Uranium versus Thorium: Synthesis and Reactivity of [η5-1,2,4-(Me3C)3C5H2]2U[η2-C2Ph2]

The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC equivalent to CPh) to the uranium phosphinidene metallocene [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U=P-2,4,6-tBu(3)C(6)H(2) (1) yields the stable uranium metallacyclopropene, [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U[eta(2)-C2Ph2] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U-(eta(2)-C=C) moiety increases significantly compared to the related Th-IV compound [eta(5)-1,2,4-(Me3C)(3)C5H2](2)Th[eta(2)-C2Ph2], which also results in more covalent bonds between the [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U2+ and [eta(2)-C2Ph2](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 [eta(5)-1,2,4-(Me3C)(3)C5H2](2)U-II when reacted with Ph2E2 (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.