Poly[[μ-bis(4-nitrophenyl) phosphato-κ2 O,O′]sodium]

The title compound, [Na(C12H8N2O8P)], consists of one Na+ cation and one bis(p-nitrophenyl)phosphate anion with a considerable distortion of the phosphate tetrahedron due to the presence of two P—O ester bonds. The anion bridges five Na+ cations whereby each cation is chelated by the nitro O atoms of one anion and bonded via a nitro O atom and phosphate O atoms to four other anions. This bridging arrangement leads to the formation of double layers parallel to (001). Adjacent layers are linked through weak C—H⋯O hydrogen bonds.

The title compound, [Na(C 12 H 8 N 2 O 8 P)], consists of one Na + cation and one bis(p-nitrophenyl)phosphate anion with a considerable distortion of the phosphate tetrahedron due to the presence of two P-O ester bonds. The anion bridges five Na + cations whereby each cation is chelated by the nitro O atoms of one anion and bonded via a nitro O atom and phosphate O atoms to four other anions. This bridging arrangement leads to the formation of double layers parallel to (001). Adjacent layers are linked through weak C-HÁ Á ÁO hydrogen bonds.

Aleksandra Gerus and Tadeusz Lis Comment
Phosphate diester hydrolysis is a reaction of continuing interest since such process is of fundamental biological importance. Currently, there is much interest in developing artificial nucleases that hydrolyze the phosphate diester bonds in RNA and DNA (Sredhera & Cowan, 2001;Belousoff et al., 2009;Branum et al., 2001). Thus, there have been numerous model studies devoted to understanding how metalloenzymes hydrolyze phosphate diesters (Mancin et al., 2005;Liu et al., 2004). In most cases the substrate of choice is the "activated" phosphate diester bis(p-nitrophenyl)phosphate (BNPP). It is considered as an activated phosphate diester because the p-nitro groups draw electron density away from the phosphorus atom as well as help to stabilize the negatively charged leaving group (Jurek & Martell, 1999).
For this reason the bis(p-nitrophenyl)phosphate anion is a popular model substrate for kinetic studies of the hydrolytic cleavage of the phosphodiester bond similar to DNA (Chang et al., 2009;Oh et al., 1996).
BNPP is commercially available as a sodium salt. There are many references concerning solid state studies of BNPP acting as a ligand in complexes or as an anion in salts. The first publications referring to BNPP describe salts of local anesthetics (Sax et al., 1970(Sax et al., , 1971Pletcher et al., 1972;Yoo et al., 1975). The structure of BNPP has been observed also in many macrocyclic complexes (Král et al., 2006;Bazzicalupi et al., 2004;Fry et al., 2003;Warden et al., 2005). Here we report the structure of BNPP as a sodium salt, [Na(C 12 H 8 N 2 O 8 P)], (I).
Compound (I) crystallizes with one bis(p-nitrophenyl)phosphate anion ( Fig. 1) and one sodium cation in the asymmetric unit. The phenyl rings are almost coplanar. The interplanar angle between two phenyl rings amounts to 2.36 (3)°. The nitro group O1-N1-O2 is rotated 2.00 (4)° from the phenyl ring C1-C6 and the second nitro group O3-N2-O4 is rotated 9.01 (4)° from the phenyl ring C11-C61. The phosphate group is highly distorted from the ideal tetrahedral geometry (Table 1). In the anion there are two shorter P-O bonds of 1.4733 (8) Å and 1.4834 (7) Å, and two longer P-O ester bonds lengths of 1.6266 (8) Å and 1.6287 (12) Å. The shortest bond is P-O31. A little longer than the P-O31 bond is the P-O41 bond, bridging Na + ions. Both O11 and O21 atoms involved in longer ester bonds are attached to aryl rings. The average P-O distance is 1.55 Å, but individual P-O bonds in the structure of compound (I) show how much the phosphate group is deformed. In previous reports containing BNPP anions the most similar deviations for P-O ester bond length in the phosphate group has been observed for these four examples (Sax et al., 1970(Sax et al., , 1971Pletcher et al., 1972;Yoo et al., 1975).
The bond angles in the phosphate group distinctly deviate from the ideal value of 109.5°. The average value for the O-P-O angle is 108.9°, however, individual angles show considerable deviations. The smallest angle is 92.20 (4)° for O11-P1-O21, that is ArO-P-OAr. This deviation can be correlated with the corresponding bond lengths. Such a small angle has not been observed in any previous report of a BNPP structure. The most comparabler value of an O-P-O angle is 95.3 (2)° (Bond et al., 1985). The largest angle is 119.67 (5) In (I), the coordination geometry of Na + ion is irregular, with an overall coordination number of six [5 + 1] . The Na + ion is coordinated by five symmetry-related BNPP anions via oxygen atoms. It is chelated by one anion in a bidentate mode (via O1 and O2), and coordinated by four anions in a monodentate manner (via O31 iv , O4 iii , O41 v and O41 vi ) (Fig. 2). The Na-O distances are in the range 2.2386 (10) to 2.9377 (18) Å (Table 2). In the structure there is also a short Na···Na distance of 3.253 (2) Å, and two sodium cations are bridged by two O atoms (denoted as O41 in Fig. 2), forming a dimeric sub-structure with a four-membered ring (Figs. 2 and 4). The cations and anions are arranged in double layers parallel to (001) (Figs. 3 and 4). Adjacent layers are linked through weak C-H···O hydrogen bonds existing between H atoms of the aromatic rings and nitro O atoms (Fig. 3, Table 2).

Experimental
The bis(p-nitrophenyl)phosphate sodium salt was purchased from Sigma-Aldrich. Yellow crystals were obtained after several days by slow evaporation of an aqueous solution.

Refinement
All H atoms were introduced in geometrically calculated positions, with C-H = 0.95 A ° and U iso (H) = 1.2Ueq(C).

Crystal data
[Na (C 12  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.