Self-assembly of a mixed-valence FeII-FeIII tetranuclear starElectronic supplementary information (ESI) available: Experimental details, crystallographic details & figures, magnetic fits and CASSCF-SO calculations details, Mössbauer and EPR spectroscopic figures. CCDC 1830689 and 1830690. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt01241e

A unique self-assembled mixed-valence Fe II -Fe III tetranuclear star has been comprehensively characterised showing a large magnetic anisotropy at the peripheral Fe II centres, ferromagnetic coupling between the iron centres and field-induced SMM behaviour. A self-assembled mixed-valence Fe II -Fe III tetranuclear star is reported that shows ferromagnetic coupling, field-induced single molecule magnetism and strong magnetic anisotropy at the peripheral Fe II centres.