Crystal structure of poly[[hexaaquatris(μ-3,6-dioxocyclohexa-1,4-diene-1,4-diolato)dierbium(III)] octadecahydrate]

The title lanthanide complex is isostructural with its La, Gd, Yb and Lu analogues. The Er3+ ion, located on a threefold rotation axis, is nine-coordinated in a distorted tricapped trigonal–prismatic geometry, which is completed by six oxygen atoms from three dhbq2− ligands and three oxygen atoms from coordinated water molecules. Each dhbq2− ligand acts as a μ2-bis(bidentate) bridging mode to connect two Er3+ ions to form honeycomb (6,3) two-dimensional sheets extending in the ab plane.


Chemical context
Over the past few decades, lanthanide-based coordination polymers (LnCPs) have attracted significant attention because their high photoluminescence efficiency and long luminescence lifetime in lighting and full-colour displays (Parker, 2000;Bü nzli & Piguet, 2005;Cui et al., 2018). Besides transition metal ions, lanthanide ions feature high coordination numbers and flexible coordination geometries, which facilitate the formation of diverse extended structures. Since lanthanide(III) ions have a high affinity to hard donor atoms, ligands containing oxygen atoms such as carboxylic acids (Xu et al., 2016), phosphoric acids (Mao, 2007), calixarenes (Ovsyannikov et al., 2017) and -diketones (Vigato et al., 2009) have been used extensively in the synthesis of new types of LnCPs. On the basis of the above considerations, we selected 2,5dihydroxy-1,4-benzoquinone (H 2 dhbq) as the ligand to react with erbium(III) nitrate hexahydrate under solvothermal conditions to construct a new erbium(III)-based CP, [Er 2 (dhbq) 3 (H 2 O) 6 ]Á18H 2 O, (I), which is isotypic with its La, Gd, Yb and Lu analogues (Abrahams et al., 2002). Herein, we report its structure.

Structural commentary
The asymmetric unit of (I) contains one third of an Er 3+ ion, half of a dhbq 2À ligand, one coordinated water molecule and three water molecules of crystallization. The Er 3+ ion is located on a threefold rotation axis, whereas the complete dhbq 2À anion is generated by a crystallographic inversion center. As can be seen from Fig. 1, the Er 3+ ion is nine-coordinated by six oxygen atoms from three different dhbq 2À ligands and three other oxygen atoms from three coordinated water molecules. The coordination polyhedron of the central Er 3+ ion can best be described as having a distorted tricapped trigonal-prismatic geometry, as depicted in Fig. 2, in which the O-Er-O bond angles range from 65.01 (5) to 139.97 (7) . The Er-O bond lengths in the title complex lie between 2.3577 (15) and 2.4567 (15) Å , mean 2.393 Å . The whole dhbq 2À anion is nearly planar: the r.m.s. deviation from the mean plane through all of the non-H atoms is 0.021 Å , with a maximum displacement from this plane of 0.033 (2) Å for atom C2. As can be seen from Fig. 3, the dhbq 2À ligand acts in a 2 -bis-(bidentate) bridging mode, connecting two Er 3+ ions to form a honeycomb (6,3) sheet extending in the ab plane, having a ErÁ Á ÁEr separation of 8.7261 (2) Å .

Supramolecular features
In the crystal, extensive O-HÁ Á ÁO hydrogen-bonding interactions (Table 1) are observed between the oxygen atoms of the coordinated (O3) and lattice (O4 and O5) water molecules as well as between the water (O5 and O6) molecules of crystallization. Other O-HÁ Á ÁO hydrogen-bonding interactions involve O6 and the dhbq 2À oxygen atom, and this interaction further links neighbouring sheets into a threedimensional supramolecular structure (Fig. 4).  Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of the title complex, showing selected atom labels. Displacement ellipsoids are drawn at the 50% probability level. Symmetry codes: (i) 1 + y À x, 1 À x, z; (ii) 1 À y, x À y, z.

Figure 2
View of the distorted tricapped trigonal-prismatic geometry of the central Er III ion in the title complex. Symmetry codes: (i) 1 + y À x, 1 À x, z; (ii) 1 À y, x À y, z. dhbq 2À ligand gave 94 hits. They include the isotypic crystal structures (Abrahams et al., 2002) with La (MIZXAU), Gd (MIZXEY), Yb (MIZXIC) and Lu (MIZXOI). In most cases, the dhbq 2À ligand acts in a 2 -bis(bidentate) bridging mode to the central metal ions. Comparing the mean Ln-O bond length and the unit-cell volume for the title complex with the La, Gd, Yb and Lu analogues (Abrahams et al., 2002), the values decrease as the ionic radius of the Ln

Synthesis and crystallization
A mixture of Er(NO 3 ) 3 Á6H 2 O (46.2 mg, 0.1 mmol) and H 2 dhbq (14.2 mg, 0.1 mmol) in distilled H 2 O (4 ml) and DMF (1 ml) was placed in a 20 ml vial and stirred at room temperature for 10 min. The mixture was sealed tightly, placed in an oven and then heated to 358 K under autogenous pressure for 12 h. After the reactor was cooled to room temperature, block-shaped dark-red crystals were filtered off, washed with deionized H 2 O and dried in air at room temperature. Yield: 57% based on Er III source.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The carbon-bound H atoms were placed in geometrically calculated positions and refined as riding with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). The hydrogen atoms of the water molecules were located from difference-Fourier maps but were refined with distance restraints of O-H = 0.84 Å and U iso (H) = 1.5U eq (O).

Figure 3
View of the honeycomb (6,3) sheet extending normal to the c-axis direction.

Funding information
Funding for this research was provided by: Thailand Research Fund (grant No. RSA5780056). NP acknowledges the NSTDA STEM Workforce (SCA-CO-2560-3565-TH).

Poly[[hexaaquatris(µ-3,6-dioxocyclohexa-1,4-diene-1,4-diolato)dierbium(III)] octadecahydrate]
Crystal data  Extinction correction: SHELXL (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00020 (2) 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. 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 > 2sigma(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.