An Unusual Bismuth Ethanedisulfonate Network

The three dimensional bismuth ethanedisulfonate framework Bi(O 3 SC 2 H 4 SO 3) 1.5 (H 2 O) 2 was synthesized under hydrothermal conditions using the bidentate ligand 1,2-ethanedisulfonate and then characterized through X-ray diffraction and elemental analyses. The bismuth cation coordinates at three different ethanedisulfonate ligands and has a coordination number of eight, which is accompanied by a distorted square antiprismatic configuration. Here, we report on the crystal structure of this bismuth metal–organic framework and its coordination behavior, which has thus far not been reported in heavier main group elements.


Introduction
Bismuth(III) complexes are used in a variety of fields including biology and medicine [1,2], materials chemistry [3,4], and catalysis [5].For this reason, the synthesis of bismuth metal-organic frameworks (MOFs) has received considerable attention [6].A handful of bismuth organic frameworks exist, which have a broad range of structures due to their different molecular topologies [7,8].By varying the set of donor atoms in the organic ligand, it is possible to enlarge this growing class of organometallic compounds.The formation of such frameworks depends significantly on the dimension of the internal cavity, rigidity of the ligands, nature of the electronegative atoms, and complex forming properties of the anion that is involved in the coordination bonding [6].Generally, Bi 3+ with its ionic radius of 1.16 Å [9] has one inert 6s 2 electron pair and forms non-transition metal center complexes with The bridging oxygen atoms (O1, O3, O4, O5, O7, O9) have slightly shorter bond lengths [10] with the bismuth atoms, ranging from 2.324(8) to 2.590(8) Å (Table 1).The bond lengths between water atoms (O10 and O11) and the bismuth lie in the range of other Bi-OH 2 bond lengths (Cambridge Database: 2.303-3.123Å).A bond valence analysis reveals a value of 2.819 for the bismuth cation, which reflects the slightly insufficient coordination environment of the cation [33,34].Consequently the ligands are spreading out in all three directions to the neighbor bismuth cations forming a metal organic framework with very small cavities.An ideal way to understand the structure is to investigate the bismuth-bridging SO 2 units and omit the ethylene groups (Figure 2).The resulting three-dimensional network built by the organosulfonate groups is cross-linked to the adjacent bismuth cation, and the MOF-like system can be seen clearly.When the whole crystal structure is examined the alternating layers of bismuth cations and ligands becomes visible.Two different sheet systems can be distinguished.One has a large distance of around 6.5 Å as measured between the bismuth layers where the ligand lies stretched out in the z-direction (Figure 3).The second system has a shorter distance between the bismuth layers of around 3.  The bulk material of compound 1 was confirmed through IR, powder X-ray-diffraction and elemental analyses.Even though compound 1 is the main product of the bulk material, a small amount of the compound [Bi 6 O 4 (OH) 4 (H 2 O) 2 ][(CH 2 ) 2 (SO 3 ) 2 ] 3 [10] was always isolated after the first filtration and discarded.
For a better insight into the thermal stability a thermal analysis (TGA) under oxygen was performed: Single crystals of 1 were crushed thus only the pure phase could be investigated: up to the first decomposition peak starting at about 320 °C only the weight loss of the two coordinated water molecules can be observed (at about 139 °C and 222 °C, observed 6.7% calculated 6.8%).Higher temperatures led to the decomposition of the organic moiety in one step (320 °C-395 °C, observed 35.9%, calculated 35.8%), followed by weight gain of 2.5% till 870 °C, which could be possibly be explained by the adsorption of oxygen molecules.The crystal parameters used in the unit cell determination, and structure refinement parameters are summarized in Table 2.

Experimental Section
All reagents were obtained commercially and were used in the form that they were received in.The reactions were performed under hydrothermal conditions in air.
A mixture of Bi 2 O 3 (1.16g, 2.4 mmol), 1,2-ethanedisulfonic acid (2.5 g, 1.3 mmol), and H 2 O (20 mL) was placed into a 25 mL teflon-lined stainless steel autoclave and kept for 3 days at 170 °C.After slowly cooling the autoclave to room temperature, a colorless solution with a little [Bi 6 O 4 (OH) 4 (H 2 O) 2 ][(CH 2 ) 2 (SO 3 ) 2 ] 3 precipitate [10] had formed.The solution was filtered, and the mother liquor was allowed to evaporate in an open beaker.After two weeks, white crystals had formed.The crystals were filtered, washed with water, and then dried in air.Yield of the pure crystals: 20%.
Single-crystal measurements were carried out on a Stoe IPDS-II diffractometer with a MoK  sealed tube at room temperature (λ = 0.71073 Å).The structure was solved using direct methods and refined in anisotropic approximation using the SHELX program suite [36].
A crystal was selected for single-crystal X-ray diffraction.The composition of the bulk material was confirmed through IR, powder X-ray-diffraction and elemental analyses.Powder X-ray diffraction (PXRD) data were recorded using a Bruker ASX D8 Advance powder diffractometer with CuK  radiation (λ = 1.5418Å).Lattice parameters of the bulk material of 1 according to the PAWLEY fit could be found as; a = 11.596,b = 110.385,c = 10.4926,β = 115.197.The reflections of the powder pattern can be assigned to compound 1 (Figure 4, Table S1).

Figure 2 .
Figure 2. The Bi-SO 3 framework of 1.The carbon and hydrogen atoms are omitted for clarity.

Table 1 .
Selected bond lengths (Å) for 1 with estimated standard deviations in parentheses.