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 cloao-3-Ph-1,2-Li₂-1,2-C₂B₁₀H₉. 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-altemate structure with phenyl substituents in an “up”, “up”, “down”, “down” fashion): monoclinic, space group P2/a, a = 24.080(1) Å, b = 13.4509(8) Å, and c = 26.054(2) Å, β = 99.488(2)°, V = 8323 ų, Z = 4, ϱ_{c aled}= 1.61 g cm^-3, R = 0.056, R_w= 0.067. 3b• ClLi (1,3-altemate structure; phenyl substituents “up”, “down”, “up”, “down”): triclinic, space group P1, a = 12.8692(9) Å, b = 18.126(1) Å, and c = 18.309(1) Å, α = 114.593(2)°, β = 100.016(2)°, and γ = 97.259(2)°, V = 3729 ų, 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) Å, b = 18.928(3) Å, and c = 10.409(1) Å, β= 112.610(4)°, V = 2984 ų, Z = 2, ϱ_calcd= 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-altemate 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 I4₁/amd, a = 17.520(2) Å, c = 35.432(5) Å, V = 10876 ų, Z = 4, ϱ_calcd= 1.23 g cm ^-3, R = 0.069, R_w= 0.095. Molecular modeling results preclude the formation of analogous 1,2-altemate 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-Li₂-1,2-C₂B₁₀H₉with Hg(O₂-CCH₃)₂- This species forms a 1:1 complex with ethanol in the solid state. Crystal data for 4 • (C₂H₅OH): monoclinic, space group P2₁/c, a = 14.542(4) Å, b = 17.861(5), c = 20.701(6) Å, β = 99.04(1)°, V = 5310 ų, Z = 4, ϱ_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.