THE PHENOMENON OF KRYPTORACEMIC CRYSTALLIZATION. PART 1. COUNTERION CONTROL OF CRYSTALLIZATION PATHWAY SELECTION. PART 4. THE CRYSTALLIZATION BEHAVIOR OF (+/−)-[Co(tren)(NO2)2]Br(I), (+/−)-[Co(tren)(NO2)2]2Br(ClO4) · H2O(II), (+/(−)-[Co(tren)(NO2)2]ClO4(III) AND ATTEMPTS TO SOLVE THE STRUCTURE OF (+/−)-[Co(tren)(NO2)2]NO3(IV)

Unbalanced racemates: solid state solutions containing enantiomeric pairs crystallizing in Sohncke space groups with (L:D) ratios other than (1:1) – illustrated with crystals of a Co(III) coordination compound

Herein we describe materials of composition [Co(NH3)4(X-leucinato)]I2·H2O in which the amino acid ligand is either L or D, and in which (a) while in pure enantiomorphic form (L), crystallizes in a Sohncke space group with Z′ = 2.0; but, whose packing closely resembles that of its racemate. Such substances are labeled a Racemic Mimic; and (b) crystals in which the L:D ratio of the amino acid ligand in the asymmetric unit is (71:29), which interestingly crystallize in the same space group and cell constants as those of the former. Moreover, the packing behavior is essentially the same in both—the difference being that the (1:1) species is fully ordered, while that with L:D (71:29) ratio has a partially disordered propyl chain. The (71:29) species we describe herein as an Unbalanced Racemate.

Revisiting the structure of (±)-[Co(en)3]I3·H2O – X-ray crystallographic and second-Harmonic results

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).

NO Generation from the Cross-Talks between Ene-diol Antioxidants and Nitrite at Metal Sites

The one-electron reduction of nitrite (NO₂^-) to nitric oxide (NO) and ene-diol oxidation are two important biochemical transformations. Employing mononuclear cobalt-nitrite complexes with Co^III and Co^II oxidation states, [(Bz₃Tren)Co^III(nitrite)₂](ClO₄) (1) and [(Bz₃Tren)Co^II(nitrite)](ClO₄) (2), this report illustrates NO release coupled to stepwise oxidation of ene-diol antioxidants such as l-ascorbic acid (AH₂) and catechol. Analysis of the AH₂ end-product reveals that the reaction with complex 1 affords dehydroascorbic acid. Intriguingly, a controlled oxidation of AH₂ with complex 2 results in a [Co^II]-bound ascorbyl radical-anion (8). Finally, NO release with the concomitant generation of metal-bound 3,5-di-tert-butyl-semiquinone radical anion from the reactions of 3,5-di-tert-butyl-catechol and [(Bz₃Tren)M^II(nitrite)](ClO₄) (2, M = Co; 4, M = Zn) provides mechanistic insights into the cross-talk between nitrite and ene-diols at the metal sites.

1D Mn(III) coordination polymers exhibiting chiral symmetry breaking and weak ferromagnetism

Mn(iii) complexes with achiral ligands, (E)-N-(2-((2-aminobenzylidene)amino)-2-methylpropyl)-5-X-2-hydroxybenzamide (HLX, X = H, Cl, Br, and I), crystallise as chiral conglomerates containing amide oxygen-bridged one-dimensional coordination polymers that exhibit weak ferromagnetism. The bulk products exhibit symmetry breaking in that they do not contain equal amounts of enantiomers, though which is the dominant species depends on the substituent X.

Is it all in the hinge? A kryptoracemate and three of its alternative racemic polymorphs of an aminonitrile

A kryptoracemate, a racemic crystal in a Sohncke space group, and three additional polymorphs, well-ordered racemic structures.

A list of organometallic kryptoracemates

The vast majority of racemic solutions of chiral compounds apparently crystallize at room temperature in non-Sohncke space groups as racemic crystals. However, kryptoracemic crystals composed of nearly enantiomeric pairs occasionally crystallize at room temperature, or appear as low-temperature phases, in Sohncke space groups. As a complement to the previously published catalog of organic kryptoracemates [Fábián & Brock (2010).Acta Cryst.B66, 94–103], 1412 chiral organometallic crystal structures have now been extracted from the Cambridge Structural Database and analyzed. 26 are listed herein as credible kryptoracemates. The possible influence of temperature is discussed, together with some problems in characterizing and classifying these structures.

A list of organic kryptoracemates

A list of 181 organic kryptoracemates has been compiled. This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations). Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups. The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors. Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures. The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect. There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot. Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule. The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule.