From the {Fe(2)(III)Ln(2)} Butterfly's Perspective: the Magnetic Benefits and Challenges of Cooperativity within 3 d-4 f Based Coordination Clusters

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(2)(III)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. This simplifies theory and anaylsis; (ii) choosing Fe allows Fe-57 Mossbauer 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 [(Fe3O)-O-III(O2CR)(6)(L)(3)](X) (X=anion, L=solvent such as H2O, py) with an ethanolamine-based ligand L ' and lanthanide salts. This allows to study analogues of [Fe(2)(III)Ln(2)(mu(3)-OH)(2)(L ')(2)(O2CR)(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(2)(III)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