Single-Molecule Magnet Properties of Transition-Metal Ions Encapsulated in Lacunary Polyoxometalates: A Theoretical Study

Abstract

Single-molecule magnet (SMM) properties of transition-metal complexes coordinated to lacunary polyoxometalates (POM) are studied by means of state of the art ab initio methodology. Three [M(γ-SiW10O36)2] (M = Mn(III), Fe(III), Co(II)) complexes synthesized by Sato et al. (Chem. Commun. 2015, 51, 4081-4084) are analyzed in detail. SMM properties for the Co(II) and Mn(III) systems can be rationalized due to the presence of low-energy excitations in the case of Co(II), which are much higher in energy in the case of Mn(III). The magnetic behavior of both cases is consistent with simple d-orbital splitting considerations. The case of the Fe(III) complex is special, as it presents a sizable demagnetization barrier for a high-spin d(5) configuration, which should be magnetically isotropic. We conclude that a plausible explanation for this behavior is related to the presence of low-lying quartet and doublet states from the iron(III) center. This scenario is supported by ab initio ligand field analysis based on complete active space self-consistent field results, which picture a d-orbital splitting that resembles more a square-planar geometry than an octahedral one, stabilizing lower multiplicity states. This coordination environment is sustained by the rigidity of the POM ligand, which imposes a longer axial bond distance to the inner oxygen atom in comparison to the more external, equatorial donor atoms.

Effect of Heteroatoms on Field-Induced Slow Magnetic Relaxation of Mononuclear FeIII (S = 5/2) Ions within Polyoxometalates

“…4,5 In particular, a mononuclear FeIII ion possessing an unusually distorted sixcoordinate octahedral geometry within a trivacant lacunary POM exhibited a unique field-induced single-molecule magnet (SMM) property for a high spin FeIII ion.4 Ab initio calculations showed that the axial ligand elongations of the octahedrally coordinated [FeO6]9-unit destabilized the dx2-y2 orbital, which lowered the sextet-quartet gap, resulting in a large magnetic anisotropy of the FeIII ion required for SMMs. 6 It is noteworthy that although a number of reports of mononuclear SMMs consisted of 3d transition metals have been reported,7 there have been only two reports on mononuclear FeIII complexes exhibiting SMM properties8 other than our mononuclear FeIII-POM, to the best of our knowledge: i.e., Mossin, Mindiola, and coworkers To date, several experimental and theoretical studies have revealed that heteroatoms of POMs play important roles in controlling the catalytic and redox properties because the anion charges and/or the coordination geometries of POMs depend on the heteroatoms.9 In magnetic molecules, a few comparative studies on POM-based SMMs with different heteroatoms have been reported;10 e.g., {Mn6} clusters in [B--XW9O34]10-(X = Si, Ge) and {Co16} clusters in [A--XW9O34]n-(X = P, Si, Ge). However, theoretical studies about the effect of heteroatoms on the magnetic properties are lacking.…”

Effect of Low Spin Excited States for Magnetic Anisotropy of Transition Metal Mononuclear Single Molecule Magnets

“…Understandably, excited states of other multiplicities (S→S ± 1) are often not considered due to their lower contribution to D. An exception to the prevalence of S→S contributions are, of course, high-spin Fe(III) complexes, where magnetic anisotropy stems from sextet-quartet excitations. There are some examples of multireference ab initio studies on magnetic anisotropy of d 5 systems [35][36][37] and one example of a mononuclear single molecule magnet featuring a high spin Fe(III) center [38]. Thus, it is interesting to explore the role of S→S − 1 contributions in the magnetic anisotropy of transition metal single molecule magnets, as they will be always present and could be beneficial or detrimental for magnetization relaxation properties.…”

Iron doped gold cluster nanomagnets: ab initio determination of barriers for demagnetization

“…[23][24][25] In addition, Aravena et al studied transition metal ions in an inorganic polyoxometalate environment. 26 To our present knowledge, our study is the rst to report calculations of magnetic properties of transition metal doped gold clusters at the level of multireference perturbation theory. In particular, we use an approach based on SA-CASSCF 27 /NEVPT2 (ref.…”