René Pellaux

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.

Crystal Structures, Magnetic Structures and Photophysics in Supramolecular Transition-Metal Oxalate Compounds

Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

Resonant excitation energy transfer in [Rh(bpy)3][NaCr(ox)3]ClO4

Resonant fluorescence line narrowing of the R1 line of the [Cr(ox)3]3− chromophore in [Rh(bpy)3][NaCr(ox)3]ClO4 at 1.6 K neither gives rise to the usual three-line pattern nor to spectral diffusion. Instead multi-line spectra with spacings equal to the zero-field splitting of the ground state are observed. This phenomenon is attributed to efficient non-radiative resonant energy transfer within the R1 line.

Polymeric Two- and Three-Dimensional Transition-Metal Networks. Crystal Structures and Magnetic Properties

Abstract A strategy for the design of molecule-based magnets consists of assembling transition-metal ions in two-dimensional (2D) or three-dimensional (3D) networks in order to maximize the interactions between the magnetic centers. While increasing the dimensionality of the compounds, an increase in Tc, the critical temperature of spontaneous magnetization should, in principal, be favoured. We are currently studying materials that are formed by the interaction of transition-metal ions with the oxalate ligand, C2O4 2-, and will present a straightforward concept for building up achiral 2D and chiral 3D homo- and bimetallic networks. Whereas a variety of metal ions can be incorporated, a correlation of the magnetic behavior and the relationship to the special topology of e.g. the chiral, cubic 3-connected 10-gon net (10,3) is not yet fully understood.

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.

Supramolecular Host/Guest Compounds and their Prospects for Multifunctional Materials

Abstract Cleverly designed molecular building blocks provide chemists with the tools of a powerful molecular-scale construction set. They enable them to engineer materials having 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 bimetallic 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.

Solid State Supramolecular Chemistry of Oxalato-Bridged Transition-Metal Compounds with Two- and Three-Dimensional Connectivities — Structure, Magnetism and Photophysics

A current challenge in the field of molecular magnetism is the control of both molecular and supramolecular structures with preferably two-dimensional (2D) or three-dimensional (3D) motifs. In case of coordination compounds, a high structural organization is ensured through multiple binding sites of transition-metal ions, which may lead to a higher dimensional architecture of extended inorganic networks. Thus, the overall topology of the extended structure is strongly influenced by the coordination algorithm of the linking metal ions as well as on the choice of the bridging ligand system. In this respect, the oxalate and thiooxalate ions exhibit a versatile, distinct bonding pattern because of their bis-chelating character leading to various polymeric molecular compounds. Consequently, numerous oxalato-bridged homo- and bimetallic assemblies have been reported over the past few years and their magnetic properties have been extensively investigated [1–6]. As a result, the design of bulk magnetic materials based on metal complexes has been one of the most stimulating subjects in the field of transition-metal chemistry.

Magnetic structure of two- and three-dimensional supramolecular compounds

Supramolecular chiral networks of oxalato-bridged transition metals show either two- or three-dimensional structural features. The magnetic structures of such compounds have been investigated by means of elastic neutron powder diffraction.

Photophysical and Photochemical Properties of Three-Dimensional Metal-Tris-Oxalate Network Structures

Abstract Chemical variation and combination of metal ions of different valencies in the oxalate backbone as well as in the tris-bpy cation of the three-dimensional network structures of the type [MII 2(ox)3][MII(bpy)3] (bpy = 2,2′-bipyridine, ox = C2O4 2-), [MIMIII(ox)3][MII(bpy)3] and [MIMIII(ox)3][MIII(bpy)3]ClO4 offer unique opportunities for studying a large variety of photophysical processes. Depending upon the relative energies of the excited states of the chromophores, excitation energy transfer either from the tris-bipyridine cation to the oxalate backbone or vice versa is observed, as for instance from [Ru(bpy)3]2+ as photo-sensitiser to [Cr(ox)3]3- as energy acceptor in the combination [NaCr(ox)3][Ru(bpy)3], or from [Cr(ox)3]3- to [Cr(bpy)3]3+ in [NaCr(ox)3][Cr(bpy)3]ClO4. In addition efficient energy migration within the oxalate backbone is observed. Furthermore, depending upon the excited state redox potentials, light-induced electron transfer processes may be envisaged.

Magnetism and photophysics in supramolecular transition-metal compounds

Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.