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.

Population growth has led to an increased demand for raw minerals and energy resources; however, their supply cannot easily be provided in the same proportions. Modern technologies contain materials that are becoming more finely intermixed because of the broadening palette of elements used, and this outcome creates certain limitations for recycling. The recovery and separation of individual elements, critical materials and valuable metals from complex systems requires complex energy-consuming solutions with many hazardous chemicals used. Significant pressure is brought to bear on the improvement of separation and recycling approaches by the need to balance sustainability, efficiency, and environmental impacts. Due to the increase in environmental consciousness in chemical research and industry, the challenge for a sustainable environment calls for clean procedures that avoid the use of harmful organic solvents. Ionic liquids, also known as molten salts and future solvents, are endowed with unique features that have already had a promising impact on cutting-edge science and technologies. This review aims to address the current challenges associated with the energy-efficient design, recovery, recycling, and separation of valuable metals employing ionic liquids.

A nonanuclear {Fe₆Dy₃} coordination cluster displaying SMM behaviour in which an unprecedented chemical transformation provides structural information for the existence of 1,1,2,2-tetrahydroxyethane is reported.