Experimental and Computational Studies of a Uranium Metallacyclocumulene

The synthesis, electronic structure, and reactivity of a uranium metallacyclocumulene were studied. Reduction of [(η5-C5Me5)2UCl2] (1) with potassium graphite (KC8) in the presence of 1,4-bis(trimethylsilyl)butadiyne (Me3SiC≡C–C≡CSiMe3) forms the uranium metallacyclocumulene [(η5-C5Me5)2U{η4-C4(SiMe3)2}] (2) in good yield. Magnetic susceptibility data confirm that 2 behaves as a U(IV) complex, and density functional theory (DFT) studies indicate a substantial 5f orbital contribution to the bonding of the metallacyclopentatriene U(η4-C═C═C═C) moiety, leading to more covalent bonds between the [(η5-C5Me5)2U]2+ and [η4-C4(SiMe3)2]2– fragments than those found in the related Th(IV) compound. Consequently, very different reactivity patterns emerge; e.g., 2 can act as a synthetic equivalent for the (η5-C5Me5)2U(II) fragment when reacted with conjugated species such as butadiyne, bipy, and diazabutadiene derivatives. Alternatively, the [(η4-Me3SiC═C═C═CSiMe3)]2– moiety in 2 may react as a nucleophile when expose...