A Porous Coordination Polymer Exhibiting Reversible Single-Crystal to Single-Crystal Substitution Reactions at Mn(II) Centers by Nitrile Guest Molecules

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.