Crystal structures of three lead(II) acetate-bridged diaminobenzene coordination polymers

The structures of three lead(II) coordination polymers are reported. One exhibits a two-dimensional structure, whereas the other two are one-dimensional. All three exhibit bidentate bridging acetate and monodentate benzene-1,2-diamine ligands. The extended structures reveal extensive hydrogen-bonding networks involving the diamine and acetate ligands.

and (III) are isotypic and have one Pb II ion in the asymmetric unit that has an O 6 N coordination sphere. Pb 2 O 2 units result from a symmetry-imposed inversion center. Polymeric chains parallel to [100] exhibit hydrogen bonding between the amine and acetate ligands. In (III), additional hydrogen bonds between cyano groups and non-coordinating amines join the chains by forming R 2 2 (14) rings.

Chemical context
Metal-organic frameworks (MOFs) are of inherent interest in areas such as gas storage, catalysis, chemical sensors and molecular separation (Dey et al., 2014;Kreno et al., 2012;Farha & Hupp, 2010). Recently, we reported the synthesis and structural characterization of two zinc MOFs possessing bridging acetate ligands and monodentate chloro-or cyanosubstituted o-phenylenediamine ligands (Geiger & Parsons, 2014). These complexes possess a ladder-chain structure with an ethanol molecule that occupies a void with a volume of approximately 224 Å 3 . The results presented here expand the structural study to Pb II analogues.
Pb II compounds often exhibit a distorted coordination sphere or open coordination site that has been attributed to stereoactive 'lone-pair' electrons (Morsali, 2004;Wang & Liebau, 2007;Park & Barbier, 2001). Indeed, hemidirected geometry is favored over halodirected geometry for Pb II when hard ligands are present, which corresponds to a greater ionic character in the metal-ligand bonding (Shimoni-Livny et al., 1998), or when one or more of the ligands is anionic (Esteban-Gó mez et al., 2006). However, hemidirected lead(II) complexes in a soft sulfur-rich environment are also known (Imran et al., 2014). The results of a reduced variational space (RVS) analysis suggest that more sterically crowded, hemidirected structures are stabilized by polarization of the lead(II) ion induced by the ligand arrangement (Devereux et al., 2011). The possibility of a distorted coordination sphere ISSN 1600-5368 enhancing the volume of void space between chains found in coordination polymers provided the impetus for the synthesis and structural characterization of the compounds reported herein. Fig. 1 shows the three acetate coordination modes displayed by (I), (II), and (III). The three modes will be referred to hereafter as types (a), (b) and (c). As seen in Fig. 2, the asymmetric unit of (I) has four symmetry-independent Pb atoms. The Pb atoms are linked by bridging acetate ligands of type (b) to form a ladder-chain parallel to [010]. Each is also coordinated to a bidentate acetate ligand of type (a) and Pb2 and Pb4 have an amine nitrogen in their coordination spheres. Finally, atoms Pb3 and Pb4 are linked by an acetato ligand of type (c). The two benzene-1,2-diamine ligands are approximately coplanar. The angle formed by the benzene mean planes is 6.1 (4) , with N1, N2, N3 and N4 being 0.051 (16), 0.013 (19), 0.074 (16), and 0.034 (16) Å from their respective planes.

Structural commentary
The asymmetric unit of (I) possesses pseudo-translational symmetry as a result of the similarity in the coordination geometries exhibited by Pb1 and Pb3 and by Pb2 and Pb4. Pb1Á Á ÁPb3 = 7.4548 (10) Å and Pb2Á Á ÁPb4 = 7.5372 (10) Å , approximately half of the Pb1Á Á ÁPb4 i = 14.989 (2) Å distance (see Table 1 for symmetry codes). Fig. 3 shows a representation of (I) in which the two pseudo-translationally related halves of the asymmetric unit are color coded. Primary differences in the two halves involve the orientation of the two   The atom-labeling scheme for (I). Anisotropic displacement parameters are drawn at the 50% probability level.

