Redetermination of (d-penicillaminato)lead(II)

In the title coordination polymer, [Pb(C5H9NO2S)]n {systematic name: catena-poly[(μ-2-amino-3-methyl-3-sulfidobutanoato)lead(II)]}, the d-penicillaminate ligand coordinates to the metal ion in an N,S,O-tridentate mode. The S atom acts as a bridge to two neighbouring PbII ions, thereby forming a double thiolate chain. Moreover, the coordinating carboxylate O atom forms bridges to the PbII ions in the adjacent chain. The overall coordination sphere of the PbII ion can be described as a highly distorted pentagonal bipyramid with a void in the equatorial plane between the long Pb—S bonds probably occupied by the stereochemically active inert electron pair. The amino H atoms form N—H⋯S and N—H⋯O hydrogen bonds, resulting in a cluster of four complex units, giving rise to an R 4 4(16) ring lying in the ab plane. The crystal structure of the title compound has been reported previously [Freeman et al. (1974 ▶). Chem. Soc. Chem. Commun. pp. 366–367] but the atomic coordinates have not been deposited in the Cambridge Structural Database (refcode DPENPB). Additional details of the hydrogen bonding are presented here.

In the title coordination polymer, [Pb(C 5 H 9 NO 2 S)] n {systematic name: catena-poly[(-2-amino-3-methyl-3-sulfidobutanoato)lead(II)]}, the d-penicillaminate ligand coordinates to the metal ion in an N,S,O-tridentate mode. The S atom acts as a bridge to two neighbouring Pb II ions, thereby forming a double thiolate chain. Moreover, the coordinating carboxylate O atom forms bridges to the Pb II ions in the adjacent chain. The overall coordination sphere of the Pb II ion can be described as a highly distorted pentagonal bipyramid with a void in the equatorial plane between the long Pb-S bonds probably occupied by the stereochemically active inert electron pair. The amino H atoms form N-HÁ Á ÁS and N-HÁ Á ÁO hydrogen bonds, resulting in a cluster of four complex units, giving rise to an R 4 4 (16) ring lying in the ab plane. The crystal structure of the title compound has been reported previously [Freeman et al. (1974). Chem. Soc. Chem. Commun. pp. 366-367] but the atomic coordinates have not been deposited in the Cambridge Structural Database (refcode DPENPB). Additional details of the hydrogen bonding are presented here.

Comment
Lead is a serious environmental contaminant. The extensive use of lead as metal and in lead compounds into modern times, e. g., in alkyl lead additives in leaded gasoline, battery manufacturing and in paints, has made lead a ubiquitous pollutant in the ecosystem. The soluble Pb II ion with its 5 d 10 6 s 2 electronic configuration in the valence shell has a very flexible coordination behaviour. It is a neurotoxic heavy metal ion that perturbs multiple enzyme systems affecting areas of the brain that regulate behavior and nerve cell development and any site with sulfhydryl groups is vulnerable (Needleman, 2004). In particular Zn(II) can be replaced in enzymes, e. g., inhibiting the heme biosynthetic pathway, even though the effective ionic radius in four-coordination of the soft Pb II ion (0.98 Å) is significantly larger than that of Zn II (0.60 Å). Pb II can also adapt to replace Ca(II) in bone (Bressler et al., 1999;Godwin, 2001).
Treatments of lead(II) poisoning are mainly based on using chelators that form strong bonds to heavy metal ions, such as the disodium salt of the calcium edta complex (CaNa 2 edta) and dimercaprol (BAL), which are injected, and DMSA (meso-2, 3-dimercaptosuccinic acid) and D-penicillamine (H 2 Pen), which are administered orally (Sinicropi et al., 2010;Casas & Sordo, 2006).
The binding of Pb II to the tridentate chelator H 2 Pen containing a sulfhydryl group is of interest for better understanding of the coordination behaviour in biological systems and for the design of specific detoxifying agents. The coordination geometry around the Pb II ion in the crystalline title compound (PbPen), which precipitates in a wide pH range from penicillamine solutions containing lead(II) ions, was previously discussed by Freeman et al., (1974). However, the atomic coordinates of the crystal structure were not reported, nor deposited in the Cambridge Structural Database (refcode: DPENPB]. Here, we report the crystal structure of PbPen, and also discuss the Pb-Pb distances in this polymeric structure with double bridged thiolate chains. Mixing Pb(NO 3 ) 2 and D-penicillamine in 1:2 molar ratio resulted in a 1:1 complex, PbPen, formed in an alkaline solution. The ligand is coordinated to the Pb II ion in a tridentate mode: Pb-N 2.444 (9) Å, Pb-O 2.451 (7) Å and Pb-S 2.714 (2) Å (Fig. 1). The sulfur atom acts as a bridge with Pb-S distances of 3.091 (2) and 3.464 (2) Å to two other neighbouring Pb II ions located at 4.363 Å relative to the original Pb II ion, forming a double thiolate chain in a polymeric structure. Moreover, the coordinated carboxylate oxygen atom forms bridges to the lead ions (Pb-O 2.720 (7) Å) in the adjacent chain with two Pb II ions at 4.663 Å relative to the central Pb II ion (Fig. 2). The coordination sphere of lead can be described as a distorted pentagonal bipyramid if the Pb-O interactions to the carboxylate oxygen atoms are considered as axial interaction opposite the short Pb-S bond (2.714 (2) Å), and also including a possible stereochemically active inert electron pair in the void in the equatorial plane between the two long Pb-S interactions, 3.091 (2) Å and 3.464 (2) Å ( Fig. 3) (Freeman et al., 1974).
The amino H-atoms of the title complex are hydrogen bonded to a S-atom (N1-H1B···S1) along the a-axis and an Oatom (N1-H1A···O2) along the b-axis resulting in a cluster of four complex units giving rise to a 16-membered ring in the ab-plane which can be best described as a R 4 4 (16) motif in the graph set notation (Bernstein et al., 1994) (Fig. 4).

Refinement
All H atoms were positioned geometrically and refined using a riding model, with N-H = 0.92 Å and C-H = 0.98 and 1.00 Å, for methyl and methylene H-atoms, respectively, and the U iso (H) were allowed at 1.5U eq (N/C). An absolute structure was determined using 966 Friedel pairs of reflections which were not merged; the Flack parameter was 0.03 (2) (Flack, 1983). The largest residual peaks in the final difference map were located in the close proximity of the Pb atom and may be attributed to inadequate absorption correction.

catena-poly[(µ-2-amino-3-methyl-3-sulfidobutanoato)lead(II)]
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.