Understanding the Scarcity of Thorium Peroxide Clusters.

The reaction of Th(NO3)4·5H2O with 3 equiv of 2,2',6',2″-terpyridine (terpy) in a mixture of acetonitrile and methanol results in formation of the trinuclear thorium peroxide cluster [Th(O2)(terpy)(NO3)2]3. This cluster is assembled via bridging by μ-η2:η2 peroxide anions between thorium centers. It decomposes upon removal from the mother liquor to yield Th(terpy)(NO3)4 and Th(terpy)(NO3)4(EtOH). The peroxide formation appears to be radiolytic in origin and is, most likely, generated from radiolysis of water by short-lived daughters generated from 232Th decay. This cluster does not form when freshly recrystallized Th(NO3)4·5H2O is used as the starting material and requires an aged source of thorium. Analysis of the bonding in these clusters shows that, unlike uranium(VI) peroxide interactions, thorium(IV) complexation by peroxide is quite weak and largely ionic. This explains its much lower stability, which is more comparable to that observed in similar zirconium(IV) peroxide clusters.