CRYSTAL-STRUCTURES OF 2 POLYMORPHIC THALLIUM(I) SALTS OF THE ANTIBIOTIC LASALOCID-A - A POLYMERIC FORM INVOLVING METAL PHENYL PI-BONDING AND A MONOMERIC FORM INVOLVING THE HALF-NAKED METAL-ION

The crystal and molecular structures of two polymorphic lasalocid A-thallium(I) salts, which were prepared from lasalocid A sodium salt and thallium nitrate, have been determined by X-ray diffraction. Salt 1, which was crystallized from aqueous methanol, forms a one-dimensional polymer, [Tl+(lasalocid A-)]n, where the metal ion is coordinated, on one side, to five oxygens of an ionophore molecule and, on the other side, to the phenyl ring of the neighboring molecule, with Tl-O distances of 2.678 (8)-3.363 (10) angstrom and a Tl-ring (centroid) distance of 3.22 angstrom. The coordinated oxygens are the hydroxyl O4 and O8, carbonyl O5, and ether O6 and O7. Salt 2, which was crystallized from aqueous methanol-ethylene glycol, forms a monomer, Tl+(lasalocid A-), where the metal ion is coordinated, on one side, to six oxygens of an anion ligand with Tl-O distances of 2.618 (8)-3.202 (8) angstrom. The coordinated oxygens involve the carboxylate 02 in addition to the five oxygens in 1. The other side of the metal ion is completely naked and exposed to a nonpolar environment provided by hydrophobic alkyl groups of neighboring molecules in the crystal structure. This is the first ionophore-metal complex that lacks three-dimensional nonpolar enclosure of the metal ion, In both salts, the conformational rigidity of the lasalocid A anion molecule is preserved: the pseudocyclic conformation is stabilized by "head-to-tail' O1...H-O8 and also by O2...H-O4 hydrogen bonds, with most of the polar oxygens directed inward to capture the metal ion and with all nonpolar groups outward. The formation of two polymorphic structures under similar crystallization conditions indicates that the Tl+-phenyl eta-6-bonding in salt 1 is weak. Crystallographic details: for 1, Tl+(C34H53O8-), space group P2(1)2(1)2(1), a = 22.851 (2) angstrom, b = 11.320 (1) angstrom, c = 13.790 (2) angstrom, V = 3567.1 (8) angstrom3, Z = 4, R(F) and R(wF) of 0.056 and 0.036, respectively, for 2573 reflections with F(o) > 5-sigma(F(o)); for 2, Tl+(C34H53O8-), space group P2(1)2(1)2(1), a = 18.928 (5) angstrom, b = 18.649 (6) angstrom, c = 10.054 (1) angstrom, V = 3549 (1) angstrom3, Z = 4, R(F) and R(wF) of 0.048 and 0.028, respectively, for 2041 reflections with F(o) > 5-sigma(F(o)).