Novel rare case of 2D + 1D = 2D polycatenation Hg(II) coordination polymer

Abstract

A novel polymeric mercury(II) thiocyanate coordination polymers, [Hg₂(µ-bpdh)(µ-SCN)₄]_n[Hg₂(µ-bpdh)(µ-SCN)₄]_2n (1) (bpdh = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene) was prepared and fully characterized. In compound 1, the bridging ligand bpdh adopts a trans-oid conformation and the network contains two interpenetrating coordination polymers, a 2D net and a 1D double chain, which results in a very rare case of 2D + 1D = 2D polycatenation.

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Metal coordination polymers have been studied widely as they represent an important interface between synthetic chemistry and materials science, with them having specific structures, properties, and reactivities that are not found in mononuclear compounds. Many attempts have been made to prepare a variety of transitional metal complexes using different spacers, and their structures and properties have been physically and chemically determined.^1–11 In contrast to coordination polymers of transition metal ions, the formation of polymers with heavy metal ions such as mercury(II) seems to be surprisingly sparse. Herein, we wish to report the synthesis of a novel mercury(II) coordination polymer based on a Schiff-base ligand, [Hg₂(µ-bpdh)(µ-SCN)₄]_n[Hg₂(µ-bpdh)(µ-SCN)₄]_2n (1); bpdh = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene. Some 2D–1D catenates have been reported.¹²

Recently, zur Loye and co-workers reported two ditopic Schiff-base ligands containing terminal 3-pyridyl groups, 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene (bpdh).¹³ The specific geometric features of these ligands, mainly the off-axis coordination orientation of the 3-pyridyl rings and the flexibility of the zigzag –(R)C=N–N=C(R)– central moiety, have allowed the authors to obtain new Co(II) and Cd(II) coordination polymers with interesting structures difficult to achieve by other bis(pyridyl) derivatives:¹³bpdb forms 1D double-bridged [M(NO₃)₂(bpdb)₂]_n, bpdb = 1,4-bis(3-pyridyl)-2,3-diaza-1,3-butadiene, coordination chains are interlinked by C–H⋯O hydrogen bonds into 2D layers. The rotations about the central N–N bond and about the two py–C bonds afford the bpdb ligand some flexibility, thus making the ligand adaptable to different structures. In the present Hg(II) complex, the bridging ligand adopts a trans-oid conformation.

The [Hg₂(µ-bpdh)(µ-SCN)₄]_n[Hg₂(µ-bpdh)(µ-SCN)₄]_2n (1) was prepared by a branched tube method.‡ With the exception of the ligand bpdh, which was prepared according to the literature procedure,¹⁴ all reagents and solvents for the synthesis and analysis were commercially available and used as received. Determination of the structure of the compound 1 by X-ray crystallography§ showed the complex crystallizes in the monoclinic space groupP2₁/c and in the solid state to be a polymeric species and contain two parts, 2D (part A) and 1D (part B) coordination polymers (see Fig. S1–S5 in ESI†).

Part A from compound 1 is a two-dimensional neutral metallopolymer as illustrated in Fig. S2† and consists of the mercury(II) ions bridged by both bpdh and thiocyanate ligands, thus forming a two-dimensional, infinite framework. On the other hand, the structure of part A may be considered a coordination polymer of mercury(II) consisting of one-dimensional linear chains, running parallel to the c axis, with a building block of [Hg(SCN)₂]. Two SCN⁻ anions doubly bridge two mercury(II) ions via the N and S atoms with bond distances of Hg(3)–N(9) = 2.665(7) Å, Hg(3)–N(10) = 2.603(7) Å, Hg(3)–S(5) = 2.4378(19) Å and Hg(3)–S(6) = 2.421(2) Å.

