Uranium(III)-Mediated C–C-Coupling of Terminal Alkynes: Formation of Dinuclear Uranium(IV) Vinyl Complexes

The previously reported uranium­(III) complex [((AdArO)3N)­UIII(DME)] (1;
Ad = adamantane,
DME = 1,2-dimethoxyethane) reacts with the terminal bis-alkynes 1,7-octadiyne
or 1,6-heptadiyne in C–C-coupling reactions to form the uranium­(IV)
vinyl complexes [{((AdArO)3N)­UIV}2(μ–η2:η1-1,2-(CH)2-cyclohexane)] (2) and [{((AdArO)3N)­UIV}2(μ–η2:η2-1,2-(CH)2-cyclopentane)] (3). With the monoalkynes 1-hexyne or
4-tbutyl-phenylacetylene, the complexes
[{((AdArO)3N)­UIV}2(μ–η2(C1):η1(C4)-2-nBu-1,3-octadiene)] (4) and [{((AdArO)3N)­UIV}2(μ–η2(C4):η1(C1)-1,3-di-(p-tBu-phenyl)­butadiene))] (5), are formed.
These are the first four examples of uranium vinyl complexes that
are reported and crystallographically characterized. In addition,
detailed DFT calculations are presented to establish a possible mechanism
for their formation and explain the differences found for the coordination
of the hydrocarbon fragments. In contrast to a previously proposed
monometallic pathway for catalytic hydroamination of alkynes and alkyne
dimerization involving a uranium vinyl intermediate at uranium­(III)
complexes, the calculations clearly support a bimetallic mechanism,
since its transition states are energetically the most favored.