Structural and conformational studies of alcohol, diol and methyl ether derivatives of dibenzo-14-crown-4. Implications for ligand and tecton design

Molecular structures are determined for six dibenzo-14-crown-4 derivatives that have one or two substituents on the central carbon(s) of the three-carbon bridge(s). The series of compounds includes three crown ether alcohols, one crown ether trans-diol, and two methoxy crown ether compounds. The crystal structures for these six crown ethers reveal that due to hydrogen-bonding and steric interactions, a hydroxy substituent is directed, at least partially, toward the crown ether cavity and an unusual intra- and intermolecular hydrogen bond network is formed between the hydroxy group protons and the ether oxygens of the crown ether ring. On the other hand, an ether group or a substituent with carbon as the first atom is oriented away from the polyether ring. The structure of sym-(methoxy)(methyl)dibenzo-14-crown-4 is markedly different from that of sym-(hydroxy)(methyl)dibenzo-14-crown-4 both in terms of the substituent orientation and very significant distortion from planarity of the four crown ether oxygens in the former. Support for an unusual conformation for sym-(methoxy)(methyl)dibenzo-14-crown-4 in solution is derived from ¹³C NMR measurements. Crown ether alcohols are hydrogen-bonding "tectons" that participate in strong, specific and directional intermolecular interactions.