A Spin-Canted Polynuclear Manganese Complex Comprised of

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A Spin-Canted Polynuclear Manganese Complex Comprised of Alternating Linkage of Cyclic Tetra-and Mononuclear Fragments Song-De Han,† Jiong-Peng Zhao,†,‡ Yong-Qiang Chen,† Sui-Jun Liu,† Xiao-Hong Miao,† Tong-Liang Hu,† and Xian-He Bu*,† †

Department of Chemistry, TKL of Metal- and Molecule-Based Material Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China ‡ School of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, China S Supporting Information *

ABSTRACT: A polynuclear manganese complex composed of alternating cyclic tetramer and monomer has been solvothermally prepared and magnetically characterized. Magnetic analyses indicate that the title compound shows spin-canting behavior.

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(SCM)-like behavior was found in polynuclear cobalt complexes assembled via serendipitous self-assembly.8a Singlemolecule magnet (SMM) behavior was observed in a discrete Fe12Sm4 cluster synthesized by a step-by-step assembly.8b A 2D coordination polymer based on Fe 20 clusters and a ferrimagnetic polynuclear cobalt complex composed of [Co24] macrocycle have also been constructed via step-by-step assembly. 8c,d A porous MOF based on nanosized Zn9 precursors was fabricated via bottom-up assembly.8e As references documented, 1H-Benzotriazole (HBTA), a good bridging/chelating ligand, has been well-utilized to generate metal cluster building units because the tri-N-donor features of the triazolate group can provide more coordination sites for metal ions to form polynuclear clusters.7e,f,9 As a continuation of our studies on the synthesis and magnetic properties of polynuclear complexes, we report herein the synthesis, structure, and magnetic properties of an unusual polynuclear manganese complex, H[Mn6(BTA)8Cl5]·(H2O)4 (1).10 Colorless block crystals of 1 were obtained by the solvothermal reaction of MnCl2·4H2O and HBTA in the presence of triethylamine (TEA) and 1H-benzimidazole-2-carboxylic acid (H2BIC) in ethanol (EtOH) solvent at 140 °C. Crystallographic studies revealed that compound 1 crystallizes in a tetragonal system with space group I4̅2d, and its asymmetric unit contains one and a half Mn2+ ions, two BTA− ligands, one and a quarter Cl− anions, and one H2O molecule. All Mn sites are six-coordinate. All the BTA− ligands are in η1:η1:η1:μ3-bridging mode (Figure 1a). The bridging Cl− anions

olynuclear transition-metal complexes have gained huge attention mainly due to their aesthetically pleasing structures and fascinating chemical and physical properties.1,2 Owing to their potential applications in catalyst and moleculebased magnets, the multinuclear manganese complexes continue to garner considerable attention and have been extensively explored.3 However, the design and synthesis of such molecular materials is still an ongoing challenge for coordination chemist, although considerable progress has been achieved in the practical and theoretic approaches. One of the possible strategies to this target is the utilization of multinuclear clusters as secondary building units (SBUs) for the synthesis of larger cluster or cluster-based networks, since such compounds may combine the novel architectures with interesting properties appearing in their SBUs.4 Furthermore, the organic components of the SBUs can also be functionalized, which may be beneficial to the formation of functional molecular materials.5 Until now, current research mainly focuses on two effective strategies to construct polymetallic SBUs. The most common strategy is serendipitous selfassembly between metal ions and suitable organic ligands.6 Although it is difficult to predict the final result, this strategy has found wide applications and has been applied successfully in many systems. Paralleling the serendipitous assembly, stepby-step assembly has also been well-utilized to construct the desirable products, especially bottom-up assembly providing the possibility of rational synthesis.7 The effectiveness and efficiency of such two synthetic routes to produce polymetallic SBUs has been well-demonstrated in recent publications.6−8 We have sought to explore the synthetic methodology toward the construction of new polymetallic clusters appearing in zero (0D)-, one (1D)-, two (2D)- and three-dimensional (3D) molecular materials.8 For example, single-chain magnet © 2013 American Chemical Society

Received: September 5, 2013 Revised: December 1, 2013 Published: December 3, 2013 2

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Figure 1. Ball-and-stick views of (a) the coordination mode of the BTA− ligands and (b) the coordination modes of the Cl− anions (black dashed lines are a guide for the eye).

falls into two groups: the first bridges four symmetry-related Mn1 atoms of the cyclic tetramer in the μ4 mode; the second links one Mn1 atom of the tetramer and one Mn2 atom of the monomer in the μ2 mode (Figure 1b). Mn1 is six-coordinate with an elongated octahedral [MnN4Cl2] geometry formed by the coordination of four N atoms from two different pairs of symmetry-related BTA− ligands and two distinct Cl− anions (Figure 2a). Mn2 is also six-coordinate with an elongated

Figure 3. Ball-and-stick view of the linkage mode of the tetramers and monomers along the c axis.

