Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in Fe^II, Co^II, and Ni^II Single‐Ion Magnets

Abstract

Since the last decade, the focus in the area of single‐molecule magnets (SMMs) has been shifting constructively towards the development of single‐ion magnets (SIMs) based on transition metals and lanthanides. Although ground‐breaking results have been witnessed for DyIII‐based SIMs, significant results have also been obtained for some mononuclear transition metal SIMs. Among others, studies based on CoII ion are very prominent as they often exhibit high magnetic anisotropy or zero‐field splitting parameters and offer a large barrier height for magnetisation reversal. Although CoII possibly holds the record for having the largest number of zero‐field SIMs known for any transition metal ion, controlling the magnetic anisotropy in these systems are is still a challenge. In addition to the modern spectroscopic techniques, theoretical studies, especially ab initio CASSCF/NEVPT2 approaches, have been used to uncover the electronic structure of various CoII SIMs. In this article, with some selected examples, the aim is to showcase how varying the coordination number from two to eight, and the geometry around the CoII centre alters the magnetic anisotropy. This offers some design principles for the experimentalists to target new generation SIMs based on the CoII ion. Additionally, some important FeII/FeIII and NiII complexes exhibiting large magnetic anisotropy and SIM properties are also discussed.

Rotaxane CoII Complexes as Field-Induced Single-Ion Magnets

“…49 The best fits yielded gaverage of 2.33, 2.31 and 2.33 and D values of -78, -59 and -95 cm -1 for complexes [Co(1-3)](ClO4)2, respectively (Table 1, see also Figure S27 for fits with different values of D). These values are considerably larger than those obtained for other pentacoordinate mononuclear cobalt(II) complexes [60][61][62] (Table S4). Because of the change in the spin population as a function of temperature for [Co(1)](ClO4)2, only the fit of the magnetization was used to estimate the parameters (i.e.…”

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

“…While 4 f orbitals barely participate in bonding and are shielded by the outer core electrons, 3d orbitals are diffuse valence orbitals whose energies are greatly affected by ligand fields, usually resulting in quenching of the OAM [7] . Nevertheless, 3d based systems are still arousing interest because of their highly‐tunable properties, contrary to lanthanide‐containing compounds [11,12] . In the past decade, remarkable 3d mononuclear SMMs were synthesized, [13] including two‐coordinate iron [14] and cobalt [15] systems, as well as highly distorted tetrahedral cobalt complexes [16–22] .…”

“…Instead, theoretical [19,24–26] or experimental magneto‐structural investigations focused on other structural variations, [26,27] such as changing the donor strength and the counter ion [19,28] or applying high‐pressure [29,30] . None of these studies considers the possibility of an ideal bite angle [12,17,24–27,29,31] . All of them are based on the common perception of compression from distorted tetrahedral D 2d to linear C ∞ geometry as the best strategy towards superior SMMs.…”

Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

“…Such a value would be expected given the high symmetry and close-to-perfect octahedral geometry around the Ni II centres. 32,33 For the CASSCF/NEVPT2 calculations, the other Ni II centres were substituted by diamagnetic Zn II ions.…”