From the Fe$^{III}$$_{2}$Ln$_{2}$ Butterfly’s Perspective: the Magnetic Benefits and Challenges of Cooperativity within 3 d–4 f Based Coordination Clusters

The well-known oxo-centred Fe triangle is used to access the vast majority of [Fe$^{III}$$_{2}$Ln$_{2}$(μ$_{3}$-OR)$_{2}$L$_{2}$(O$_{2}$CR)$_{6}$] butterflies, since it provides a di-iron central body unit and carboxylate bridges to the wingtips, which are occupied by supplied Ln ions. The remaining coordination sites of the lanthanide are occupied by a multidentate ligand, which can also provide further bridging. This system provides a useful testbed for exploring multiple variations, for example changing the Ln, the bridging carboxylate and the nature of the chelating ligand.
In this Review we discuss the tuning handles which can be used to steer the magnetic properties of Fe$^{III}$-4 f “butterfly” compounds. The majority of presented compounds were produced in the context of project A3 “Di- to tetranuclear compounds incorporating highly anisotropic paramagnetic metal ions” within the SFB/TRR88 “3MET”. These contain {Fe$^{III}$$_{2}$Ln$_{2}$} cores encapsulated in ligand shells which are easy to tune in a “test-bed” system. We identify the following advantages and variables in such systems: (i) the complexes are structurally simple usually with one crystallographically independent Fe$^{III}$ and Ln$^{III}$ respectively. ... mehrThis simplifies theory and anaylsis; (ii) choosing Fe allows $^{57}$Fe Mössbauer spectroscopy to be used as an additional technique which can give information about oxidation levels and spin states, local moments at the iron nuclei and spin-relaxation and, more importantly, about the anisotropy not only of the studied isotope, but also of elements interacting with this isotope; (iii) isostructural analogues with all the available (i. e. not Pm) 4 f ions can be synthesised, enabling a systematic survey of the influence of the 4 f ion on the electronic structure; (iv) this cluster type is obtained by reacting [Fe$^{III}$$_{3}$O(O$_{2}$CR)$_{6}$(L)$_{3}$](X) (X=anion, L=solvent such as H$_{2}$O, py) with an ethanolamine-based ligand L′ and lanthanide salts. This allows to study analogues of [Fe$^{III}$$_{2}$Ln$_{2}$(μ$_{3}$-OH)$_{2}$(L′)$_{2}$(O$_{2}$CR)$_{6}$] using the appropriate iron trinuclear starting materials. (v) the organic main ligand can be readily functionalised, facilitating a systematic investigation of the effect of organic substituents on the ligands on the magnetic properties of the complexes. We describe and discuss 34 {M$^{III}$$_{2}$Ln$_{2}$} (M=Fe or in one case Al) butterfly compounds which have been reported up to 2020. The analysis of these gives perspectives for designing new SMM systems with specific electronic and magnetic signatures