A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions

Reaction of Zn(OAc) 2 ⋅ 2 H 2 O with 2,6‐diisopropylphenyl phosphate (dippH 2 ) in the presence of pyridine‐4‐carboxaldehyde (Py‐4‐CHO) in methanol resulted in the isolation of a tetrameric zinc phosphate cluster [Zn(dipp)(Py‐4‐CH(OH)(OMe))] 4 ⋅ 4 MeOH ( 1 ) with four hemiacetal moieties stabilized on the double‐4‐ring inorganic cubane cluster. The change of solvent from methanol to acetonitrile leads to the formation of [Zn(dipp)(Py‐4‐CHO)] 4 ( 2 ), in which the coordinated Py‐4‐CHO retains its aldehydic form. Dissolution of 1 in CD 3 CN readily converts it to the aldehydic form and yields 2 . Similarly 2 , which exists in the aldehyde form in CD 3 CN, readily converts to the hemiacetal form in CD 3 OD/CH 3 OH. Compound 1 is an unprecedented example in which four hemiacetals have been stabilized on a single molecule in the solid state retaining its stability in solution as revealed by its 1 H NMR spectrum in CD 3 OD. The solution stability of 1 and 2 has further been confirmed by ESI‐MS studies. To generalize the stabilization of multiple hemiacetals on a single double‐four‐ring platform, pyridine‐2‐carboxaldehyde (Py‐2‐CHO) was used as the auxiliary ligand in the reaction between zinc acetate and dippH 2 , leading to isolation of [Zn(dipp)(Py‐2‐CH(OH)(OMe))] 4 ( 3 ). Understandably, recrystallization of 3 from acetonitrile yields the parent aldehydic form, [Zn(dipp)(Py‐2‐CHO)] 4 ( 4 ). Single‐crystal X‐ray diffraction studies reveal that supramolecular bonding, aided by hydrogen‐bonding interactions involving the hemiacetal functionalities (C−OH, C−OMe, and C−H), are responsible for the observed stabilization. The hemiacetal/aldehyde groups in 1 and 2 readily react with p ‐toluidine, 2,6‐dimethylaniline, and 4‐bromoaniline to yield the corresponding tetra‐Schiff base ligands, [Zn(dipp)(L)] 4 (L=4‐methyl‐ N ‐(pyridin‐4‐ylmethylidene)aniline ( 5 ), 2,6‐dimethyl‐ N ‐(pyridin‐4‐ylmethylene)‐aniline ( 6 ), and 4‐bromo‐ N ‐(pyridin‐4‐ylmethylene)aniline ( 7 )). Isolation of 5 – 7 opens up further possibilities of using 1 and 2 as new supramolecular synthons and ligands.