2D Hydrogen-bonded polymer assembled by zinc(II) tetraaza macrocyclic complex and 1,2-cyclopentanedicarboxylic acid

Introduction

The self-assembly of the coordination polymers with specific network topologies has received great attention due to its potential usage in many areas of science and technology (Tecila et al., 1990; Tsukube et al., 1991; Lehn et al., 1992; Lawrence et al., 1995; Choi et al., 1999a,b; Cotton et al., 2001; Lu et al., 2001; Park et al., 2007a,b; Kim et al., 2008; Kwag et al. 2010a,b; Sen et al., 2013). Especially, hydrogen bonding is one of the key interactions for the process of molecular aggregation and recognition in nature, which creates novel structures of molecular assembly (Lehn et al., 1992). For example, the compound [Ni(L)(H₂O)₂]_0.5[Ni(L)(H₂O)₂]_0.5 (L=3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,0^1.18,0^7.12]docosane) assembles in the solid state to form a 1D chain linked by an intermolecular hydrogen bond (Choi et al., 1999b). In addition, a 3D supramolecular network [Ni(cyclam)(H₂O)₂]₃[(btc)₃]₂⋅24H₂O (btc=1,3,5-benzenetricarboxylate) was assembled by [Ni(cyclam)(H₂O)₂]²⁺ and btc^3- ligand in water via hydrogen bonds, which is regarded as a molecular floral lace (Choi et al., 1999a). The hydrogen-bonding interactions, therefore, play a significant role in aligning the molecules and the polymer strands in the crystalline solids.

In the present paper, we report the synthesis, crystal structure, and chemical properties of the 2D hydrogen bonded polymer [Zn(L)(Hcpdc)₂] (1), which was assembled by [Zn(L)(H₂O)₂]·Cl₂ (L=3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,0^1.18,0^7.12]docosane) and 1,2-cyclopentanedicarboxylic acid (H₂ cpdc). The structure of L ligand is shown in Scheme 1.

Scheme 1

3,14-dimethyl-2,6,13,17-tetraazatricyclo[14,4,0^1.180^7.12]-docosane (L).

Results and discussion

An Oak Ridge Thermal Ellipsoid Plot (ORTEP) (Farrugia, 1997) of [Zn(L)(Hcpdc)₂] (1) with the atomic numbering scheme is shown in Figure 1. selected bond lengths and angles are listed in Table 1. The macrocyclic ligand skeleton of the present compound takes the trans-III(R,R,S,S) conformation with two chair six-membered and two gauche five-membered chelate rings. The zinc atom lies on a center of inversion. The coordination environment around the central zinc(II) ion is described as a distorted octahedron with four Zn-N bonds from the macrocycle and two Zn-O bonds from Hcpdc^- ligand. The zinc atom and four N atoms of the macrocycle are exactly in a plane. The Zn-N (secondary amine) distance of 2.077(2) and 2.118(2) Å is similar to that observed for octahedral zinc(II) complexes with 14-membered tetraaza macrocycle ligands (Choi, 1998a; Choi et al., 1997, 1998b). The axial Zn-O(1) distance of 2.290(1) Å is slightly longer than the equatorial Zn-N bond distances, giving an axially elongated octahedral geometry. The N-Zn-N angles of the six-membered chelate ring are larger than those of the five-membered chelate ring. Furthermore, the axial Zn-O(1) linkage is not perpendicular to the ZnN₄ plane with N(1)-Zn-O(1) and N(2)-Zn-O(1) angles of 97.58(6) and 86.88(6)°, respectively. The Zn-O(1)-C(11) angle and C(11)-O(1) distance relative to the Hcpdc^- ligand are 126.1(1)° and 1.260(2) Å, respectively. The deprotonated one among the two Hcpdc^- carboxylic groups is bonded to the zinc atom. Interestingly, the two secondary amines N(1) and N(2) form intramolecular hydrogen bonds with the carboxylate oxygen atoms of Hcpdc^- ligand [N(1)-H(1)…O(4)^iv 3.022(2) Å, 157.5°; N(2)-H(2)…O(2)^iv 2.799(2) Å, 153.0°; symmetry code (iv): -x+1, -y, -z+2]. Furthermore, the protonated oxygen atom O(3) in the Hcpdc^- ligand forms the intermolecular hydrogen bond to an adjacent deprotonated oxygen atom O(2) of the Hcpdc^- [O(3)-H(3)…O(2)^v; 2.515(2) Å, 159(3)°; symmetry code (v): -x+1, y-1/2, -z+3/2-x+3/2, y+1/2, -z+3/2]. This interaction gives rise to a 2D hydrogen-bonded polymer (Figure 2 and Table 2).

Figure 1

An ORTEP drawing (30% probability ellipsoids) of [Zn(L)(Hcpdc)₂] (1) with the atomic numbering scheme.

The hydrogen bonds are shown as dashed lines.

Figure 2

A packing diagram of [Zn(L)(Hcpdc)₂] (1).

The hydrogen bonds are shown as dashed lines.

The IR spectrum of 1 shows a band at 3126 cm^-1 corresponding to the ν(NH) of the coordinated secondary amines of the macrocycle. Two strong bands exhibit ν_as(COO) stretching frequency at 1617 cm^-1 and ν_sym(COO) at 1372 cm^-1, respectively. The value of Δν (245 cm^-1) indicates that the carboxylate groups coordinated to the zinc(II) ion only as a monodentate ligand (Bakalbassis et al., 1991; Cao et al., 2002). The TGA diagram of 1 further supports the structure determined by X-ray diffraction method (Figure 3). The compound was heated in the temperature range 25–900°C in nitrogen gas. The first weight loss is observed from 266 to 342°C, which is due to the loss of two Hcpdc^- ligands (observed, 40.3%; calculated, 41.6%). A second weight loss corresponding to the macrocycle (observed, 46.1%; calculated, 47.0%) is found in the temperature range 342–510°C. Further weight loss is observed in the temperature range 510–900°C corresponding to the ZnO residue (observed, 11.1%; calculated, 11.4%). The formation of ZnO accompanies the decomposition of the macrocycle ligand in the zinc(II) complex (Dong et al., 1999).

Figure 3

Thermogravimetric curve of [Zn(L)(Hcpdc)₂] (1).

Conclusions

The crystal structure of complex 1 shows a distorted octahedral coordination geometry around the zinc (II) ion, with the four secondary amines of the macrocycle and two carboxylate oxygen atoms of the Hcpdc^- ligand in the trans position. Interestingly, the intramolecular and intermolecular hydrogen bonds in the complex generate a 2D hydrogen-bonded polymer. The IR spectrum and TGA behavior of the compound are significantly affected by the Hcpdc^- ligand.

Experimental section