Erdmann’s anion [1,6-diammino tetranitrocobaltate(III)] is useful in the isolation and crystallization of recently confiscated street drugs needing to be identified and catalogued. The protonated form of such drugs forms excellent crystals with that anion; moreover, Erdmann’s salts are considerably less expensive than the classically used AuCl₄⁻ anion to isolate them, while preparation of high-quality crystals is equally easy in both cases. We describe the preparation and structures of the K⁺CoH₆N₆O₈⁻ and NH₄⁺CoH₆N₇O₈⁻_, salts of Erdmann’s. In addition, herein are described the preparations of this anion’s salts with cocaine (C₁₇H₂₈CoN₇O₁₂), with methamphetamine (C₁₀H₂₂CoN₇O₈), and with methylone (C₂₂H₃₄CoN₈O₁₄), whose preparation and stereochemistry had been characterized by the old AuCl₄⁻ salts methodology. For all species in this report, the space groups and cell constants were determined at 296 and 100 K, looking for possible thermally induced polymorphism—none was found. Since the structures were essentially identical at the two temperatures studied, we discuss only the 100 K results. Complete spheres of data accessible to a Bruker ApexII diffractometer with Cu–Kα radiation, λ = 1.54178 Å, were recorded and used in the refinements. Using the refined single crystal structural data for the street drugs, we computed their X-ray powder diffraction patterns, which are beneficial as quick identification standards in law enforcement work.

The title compound [Cu₂(DPA)₂(μ-bpe)(ClO₄)₄]·2H₂O, I where DPA = di(2-methylpyridyl)amine and bpe = 1,2-bis(4-pyridyl)ethane was studied by single crystal X-ray crystallography at different temperatures, spectroscopic methods, differential scanning calorimetry (DSC) and by magnetic susceptibility methods. DSC studies were recorded between 125 K and 647 K and magnetic studies between 2 K and 300 K. The X-ray diffraction studies were carried out at 293, 200, 150 and 100 K. Cooling the crystal from 293 K to 100 K produces a significant change in cell constants but no change in space group. X-ray and DSC studies revealed that significant phase transitions occur at temperatures between 200 K and 100 K that are described herein. The X-ray studies revealed a thermally-induced case of polymorphism whereby the cell volume triples on going from 293 to 100 K. This transition was reversible even though there is a marked hysteresis in the process. The major structural change between 200 K and 100 K is due to the displacement of the central bpe pyridyl rings relative to the plane defining CuN1N2N3 and the bending angle Cu–N1–C3. This transitional phase change was indirectly supported by quantum mechanical calculations which indicated the absence of significant π–π stacking interaction between the central bpe pyridyl rings forming the two parallel dinuclear species. This was attributed to the long 8.28 Å distances between the two indicated moieties. Lack of energetic and spatial constraints justified the pyridyl ring displacement along the N–C3 axis. The temperature dependence of the magnetic behavior of I was that of a simple paramagnet down to 2 K. The complex revealed a very weak to non-existent magnetic coupling between the Cu(II) centers (J = −0.34 cm⁻¹).

The title species has been studied in two crystalline forms resulting from reports describing (1) a centrosymmetric form [ P 2 ₁ / n , Z ′ = 1.0] and (2) a Sohncke form [ P 2 ₁ , Z ′ = 2.0; Flack parameter = −0.04(2)]. They are related by the fact that (a) the two have, approximately, the same cell constants, while (b) Z ′ is doubled in the latter in the course of losing the n-glide plane. The existence of species displaying such a remarkable relationship was noted approximately 75 years ago, but, ignored probably because the discoverers (see below) never completed the necessary experimental work to fully document the phenomenon, inasmuch as (1) crystallography was in its infancy then and (2) synthesizing the necessary, pure, chiral species and growing the necessary high-quality crystals was a major challenge which has, not even yet, been accomplished. Moreover, O. Hassel, the major player in this field, was distracted by his work on the chair-boat equilibrium in cyclohexane and related rings systems for which he eventually won the Nobel Prize (1969). Herein, we have attempted to contribute a chapter to this curious crystallographic phenomenon, which has been made even more fascinating by the observation that a ruthenium Grubbs catalyst belongs in the racemic mimic class.

As described in the Introduction, we became interested in the existing literature for the crystallization behavior of (±)-[Co(en)₃]I₃·H₂O and the absolute configuration of its enantiomers because of our project on the historical sequence of chemical studies leading Werner to formulate his Theory of Coordination Chemistry. In so doing, we discovered a number of interesting facts, including the possibility that the published “Pbca” structure of the (±)-[Co(en)₃]I₃·H₂O was incorrect, and that it really crystallizes as a kryptoracemate in space group P2₁2₁2₁. Other equally interesting facts concerning the crystallization behavior of [Co(en)₃]I₃·H₂O are detailed below, together with an explanation why Platon incorrectly selects, in this case, the space group Pbca instead of the correct choice, P2₁2₁2₁. As for the Flack parameter, (±)-[Co(en)₃]I₃·H₂O provides an example long sought by Flack himself – a challenging case, differing from the norm. For that purpose, data sets (for the pure enantiomer and for the racemate) were collected at 100 K with R-factors of 4.24 and 2.82%, respectively, which are ideal for such a test. The fact that Pbca is unacceptable in this case is documented by the results of Second-Harmonic Generation experiments. CCDC nos: 1562401 for compound (I) and 1562403 for compound (II).