Theoretical study of the structural and fluxional behavior of copper(I)-octahydrotriborate complex

The geometrical structures of [B3H8](-) and [ClCuB3H8](-) compounds have be en investigated by means of ab initio calculations, using the MP2 level of theory, and density functional theory (DFT) method. The [ClCuB3H8](-) (3a) complex structure, in which two boron atoms were bound to the copper atom v ia two B-H-Cu bridge hydrogen bonds (one bridge bond for each boron), was t he most stable. Its MP2 and DFT calculated geometry is comparable to that o f B4H10. In addition, the DFT calculated vibrational frequencies are in goo d agreement with experimental values. On the other hand, the most favorable interconversion of 3a structure was found. to proceed with a low activatio n barrier (6.5 kcal mol(-1)). This is consistent with the NMR spectra and c onfirms the rapid fluxional behavior for this complex. Moreover, the isomer ization leading to 3a, from the less stable structure 3c, also proceeds wit h a low energy barrier (5.2 kcal mol(-1)), whereas the activation barrier o f reverse rearrangement is negligible. The calculated energy barriers of th e rearrangement between the less stable structures 3b and 3c are also very small (1.2 kcal mol(-1)).