Magnetostructural correlations in the tetranuclear series of{Fe3LnO2}butterfly core clusters: Magnetic and Mössbauer spectroscopic study

Abstract

Six tetranuclear complexes $[\text{Fe}{(\text{III})}_{3}\text{Ln}{({\ensuremath{\mu}}_{3}\text{-O})}_{2}{({\text{CCl}}_{3}\text{COO})}_{8}({\text{H}}_{2}\text{O}){(\text{THF})}_{3}]\ensuremath{\cdot}\text{THF}\ensuremath{\cdot}{\text{C}}_{7}{\text{H}}_{16}$ [$\text{Ln}=\text{Gd}(\text{III})$ (1), Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5), and Lu(III) (6)] have been studied by magnetic susceptibility and M\ossbauer spectroscopy. These isostructural molecules have a ``butterfly'' structure core consisting of two ${\text{Fe}}_{2}\text{Ln}({\ensuremath{\mu}}_{3}\text{-O})$ triangular ``wings'' which share a common Ln-Fe ``body''; the dihedral angle between the wings is ca. $148\ifmmode^\circ\else\textdegree\fi{}$. 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 ${J}_{\text{Fe-Fe}}=\ensuremath{-}50\text{ }\text{K}$ 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 ${{\text{Fe}}_{3}{\text{TbO}}_{2}}$, ${{\text{Fe}}_{3}{\text{DyO}}_{2}}$, and ${{\text{Fe}}_{3}{\text{HoO}}_{2}}$. The activation energy of a Debye process describing the magnetization reversal has been determined to be, ${E}_{a}\ensuremath{\approx}8$, 9, and 10 K for the $\text{Ln}=\text{Tb}$, Dy, and Ho, respectively, and the prefactor ${\ensuremath{\tau}}_{0}\ensuremath{\approx}{10}^{\ensuremath{-}7}\text{ }\text{s}$. The high spin states of the Fe(III) centers were confirmed by the M\ossbauer 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 M\ossbauer 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 M\ossbauer spectra showed a state with blocked spins (sextets) for the ${{\text{Fe}}_{3}{\text{TbO}}_{2}}$ and ${{\text{Fe}}_{3}{\text{DyO}}_{2}}$ cases. From the ${E}_{a}$ and ${\ensuremath{\tau}}_{0}$, determined in the ac susceptibility, the relaxation time at 3 K is estimated as $\ensuremath{\tau}\ensuremath{\approx}{10}^{\ensuremath{-}5}--{10}^{\ensuremath{-}6}\text{ }\text{s}$ much longer than the time window of M\ossbauer 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.