Dodecahydroxy-closo-dodecaborate(2-).

Abstract

The cesium salt of the icosahedral borane anion dodecahydroxy-closo-dodecaborate(2-), Cs(2)[closo-B(12)(OH)(12)], Cs(2)1, was prepared by heating cesium dodecahydro-closo-dodecaborate(2-), Cs(2)[closo-B(12)H(12)], Cs(2)2, with 30% hydrogen peroxide. The other alkali metal salts A(2)1 (A = Li, Na, K, Rb) precipitated upon addition of ACl to warm aqueous solutions of Cs(2)1. The ammonium salt, [NH(4)](2)1, and the (mu-nitrido)bis(triphenylphosphonium) salt, [PPN](2)1, were obtained similarly. The [H(3)O](2)1 salt precipitated upon acidification of aqueous solutions of Cs(2)1 with hydrochloric acid. The solubility of these salts in water was determined by measuring the boron content of saturated aqueous solutions of A(2)1 (A = Li, Na, K, Rb, Cs), [H(3)O](2)1, and [NH(4)](2)1 using ICP-AES. Although these salts are derived from a dianion with twelve pendant hydroxyl groups, the alkali metal salts surprisingly displayed low water solubilities. Water solubility decreases with a decrease in the radius of A(+), except for the lithium salt, which is slightly more soluble than the potassium salt. The [H(3)O](2)1 and the [NH(4)](2)1 salts provide rare examples of water-insoluble hydronium and ammonium salts. The low water solubility of the A(2)1 salts is attributed to the dianion's pendant hydroxyl groups, which appear to function as cross-linking ligands. Four alkali metal salts, A(2)1 (A = Na, K, Rb, Cs), were characterized in the solid state by single-crystal X-ray crystallography. These data revealed intricate networks in which several anions are complexed through their hydroxyl groups to each alkali metal cation. In addition, the anions are engaged in hydrogen bonding with each other and, if present, with water of hydration. This cross-linking results in the precipitation of aggregated salts. Cation coordination numbers decrease with cation radius. Thus, cesium and rubidium are ten-coordinate, whereas potassium is seven-coordinate and sodium is six-coordinate. The geometry of anion 1(2)(-) is independent of cation identity; the B-B and B-O bond lengths of the various A(2)1 salts (A = Na, K, Rb, Cs) are identical.