Rerefinement of poly[diaquabis(μ3-2-methylpropanoato-κ4 O:O,O′:O′)bis(μ3-2-methylpropanoato-κ3 O:O:O)(μ2-2-methylpropanoato-κ3 O:O,O′)(2-methylpropanoato-κ2 O,O′)trilead(II)]

The crystal structure of [Pb3(C4H7O2)6(H2O)2] n was redetermined, revealing details of the disorder of one of the hydrocarbon chains in one of the six independent 2-methylpropanoate anions.

The crystal structure of the title complex, [Pb 3 (C 4 H 7 O 2 ) 6 (H 2 O) 2 ] n , was redetermined on basis of modern CCD-based single-crystal X-ray data at 120 K. The current study basically confirms the previous report [Fallon et al. (1997). Polyhedron,16,[19][20][21][22][23] at 190 K, but with higher accuracy and precision. In particular, positional disorder of one of the 2-methylpropanoate anions over two sets of sites was resolved, showing a refined ratio of the disorder components of 0.535 (9):0.465 (9). The three independent cations in the structure have coordination numbers of [7 + 1], [6 + 1], and [5 + 3], with O atoms belonging either to carboxylate groups or water molecules. This arrangement leads to the formation of sheets parallel to (101), whereby the hydrophobic 2-methylpropanyl groups of the anions are oriented above and below the hydrophilic sheets to form a layered structure. Within a sheet, hydrogen bonds of the type O water -HÁ Á ÁO are formed, whereas the hydrophobic groups between adjacent layers interact through van der Waals forces.

Structure description
The structural features of metal carboxylates, except formates and acetates, are strongly affected by voluminous hydrophobic chains [cf. Duruz & Ubbelohde (1972) and Dumbleton & Lomer (1965)] which tend to be separated from the hydrophilic parts of these structures. The latter parts are composed of the cations, which are coordinated by the carboxylate or water oxygen atoms. The hydrophilic parts can take the form of clustered aggregates, columns or planes, which then are surrounded by the hydrophobic parts [see Samolová & Fá bry (2020)]. In some cases there is a positional disorder of data reports hydrophobic chains realised, e.g. in barium dicalcium hexakis(propanoate) (Stadnicka & Glazer, 1980), or in the crystal structure of the title compound, [Pb 3 (C 4 The title structure has been determined previously by Fallon et al. (1997) without details regarding atomic coordinates and displacement parameters in the original publication. The current redetermination was undertaken because the deposited data in the Cambridge Structural Database (Groom et al., 2016;version 5.41 from November 2019 with updates until August 2020), refcode REXBAX, is also incomplete. Here only atomic coordinates are given, and occupation factors of the disordered hydrocarbon chains are missing as well. In general, the quality of the study by Fallon et al. (1997) with a reliability factor R = 0.071, wR = 0.092 is below current standards. For example, the differences between the positions of the corresponding atoms in the original and the preset study is as large as 0.3 Å . However, it should be taken into account that the re-refined structure is based on data measured at 120 K with all non-H atoms refined with anisotropic displacement parameters compared to the previous determination at 193 K.

Figure 1
View of the core motif in the title structure showing the environments of the cations. Displacement ellipsoids of the Pb (dark green), O (red) and C (grey) atoms are shown at the 30% probability level while H atoms are shown as spheres of arbitrary radius. The positional disorder is shown. This involves the groups attached to C17 and C17 i . Three terminal methyl groups are present.

Figure 2
View of the (101) sheets in a view along the b axis. Displacement ellipsoids of the Pb, O and C atoms are shown at the 50% probability level while H atoms are shown as spheres of arbitrary radius. O-HÁ Á ÁO hydrogen bonds are shown as dashed yellow lines; colour codes are as in Fig. 1.
ring composed of the atoms Pb1\O2 i \Pb3\O4\Pb1\O2\Pb3 i \O4 i ( Fig. 1) [symmetry code (i): Àx + 1, Ày + 1, Àz + 1]. Symmetry-equivalent Pb2 2+ cations including their coordinating molecules are attached to this core. The cations, carboxylate oxygen atoms and water molecules form the hydrophilic part of the structure that is characterized by sheets parallel to (101) (Fig. 2). Each of the water molecules is involved in an O water -HÁ Á ÁO hydrogen bond of moderate strength (Gilli & Gilli, 2009) within a sheet (Table 2). These sheets are surrounded by hydrophobic layers composed of 2-methylpropanoic chains. Two methyl groups centered on the atoms C3 and C12 are protruding into the cation-oxygen sheet. The methyl group C19 is disordered over two sets of sites (split into C19a and C19b). The distances C methyl Á Á Á C methyl or C methanetriyl Á Á ÁC methyl indicate the presence of van der Waals interactions. The shortest distance of this kind regards the contact C3Á Á ÁC10(Àx + 1 2 , y À 1 2 , Àz + 1 2 ) and equals 3.713 (5) Å .

Synthesis and crystallization
The title structure was prepared by by disolution of 1.18 g of PbCO 3 in a water solution (100 ml) of 0.78 g of 2-methylpropanoic acid (molar ratio 1:2). The pH of the solution was adjusted to $6 by adding 2-methylpropanoic acid. The solution was then filtered and concentrated at 313 K. After a crust had started to appear on the surface of the solution, heating was stopped and elongated colourless crystals appeared.

methylpropanoato-κ 3 O:O,O′)(2-methylpropanoato-\ κ 2 O,O′)trilead(II)]
Crystal data Special details Refinement. The non-hydrogen atoms were determined by SHELXT (Sheldrick, 2015). The methanetriyl hydrogen was placed into the calculated positions and refined under the following constraints: Cmethanetriyl-Hmethanetriyl = 1.00?Å, Uiso(Hmethanetriyl) = 1.2Ueq(Cmethanetriyl). After the anisotropic refinement of the non-hydrogen atoms with the methanetriyl hydrogen had been carried out the difference electron density map revealed other hydrogens. These hydrogens were refined under the following constraints: Cmethyl-Hmethyl = 0.98?Å, Uiso(Hmethyl) = 1.5Ueq(Cmethyl). The water hydrogen were refined using the distance restraints Owater-Hwater = 0.840?(1)?Å and the constraints Uiso(Hwater) = 1.5Ueq(Owater). The occupancies regarding the atoms C19a and C19b were treated in such a way that their sum equalled to 1 while the occupational parameter of C19a was refined. The attached hydrogens tothe atoms C19a and C19b were assigned the pertinent occupancies. The same holds for the methanetriyl hydrogens H1C18 and H1C18d which were assigned the occupancies of the atoms C19a and C19b, respectively. For the treatment of the disorder a dummy atom C18d was introduced with the same positional and displacement parameters as the atom C18.