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 comp lexes of a set of isomeric hydrophobic tetraphenyl [12]mercuracarboran d-4 hosts having sterically encumbered cavities are described. These c ompounds [(3-C6H5-1,2-C2B10H9Hg)(4)]. XLi (3 . XLi, X = CL(-) and I-) were prepared by the reaction of the corresponding mercuric halides wi th 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 detec ted by HPLC analysis of the reaction mixtures, separated by recrystall ization, identified as the desired species by the negative ion FAB mas s spectrum, and structurally characterized by crystallography, X-ray d iffraction data are as follows: 3a . ClLi (1,2-alternate structure wit h 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 = 8 323 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) Angstr om, 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 . Cl Li (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 co mplex was detected by HPLC analysis of the reaction mixtures. This spe cies, 3b . ILi, was found to be a 1,3-alternate structure by X-ray ana lysis (phenyl substituents ''up'', ''down'', ''up'', ''down'') and was further characterized by multinuclear NMR and negative ion FAB mass s pectroscopy. Crystal data: tetragonal, space group I4(1)/amd, a = 17.5 20(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 mod eling results preclude the formation of analogous 1,2-alternate or par tial cone structures of the iodide ion complex. An acyclic host, 4, ha ving two Lewis-acidic mercury atoms and three carborane units was prep ared by reacting 3-Ph-1,2-Li-2-1,2-C2B10H9 with Hg(O2CCH3)(2) This spe cies 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) Angstro m, 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 di scussed.