Nonconventional Supramolecular Self-Assemblies of Zinc(II)-Salphen Building Blocks

In the presence of methanol (or ethanol) and acetone molecules, a zinc(II)–salphen [salphen = N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐phenylenediamine] building block gives rise to discrete supramolecular dimers involving both intermolecular hydrogen bonding and unique oxygen···oxygen interactions, that is, the O···O distances are much shorter than twice the van der Waals radius of an oxygen atom. These types of unexpected self‐assembled structures have been studied thoroughly through a combination of experimental (X‐ray diffraction, IR spectroscopy, solid‐state UV/Vis absorption spectroscopy, and elemental analysis) and theoretical [DFT and natural bond orbital (NBO) calculations] studies. However, their solution structures involve different equilibrating species according to variable‐temperature 1H NMR spectroscopy. The replacement of the methanol (or ethanol) and acetone molecules by bulkier isopropyl alcohol and 2‐butanone, respectively, leads to structures with no oxygen···oxygen interactions. An infinite supramolecular polymeric chain stabilized by hydrogen bonding is obtained in the presence of water. The present study highlights that even well‐explored molecules can provide extremely interesting possibilities for metallo‐supramolecular chemistry. Weak interactions involving hydrogen bonding and unprecedented oxygen···oxygen interactions are responsible for the self‐assembly of zinc(II)–salphen building blocks into dimeric supramolecular structures. Such an unexpected rearrangement is established unambiguously by a combination of experimental and theoretical studies.