Sterically hindered aromatic tethered carboxylic acids: what is the critical length of the tether for adoption of centrosymmetric dimer synthon?

Abstract

The synthesis and X-ray structural investigations of sterically hindered aromatic tethered carboxylic acids have been carried out to examine the extent to which the tether modifies the molecular structures and the occurrence of strong O-H···O hydrogen bond-mediated synthons of the carboxyl groups. The diacids based on mesitylene- and isodurene carboxylic acids (MA2 and DA2) containing a C₂-tether were found to exhibit pseudopolymorphism/solvatomorphism. Whereas a C₃-tethered dimesitoic acid MA3 crystallized only in the presence of 2,4,6-collidine as a guest, the isodurene analogue DA3 resisted crystallization. The adoption of the centrosymmetric dimer motif was observed for the butylene-tethered dimesitoic acid MA4. The situation appears to be a little relaxed for unhindered diacids such that a C₃-tether is sufficient for adoption of the centrosymmetric dimer synthon in the crystal lattice, as revealed by the crystal structure of 1,3-bis(m-carboxyphenyl)propane, BA3. The topological changes, that is, skewed (for odd) and extended (for even), imparted by the tether appear to render the close packing with simultaneous exploitation of the strong O-H···O hydrogen bonds difficult even for diacids with a C₂-tether. Thus, for short chain lengths, the tether typically behaves like a functional group, which perturbs the molecular association based on O-H···O hydrogen bonds of the otherwise strongly interacting carboxyl groups.