Spectroscopic Investigation of a Metal–Metal-Bonded Fe₆ Single-Molecule Magnet with an Isolated S = ¹⁹/₂ Giant-Spin Ground State

Abstract

The metal−metal-bonded molecule [Bu 4 N]-[( H L) 2 Fe 6 (dmf) 2 ] (Fe 6 ) was previously shown to possess a thermally isolated spin S = 19 / 2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc. 2015, 137, 13949−13956]. Here, we present a comprehensive spectroscopic investigation of this unique singlemolecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequencydomain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe 6 . Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin−spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn 12 -acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe 6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal−metal bonds and strong ferromagnetic spin−spin (exchange) interactions.

A New Look at Molecular and Electronic Structure of Homoleptic Diiron(II,II) Complexes with N,N‐Bidentate Ligands: Combined Experimental and Theoretical Study

“…[72] Another strategy could involve the preparation of higher-nuclearity clusters with metalÀ metal bonds as demonstrated by Betley et al, or the oxidation of the diiron(II,II) compounds to non-integer spin systems diiron(II,III). [54] Theoretical investigations: While the majority of [M 2 L 4 ] paddlewheel-type complexes exhibit a D 4h symmetry, the tetragonal diiron(II,II) complex 1 has a lower D 2d symmetry and a putative high-spin electronic configuration (shown in Figure 2a), supported by previous density functional theory (DFT) calculations. [27] However, so far neither comprehensive experimental characterisation nor high-level ab-initio calculations have been taken to examine the electronic structure of this complex.…”

“…Further ligand modification towards perfectly axial geometry with D 4d symmetry (or higher) may further enhance the slow relaxation of the magnetisation, leading to the opening of the magnetic hysteresis loop even at zero magnetic field for this class of compounds [72] . Another strategy could involve the preparation of higher‐nuclearity clusters with metal−metal bonds as demonstrated by Betley et al., or the oxidation of the diiron(II,II) compounds to non‐integer spin systems diiron(II,III) [54] …”

“…While the majority of high‐performance single molecular magnets (SMMs) achieve their high ground state by superexchange coupling mechanism at low temperatures, [40–53] the Fe−Fe bonded complexes may be an alternative pathway to the design of SMMs combining high spin ground state even at room temperature due to the direct overlap of the metal valence orbitals with significant magnetic anisotropy. Indeed, Betley and co‐workers investigated mixed‐valent tri‐ (S=11/2) and hexanuclear (from S=9/2 to S=11) multiiron clusters, which show slow magnetic relaxation and maintain a thermally persistent high‐spin ground state owing to both direct and double exchange mechanisms between the iron centres [54–57] …”

“…Indeed, Betley and co-workers investigated mixedvalent tri-(S = 11/2) and hexanuclear (from S = 9/2 to S = 11) multiiron clusters, which show slow magnetic relaxation and maintain a thermally persistent high-spin ground state owing to both direct and double exchange mechanisms between the iron centres. [54][55][56][57] Herein, we examined the nature of the metalÀ metal interactions in high-spin diiron formamidinates [Fe 2 (μ-DPhF) 4 ]…”

[Ni30S16(PEt3)11]: an open-shell nickel sulfide nanocluster with a “metal-like” core

“…The resulting APNCs would be potentially useful in a number of magnetic applications. 12,26 In an effort to test this hypothesis, we endeavoured to synthesize and characterize even larger open-shell Ni APNCs. Herein, we report the largest open-shell Ni APNC yet known, namely, [Ni 30 S 16 (PEt 3 ) 11 ] (1).…”