Magnetostructural Correlations in Tetrairon(III) Single‐Molecule Magnets

Abstract

Tunable single-molecule magnets: The spin-level landscape in a series of Fe(III) (4) single-molecule magnets with propeller-like structure was analyzed by means of high-frequency EPR spectroscopy. The zero-field splitting parameter D of the ground S=5 spin state correlates strongly with the pitch of the propeller gamma (see picture), and thus provides a simple link between molecular structure and magnetic behavior.We report three novel tetrairon(III) single-molecule magnets with formula [Fe(4)(L)(2)(dpm)(6)] (Hdpm=2,2,6,6-tetramethylheptane-3,5-dione), prepared by using pentaerythritol monoether ligands H(3)L=R'OCH(2)C(CH(2)OH)(3) with R'=allyl (1), (R,S)-2-methyl-1-butyl (2), and (S)-2-methyl-1-butyl (3), along with a new crystal phase of the complex containing H(3)L=11-(acetylthio)-2,2-bis(hydroxymethyl)- undecan-1-ol (4). High-frequency EPR (HF-EPR) spectra at low temperature were collected on powder samples in order to determine the zero-field splitting (zfs) parameters in the ground S=5 spin state. In 1-4 and in other eight isostructural compounds previously reported, a remarkable correlation is found between the axial zfs parameter D and the pitch gamma of the propeller-like structure. The relationship is directly demonstrated by 1, which features both structurally and magnetically inequivalent molecules in the crystal. The dynamics of magnetization has been investigated by ac susceptometry, and the results analyzed by master-matrix calculations. The large rhombicities of 2 and 3 were found to be responsible for the fast magnetic relaxation observed in the two compounds. However, complex 3 shows an additional faster relaxation mechanism which is unaccounted for by the set of spin Hamiltonian parameters determined by HF-EPR.

Origin and spectroscopic determination of trigonal anisotropy in a heteronuclear single-molecule magnet

“…These levels were identified by a preliminary analysis of the field dependent energy pattern using anisotropy parameters from previous works. conclusion, which fully supports previous studies, 36,40,42 implies that the hard axis (z) of each D Fe(i) tensor is normal to the corresponding Cr-Fe(i) direction, with no symmetryimposed restriction on the angle β between z and Z. However, γ can have only two possible values, 0 or 90°, depending on whether y or x is found along Cr-Fe(i).…”

The Role of Anisotropic Exchange in Single Molecule Magnets: A CASSCF/NEVPT2 Study of the Fe4 SMM Building Block [Fe2(OCH3)2(dbm)4] Dimer

“…For the study of the magneto-structural correlations, we produced six distorted structures corresponding at ±1q, ±2q and ±3q displacements, expressed in terms of normal mode unit, with respect to the equilibrium geometry. The ensemble of angles spanned by these structures is rather large (94.9 • ÷ 106.4 • ) and fully includes all the experimental scenarios encountered so far in both propeller-shaped SMMs and this oxo-bridged Fe2 dimer family [18,37]. As expected, the J 12 value nicely correlates with the FeOFe angle and the reduced value calculated for the optimised geometry is fully accountable to the reduction of the FeOFe during the optimisation.…”

“…As reported several times in the literature [17,18,37], the most interesting collective variable connected to the isotropic exchange coupling is the FeOFe angle and in this work we further extend the study of this correlation including the assessment of the anisotropic part of the exchange. In the attempt to address magneto-structural correlations by mean of solely first principle calculations, we did not use any experimental reference structure, differently from what previously done [17].…”

Torque-detected ESR of a tetrairon(III) single molecule magnet

“…However, by properly adjusting the field orientation, TDESR can be made several orders of magnitude more sensitive than FDMRS. We herein present a combined study by TM, TDESR and HFESR on a single crystal of [Fe 4 (L) 2 (dpm) 6 ] (1) where Hdpm is 2,2,6,6-tetramethyl-3,5-heptanedione (also known as dipivaloylmethane) and H 3 L is the tripodal ligand (R,S)-2-hydroxymethyl-2-(2-methyl-butoxymethyl)propane-1,3-diol [21]. Among SMMs of the Fe 4 family, this particular derivative was chosen because it exhibits very narrow lines in the HFESR spectra recorded on a powder sample [21].…”

“…We herein present a combined study by TM, TDESR and HFESR on a single crystal of [Fe 4 (L) 2 (dpm) 6 ] (1) where Hdpm is 2,2,6,6-tetramethyl-3,5-heptanedione (also known as dipivaloylmethane) and H 3 L is the tripodal ligand (R,S)-2-hydroxymethyl-2-(2-methyl-butoxymethyl)propane-1,3-diol [21]. Among SMMs of the Fe 4 family, this particular derivative was chosen because it exhibits very narrow lines in the HFESR spectra recorded on a powder sample [21]. Moreover, all molecules in the crystal are magnetically equivalent and iso-oriented, a most favorable situation for singlecrystal studies (see below).…”