Chiral, Racemic, and Meso-Lithium Tartrate Framework Polymorphs: A Detailed Structural Analysis

Following our previous report of five anhydrous lithium tartrates 1–5 (tart = C4H4O6 2–), we have synthesized and solved the single crystal structures of four new I1O2 inorganic–organic frameworks, all with the same chemical formula, Li2(tart). Reactions between lithium acetate and the meso, chiral, and racemic isomers of tartaric acid in water:ethanol mixtures have yielded two new polymorphs of Li2(meso-tart) in space groups P21/c 6 and Cc 7, racemic Li2(d,l-tart) in P21/c 8, and chiral Li2(l-tart) in C2 9. Hydrogen bond graph set analysis was adapted for use with framework materials and employed here to examine the motifs displayed by the eight anhydrous dilithium tartrates 2–9. A variety of hydrogen-bonding patterns and dimensionalities are observed in this system, and the relative hydrogen bond strengths are found to correlate well with O–H stretching frequency shifts in the FTIR spectra. The relative formation energies of the framework isomers have been calculated by DFT methods, using schemes that include dispersion correction, zero-point vibrational energy, and thermal vibrations at room temperature. Although the energy ordering depends slightly on the scheme used, it is generally found to relate to the differences in crystallographic density and hydrogen bond strength rather than other structural features.