Eight-vertex polyhedral monocarbaborane chemistry. Three closo anions, [CB7H8]–, [CB7H7I]– and [CB7H6I2]–. Preparation and structural studies

Abstract

Salts of the eight-vertex [closo-1-CB₇H₈]^– monoanion can be prepared from arachno nine-vertex ligand adducts exo-6-L-4-CB₈H₁₂ by base-degradation reactions. Iodination of [closo-1-CB₇H₈]^– with elemental iodine yields the [1-CB₇H₇-7-l]^– and [1-CB₇H₆-7,8-l₂]^– monoanions, characterised by single-crystal X-ray diffraction analysis. Crystals of [PPh₄][1-CB₇H₇-7-I] were triclinic, space group P, with a= 7.4703(7), b= 12.766(2), c= 13.9102(12)Å, α= 100.973(10), β= 92.886(7), γ= 93.005(10)° and Z= 2; structure refined for 3867 data collected at 150 K to wR₂= 0.0348. Crystals of [PPh₄][1-CB₇H₆-7,8-I₂]^– were triclinic, space group P, with a= 7.8389(8), b= 13.4489(11), c= 13.6337(13)Å, α= 98.009(9), β= 93.060(7), γ= 92.939(7)° and Z= 2; structure refined for 4284 data collected at 200 K to wR₂= 0.0504. The {CB₇} cluster geometries of the two anions approximate closely to regular triangulated dodecahedral, with no sign of any opening. The three anions are fluxional to ca. 180 K; structures for the possible intermediates are briefly discussed.