Abstract
Abstract
While oxygen and nitrogen are ubiquitous as bridging ligators in molecule-based magnetic systems, fluoride is much less explored and studied in this respect. In this project, new polynuclear complexes and one-dimensional polymeric systems, based on fluoride linkages between transition metal ions and between transition metal and lanthanide ions, have been synthetized and thoroughly characterized. Assembly of kinetically robust 3d fluoride
complexes with various lanthanide precursors has proven to be a convenient route to small heterometallic complexes. However, the use of more labile precursors was found to be possible despite the insoluble nature of lanthanide fluorides. The propensity of fluoride for linear bridging also aids in bringing topological control into the synthesis of 3d-4f clusters. The resulting, structurally simple, systems have allowed for modeling of their magnetic
properties and for the first quantifications of magnetic exchange coupling parameters between 3d and 4f centers across a fluoride bridge. This includes determination of the sign and magnitude of energy of interaction in notoriously complicated, orbitally degenerate, lanthanide ion systems, by X-ray magnetic circular dichroism. By using the derived magnetostructural correlations, new systems with large spin ground states and negligible magnetic anisotropy were synthesized and characterized. Herein, large magneto-caloric effects were
observed and it was experimentally confirmed that vanishing magnetic exchange interactions in a lower total spin polynuclear complex had a larger magnetic entropy change during a adiabatic demagnetization than an, all ferromagnetically coupled, complex with a larger spin ground state.
Diffuse orbitals and strong magnetic anisotropy resulting from large values of the spinorbit coupling parameter make complexes with central ions from the 4d and 5d series particularly interesting building blocks for magnetic materials. The main obstacle is the common inherent lability of hexafluoridometallates towards hydrolysis, a tendency that is strongly diminished for several 4d and 5d [MF6]2– complexes. We herein present the use of such simple [MF6]2– (M = Zr, Re, Ir) anions as modules for magnetic architectures of various dimensionality. For instance, for rhenium(IV), using direct current (dc) and alternating current
(ac) magnetic susceptibility measurements, inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) spectroscopies, the magnetic properties of the isolated [ReF6]2– unit in (PPh4)2[ReF6]·2H2O have been fully studied including the slow relaxation of the magnetization observed below ca. 4 K. This slow dynamic is preserved for the tetragonal, one-dimensional coordination polymer [Zn(viz)4ReF6] (viz = 1-vinylimidazole),
demonstrating the irrelevance of low symmetry for such magnetization dynamics in systems with easy-plane magnetic anisotropy. The ability of fluoride to mediate significant exchange interactions is exemplified by the isostructural [M(viz)4ReF6] (M = Mn, Fe, Co, Ni) analogues in which the magnitude of the MII–ReIV interactions are comparable to the coupling present in related cyanide-bridged systems.
The phenomenological Heisenberg-Dirac-van Vleck Hamiltonian, commonly employed for the modeling of magnetic super-exchange, fails to describe cases where orbital degeneracy is present. In molecular magnetic systems, the orbital degeneracy is normally lifted by the low local symmetry of the ligand field. However, in certain cases where the energy splittings resulting from slight departure from perfect, e.g. cubic symmetry, are small compared to the
general energy scheme, the orbital angular momentum may be left unquenched. This effect becomes particularly important for the heavier 4d and 5d transition elements, where ligand field splittings exceed those of the 3d metal ions. Herein, we have contributed to the active discussion on anisotropic exchange in orbitally degenerate systems by studying the series of
(NEt4)[{MnIII(5-Brsalen)2(MeOH)}2{MIII(CN)6}] (M = Ru, Os, Ir, 5-Brsalen = N,N′-
ethylenebis(5-bromosalicylidene)iminate) complexes by magnetic measurements, INS and frequency-domain Fourier-transform THz electron paramagnetic resonance spectroscopy.
Lanthanides are interesting components for magnetic materials and have a huge
underexplored potential in molecular magnetic systems. Advancements are, however, significantly hindered by their complicated electronic and magnetic nature. In this project, we have studied the properties of a class of simple lanthanide single-ion magnets with crystallographic or approximate trigonal symmetry. The combination of high symmetry, surprisingly intense inelastic neutron scattering excitations, optical transparency, and the possibility to grow large single-crystals of the Ln(trensal) (H3trensal = 2,2′,2″-tris(2-
(salicylideneimino)ethyl)amine) systems have provided us with a possibility to reach an experiment-based understanding of the electronic structure, as well as static and dynamic magnetic properties, that is nearly unprecedented in this field of research. Indeed, we have found no indications of the relevance of the thermally activated Orbach process for magnetization relaxation that is commonly a priori assumed to be of importance in related systems. Furthermore, the chemical robustness of the Er(trensal) system has allowed for outof-crystal studies of vapor- and solution-deposited films and monolayers on metallic substrates and of frozen solution.
