Joan Cirujeda

Control of the structural dimensionality in hydrogen-bonded self-assemblies of open-shell molecules. Extension of intermolecular ferromagnetic interactions in α-phenyl nitronyl nitroxide radicals into three dimensions

The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different hydrogen-bonded molecular self-assemblies with distinct dimensionalities; the mono-ortho-substituted isomer is most remarkable since it shows a three-dimensional network of weak hydrogen bonds exhibiting a bulk ferromagnetic transition at 0.45 K.

Structure–Magnetism Relationships in α-Nitronyl Nitroxide Radicals

Statistical analysis of the packing of all known α-nitronyl nitroxide crystals (see diagram) with dominant ferro- and antiferromagnetic properties shows that there are no differences in the geometrical distribution of the NO⋅⋅⋅ON, C(sp3)−H⋅⋅⋅O−N and C(sp3)−H⋅⋅⋅O−N contacts in these two subsets. The magnetic properties are a result of the relative orientation of the molecules, that is, of their packing patterns (or synthons) and not of the geometry of an individual contact.

Role of hydrogen bonds in the propagation of ferromagnetic interactions in organic molecular solids. Part 1.—The p-hydroxyphenyl α-nitronyl aminoxyl radical case

The role of hydrogen bonds in the obtention of magnetic molecular materials is investigated in a purely organic openshell molecular solid. The structural and magnetic properties of the 2-(4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-yloxyl 3-oxide radical (1) are studied in detail. The intermolecular hydrogen bonds in 1 are found to be able to control the crystal packing (i.e. the relative disposition of the radical molecules within the crystal lattice) and, at the same time, to propagate ferromagnetic intermolecular interactions along certain directions of the solid. Molecular orbital (MO) calculations, structural studies about the crystal packing, static magnetic susceptibility measurements, electron paramagnetic resonance (EPR) solution spectra and a detailed EPR study of an oriented single crystal of the title compound are presented and discussed. All these studies point strongly towards the fact that this material behaves as a quasi-two-dimensional ferromagnet, in which hydrogen bonds of O–H ⋯ O–N and C–H ⋯ O–N types are found to be responsible for the ransmission of the ferromagnetic interactions within the (a, b) crystallographic plane.

Stereochemistry of Phenylα-Nitronyl Nitroxide Radicals

An extensive investigation of the conformations adopted by the family of phenyl alpha-nitronyl nitroxides has been carried out. A database containing 110 crystal structures was used in a statistical study of the solid-state geometries and conformations of these radicals. This study revealed that the favoured conformations involve a twisted distortion in the imidazolyl rings and a twist between the aromatic and heterocyclic rings in the molecules. As a consequence, these radicals show two types of preferred conformations in the solid state: the pseudo-anti enantiomeric pair and the pseudo-eclipsed pair, the latter type being the most statistically probable. A new chiral member of this group of radicals that bears a lactate moiety, (R)-1, and its corresponding racemic compound, (R,S)-1, have been prepared in order to study the influence of chiral induction from the stereogenic centre on the torsion angle between the aromatic and heterocyclic rings of the alpha-nitronyl nitroxides. The X-ray crystal structures of the enantiopure and racemic compounds, which both reveal chains of molecules sustained by strong O-H...O hydrogen bonds between the carboxylic acid group and the ON group of the radical in the solid, as well as their magnetic properties have been determined. Remarkably, the molecules with a given stereogenic centre have a single helical sense between their component rings, even in the racemic crystal. Chiral induction from the stereogenic centre to the radical unit has also been proved by CD spectroscopy in the solid state. The results of these experiments have been rationalised by ab initio calculations of the spectra.

The Hydrogen Bonding Strategy. A New Approach Towards Purely Organic/Molecular Ferromagnets

Abstract The variation of the number and position of OH substituents at the phenyl ring of α-phenyl nitronyl nitroxide radicals yields different H-bonded molecular self-assemblies in the solid state but do not change essentially their molecular spin density distributions. In accordance with the distinct structural dimensionalities, the obtained radicals show a large variety of magnetic behaviors, being the mono ortho substituted isomer the most remarkable one since it shows a 3-D network of weak H-bonds exhibiting a bulk ferromagnetic transition below 0.45K.