The intrachain Hg⋯Hg distances within the [Hg(SCN)₂]_n chains are 5.854 Å. The individual polymeric chains are almost parallel to each other and further bridged by bidentate bridging bpdh ligands with bond distances of Hg(3)–N11 = 2.336(6) Å, resulting in a two-dimensional framework as shown in Fig. S2.† The intra–chain Hg⋯Hg distance by the bpdh ligands is 14.785 Å. Each Hg(II) atom has a coordination number of five and the environment is a distorted square pyramid with a N₃S₂ coordination sphere; the apical positions are occupied by the nitrogen atoms of the bpdh ligand; the equatorial positions are taken by two sulfur and two nitrogen atoms of the thiocyanate anions. In part B the structure may also be considered to be a coordination polymer of mercury(II) consisting of linear double chains formed by bridging bpdh ligands, thus building a one-dimensional array of Hg(II) and bpdh in a 2 : 1 stoichiometry, resulting from a connection of the thiocyanate-bridged {Hg₂(SCN)₄} nodes with the bpdh rods.

Single-crystal X-ray diffraction analysis of compound 1 reveals that there are two different mercury(II) ions in part B, Hg1 and Hg2, are coordinated by four thiocyanate anions and one neutral bpdh molecule, resulting in a five-coordinate complex with a HgN₃S₂chromophore.

Although the coordination geometry around the mercury(II) ion is irregular, presumably associated with the steric constraints arising from the shape of the present ligands, it is best described as a distorted square pyramid. The S–Hg–S and N– Hg –N bonds with SCN⁻ form the basal plane while the N atom occupies the apical position. The two individual adjacent 1D (part B) and 2D (part A) in compound 1 are almost parallel to each other and further linked by weak Hg⋯S (Fig. 1) with bond distances of 3.371 and 3.665 Å. The weak Hg⋯S_thiocyanate interactions in compound 1 extend the structure into three-dimensional coordination polymer (Fig. 1). Interestingly the 2D net (part A) is polycatenated by the 1D double chain (part B) (Fig. 2) which is a very rare case of 2D + 1D = 2D polycatenation.¹⁵ If the weak S⋯Hg interaction is taken into account the “ladders” are connecting every other two layers with the result of 2-fold interpenetrated nets with a complex topology (Fig. 1).

Fig. 1 Representation of a 2-fold interpenetrated nets generated by weak S=Hg interaction in compound 1.

Fig. 2 Representation of 2D + 1D = 2D polycatenation in compound 1.

It contrasts to the twisted conformation found in the Co(II) and Cd(II) complexes and the Mn(II) complex, the bridging ligand adopts a trans-oid conformation reported previously,¹⁶ in which the central portion of bpdh is nonplanar, and the two pyridylimine [PyC(Me)=N] moieties are rotated about the central N–N bond, leading to dihedral angles of ca. 153.31, 180 and 180° between the two pyridyl rings in the compounds 1 and [Hg(bpdb)(SCN)₂]_n and [Hg(bpdb)Br₂] (see ref. 17) respectively.

In summary, the important point about the interpenetration is the dimensionality of the individual nets and overall dimension¹² as well as interpenetration of nets of different dimensions is rare. For interpenetration between 1D nets and 2D nets, the nets can be either parallel or inclined, and can give an overall 2D or 3D entanglement. A example of parallel 1D + 2D = 2D interpenetration has been observed in a supramolecular case¹⁸ but there have been no examples to date of covalent 1D + 2D = 2D parallel interpenetration.¹² However, the interpenetration which exists in compound 1 is the first one of 1D + 2D = 2D parallel interpenetration.

Acknowledgements

This work was supported by the Tarbiat Modares University.

