Chemical and structural diversity in chiral magnesium tartrates and their racemic and Meso analogues

Nine crystalline modifications of magnesium tartrate (tart(2-) = 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 degrees C. Two racemic crystals, a dimeric Mg(D,L-tart)(H2O)(2)center dot 3H(2)O in space group Pban, (1), and a two-dimensional coordination polymer Mg(D,L-tart)(H2O)center dot 3H(2)O, P2/c, (2), were obtained from the D,L-acid reactions at 100 degrees C and 125-150 degrees C, respectively. Two chiral structures were obtained from the D-acid reactions at these temperatures: a layered Mg(D-tart)(H2O)center dot 1.5H(2)O, P2(1)2(1)2(1), (3), and a three-dimensional (3-D) network, Mg(D-tart)center dot 1.5H(2)O, C222(1), (4). In addition, an anhydrous 3-D coordination polymer, Mg(D-tart), I222, (5), was obtained by dehydrating 3 under vacuum at 85 degrees C. Finally, four achiral structures were obtained from reactions with the meso-acid at 100 degrees C, 125-150 degrees C, 180 degrees C, and 200 degrees C, respectively: three one-dimensional (1-D) chains, Mg(meso-tart)(H2O)(2)center dot H2O (6, P2(1)/n, and 7, Pbca) and Mg(meso-tart)(H2O)(2), P2(1)/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 degrees 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.