Structural Diversity and Energetics in Anhydrous Lithium Tartrates: Experimental and Computational Studies of Novel Chiral Polymorphs and Their Racemic and Meso Analogues

Five novel anhydrous inorganic−organic frameworks based on crystalline modifications of lithium tartrate (tart2− = C4H4O62−) have been synthesized by solvothermal means and investigated by single crystal X-ray and computational methods. Reactions between lithium acetate dihydrate and l-tartaric acid yielded three chiral compounds, LiH(l-tart) in space group P21 (1), Li2(l-tart), P212121 (2), and Li2(l-tart), C2221 (3), a polymorph of 2. Two achiral compounds, Li2(meso-tart), P21/c (4) and Li2(d,l-tart), C2/c (5), were obtained from the reaction of lithium acetate dihydrate with meso-tartaric acid and d,l-tartaric acid, respectively. The singly deprotonated tartrate 1 contains isolated LiO4 tetrahedra, while the tetrahedra in the doubly deprotonated tartrates 2−5 share corners to form parallel chains. The tartaric acid ligands employ a variety of binding modes to connect the inorganic moieties, resulting in nonporous three-dimensional frameworks in all cases. The relative formation energies of structures 2−5 were calculated using density functional theory (DFT) methods and are found to be within the narrow range of ca. 5 kJ mol−1 when electronic energies only are considered. However, when the contributions of zero point vibrational energy (ZPVE) and thermal vibrational energy are considered, the relative energy range increases to ca. 15 kJ mol−1. Analysis of the vibrational modes shows that the non-negligible difference in ZPVE arises from degrees of freedom associated with the heavy backbone atoms rather than, for example, O−H modes.