Reactions and Structure of Electron-Precise Triborane(5) and Tetraborane(6) Derivatives

An improved synthesis of B3(NMe2)5 (2), B4(NMe2)6 (1), and B6(NMe2)8 (4) is reported. From each compound two of the four terminal dimethylamino groups are readily replaced by halogen atoms yielding alpha,omega-dihalides B(n)(NMe2)(n)X2 (n = 3, 4; X = Cl, Br, I). These are used in turn to replace the substituent X by groups like RO, RS, RHN, R2P, and R. Crystal structure determinations on several of these compounds reveal as a common feature planar (C2N)BB2 and (C2N)B(B)X groups which are arranged in an approximately orthogonal manner to one another with NBBN dihedral angles ranging from 69 to 104.6-degrees. Consequently, there is no significant pi bonding between the boron atoms.