STEREOSELECTIVE ANION TEMPLATE EFFECTS - SYNTHESES AND MOLECULAR-STRUCTURES OF TETRAPHENYL [12]MERCURACARBORAND-4 COMPLEXES OF HALIDE-IONS

The synthesis and characterization of the chloride and iodide ion complexes of a set of isomeric hydrophobic tetraphenyl [12]mercuracarborand-4 hosts having sterically encumbered cavities are described. These compounds [(3-C6H5-1,2-C2B10H9Hg)(4)]. XLi (3 . XLi, X = CL(-) and I-) were prepared by the reaction of the corresponding mercuric halides with closo-3-Ph-1,2-Li-2-1,2C2B10H9. The presence of phenyl substituents in this system requires the existence of isomers in which the phenyl substituents are arrayed about the periphery of the host cavity. Three of four possible stereoisomers of the chloride ion complex were detected by HPLC analysis of the reaction mixtures, separated by recrystallization, identified as the desired species by the negative ion FAB mass spectrum, and structurally characterized by crystallography, X-ray diffraction data are as follows: 3a . ClLi (1,2-alternate structure with phenyl substituents in an ''up'', ''up'', ''down'', ''down'' fashion): monoclinic, space group P2/a, a = 24.080(1) Angstrom, b = 13.4509(8) Angstrom, and c = 26.054(2) Angstrom, beta = 99.488(2)degrees, V = 8323 Angstrom(3), Z = 4, rho(calcd) = 1.61 g cm(-3), R = 0.056, R(w) 0.067. 3b . ClLi (1,3-alternate structure; phenyl substituents ''up'', ''down'', ''up'', ''down''): triclinic, space group P (1) over bar, a = 12.8692(9) Angstrom, b = 18.126(1) Angstrom, and c = 18.309(1) Angstrom, a = 114.593(2)degrees, beta 100.016(2)degrees, and gamma 97.259(2)degrees, V = 3729 Angstrom(3), Z = 2, R = 0.076, R(w) = 0.097. 3c . ClLi (partial cone structure; phenyl substituents ''up'', ''up'', ''up'', ''down''): monoclinic, space group Cm, a 16.405(2) Angstrom, b = 18.928(3) Angstrom, and c = 10.409(1) Angstrom, beta = 112.610(4)degrees, V = 2984 Angstrom(3), Z = 2, rho(calcd), and 1.91 g cm(-3), R = 0.032, R(w) = 0.045. This molecule has a mirror plane. In contrast to these results, only one of four possible stereoisomers of the iodide ion complex was detected by HPLC analysis of the reaction mixtures. This species, 3b . ILi, was found to be a 1,3-alternate structure by X-ray analysis (phenyl substituents ''up'', ''down'', ''up'', ''down'') and was further characterized by multinuclear NMR and negative ion FAB mass spectroscopy. Crystal data: tetragonal, space group I4(1)/amd, a = 17.520(2) Angstrom, c = 35.432(5) Angstrom, V = 10876 Angstrom(3), Z = 4, rho(calcd) and = 1.23 g cm(-3), R = 0.069, R(w) = 0.095. Molecular modeling results preclude the formation of analogous 1,2-alternate or partial cone structures of the iodide ion complex. An acyclic host, 4, having two Lewis-acidic mercury atoms and three carborane units was prepared by reacting 3-Ph-1,2-Li-2-1,2-C2B10H9 with Hg(O2CCH3)(2) This species forms a 1:1 complex with ethanol in the solid state. Crystal data for 4 .(C2H5OH): monoclinic, space group P2(1)/c, a 14.542(4) Angstrom, b = 17.861(5), c = 20.701(6) Angstrom, beta = 99.04(1)degrees, V = 5310 Angstrom(3), Z = 4, rho(calcd) = 1.21 g cm(-3), R = 0.072, R(w) = 0.083. The halide ion-specific kinetic template effects displayed by Cl- and I- ions during the stereoselective cyclization reaction are discussed.