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

Tetrairon(III) single-molecule magnets [Fe4 (pPy)2 (dpm)6 ] (1) (H3 pPy=2-(hydroxymethyl)-2-(pyridin-4-yl)propane-1,3-diol, Hdpm=dipivaloylmethane) have been deliberately organized into supramolecular chains by reaction with Ru(II) Ru(II) or Ru(II) Ru(III) paddlewheel complexes. The products [Fe4 (pPy)2 (dpm)6 ][Ru2 (OAc)4 ](BF4 )x with x=0 (2 a) or x=1 (2 b) 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 2 a act as effective seff =0 spins and lead to negligible intrachain communication. By contrast, the mixed-valent bridges (s=3/2, seff =1/2) in 2 b 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.