The Quest for Optimal 3 d Orbital Splitting in Tetrahedral Cobalt Single‐Molecule Magnets Featuring Colossal Anisotropy and Hysteresis

Abstract

Only a few four‐coordinated Co2+‐complexes show single‐molecule magnet (SMM) properties without an applied dc field. Common for those is a large magnetic anisotropy generated by the close proximity of the dxy4pt and dx2-y2 orbitals. This type of magnetic anisotropy is strongly correlated with the extent of structural distortion from ideal tetrahedral towards linear coordination. Quantification of the governing magneto‐structural correlations is hence crucial for the development of better SMMs. For this purpose, we synthesized and analyzed four significantly distorted tetrahedral cobalt complexes that exhibit extremely large magnetic anisotropies and slow relaxation of magnetization in zero field. The N−Co−N bite angles vary from 70.8 to 72.7°, and magnetic measurements show strong anisotropy with D‐values in the range from - 75 to - 114 cm−1. Ab initio calculations supported the experimental results and highlighted a significant increase of D up to - 141 cm−1, correlated with the reduction of the energy difference between the dxy4pt and dx2-y2 orbitals. Based on these magneto‐structural correlations and in contrast to the current assumption that the smaller bite angle is the better, we predict that with an ideal bite angle in the range from 76–78° in distorted Co(N2R)2 complexes, the energy difference between the two important d‐orbitals is at a minimum and hence the magnetic anisotropy is maximized.

Enhancing Steric Hindrance via Ligand Design in Dysprosium Complexes: From Induced Slow Relaxation to Zero-Field Single-Molecule Magnet Properties

“…We currently explore the possibilities of exchanging lithium to larger alkali-metals prior to the lanthanide salt addition. In complexes 1 and 2 , the sulfur atom is in the oxidation state (IV), but as expected, the reduced electropositivity, in comparison to complex 3 , has no significant impact on the Dy–N bond lengths (shortest 2.35 to longest 2.41 Å both in 1 ), similar to 3d metal complexes. ,,− The rising oxidation state is only mirrored in the S–N distances. The dysprosium-coordinated S­(IV)–N distances are 1.63 Å long and hence about 0.03 Å longer than the S­(VI)–N distances.…”

“…Even more challenging is to find ligands that may induce better magnetic performances, for example by entirely suppressing alternative relaxation processes such as Raman relaxations and underbarrier quantum tunneling of the magnetization (QTM). , Currently, it seems that the experimental exploration of further cyclopentadienyl substitutions is stalling. New strategies to include heavier , and softer main group elements ,, still mostly rely on p-block substituted cyclopentadienyl ligands for lanthanide-based SMMs. − Since the obtained f-element containing complexes do not necessarily show better SMM performances, investigation of ligands, whose design is further away from cyclopentadienyl derivatives, seems a challenging yet promising alternative. ,, Notably, the large family of SN ligands − provides a versatile and Galore tunable platform toward the design of SMMs. , The extreme flexibility of sulfur coupled with the hardness of the nitrogen donor atoms offers a promising compromise for obtaining an appropriate environment around the paramagnetic center, as demonstrated recently. , It further allows a fine-tuning of the SN ligands toward a well-controlled synthesis of novel mono- and multinuclear SMMs. Starting from selected SN ligands, we recently obtained binuclear systems with antiferromagnetically coupled manganese and cobalt centers .…”

“…Starting from selected SN ligands, we recently obtained binuclear systems with antiferromagnetically coupled manganese and cobalt centers . Additionally, the design and synthesis of distorted tetrahedral cobalt complexes also proved successful for obtaining highly anisotropic SMMs with hysteresis . We now investigate the potential of SN ligands with lanthanides to benefit from their intrinsic magnetic anisotropy.…”

Pentacoordinate cobalt(ii) single ion magnets with pendant alkyl chains: shall we go for chloride or bromide?

“…This is in agreement with our expectations, since the magnetic moment of the E-terms in contrast with that of the A-terms can be increased by orbital angular momentum. 29,30 Additional cooling of 2LT causes gradual lowering of χT down to 1.9 cm 3 mol −1 K −1 at 2 K. Measurements in the heating and cooling modes proved the reversibility of the transition between 2LT and 2HT phases, which is accompanied by a small hysteresis loop (Δ T ≈ 4 K, Fig. 3b inset).…”

“…As a matter of fact, the authors use almost exclusively the ZFS spin Hamiltonian (SH, vide infra ) to analyze the experimental magnetization functions. 20,29,30,32–41 The main problem is, however, that its parameters have a straightforward physical meaning only if the ground state is orbitally non-degenerate and well separated from the closest excited states, 42 and none of these assumptions is fulfilled in the mentioned class of systems (see Computational study). Another option is the Griffith–Figgis Hamiltonian (GFH, vide infra ), which was originally designed for octahedral systems possessing (or approaching) an orbitally triple-degenerate ground state.…”