Trigonal Prismatic Tris-pyridineoximate Transition Metal Complexes: A Cobalt(II) Compound with High Magnetic Anisotropy

Abstract

High magnetic anisotropy is a key property of paramagnetic shift tags, which are mostly studied by NMR spectroscopy, and of single molecule magnets, for which magnetometry is usually used. We successfully employed both these methods in analyzing magnetic properties of a series of transition metal complexes, the so-called clathrochelates. A cobalt complex was found to be both a promising paramagnetic shift tag and a single molecule magnet because of it having large axial magnetic susceptibility tensor anisotropy at room temperature (22.5 × 10 m mol) and a high effective barrier to magnetization reversal (up to 70.5 cm). The origin of this large magnetic anisotropy is a negative value of zero-field splitting energy that reaches -86 cm according to magnetometry and NMR measurements.

A Trigonal Prismatic Cobalt(II) Complex as a Single Molecule Magnet with a Reduced Contribution from Quantum Tunneling

“…Reducing its efficiency is an established approach to increase the effective barrier to magnetization reversal, [45] which can be done by applying an external magnetic field. As it does not affect dramatically the relaxation time for the complex 1, the quantum tunneling has a negligible contribution to the magnetization relaxation, as does the direct mechanism, in contrast to a previously published cobalt(II) tris-pyridineoximate clathrochelate [19] due to different crystal packing [4] or lower rhombicity [46] of 1. At temperatures below 6 K, the relaxation times for 1 cannot be reliably obtained from the ac-magnetometry data, as the position of the maximum in frequency dependence of out-of-phase χ'' component of the magnetic susceptibility is at frequencies beyound the range of the magnetometer used ( Figures S4, S5).…”

“…The cobalt(II) clathrochelate 1 and its diamagnetic analogue 1 a , the isostructural complex of zinc(II) (Figure S1), were easily obtained (Scheme ) in boiling ethanol by the template condensation of 1‐(1‐methyl‐1H‐imidazol‐2‐yl)ethanone oxime with phenylboronic acid on the corresponding metal ion as a matrix, as previously described for 2‐acetylpyridineoximate clathrochelates …”

“…Magnetometry in alternating current magnetic fields ( ac ‐magnetometry) is the most popular method in SMM research, which directly accesses the magnetization relaxation time; other techniques of choice are dc‐magnetometry,, far‐infrared spectroscopy, magnetic circular dichroism, NMR spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy ,. The latter is rarely used for studying SMMs.…”

“…The cobalt(II) clathrochelate 1 and its diamagnetic analogue 1 a, the isostructural complex of zinc(II) ( Figure S1), were easily obtained (Scheme 1) in boiling ethanol by the template condensation of 1-(1-methyl-1H-imidazol-2-yl)ethanone oxime with phenylboronic acid on the corresponding metal ion as a matrix, as previously described for 2-acetylpyridineoximate clathrochelates. [19] The obtained cobalt(II) complex 1 shows a rather large ZFS value determined by THz-EPR spectroscopy. In the magnetic field division spectra (MDS), [42] obtained by dividing a spectrum measured at B 0 by a spectrum measured at B 0 + 1 T, a maximum at 206 cm À 1 is observed in the zero magnetic field (Figure 1), and a corresponding minimum at a lower energy is broad and shallow.…”

“…[2][3][4][5][6][7][8][9][10] An SMM, after it has been magnetized in an applied magnetic field, keeps its magnetization over time as a result of the magnetization reversal energy barrier U between the states with M S = + S and M S = À S. The higher this barrier, the longer the magnetization is kept if the Orbach 'over-thebarrier' relaxation mechanism prevails. Magnetometry in alternating current magnetic fields (acmagnetometry) is the most popular method in SMM research, which directly accesses the magnetization relaxation time; other techniques of choice are dc-magnetometry, [8,9] far-infrared spectroscopy, [11] magnetic circular dichroism, [12] NMR spectroscopy, [13][14][15][16][17][18][19][20] and electron paramagnetic resonance (EPR) spectroscopy. [21,22] The latter is rarely used for studying SMMs.…”