[Mn(2,2′-bipy)2S6]n: A Homochiral Hybrid of Manganese Polysulfido

[Mn(2,2′-bipy)2S6]n: A Homochiral Hybrid of Manganese Polysulfido Compound with a One-Dimensional Mn-S6-Mn Helical Chain Ming-Lai Fu, Guo-Cong Guo,* Xi Liu, Jian-Ping Zou, Gang Xu, and Jin-Shun Huang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 12 2387–2389

ReceiVed July 11, 2007; ReVised Manuscript ReceiVed October 1, 2007

ABSTRACT: A novel manganese polysulfido hybrid, [Mn(2,2′-bipy)2S6]n, which is the first example with Qn2– (Q ) S, Se, Te, n g 1) as a bridging ligand connecting metal atoms to form a one-dimensional helical chain, has been synthesized and structurally characterized. The compound exhibits semiconducting properties (Eg ) 1.67 eV) and strong blue photoluminescence with the emission maximum occurring around 400 nm, and the variable-temperature magnetic susceptibility data suggest a weak antiferromagnetic interaction among Mn2+ ions in the compound. Helical and nonracemic structures play a fundamental role in many natural systems, such as the R-helix of peptides and the double helix of DNA structures. In recent years, much effort has been devoted to the design and syntheses of inorganic coordination materials with enantiopure topology due to their potential impact on important areas of research such as enantioselective separation and catalysis.1 Transition metal (TM) complexes as templates have been used in the syntheses of chiral inorganic materials2 because they are potentially used to construct objects with a wide variety of shapes, charges, hydrogen bonding sites, and particularly chirality.2b Furthermore, it has been demonstrated that the chirality of the metal complexes can be transferred into the inorganic frameworks.2a,c In the domain of inorganic–organic hybrid polymers in which the metal centers are coordinated by organic ligands and connected by inorganic bridges,3 only two examples with enantiopure [Ni2O(L-Asp)(H2O)2] · H2O and Cd2Cl4(C27H27N3O4) have recently been reported.4 We are interested in introducing TM amine complexes into the inorganic frameworks, especially the main group chalcogenide framework, to understand their role as structure director in constructing the inorganic framework.5 During our pursuit, a chiral polysulfido hybrid, [Mn(2,2′-bipy)2S6]n, containing S62– as bridging ligand, has been obtained, which is, to our knowledge, the first example of a one-dimensional (1D) chiral compound with an extended TM-Qn-TM (Q ) S, Se, Te, n g 1) helical chain. Herein, we report the synthesis, structure, and properties of the compound. [Mn(2,2′-bipy)2S6]n was prepared from a mixture of MnCO3 (115 mg, 1 mmol), antimony (122 mg, 1 mmol), sulfur (112 mg, 3.5 mmol), and 2,2′-bipy (78 mg, 0.5 mmol) in 5 mL of distilled water, which was sealed in a 25 mL poly(tetrafluoroethylene)-lined stainless steel container under autogenous pressure and then heated at 180 °C for 7 days and cooled to room temperature for over 3 h. It is worth noting that the existence of antimony is necessary for the preparation of the title compound. Other metal, such as zinc and iron, can be used instead of antimony, which may act as a reductant in the reaction. The red crystals of the title compound were filtered, washed with dry diethyl ether, and collected in ca. 80% yield (based on manganese). Anal. Calcd. (%): C, 42.92; H, 2.88; N, 10.01; found (%): C, 42.32; H, 2.77; N, 10.32. FT-IR (KBr, cm-1): 1600–1400 (CdC and CdN). The crystal structure6 of [Mn(2,2′-bipy)2S6]n consists of a neutral helical chain, which is composed of [Mn(2,2′-bipy)2]2+ fragments bridged by S62– groups with a Mn · · · Mn distance of 10.538(1) Å. As shown in Figure 1, each Mn atom is chelated by two 2,2′-bipy ligands and coordinated by two S atoms from two S62– groups to form a distorted octahedron. The Mn1–N bond distances vary from * Author to whom correspondence should be addressed. E-mail: [email protected].

