Functionalization of Carbon Dioxide and Carbon Disulfide Using a Stable Uranium(III) Alkyl Complex

A rare uranium(III) alkyl complex, Tp*2U(CH2Ph) (2) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by salt metathesis from Tp*2UI (1) and KCH2Ph and fully characterized using 1H NMR, infrared, and electronic absorption spectroscopies as well as X-ray crystallography. This complex has a uranium−carbon distance of 2.57(2) Å, which is comparable to other uranium alkyls reported. Treating this compound with either carbon dioxide or carbon disulfide results in insertion into the uranium−carbon bond to generate Tp*2U(κ2-O2CCH2Ph) (3) and Tp*2U(SC(S)CH2Ph) (4), respectively. These species, characterized spectroscopically and by X-ray crystallography, feature new carboxylate and dithiocarboxylate ligands. Analysis by electronic absorption spectroscopy supports the trivalent oxidation state of the uranium center in both of these derivatives. Addition of trimethylsilylhalides (Me3SiX; X = Cl, I) to 3 results in the release of the free silyl ester, Me3SiOC(O)CH2Ph, forming the initial uranium monohalide species, Tp*2UX, which can then be used over multiple cycles for the functionalization of carbon dioxide.