Enantiopure and Racemic Sandwich-like Networks with Dehydration, Readsorption, and Selective Guest-Exchange Phase Transformations

The room-temperature solution reactions of R- or S-H(2)opa (H(2)opa = 2-hydroxy-2-phenylacetic acid) with freshly prepared [Ag(NH3)(2)](OH) in the presence of 1,3-di(4-pyridyl)propane (bpp) yielded two enantiomeric compounds, [Ag-2(R-Hopa)(bPP)(2)](R-Hopa)center dot 3H(2)O (1 center dot 3H(2)O) and [Ag-2(S-Hopa)(bpp)(2)](S-Hopa)center dot 3H(2)O (2 center dot 3H(2)O). While a similar solution reaction was carried out with racemic H(2)Opa instead of R- or S-H(2)opa, a racemic compound [Ag-2(R-Hopa)(S-Hopa)(bpp)(2)]center dot 4H(2)O (3 center dot 4H(2)O) was formed. When bulk crystals of 3 center dot 4H(2)O were exposed to the air at ambient temperature, a partially dehydrated compound [Ag-2(R-Hopa)(S-Hopa)(bpp)(2)]center dot H2O (4 center dot H2O) was found, which would transform to [Ag-2(R-Hopa)(S-Hopa)(bpp)(2)]center dot 2MeOH (5 center dot 2MeOH) by exposing the crystals of 4 center dot H2O in mixed MeOH-EtOH-DMF solvent or in methanol. Compounds 1-5 have characteristic hydrogen-bonding-directed sandwich-like networks consisting of Ag-bpp layers and Hopa(-)-ROH chains (R = H for 1-4 and Me for 5). The different structural features come from different hydrogen bonding interactions between Hopa(-) and the lattice water or MeOH molecules. In 1 center dot 3H(2)O and 2 center dot 3H(2)O, R-Hopa(-) or S-Hopa(-) form left-handed or right-handed helices with water molecules running in parallel to the Ag-bpp layers while in 3 center dot 4H(2)O, the centrosymmetric hydrogen bonding chains are composed of two enantiomers of Hopa(-). Upon dehydration, the Ag center dot center dot center dot Ag dimeric units in [Ag-2(bpp)(2)](n)(2n+) in 3 center dot 4H(2)O were broken and the positions of the Ag-bpp chains were adjusted to form new Hopa(-)-H2O Chains between the silver layers. After absorbing methanol, the Ag-bpp chains restored to their original layer structures and 5 center dot 2MeOH is pseudopolymorphic to 3 center dot 4H(2)O. The reversible dehydration, readsorption, and guest-exchange among 3 center dot 4H(2)O, 4 center dot H2O, and 5 center dot 2MeOH is an interesting example of phase transformation for racemic coordination polymers. The chirality of 1 center dot 3H(2)O and 2 center dot 3H(2)O was further conformed by solid-state CD spectra.