Bis(μ-2,4-dihydroxybenzoato-κ2 O:O′)bis[aqua(2,4-dihydroxybenzoato-κO)(1,10-phenanthroline-κ2 N,N′)cadmium(II)]

In the title centrosymmetric dimeric CdII complex, [Cd2(C7H5O4)4(C12H8N2)2(H2O)2], the CdII cation is coordinated by a bidentate phenanthroline (phen) ligand, three dihydroxybenzoate (dhba) anions and one water molecule in a distorted CdN2O4 octahedral geometry. Among the dhba anions, two anions bridge two CdII cations to form the dimeric complex with significant different Cd—O bond distances of 2.2215 (19) and 2.406 (2) Å. The centroid–centroid distance of 3.4615 (19) Å between two nearly parallel benzene rings of the dhba and phen ligands coordinating to the same CdII cation indicates the existence of intramolecular π–π stacking in the complex. Extensive O—H⋯O hydrogen bonding and intermolecular weak C—H⋯O hydrogen bonding help to stabilize the crystal structure. One hydroxy group of the monodentate dhba ligand is disordered over two sites with a site-occupancy ratio of 0.9:0.1.

In the title centrosymmetric dimeric Cd II complex, [Cd 2 (C 7 H 5 O 4 ) 4 (C 12 H 8 N 2 ) 2 (H 2 O) 2 ], the Cd II cation is coordinated by a bidentate phenanthroline (phen) ligand, three dihydroxybenzoate (dhba) anions and one water molecule in a distorted CdN 2 O 4 octahedral geometry. Among the dhba anions, two anions bridge two Cd II cations to form the dimeric complex with significant different Cd-O bond distances of 2.2215 (19) and 2.406 (2) Å . The centroid-centroid distance of 3.4615 (19) Å between two nearly parallel benzene rings of the dhba and phen ligands coordinating to the same Cd II cation indicates the existence of intramolecularstacking in the complex. Extensive O-HÁ Á ÁO hydrogen bonding and intermolecular weak C-HÁ Á ÁO hydrogen bonding help to stabilize the crystal structure. One hydroxy group of the monodentate dhba ligand is disordered over two sites with a site-occupancy ratio of 0.9:0.1.

Related literature
For the correlation betweenstacking and electrontransfer processes in some biological systems, see: Deisenhofer & Michel (1989). For general background tostacking, see: Li et al. (2005). Forstacking involving a dihydroxybenzoate ligand in an Ni complex, see: Yang et al. (2006). Intramolecularstacking was previously observed in a Sr complex with a hydroxybenzoate ligand, see: Su et al. (2005).

Comment
The π-π stacking between aromatic rings is an important non-covalent interaction and correlated with the electron transfer process in some biological systems (Deisenhofer & Michel, 1989). As a continuous work of the series investigation on the nature of π-π stacking (Li et al., 2005), we prepared the title Cd II complex which contains a double-hydroxy substituted benzoate ligand. We present here its crystal structure to compare with those incorporating monohydroxybenzoate ligand and to show the effect of hydroxy substitution on the benzene ring to π-π stacking between aromatic rings.
The molecular structure of the title compound is shown in Fig. 1. The dimeric Cd II complex locates across on an inversion center. Each Cd II atom is coordinated by one phenanthroline (phen), one water molecule and three dihydroxybenzoate (dhba) dianions in a distorted octahedral geometry ( Table 1). The C1-containg dhba is monodentately coordinated to a Cd II atom and is well coplanar with the Cd atom [the maximum atomic deviation 0.071 (3) Å (Cd)] while the C8-conating dhba bridges two Cd atoms to form the dimeric complex.
It is notable that intra-molecular π-π stacking exists in the dimeric complex. Partially overlapped arrangement between nearly parallel C9-containg benzene ring and phen ring system [dihedral angle 6.56°] is observed in the molecular structure; the shorter centroids distance of 3.4615 (19) Å between C9-benzene and C19-benzene rings suggests the existence of intramolecualr π-π stacking. The perpendicular distance of centroid of C9-benzene ring on C19-benzene ring and perpendicular distance of centroid of C19-benzene ring on C9-benzene ring are 3.365 and 3.388 Å. Intra-molecular π-π stacking was previously observed in a Sr II complex with hydroxybenzoate ligand (Su et al., 2007). π-π stacking involving dhba ligand in a Ni II complex was reported by Yang et al. (2006).
Intermolecular π-π stacking is also present in the crystal structure; the centroid distance between C9-ring and C19 i -ring Extensive O-H···O hydrogen bonding and intermolecular weak C-H···O hydrogen bonding ( 2.5H 2 O (0.46 g, 2 mmol), Na 2 CO 3 (0.21 g, 2 mmol) and 2,4-dihydroxybenzoic acid (0.31 g, 2 mmol) was dissolved in a water/ethanol solution (20 ml, 1:3); then phenanthroline (0.36 g, 2 mmol) was added to above solution. The mixture was refluxed for 3 h, and then filtered after cooling to room temperature. Colorless single crystals of the title compound were obtained after one week.

Refinement
One hydroxy group of C1-containing dhba ligand is disordered, site occupancies for O3 and O3' atoms were initially refined and converged to 0.895 (3) and 0.105 (3), they were fixed as 0.9 and 0.1 respectively in the final refinement. Water H atoms were located in a difference Fourier map and refined with O-H distance restrains, U iso (H) = 1.2U eq (O). Other H atoms were placed in calculated positions with O-H = 0.82 and C-H = 0.93 Å, and refined in riding mode with U iso (H) = 1.2U eq (O,C). In the final refinement the C6-C7 distance was constrained to be 1.38±0.01 Å.