Tetraaquabis(4,4′-bipyridine)zinc(II) bis(trans-4-hydroxycinnamate)

The title complex, [Zn(C10H8N2)2(H2O)4](C9H7O3)2, was obtained by the hydrothermal reaction of zinc sulfate with mixed 4-hydroxylcinnamic acid (H2 L) and 4,4′-bipyridine (4,4′-bipy) ligands. The complex consists of a centrosymmetric [Zn(4,4′-bipy)2(H2O)4]2+ cation with the metal centre in a distorted ZnN2O4 coordination, and of two HL − anions. Extensive O—H⋯O and O—H⋯N hydrogen-bonding interactions between the constituents lead to the formation of a three-dimensional network.


Related literature
The main strategy used in the design and synthesis of novel coordination architectures is the building-block approach, see: Han et al. (2005); Wen et al. (2005); Yaghi et al. (1998). For the isostructural nickel analog, see: Zhou et al. (2006).

Comment
The main strategy widely used in design and synthesis of novel coordination architectures is the building-block approach (Yaghi et al., 1998;Han et al., 2005;Wen et al., 2005). 4-Hydroxylcinnamic acid (H 2 L) is considered as suitable multidentate ligand is based on the following considerations: (a) It has multiple coordination sites, carboxylate group and phenolic hydroxyl group, that may generate structures of higher dimensions. (b) Hydroxyl group can also introduce hydrogen bond in the framework construction. Here, we combined H 2 L and auxiliary ligand 4,4-bipy as a mixed ligand system to react metal ions. A new Zn(II) complex, [Zn(4,4'-bipy) 2 (H 2 O) 4 ].2HL, (I), was obtained unexpected. In this complex, HL ligand is non-coordinated and acts as a dissociative anion.
The X-ray diffraction study shows that the asymmetric unit of (I) is composed of half a Zn atom, one 4,4'-bipy ligand, two coordinated water molecules and one HL ligand. As shown in Fig.1, the Zn II center is six-coordinated by four water molecules and two N atoms of 4,4'-bipy, and displays a slightly distorted [ZnO 4 N 2 ] octahedral coordination geometry. Four water molecules form a relatively normal equatorial plane of the octahedron, and the Zn1 atom is located in this plane, while two N atoms occupy the axial positions, with an N-Zn-N angle of 180 °. The bond lengths of Zn-O water are 2.0878 (10) and 2.0881 (10) Å, Zn-N is 2.1728 (12) Å, respectively.
There are extensive hydrogen-bonding interactions involving the HL oxygen atoms, coordinated water molecules and uncoordinated 4,4'-bipy N atoms. A three-dimensional network is formed by these hydrogen-bonding interactions, as shown in Fig. 2. Complex (I) is isostructural with its nickel analog (Zhou et al., 2006).

Experimental
A mixture of 4-hydroxylcinnamic acid (0.1642 g, 1 mmol), ZnSO 4 .7H 2 O (0.1438 g, 0.5 mmol), Na 2 CO 3 (0.053 g, 0.5 mmol) and H 2 O (15 mL) was sealed in a 25 ml stainless-steel reactor with a Telflon liner and was heated at 433 K for 3 d. On completion of the reaction, the reactor was cooled slowly to room temperature and the mixture was filtered, giving colourless single crystals suitable for X-ray analysis in yield 30% (based on Zn).

Refinement
The Carbon-bound H-atoms were positioned geometrically and included in the refinement using a riding model [C-H = 0.93 Å U iso (H) = 1.2U eq (C)]. The water and hydroxyl H atoms were located from different maps, and refined with O-H and H-H distances retrained to 0.85 (2) Å and 1.35 (2) Å, and U iso (H) values of 1.5U eq (O water, hydroxyl ).

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.