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

• Published in: Chemistry : a European journal 2016-05, Vol.22 (20), p.6863-6875
• Main Authors: Kalita, Alok Ch, Gupta, Sandeep K., Murugavel, Ramaswamy
• Format: Article
• Language: English
• Subjects: Acetonitrile; Aldehydes; Aniline; Chemical bonds; Chemistry; Clusters; Cubane; Dipping; Dissolution; hemiacetals; Hydrogen; Hydrogen bonding; imines; Ligands; Methanol; Methyl alcohol; organophosphates; p-Toluidine; Platforms; Pyridines; Recrystallization; Single crystals; Solid state; Solvents; Stability; Stabilization; supramolecular chemistry; Toluidine; X-ray diffraction; Yield; Zinc; Zinc acetate; Zinc phosphate
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201600409

Reaction of Zn(OAc)2⋅2 H2O with 2,6‐diisopropylphenyl phosphate (dippH2) 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 CD3CN readily converts it to the aldehydic form and yields 2. Similarly 2, which exists in the aldehyde form in CD3CN, readily converts to the hemiacetal form in CD3OD/CH3OH. 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 1H NMR spectrum in CD3OD. 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 dippH2, 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. Solvent switch: Multiple hemiacetal groups have been stabilized on a single double‐4‐ring zinc organophosphate platform. Solvent‐assisted reversible transformation into the aldehyde functionality (see figure) and the solid‐ and solution‐phase stability are probed. Proof‐of‐concept for the utilization of these compounds as precursors for framework solids is presented.