Christos E. Kefalidis

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

Reactions of β-diketiminato magnesium and calcium hydrides with 1 atm of CO result in a reductive coupling process to produce the corresponding derivatives of the cis-ethenediolate dianion. Computational (DFT) analysis of this process mediated by Ca, Sr, and Ba highlights a common mechanism and a facility for the reaction that is enhanced by increasing alkaline earth atomic weight. Reaction of CO with PhSiH₃ in the presence of the magnesium or calcium hydrides results in catalytic reduction to methylsilane and methylene silyl ether products, respectively. These reactions are proposed to ensue via the interception of initially formed group 2 formyl intermediates, an inference which is confirmed by a DFT analysis of the magnesium-centered reaction. The computational results identify the rate-determining process, requiring traversal of a 33.9 kcal mol^–1 barrier, as a Mg–H/C–O σ-bond metathesis reaction, associated with the ultimate cleavage of the C–O bond. The carbonylation reactivity is extended to a variety of magnesium and calcium amides. With primary amido complexes, which for calcium include a derivative of the parent [NH₂]⁻ anion, CO insertion is facile and ensues with subsequent nitrogen-to-carbon migration of hydrogen to yield a variety of dinuclear and, in one case, trinuclear formamidate species. The generation of initial carbenic carbamoyl intermediates is strongly implicated through the isolation of the CO insertion product of a magnesium N-methylanilide derivative. These observations are reinforced by a DFT analysis of the calcium-centered reaction with aniline, which confirms the exothermicity of the formamidate formation (ΔH = −67.7 kcal mol^–1). Stoichiometric reduction of the resultant magnesium and calcium formamidates with pinacolborane results in the synthesis of the corresponding N-borylated methylamines. This takes place via a sequence of reactions initiated through the generation of amidatohydridoborate intermediates and a cascade of reactivity that is analogous to that previously reported for the deoxygenative hydroboration of organic isocyanates catalyzed by the same magnesium hydride precatalyst. Although a sequence of amine formylation and deoxygenation may be readily envisaged for the catalytic utilization of CO as a C₁ source in the production of methylamines, our observations demonstrate that competitive amine–borane dehydrocoupling is too facile under the conditions of 1 atm of CO employed.

The capture and utilization of CO₂ to prepare high-value compounds is very attractive chemically and highly desirable socially. Indenediide-based Pd SCS pincer complexes are shown here to promote the carboxylative cyclization of propargylamines leading to 2-oxazolidinones under mild conditions (0.5–1 bar of CO₂, DMSO, 40–80 °C, 1–5 mol % Pd loading). The indenediide Pd complex is competitive with known catalysts. It proved successful for a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines, and propargylanilines. Thorough experimental (NMR) and computational (DFT) investigations were undertaken to gain mechanistic insights. Accordingly, (i) the resting state of the catalytic cycle is a Pd DMSO complex; (ii) the indenediide backbone and Pd center act in concert to activate the carbamic acid intermediate and promote its cyclization; (iii) proton shuttling is essential to lower the activation barriers of the initial amine carboxylation as well as of the proton transfers between the ligand backbone and the organic fragments at Pd.

The reactivity of terminal uranium(V/VI) nitrides with CE₂ (E=O, S) is presented. Well‐defined C=E cleavage followed by zero‐, one‐, and two‐electron redox events is observed. The uranium(V) nitride [U(Tren^TIPS)(N)][K(B15C5)₂] (1, Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃; B15C5=benzo‐15‐crown‐5) reacts with CO₂ to give [U(Tren^TIPS)(O)(NCO)][K(B15C5)₂] (3), whereas the uranium(VI) nitride [U(Tren^TIPS)(N)] (2) reacts with CO₂ to give isolable [U(Tren^TIPS)(O)(NCO)] (4); complex 4 rapidly decomposes to known [U(Tren^TIPS)(O)] (5) with concomitant formation of N₂ and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS₂ to give [U(Tren^TIPS)(κ²‐CS₃)][K(B15C5)₂] (6), 2, and [K(B15C5)₂][NCS] (7), whereas 2 reacts with CS₂ to give [U(Tren^TIPS)(NCS)] (8) and “S”, with the latter trapped as Ph₃PS. Calculated reaction profiles reveal outer‐sphere reactivity for uranium(V) but inner‐sphere mechanisms for uranium(VI); despite the wide divergence of products the initial activation of CE₂ follows mechanistically related pathways, providing insight into the factors of uranium oxidation state, chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include common one‐electron reactions and a less‐common two‐electron redox event. Caution, we suggest, is warranted when utilising CS₂ as a reactivity surrogate for CO₂.

Uranium nitrides are important materials with potential for application as fuels for nuclear power generation, and as highly active catalysts. Molecular nitride compounds could provide important insight into the nature of the uranium–nitride bond, but currently little is known about their reactivity. In this study, we found that a complex containing a nitride bridging two uranium centers and a cesium cation readily cleaved the C≡O bond (one of the strongest bonds in nature) under ambient conditions. The product formed has a [CsU₂(μ‐CN)(μ‐O)] core, thus indicating that the three cations cooperate to cleave CO. Moreover, the addition of MeOTf to the nitride complex led to an exceptional valence disproportionation of the CsU^IV–N–U^IV core to yield CsU^III(OTf) and [MeN=U^V] fragments. The important role of multimetallic cooperativity in both reactions is illustrated by the computed reaction mechanisms.

The first examples of essentially two-coordinate, monomeric zinc hydride complexes, LZnH (L = –N(Ar)(SiR₃)) (Ar = C₆H₂{C(H)Ph₂}₂R′-2,6,4; R = Me, R′ = Prⁱ (L′); R = Prⁱ, R′ = Me (L*); R = Prⁱ, R′ = Prⁱ (L^†)) have been prepared and shown by crystallographic studies to have near linear N–Zn–H fragments. The results of computational studies imply that any Ph⋯Zn interactions in the compounds are weak at best. Preliminary reactivity studies reveal the compounds to be effective for the stoichiometric hydrozincation and catalytic hydrosilylation of carbonyl compounds.

The mechanistic investigation of the reductive coupling vs. reductive disproportionation of CO ₂ using magnesium( i ) dimers bearing tripodal ligands has been carried out using DFT computational methods.