Two-electron versus one-electron reduction of chalcogens by uranium(III): synthesis of a terminal U(V) persulfide complex

The reaction of the tripodal tris-amido U(III) complex [U{(SiMe2NPh)(3)-tacn}] (tacn = 1,4,7-triazacyclononane), 1, with 0.0625 and 0.25 equiv. of elemental sulfur affords the sulfide-bridged U(IV) complex [{U((SiMe2NPh)(3)-tacn)}(2)(mu-S)], 2, and the terminal persulfide U(V) complex [U{(SiMe2NPh)(3)-tacn}(eta(2)-S-2)], 4, respectively, in good yield. Two different electronic structures, U(V) persulfide and U(IV) supersulfide, were computed for complex 4 at the DFT level. The results show that complex 4 is best described as a U(V) persulfide species with a significant sulfur contribution. X-ray, magnetism and electrochemistry data support this description. Complex 4 is the first example of a terminal U(V) persulfide and of a two-electron reduction of S-8 by a U(III) complex. Complex 4 behaves as a S-atom transfer agent when reacted with PPh3, affording the persulfide-bridged diuranium(IV) complex [{U((SiMe2NPh)(3)-tacn)}(2)(mu-eta(2):eta(2)-S-2)], 5, and S=PPh3.