In Pursuit of Homoleptic Actinide Alkyl Complexes

This Forum Article describes the pursuit of isolable homoleptic actinide alkyl complexes, starting with the pioneering work of Gilman during the Manhattan project. The initial reports in this area suggested that homoleptic uranium alkyls were too unstable to be isolated, but Wilkinson demonstrated that tractable uranium alkyls could be generated by purposeful “ate” complex formation, which serves to saturate the uranium coordination sphere and provide the complexes with greater kinetic stability. More recently, we reported the solid-state molecular structures of several homoleptic uranium alkyl complexes, including [Li­(THF)₄]­[U­(CH₂^tBu)₅], [Li­(TMEDA)]₂[UMe₆], [K­(THF)]₃[K­(THF)₂]­[U­(CH₂Ph)₆]₂, and [Li­(THF)₄]­[U­(CH₂SiMe₃)₆], by employing Wilkinson’s strategy. Herein, we describe our attempts to extend this chemistry to thorium. The treatment of ThCl₄(DME)₂ with 5 equiv of LiCH₂^tBu or LiCH₂SiMe₃ at −25 °C in THF affords [Th­(CH₂^tBu)₅] (<b>1</b>) and [Li­(DME)₂]­[Th­(CH₂SiMe₃)₅ (<b>2</b>), respectively, in moderate yields. Similarly, the treatment of ThCl₄(DME)₂ with 6 equiv of K­(CH₂Ph) produces [K­(THF)]₂[Th­(CH₂Ph)₆] (<b>3</b>), in good yield. Complexes <b>1</b>–<b>3</b> have been fully characterized, while the structures of <b>1</b> and <b>3</b> were confirmed by X-ray crystallography. Additionally, the electronic properties of <b>1</b> and <b>3</b> were explored by density functional theory