Crystal Structures of Rubidium and Cesium Anthranilates and Salicylates

In an attempt to probe a potential template role of the large alkali-metal cations rubidium and cesium in the organization
of biorelevant ligands, salicylate and anthranilate complexes of the two elements were prepared and structurally
investigated. The studies were also expected to show the marked structural differences compared to the corresponding
thallium(I) compounds. Rubidium anthranilate and cesium salicylate could be crystallized as the monohydrates
Rb(Anth)(H2O) and Cs(Sal)(H2O). Both have layer structures containing the cations and the polar groups of the
ligands in core domains sandwiched by the aromatic rings above and below. The metal atoms have coordination
numbers 7 and 8, respectively, with an irregular coordination sphere made up exclusively of oxygen atoms. Crystalline
material with a 1:2 stoichiometry, Cs[H(Anth)2], is obtained from aqueous solutions of Cs(Anth) upon absorption of
carbon dioxide with concomitant formation of cesium bicarbonate, Cs(HCO3). The crystal structure of Cs(HCO3)
was redetermined to obtain precise benchmark data for cesium carbonates and carboxylates. The cesium hydrogen
bisanthranilate also has a layer structure with eight-coordinate cesium atoms. The coordination sphere includes
one nitrogen donor atom. The organization of all layer structures appears to be governed mainly by steric effects
and electrostatic forces with very little directional influence of the cations. This result suggests that the large alkali
metals have no efficient template effect for the organization of biological substrates and can explain the low toxicity
of rubidium and cesium salts.