Phase selection and energetics in chiral alkaline earth tartrates and their racemic and Meso analogues: Synthetic, structural, computational, and calorimetric studies

The hydrothermal reactions of calcium, strontium, and barium with L-, meso-, and D,L-tartaric acid were examined from room temperature to 220°C. We report the synthesis of 13 new phases and crystal structures of 11 alkaline earth tartrates, including an unusual I³O⁰ framework, [Ba(D,L-Tar)] (Tar = C₄H₄O₆^2-), with 3-D inorganic connectivity. Each alkaline earth exhibits different phase behavior in the reactions with the three forms of tartaric acid. Calcium forms unique L-, meso-, and D,L-tartrate phases which persist to 220°C. Strontium forms three unique phases at lower temperatures, but above 180°C reactions with L- and D,L-tartaric acid yield the meso phase. Likewise, Ba forms three unique low-temperature phases, but above 200°C reactions with L- and meso-tartaric acid yield the D,L phase. Computational and calorimetric studies of the anhydrous calcium phases, [Ca(L-Tar)] and [Ca(meso-Tar)], strontium phases, [Sr(L-Tar)] and [Sr(meso-Tar)], and barium phases, [Ba(L-Tar)] and [Ba(D,L-Tar)], were performed to determine relative phase stabilities and elucidate the role of thermodynamic and kinetic factors in controlling phase behavior. The computational and calorimetric results were in excellent agreement. The [Ca(meso-Tar)] phase was found to be 9.1 kJ/mol more stable than the [Ca(L-Tar)] phase by computation (total electronic energies) and 2.9 ± 1.6 kJ/mol more stable by calorimetry (enthalpies of solution). The [Sr(meso-Tar)] phase was found to be 13.4 and 8.1 ± 1.4 kJ/mol more stable than [Sr(L-Tar)] by computation and calorimetry, respectively. Finally, the [Ba(L-Tar)] phase was found to be 6.4 and 7.0 ± 1.0 kJ/mol more stable than the [Ba(D,L-Tar)] phase. Our results suggest that the calcium and strontium meso phases are the most thermodynamically stable phases in their systems over the temperature range studied. The phase transitions are controlled by relative thermodynamic stabilities but also by a kinetic factor, likely the barrier to isomerization/racemization of the tartaric acid, which is hypothesized to preclude phase transformations at lower temperatures. In the barium system we find the [Ba(L-Tar)] phase to be the most thermodynamically stable phase at low temperatures, while the [Ba(D,L-Tar)] phase becomes the thermodynamic product at high temperatures, due to a larger entropic contribution.