A porous coordination polymer exhibiting reversible single-crystal to single-crystal substitution reactions at Mn(II) centers by nitrile guest molecules

Abstract

The porous coordination polymer {[Mn(L)(H₂O)](H₂O)_1.5(DMF)}_n (1) containing a water molecule coordinated at the apical position of each distorted octahedral Mn(II) center has been synthesized using the solvothermal technique by reacting Mn(NO₃)₂·4H₂O with a new flexible ligand (LH₂) having isophthalic fragment and pyridine donors at the two ends. The coordinated water molecule could be substituted by nitrile guest molecules such as acetonitrile, acrylonitrile, allylnitrile, and crotononitrile (affording compounds 2-5, respectively) without loss of crystallinity. Interestingly, compound 1 selectively captures cis-crotononitrile into its cavity from a mixture of cis and trans isomers. Hence, the cis isomer can be separated from the trans isomer. In each case, 1.5 lattice water molecules and a dimethylformamide (DMF) molecule are also simultaneously replaced by certain numbers of these guest molecules. When these first-generation compounds 2-5 are dipped in DMF at room temperature with the lid of the vial open to the atmosphere, the mother crystal 1 is regenerated in each case. Thus, all of these substitution reactions are completely reversible. Also, the first-generation compounds 2-5 can be interconverted among one another by dipping them in appropriate nitrile guests. All of these phenomena could be observed in single-crystal to single-crystal fashion.