Experimental and Computational Studies of a Uranium Metallacyclocumulene

Abstract

The synthesis, electronic structure, and reactivity of a uranium metallacyclocumulene were studied. Reduction of [(η 5 -C 5 Me 5 ) 2 UCl 2 ] ( 1 ) with potassium graphite (KC 8 ) in the presence of 1,4-bis(trimethylsilyl)butadiyne (Me 3 SiC≡C–C≡CSiMe 3 ) forms the uranium metallacyclocumulene [(η 5 -C 5 Me 5 ) 2 U{η 4 -C 4 (SiMe 3 ) 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(η 4 -C═C═C═C) moiety, leading to more covalent bonds between the [(η 5 -C 5 Me 5 ) 2 U] 2+ and [η 4 -C 4 (SiMe 3 ) 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 (η 5 -C 5 Me 5 ) 2 U(II) fragment when reacted with conjugated species such as butadiyne, bipy, and diazabutadiene derivatives. Alternatively, the [(η 4 -Me 3 SiC═C═C═CSiMe 3 )] 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.