Mechanism of the opening of the closo-NB11 clusters by bases

Abstract

The icosahedral aza-closo-dodecaboranes RNB11H11 (R = H, Me, Ph) are opened by neutral bases L or anionic bases X- to give the nido-species RNB11H11L or [RNB11H 11X]-, respectively, with a nonplanar pentagonal aperture; the N atom and a BHB hydrogen bridge are situated at opposite sides of the aperture. The BL or the BX vertex is found in the aperture either adjacent to the hydrogen bridge (type 1; C1) or adjacent to nitrogen (type 2; C1) or off the aperture adjacent to nitrogen on a mirror plane (type 3; Cs). At any rate, the isomer of type 1 is the primary product, which may rearrange to yield an isomer of type 3 via an isomer of type 2. Working in deuteromethanol shows that the bridging H atom originates from the primarily attacked BH vertex. The process from RNB11H 11 to its base adduct of type 3 includes the opening and the closure of skeletal BN bonds and the jumping of the extra-H atom from endo into bridging positions and vice versa, whereas the base does not alter its position. The application of the opening process to a series of aza-closo-dodecaboranes with non-hydrogen boron ligands confirms that only atoms of the ortho-belt of the starting material are involved in structural changes. The elementary steps from the closo-species to the three isomers are identified as a [3c,1c] collocation and subsequent [3c,2c] translocations in the picture of molecular orbitals localized over three or two centers or to one center (lone pair).