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 exposed to a variety of simple heterounsaturated molecules such as aldehydes, ketones, nitriles, isothiocyanates, carbodiimides, pyridines, and organic azides. DFT studies are included to complement the experimental observations.