Guillermo Antorrena

MULTIFUNCTIONAL COORDINATION COMPOUNDS: DESIGN AND PROPERTIES

Abstract Cleverly designed molecular building blocks provide chemists with the tools of a powerful molecular-scale construction set. They enable them to engineer materials that possess a predictable order and useful solid-state properties. Hence, it is in the realm of supramolecular chemistry to follow a strategy for synthesizing materials which combine a selected set of properties, for instance from the areas of magnetism, photophysics and electronics. As a possible approach, host/guest solids which are based on extended anionic, homo- and bi-metallic oxalato-bridged transition-metal compounds with two- and three-dimensional connectivities are investigated. In particular, we report herein in detail about their structural properties and their multifunctional characteristics in the area of molecular magnetism and photophysics.

Probing magnetic exchange interactions in molecular magnets: an inclusion compound of a dithiadiazolyl radical

Co-sublimation of the dithiadiazolyl radical, p-NCC 6 H 4 CNSSN 2 with the inclusion-forming host compound perhydrotriphenylene (PHTP) leads to trichroic crystals of the host-guest complex [PHTP-2]. Molecules of 2 are linked through a CN‥S interaction (ca. 3.03 A) to form polar chains within the channels of the PHTP host lattice. The host-guest ratio is approximately 5:1. Pyroelectric and non-linear optic responses indicate that crystals of [PHTP-2] are macroscopically polar. A combination of variable temperature solid state EPR spectroscopy and magnetic susceptibility measurements are used to probe the electronic properties of [PHTP-2]; whilst [PHTP-2] is paramagnetic, no exchange coupling between neighbouring molecules of 2 within the channels could be established. The implications of these results on the magnetic exchange pathway of the weak ferromagnet, β-p-NCC 6 F 4 CNSSN β-1, are discussed.

Thermal and magnetic study of mixed-metal oxalate-bridged 2D magnets

Abstract The mixed-metal compounds [P(Ph) 4 ][M II Cr(C 2 O 4 ) 3 ] (Ph = Phenyl; M = Mn, Fe) have been studied by calorimetric and magnetic AC and DC techniques. The Mn-derivative presents low dimensional magnetic character and shows evidence of the onset of a 3D-magnetic ordering. However, experimental results in the Fe-derivative point to a spin-glass-like behavior. The differences observed in the magnetic behavior of both compounds are assigned to the effect of random anisotropy due to the presence of structural disorder.

Polymeric, H-bonded, and chelatable phenoxyl and nitroxide radicals

Abstract Various examples are described of a multi-strategy approach to improvement of multispin exchange design. For example, H-bonded benzimidazole 4-t-butylnitroxide shows apparent simple planar 2D-type interaction, with a Weiss constant of Θ = −4.24 K and an intraplane exchange interaction of J = −1.60 K.

Multi-Functional Magnetic Materials Based on Dithiadiazolyl Free Radicals

Abstract The crystal structures of a series of dithiadiazolyl free radicals, p-XC6F4CNSSN (X˭F,Cl,Br and CN) are described in terms of dipolar interactions which induce chain-like motifs in the solid state. When X˭CN two morphologies are observed; the β-phase is polar and undergoes a phase transition to a weakly ferromagnetic state at 36K, an unprecedented temperature for an organic radical. The polar structure which gives rise to weak ferromagnetism in β-p-NCC6F4CNSSN, also provides a number of other potential solid state properties which may interact independently or cooperatively with the magnetic behaviour.

Spontaneous Magnetisation at 36k in a Sulfur-Nitrogen Radical

Abstract The radical p-NCC6F4CNSSN, 1, can be obtained in two different phases depending upon sublimation conditions. The α-phase, lα, crystallises in space group P 1, and the molecules are related through an inversion centre. The β- phase, lβ crystallises in the orthorhombic space group Fdd2 and the molecules are not related by an inversion centre. The latter shows a sharp rise in the magnetic susceptibility at 36K which is accompanied by an out-of-phase component in the a.c. susceptibility. Heat capacity measurements show a distinct peak at 35.5K indicative of the long range ordering of the magnetic moments. The magnetic ordering of the compound is also confirmed by neutron diffraction experiments. Magnetisation isotherms indicate the presence of a small residual moment that saturates below 100 Oe. Extrapolation of the residual moment at T= 0K gives a value for the spontaneous magnetisation of 1.5×l0−3 μB/molecule. The behaviour is interpreted in terms of weak ferromagnetism.

Preparation and characterization of oriented thin films of a sulfur–nitrogen radical

Abstract Highly oriented thin films of the organic radical p- NC.C 6 F 4 .CNSSN have been prepared by thermal evaporation in high vacuum on glass and on highly oriented pyrolytic graphite (HOPG) substrates. The films are polycrystalline and crystallize in one of the known polymorphs, in the triclinic α -phase. The films exhibit a high degree of orientation with their (112) planes parallel to the surface of the substrates. No in-plane texture is observed.

A novel paramagnetic dithiadiazolyl radical: Crystal structure and magnetic properties of p-BrC6F4CNSSN•

The dithiadiazolyl radical p-BrC6F4CNSSN· 2 retains its monomeric nature in the solid state; variable temperature magnetic studies on 2 indicate Curie–Weiss behaviour (θ = –27 K) above 60 K with an effective magnetic moment of 1.45 µB at room temperature; the absence of long-range magnetic order down to 1.8 K is attributed to the low dimensionality of the magnetic exchange pathway predicted on the basis of the inter-molecular S‥N interactions.

Modification of molecular packing: crystal structures and magnetic properties of monomeric and dimeric difluorophenyl-1,2,3,5-dithiadiazolyl radicals

Two dithiadiazolyl radicals of formula [F 2 C 6 H 3 CNSSN] ˙ were prepared and characterised by X-ray crystallography; in the solid state the 2′,3′-difluoro isomer forms discrete twisted dimers through a spin-paired singly occupied–singly occupied molecular orbital interaction with an intra-dimer S · · · S distance of 3.020(4) A, whereas the 2′,5′-isomer is composed of uniform stacks with unexpectedly long intra-stack S · · · S contacts [3.544(3) A].

Novel Magnetic Materials Based on Sulfur-Nitrogen Radicals

The application of sulfur-nitrogen free-radicals as molecular ‘bricks’ for the construction of novel magnetic materials is described and exemplified by the dithiadiazolyl radical, which becomes weakly ferromagnetic at 36K.