Uranium versus Thorium: Synthesis and Reactivity of [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U[η2 -C2 Ph2 ].

The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC≡CPh) to the uranium phosphinidene metallocene [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U=P-2,4,6-tBu3 C6 H2 (1) yields the stable uranium metallacyclopropene, [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U[η2 -C2 Ph2 ] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U-(η2 -C=C) moiety increases significantly compared to the related ThIV compound [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 Th[η2 -C2 Ph2 ], which also results in more covalent bonds between the [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U2+ and [η2 -C2 Ph2 ]2- fragments. Although the thorium and uranium complexes are structurally closely related, different reaction patterns are therefore observed. For example, 2 reacts as a masked synthon for the low-valent uranium(II) metallocene [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 UII when reacted with Ph2 E2 (E=S, Se), alkynes and a variety of hetero-unsaturated molecules such as imines, ketazine, bipy, nitriles, organic azides, and azo derivatives. In contrast, five-membered metallaheterocycles are accessible when 2 is treated with isothiocyanate, aldehydes, and ketones.