Host–guest chemistry of NiII coordination compounds with PDC and (py)2CO: reversible crystal-to-amorphous transformations induced by solvent exchange

The host-chemistry of four coordination networks has been studied by means of X-ray diffractometry (XRD), thermogravimetry (TG) and X-ray thermodiffractometry (TDX). The four compounds exhibit the general formula [Ni(PDC)((py)2C(OH)2)(ROH)]·xROH, where PDC is pyridine-2,5-dicarboxylate(2−), ((py)2C(OH)2) is the gem-diol of di-2-pyridyl ketone ((py)2CO), and R = H (1a), Me (3m), Et (3e), iPr (3p), and x = 2 when R = H, and x = 1 when R = Me, Et, iPr. The compounds are monomers packed through hydrogen bonds and π interactions. Dehydration of compound 1a produces an amorphous compound (2) that recovers crystallinity by re-adsorption of water, and also by adsorption of alcohols (methanol, ethanol, and isopropanol). Desorption of those alcohols can also be produced, yielding again the amorphous compound, which can be used again as host framework. This is, adsorption–desorption has been observed to be completely reversible not only for a particular guest molecule but also for the exchange of different guests. Compound 2 becomes, therefore, a host framework for at least four exchangeable molecules of solvent. This host–guest chemistry is based on the synergic effect of three simple processes: the exchange of crystallisation molecules, the labile coordination of the solvent molecules to the octahedral coordination sphere, and the sp2 → sp3 rehybridization of the carbonyl group in (py)2CO in the presence of different solvents.