Contributions to the Chemistry of Boron, LXXV The Crystal and Molecular Structure of a Tetraazadiborine: 3,6-Bis(dimethylamino)-1,2,4,5-tetramethyl-1,2,4,5,3,6-tetraazadiborine

Abstract

The title compound crystallizes in the monoclinic system with dimensions of the unit cell a = 14.09(2), b = 8.60(1), c = 11.34(1) Å and β = 101.7°. The space group was determined to be A 2/a with four molecules in the unit cell. The structure was solved by direct methods, and all hydrogen atoms were located. Full matrix refinement led to R = 0.059. The B2N4 six membered ring is nonplanar as expected for an 8π electron heterocycle. The molecule possesses C2 symmetry but the ring itself is near the D2 symmetry as derived from earlier CNDO/S-calculations. The BN bond lengths are 1.43 A, the NN distances 1.43 Å and the average CN distances 1.44 A. The boron atoms and four of the six nitrogen atoms are trigonal-planarly coordinated while the remaining two N-atoms (N1,N2) deviate little, but significantly, from sp2 hybridisation. The structure of the molecule can be interpreted as the best compromise between ΒΝ-π-bonding, nonbonding repulsion between adjacent CH3 groups, and the interaction between neighbouring ‘free pairs of electrons’ of the ring N-N-units.