A redetermination of the structure of poly[[μ4-(R)-2-ammonio-3-sulfonatopropanoato]aquasodium], originally reported as poly[[μ7-l-cysteato(2−)]disodium]

The structure originally reported as poly[[μ7-l-cysteato(2−)]disodium], [Na2(C3H5NO5S)]n [Liu (2002). Acta Cryst. E67, m1346–m1347], has been redetermined with one of the sodium atoms replaced with a water molecule and an additional proton attached to the amine group, resulting in the revised formula [Na{CO2CH(CH2SO3)NH3}(H2O)]n. The agreement index, wR, has been reduced from 0.159 to 0.087 and the global instability index from 0.56 vu (valence units) to the acceptable value of 0.11 vu.

The structure originally reported as poly[[ 7 -l-cysteato(2À)]disodium], [Na 2 (C 3 H 5 NO 5 S)] n [Liu (2002). Acta Cryst. E67, m1346-m1347], has been redetermined with one of the sodium atoms replaced with a water molecule and an additional proton attached to the amine group, resulting in the revised formula [Na{CO 2 CH(CH 2 SO 3 )NH 3 }(H 2 O)] n . The agreement index, wR, has been reduced from 0.159 to 0.087 and the global instability index from 0.56 vu (valence units) to the acceptable value of 0.11 vu.

Related literature
The original structure determination of this compound was reported by Liu (2011). The bond-valence methods are described in Brown (2002).
Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO; data reduction: CrystalStructure (Rigaku/ MSC, 2002); method used to solve structure: coordinates taken from the previous refinement (Liu, 2011); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: publCIF (Westrip, 2010). In a recent issue of this journal, Liu (2011) reported the structure of the title compound, but a close examination of this structure shows a serious problem with the environment of one of the sodium atoms, Na2. In the Comment the author describes this atom as 'tetracoordinated within an NO 3 coordination sphere. The Na + ion binds to the amino N atom, to one of the O atom of the carboxylic residue and to two O atoms of the sulfonate group in a distorted tetrahedral arrangement′. The Comment does not point out that the atomic displacement parameter of Na2 is almost three times larger than the next largest atomic displacement parameter, nor does it point out that the lengths of the four bonds around Na2 all lie in the range 2.90 to 3.03 Å, distances whose bond valence sum of 0.21 vu (valence unit) indicates that they are much longer than would be expected for a four-coordinate sodium cation. The global instability index [root mean square deviation of the bond valence sums of all atoms from their atomic valence, (Brown, 2002)] is 0.56 vu, much higher than the generally accepted limit of 0.20 vu for a stable structure. The environment of Na2 is, however, one that would be expected for a water molecule that forms moderate to weak hydrogen bonds. An additional hydrogen ion is required for charge neutrality, but protonating the water molecule is unlikely as this would require much shorter hydrogen bonds than are observed for this site, but protonating the amine group would not only increase the bond valence sum around the nitrogen from 2.51 vu to a value closer to the expected 3.00 vu, it would also result in an N-H bond positioned to form a hydrogen bond with the water molecule.
This proposed model has been refined and is reported in this paper. The bond valence sum of 0.21 vu around the original Na2 has been replaced by a sum of 1.94 vu around the oxygen of water, and the sum around the N1 atom has been increased to 3.17 vu. Moreover, Na1 now has a meaningful octahedral environment (Fig. 1) with a Na1-O1W contact of 2.450 (2) Å instead of a Na1-Na2 contact as in the original model.

Experimental
The preparation of the compound is detailed in the original report (Liu, 2011).

Refinement
The structure was refined using the original diffraction measurements of Liu (2011) to the ammonium group were constrained to bond lengths N-H = 0.89 Å with U iso (H) = 1.5 times U eq (N). The water H atoms were restrained to bond lengths of 0.85 (1) Å.

poly[[µ 4 -(R)-2-ammonio-3-sulfonatopropanoato]aquasodium]
Crystal data 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 > σ(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.