Exotic Structures of Si₂B₂H₄

Abstract

The potential energy surface of Si₂B₂H₄, explored ab initio at the MP2/6-31G and density functional theory, Becke3LYP/6-311+G^∗∗ levels, is rich in detail. Twelve minima were found out of the 39 stationary points located. 1,3-Disiladiboretene (1a), with a puckered four-membered ring, is predicted to be the global minimum. Its calculated inversion barrier (1.2 kcal/mol), however, is much lower than that of the analogous 1,3-diboretene (17.0 kcal/mol). The relative energies of two other Si₂B₂H₄ isomers (5 and 7) with B-H-Si bridges are only 2.0 kcal/mol above 1a. The relative energies of 9 and 15a, both with planar pentacoordinate borons, are 7.3 and 27.1 kcal/mol. The acyclic (18) and linear (22a,b) isomers are noncompetitive energetically. Many other Si₂B₂H₄ silaboranes, possessing similar structures and bonding as the corresponding carboranes, are calculated to be saddle points. In general, hydrogen bridging and cyclic π-delocalization stabilize Si₂B₂H₄ isomers, while structures with multiply bonded silicon are unfavorable.