Theoretical study of the effect of lewis acids on dihydrogen elimination from niobocene trihydrides

Dihydrogen elimination from polyhydride metallocene complexes is usually a process of high-energy cost. In this paper we study, from a theoretical point of view, the effect in the loss of dihydrogen from the niobocene [Cp2NbH3] complex upon addition of three different Lewis acids (in increasing order of acidity: HBO2C2H2 as a model of catecholborane, BF3, and BH3). Our DFT calculations show that the Lewis acid can interact either with the central or the lateral hydride leading to two different minimum- energy adducts. The lateral adduct is still a dihydride complex in the HBO2C2H2 case, whereas it shows a clear dihydrogen structure in the other two cases. This adduct is the one that leads to dihydrogen elimination. The transition states for this process show that the stronger the Lewis acid, the lower the energy barrier. In all the cases the Lewis acid favors the dihydrogen elimination process as compared with the noncatalysed H2 elimination from the niobocene trihydride. The products are also greatly stabilized as the presence of a HBR2 Lewis acids allow the final complex to remain coordinatively saturated upon formation of an η2-BH2R2 complex. In BF3 this complex cannot strictly be formed, but here the fluorine plays the role of the missing hydrogen. Finally, the implications of the different energy profiles in the kinetics of the whole process are discussed.