Slow magnetic relaxation and charge-transfer in cyano-bridged coordination clusters incorporating [Re(CN)(7)](3-/4-).
Résumé
Treatment of the cyanometalate building unit [Re(CN)(7)](3-) with [(PY5Me(2))M(MeCN)](2+) (M = Co, Ni, Cu) affords a series of pentanuclear clusters of formulas [(PY5Me(2))(4)M(4)Re(CN)(7)](5+) (M = Co, Ni, Cu) and [(PY5Me(2))(4)Cu(4)Re(CN)(7)](4+). Single crystal X-ray diffraction analyses of the clusters reveal a star-like structure in which four [(PY5Me(2))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)(4)Re(IV) cluster, giving a Co(II)(3)Co(III)Re(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)(4)Re(IV) cluster to give a Cu(II)(4)Re(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)(4)Re(IV) and Cu(II)(4)Re(IV) clusters.