B=N Bonds and BCN Rings – Reactivity and Charge Density Studies †‡

(F5C6)(2)BNMe2 (1) was prepared and (F3C)(2)BNMe2 (2) was resynthesized. The crystal structures of both compounds were determined. In contrast to 2, compound 1 does not react with diazomethanes under carbine insertion into the BN bond. In this way the known (F3C)(2)BCH(SiMe3)NMe2 (3) was resynthesized from 2 and its solid-state structure was determined. The electron densities of compounds 1, 2, and 3 and those of the simpler model compound (F3C)(2)BCH2NMe2 (4) as well as that of the BN/CC isoelectronic compound (F3C)(2)CCH2CMe2 (5) for comparison were calculated and their topologies analysed in terms of the Quantum Theory of Atoms in Molecules (QTAIM). Despite 2 has a shorter B-N bond in calculations and in the crystal than 1, the atomic charges and electron density parameters to QTAIM are very similar for both compounds and cannot explain the different reactivity. The three-membered rings 3 and 4 do not show bond-critical points for their B-N bonds. The nature of the B-N bond region was also analysed in terms of the source function contributions in QTAIM, which demonstrated the importance of the close proximity of a third atom in three-membered rings for the charge density topologies in such systems.