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 poi nt of view, the effect in the loss of dihydrogen from the niobocene [Cp2NbH 3] complex upon addition of three different Lewis acids (in increasing orde r 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. Th e 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 ba rrier. In all the cases the Lewis acid favors the dihydrogen elimination pr ocess as compared with the noncatalysed H-2 elimination from the niobocene trihydride. The products are also greatly stabilized as the presence of a H BR2 Lewis acids allow the final complex to remain coordinatively saturated upon formation of an eta(2)-BH2R2 complex. In BF3 this complex cannot stric tly be formed, but here the fluorine plays the role of the missing hydrogen . Finally, the implications of the different energy profiles in the kinetic s of the whole process are discussed.