Structure and Nonrigidity of B9H92- and B9H10-. Comparisons of BnHn2- and BnHn+1- Systems

Ab initio calculations at MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311+G** levels indicate relatively high potential barriers for the intramolecular rearrangement of B 9 H 9 2 - via the single DSD (diamond−square−diamond) or the double DSD mechanism, 28.4 and 21.3 kcal/mol, respectively. However, its open face-protonated form, B 9 H 10 - ( 2b ) is highly fluctional. Two other minima, 2a ( C 2 v ) and 2c ( C 3 v ) have energies 1.1 and 9.7 kcal/mol higher than 2b . Facile rotation of the BH 2 group in 2a is coupled with reversible opening of the boron cage from the closo to a nido form. Structure 2c, with a face-bound proton, is a very shallow minimum along the degenerate rearrangement pathway of 2b . None of the optimized structures of B 9 H 10 - give calculated 11 B chemical shifts corresponding to experiment; the NMR assignments and interpretation need to be refined. Members of the B n H n +1 - series ( n = 6−8, 10, and 12) have both common as well as specific features. All B n H n +1 - species are fluxional, due to rapid proton migration. The additional hydrogen H* rotates over the whole boron octahedron in B 6 H 7 - and over one side of the bipyramid in B 7 H 8 - . In B 10 H 11 -, the “extra” hydrogen H* migrates around the faces of a “polar region” near the apical borons more readily than from pole to the other; as in B 9 H 10 -, H* migration in B 8 H 9 - is accompanied by a skeletal rearrangement. The proton affinities (PA) in the B n H n 2 - series decrease with increasing size, since the Coulomb repulsion in the dianions becomes smaller and the coordination number at the protonation sites increases.