Quantum Chemical Studies of 6-Vertex hypercloso- Heteroboranes

Abstract

Boron hydride clusters have always fascinated chemists because of their electron-deficient chemistry, aesthetically pleasing three dimensional cluster shapes and because of numerous applications. Boron hydride clusters have several cluster types i.e. hypercloso-, closo- nido-, arachno- and hypho- clusters. There has been recent interest in determining the structure of the thermodynamically most stable 6-vertex-hypercloso-carboranes. In this thesis computational quantum mechanical studies have been extended to 6-vertex hypercloso-X2B4H4 (X = C, Si, Ge) clusters. The computations are carried out using DFT for these hypercloso-heteroboranes of carbon, silicon and germanium at the B3LYP/LANL2DZ+ZPE level of theory. A large number X2B4H4 (X = C, Si, Ge) isomers were optimized. The optimization results of hypercloso silaboranes (Si2B4H4) and germaboranes (Ge2B4H4) indicate that unlike carboranes (C2B4H4), the thermodynamically most stable sila- and germaborane structures constitute themselves in cluster shape (and not planar form). The reason behind this structural transition in the cluster shape upon heteroatom replacement is the degree of electron-localization (electronegativity) of the heteroatom. When less electronegative silicon and germanium atoms are placed in the 6-vertex hypercloso-heteroborane structures, the thermodynamically most stable structure adapts cluster shape. This is because more electronegative carbon heteroatom has strong destabilizing effect on the cluster shape in the electron deficient environment of hypercloso-C2B4H4 cluster, whereas less electronegativity of silicon and germanium atoms can be tolerated in the hypercloso-cluster. The results are completely consistent with previous work on 12-vertex closo-heteroboranes.