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Abstract
Racemic aqueous solutions of (+/−)-[Co(tren)(NO2)2]Br (I), (+/−)-[Co(tren)(NO2)2]ClO4 (III) and [Co(tren)(NO2)2]NO3(IV) crystallize as racemates. By contrast, the double salt, (+/−)-[Co(tren)(NO2)2]2Br(ClO4) · H2O(II), produces kryptoracemic crystals belonging to the enantiomorphic space group P212121 (No. 19). The former three species crystallize with one molecule in the asymmetric unit; in the latter, a racemic pair is the asymmetric unit, a fact which is hidden by the enantiomorphic nature of its space group – thus the name of the crystallization phenomenon reported. In (II) pairs of cations are related by an approximate, non-crystallographic, inversion center. The crystal structure and polarity of (I) and the absolute configuration of (II) were determined by refinement. The crystalline contents of (I) to (III) consist of infinite strings of hydrogen bonded cations, the counter ions and (where relevant) waters of crystallization acting as a hydrogen-bonding glue linking the spiral strings
In (II), the N-CH2-CH2-NH2-Co rings of Co(1) are (δδλ) and those of Co(2) are (λλδ) and adjacent strings are linked by the counter-anions and the water of crystallization. Pairs of Co(1) and of Co(2) cations are hydrogen bonded to one another by two N-O···H-N linkages. Finally, pairs of composition Co(1)-Co(1) as well as of Co(2)-Co(2) share another pseudo-inversion center which is approximately valid for the CoN6 portion of each cation. Since the atoms of the cation are ordered, it is impossible for the pseudo-inversion center to be valid for the -CH2-CH2- fragment of the Co(1)-Co(1) or of Co(2)-Co(2) pairs. (I) and (III) crystallizes as racemates whose five-membered rings have chiroptical symbols (δδλ), or its enantiomer.
