A Redox-Switchable Single-Molecule Magnet Incorporating [Re(CN)₇]³⁻

Reaction of [Re(CN) 7 ] 3- ( S = 1 / 2 ) with 4 equiv of [(PY5Me 2 )Mn(MeCN)] 2+ ( S = 5 / 2 ) in acetonitrile generates a blue solution containing the cyano-bridged cluster [(PY5Me 2 ) 4 Mn 4 Re(CN) 7 ] 5+ . At room temperature, the color of the reaction rapidly changes from blue to green to yellow, spontaneously generating the one-electron reduced species [(PY5Me 2 ) 4 Mn 4 Re(CN) 7 ] 4+ . Crystal structures show these two clusters share a star-like geometry, wherein four [(PY5Me 2 )Mn] 2+ units are appended to a central, pentagonal bipyramidal [Re(CN) 7 ] 3-/4- complex. The dc magnetic properties of the 5+ cluster indicate the presence of ferromagnetic coupling to give an S = 21 / 2 ground state with an axial zero-field splitting of D = −0.44 cm -1 . Ac magnetic susceptibility measurements reveal a spin relaxation barrier of U eff = 33 cm -1, the largest barrier yet observed for a cyano-bridged single-molecule magnet. Upon reduction by one electron, the spin at the Re center is lost and the cluster switches behavior to that of a simple paramagnet containing four noncoupled Mn II centers.