Slow magnetic relaxation and charge-transfer in cyano-bridged coordination clusters incorporating [Re(CN)₇]^3-/4-

Treatment of the cyanometalate building unit [Re(CN)₇] ^3- with [(PY5Me₂)M(MeCN)]²⁺ (M = Co, Ni, Cu) affords a series of pentanuclear clusters of formulas [(PY5Me₂) ₄M₄Re(CN)₇]⁵⁺ (M = Co, Ni, Cu) and [(PY5Me₂)₄Cu₄Re(CN)₇]⁴⁺. Single crystal X-ray diffraction analyses of the clusters reveal a star-like structure in which four [(PY5Me₂)M]²⁺ moieties are linked to a central [Re(CN)₇]^3- unit via bridging cyanide ligands. An intramolecular Co^II → Re^IV charge-transfer accompanies the formation of the Co^II ₄Re^IV cluster, giving a Co^II ₃Co ^IIIRe^III species. Spectroelectrochemical methods and irradiation experiments are used to characterize the metal-metal charge-transfer bands of this compound. A rhenium-based thermally induced one-electron reduction is observed for the Cu^II ₄Re^IV cluster to give a Cu^II ₄Re^III complex; however, this reduction may be forestalled at low temperature. Finally, magnetic measurements reveal intracluster ferromagnetic exchange coupling, strong uniaxial magnetic anisotropy, and slow magnetic relaxation in the Ni^II ₄Re^IV and Cu^II ₄Re^IV clusters.