Comparison of the crystal structures of the low- and high-temperature forms of bis[4-(dimethylamino)pyridine]dithiocyanatocobalt(II)

Single crystals of the high-temperature form I of [Co(NCS)2(DMAP)2] (DMAP = 4-dimethylaminopyridine, C7H10N2) were obtained accidentally by the reaction of Co(NCS)2 with DMAP at slightly elevated temperatures under kinetic control. This modification crystallizes in the monoclinic space group P21/m and is isotypic with the corresponding Zn compound. The asymmetric unit consists of one crystallographically independent Co cation and two crystallographically independent thiocyanate anions that are located on a crystallographic mirror plane and one DMAP ligand (general position). In its crystal structure the discrete complexes are linked by C—H...S hydrogen bonds into a three-dimensional network. For comparison, the crystal structure of the known low-temperature form II, which is already thermodynamically stable at room temperature, was redetermined at the same temperature. In this polymorph the complexes are connected by C—H...S and C—H...N hydrogen bonds into a three-dimensional network. At 100 K the density of the high-temperature form I (ρ = 1.457 g cm−3) is lower than that of the low-temperature form II (ρ = 1.462 g cm−3), which is in contrast to the values determined by XRPD at room temperature. Therefore, these two forms represent an exception to the Kitaigorodskii density rule, for which extensive intermolecular hydrogen bonding in form II might be responsible.

Salts of octabismuth(2+) polycations crystallized from Lewis-acidic ionic liquids

The reaction of Bi and BiCl3 with RbCl or CsCl in the Lewis-acidic ionic liquid (IL) [BMIm]Cl·4AlCl3 at T = 200 °C yielded air-sensitive, shiny black crystals. X-ray diffraction on single crystals revealed the hexagonal structures of two new salts (Bi8)M[AlCl4]3 (M = Rb, Cs), which are isostructural to the high-temperature form of (Bi8)Tl[AlCl4]3. The known (Bi8)2+ polycation is a square antiprism and can be interpreted as an arachno cluster following modified Wade rules. The crystal structure is a complex variant of the hexagonal perovskite structure type ABX 3 with A = (Bi8)2+, B = M + and X = [AlCl4]–. Chiral strands ∞ { M [ AlCl 4 ] 3 } 2 − ∞ 1 $\infty {}_{\infty }{}^{1}{\left\{M{\left[{\text{AlCl}}_{4}\right]}_{3}\right\}}^{2-}$ (M = Rb, Cs) run along [001]. The (Bi8)2+ polycations are only weakly coordinated inside a cage of 24 chloride ions and show dynamic rotational disorder at room temperature. Upon slow cooling to 170 K, the reorientation of the clusters was frozen, yet no long-range order was established.

Principles of weakly ordered domains in intermetallics: the cooperative effects of atomic packing and electronics in Fe2Al5

Many complex intermetallic structures feature a curious juxtaposition of domains with strict 3D periodicity and regions of much weaker order or incommensurability. This article explores the basic principles leading to such arrangements through an investigation of the weakly ordered channels of Fe2Al5. It starts by experimentally confirming the earlier crystallographic model of the high-temperature form, in which nearly continuous columns of electron density corresponding to disordered Al atoms emerge. Then electronic structure calculations on ordered models are used to determine the factors leading to the formation of these columns. These calculations reveal electronic pseudogaps near 16 electrons/Fe atom, an electron concentration close to the Al-rich side of the phase's homogeneity range. Through a reversed approximation Molecular Orbital (raMO) analysis, these pseudogaps are correlated with the filling of 18-electron configurations on the Fe atoms with the support of isolobal σ Fe–Fe bonds. The resulting preference for 16 electrons/Fe requires a fractional number of Al atoms in the Fe2Al5 unit cell. Density functional theory–chemical pressure (DFT-CP) analysis is then applied to investigate how this nonstoichiometry is accommodated. The CP schemes reveal strong quadrupolar distributions on the Al atoms of the channels, suggestive of soft atomic motions along the undulating electron density observed in the Fourier map that allow the Al positions to shift easily in response to compositional changes. Such a combination of preferred electron counts tied to stoichiometry and continuous paths of CP quadrupoles could provide predictive indicators for the emergence of channels of disordered or incommensurately spaced atoms in intermetallic structures.

