Uranium versus Thorium: Synthesis and Reactivity of [η(5) -1,2,4-(Me(3) C)(3) C(5) H(2) ](2) U[η(2) -C(2) Ph(2) ].

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