Ni© 2012 American Chemical Society

Received: July 13, 2012 Revised: September 14, 2012 Published: September 18, 2012 4712

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Figure 1. Coordination environment of Ni(II) centers in the pentanuclear cluster (a) and neutral component (b).

angles are 98.58(13)° for Ni2−O11−Ni3, 128.31(16)° for Ni2−O11−Ni4, 90.28(11)° for Ni3−O9−Ni2, 99.43(14)° for Ni3−O11−Ni4, and 91.68(12)° for Ni3−O12−Ni4A. There are two kinds of aze ligands in the pentanuclear cationic cluster component. One carboxylate group (O7, O8) of an aze ligand has a bidentate coordination mode bridging two Ni(II) ions (Ni2 and Ni4) of the pentanuclear Ni5 cluster. The other carboxylate (O9, O10) shows a tridentate coordination mode bridging three Ni(II) ions (Ni2, Ni3, and Ni4) of the other pentanuclear Ni5 cluster. For another aze ligand, one of its carboxylate groups (O12, O13) displays a tridentate coordination mode bridging three Ni(II) ions (Ni2, Ni3, and Ni4) of the pentanuclear Ni5 cluster, while the other carboxylate group (O14, O15) is nondeprotonated and forms the hydrogen bond with the O1 atom of the neutral component: O15−H15A···O1 (2.510(16) Å, 132.1°) (Table S2). For the L ligand, there are also two coordinated modes in the pentanuclear cationic cluster unit; one exhibits cis fashion, bridging Ni2 and Ni4 atoms of two Ni5 cluster units via N5 and N8 atoms with the dihedral angle of two imidazole rings being 16.37°, and another displays trans fashion, bridging Ni2 and Ni4 atoms of two Ni5 cluster units via N9 and N12 atoms, with the two imidazole rings being nearly coplanar (dihedral angle = 4.55°). Each Ni5 cluster is connected with six other identical Ni5 clusters by four aze and eight L ligands, forming a 3D host-framework, which represents as an α-Po net with double edges. This is the first α-Po net with double edges containing pentanuclear Ni5 clusters.9 The pentanuclear core [Ni5(μ3-OH)2(COO)6]2+ in complex 1 is similar to the [Ni5(μ3-O)2(O2C)6] core in reported complex [Ni5O2(btb)2(def)21(H2O)21] (btb = benzene-1,3,5-tribenzoate),8a only the two μ3-O2− bridges are replaced by two μ3-OH bridges; therefore, the [Ni5(μ3-O)2(O2C)6] core is neutral, while the [Ni5(μ3-OH)2(COO)6]2+ core in 1 is cationic. In the neutral moiety, there is only one independent Ni(II) center; it shows a slightly distorted octahedral geometry coordinated by two aze oxygen atoms (O2, O3), two oxygen atoms from two coordinated water molecules (O5, O6), and two L nitrogen atoms (N1, N4) (Figure 1b). The Ni−O/N bond lengths are in the range of 2.043(5)−2.094(5) Å (Table S1 of the Supporting Information). Unlike that in the pentanuclear cluster, both carboxyl groups of the aze ligand show the monodentate coordination mode. The L ligands exhibit trans fashion, with the two imidazole rings being nearly coplanar (dihedral angle =4.97°). The Ni(II) ions are linked by aze2− to form 1D left- and right-handed single-helical chains of {Ni(aze)}n with a period of 13.484 Å (Figure 2a). The L ligand situated the spaces between the adjacent two pitches of the helical species (Figure 2b). Interestingly, such left- and right-

Figure 2. (a) Perspective and space-filling views of the left- and righthanded helical chains of {Ni(aze)}n from the neutral component. (b) The helical chains with the L ligands situated the spaces between the adjacent two pitches.

