Molecular motion in zinc hydrazone grid complexes

Four new grid forming hydrazone ligands substituted with methoxy and dimethylamino groups were synthesised. Combination of these ligands with zinc triflate in acetonitrile resulted in self-assembly to form grids as indicated by H-1 NMR and ES-MS. H-1 NMR also showed thermally induced rotation of the intercalated phenyl ring in both the new grids, and in three previously reported grid compounds. Of these seven, five were amenable to study by variable temperature H-1 NMR. Though the observed rates varied by more than an order of magnitude depending upon ligand structure and level of deprotonation, activation energies were similar (similar to 60 kJ/mol) for all complexes studied. This suggests a model in which dissociation of the central pyrimidine ligand precedes phenyl group rotation with an enthalpy of dissociation near zero. The rate of rotation of the phenyl ring increases with an introduction of electron-donating substituents on the phenyl ring, possibly due to an increased repulsion between pi systems.