Crystal structure of di-μ-hydroxido-bis{aqua[ethyl (1,10-phenanthrolin-3-yl)phosphonato-κ2 N,N′]copper(II)} heptahydrate

In the binuclear complex, [Cu2(OH)2(C12H7N2(PO3C2H5))2(H2O)2]·7H2O, the two Cu2+ cations each have a square-pyramidal geometry and are bridged by two hydroxide groups. The phenanthroline ligand in this complex acts as a counter-ion due to a negatively charged monoethylphosphoryl group. In the crystal, O—H⋯O hydrogen bonds link the cations, P(O)(O−)(OEt) group and water molecules of crystallization into a three-dimensional supramolecular architecture.


Chemical context
Although there are only a few examples reporting the synthesis of three-dimensional coordination polymers from monoalkylphosphonates in the literature, the known examples have interesting properties including enhanced water stability (Taylor et al., 2012) and oxygen absorption (Iremonger et al., 2011). Recently, we have synthesized a new class of phenanthroline ligands bearing diethoxyphosphoryl groups (Mitrofanov et al., 2012) and found that they form different supramolecular architectures, such as dimers and one-dimensional polymers with copper(II) cations, in which the metal can coordinate to both the nitrogen atoms of the phenanthroline core and the oxygen atoms of the diethoxyphosphoryl group (Mitrofanov et al., 2016). As part of a systematic study to generate stable supramolecular architectures based on copper(II) cations and phosphoryl-1,10-phenanthrolines, we decided to investigate the use of monoesters of phosphoryl-1,10-phenanthrolines as ligands. During these studies, the title compound, which contains centrosymmetric copper(II)-based dimers and uncoordinated water molecules was obtained unexpectedly.

Structural commentary
The title complex crystallizes in the monoclinic crystal system in space group C2/c. The asymmetric unit of the compound (Fig. 1) contains one copper(II) cation, one coordinated water molecule, one hydroxyl bridging group, one phenanthroline molecule and 3.5 water molecules. The copper(II) cation has a square-pyramidal geometry with pseudo-C 4v symmetry (Fig. 2). The spherical square-pyramidal geometry was confirmed by shape analysis using SHAPE software (Llunell et al., 2013). The basal plane of the square-based pyramid is formed by coordination of the Cu 2+ ion to two nitrogen atoms of the phenanthroline ligand (N1, N2) and to the oxygen atoms of two symmetry-related hydroxyl groups (O2). The coordination of the copper atom is completed by the oxygen atom from a water molecule at the apex of the square pyramid (O1). The axial Cu1-O1 distance [2.198 (2) Å ] is rather longer than the equatorial Cu1-O2 bond lengths [1.948 (2) and 1.945 (2) Å ], as expected from the Jahn-Teller theorem. Two of the copper centres are connected through the two bridging hydroxyl groups to form the centrosymmetric complex (Fig. 3). The pair of copper centres forms a fourcornered, planar Cu 2 O 2 core. The two 1,10-phenanthroline molecules are trans oriented with respect to the Cu 2 O 2 core, forming five-membered chelate rings with the Cu atoms.
An interesting feature of the title complex is the short intermetallic distance between the copper atoms in the dimer [2.8915 (9) Å ]. This value is amongst the shortest Cu II Á Á ÁCu II distances reported in the CSD ((version 5.39, updatel May 2018;Groom et al., 2016) for complexes of this type [mean value of 2.904 (13) Å for the structures reported by Zhang et al. (2005); Li et al. (2008); Lu et al. (2003Lu et al. ( , 2004; Arias-Zá rate et al. The elongation of the apical bond length in these complexes is of comparable magnitude to that observed in the previously reported complexes. The N1-Cu-N2 angle, corresponding to the angle formed by the copper ion and the two N atoms of the 1,10-phenanthroline unit, is 82.06 (10) for the title complex and is similar to the value for complexes of copper(II) with ligands having N and O donor atoms [mean value of 82.1 (5) for the above-mentioned structures in the CSD].   ORTEP view of the hydrogen-bonding interactions (dashed lines; see Table 1) in the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 1
ORTEP view of the asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Supramolecular features
The crystal structure features a three-dimensional network of hydrogen bonds (Table 1) involving the complex molecules and uncoordinated water molecules (Figs. 3 and 4). Atom O1 of the coordinating water molecule acts as a hydrogen-bond donor to O7 of a water molecule and O3 of the phosphonate group. The bridging hydroxide group (O2) acts as a hydrogenbond donor to atom O9 of an uncoordinated water molecule and a hydrogen-bond acceptor with water oxygen atom O6. The phosphonate atoms O3 and O4 both form hydrogen bonds with two water molecules, namely O1 and O9, and O7 and O8, respectively. The uncoordinated water molecules also form hydrogen bonds with each other: oxygen atoms O6 with O7, and O9 with O8.

Synthesis and crystallization
The lithium salt of monoethyl 1,10-phenanthrolin-3-ylphosphonate was obtained from diethyl 1,10-phenanthrolin-3ylphosphonate by monodealkylation with lithium bromide in 2-hexanone at 353 K according to a literature procedure (Krawczyk, 1997). The lithium salt (29.4 mg, 0.1 mmol) was stirred with copper(I) iodide (19.1 mg, 0.1 mmol) in 1 ml of distilled water in air at room temperature. The resulting mixture was left overnight without stirring after which time, clear blue prismatic crystals were formed. The yield could not be determined because of the poor stability of the crystals out of solution.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. The ethyl group linked to O5 exhibits disorder and was modelled over three sites with occupancies of 0.455, 0.384 and 0.161 for C13/C14, C13A/ C14A and C13B/C14B, respectively. The geometric parameters of the disordered components in each group were restrained by using SADI (Sheldrick, 2015) restraints. Similar U eq constraints were applied within the disordered parts to maintain a reasonable model with two free variable (see res file included in the CIF). Anisotropic thermal parameters were used for non-hydrogen atoms, except for the disordered ethyl group.