Poly[[hexaaquatris[μ2-2,5-dihydroxy-1,4-benzoquinonato(2−)]diholmium(III)] octadecahydrate]

In the polymeric title compound, {[Ho2(C6H2O4)3(H2O)6]·18H2O}n, the HoIII ion is nine-coordinated by six O atoms derived from three bidentate 2,5-dihydroxy-1,4-benzoquinonate (DHBQ2−) ligands and three O atoms from three water molecules. The HoIII ions are connected via three ligands, resulting in the formation of a two-dimensional honeycomb layer parallel to the ab plane. The layer is racemic in which Δ- and Λ-coordination geometries around HoIII ions are alternately arranged. The asymmetric unit comprises a third of a HoIII ion, located on a threefold axis, one-half of a DHBQ2− ion, located on a centre of inversion, one coordinated water molecule and three uncoordinated water molecules.

In the polymeric title compound, {[Ho 2 (C 6 H 2 O 4 ) 3 (H 2 O) 6 ]Á-18H 2 O} n , the Ho III ion is nine-coordinated by six O atoms derived from three bidentate 2,5-dihydroxy-1,4-benzoquinonate (DHBQ 2À ) ligands and three O atoms from three water molecules. The Ho III ions are connected via three ligands, resulting in the formation of a two-dimensional honeycomb layer parallel to the ab plane. The layer is racemic in which Áand Ã-coordination geometries around Ho III ions are alternately arranged. The asymmetric unit comprises a third of a Ho III ion, located on a threefold axis, one-half of a DHBQ 2À ion, located on a centre of inversion, one coordinated water molecule and three uncoordinated water molecules.
The asymmetric unit comprises a third of a Ho III ion, being located on a three-fold axis, one-half of a DHBQ 2ion, being disposed about a centre of inversion, one coordinated water molecule, and three zeolitic water molecules.
The C-O distances of 1.276 (4) Å (C1-O1) and 1.274 (3) Å (C2-O2) in this compound agree with those of the DHBQ 2ligands which were previously reported, e.g. 1.276 (6) Å (Weiss et al. 1986). In the coordination geometry, a Ho III ion is coordinated to six O atoms from three bidentate DHBQ 2ligands and three O atoms from three water molecules ( Fig.   1 and Table 1). The Ho III ion is connected via three ligands, which results in a two-dimensional honeycomb layer with a diameter of 16.6 Å. The layer has a racemic structure in which Δ-and Λ-coordination geometries around Ho III ions are alternately arranged (Fig. 2). The zeolitic water molecules occupy regions between the layers.
The product of the molar magnetic susceptibility (χ M ) and temperature (T), χ M T, values at room temperature was 13.6 cm 3 K mol -1 . This value nearly corresponds to the expected value of 13.9 cm 3 K mol -1 due to Ho III ions (J = 8, L = 6, S = 2, and g = 5/4).
All known compounds of this type have two specific structures, honeycomb or racemic, and show paramagnetism. Mixing lanthanide ions, the chiral lanthanide assemblies with Δ-or Λ-coordination geometries are targeted for synthesis, which should show a magneto-chiral dichroism. A study to clarify this hypotheses, work is currently under way.

Experimental
Under air, aqueous solutions of 0.1 M Ho(NO 3 ) 3 and 0.4 M 1,2,4,5-tetrahydroxybenzene (THB) (Weider et al., 1985) mixed. THB was gradually oxidized to 2,5-dihydroxy-1,4-benzoquinone (DHBQ) in the mixed solution, and slow complexation of Ho(NO 3 ) 3 and DHBQ) produced red crystals of the title polymer in 24% yield within a week. The obtained poly- difference of zeolitic water molecules between the elemental analysis and the crystallographic formulation because zeolitic water molecules are easy to be lost from the crystals and their number depends on the drying processes.

Refinement
The H atoms were placed in their calculated positions,with C-H = 0.95 Å, and refined using a riding model, with U iso (H) = 1.2 U eq (C). Fig. 1. A part of the title polymeric compound, thermal ellipsoids are shown at the 50% probability level. Blue, gray, and red represent Ho, C and O atoms, respectively. The labeled atoms indicate all independent atoms.