Hetero‐Seeding and Solid Mixture to Obtain New Crystalline Forms

Para‐methyl benzyl alcohol (p‐MeBA II) and para‐chloro benzyl alcohol (p‐ClBA) are quasi‐isostructural and share the same hydrogen‐bond patterns, but their crystals are not isomorphous. No new polymorphs could be obtained by conventional polymorph screening based on different solvents and different crystallization conditions. Formation of a new polymorph of p‐MeBA named p‐MeBA I, isomorphous with the crystal of p‐ClBA, was induced by hetero‐seeding with a small quantity of powdered p‐ClBA added to a supersaturated solution of p‐MeBA in hexane, while seeding of p‐ClBA with p‐MeBA II failed to give a new phase of p‐ClBA isomorphous with known crystalline p‐MeBA II. Mixed crystals of p‐MeBA and p‐ClBA were also prepared with different p‐MeBA/p‐ClBA ratios to understand the role of the different functional groups in the crystal structure. Crystal phases were characterized by combined use of single‐crystal and powder X‐ray diffraction, differential scanning calorimetry, and solid‐state NMR spectroscopy.     

Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds. Part III. Crystallographic and dynamic evidence for the intermediate of the ‘merry-go-round’ process in nonacarbonyl-μ3-(1,3,5-trithiane)-tetrairidium

The title complex crystallises in two C_3v, isomeric forms differing in carbonyl-ligand arrangement. In solution, the isomer 1b with three edge-bridging carbonyls on a common face of the metal tetrahedron converts via an endothermic equilibrium into the isomer 1u with no bridging carbonyls. The latter was shown by ¹³C-NMR to be the intermediate of the ‘merry-go-round’ process which exchanges the sites of the basal CO's.     

Gel-like lyomesophases formed in organic solvents by self-assembled guanine ribbons

Lipophilic guanosine derivatives are self-assembled into ribbonlike aggregates, both in the crystal state and in solution. The structure of the ribbons has been characterised by single-crystal X-ray diffraction and, in solution, by NMR spectroscopy and ESI-MS. Two different ribbons with different patterns of hydrogen bonds are present in the solid state and in chloroform solutions. The gel-like phases obtained in hexadecane, toluene and chloroform have been investigated by optical microscopy and small-angle X-ray diffraction: the type of phase observed is related to the molecular structure of the compounds and depends dramatically on the solvent. The structures of the phases are discussed, with the presence of the two different ribbons being taken into account.     

Shape persistence and bistability of planar three-fold core polyphenylene dendrimers: a molecular dynamics study

We present an atomistic molecular dynamics investigation of the structural time evolution of isolated polyphenylene dendrimers, carbon based dendrimers with a planar core formed by a 1,3,5 trisubstituted benzene ring. Simulations are carried out at low (80 K) and room temperature. A general classification of the conformations (core conformations) assumed by the three dendrimer branches with respect to the planar core is presented. It is found that out of the six possible core conformations only four are stable, the remaining two being unstable for steric reasons. For second generation dendrimers, two of the four accessible core conformations are associated with an open arrangement of the three branches attached to the planar 3-fold core of the dendrimer, whereas the remaining two are associated with a collapsed arrangement of two branches. At low temperature the initial conformation is generally conserved whereas at room temperature jumps among the four possible core conformations are observed in the nanosecond time range. For second generation dendrimers the core conformation jumps are associated with an oscillation between two global shape states: open and collapsed. The computed bistability of the global shape suggests additional possible functional uses for some of these carbon based dendrimers.     

A variable temperature study of Ru3(CO)12 in the solid state and the generation of alternative crystal structures

Summary The molecular and crystal structure of Ru₃(CO)₁₂ has been reinvestigated on single-crystal X-ray diffraction data collected at 100 and 150 K. Thermal motion analysis on high-order diffraction data have been used to characterize the motion about equilibrium positions of the molecule as a whole and of groups of atoms. It has been shown that, while the molecule undergoes an almost isotropic rigid-body librational and translational motion, the three Ru(CO)₄ units undergo different librational motions about the axes passing between each Ru atom and bisecting the opposite 615-01 bond. Packing potential energy calculations and computer graphics have been used to show that the most favoured packing motif in the experimental crystal is based on interlocking between tetracarbonyl units formed by two pairs of CO ligands at right angles along a 615-02 edge. Starting from the structure of Ru₃(CO)₁₂, alternative crystal arrangements have been generated and compared with the experimental crystal structure in terms of packing efficiency and cohesive energy.     

Dinuclear ruthenium bipyridine complexes with a bis(iminodioxolene)-meta-phenylene ligand: magnetic coupling and mixed valence character of the semiquinonato species

The dinuclear bis-iminosemiquinonato [Ru(bpy)(2)(L)(2)Ru(bpy)(2)](PF(6))(2) complex where L is the deprotonated form of N,N'-bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine (H(4)L), has been synthesised. For comparison purposes, the mononuclear iminosemiquinonato [Ru(bpy)(2)L']PF(6) formed by the N-phenyl-o-aminophenol (H(2)L') was also investigated. Magnetic investigation evidences that the bis-semiquinonato species is characterised by a moderately strong ferromagnetic interaction (H=JS(1) x S(2), J=-85(5) cm(-1)). This result is the theoretically expected one and it is contrasting with the antiferromagnetic interaction observed in previously investigated complexes containing the same ligand. It is suggested that the different behaviour is determined by the different coordination requirements occurring in these complexes. The spectroelectrochemical analysis provides a clear evidence of the valence trapped character of the mono-semiquinonato forms of the ligand. The two mixed valence semiquinonato species containing the quinonato-semiquinonato and semiquinonato-catecholato bridging ligand can be described as class II, but show a significantly different electronic coupling. The electronic factors determining this behaviour are discussed.