Crystal structure of the high-temperature form of the trisulfide Cs2S3 and the (3+1)D modulated structure of the telluride K37Te28

The high-temperature polymorph of the trisulfide Cs2S3, which has been synthesized from Cs2S2 and elemental sulfur, crystallizes in a new structure type (monoclinic, space group P21/c, a=999.97(4), b=1029.30(5), c=2642.07(12) pm, β=90.083(2)°, Z=16, R1=0.0324). The structure contains four crystallographically independent angled ${\rm{S}}_3^{2 - }$ trisulfide ions with S–S distances of 205.7–208.3 pm. The distorted b.c.c. packing of the anions and their insertion in the five-membered rings of 3.53+3.5.3.5. (1:1) Cs nets are similarly found in the r.t. form (Cmc21, K2S3-type structure) and the two polymorphs differ mainly in the orientation of the S3 groups. The second title compound, K37Te28, was synthesized from stoichiometric melts of the elements. It forms a complex (3+1)D modulated tetragonal structure (space group I41/amd (00σ3)s0s0, q=(0, 0, 0.5143), a=1923.22(2), c=2626.66(4) pm, Z=4, R1all=0.0837). According to K37Te28=K37[Te(1X)]8[Te(2X)2]6[Te(3X)8] the structure contains three different types of Te anions: The two crystallographically different isolated telluride anions [Te(1X)]2− are coordinated by 9/10 K+ cations. Three [Te(2X)2]2− dumbbells (dTe-Te=277.9/286.4 pm) are arranged to ‘hexamers’. The Te(31) and Te(32) atoms are located in columns of face-sharing K square antiprisms. Their z position modulation, which is accompanied by a smaller shift of the surrounding K+ cations, results in the decomposition of the [Te(3X)8]2 chain in a sequence |:Te3–Te2–Te2–Te3–Te2–Te2–Te2:| of dumbbells Te22− (dTe–Te=304 pm) and hypervalent linear trimers Te34− (dTe–Te=325 pm).

Mechanochemical synthesis ofN-salicylideneaniline: thermosalient effect of polymorphic crystals

Polymorphs of the dichloro derivative ofN-salicylideneaniline exhibit mechanical responses such as jumping (Forms I and III) and exploding (Form II) in its three polymorphs. The molecules are connectedviathe amide N—H...O dimer synthon and C—Cl...O halogen bond in the three crystal structures. A fourth high-temperature Form IV was confirmed by variable-temperature single-crystal X-ray diffraction at 180°C. The behaviour of jumping exhibited by the polymorphic crystals of Forms I and III is due to the layered sheet morphology and the transmission of thermal stress in a single direction, compared with the corrugated sheet structure of Form II such that heat dissipation is more isotropic causing blasting. The role of weak C—Cl...O interactions in the thermal response of molecular crystals is discussed.

Two reversible enantiotropic phase transitions in a pentacoordinate silicon complex with anO,N,O′-tridentate valinate ligand

Dimethyl[N-(4-oxidopent-3-en-2-ylidene)valinato-κ3O,N,O′]silicon(IV), C12H21NO3Si, (II), crystallizes in the orthorhombic space groupP212121. The chiral compound undergoes two sharp enantiotropic phase transitions upon cooling. The first transformation occurs at 163 K to yield a unit cell with one axis having double length. This intermediate-temperature form has the monoclinic space groupP21. The second transition takes place at 142 K and converts the single crystal into the low-temperature form in the orthorhombic space groupP212121. This transition proceeds under tripling of theaaxis of the high-temperature form. Both phase transitions are fully reversible and correspond to order–disorder transitions of the isopropyl group of the valine unit in the ligand backbone. The phase transitions presented here raise questions, since they do not fit into the rules of group–subgroup relationships.