References

1. E. Coronado, J. R. Galán-Mascarós, C. J. Gómez-Garca and V. Laukhin, Nature, 2000, 408, 447.
2. E. Corey, J. Chem. Soc. Rev., 1988, 17, 111.
3. B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629 , and references therein.
4. G. R. Desiraju, Angew. Chem., 1995, 21, 2541 ( Angew. Chem., Int. Ed. Engl. , 1995 , 34 , 2311 ).
5. V. A. Russell, C. C. Evans, W. J. Li and M. D. Ward, Science, 1997, 276, 575.
6. D. Venkataraman, S. Lee, J. Zhang and J. S. Moore, Nature, 1994, 371, 591.
7. V. A. Russell, M. C. Etter and M. D. Ward, J. Am. Chem. Soc., 1994, 116, 1941.
8. T. Kuroda-Sowa, T. Horrino, M. Yamamoto, Y. Ohno, M. Maekawa and M. Munakata, Inorg. Chem., 1997, 36, 6382.
9. (a) D. R. Turner and S. R. Batten, CrystEngComm, 2008, 10, 170; (b) Q.-G. Zhai, C.-Z. Lu, X.-Y. Wu and S. R. Batten, Cryst. Growth Des., 2007, 7, 2332.
10. A. S. R. Chesman, D. R. Turner, D. J. Price, B. Moubaraki, K. S. Murray, G. B. Deacon and S. R. Batten, Chem. Commun., 2007, 3541.
11. A. J. Blake, N. R. Champness, P. Hubberstey, W.-S. Li, M. A. Withersby and M. Schröder, Coord. Chem. Rev., 1999, 183, 117.
12. L. Carlucci, G. Ciani and D. M. Proserpio, CrystEngComm, 2003, 5, 269.
13. Y.-B. Dong, M. D. Smith, R. C. Layland and H.-C. zur Loye, Chem. Mater., 2000, 12, 1156; Y.-B. Dong, M. D. Smith and H.-C. zur Loye, Inorg. Chem., 2000, 39, 4927; E.-Q. Gao, A.-L. Cheng, Y.-X. Xu, C.-H. Yan and M.-Y. He, Cryst. Growth Des., 2005, 5, 1005.
14. X.-M. Ouyang, B.-L. Fei, T.-a. Okamur, H.-W. Bu, W.-Y. Sun, W.-X. Tang and N. Ueyama, Eur. J. Inorg. Chem., 2003, 618.
15. L. Carlucci, G. Ciani and D. M. Proserpio, Coord. Chem. Rev., 2003, 246, 247.
16. M. Cattaneo, F. Fagalde and E. N. Katz, Inorg. Chem., 2006, 45, 127.
17. G. Mahmoudi, A. Morsali, A. D. Hunter and M. Zeller, CrystEngComm, 2007, 9, 704.
18. K. V. Domasevitch, G. D. Enright, B. Moulton and M. J. Zaworotko, J. Solid State Chem., 2000, 152, 280.

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Footnotes

† Electronic supplementary information (ESI) available: Molecular views. CCDC reference number 661152. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b813701n

‡ 1,4-Bis(3-pyridyl)-2,3-diaza-1,3-hexadiene (bpdh) (0.5 mmol, 0.119 g) and mercury(II) thiocyanate (0.159 g, 0.5 mmol) were placed in the main arm of the branched tube. Methanol was carefully added to fill the arms, the tube was sealed and the ligand- and mercury-salt-containing arm immersed in an oil bath at 60 °C while the other arm was kept at ambient temperature. After 4–5 days, yellow crystals, m.p. 160.5 °C. Yield: 0.980 g, 75%. (Found: C, 24.50; H, 1.55; N, 12.80%. Calcd for C₂₇H₂₁Hg₃N₁₂S₆: C, 24.77; H, 1.60; N, 12.85%). IR (cm⁻¹) selected bands: 532(m), 799(m), 1035(m), 1189(m), 1411(m), 1600(m), 2075(vs), 2900(m), 3030(m).

§ Crystallographic measurements were made at 100(2) K for compound 1 using a Bruker AXS SMART APEX CCD diffractometer. The intensity data were collected using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). Crystal data for 1: monoclinic space groupP2₁/c, a = 11.6950(7) Å, b = 14.2219(8) Å, c = 22.1864 (14) Å, β = 102.4420(10), V = 3603.5(4) ų, Z = 2, T = 100(2) K, R(int) = 0.0485. The refinement of 436 parameters on the basis of 7050 independent reflections (of a total of 8835) converged at R1 = 0.0430, wR2 = 0.0977.†

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