Dodecahedryl anion formation and an experimental determination of the acidity and C-H bond dissociation energy of dodecahedrane

Dodecahedryl anion (1a) was generated in a Fourier transform mass spectrometer by deprotonation of dodecahedrane (1). Examination of the acid and base behavior of 1 and 1a, respectively, enabled the acidity of 1 to be determined (ΔH°(acid) = 402 ± 2 kcal/mol). Good agreement is found with the 298 K computed MP2/6-31+G(d)//HF/6-31+G(d) value of 405.1 kcal/mol. In a similar manner, the electron affinity of dodecahedryl radical was measured (EA = 4 ± 2 kcal/mol). These results were combined in a thermodynamic cycle to afford the C-H bond dissociation energy of dodecahedrane (BDE = 92 ± 3 kcal/mol), which is reasonably well reproduced (96.7 kcal/mol) at the MP2//HF level but leads to the suggestion that the reported heat of hydrogenation of dodecahedrene is in error. The DePuy kinetic method for measuring the acidity of 1 also was explored. It was found that this approach works well with triphenylsilyldodecahedrane but gives poor results with triethylsilyldodecahedrane. This latter failure is attributed to steric effects, and provides a rationale for several problem cases and a means to overcome these difficulties.