Despite the long history of the Fischer-Tropsch reaction, carbon monoxide has proven remarkably resistant to selective homologation under mild conditions. Here, we find that an organouranium(III) complex induces efficient reductive trimerization of carbon monoxide at room temperature and pressure. The result is a triangular, cyclic C3O(2-)3, or deltate, dianion held between two uranium(IV) units. The bonding within the C3O(2-)3 unit and its coordination to the two U centers have been analyzed by x-ray diffraction and density functional theory computational studies, which show a stabilizing C-C agostic interaction between the C3 core and one U center. Solution nuclear magnetic resonance studies reveal a rapid equilibration of the deltate unit between the U centers.
The U(III) mixed-sandwich compound [U(eta-C5Me4H)(eta-C8H6{SiiPr3-1,4}2)(THF)] 1 may be prepared by sequential reaction of UI3 with K[C5Me4H] in THF followed by K2[C8H6{SiiPr3-1,4}2]. 1 reacts with carbon monoxide at -30 degrees C and 1 bar pressure in toluene solution to afford the crystallographically characterized dimer [(U(eta-C8H6{SiiPr3-1,4}2)(eta-C5Me4H)]2(mu-eta2: eta2-C4O4) 2, which contains a bridging squarate unit derived from reductive cyclotetramerization of CO. DFT computational studies indicate that addition of a 4th molecule of CO to the model deltate complex [U(eta-COT)(eta-Cp)]2(mu-eta1: eta2-C3O3)] to form the squarate complex [U(eta-COT)(eta-Cp)]2(mu-eta2: eta2-C4O4)] is exothermic by 136 kJ mol-1.
The mixed-sandwich U(III) complexes [U(eta-C8H6{SiiPr(3)-1,4}2)(eta-CpR)(THF)] (R=Me5, Me4H) react with CO2 to give free CO and the U(IV) carbonate products [U(eta-C8H6{SiiPr(3)-1,4}2)(eta-CpR)]2(micro-eta1:eta2-CO3)]; the latter has been structurally characterised for R=Me4H; a 25% molar excess of the U(III) reductant gives a mixture of the carbonate and squarate [U(eta-C8H6{SiiPr(3)-1,4}2)(eta-C5Me4H)]2(micro-eta2:eta2-C4O4) products-the first synthesis of an oxocarbon from a CO2 carbon source.
The selectivity of the mixed-sandwich U(III) complexes of the type [U(η-C 8 H 6 {Si i R 3 -1,4} 2 )(η-Cp R ′)] (R = Me, i Pr; R′ = Me 4 H, Me 5 , Me 4 i Pr, Me 4 SiMe 3 , Me 4 Et) toward the reductive coupling of CO to form uranium-bound oxocarbons has been explored. In this context, the new U(III) mixed-sandwich complexes [U(η-C 8 H 6and [U(η-C 8 H 6 {SiMe 3 -1,4} 2 )(η-Cp Me4TMS )] have been prepared and structurally characterized. The reactivity toward CO is dominated by the "global" sterics around the uranium center, while selectivity for oxocarbon formation is largely regulated by the steric bulk of the Cp R ′ ligand. Accordingly, with excess CO [U(η-C 8 H 6 {Si i Pr 3 -1,4} 2 )(η-Cp Me4TMS )] and [U(η-C 8 H 6 {Si i Pr 3 -1,4} 2 )(η-Cp Me4iPr )] show no reactivity, [U(η-C 8 H 6 {SiMe 3 -1,4} 2 )(η-Cp Me4TMS )] is completely selective for the formation of the ynediolate complex, and [U(η-C 8 H 6 {SiiPr 3 -1,4} 2 )(η-Cp Me4H )] gives solely the squarate complex [U(η-C 8 H 6 {SiiPr 3 -1,4} 2 )(η-Cp Me4H )] 2 (μ-η 2 :η 2 -C 4 O 4 ). Additionally, the squarate moiety has been removed from the uranium center in the last complex by reaction with Me 3 SiCl to afford the TMS ester of squaric acid, C 4 O 2 (OTMS) 2 .Article pubs.acs.org/Organometallics
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