Abstract
The magnetic properties of 3d monometallic complexes can be tuned through geometric control, owing to their synthetic accessibility and relative structural simplicity. Monodentate ligands offer great potential for fine-tuning the coordination environment to engineer both the axial and rhombic zero-field splitting (ZFS) parameters. In [CoCl3(DABCO)(HDABCO)] (1), the trigonal bipyramidal Co(ii) centre has two bulky axial ligands and three equatorial chloride ligands. An in-depth experimental and theoretical study of 1 reveals a large easy-plane magnetic anisotropy (+ve D) with a negligible rhombic zero-field splitting (E) due to the strict axial symmetry imposed by the C 3 symmetric ligand and trigonal space group. The large easy-plane magnetic anisotropy (D = +44.5 cm-1) is directly deduced using high-field EPR and frequency-domain magnetic resonance (FDMR) studies. Ab initio calculations reveal a large positive contribution to the D term arising from ground state/excited state mixing of the 4E'' states at ∼4085 cm-1 and a minor contribution from the spin-flip transition as well. The nature of the slow relaxation in 1 is elucidated through analysis of the rates of relaxation of magnetisation, taking into account Raman and direct spin-lattice relaxation processes and Quantum Tunnelling of the Magnetisation (QTM). The terms relating to the direct process and QTM were found based on the fit of the field-dependence of τ at 2 K. Subsequently, these were used as fixed parameters in the fit of the temperature-dependence of τ to obtain the Raman terms. This experimental-theoretical investigation provides further insight into the power of FDMR and ab initio methods for the thorough investigation of magnetic anisotropy. Thus, these results contribute to design criteria for high magnetic anisotropy systems.
Highlights
The investigation of molecules that retain their magnetisation in the absence of an applied magnetic eld, single-molecule magnets (SMMs), has been driven by the need to nd new materials for high-density data storage and quantum computing.1. Recent work in this eld has focused on engineering high magnetic anisotropy in complexes containing a single paramagnetic ion, which has led to the rst examples of SMMs showing slow relaxation of their magnetisation above the temperature of liquid nitrogen.2. This approach requires that we develop an understanding of how molecular geometry can be tailored to achieve large spin–orbit coupling (SOC) contributions to magnetic anisotropy, and how undesired relaxation processes can be controlled
Complex 1 crystallizes in the trigonal R32 space group and comprises a central Co(II) ion in a trigonal bipyramidal coordination environment (D3h) with two axial DABCO ligands and three equatorial chloride ligands
Continuous shape measures (CShMs) were calculated using the program SHAPE to quantify the degree of distortion around the CoII center from the ideal trigonal bipyramidal coordination geometry,7 and the value of 0.015 obtained for 1 indicates an almost perfect TBP environment
Summary
The investigation of molecules that retain their magnetisation in the absence of an applied magnetic eld, single-molecule magnets (SMMs), has been driven by the need to nd new materials for high-density data storage and quantum computing. Recent work in this eld has focused on engineering high magnetic anisotropy in complexes containing a single paramagnetic ion, which has led to the rst examples of SMMs showing slow relaxation of their magnetisation above the temperature of liquid nitrogen. This approach requires that we develop an understanding of how molecular geometry can be tailored to achieve large spin–orbit coupling (SOC) contributions to magnetic anisotropy, and how undesired relaxation processes can be controlled. monometallic 3d complexes have generated signi cant interest in this area, with simple modi cation of the ligands in uencing the zero- eld splitting parameters and, the observation of 6354 | Chem. The investigation of molecules that retain their magnetisation in the absence of an applied magnetic eld, single-molecule magnets (SMMs), has been driven by the need to nd new materials for high-density data storage and quantum computing.1 Recent work in this eld has focused on engineering high magnetic anisotropy in complexes containing a single paramagnetic ion, which has led to the rst examples of SMMs showing slow relaxation of their magnetisation above the temperature of liquid nitrogen.. If the rhombic zero- eld splitting (ZFS) parameter E is zero, as is the case in a high symmetry molecule, the contribution to the relaxation from quantum tunneling transitions mediated by hyper ne or dipole elds will be further inhibited, isolating spin–lattice relaxation processes. Strict D3h symmetry, in the rst coordination sphere, is imposed by the molecules packing in the R32 space group. We use a combination of magnetic susceptibility measurements, ab initio calculations, high eld electron paramagnetic resonance (HF-EPR), and frequency-domain magnetic resonance (FDMR) spectroscopy to elucidate the relaxation mechanisms and the large easy-plane magnetic anisotropy for Co(II) in a strict trigonal bipyramidal (TBP) coordination environment
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