Slow magnetic relaxation in a {Co^IICoIII2} complex containing a high magnetic anisotropy trigonal bipyramidal Co^II centre

Abstract

We report a trinuclear mixed-valence {CoIICoIII2} complex, where the CoII centre adopts a trigonal bipyramidal geometry, leading to a large, easy-plane magnetic anisotropy and field-induced slow magnetic relaxation with a Raman-like relaxation process.

Trapping of a Pseudotetrahedral CoIIO4 Core in Mixed-Valence Mixed-Geometry [Co5] Coordination Aggregates: Synthetic Marvel, Structures, and Magnetism

“…First attempts to fit 1/τ vs T data including the Orbach contribution (see Figure S11, left, in the Supporting Information) yield an energy barrier value ((Δ E / k B ) 1 ≈ 50 K) significantly lower than that calculated using the D and E values obtained from the ab initio study ( ) . This can be ascribed to quantum tunneling of the magnetization, promoted by the rhombic term E in previous studies on Co­(II)-based SIMs or other possible spin-phonon relaxation mechanisms, as shown in Fe­(II) SIMs. , However, the absence of a real state at the energy gap proposed by the fit suggests that the reversal of the magnetization should occur via relaxation processes alternative to Orbach. − Therefore, and in order to avoid overparameterization, the fit of the 1/τ vs T data at H dc = 500 Oe is performed considering only the Raman contribution (see eq ). …”

Trigonal to Pentagonal Bipyramidal Coordination Switching in a Co(II) Single-Ion Magnet

“…This could be due to the phase transition (α → β) observed in the X-ray diffraction studies, arising from the change from a trigonal bipyramidal to a pentagonal bipyramidal geometry. However, it is not surprising that the difference in the χ m T data around the temperature of the phase transition is not particularly prominent based on previous measurements conducted on TBP and PBP Co­(II) complexes reported in the literature. ,,− Furthermore, this feature would be quite difficult to spot in the absence of the variable temperature X-ray study, which would lead to an erroneous analysis of the low temperature magnetic data as arising from a trigonal bipyramidal Co­(II) complex. The χ m T vs T data then gradually decrease until 40 K is reached, after which a sharp maximum is observed with χ m T max = 3.29 cm 3 mol –1 K observed at 12 K. The χ m T value then rapidly drops with χ m T min = 2.09 cm 3 mol –1 K recorded at 2 K. In the low temperature β phase of 1 , the shortest intermolecular Co··Co distance is ∼6.8 Å, where there is a hydrogen bonding interaction between a coordinated water molecule and a nitrate ligand on an adjacent molecule, {Co–H 2 O···NO 3 –Co}.…”

“…This is common for easy-axis (+ D ) systems with small E terms, where relaxation is mediated instead via the hyperfine interaction of the nuclear spin and molecular vibrations. ,− Therefore, the fit was reconsidered using the extended Debye model shown in eq (see Figure S15). This model takes into account the spin–lattice relaxation Raman (τ ∝ T n ) processes, as well as quantum tunneling of magnetization (QTM) given as the first and second terms, respectively. ,, The two phonon Raman process (∝ T n ) was found to be dominant in the high temperature region, with C = 0.23 (0.05) s –1 K – n and n = 7.65 (0.11) obtained from the fit. Although n should equal 9 for a Kramers ion, this value may be lower if optical and acoustic phonons are taken into account. − Inclusion of the term relating to a QTM relaxation pathway was necessary to fit the data (QTM = 5.92(1.50) × 10 –3 s –1 ), as expected given the indication of quantum and thermally independent processes in the Arrhenius plot. …”

“…This model takes into account the spin−lattice relaxation Raman (τ ∝ T n ) processes, as well as quantum tunneling of magnetization (QTM) given as the first and second terms, respectively. 9,56,57 The two phonon Raman process (∝ T n ) was found to be dominant in the high temperature region, with C = 0.23 (0.05) s −1 K −n and n = 7.65 (0.11) obtained from the fit. Although n should equal 9 for a Kramers ion, this value may be lower if optical and acoustic phonons are taken into account.…”

“…In a molecular magnetism context, Co­(II)-based complexes are particularly attractive to study as they can show slow relaxation of the magnetization in a range of coordination environments. Co­(II) can help address two requirements for successful single-molecule magnets (SMMs)a significant spin–orbit coupling (SOC) contribution to the magnetic anisotropy (first or second order depending on geometry) and control over undesirable relaxation processes. − The latter is a result of the Kramers Theorem, where quantum tunneling of the magnetization (QTM) and direct spin-phonon relaxation between the ground state Kramers doublet is formally forbidden for the half-integer spin Co­(II) ion, regardless of the sign of the axial zero-field splitting (ZFS) parameter D . − Both of these factors contribute to the success of Co­(II) in polynuclear 3d, 3d–4f, and mononuclear 3d cobalt SMMs. − For mononuclear Co­(II) complexes, the focus is firmly on enhancing the axial magnetic anisotropy of the system . However, in order to understand the magnetic anisotropy in Co­(II) mononuclear complexes, which tends to dictate the resulting magnetic properties, a thorough understanding of the geometrical environment of the Co­(II) is required .…”