STRUCTURE OF INTRAMOLECULAR BORON-AMINE COMPLEXES AND PROPOSAL OF TETRAHEDRAL CHARACTER FOR CORRELATION BETWEEN MOLECULAR-STRUCTURE AND BARRIER TO DISSOCIATION OF THE N-B BONDS

The molecular structures of a set of intramolecular boron-amine complexes, 9-[2-(dialkylaminomethyl)phenyl]-9-borabicyclo[3.3.1]nonanes (alkyl=Me and Et) and 2-[2-(dimethylaminomethyl)phenyl]-4,4-diphenyl-1,3,2-dioxaborolane, were determined by the X-ray analyses. The boron atom has a tetrahedral geometry and the five-membered ring is puckered with the nitrogen atom out of the plane in every complex. The distances of the N-B coordination bonds, which are in the range of 1.74-1.77 angstrom, are not always correlated to the barrier to dissociation of the N-B bonds. In order to correlate the molecular structures to the strength of the N-B coordination bonds, the tetrahedral character, which is calculated from bond angles at a boron atom, is proposed. The usefulness as well as the limitation of this parameter are discussed.