Hydroxycarboxylate Complexes of Bismuth

Bismuth compounds containing hydroxycarboxylate ligands have been most extensively studied in the context of medicinal applications. Despite the 'sub' terminology in BSS (Pepto-Bismol®) and CBS (De-Nol®), experimental data are inconclusive for the presence of the bismuthyl (BiO+) cation in examples of these compounds. Hydroxycarboxylic acids that are observed to form complexes with bismuth are shown in Figure 27.2. Most complexes involve conjugate bases, which are abbreviated according to the degree of deprotonation of the acid; for example, (sal-H)~ is a monoanionic conjugate of salicylic acid and lactic acid O (lac) 0 OH

O OH

malic acid (mal)

tartaric acid (tar)

O OH

O OH

citric acid (cit) O OH O

HO O

salicylic acid (sal) O

OH O

Figure 27.2 Drawings and abbreviations for hydroxycarboxylic acids that are observed to form complexes with bismuth

(mal-3H)3~ is a trianionic conjugate of malic acid. Some formulae assignments for salicylate and citrate complexes of bismuth are based on elemental analysis data, and conclusions are often impeded by variable water content in solid samples.

The only structural reports of bismuth-salicylate complexes involve 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) as auxiliary ligands,16 which are believed to provide a stabilizing influence or disruption of the coordination polymeric lattice that is responsible for the low solubility of BSS. The solidstate structures of [Bi(sal-H)3(bipy)]2 and [Bi(sal-H)(sal-2H)(phen)]2 are illustrated in Figure 27.3. Both involve a hydroxycarboxylate ligand interacting with bismuth through the carboxylate functionality, as either a terminal four-membered chelate or a bridging carbonyl. In the latter, a dianionic salicylate (sal-2H)2~ behaves as an alkoxycarboxylate-chelating bridge. The potential structural complexity for these types of compounds is demonstrated by the octanuclear cluster observed for Bi8(tsal-2H)12(DMF)12, which involves dianionic thiosalicylate (Figure 27.1) ligands engaging bismuth with thiolate and carboxylate donors (DMF = N,N-dimethylformamide).17

Bismuth citrate is essentially insoluble in water, but a dramatic increase in solubility is observed with increasing pH, yielding the 'bismuth citrate' anion. Formulation of these solutions is complicated and various examples have been determined by X-ray crystallography and classified in the literature as CBS.18 26 In this context, the Merck index uses the non-stoichiometric name 'tripotassium dicitrato bismuthate'.27 The reported structures of CBS are closely related and are generally constructed from the monoanionic complex of Bi3+ with a tetra-anionic (tricarboxylate/alkoxide) citrate ligand (cit-4H)4~. As shown in Figure 27.4, two carboxylate oxygen centers and the alkoxide effect a tridentate interaction with bismuth. The third carboxylate is pendant due to the required tetrahedral geometry of the central carbon center in the citrate framework, and is

Figure 27.3 Solid-state structures of (a) [Bi(sal-H)3(bipy)]2 and (b) [Bi(sal-H)(sal-2H) (phen)]2
Figure 27.4 A dianionic dimeric unit observed in the solid-state structures of 'colloidal bismuth subcitrate' compounds
Figure 27.5 A section of the polymeric solid-state structure of Bi(lac-H)3

available for interaction with a second bismuth center that involves a bidentate chelation. As a result, well-defined dimeric arrangements exist in most structures and the formulations are distinguished by differences in the interactions between the dimeric units and the degree of hydration.

The structural features of these salicylate and citrate complexes can be compared with those for bismuth complexes of other familiar hydroxycarboxy-lates. The multifunctional nature of conjugates from lactic acid, malic acid and tartaric acid (Figure 27.2) provide for a variety of structures that include intermolecular Bi—O coordination polymeric interactions. Tartrate engages bismuth as both mono- and di-anionic ligands in Bi(tar-H)(tar-2H)H2O28 and [NH4][Bi(tar-2H)2H2O].29 Bismuth is chelated by carboxylate and hydroxy oxygen centers in the lactate salt Bi(lac-H)3, as illustrated in Figure 27.5.30 Three additional oxygen centers from carboxylate functionalities of neighboring complexes impose a coordination number of nine on bismuth. The multifunctional trianionic malate ligand also imposes nonacoordination on bismuth in Bi(mal-3H)28 by intermolecular interactions and interaction with a water molecule (Figure 27.6).

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