Six tetranuclear complexes $\left[\text{Fe}{\left(\text{III}\right)}_3\text{Ln}{\left({\mu }_3\text{-O}\right)}_2{\left({\text{CCl}}_3\text{COO}\right)}_8\left({\text{H}}_2\text{O}\right){\left(\text{THF}\right)}_3\right]\cdot \text{THF}\cdot {\text{C}}_7{\text{H}}_{16}$ [$\text{Ln}=\text{Gd}\left(\text{III}\right)$ (1), Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5), and Lu(III) (6)] have been studied by magnetic susceptibility and Mössbauer spectroscopy. These isostructural molecules have a “butterfly” structure core consisting of two ${\text{Fe}}_2\text{Ln}\left({\mu }_3\text{-O}\right)$ triangular “wings” which share a common Ln-Fe “body”; the dihedral angle between the wings is ca. $148°$. The coordination spheres of the iron ions are essentially distorted octahedral. The lanthanides are eight-coordinate with coordination polyhedra that may be described as distorted tetragonal bipyramids. Variable-temperature solid-state magnetic susceptibility in the temperature range 1.8–300 K and magnetization at 1.8 K for compounds 1–6 were measured. The spin state of Fe is $S=5/2$ in all cases. In compounds 5 and 6, where Ln(III) (Y and Lu, respectively) is diamagnetic, the three Fe atoms form an obtuse isosceles triangle with antiferromagnetic interactions $J_{\text{Fe-Fe}}=-50\text{K}$ between the wing-tip ${\text{Fe}}_w$ and body ${\text{Fe}}_b$ atoms, and negligible interaction between the ${\text{Fe}}_w$’s, resulting in a ground state of effective spin $S=5/2$ per cluster. In the complexes with paramagnetic lanthanide ions, the interaction between the ${\text{Fe}}_3$ triangle and the Ln(III) center is described by an effective exchange constant which is antiferromagnetic and 1 order of magnitude weaker. Besides, at 3 K incipient spin blocking, characteristic of single molecule magnets, was found to occur in the out-of-phase component of the ac susceptibility in $\left\{{\text{Fe}}_3{\text{TbO}}_2\right\}$, $\left\{{\text{Fe}}_3{\text{DyO}}_2\right\}$, and $\left\{{\text{Fe}}_3{\text{HoO}}_2\right\}$. The activation energy of a Debye process describing the magnetization reversal has been determined to be, $E_a\approx 8$, 9, and 10 K for the $\text{Ln}=\text{Tb}$, Dy, and Ho, respectively, and the prefactor ${\tau }_0\approx {10}^{-7}\text{s}$. The high spin states of the Fe(III) centers were confirmed by the Mössbauer spectra, in which two distinguishable Fe sites could be resolved above 80 K, corresponding to the ${\text{Fe}}_w$ and ${\text{Fe}}_b$ sites, respectively. Relatively larger values of the quadrupole splitting of the Mössbauer spectra were observed for the ${\text{Fe}}_w$ pair as compared with that for the ${\text{Fe}}_b$, and both quadrupole splittings diminished with increasing temperature. At 3 K the Mössbauer spectra showed a state with blocked spins (sextets) for the $\left\{{\text{Fe}}_3{\text{TbO}}_2\right\}$ and $\left\{{\text{Fe}}_3{\text{DyO}}_2\right\}$ cases. From the $E_a$ and ${\tau }_0$, determined in the ac susceptibility, the relaxation time at 3 K is estimated as $\tau \approx {10}^{-5}–{10}^{-6}\text{s}$ much longer than the time window of Mössbauer spectroscopy and compatible with the single molecule magnet behavior. In the presence of a strong magnetic field the moments of the Ln(III) cation and the ${\text{Fe}}_3$ triangle are polarized. For some compounds at low temperature a magnetic pattern (sextet) for each of the three Fe sites appeared, and the antiferromagnetic coupling within the ${\text{Fe}}_3$ cluster was directly proved by the opposite trend of the field dependence of the two ${\text{Fe}}_w$ sextets as compared with the ${\text{Fe}}_b$ third one.

• Received 8 April 2009

DOI:https://doi.org/10.1103/PhysRevB.80.014430