Redox-Controlled Exchange Bias in a Supramolecular Chain of Fe4 Single-Molecule Magnets

Tetrairon(III) single-molecule magnets [Fe-4(pPy)(2)(dpm)(6)] (1) (H(3)pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane) have been deliberately organized into supramolecular chains by reaction with (RuRuII)-Ru-II or (RuRuIII)-Ru-II paddlewheel complexes. The products [Fe-4(pPy)(2)(dpm)(6)][Ru-2(OAc)(4)](BF4)(x) with x=0 (2a) or x=1 (2b) differ in the electron count on the paramagnetic diruthenium bridges and display hysteresis loops of substantially different shape. Owing to their large easy-plane anisotropy, the s=1 diruthenium(II,II) units in 2a act as effective s(eff)=0 spins and lead to negligible intrachain communication. By contrast, the mixed-valent bridges (s=3/2, s(eff)=1/2) in 2b introduce a significant exchange bias, with concomitant enhancement of the remnant magnetization. Our results suggest the possibility to use electron transfer to tune intermolecular communication in redox-responsive arrays of SMMs