Polymorphism of a Model Arylboronic Azaester: Combined Experimental and Computational Studies

Two polymorphic structures of the model arylboronic azaester 6-buty1-2-(4'-bromophenyl)-(N-B)-1,3,6,2-dioxazaborocane [p-BrC6H4BO2(C2H4)(2)NBu] give different C-13 CP/MAS solid-state NMR spectra with the carbon chemical shifts systematically shifted to the higher frequencies in form I relative to form II. Molecular geometries of both forms are similar; however, the molecules in both forms have different conformations of the bicyclic moieties. The most spectacular difference between the molecules are present in Hirshfeld surfaces and fingerprint plots. Also, changes of the thermal-expansion tensor and multi-temperature unit-cell X-ray measurements show significant differences between the forms. Theoretical calculations of lattice energies using the PIXEL and CRYSTAL09 programs are in perfect agreement, confirming that form I is slightly more stable than form II. They demonstrate the importance of a dispersion correction. The stability of the polymorphs is also reflected in the kinetics of the crystallization process and melting point temperatures.