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.