Figure 4. The uninodal 4-connected 3D framework with Schläfli symbol 66. Figure 2. Ball-and-stick views of (a) the coordination environments of the Mn2+ ions and (b) the cyclic Mn4 tetramer along the c axis (H atoms and solvent molecules are omitted for clarity, similarly hereafter). Symmetry code: A, x, 0.5 − y, 0.25 − z.

among the various cluster-based complexes.12 The successful synthesis of 1 not only adds a new number to the family of multinuclear manganese complex but also provides a rare example constructed from cyclic SBUs. Magnetic measurements were carried out on a crystalline sample of 1, and the phase purity was confirmed by XRPD (Figure S1 of the Supporting Information). The temperaturedependent magnetic susceptibility of 1 was investigated in the range of 2−300 K with a 1 KOe applied field (Figure 5). The χMT product of complex 1 at 300 K is 19.24 cm3 mol−1 K, which is smaller than the spin-only values (g = 2.0) expected for 6 isolated high-spin d5 ions (26.25 cm3 mol−1 K), indicating the antiferromagnetic (AF) coupling between Mn2+ ions. As the temperature is lowered, χMT decreases gradually to 9.09 cm3 mol−1 K at 46 K, and then it increases sharply to a maximum value of 19.53 cm3 mol−1 K at 34 K. Below this temperature, it falls again to 1.83 cm3 mol−1 K at 2 K. The temperature dependence of molar susceptibility in 50−300 K is welldescribed by the Curie−Weiss law, with the Curie constant, C = 24.51 cm3 mol−1 K, and Weiss constant, θ = −76.31 K (Figure S2 of the Supporting Information), which indicates an overall AF interaction. The initial decrease of χMT could be a result of AF coupling of 1. The steep increase of χMT below 46 K is clearly indicative of ferrimagnetism or spin-canting. The field-dependent magnetizations at 2 K show a trend of linear

octahedral [MnN4Cl2] geometry formed by the coordination of four N atoms from two different pairs of symmetry-related BTA− ligands and two symmetry-related Cl− anions (Figure 2a). The Mn−N bond distances range from 2.191(6) to 2.248(6) Å, while the Mn−Cl bond distances are from 2.475(2) to 2.687(2) Å. Four Mn12+ ions are cobridged by four same pairs of symmetry-related BTA− ligands and one μ4Cl− anion to form a cyclic tetrameric [Mn4] entity (Figure 2b). Bridged by the remaining N atoms (N2, N5) of BTA− ligands and the μ2-Cl(2) anions, each tetrameric [Mn4] entity is connected to four neighboring [Mn4] entities by monometric [Mn2] octahedral units, while each monomeric unit connects two cyclic tetrameric units. The strictly alternating arrangement of tetrameric and monomeric units gives rise to the final 3D framework (Figure 3). From the viewpoint of structural topology, if the cyclic Mn4 tetramers are considered as 4-connected nodes (extended point symbol is 62·62·62·62·62·62), the whole 3D structure can be simplified as a uninodal dia net with point (Schläfli) symbol 66 calculated with TOPOS (Figure 4).11 It is notable that complexes based on cyclic or wheel-type SBUs are scarce 3

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ACKNOWLEDGMENTS This work was supported by the 973 Program of China (Grant 2012CB821700) and the NSF of China (Grants 21031002, 51073079, and 21290171).

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Figure 5. Plot of χMT vs T of 1.

increase along with the increasing field without saturation (Figure S3 of the Supporting Information). In contrast with ferrimagnetism (the uncompensated magnetic moment during AF interaction), residual spin resulting from perturbation of antiparallel or parallel coupling may induce spin canting. It is well-known that antisymmetric interactions and single ions anisotropy are the origins of such magnetic behaviors.13 With consideration of the magnetic exchange ways between the neighboring MnII ions of 1, 1 probably shows spin-canting behavior, which is further corroborated by the plot of M versus H at 2 K:2.61 Nβ at 7 T (much smaller than 10 Nβ expected for ferrimagnetic behavior). The spin-canting behavior of 1 may be ascribed to the lack of inversion center in local symmetry between the two adjacent spin-carriers. Below 40 K, there is a split-up between a field-cooled (FC) and zero-field-cooled (ZFC) plot (Figure S4 of the Supporting Information), indicating the presence of long-range order. No obvious loop was observed at 2 K (Figure S5 of the Supporting Information). In conclusion, a polynuclear manganese complex based on alternating cyclic tetramers and monomers has been solvothermally prepared and magnetically characterized. Magnetic measurements indicate that 1 shows spin-canting behavior. The successful synthesis of 1 not only enriches the existing field of multinuclear manganese complexes but also provides a rare example constructed from cyclic SBUs. Further investigation on this work is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic details (CIF), experimental section, supplementary tables and structural figures, and additional characterizations and magnetic properties of three isomorphous complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-22-23502458. Notes

The authors declare no competing financial interest. 4

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