Gas-Phase Positive-Ion Chemistry of Trimethylboron and Trimethylaluminum

Abstract

Me2B+ and Me2A1+ are the principal ions produced by electron impact on Me3B and Me3Al, respectively. Study of halide transfer to these species leads to determination of the halide affinities: D(Me2B+−F−) = 238.5 ± 3 kcal/mol, D(Me2B+−Cl−) = 192.5 ± 3 kcal/mol, D(Me2Al+−F−) = 228 ± 8 kcal/mol, and D(Me2Al+−Cl−) = 185 ± 5 kcal/mol. These show about 9 kcal/mol greater stability for Me2Al+ compared to Me2B+. Me3B reacts by methyl anion transfer with CF3+ and CF2Cl+ but not with CCl3+, C3H7+, or CHCl2+, establishing a value for the methyl anion affinity of Me2B+ of D(Me2B+−CH3−) = 250 ± 15 kcal/mol. In a related reaction also involving carbon-carbon bond formation, CHCl2+ and CFCl2+ are alkylated by Me3Al to C2H4Cl+ and C2H3FCl+, respectively. Other thermochemical determinations include the hydride affinity of Me2B=CH2+: D(Me2BCH2+−H−) = 260.5 ± 5 cal/mol. A variety of molecules (L) condense with Me2B+ to yield one-ligand complexes and with Me2Al+ to yield two-ligand complexes. The relative order of ligand binding energies for these molecules to both species is determined from the preferred direction of displacement reactions: for Me2B(L)+, Me2S < MeCN < C6H5OMe < Me2O < C6H5CN < pyridine, and for Me2Al(L)2+, Me3Al < Me2S < C6H5OMe < Me2O < MeCN < Et2O ≃ (i-C3H7)2O < C6H5CN < Me3N < pyridine. The relative position of MeCN in these series indicates that Me2Al+ is a relatively softer acid than Me2B+ or Al+. © 1982, American Chemical Society. All rights reserved.