Abstract
In molecular magnetism and single-ion magnets in particular, the observation of slow relaxation of the magnetization is intimately linked to the coordination environment of the metal center. Such systems typically have blocking temperatures well below that of liquid nitrogen, and therefore detailed magnetic characterization is usually carried out at very low temperatures. Despite this, there has been little advantage taken of ultralow temperature single-crystal X-ray diffraction techniques that could provide a full understanding of the crystal structure in the same temperature regime where slow magnetic relaxation occurs. Here, we present a systematic variable temperature single crystal X-ray diffraction study of [CoII(NO3)3(H2O)(HDABCO)] (1) {DABCO = 1,4-diazabicyclo[2.2.2]octane} conducted between 295 to 4 K. A reversible and robust disorder-to-order, single-crystal to single-crystal phase transition was identified, which accompanied a switching of the coordination geometry around the central Co(II) from 5- to 7-coordinate below 140 K. The magnetic properties were investigated, revealing slow relaxation of the magnetization arising from a large easy-plane magnetic anisotropy (+D) in the Co(II) pentagonal bipyramidal environment observed at low temperatures. This study highlights the importance of conducting thorough low temperature crystallographic studies, particularly where magnetic characterization is carried out at such low temperatures.
Highlights
Cobalt is widely used in chemistry, from biological applications such as antibacterial and therapeutic agents to electrochemical catalysis and molecular magnetism.[1−4] This diverse range of applications reflects cobalt’s rich and interesting behavior, which stems from extensive redox, optical, and magnetic properties
Co(II) can help address two requirements for successful single-molecule magnets (SMMs) a significant spin−orbit coupling (SOC) contribution to the magnetic anisotropy and control over undesirable relaxation processes.[5−11] 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.12−20 Both of these factors contribute to the success of Co(II) in polynuclear 3d, 3d−4f, and mononuclear 3d cobalt SMMs.[21−26] For mononuclear Co(II) complexes, the focus is firmly on enhancing the axial magnetic anisotropy of the system.[27]
We present a systematic variable temperature single crystal X-ray diffraction study of [CoII(NO3)3(H2O)(HDABCO)] (1),[30] evidencing a disorder-to-order, singlecrystal to single-crystal phase transition accompanied by a clear coordination switch from a 5- to a 7-coordinate complex below 140 K
Summary
Cobalt is widely used in chemistry, from biological applications such as antibacterial and therapeutic agents to electrochemical catalysis and molecular magnetism.[1−4] This diverse range of applications reflects cobalt’s rich and interesting behavior, which stems from extensive redox, optical, and magnetic properties. In a molecular magnetism context, Co(II)-based complexes are 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.[5−11] 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.12−20 Both of these factors contribute to the success of Co(II) in polynuclear 3d, 3d−4f, and mononuclear 3d cobalt SMMs.[21−26] For mononuclear Co(II) complexes, the focus is firmly on enhancing the axial magnetic anisotropy of the system.[27] 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.[28] In particular, where low temperature magnetic measurements are used to confirm certain properties, it is important to be certain of the structure in the temperature range at which the measurement is carried out (typically < 10 K).[29]. We stress the merits of thorough low temperature crystallographic investigations of single-ion magnets, especially where the ligands involved can display a variety of coordination modes
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