While oxygen and nitrogen are ubiquitous as bridging ligators in molecule-based magnetic systems, fluoride is much less explored and studied in this respect. In this project, new polynuclear complexes and one-dimensional polymeric systems, based on fluoride linkages between transition metal ions and between transition metal and lanthanide ions, have been synthetized and thoroughly characterized. Assembly of kinetically robust 3d fluoride
complexes with various lanthanide precursors has proven to be a convenient route to small heterometallic complexes. However, the use of more labile precursors was found to be possible despite the insoluble nature of lanthanide fluorides. The propensity of fluoride for linear bridging also aids in bringing topological control into the synthesis of 3d-4f clusters. The resulting, structurally simple, systems have allowed for modeling of their magnetic
properties and for the first quantifications of magnetic exchange coupling parameters between 3d and 4f centers across a fluoride bridge. This includes determination of the sign and magnitude of energy of interaction in notoriously complicated, orbitally degenerate, lanthanide ion systems, by X-ray magnetic circular dichroism. By using the derived magnetostructural correlations, new systems with large spin ground states and negligible magnetic anisotropy were synthesized and characterized. Herein, large magneto-caloric effects were
observed and it was experimentally confirmed that vanishing magnetic exchange interactions in a lower total spin polynuclear complex had a larger magnetic entropy change during a adiabatic demagnetization than an, all ferromagnetically coupled, complex with a larger spin ground state.
Diffuse orbitals and strong magnetic anisotropy resulting from large values of the spinorbit coupling parameter make complexes with central ions from the 4d and 5d series particularly interesting building blocks for magnetic materials. The main obstacle is the common inherent lability of hexafluoridometallates towards hydrolysis, a tendency that is strongly diminished for several 4d and 5d [MF6]2– complexes. We herein present the use of such simple [MF6]2– (M = Zr, Re, Ir) anions as modules for magnetic architectures of various dimensionality. For instance, for rhenium(IV), using direct current (dc) and alternating current
(ac) magnetic susceptibility measurements, inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) spectroscopies, the magnetic properties of the isolated [ReF6]2– unit in (PPh4)2[ReF6]·2H2O have been fully studied including the slow relaxation of the magnetization observed below ca. 4 K. This slow dynamic is preserved for the tetragonal, one-dimensional coordination polymer [Zn(viz)4ReF6] (viz = 1-vinylimidazole),
demonstrating the irrelevance of low symmetry for such magnetization dynamics in systems with easy-plane magnetic anisotropy. The ability of fluoride to mediate significant exchange interactions is exemplified by the isostructural [M(viz)4ReF6] (M = Mn, Fe, Co, Ni) analogues in which the magnitude of the MII–ReIV interactions are comparable to the coupling present in related cyanide-bridged systems.
The phenomenological Heisenberg-Dirac-van Vleck Hamiltonian, commonly employed for the modeling of magnetic super-exchange, fails to describe cases where orbital degeneracy is present. In molecular magnetic systems, the orbital degeneracy is normally lifted by the low local symmetry of the ligand field. However, in certain cases where the energy splittings resulting from slight departure from perfect, e.g. cubic symmetry, are small compared to the
general energy scheme, the orbital angular momentum may be left unquenched. This effect becomes particularly important for the heavier 4d and 5d transition elements, where ligand field splittings exceed those of the 3d metal ions. Herein, we have contributed to the active discussion on anisotropic exchange in orbitally degenerate systems by studying the series of
(NEt4)[{MnIII(5-Brsalen)2(MeOH)}2{MIII(CN)6}] (M = Ru, Os, Ir, 5-Brsalen = N,N′-
ethylenebis(5-bromosalicylidene)iminate) complexes by magnetic measurements, INS and frequency-domain Fourier-transform THz electron paramagnetic resonance spectroscopy.
Lanthanides are interesting components for magnetic materials and have a huge
underexplored potential in molecular magnetic systems. Advancements are, however, significantly hindered by their complicated electronic and magnetic nature. In this project, we have studied the properties of a class of simple lanthanide single-ion magnets with crystallographic or approximate trigonal symmetry. The combination of high symmetry, surprisingly intense inelastic neutron scattering excitations, optical transparency, and the possibility to grow large single-crystals of the Ln(trensal) (H3trensal = 2,2′,2″-tris(2-
(salicylideneimino)ethyl)amine) systems have provided us with a possibility to reach an experiment-based understanding of the electronic structure, as well as static and dynamic magnetic properties, that is nearly unprecedented in this field of research. Indeed, we have found no indications of the relevance of the thermally activated Orbach process for magnetization relaxation that is commonly a priori assumed to be of importance in related systems. Furthermore, the chemical robustness of the Er(trensal) system has allowed for outof-crystal studies of vapor- and solution-deposited films and monolayers on metallic substrates and of frozen solution.
| Originalsprog | Engelsk |
|---|
| Forlag | Department of Chemistry, Faculty of Science, University of Copenhagen |
|---|---|
| Antal sider | 474 |
| Status | Udgivet - 2014 |