Halocyclopropenium-Halide Halogen-Bonded Ion Pairs and Their Hydrogen-Bonded Halide Solvates

A series of salts with a diaminohalocyclopropenium cation and halide anion [C₃(NⁱPr₂)₂X]X (X=Cl ([ 1 ]Cl) or Br ([ 2 ]Br) were isolated with a variety of solvates and, in one case, as a co-crystal with hydronium chloride. In particular, the initial synthesis of [ 1 ]Cl formed a co-crystal with hydronium and with CH₂Cl₂ solvate ([ 1 ]₂[OH₃Cl₃] ⋅ CH₂Cl₂) upon isolation from acetone/CH₂Cl₂. Recrystallization of this from chloroform gave a dichloroform adduct [ 1 ]Cl ⋅ 2CHCl₃, whereas treatment with ICl formed an octahalide cluster [ 1 ]₂I₄Cl₄. The bromine salt [ 2 ]Br ⋅ C₂H₄Br₂ was prepared by treatment of [ 1 ]Cl with dibromoethane and was isolated as a solvate. The hydronium cation was found as part of a hydronium trichloride cluster [OH₃Cl₃]²⁻ and this, along with a partially-deuterated analogue of [OHD₂Cl₃]²⁻ and [OD₃Cl₃]²⁻, was studied computationally and by mid- and far-infrared spectroscopy. Significant halogen bonds were found between 1 ⁺ or 2 ⁺ and chloride or bromide, respectively. On the other hand, the distance to the octahalide [I₄Cl₄]²⁻ is too long for a halogen bond. Hydrogen bonding from the halides to the halomethane solvates is also significantly stronger than to the cation isopropyl groups. The geometries formed at the halide ions with respect to the halogen bond and strong hydrogen bonds are pyramidal with approximately orthogonal angles.