Tetraaquabis[(1-carboxylatomethyl-1,3-benzimidazol-3-ium-3-yl)acetato-κO]palladium(II) dihydrate

In the title compound, [Pd(C11H9N2O4)2(H2O)4]·2H2O, the palladium(II) cation lies on an inversion centre and is hexacoordinated by two carboxylate O atoms from two (1-carboxylatomethyl-1,3-benzimidazol-3-ium-3-yl)acetate ligands and four water molecules, with a slightly distorted octahedral geometry. O—H⋯O hydrogen bonds link the molecules together.


Comment
In recent years, carboxylates have been widely used as polydentate ligands, which can coordinate to transition or rare earth ions yielding complexes with interesting properties that are useful in materials science (Church & Halvorson, 1959;Chung et al., 1971) and in biological systems (Okabe & Oya, 2000;Serre et al., 2005;Pocker & Fong, 1980;Scapin et al., 1997).
For example, Kim et al. (2001) focused on the syntheses of transition metal complexes containing benzenecarboxylate and rigid aromatic pyridine ligands in order to study their electronic conductivity and magnetic properties. The importance of transition metal dicarboxylate complexes motivated us to pursue synthetic strategies for these compounds, using sodium 1-carboxymethyl-1,3-benzimidazol-3-ium-3-acetate as a polydentate ligand. Here we report the synthesis and X-ray crystal structure analysis of the title compound.
The molecular structure of the title compound is shown in Fig. 1. The palladium(II) cation lies on an inversion center and is hexacoordinated by two carboxylate oxygen atoms from two 1-carboxymethyl-1,3-benzimidazol-3-ium-3-acetato ligands and four water molecules, with a slightly distorted octahedral geometry. The Pd-O bond distances are in the range 2.2608 (19)-2.276 (2) Å. The packing involves hydrogen bonds, shown in Table 1 and Figure 2.

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 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.