A Family of Enneanuclear Iron(II) Single‐Molecule Magnets

Abstract

Complexes [Fe9(X)2-(O2CMe)8{(2-py)2CO2}4] (X(-)=OH(-) (1), N3(-) (2), and NCO(-) (3)) have been prepared by a route previously employed for the synthesis of analogous Co(9) and Ni(9) complexes, involving hydroxide substitution by pseudohalides (N3(-), NCO(-)). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3, leading to higher ground spin states compared to that of 1. Variable-field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out-of-phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single-molecule magnets, with energy barriers of 41 K for 2 (tau 0=3.4 x 10(-12) s) and 44 K for 3 (tau 0=2.0 x 10(-11) s). Slow magnetic relaxation has also been observed by zero-field 57Fe Mössbauer spectroscopy. Characteristic integer-spin electron paramagnetic resonance (EPR) signals have been observed at X-band for 1, whereas 2 and 3 were found to be EPR-silent at this frequency. 1H NMR spectrometry in CD3CN has shown that complexes 1-3 are stable in solution.

Coordination Clusters of 3d-Metals That Behave as Single-Molecule Magnets (SMMs): Synthetic Routes and Strategies

“…These ligands most often propagate antiferromagnetic exchange interactions. Our group (Papaefstathiou et al, 2001a , b ; Boudalis et al, 2004 , 2008 ), in collaboration with Escuer's group, have developed a general strategy for the synthesis of high-spin clusters, which often “switches on” SMM behavior. The strategy is based on the substitution of bridging hydroxido or/and alkoxido groups in pre-formed coordination clusters (generally low-spin due to antiferromagnetic interactions) by EO azido groups (2.20, 3.30, 4.40) or N-bonded cyanato (isocyanato) bridging groups (2.02, 3.03, 4.04).…”

“…The method works better for divalent 3d metals. The strategy is illustrated with one example (Boudalis et al, 2004 , 2008 ).…”

Rare “Janus”-faced single-molecule magnet exhibiting intramolecular ferromagnetic interactions

“…Only a few trivalent Fe III SMMs16 have been reported to date and divalent Fe II examples are limited to three examples, viz ., an ,17 a family of cubanes,18 and a nonanuclear compound 19. Although organic bridging ligands assist in the thermodynamic stability, solubility, and crystallinity of 0-D compounds, their presence as neutral linkers often limits the magnetic properties and dynamics of SMMs, leading to competing magnetic interactions, moderate-to-weak magnetic couplings, and low-lying excited states, among others 11–14,16–19. A potential solution to these obstacles in preparing molecules with large S values and enhanced SMM properties is the discovery of new synthetic avenues to 3d compounds with exclusively ferromagnetic couplers.…”

A Strongly Spin‐Frustrated FeIII7 Complex with a Canted Intermediate Spin Ground State of S=7/2 or 9/2

“…57 Fe Mössbauer spectroscopy is proving to be a useful probe for the spin structure of CCs with high nuclearities, such as Fe III 7 Ln III 4 13a and Fe III 16 ,13b given that calculation of the spin structures of antiferromagnetically coupled high‐nuclearity Fe III systems can be difficult, if not indeed currently impossible, by using numerical or modelling techniques 14. Furthermore, 57 Fe Mössbauer spectroscopy can be used to assign oxidation levels and spin states,15 local or delocalised moments at the iron nuclei and spin‐relaxation dynamics,16 structural changes in the coordination sphere,17 and the anisotropy not only of the studied isotope but of the elements that interact with this isotope as well 18…”