Mechanism of the opening of the closo-NB11 clusters
by bases*
Peter Paetzold, Jens M�ller, Franc Meyer, and Petra Lomme
Institut f�r Anorganische Chemie, Technische Hochschule
Aachen, D-52056 Aachen, Germany
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 [RNB11H11X]�, 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 RNB11H11
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).
*Lecture presented at the XIth International Meeting
on Boron Chemistry (IMEBORON XI), Moscow, Russia, 28 July - 2 August
2002. Other presentations are published in this issue,
pp. 1157-1355.
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