Single metal four-electron reduction by U(ii) and masked "U(ii)" compounds.

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)][{((MeSi)N)U}(μ-O)], , effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U( -CPh){N(SiMe)}], , and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh){N(SiMe)}], , respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe)}], . Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the -hydrazide intermediate [K(2.2.2-cryptand)][U(NPh){N(SiMe)}], , corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a "clear-cut" single metal four-electron transfer in f-element chemistry.