Exploring the effect of substitution patterns on the symmetry of hydrogen-bonded supramolecular motifs in functionalized benzosiloxaboroles

Crystal structures of a series of 26 functionalized 3-hydroxybenzo[c][1,2,3]siloxaboroles were compared taking into account electronic and steric effects of substituents at the aromatic ring on the hydrogen-bond (HB) motifs involving B-OH groups. The supramolecular assemblies of those compounds show strong variation depending on the number, position and type of substituents. Thus, HB dimers, trimers, tetramers and chains are formed. Most 7-substituted derivatives are isomorphous and crystallize in the I41/a tetragonal space group of symmetry featuring cyclic propeller-like tetramers as a characteristic structural motif. DFT calculations revealed that all observed HB motifs are characterized by similar stabilization energies ranging from -25 to -35 kJ mol-1 per molecule, which rationalizes the strong diversification of HB motifs in the studied structures. A comparative analysis of crystal structures of a series of 26 functionalized 1,1-dimethyl-3-hydroxybenzo[c][1,2,3]siloxaboroles revealed a strong diversification of hydrogen-bond supramolecular assemblies involving BOH groups.