Field-Induced Slow Magnetic Relaxation in a Six-Coordinate Mononuclear Cobalt(II) Complex with a Positive Anisotropy

Abstract

The novel mononuclear Co(II) complex cis-[Co(II)(dmphen)(2)(NCS)(2)]·0.25EtOH (1) (dmphen = 2,9-dimethyl-1,10-phenanthroline) features a highly rhombically distorted octahedral environment that is responsible for the strong positive axial and rhombic magnetic anisotropy of the high-spin Co(II) ion (D = +98 cm(-1) and E = +8.4 cm(-1)). Slow magnetic relaxation effects were observed for 1 in the presence of a dc magnetic field, constituting the first example of field-induced single-molecule magnet behavior in a mononuclear six-coordinate Co(II) complex with a transverse anisotropy energy barrier.

Origin of slow magnetic relaxation in Kramers ions with non-uniaxial anisotropy

“…1b), whose up and down field-sweeping branches merge as H-0. This behaviour resembles closely that found for some other Co II ions with dominant easy-plane anisotropy [5][6][7][8][9] . Two intriguing questions that need to be addressed are, on the one hand, why do Co II spins with D40 relax so slowly at low T and, on the other, why is this relaxation only visible under finite magnetic fields.…”

“…The results (see Table 1) give a positive D value together with a relative large E. The inclusion of dynamic correlation with the CASPT2 method gives rise to a smaller spin-orbit effect resulting in smaller anisotropy parameters. The splitting between the two Kramers doublets remains nevertheless larger than 130 cm À 1 (see Table 1), a value that is within the characteristic range observed for other hexacoordinate Co II complexes 5,16 . The sign of D agrees also with that predicted by previous DFT calculations 17 , although the latter predict a weaker magnetic anisotropy.…”

“…The same or similar behaviour has thereafter been reported for a number of other Co II complexes [5][6][7][8][9] as well as for lineal Fe II complexes 10 . However, a detailed microscopic interpretation of this phenomenon is still missing.…”

“…Intensities were collected with graphite monochromatized Mo Ka radiation, using j scan technique. Reflections (5,119) were measured in the range 1.93ryr32.06, 1,702 of which were non-equivalent by symmetry (Rint(on I) ¼ 0.070 ). Reflections (1,427) were assumed as observed applying the condition I42s(I).…”

“…However, a detailed microscopic interpretation of this phenomenon is still missing. In some cases 5 the temperature dependence of the spin-lattice relaxation rate could be fitted using the exponential dependence expected for a thermally activated, or Orbach 11 , process, with activation energies U that are not far from the classical energy barrier ES 2 associated with the in-plane anisotropy. However, this interpretation is incompatible with the energy level schemes of Co II complexes, which consist of only two Kramers doublets separated by an energy gap D 44 ES 2 and with the fact that Orbach processes imply phonon-induced transitions to 'real' intermediate spin states 11 .…”