Novel tetracationic template and application in construction metal thiocyanate polymers

2.1 Materials and methods

tepb⁴⁺ = 1, 2, 4, 5-tetra (N-pyridiniummethyl) bezene tetrabromide (tepb·4Br) was prepared from refluxing reaction of 1, 2, 4, 5-tetra(bromomethyl)benzene with excessive pyridinium in toluene under 110°C, similar to a literature approach [19]. Other chemicals and solvents were obtained from commercial sources and used as received without further purification. The IR spectrum was recorded on a Shimadzu IR435 spectrometer as KBr disk (0–400cm^-1). Elemental analysis for C, H, and N was performed on a Perkin–Elmer 240 elemental analyzer. A model NETZSCHTG209 thermal analyzer was used to record simultaneous TG curves in the flowing air atmosphere of 20 mL min^-1 at a heating rate of 5°C min^-1 in the temperature range from room temperature to 800°C using aluminum crucibles.

2.2 Preparation of {(tepb)[Ag₄Br₈]}(1)

A solution of tepb·4Br (0.1 mmol) in DMF was added to a stirring solution of AgSCN (0.1 mmol) dissolved in 10 mL DMF in the presence of excess KSCN (0.4 mmol). The resulting mixture was stirred for 5 min and filtered. Then the solution was slowly evaporated in a vial at room temperature. The light brown crystals of {(tepb)[Ag₄Br₈]} suitable for X-ray analysis were obtained after two weeks in about 30% yield. The product was not soluble in common solvents. IR(KBr): 3118.62(m), 3066.76(m), 3046.51(s), 2956.77(m), 2077.64(w), 1630.66(s), 1576.98(w), 1528.24(w), 1483.36(vs), 1438.34(w), 1303.07(w), 1140.98(vs), 953.62(w), 886.06(w), 865.55(w), 823.29(w), 807.21(w), 765.66(w), 680.31(m), 618.30(m), 470.10(w), 438.92(w) cm^-1.Anal calcd for C₃₀H₃₀N₄Ag₄Br₈: H,1.99; C,23.75; N,3.69. Found: H, 1.94; C, 23.83; N, 3.62.

2.3 Preparation of {(tepb)[Cu₂(SCN)₆]}(2)

The procedure was similar to the synthesis of 1, except that AgSCN was replaced by CuSCN (0.1 mmol) and tepb·4BF₄ was used instead of tepb·4Br. Red crystals of {(tepb)[Cu₂(SCN)₆]} suitable for X-ray analysis were obtained about 83% yield. IR(KBr): 3115.67(m), 3055.27(s), 2967.97(m), 2079.25(vs), 2065.04(vs), 1625.90(s), 1481.12(s), 1144.44(m), 757.83(s), 674.38(s),605.61(w), 461.47(w) cm^-1.Anal calcd for C₃₆H₃₀Cu₂N₁₀S₆: H, 3.28; C, 46.89; N, 15.19. Found: H, 3.24; C, 46.82; N, 15.25.

2.4 X-ray structure data collections

Crystallographic data for the title three compounds were collected at 100(2) K on a Bruker APEX-II area-detector diffractometer equipped with graphite- monochromatized Mo-Kα radiation (λ= 0.71073Å). Their structures were solved by direct methods and expanded using Fourier techniques. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical constraints. The structures were refined with full-matrix least-squares techniques on F² using the SHELXTL-97 program package [20, 21]. Crystal data are summarized in detail in Table 1.

3.1 Structure description of {(tepb)[Ag₄Br₈]}(1)

Bromine atoms preferentially coordinate the silver atoms, so SCN^- groups do not participate in the coordination in 1. Compound 1 crystallizes in the monoclinic space group P2₁/n with its asymmetric unit including one tetrameric inorganic anion [Ag₄(μ₂-Br)₆(t-Br)₂]^4- and tetracation tebp⁴⁺. In the structure each Ag(1) coordinates to three μ ₂ -Br and one t-Br (Fig. 1). In contrast, Ag(2) is coordinated by three μ₂-Br. All the Ag-μ ₂ -Br distances vary from 2.634 Å to 2.937 Å and the bond length of Ag(1)-t-Br is 2.590 Å. The distance of Ag(1)···Ag(2) (3.295 Å), Ag(1)···Ag(2)^# (3.213 Å), or Ag(2)···Ag(2)^# (2.852 Å), has been proved shorter than double Van der Waals radius of Ag atom (3.44 Å), indicating the existence of metal-metal interactions. The tetracations take trans-configuration and it is the C-H···Br hydrogen bonds and electrostatic interaction that hold positive and negative ions together.

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Fig.1

The Ag-Br unit in compound 1.

Compound 2 features a centrosymmetric dimeric inorganic phase and disperses four-charged cations as illustrated in Fig. 2a. Each cuprous center adopts the different circumstance of 2N2S with Cu-N(1) = 1.957 Å, Cu-N(2) = 1.957 Å, Cu-S(2) = 2.505 Å and Cu-S(3) = 2.364 Å. The bond angles of Cu-N(1)-C(3)^# and Cu-N(2)-C(1) are 158.19° and 171.08°. The angles of Cu-S(2)-C(3) and Cu-S(3)-C(2) are 98.64° and 95.57°.

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Fig.2

The structural unit (a) and the monolayer (b) of compound 2.

The distance of two pairs of pyridinium rings in one polycation is 3.590 Å, meaning a weak π-π stacking caused by high polarized ambient. Delicate perception tells two kinds of similar but different orientations of the structural unit, of which the angle is approximately 74.13°. Each orientation extends into independent layers along ab-plane as shown in Fig. 2b. It is the alternative arrangement of the two orientated layers in space that piles the layers up into three-dimensional arrangement along c-axle.

3 crystallizes in orthorhombic space group Acam. It features tebp⁴⁺ threaded in the two dimensional inorganic network and water molecular distributed between layers. X-ray crystallographic analysis reveals only one independent Ag⁺ center in the inorganic moiety: Ag(1) is four-coordinated by three μ₂-Br and one end-on S-μ ₂ -SCN^-. The bond lengths of Ag-Br vary from 2.623 Å to 2.777 Å and Ag-S is as long as 2.592 Å. The distance of Ag(1)···Ag(1)^# is 3.373 Å, indicating a week metal-metal interaction. As shown in Fig. 3a, the SCN^- groups act as the connecter bridging two [Ag₂Br₂] by coordinating to silver ions. Due to the flexibility caused by Br(1), hexagonal rings compose each inorganic layer viewing along b-direction, as shown in Fig. 3b. From Fig. 3c we know that the tetracations(tebp⁴⁺) take a classical trans-configuration in 3. It is made up of infinite 2D inorganic networks with the corresponding tetracations trapped inside to form a penetrating model. Polyrotaxane architecture consisting of a 2D inorganic network perforated with organic molecules was few reported. From this point of view, 3 can be regarded as the rare representative example of 2D polypseudorotaxane polymers. The topological diagram is shown in Fig. 3d.

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Fig.3

The diagram of inorganic unit (a), hexagonal ring in each inorganic layer (b), transfixion model (c), topological diagram (d) of compound 3.

To estimate the stability of 1, 2 and 3, the TGA experiments were carried out up to 800° in flowing air atmosphere and the TG curves of three compounds are shown in Fig. 4. Results show the organic moieties were largely lost at around 190°C, which is approximately equal to that of honeycomb-like hybrids, meaning a similar protection imposed on the interior organic cations by the inorganic moieties. This is ascribable to the analogous threading and exposure of inner cations through the annulus of inorganic macrocycles [22].

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Fig.4

The TG curves of compounds 1-3.