Boratranes with all six-membered rings or with two different ring sizes: Synthesis, characterization, and X-ray crystal structures

New types of boratranes B[(O-o-ArCH2)3−nN(CH2CH2O)n] (Ar = 2,4-di-MeC6H2; n = 0, 1; n = 1, 2; n = 2, 3) featuring three six-membered chelating ring (1) to two five-membered chelating rings (3) in a stepwise fashion through 2 have been synthesized by the reaction of B(OMe)3 with a series of tetradentate ligands such as (HO-o-ArCH2)3−nN(CH2CHOH)n (Ar = 2,4-di-MeC6H2; n = 0, L1H3; n = 1, L2H3; n = 2, L3H3). Compounds 1–3 have been characterized by solution 1H, 13C{1H} and, 11B NMR and the crystal structures of 1 and 3 have been determined by single crystal X-ray diffraction. Compounds 1 and 3 are the first structurally characterized boratrane derivatives bearing all six-membered rings and mixed ring systems in the atrane cage, respectively. Especially, the molecular structures of 1 revealed that its transannular N–B bond length of 1.6322(19) Å is the shortest among the previously reported boratrane and its derivatives.

Graphical abstractA novel series of three boratrane derivatives featuring a stepwise change in ring size from three six-membered rings to one six-membered and two five-membered rings were prepared. X-ray analysis revealed that the coordination around boron and nitrogen was a slightly distorted tetrahedral geometry and the tricyclic cage moiety resembled a three-bladed turbine of C3 or Cs symmetry.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► (HO-o-ArCH2)3−nN(CH2CH2OH)n (n = 0–2) were used as ligands. ► B(OMe)3 was used as a boron precursor. ► Boratrane derivatives with (HO-o-ArCH2)3−nN(CH2CH2OH)n (n = 0–2) were prepared. ► The tricyclic cage moiety resembled a three-bladed turbine of C3 or Cs symmetry. ► The N–B bond length of 1.6322(19) Å is the shortest among reported boratranes.