(μ-Methylenediphosphonato-κ4 O,O′:O′′,O′′′)bis[(ethylenediamine-κ2 N,N′)palladium(II)] tetrahydrate

The binuclear title molecule exhibits point group symmetry 2, with the PdII atom in a square-planar coordination environment defined by two N atoms from ethylenediamine and two O atoms from methylenediphosphonate ligands.

The title compound, [Pd 2 (C 2 H 8 N 2 ) 2 (CH 2 O 6 P 2 )]Á4H 2 O, comprises of a binuclear molecule (point group symmetry 2), with a twofold rotation axis running through the central C atom of the methylenediphosphonate (MDP) anion. The Pd II atom has a square-planar coordination environment defined by the N atoms of a bidentate ethylenediamine (en) ligand and two O atoms of the bridging MDP anion. In the crystal structure, metal complexes are arranged in layers parallel (001) and are sandwiched between layers containing disordered water molecules of crystallization. Extensive intralayer hydrogen bonds of the type N-HÁ Á ÁO in the metal complex layer and O-HÁ Á ÁO in the water layer, as well as O-HÁ Á ÁO hydrogen bonds between the two types of layers, lead to the formation a three-dimensional network structure. The two lattice water molecules are each equally disordered over two positions.

Chemical context
Platinum drugs are some of the most important and clinically applied anti-cancer agents. Diphosphonic acids are therapeutic agents for treating osteoporosis and metastatic bone diseases. Therefore new complexes designed with a combination of platinum group metals with diphosphonic acid (or derivatives thereof) as bone-targeting groups can improve the chemotherapeutic efficacy in the treatment of bone cancer and can reduce adverse effects. Methylenediphosphonic acid (medronic acid, MDP, H 4 L) is the smallest bisphosphonate, which accumulates on the sites of osteoid mineralization and can be used in combination with platinum metals to treat cancer and metastatic bone diseases. Platinum-bisphosphonate complexes, including bis{ethylenediamine)platinum(II)}medronate, as novel Pt-prodrugs in the local treatment of bone tumors have been reported (Wani et al., 2016;Iafisco et al., 2009;Palazzo et al., 2007;Iafisco & Margiotta, 2012;Margiotta et al., 2009). The preparation and structure determination of [Pt 2 (cis-diaminohexane) 2 (methylenediphosphonate)] was reported by Bau et al. (1988). ISSN 2056-9890 In this work, we synthesized a new platinum metal complex, viz. [Pd 2 (C 2 H 8 N 2 ) 2 (CH 2 O 6 P 2 )]Á4H 2 O or [Pd 2 (en) 2 (MDP)]Á-4H 2 O, and report here its molecular and crystal structures.

Structural commentary
The binuclear [Pd 2 (en) 2 (MDP)] complex molecule is uncharged and exhibits point group symmetry 2, with the twofold rotation axis passing through the central C atom of the MDP ligand (Fig. 1). The Pd II atom has a square-planar environment defined by two nitrogen atoms (N1 and N2) of the chelating en ligand and two oxygen atoms (O2 and O3) of the bis-bidentately coordinating MDP ligand that bridges two symmetry-related Pd II atoms into the binuclear complex. The deviation of the Pd1 site from the least-squares plane involving the ligand atoms N1, N2, O2 and O3 is 0.06 Å . The Pd-N and Pd-O bond lengths are in typical ranges whereby the Pd-N bonds are about 0.02 Å shorter than the Pd-O bonds ( Table 1) Table 1) and indicate a slight distortion from a square-planar coordination. In general, the structural features of the [Pd 2 (en) 2 (MDP)] complex are similar to those of the related complex [Pd 2 (en) 2 EDP] where EDP is 1-hydroxyethane 1,1-diphosphonate or etidronate (Kozachkova et al., 2018).

Figure 1
Molecular structure of the title complex and surrounding water molecules with displacement ellipsoids drawn at the 50% probability level.

Synthesis and crystallization
[Pd 2 (C 2 H 8 N 2 ) 2 (CH 2 O 6 P 2 )]Á4H 2 O was prepared as previously reported in the literature (Kozachkova et al., 2018). A solution of AgNO 3 (0.4 mmol, 0.0679 g) in water (3 ml) was added to a suspension of [Pd(en)Cl 2 ] (0.2 mmol, 0.0474 g) in water (6 ml) under constant stirring and heating at 333 K for 30 min. The resulting suspension was refrigerated to facilitate the precipitation of AgCl. The solid material was removed by suction filtration. MDP (0.1 mmol, 0.0176 g) was added to the filtrate, and the pH was adjusted to 6 by addition of KOH (0.1 mol l À1 ). The obtained slightly yellow solution was heated at 333 K for 20 min and then left to evaporate at room temperature. Yellow rectangular crystals of the title compound suitable for crystallographic studies were grown by slow evaporation of an aqueous solution at room temperature. Compound [Pd 2 (en) 2 (MDP)]Á4H 2 O: yield 86%. Analysis found: C, 11.5; H, 3.6; N, 10.9; P, 11.8; Pd, 40.8. Calculated for C 5 H 26 N 4 O 6 P 2 Pd 2 : C, 11.9; H, 3.9; N, 11.1; P, 12.3; Pd, 41.9%. The 31 P-{H} NMR spectrum of an aqueous solutions of the synthesized compound exhibited a singlet with P 35.0 ppm.

(µ-Methylenediphosphonato-κ 4 O,O′:O′′,O′′′)bis[(ethylenediamine-κ 2 N,N′)palladium(II)] tetrahydrate
Crystal data  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )