The phenomenon of conglomerate crystallization.: Part 56.: The crystallization behavior of [(tren)Co(N3)2]X (X=Br, I):: centrosymmetric to Sohnke space group phase transition caused by cooling

[(tren)Co(N-3)(2)]Br (I) and [(tren)Co(N-3)(2)]I (III) was prepared and its crystal structure determined at 293 and at 120 K. At room temperature (293 K), (I) crystallizes as a conglomerate in space group P2(1)2(1)2(1) and remains so at 120 K, (data sets Ia and Ib, respectively). The iodide crystallizes as a racemate, space group P2(1)/n (z = 4), data set (IIIa); but, cooling to 120 K results in a phase change whereby the crystal is enantiomorphic (IIIb) + (IIIc), space group P2(1) (z = 4). There is no damage to the crystal upon undergoing the phase transition (IIIa) --> (IIIb) + (IIIc). In fact, the same crystals were used to collect the data sets which were cycled as follows: 293 --> 120 --> 293 K. The two molecules in the asymmetric unit of (IIIb and IIIc) are homochiral demanding that the phase transition observed causes the inversion of the axial chirality of one of the five-membered rings of the iodide. This result is reproducible and was observed for all crystals cooled to 120 K. Differential scanning calorimetry confirms the existence of phase changes in the crystalline material as a result of cooling. (C) 2003 Elsevier Science B.V. All rights reserved.