Coligand Effects on the Field-Induced Double Slow Magnetic Relaxation in Six-Coordinate Cobalt(II) Single-Ion Magnets (SIMs) with Positive Magnetic Anisotropy

Abstract

Two mononuclear cobalt­(II) compounds of formula [Co­(dmphen)2(OOCPh)]­ClO4·1/2H2O·1/2CH3OH (1) and [Co­(dmbipy)2(OOCPh)]­ClO4 (2) (dmphen = 2,9-dimethyl-1,10-phenanthroline, dmbipy = 6,6′-dimethyl-2,2′-bipyridine and HOOCPh = benzoic acid) are prepared and magnetostructurally investigated. Each cobalt­(II) ion is six-coordinate with a distorted octahedral CoN4O2 environment. The complex cations are interlinked leading to supramolecular chains (1) and pairs (2) that grow along the crystallographic c-axis with racemic mixtures of (Δ,Λ)-Co units. FIRMS allowed us to directly measure the zero-field splitting between the two lowest Kramers doublets, which led to axial anisotropy values of 58.3 cm–1 ≤ D < 60.7 cm–1 (1) and 63.8 cm–1 ≤ D < 64.1 cm–1 (2). HFEPR spectra of polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and provide an estimate of the E/D quotient [0.147/0.187 (1) and 0.052 (2)]. Detailed ac and dc magnetic studies reveal that 1 and 2 are new examples of field-induced single-ion magnets (SIMs) with small transversal anisotropy. CASSCF/NEVPT2 calculations support these results. Two Orbach processes or one Orbach plus a direct relaxation mechanism provide similar agreements with the nonlinear experimental Arrhenius plots at H dc = 500 and 2500 G for 1. Two independent relaxation processes occur in 2, but in contrast to 1, an observed linear dependence of ln­(τ) vs 1/T substantiates Orbach processes against the most widely proposed Raman and direct mechanisms. The analysis of each relaxation process in 2 provided values for E a and τ0 that are very close to those found for 1, validating the predominant role of the Orbach relaxations in both compounds and, probably, also in other cobalt­(II) SIMs. A mechanism based on a spin-phonon coupling is proposed to account for the SIM behavior in 1 and 2 with any Raman or direct processes being discarded.

Tuning magnetic anisotropy by the π-bonding features of the axial ligands and the electronic effects of gold(i) atoms in 2D {Co(L)2[Au(CN)2]2}nmetal–organic frameworks with field-induced single-ion magnet behaviour

“…These FIRMS/HFEPR results are quite similar to those obtained for other cobalt(II) complexes with a distorted octahedral CoN 4 O 2 environment. 38 It is worth mentioning that the obtained D value is very close to that obtained from dc magnetic measurements of 68 cm −1 . The FIRMS spectra also reveal another magnetic transition at 793 cm −1 , which corresponds to the excited Kramers doublet next to the S = 3/2 level.…”

“…Recent theoretical calculations of the lowest energy vibrational modes for two field-induced distorted octahedral Co(II) SIMs have demonstrated that their energies are close to the experimental thermal energy barriers for Orbach relaxation mechanisms. 38 This fact led to a suggestion that the thermal energy barrier is tied to the lowest vibrational modes of the metal complex, so when these vibrational modes are thermally populated, the magnetization reversal becomes faster, promoting the elimination of the magnetization blocking. Moreover, it has been suggested that the direct relaxation between two quasi-degenerate ground states is accelerated in structurally flexible SMMs.…”

Insertion of single-ion magnets based on mononuclear Co(ii) complexes into ferromagnetic oxalate-based networks

“…The values for the pre-exponential factor and activation energy are consistent with those of the other Co( ii ) complexes showing this behavior. 34–36…”

Single molecule magnets of cobalt and zinc homo- and heterometallic coordination polymers prepared by a one-step synthetic procedure

“…[73][74][75][76] The observed behavior and anisotropic values are similar to other published mononuclear Co(II) complexes in pseudo-octahedral geometry and also in other Co(II) CPs. [77][78][79][80][81][82] Dynamic susceptibility measurements were performed to study the possibility of slow relaxation of the magnetization. Without an applied static dc eld, no c 0 signal was observed; however, when an external dc eld was used, a dependence of c 0 appeared.…”