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.

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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

A shift to the hydrogen economy requires the development of an effective hydrogen fuel carrier with high volumetric and gravimetric storage capacity. Ammonia borane (AB) has emerged as a leading candidate due to its light weight and multiple protic (N-H) and hydridic (B-H) hydrogens. As a consequence, much work has been directed towards fine tuning the release of H2 from AB, in addition to its regeneration from the dehydrogenated "spent fuel" materials. This review summarizes the development of these regeneration methodologies.

In this review we detail recent developments in the synthesis of lanthanoid complexes (group III and lanthanide elements) containing multiple bonding interactions with tetrel, pnictogen and chalcogen main group elements.

The new pentalene antibimetallic compounds [MCp*(THF)] 2 (μ,η 5 :η 5 -C 8 H 4 1,4-Si i Pr 3) were prepared for M = Eu (1), Yb (2) from the one-pot reaction of MI 2 (THF) x and KCp* and the subsequent addition of 1 / 2 equiv of C 8 H 4 1,4-Si i Pr 3[K] 2 in THF. The related series of COT 1,4-Si i Pr 3 -bridged triple-deckers [MCp*(THF) x ] 2 (μ,η 8 :η 8 -COT 1,4-Si i Pr 3 ) (M=Eu, x=0 (3); M=Yb, x=0 (4); M=Sm, x=1 (5)) were synthesized similarly; additionally, the base-free derivative with M=Sm and x=0 (6) could be prepared by reaction of [SmCp*(μ-I)(THF) 2 ] 2 with COT 1,4-Si i Pr 3 [K] 2 in toluene with heating. The solid-state structures, as determined by X-ray diffraction, show antibimetallic arrangements in which the divalent lanthanide centers are held on opposing sides of a planar bridging ligand. The pentalene ligand coordinates in an approximate η 5 :η 5 mode, with the metal centers slipped toward the wingtip carbons, whereas the COT ligand is bound in an η 8 :η 8 fashion with the metal centers aligned with the centroid of the bridging ligand. Electronic spectroscopy suggests the Eu and Yb pentalene complexes have a smaller f-d gap than their COT analogues, indicating a greater extent of through-ligand metal-metal interaction in the pentalene species. [EuCp*(THF) x ] 2 (μ-COT 1,4-Si i Pr 3 ) displays a weak green-yellow emission in THF solution (λ max 509 nm, Φ em < 0.1%) upon excitation in the UV, consistent with a 4f 6 5d 1 f 4f 7 emission process, with a short lifetime indicative of Eu-Eu coupling through the bridging COT ligand. Cyclic voltammetry reveals that 1, 3, and 4 decompose rapidly upon oxidation, although the monocation 2 þ appears to be stable in THF solution. Through-ligand Yb-Yb coupling is suggested by the electrochemical data for 2, of magnitude similar to that observed for its transition-metal analogues. However, unfortunately, attempts to further quantify this conclusion by spectroscopic investigation of 2 þ were unsuccessful.