A new approach to the synthesis of heteronuclear propeller-like single molecule magnets

Abstract

Propeller-like [Fe(4)(L)(2)(dk)(6)] complexes, in which Hdk is a β-diketone and H(3)L is a tripodal alcohol, R-C(CH(2)OH)(3), exhibit tunable magnetic anisotropy barriers and retain their magnetic memory effect when chemically anchored on metal surfaces. Heteronuclear analogues of these M(4) complexes have been sought to afford a library of compounds with different total spin (S) values, but synthetic efforts described so far gave solid solutions containing M(4) in addition to the desired M(3)M' species. We now present a novel synthetic route to M(3)M' complexes featuring a central chromium(III) ion. The three-step preparation goes through coordination of Cr(III) by two equivalents of tripodal alkoxide (R = Et and Ph), followed by reaction of this complex "core" with the peripheral +III metal ions. Products have been characterised by chemical analyses together with (1)H-NMR, FTIR, W-band EPR, DC/AC magnetic susceptibility measurements and single crystal X-ray diffractometry. Due to the chemical inertness of Cr(III), this route yields 100% pure Fe(3)Cr complexes without metal scrambling; what is more, it is suitable for designing novel heteronuclear single molecule magnets (SMMs) with a variety of d- and f-metals and R groups.

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

“…The energies of the two excited multiplets, and the degeneracy imposed by the trigonal symmetry of 1, are in excellent agreement with magnetic susceptibility data reported for related Fe 3 Cr derivatives. 42,44 The broadening of the lines at the extremes of the spectra, which is often observed in the EPR spectra of SMMs, 11,31,48,49 was attributed to a distribution in the axial zfs parameters D, and could be correctly reproduced assuming for each multiplet a specific distribution width σ D (the complete set of best fit parameters can be found in Table 1). …”

“…The synthesis of 1 as a pure Fe 3 Cr phase followed the procedure reported in Ref. 44 using H 3 L = 2-hydroxymethyl-2-methyl-propane-1,3-diol in place of 2-hydroxymethyl-2-ethyl-propane-1,3-diol, with recrystallization by slow evaporation of a n-hexane solution. The efficiency of this recently-developed synthetic route for the isolation of pure heterometallic species was here further confirmed by measuring the ac susceptibility of a powder sample of 1 in the range 0.03-60 kHz with a home developed probe based on the Oxford Instruments MAGLAB platform.…”

“…44 while dipolar contributions were calculated within the point-dipole approximation. Due to the aforementioned symmetry-imposed constraints, the number of free parameters to be determined in Eq.…”

“…42,43 Much better suited for detailed spectroscopic studies are samples prepared through a three-step synthetic approach, which reduces the amount of Fe 4 to below the detection limit. 44 Of particular interest among these new heterometallic systems are Fe 3 Cr complexes characterized by an S = 6 ground state and an axial zero field splitting (zfs) parameter D ≅ -0.18 cm -1 .…”

Experimental and Theoretical Studies on the Magnetic Anisotropy in Lanthanide(III)‐Centered Fe3Ln Propellers

“…By using 3d metals, derivatives of 2‐hydroxymethyl‐propane‐1,3‐diol work well as tripodal ligands (Scheme ). Thus, Fe 4 ,13, 14 Fe 3 Cr,15, 16 and Fe 3 V17 species were isolated, which feature antiferromagnetic FeM interactions and ground total spin values ranging from S =5 to 13/2. These molecules conform to either rigorous or idealized D 3 symmetry and display easy‐axis anisotropy along the normal to the metal plane.…”