Structure and Electronic Properties of Large Oligomeric Heterometallic 3d/Ce^IV Polyoxometalates

Merging the rich chemistry of Ce(IV) polyoxometalates (POMs) with that of 3d polyanions remains a challenge due to the strong competition between these highly oxophilic lanthanide cations and 3d metallic ions for coordination to lacunary molecular metal oxides. We report herein the characterization of an unprecedented water stable hexameric Ce-IV/Co-II POM (Ce12Co6) made of two {(SiW9)(2)Ce-6} units connected to a {(SiW10)(2)Co-6(PO4)(2)} core. In addition, the pentameric Ce-IV/Ni-II compound Ce6Ni8, where two {PW9Ni3W} and a {PW10Ni2} fragments are grafted on a {(PW9)(2)Ce-6} moiety, has been obtained. Magnetic studies of Ce6Ni8 revealed ferromagnetic interactions between the Ni-II centers constituting the {Ni3PW10} fragments, in agreement with the geometry of such a trinuclear cluster. Related insoluble barium salts of Ce12Co6 and Ce6Ni8 were also prepared, allowing their solid-state electrochemical investigations and showing in particular that in Ce12Co6, both the cobalt, cerium, and silicotungstate moieties are electroactive. Finally, photophysical studies demonstrate the formation of long-lived reduced POMs photosensitized by [Ru(bpy)(3)](2+), suggesting that Ce12Co6 and Ce6Ni8 could be used as efficient reservoirs of reduction equivalents for photocatalytic reactions.