On the nature of the B⋯N interaction and the conformational flexibility of arylboronic azaesters

Abstract

It is shown that the structural and physicochemical properties of arylboronic azaesters result from the properties of the key B–N bond. This bond is different in arylboronic azaesters from typical single/double/triple B–N bonds present in other boron–nitrogen compounds. By studying a model example, 6-tert-butyl-2-(3′,5′-difluorophenyl)-(N–B)-1,3,6,2-dioxazaborocane, it is proved that the molecule adopts the closed form in the solid state with the B⋯N bond length equal to 1.7646(3) Å. This represents the longest B–N bond length for an arylboronic ester reported in literature. According to the results of experimental charge density studies and QTAIM analysis, the B⋯N bond/contact shows good separation of the respective atomic basins and significant flexibility. The source function and NBO analyses indicate an evident involvement of the oxygen atoms in the creation of the B–N bonding. According to our DFT calculations the isolated model molecule exists in the open conformer. The constrained energy scans reveal an E(d_B–N) potential energy surface which depends strongly on the nature of the substituents at the nitrogen and aromatic carbon atoms. It appears that an appropriate substitution can produce either conformer. A dipole moment analysis provides a good insight into the reactivity properties of the compound in solution. The opening of the azaester cage is associated with a breaking of the B–N bond, thus decreasing the magnitude of the dipole moment. The existence of the open conformer is confirmed by multi-temperature NMR studies. When decreasing the temperature of the measurements the ¹H and ¹¹B NMR spectra show features corresponding to the open form of the model arylboronic azaester.