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.
ABSTRACT
Bis(tetraethylammonium) dodecamethyl-closo-dodecaborate(2-), [NEt(4)](2)[closo-B(12)Me(12)], [NEt(4)](2)2, was prepared employing modified Friedel-Crafts reaction conditions from [NEt(4)](2)[closo-B(12)H(12)], [NEt(4)](2)1, trimethylaluminum, and methyl iodide. The [NEt(4)](2)2 salt provides sufficient solubility in water to allow the synthesis of the important alkali metal salts A(2)2 (A = Li, Na, K, Rb, Cs) using cation-exchange procedures. The solid state structure of colorless [AsPh(4)](2)2 reveals a nearly perfect icosahedral B(12) cluster with B-B bonds ranging from 1.785(3) to 1.807(3) A and B-C bonds of 1.597(3)-1.625(3) A. In contrast, the crystal structure of dark-red [Py(2)CH(2)]2 (obtained from [NEt(4)](2)2 and [Py(2)CH(2)]Br(2)) contains a distorted icosahedral dianion [B-B = 1.740(13)-1.811(14) A, B-C = 1.591(13)-1.704(13) A]. In the [Py(2)CH(2)]2 salt, the dianion 2(2-) and its dipositive dipyridiniomethane counterion form a red charge-transfer complex. One-electron oxidation of 2(2)(-) by ceric(IV) ammonium nitrate affords the blue, air-stable radical [hypercloso-B(12)Me(12)](*-), dodecamethyl-hypercloso-dodecaborate(1-), 2(*-), isolated as the PPN salt. X-ray crystallography reveals that the geometries of the B(12) clusters observed in hypercloso-[PPN]2 and closo-[AsPh(4)](2)2 are identical and essentially undistorted icosahedra. The anion in the [PPN]2 structure contains B-B bonds ranging from 1.784(8) to 1.806(7) A and a range of B-C bonds from 1.596(7) to 1.616(7) A.
ABSTRACT
The closo-[B12H12-n(OH)n]2- (n = 1-4) ions have been synthesized by the reaction of cesium dodecahydro-closo-dodecaborate(2-), Cs21, with aqueous sulfuric acid. Variation of the reaction temperature, time, and acid concentration results in the stepwise introduction of from one to four hydroxyl groups. Each individual hydroxylation step proceeds regioselectively, affording only one isomer per step. Further substitution of the hydroxylated cluster preferentially takes place at a B-H vertex meta to a B-OH vertex. The closo-[B12H12-n(OH)n]2- (n = 1-4) species, designated 2-5, respectively, are characterized by one- and two-dimensional 11B NMR spectroscopy, IR spectroscopy, and high-resolution fast atom bombardment (FAB) mass spectrometry. A rationale that qualitatively explains the influence of the hydroxyl group on the chemical shifts of the individual boron vertices is developed. Furthermore, the solid state structures of closo-[B12H11(OH)]2-, 2, and closo-1,7-[B12H10(OH)2]2-,3, are determined by X-ray diffraction. Crystallographic data are as follows: For [MePPh3](2)2, monoclinic, space group P2(1)/n, a = 890.1(5) pm, b = 1814(1) pm, c = 1270.5(7) pm, beta = 101.66(2) degrees, Z = 2, R = 0.055; for [MePPh3](2)3, monoclinic, space group P2(1)/n, a = 887.6(4) pm, b = 1847.2(8) pm, c = 1271.1(5) pm, beta = 101.17(1) degrees, Z = 2, R = 0.065. In addition, synthetic routes to O-derivatized species of the anions 2-5 such as closo-[B12H11(OTiCpCl2)]2-, 7, closo-1,7-[B12H10(OTiCpCl2)2]2-, 8, closo-1,7,9-[B12H9(OTiCpCl2)3]2-, 9, closo-[B12H11(OCONHPh)]2-, 10, and closo-1,7-[B12H10(OSO2Me)2]2-, 11, are described. The crystal structures of 7 and 11 are determined by single-crystal X-ray diffraction. Crystallographic data are as follows: For [MePPh3](2)7, monoclinic, space group Cc, a = 2530.5(2) pm, b = 1653.3(1) pm, c = 1281.3(1) pm, beta = 118.79(2) degrees, Z = 4, R = 0.085; for [HPy](2)11, monoclinic, space group P2(1)/n, a = 1550.9(8) pm, b = 993.1(5) pm, c = 1726.5(9) pm, beta = 112.36(2) degrees, Z = 4, R = 0.061.
ABSTRACT
No explosion, but per-B-hydroxylation occurs if the icosahedral boron hydrides [closo-B12 H12 ](2-) (see picture), [closo-CB11 H12 ](-) , or closo-1,12-(CH2 OH)2 -1,12-C2 B10 H10 are refluxed in 30 % hydrogen peroxide. Thus, the three isoelectronic species [closo-B12 (OH)12 ](2-) , [closo-1-H-1-CB11 (OH)11 ](-) , and closo-1,12-H2 -1,12-C2 B10 (OH)10 were obtained. â=BH, â=BOH.
