Synthesis and Reactivity of a U(IV) Dibenzyne Complex

Reaction of UCl4 with
6 equiv of 2-Li-C6H4CH2NMe2 affords the U­(IV) dibenzyne
complex [Li]2[U­(2,3-C6H3CH2NMe2)2(2-C6H4CH2NMe2)2] (1), which can be isolated
as a dark blue solid in 40% yield. Complex 1 represents
a rare example of a structurally characterized dibenzyne complex,
and its solid-state metrical parameters suggest that the two benzyne
ligands are best described with the dianionic metallacyclopropene
resonance form. The reactivity of 1 with a variety of
electrophiles and oxidants, including benzophenone, 1-azidoadamantane,
and benzonitrile, was also explored. Reaction of 1 with
2 equiv of benzophenone affords the insertion product [Li]­[U­(2-C6H3CH2NMe2-3-COPh2)2(2-C6H4CH2NMe2)] (3) as a red-orange solid in 61% yield,
concomitant with formation of 1 equiv of 2-Li-C6H4CH2NMe2. Reaction of 1 with 2
equiv of PhCN also affords an insertion product, [Li]­[Li­(Et2O)]­[U­(2,3-C6H3CH2NMe2)­(2-C6H3CH2NMe2-3-C­(Ph)N)­(2-C6H4CH2NMe2)2] (4), as a green-brown crystalline solid
in 21% yield. In an attempt to oxidize 1 to U­(VI), the
reaction of 1 with 2 equiv of AdN3, in the
presence of 2 equiv of 12-crown-4, was probed. This reaction only
yields the U­(IV) insertion product [Li­(12-crown-4)2]­[Li]­[U­(2-C6H3CH2NMe2-3-(N-NN-Ad))2(2-C6H4CH2NMe2)2] (5) as a red crystalline solid in 42% yield. No evidence for the formation
of a U­(VI) imido complex is observed in the reaction mixture.