Magnetic crystallography comes of age.

A special issue of Acta Crystallographica Section B reports the great progress made recently in the determination, reporting, and archiving of magnetic structures, of which there are now more than 2000. The infrastructure needed to support the field is now in place. The special issue also highlights new science made possible by these developments.

Uri Shmueli (1928-2023).

Obituary for Uri Shmueli.

Pervasive approximate periodic symmetry in organic P1 structures.

The goal of this project was to identify the prevalence of approximate symmetry in organic P1 structures. In the November 2019 version of the Cambridge Structural Database (CSD), there are 2592 organic, P1, R ≤ 0.050 structures; complete, unique entries are available for 1407 Z = Z' > 1 and 1049 Z = Z' = 1 structures.

Approximate symmetry in the third reported structure of a metal complex of L-DOPA.

Although l-DOPA is a major drug that has been used for more than 50 years to treat the motor symptoms of Parkinson’s disease, the Cambridge Structural Database contains only two structures in which it is coordinated to a metal. Why are there not more? The new structure is also notable for an approximate 4 axis.

Identifying and characterizing translationally modulated molecular crystal structures.

Most structural (i.e. displacive) modulations make molecules independent that had been related by translation in a phase having a smaller or centered unit cell.

High-Z' structures of organic molecules: their diversity and organizing principles.

A list has been compiled of 284 well determined organic structures having more than four crystallographically independent molecules or formula units (i.e. Z' > 4).

An exceptional 5:4 enantiomeric structure.

The only crystals that could be grown from racemic solutions of the PF6(-) salt of the resolvable cation [Ru(2,9-dimethyl-1,10-phenanthroline)2(dipyrido[3,2-d:2',3'-f]quinoxaline)](2+) have translational symmetry only (space group P1), contain nine independent sets of ions, and include numerous independent solvent molecules (11 acetone, one diethyl ether and possibly several water molecules). Layers of hydrophobic cations alternate with layers containing most of the anions and solvent molecules. All nine cations have the same basic conformation, which is distorted by the presence of the methyl substituents on the two 1,10-phenanthroline ligands.

C6H4S2AsCl: description and interpretation of an incommensurately modulated molecular crystal structure.

Crystals of 2-chloro-benzo-1,3,2-dithiarsole have a strongly modulated structure that can be solved and refined with relative ease in a P(1), Z' = 17 approximate supercell but that is better described as incommensurate. Two conventional refinements (different superstructure approximations that differ in the placement of their crystallographic inversion centers) and a (3 + 1)-dimensional superspace refinement are all nearly equally successful, at least as measured by the usual agreement factors; the data integration, however, shows that the incommensurate description is preferable. The overall packing is determined by the stacking of the aromatic rings and probably by the segregation of As and Cl atoms to give short As.

[Ni(MeCN)(H2O)2(NO3)2]·(15-crown-5)·MeCN: detailed study of a four-phase sequence that includes an intermediate modulated phase.

A sequence of four phases has been found for an acetonitrile-solvated co-crystal with 15-crown-5 of the nickel complex [acetonitrilediaqua-κ(1)O-nitrato-κ(2)O-nitratonickel(II)]. The structure could be determined at intervals of ca 10 K in the range 90-273 K because crystals remain single through the three transitions. In phase (I) (T ≥ ca 240 K; P2(1)/m, Z' = ½), there is extensive disorder, which is mostly resolved in phase (III) (ca 230-145 K; P2(1)/c, Z' = 1).

[Ni(H2O)6](NO3)2·(15-crown-5)·2H2O: three phase transitions and an intermediate modulated phase stable over a range of ca 40 K.

In the range 308-90 K single crystals of [Ni(H(2)O)(6)](NO(3))(2)·(15-crown-5)·2H(2)O continue to diffract well as they undergo three phase transitions. Structures have been determined at 28 different temperatures; data were collected at six more but could not be refined satisfactorily. The transitions identified structurally are in good agreement with those found by thermal analysis.

Detailed study of the phase transition in [Ni(H2O)6](NO3)2·(15-crown-5)·H2O and analysis in terms of mean-field theory.

The transition between 190 and 200 K in [Ni(H(2)O)(6)](NO(3))(2)·(15-crown-5)·H(2)O has been followed by determining the structure at 22 temperatures in the range 90-273 K. The structural change is a zone-boundary transition with a critical point at (½, 0, ½) in the Brillouin zone of the high-temperature phase; both phases have space-group symmetry P2(1) but the volume of the unit cell is halved when a crystal is heated through the transition. The only obvious disorder in the high-temperature phase is of the lattice water molecule, which occupies two sites; some disorder persists below the transition.

Failures of fractional crystallization: ordered co-crystals of isomers and near isomers.

A list of 270 structures of ordered co-crystals of isomers, near isomers and molecules that are almost the same has been compiled. Searches for structures containing isomers could be automated by the use of IUPAC International Chemical Identifier (InChI™) strings but searches for co-crystals of very similar molecules were more labor intensive. Compounds in which the heteromolecular A···B interactions are clearly better than the average of the homomolecular A···A and B···B interactions were excluded.

More examples of the 15-crown-5...H2O-M-OH2...15-crown-5 motif, M = Al3+, Cr3+ and Pd2+.

