The large closo-borane dianions, BnHn2- (n = 13-17) are aromatic, why are they unknown?

The relative stabilities of the unknown larger closo-borane dianions BnHn2- (n = 13-17), were evaluated at the B3LYP/6-31G* level of density functiona l theory by comparing the average energies, E/n, and also by the energies u sing the model equation: Bn-1Hn-12- + B6H10 --> BnHn2- + B5H9 (n = 6-17). S tarting with the small closo-borane, B5H52-, the sequential addition of BH groups is represented by formal transfer from B6H10 to build up larger and larger clusters. Most of the energies for these sequential steps are exothe rmic, but not for the B12H122- to B13H132- and the B14H142- to B15H152- sta ges. The cumulative total energies (Delta H-add) Of these BH group addition s, based on B5H52- as the reference zero, tend to increase with increasing cluster size. Delta H-add indicates that the larger unknown closo-boranes B 13H132- to B17H172- are more stable than B9H92-, B10H102-, and B11H112-; th is agrees with E/n and with Lipscomb's earlier conclusion based on the PRDD O average energies. B13H132-, B14H142-, and B15H152- are less stable than B 12H122-, which has the lowest average energy on a per vertex basis among th e closo-borane dianions. However, the total Delta H-add treatment indicates the larger B16H162- and B17H172- to be favorable relative to B12H122-, bec ause of the larger number of vertexes. The formation of B13H132- from B12H1 22- is especially unfavorable. The further formation of B14H142- and B15H15 2- via BH transfer also is endothermic. These are not the only thermodynami c difficulties in building up large closo-borane dianions beyond B12H122-. The highly exothermic disproportionation of larger and smaller closo-borane dianions, e.g., B12+nH12+n2+ B12-nH12-n2- --> 2B(12)H(12)(2-) (n = 1-5), a lso indicate possible synthetic problems in preparing larger closo-boranes with more than 12 vertexes under condition where smaller boranes are presen t. All the larger closo- BnHn2- (n = 13-17) cluster exhibit "three-dimensio nal aromaticity", judging from the computed Nucleus Independent Chemical Sh ifts (NICS), which range from -30.9 to -36.5 ppm. The trends in MCS values are similar to the variations in the bond length alternations, Delta r: Thu s, the qualitative relationships between geometric and magnetic criteria of aromaticity found earlier for the smaller clusters extends to the larger c loso-borane dianions, BnHn2- (n = 13-17).