A novel inorganic–organic hybrid borate, poly{[Na2(C4H2O4)(H3BO3)(H2O)4]·H3BO3}

The structure of the title compound, catena-poly[[[di-μ-aqua-μ-fumarato-μ-(boric acid)-disodium]-di-μ-aqua] boric acid monosolvate], contains two crystallographically independent Na+ cations, each being six-coordinated by one fumarate O atom, one boric acid O atom and four water O atoms in a distorted octahedral geometry. Adjacent [NaO2(OH2)4] units share edges and are linked into chains propagating parallel to [100]. The free boric acid molecules are connected to the chains through strong intermolecular O—H⋯O hydrogen bonds. Additional O—H⋯O hydrogen bonds between the water molecules, the free and coordinated boric acid molecules and the fumarate anion lead to the formation of a three-dimensional supramolecular structure. With the exception of the two water molecules, all other atoms lie on mirror planes.


Experimental
Crystal data [Na 2 (C 4   is fumaric acid), (I), which represents the first one-dimensional Na + coordination polymer involving both boric acid and an organic anion.
As shown in Fig. 1, the asymmetric unit of the structure of compound (I) contains two crystallographically independent Na + cations (Na1 and Na2). Each Na atom is six-coordinated in the form of a distorted octahedron by two oxygen atoms (one from the carboxyl group of the fumarate (fum) anion, one from the hydroxyl group of the coordinated boric acid molecule) that occupy the axial positions, and by four water molecules in the equatorial plane. Both the fumarate anion and the coordinated boric acid act as bidentate bridging ligands to link two neighboring Na + ions.  (Li et al., 1999;Andrews et al., 1983;Roy et al., 2002).
The most striking structural feature of the title compound is the oxygen-bridged one-dimensional Na infinite chain. To the best of our knowledge, no previous carboxylato-MBO (MBO is a metal borate with M being an alkali metal) one-dimensional coordination polymer has been reported. There is only one report about the crystal structure of a B(OH) 3 unit bridging metal ions (Tombul et al., 2007). In this example, the B(OH) 3 molecule may be considered as coexisting with the dipotassium supplementary materials sup-2 maleate salt. However, in structure (I), the coordinated B(OH) 3 molecule distinctly acts as a bidentate ligand bridging two neighboring Na + ions into an infinite chain, and such a coordination mode for B(OH) 3 is unprecedented until now.

Experimental
A mixture of borax (0.604 g), fumaric acid (0.234 g) and water (15 ml) was homogenized at 373 K for 1 h, producing a colourless solution. Colourless transparent block-like crystals of (I) were obtained by slow evaporation from a concentrated solution of the compound in water after standing for two weeks.

Refinement
H atoms of the coordinated boric acid, water molecules and O8 were located in a difference Fourier map and were allowed for as riding parent atoms with U iso (H) = 1.5U eq (O) and 1.2U eq (C). Other H atoms were placed in calculated positions and were included in the refinement in the riding-model approximation, with hydroxyl O-H = 0.84 Å, methylene C-H = 0.99 Å, and with U iso (H) = 1.5U eq (O), 1.2U eq (C). supplementary materials sup-3 Fig. 3. The three-dimensional supramolecular network in the structure of (I).