2.265(2) to 2.300(2) Å, and the Mn1–S bond distance of 2.553(1) Å are comparable with those reported before.5cd The narrow range of Mn–N bond distances in the title compound is due to the rigidity of the 2,2′-bipy ligand, as can also be found in our previously reported compounds.5d The significantly distorted Mn octahedron is evident with the axial angles ranging from 157.1(1) to 169.2(5)°, which are usually found for the Mn octahedral geometry coordinated by S and N atoms. The S–S bond distances in the S62– group vary from 2.0448(10) to 2.0899(12) Å, and the S–S–S bond angles are in the range of 107.42(3) to 111.12(4)°, which are similar to the those in the literature.7 Nonoxidic materials, particularly based on metal chalcogenides, are highly attractive. For the last few decades, various metal chalcogenides with fascinating structures and interesting properties have been prepared.8 Notably, a number of metal polysulfides,9 polyselenides, and polytellurides with diverse structures have been synthesized, of which the metal polysulfides are mainly investigated. The polysulfido ligands usually serve as chelating ligands,10 while those acting as bridging ligands to form extended structure are scarce.11 The S62– group as a bridging ligand connecting metal ions to form a 1D chain is found for the first time. An interesting feature is that the crystal structure of the title compound crystallizes in the orthorhombic chiral space group C2221, and there is a 2-fold screw axis along the crystallographic c axis, which presents the chiral nature of the title compound. The chiral compounds can be generally synthesized by using chiral reactants or by the influence of a chiral physical environment such as polarized light.12 Although achiral ligands can be used to synthesize chiral coordination compounds by spontaneous chiral resolution,13 which is more attractive since usually such ligands are more common, the construction of chiral coordination polymers using achiral ligands is currently a challenging research field, especially that of helical chains.14 The phenomenon of spontaneous chiral resolution is unusual in coordination chemistry as the products are normally a racemic mixture of opposite handed helical chains.15 Until now, it is not well understood how homochiral packing in crystals can be induced.16 Although organic 2,2′-bipy and inorganic S62– ligands are both achiral, the resulting hybrid is homochiral with an inorganic right-handed helical chain with helical pitch of 19.503(2) Å, which may be due to the in situ formation of the structure directing agent of TM fragment, [Mn(2,2′-bipy)2]2+ (Figure 1b). The chirality of bulk material of the compound can also be confirmed by the presence of optical activity in 1 (Figure S2, Supporting Information). Thermogravimetric analysis revealed two distinct steps with weight changes of about 64.74 and 20.42% until 400 °C (Figure S3, Supporting Information). The observed total weight losses (85.16%) are comparable with the removal of 2,2′-bipy ligands

10.1021/cg070637v CCC: $37.00  2007 American Chemical Society Published on Web 11/03/2007

2388 Crystal Growth & Design, Vol. 7, No. 12, 2007

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Figure 1. (a) ORTEP view of [Mn(2,2′-bipy)2S6]n neutral helical chain in the title compound with 50% thermal ellipsoids. (b) The hybrid with right-handed helical inorganic chain view along the a axis. Hydrogen atoms are omitted for clarity.

among Mn2+ ions in the title compound. To simulate the experimental magnetic behavior, we used the analytical experimental expression deduced for Fisher expression.18

[ 11 -+ uu ]

χm ) S(S + 1) u ) coth

Figure 2. χM and µeff versus T plots for the title compound. The solid lines represent the best fit of the experimental data of the Curie–Weiss law.

(calcd. 55.81%) and sulfur (calcd. 28.64%). The residue of the compound is calculated to be MnS (exper. 14.84%, calcd. 15.55%). The title compound is a photoluminescent material with an emission maximum occurring around 400 nm (λex ) 340 nm) (Figure S4, Supporting Information), which is similar to those found in the main group chalcogenides.5c,d,8a Furthermore, the diffuse reflectance spectrum suggests that the title compound is a semiconductor with Eg ) 1.67 eV (Figure S5, Supporting Information), which is consistent with the red color of the crystals. The electronic transition is likely a result of charge transfer from the S62–dominated valence band to the Mn2+ conduction band.17 The variable-temperature magnetic susceptibility data of the title compound were measured on a crystalline sample in a field of 1 T and in 2–300 K as shown in Figure 2 in the χM and µeff versus T plots. The data were fitted by using the Curie–Weiss equation above 50 K: χM ) C/(T – θ) + χ0, χ0 ) -2.1(1) × 10-4 emu/mol, C ) 4.46(1) emu · K/mol and θ ) -2.2(1) K. The effective magnetic moment, calculated from the equation µeff ) 2.828C1/2, is 5.97(3) µB, which is close to the value of 5.92 µB for one independent unpaired electrons per formula unit. The slow decreases in the µeff value with decreasing temperature, and small negative θ value clearly indicates the presence of weak antiferromagnetic interaction

-JS(S + 1) -2JS(S + 1) kT kT

where N is Avogadro’s number, β is Bohr’s magneton, and k is Boltzmann’s constant. The best least-squares fitting of the theoretical equation to experimental data leads to g ) 2.0199(9), J/k ) -0.26(1) K, and the agreement factor R ) 3.03 × 10-5 (R ) Σ[(χMT)obs - (χMT)calc]2/Σ[(χMT)obs]2). The negative J value suggests a weak antiferromagnetic interaction exists in the title compound, which is agreement with the result of the fitting via the Curie–Weiss’s Law. In summary, a chiral polysulfido hybrid, [Mn(2,2′-bipy)2S6]n, in which the S62– as a bridging ligand connects metal atoms to form a 1D helical chain, has been synthesized and structurally characterized. The title compound exhibits semiconducting properties and strong blue photoluminescence. It can be expected to be the first example of a new series of hybrid formulated as Mn(L)2Qn (L ) neutral chelated ligands; n g 1; Q ) S, Se, Te).

Acknowledgment. We gratefully acknowledge the financial support of the NSF of China (20571075, 20521101), the NSF for Distinguished Young Scientist of China (20425104), and the NSF of Fujian Province (2006J0013). Supporting Information Available: X-ray crystallographic files in CIF format for the structure determination, TGA curve, IR spectrum, optical absorption spectrum, solid emission spectrum, magnetic susceptibility data of the title compound. These materials are available free of charge via the Internet at http://pubs.acs.org.

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