Chemical and Structural Diversity in Chiral Magnesium Tartrates and their Racemic and <i>Meso</i> Analogues

Nine crystalline modifications of magnesium tartrate (tart2- = C4H4O6) have been synthesized by hydrothermal reactions between magnesium acetate tetrahydrate and the d,l-, d-, and meso-forms of tartaric acid in the temperature range 100−200 °C. Two racemic crystals, a dimeric Mg(d,l-tart)(H2O)2·3H2O in space group Pban, (1), and a two-dimensional coordination polymer Mg(d,l-tart)(H2O)·3H2O, P2/c, (2), were obtained from the d,l-acid reactions at 100 °C and 125−150 °C, respectively. Two chiral structures were obtained from the d-acid reactions at these temperatures: a layered Mg(d-tart)(H2O)·1.5H2O, P212121, (3), and a three-dimensional (3-D) network, Mg(d-tart)·1.5H2O, C2221, (4). In addition, an anhydrous 3-D coordination polymer, Mg(d-tart), I222, (5), was obtained by dehydrating 3 under vacuum at 85 °C. Finally, four achiral structures were obtained from reactions with the meso-acid at 100 °C, 125−150 °C, 180 °C, and 200 °C, respectively: three one-dimensional (1-D) chains, Mg(meso-tart)(H2O)2·H2O (6, P21/n, and 7, Pbca) and Mg(meso-tart)(H2O)2, P21/c, (8), and one 3-D framework, Mg(meso-tart), C2/c, (9). Within each family, the structures become less hydrated and more condensed with increasing temperature, indicating that the reactions proceed under thermodynamic control. The chiral d-tartrates, 3 and 4, adopt entirely different architectures compared to the racemic d,l-tartrates, 1 and 2, with the former exhibiting higher dimensionalities and lower levels of hydration than their d,l-analogues formed at the same temperatures. At 180 and 200 °C, the d,l-magnesium tartrate undergoes spontaneous resolution to form a conglomerate corresponding to one of the lower temperature D-phases (4). The unexpected complexity of this ostensibly simple system points to the extraordinary richness of the emerging field of chiral inorganic−organic framework structures.