From an icosahedron to a plane: Flattening dodecaiodo-dodecaborate by successive stripping of iodine

It has been shown by electrospray ionization-ion-trap mass spectrometry that B ₁₂I ₁₂ ^2- converts to an intact B ₁₂ cluster as a result of successive stripping of single iodine radicals or ions. Herein, the structure and stability of all intermediate B ₁₂I _n ^- species (n=11 to 1) determined by means of first-principles calculations are reported. The initial predominant loss of an iodine radical occurs most probably via the triplet state of B ₁₂I ₁₂ ^2-, and the reaction path for loss of an iodide ion from the singlet state crosses that from the triplet state. Experimentally, the boron clusters resulting from B ₁₂I ₁₂ ^2- through loss of either iodide or iodine occur at the same excitation energy in the ion trap. It is shown that the icosahedral B ₁₂ unit commonly observed in dodecaborate compounds is destabilized while losing iodine. The boron framework opens to nonicosahedral structures with five to seven iodine atoms left. The temperature of the ions has a considerable influence on the relative stability near the opening of the clusters. The most stable structures with five to seven iodine atoms are neither planar nor icosahedral. Flat out: When icosahedral B ₁₂I ₁₂ ^2- is fragmented in mass spectrometry and its iodine atoms are stripped off one by one, it eventually forms a planar B ₁₂ unit (see figure). This transition occurs when seven to five iodine atoms remain.