handed single-helical chains act as the guest penetrating the 1D channels of the host-framework alternately along the b direction (Figure 3); this case is rather rare for the 3D framework, with two types of helices as the guests. Between the 1D channels of the host-framework and the chiral helices, there exist the free chlorine anions and free water molecules, which form a series of hydrogen bonds connecting the hostframework and the guest helices (Table S2). For complex 1, the pentanuclear component acts as the host, with the mononuclear component as the guest; to the best of our knowledge, such an interesting combination is the first example for Ni complexes. The variable-temperature magnetic susceptibility measurement of 1 was performed in the temperature range of 2−300 K under a field of 1000 Oe. At room temperature, the χMT value is 8.12 cm3 K mol−1 (Figure 4), larger than the spin-only value of 7.0 cm3 K mol−1 for seven isolated Ni(II) centers, which might be caused by the spin−orbit coupling characteristic for Ni(II) complexes with an 3A2 g ground state resulting in an increasing g factor.10 Upon cooling to 60 K, the χMT value decreases continuously to 6.23 cm3 K mol−1, indicating antiferromagnetic interaction between the Ni(II) ions. And then, the χMT value increases with the decrease of temperature; it reaches the maximum of 6.55 cm3 K mol−1 at 8 K, with such weak ferromagnetism possibly being due to intercluster ferromagnetic interactions or spin-canting effects.11 Below 8 K, the corresponding value decreases rapidly, which should be assigned to zero-field splitting within the ground state at low temperature, as often found in Ni(II) complexes.8b,12 The magnetization data collected at 2.0 K (Figure S1 of the Supporting Information) show a gradual increase of M values as the fields increase; no saturation was observed up to an external 4713

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Figure 3. The 3D host-framework consisting of the pentanuclear cationic cluster with the α-Po net viewed along the a direction (a) and the b direction (b); the left- and right-handed helical chains penetrate the 1D channels of the host-framework alternately along the b direction.

occurrence of the phase transition near 8 K. Further, the zero-field ac magnetic susceptibility for complex 1 (Figure 5b) exhibits the maximum in χM′ at 8 K but without the maximum in χM″, which clearly revealed the occurrence of the magnetic phase transition and agreed with the previous magnetic studies. And the absence of a frequency dependence of ac susceptibility precludes the possibility of the spin glass behavior in 1. Therefore, it suggests spin-canting behavior for the magnetic property of complex 1.13 The thermal study of complex 1 (Figure S2 of the Supporting Information) shows a weight loss of 7.23% from 128 to 310 °C, corresponding to the loss of free and coordinated water molecules and the free chlorine ions (calcd 7.30%), and after that, it began to decompose upon further heating. The powder X-ray diffraction pattern (PXRD) was also performed to confirm the phase purity of bulk samples of 1 (Figure S3). In summary, this work presents a polynuclear Ni(II) complex containing both pentanuclear [Ni5(μ3−OH)2(COO)6]2+ core and mononuclear [Ni(aze)(L)(H2O)2] components. The Ni5 clusters construct a 3D host-framework. The mononuclear components form left- and right-handed helices penetrating the 1D channels of the host net along the b direction. Magnetic studies reveal that complex 1 shows a magnetic phase transition near 8 K induced by a spin-canting effect. It is believed that the presented results provide new information regarding polynuclear nickel chemistry and magnetic chemistry.

Figure 4. χM and χMT vs T curves for complex 1.

field of 80 kOe. The M value at 80 kOe is 12.5 Nβ, which is lower than the expected ferromagnetic value of 14.0 Nβ for seven Ni(II) ions with ground state S = 1.0. This nature also indicates that weak ferromagnetism should be induced by a spin-canting effect for the χMT vs T abnormality at low temperature in complex 1.11 No detected hysteresis for isothermal magnetization was observed at 2 K (Figure S1 of the Supporting Information, inset). For further proof of the results above, zero-field-cooled (ZFC) and field-cooled (FC) magnetization experiments were carried out at an applied field of 30 Oe (Figure 5a). The presence of the bifurcation point between the FC and ZFC magnetic susceptibility curves below 8 K indicates the

Figure 5. (a) Field-cooled and zero-field-cooled magnetic susceptibility curves for 1. (b) Temperature dependence of ac susceptibility at 499 and 4999 Hz. 4714

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ASSOCIATED CONTENT

S Supporting Information *

The experimental section, X-ray crystallographic files in CIF format, tables of selected bond lengths and angles (Table S1) and those of the hydrogen bonds (Table S2), and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-22-23508056. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (50872057).

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REFERENCES

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