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

Abstract

In this paper, the synthesis and magnetic properties of mononuclear Fe-containing polyoxometalates (POMs) with different types of heteroatoms, TBAH[(A-α-XWO)Fe] (II, X = Ge, Si; TBA = tetra- n-butylammonium), are reported. In these POMs, mononuclear highly distorted six-coordinate octahedral [FeO] units are sandwiched by two trivacant lacunary units [A-α-XWO] (X = Ge, Si). These POMs exhibit field-induced slow magnetic relaxation based on the single high-spin Fe magnetic center ( S = 5/2). Combining experiment and ab initio calculations, we investigated the effect of heteroatoms of the lacunary units on the field-induced slow magnetic relaxation of these POMs. By changing the heteroatoms from Si (II) to Ge (II), the coordination geometry around the Fe ion is mildly changed. Concretely, the axial Fe-O bond length in II is shortened compared with that in II, and consequently the distortion of the [FeO] unit in II from the ideal octahedral coordination geometry becomes larger than that in II. The effective demagnetization barrier of II (11.4 K) is slightly larger than that of II (9.2 K). Multireference ab initio calculations predict zero-field splitting parameters in good agreement with experiment. Although the differences in the coordination geometries and magnetic properties of II and II are quite small, ab initio calculations indicate subtle changes in the magnetic anisotropy which are in line with the observed magnetic relaxation properties.

Single‐Ion Magnetic Behaviour in an Iron(III) Porphyrin Complex: A Dichotomy Between High Spin and 5/2–3/2 Spin Admixture

“…A high‐spin 3d 5 complex should be magnetically isotropic ( D =0), but the proximity of the excited quartet ( S= 3/2) states, which depends on the ligand fields, can significantly affect the ground state, inducing anisotropy. Only recently, SIM behaviour (with D <0), was observed for S= 5/2 iron(III) compounds [58–60] . The ground state in iron(III) porphyrins in particular can be tuned by the ligand field, as observed in such complexes showing S= 1/2, 3/2, and 5/2 ground states, [34–36] including those with S= 5/2 and positive and sizable D values [61–63] .…”

Multireference Methods are Realistic and Useful Tools for Modeling Catalysis

“…[33] That is, MR energies are computed as single-point calculations on structures optimized at the DFT level (and MR spin ladders may simultaneously be used to inform the choice of DFT functional used to generate the reaction coordinate). This approach has been applied to study catalysis by vanadium oxide [34] and bimetallic cobalt oxide clusters, [35] iron fluorides, [36] polyoxometalates, [37] transition metal organometallic complexes (e. g. MnN4py, [28c] [(Me 2 Py 2 TACN)FeN] 2 + , [38] porphyrins, [26a,33c,e,39] iron and ruthenium imido [40] and aquo complexes, [41] ruthenium diolefin diazadienes, [42] iridium-based systems, [43] and complexes featuring short metal-metal bonds such as Mn 2 (C 6 H 6 ) 2 [44] and N,N,N-tri(2-(2pyridylamino)ethyl)amine complexes [45] ), metal-organic frameworks (MIL-100(Fe), [10b] Fe 0.1 Mg 1.9 (dobdc) 2 , [10a] ZIF-8(Fe) [31] ), and iron-based zeolites (Fe-BEA*, [46] Fe-CHA, [30] Fe-FER, [46b] Fe-ZSM5). [46b] For any given study, a significant complication is the choice of orbitals to include in the active space.…”

Incorporation of CrIII into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {CrIIIO4} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties

“…The possibility of constructing nanomagnets using a single lanthanoid ion has been demonstrated for both polyoxotungstates ([Ln­(W 5 O 18 ) 2 ] 9– , [Ln­(β 2 -SiW 11 O 39 ) 2 ] 13– , [LnW 30 O 110 ] 12– ) and polyoxomolybdates ([Ln­(β-Mo 8 O 26 ) 2 ] 5– , [Ln­{Mo 5 O 13 (OMe) 4 NNC 6 H 4 -p-NO 2 } 2 ] 3– ). Although a growing number of first-row d-block single-ion magnets (SIMs) has been reported, d-block SIMs based on POMs are particularly rare. , Even though there are a few examples of heteropolynuclear SMMs containing chromium­(III) centers, there are no reported chromium­(III) SIMs.…”

“…Only two cases of POM-based d-block (Fe III and Co II ) SIMs have been described in literature. , The single-ion magnet behavior is unusual in high-spin Fe III complexes and rarely encountered for the systems based on POMs; however, this effect is well-known for Fe III porphyrin complexes . In contrast, in CrW 12 , the zfs is induced through a unique tetrahedral geometry for a d 3 ion.…”

“…Although a growing number of first-row d-block single-ion magnets (SIMs) has been reported, 15 d-block SIMs based on POMs are particularly rare. 16,17 Even though there are a few examples of heteropolynuclear SMMs containing chromium(III) centers, 18 there are no reported chromium(III) SIMs.…”