An ab initio study of protonation of B_(12)H_(12)~(2-). Structure and non-rigidity of B_(12)H_(13)~- and formation of B_(12)H_(11)~- and B_(24)H_(23)~(3-)

摘要

Protonation of B_(12)H_(12)~(2-) (1) has been studied using ab initio MP2/6-31G~* and density functional B3LYP/6-31G~* and B3LYP/6-31G~(**) calculations and GIAO computation of the ~(11)B NMR chemical shifts. The protonation of 1 leads to the formation of B_(12)H_(13)~- (t, 2a) where the additional hydrogen H~* is located over the center of a boron cage face and the calculated PA is 366.2 kcal/mol, the lowest value in the B_nH_n~(2-) (n = 6 - 10, 12) set. Notably, 2a is non-rigid with respect to migration of H~* which can move from one face to another over the whole boron cage with a barrier of 3.2 kcal/mol via transition state 2b. This migration is expected to occur even at very low temperatures and to render all B atoms equivalent on the NMR time scale. With increasing temperature 2a can rearrange into the B_(12)H_(11)~- centre dot H_2 complex (v, 2c) overcoming a barrier of 8.0 kcal/mol (TS 2d). A local minimum for B_(12)H_(13)~-, 2c is 3.2 kcal/mol above 2a. B_(12)H_(11)~- centre dot H_2, where one of the cage boron atoms shares the electrons of a hydrogen molecule #sigma# pair, has a peculiar structure with acute HBH angle and short HH distance in the BH_2 group. H_2 is lost from 2c to form B_(12)H_(11)~- (3) readily; the endothermicity is only 4.3 kcal/mol. The interaction of 3 and 1 gives the B_(24)H_(23)~(3-) complex, 4. B_(24)H_(23)~(3-) has a D_(5d)-symmetric structure with a symmetric linear BHB bridge connecting two boron cages but is expected to be fluxional with respect to the rotation around the BHB bridge and the BHB bending. In the presence of counterions (e.g. in Li_3B_(24)H_(23)) the BHB bridge bends (152.5 deg) and further interaction with solvent can