Bis[μ-2-(2-naphthoxy)acetato]bis{aqua[2-(2-naphthoxy)acetato]zinc(II)}

The title binuclear ZnII compound, [Zn2(C12H9O3)4(H2O)2], is centrosymmetric. Each Zn atom is coordinated by two bridging 2-naphthoxyacetate anions, one terminal 2-naphthoxyacetate anion and one water molecule in a distorted ZnO4 tetrahedral geometry. The naphthalene system of the bridging ligand is nearly perpendicular to the naphthalene of the terminal ligand, with a dihedral angle of 78.26 (6)°. Within the binuclear molecule the Zn⋯Zn separation is 3.815 (5) Å. In the crystal structure, intermolecular O—H⋯O hydrogen bonding between the water molecule and carboxylate groups helps to stabilize the crystal structure.

The title binuclear Zn II compound, [Zn 2 (C 12 H 9 O 3 ) 4 (H 2 O) 2 ], is centrosymmetric. Each Zn atom is coordinated by two bridging 2-naphthoxyacetate anions, one terminal 2-naphthoxyacetate anion and one water molecule in a distorted ZnO 4 tetrahedral geometry. The naphthalene system of the bridging ligand is nearly perpendicular to the naphthalene of the terminal ligand, with a dihedral angle of 78.26 (6) . Within the binuclear molecule the ZnÁ Á ÁZn separation is 3.815 (5) Å . In the crystal structure, intermolecular O-HÁ Á ÁO hydrogen bonding between the water molecule and carboxylate groups helps to stabilize the crystal structure.

S1. Comment
The synthesis of metal-organic hybrid materials has been deeply researched as their interesting structural diversity and potential functions (Harrison et al., 2002). In particular, carboxylate ligands, especially aromatic carboxylate ligands, have been shown to be good building blocks in the synthesis of metal-organic materials with desired topologies, owing to their rich coordination modes. The coordination chemistry of flexible aromatic carboxylic acids such as 2naphthoxyacetic acid (Ma et al., 2004) has captured the attention of chemist for many years. Herein we report the crystal structure of the title compound incorporating 2-naphthoxyacetate ligands.
The binuclear molecule of the title complex is centrosymmetric. The coordination environment of the Zn atom displays a distorted ZnO 4 tetrahedron (Fig. 1). The molecule contains two Zn atoms connected by two bridge 2-naphthoxyacetate anions and two terminal 2-naphthoxyacetate anions and two coordinate water molecules. Within the binuclear molecule the dihedral angle between bridge naphthalene ring systems is 1.77 (3)°, and that between terminal naphthalene systems is 2.59 (2)°. The C1-containg naphthalene is nearly perpendicular to the C13-containig naphthalene with a dihedral angle of 78.26 (6)°. The bond angles at the Zn center range from 95.10 (5)° to 138.94 (5)°. The coordinate bond distances (Table 1) range from 1.949 (3) to 2.014 (4) Å, which are comparable to those found in a Zn II complex (Li et al., 2008).
In the crystal structure there are intermolecular O-H···O hydrogen bonds between water O and 2-naphthoxyacetate O atoms (Table 2), which helps to stabilize the crystal structure.

S2. Experimental
A mixture of Zn(CH 3 COO) 2 .2H 2 O (0.2195 g, 1 mmol), NaOH (0.021 g, 0.5 mmol), 2-naphthoxyacetic acid (0.202 g, 1 mmol), 2,2′-bipyridine (0.078 g, 0.5 mmol) was dissolved in 17 ml of 15:2 water/ethanol. The solution was placed in a 25 ml Teflon-lined stainless steel bomb. The bomb was heated to 433 K for 3 d. Then it was cooled to room temperature over 3 d. The colorless crystals of the title compound suitable for X-ray diffraction structure analysis were isolated from the solution.

S3. Refinement
The carbon-bound H atoms were positioned geometrically and included in the refinement using a riding model with C-H = 0.93 Å for aromatic and C-H = 0.97 Å for the others, and U iso (H) = 1.2U eq (C). The water H atoms were located from a different map and their positions were refined with restraints of O-H = 0.80 (2) Å and H···H = 1.30 (2) Å, their displacement parameters were set to 1.5U eq (O).  The molecular structure of the title compound, showing 30% probability displacement ellipsoids [symmetry code: (A) -x + 2, -y + 1, -z + 1]. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.37 e Å −3 Δρ min = −0.24 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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.