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 CoII → ReIV charge-transfer accompanies the formation of the CoII4ReIV cluster, giving a CoII3CoIIIReIII 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 CuII4ReIV cluster to give a CuII4ReIII 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 NiII4ReIV and CuII4ReIV clusters.