Five structures of co-crystals grown from aqueous solutions equimolar in 15-crown-5 (or 15C5) and [M(H(2)O)(6)](NO(3))(n), M = Al(3+), Cr(3+) and Pd(2+), are reported. The hydrogen-bonding patterns in all are similar: metal complexes including the fragment trans-H(2)O-M-OH(2) alternate with 15C5 molecules, to which they are hydrogen bonded, to form stacks. A literature survey shows that this hydrogen-bonding pattern is very common.

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.

Five more phases of the structural family [M(H2O)2(15-crown-5)](NO3)2.

The structures of five more phases of the structural family of compounds [M(H2O)2(15-crown-5)](NO3)2 have been determined. All of these phases are stable at room temperature or above, but transform to other phases if cooled slowly. All five phase transitions take place without significant damage to the single crystal.

The elusive [Ni(H2O)2(15-crown-5)]2+ cation and related co-crystals of nickel(II) hydrates and 15-crown-5.

Initial attempts to make [Ni(H2O)2(15-crown-5)](NO3)2, i.e. to insert the Ni2+ ion into the 15-crown-5 macrocycle, gave the mono- (two polymorphs) and dihydrate of a co-crystal of[Ni(H2O)6](NO3)2 and 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane=15C5).

An unexpected co-crystal with a variable degree of order: 1:1 rac-1,2-cyclohexanediol/triphenylphosphine oxide.

A 1:1 co-crystal of rac-trans-1,2-C(6)H(10)(OH)(2) and (C(6)H(5))(3)PO has been found that is unusual because there are no strong interactions between the two kinds of molecules, which are segregated into layers. Furthermore, neither pure rac-1,2-cyclohexanediol (CHD) nor pure triphenylphosphine oxide (TPPO) has any obvious packing problem that would make the formation of inclusion complexes likely. The TPPO layers are very much like those found in two of the four known polymorphs of pure TPPO.

Solid-state compounds of stereoisomers: cis and trans isomers of 1,2-cyclohexanediol and 2,3-tetralindiol.

The phases of 1,2,3,4-tetrahydro-2,3-naphthalenediol (or 2,3-tetralindiol) and of 1,2-cyclohexanediol have been investigated. The structure of a very stable 1:1 compound (or co-crystal) of the cis and trans isomers of 2,3-tetralindiol, the existence of which has been known for nearly a century, has finally been determined. No evidence of any analogous compound between the cis and trans isomers of 1,2-cyclohexanediol has been found.

S,S-1,2-Dicyclohexylethane-1,2-diol and its racemic compound: a striking exception to Wallach's rule.

The structures of enantiopure S,S-1,2-dicyclohexylethane-1,2-diol and its racemic compound (rac-S,S-1,2-dicyclohexylethane-1,2-diol) have been determined at 295 and 173 K. The crystals of the enantiopure material are more than 4% denser than the crystals of the racemic compound, but the melting points indicate that the crystals of the less dense racemic compound are considerably more stable than those of the racemic conglomerate. This apparent exception to the correlation of crystal density and melting point is explained.

The unusual phases of anhydrous and hydrated pinacol.

The structure of highly twinned pinacol (2,3-dimethyl-2,3-butanediol) monohydrate, the existence of which has been known since 1922, has been determined, and the structures of anhydrous pinacol and its two other known hydrates have been reinvestigated. All the phases are unusual. The anhydrous phase is exceptional among molecular crystals in having molecules located on three different symmetry sites (1, \overline 1 and 2).

Three modulation patterns in four related [M(H2O)2(15-crown-5)](NO3)2 structures.

The structures of [M(H2O)2(15-crown-5)](NO3)2, M = Cu, Zn, Mg and Co, and 15-crown-5 = 1,4,7,10,13-pentaoxacyclopentadecane, have been redetermined at 294 and 90 K. The four structures, and a second form of the Cu structure, have been reported in the literature, but are all incorrect in some significant way. The structures, which all have at least two independent formula units (i.

A helical structure with Z' = 10.

The structure of N''-cyano-N,N-diisopropylguanidine, a push-pull nitrile that includes the H(2)N-C=N-CN fragment, has been determined. Although the number of molecules in the asymmetric unit is exceptionally high (Z' = 10), there were no special crystallographic difficulties associated with data collection, structure solution or structure refinement because the individual molecules are small (12 non-H atoms) and because there is no important crystallographic pseudosymmetry. A complete set of N-H.

Packing conflicts in the Z' = 5 structure of CF(3)(CF(2))(3)(CH(2))(10)COOH.

Dimers of the partially fluorinated amphiphile 1-(perfluorobutyl)undecanoic acid, CF(3)(CF(2))(3)(CH(2))(10)COOH, crystallize in the same basic packing arrangement found for many n-alkyl carboxylic acids. The unit cell of the fluorinated compound, however, contains five independent molecules rather than the one found for most n-alkyl carboxylic acids including CH(3)(CH(2))(13)COOH. The structures of the two C(15) acids are compared.

2-Biphenylol revisited.

The substantially disordered structure of 2-biphenylol [Perrin et al. (1987). Acta Cryst.