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

Reaction of [Re(CN)7]3- (S = 1/2) with 4 equiv of [(PY5Me2)Mn(MeCN)]2+ (S = 5/2) in acetonitrile generates a blue solution containing the cyano-bridged cluster [(PY5Me2)4Mn4Re(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 [(PY5Me2)4Mn4Re(CN)7]4+. Crystal structures show these two clusters share a star-like geometry, wherein four [(PY5Me2)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 Ueff = 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 MnII centers.