Figure 3
A view of (I) in which the two halves of the asymmetric unit related by the pseudo-translation are color coded. H atoms have been omitted for clarity. non-coordinating amine groups, one less acetate type (c) on Pb1 than on Pb3, and a type (c) acetate ligand on Pb2 replaced by a type (a) acetate ligand on Pb4.
(II) and (III) are isotypic if the nitrile function in (III) is considered as a large one-atomic group and replaces the Cl atom in (II). Fig. 4 shows the atom-labeling scheme for (II) and Fig. 5 shows the atom-labeling scheme for (III). Each Pb atom has two bidentate acetate ligands, one of type (a) and one of type (b). The type (b) ligands result in chains parallel to [100], with Pb 2 O 2 cores related by inversion centers. The substituted benzene-1,2-diamine ligands are essentially planar. For (II), N1 and N2 are below the plane by 0.056 (14) and 0.066 (18) Å , respectively, and Cl1 is 0.020 (14) Å above the plane. In (III), N1 and N2 are 0.073 (17) and 0.05 (2) Å out of the plane. The C7-N3-C4 angle is 177.7 (16) and N3 is 0.12 (2) Å out of the plane.
The coordination spheres are O 6 , O 6 N, O 7 , and O 6 N for Pb1, Pb2, Pb3, and Pb4, respectively, for (I), and O 6 N for (II) and (III). Representations of the coordination spheres are shown in Fig. 6 and pertinent bond distances are found in Tables 1, 2 and 3. The coordination is clearly hemidirected for each Pb and the Pb-O bond lengths are asymmetrical, as is often found for hemidirected compounds (Shimoni-Livny et al., 1998). The average Pb-O bond lengths are 2.60 (13), 2.59 (11), and 2.58 (12) Å for (I), (II) and (III), respectively, or 2.59 (12) Å overall, and range from 2.380 (6) to 2.901 (6) Å . The average Pb-N bond length for the three compounds is 2.84 (5) Å . In all cases, the Pb-O(N) bond lengths are longer for those ligand atoms adjacent to the open coordination site. This is consistent with structural results for other hemidirected coordination modes involving O-and N-donor atoms (cf. Shimoni-Livny et al., 1998;Morsali et al., 2005;Esteban-Gó mez et al., 2006;Morsali, 2004).
In compounds (II) and (III), chains parallel to [100] are observed. An extensive N-HÁ Á ÁO hydrogen-bonding network is found along the chains (see Tables 5 and 6). For (III), the nitrile group affords the opportunity for additional hydrogen bonding. As seen in Fig. 8, this results in R 2 2 (14) rings involving N-HÁ Á ÁN C hydrogen bonds between adjacent chains.

Figure 6
Representation of the Pb II coordination environments observed in (I), (II), and (III). Symmetry identifiers are those used in Tables 1, 2 and 3.

Preparation of (II)
4-Chlorobenzene-1,2-diamine (0.106 g, 0.75 mmol) was dissolved in boiling ethanol (10 ml) and lead(II) acetate trihydrate (0.134 g, 0.35 mmol) was added with stirring. The resulting solution was refluxed for 4 h, removed from the heat and the solvent was allowed to slowly evaporate. The residue obtained was dissolved in hot methanol and passed through a 45 mm pore filter. Crystals suitable for X-ray analysis were obtained after slow evaporation of the solvent. Further solvent reduction resulted in precipitation of excess diamine and so the overall yield was not determined. Selected IR bands (diamond anvil, cm À1 ): 3334 (br), 1537 (s), 1393 (s), 1337 (s), 1018 (s).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. All H atoms were observed in difference Fourier maps. C-bonded H atoms were refined using a riding model, with C-H = 0.98 Å for the methyl groups and 0.95 Å for the aromatic ring. The C-H hydrogen isotropic displacement parameters were fixed using the approximation U iso (H) = 1.5U eq (C) for the methyl H atoms and 1.2U eq (C) for the aromatic H atoms. The atomic coordinates for the amine H atoms were refined using an N-H bond-distance restraint of 0.88 (2) Å and the H-atom isotropic displacement parameters were set using the approximation U iso (H) = 1.5U eq (N). Late in the refinement, a correction for extinction was applied for each of the structures. For (I), the highest residual electron-density peak is 0.94 Å from Pb2 and the deepest hole is 1.20 Å from Pb3. The highest residual electron-density peak is 0.89 Å and the deepest hole is 0.91 Å from Pb1 in (II). For (III), the highest residual electron-density peak and the deepest hole are 0.92 Å and 0.82 Å , respectively, from Pb1.  (14), 29.694 (4), 11.8597 (14) 7.3623 (10), 7.6177 (10), 13.1413 (17) 7.3724 (8) For all compounds, data collection: APEX2 (Bruker, 2013); cell refinement: APEX2 (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2008); molecular graphics: PLATON (Spek, 2009), Mercury (Macrae et al., 2006) and ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: publCIF (Westrip, 2010).