Magnetic transitions in the positional isomers (4-HOPNN and 2-HOPNN) of an organic nitronyl nitroxide radical using muon–spin rotation

The zero-field muon–spin rotation/relaxation technique has been used in the direct observation of spontaneous magnetic order below the Curie temperature of TC∼500 mK in 2-hydroxyphenyl-α-nitronyl nitroxide (2-HOPNN) and two successive magnetic transitions in 4-hydroxyphenyl-α-nitronyl nitroxide (4-HOPNN) at TC1≈700 and TC2∼100 mK. The different position of the hydroxyl group in these positional isomeric compounds causes different crystal packings and magnetic dimensionalities. The results reveal different magnetic transitions in these two α-nitronyl nitroxides radicals suggesting a 2D–3D magnetic crossover in 4-HOPNN.

Theoretical analysis of the crystal packing of nitronyl nitroxide radicals: the packing of the α-2-hydro nitronyl nitroxide radical

Abstract The crystal packing of the simplest member of the nitronyl nitroxide familyof compounds, the 2-hydro nitronyl nitroxide radical in its α phase, is analyzed by means of a combination of distance analysis and accurate ab initio computations using correlated methods and extended basis sets. The packing is found to be driven by two types of intermolecular interactions, the C(sp 2 )H ··· ON and C(sp 3 )H ··· ON interactions, both of them found to be stable by ab initio computations (by −3.71 and −0.40 kcal/mol, respectively). The packing can be rationalized as planes kept together by C(sp 2 H ··· ON interactions. The planes are then linked by C(sp 3 )H ··· ON interactions.

Magnetostructural study of substituted α-nitronyl aminoxyl radicals with chlorine and hydroxy groups as crystalline design elements

We present a new family of phenyl substituted α-nitronyl aminoxyl radicals which contain hydroxy- and chlorine-substituents as crystal engineering tools. The magnetic behaviour of these radicals strongly differs in dimensionality and strength, showing in all cases antiferromagnetic interactions. We have determined the X-ray crystal structure and analysed the crystal packings of these radicals. From this analysis all the observed magnetic properties can be conveniently rationalized by only considering the close contacts of NO groups of neighbouring molecules according to the generally accepted mechanisms for intermolecular magnetic interactions. Beside the strong O–H‥O(–N) hydrogen bonds and the weaker C–H‥O(–N) hydrogen bonds, weak Cl‥H bonds also seem to play a significant role in determining the molecular arrangement in the solid state and, therefore, the magnetic properties. In only one case are close Cl‥Cl contacts observed, pointing to attractive interactions between chlorine atoms.

Ab Initio Computation of the Spin Population of Substituted α-Nitronyl Nitroxide Radicals

Abstract The ability of various ab initio methods (UHF, ROHF, MCSCF, MP2, CISD, CCD) to reproduce the experimental information on the spin distribution over various substituted α-nitronyl nitroxide radicals was tested using various basis sets which range from single zeta to near Hartree-Fock limit quality.

A Theoretical Analysis of the Packing and Polymorphism of the 2-Hydro Nttronyl Nitroxide Crystal

Abstract The packing of the two crystallographics forms the 2-hydro nitronyl nitroxide (HNN) molecule has been studied. Using the ab initio methodology described in the accompanying paper, the packing is rationalized. In both crystals the primary structure are the HNN dimers linked trough C(sp2)-H…O-N contacts. These dimers form planes of molecules, which constitute the secondary structure. The structure and stability of the planes are different in the two crystals. Finally, the planes form stacks, the tertiary and last structure in both crystals. The forces involved in each type of structure have been quantified. The relationship between the two polymorph crystal forms have been analyzed and rationalized.