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] (<b>1</b>) 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}] (<b>2</b>) in good yield. Magnetic susceptibility data confirm that <b>2</b> 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., <b>2</b> 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 <b>2</b> 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