Heteroleptic actinocenes: a thorium(iv)-cyclobutadienyl-cyclooctatetraenyl-di-potassium-cyclooctatetraenyl complex

Pyramidanes: newcomers to the anti-van't Hoff-Le Bel family

In this feature article, an overview of the chemistry of pyramidanes, as a novel class of main group element clusters, is given. A general introduction sets the scene, briefly presenting the non-classical pyramidal geometry of tetracoordinate carbon, as opposed to the classical tetrahedral configuration. Pyramidanes, as the simplest organic compounds possessing a pyramidal carbon atom, are then discussed from both computational and experimental viewpoints, to show the theoretical predictions on the stability and thus the feasibility of pyramidanes has finally culminated in the isolation of the first stable representatives of the pyramidane family featuring heavy main group elements at the apex of the square pyramid. Synthetic strategies towards pyramidanes, as well as their peculiar structural features, non-classical bonding situations, and specific reactivity, are presented and discussed in this review.

Stable Silapyramidanes

Starting from tetrakis(trimethylsilyl)cyclobutadiene and an amidinate-supported silylene of the Roesky-type, a sequence of addition and reduction cleanly gives the elusive silapyramidane via an isolable cyclobutene intermediate with an exocyclic Si═C bond. The silapyramidane features an unusually shielded ²⁹Si NMR resonance at -448.3 ppm for the apex silicon atom. Treatment with Fe₂(CO)₉ results in the formation of the corresponding silapyramidane-iron complex. Silapyramidane also reacts with the cyclobutadiene starting material to cleanly afford a fluorescent spirobis(silole).

Thorium- and Uranium-Mediated C-H Activation of a Silyl-Substituted Cyclobutadienyl Ligand

Cyclobutadienyl complexes of the f-elements are a relatively new yet poorly understood class of sandwich and half-sandwich organometallic compounds. We now describe cyclobutadienyl transfer reactions of the magnesium reagent [(η⁴-Cb'''')Mg(THF)₃] (1), where Cb'''' is tetrakis(trimethylsilyl)cyclobutadienyl, toward thorium(IV) and uranium(IV) tetrachlorides. The 1:1 stoichiometric reactions between 1 and AnCl₄ proceed with intact transfer of Cb'''' to give the half-sandwich complexes [(η⁴-Cb'''')AnCl(μ-Cl)₃Mg(THF)₃] (An = Th, 2; An = U, 3). Using a 2:1 reaction stoichiometry produces [Mg₂Cl₃(THF)₆][(η⁴-Cb'''')An(η³-C₄H(SiMe₃)₃-κ-(CH₂SiMe₂)(Cl)] (An = Th, [Mg₂Cl₃(THF)₆][4]; An = U [Mg₂Cl₃(THF)₆][5]), in which one Cb'''' ligand has undergone cyclometalation of a trimethylsilyl group, resulting in the formation of an An-C σ-bond, protonation of the four-membered ring, and an η³-allylic interaction with the actinide. Complex solution-phase dynamics are observed with multinuclear nuclear magnetic resonance spectroscopy for both sandwich complexes. A computational analysis of the reaction mechanism leading to the formation of 4 and 5 indicates that the cyclobutadienyl ligands undergo C-H activation across the actinide center.

Revisiting the Bonding Model for Gold(I) Species: The Importance of Pauli Repulsion Revealed in a Gold(I)-Cyclobutadiene Complex

Understanding the bonding of gold(I) species has been central to the development of gold(I) catalysis. Herein, we present the synthesis and characterization of the first gold(I)-cyclobutadiene complex, accompanied with bonding analysis by state-of-the-art energy decomposition analysis methods. Analysis of possible coordination modes for the new species not only confirms established characteristics of gold(I) bonding, but also suggests that Pauli repulsion is a key yet hitherto overlooked element. Additionally, we obtain a new perspective on gold(I)-bonding by comparison of the gold(I)-cyclobutadiene to congeners stabilized by p-, d-, and f-block metals. Consequently, we refine the gold(I) bonding model, with a delicate interplay of Pauli repulsion and charge transfer as the key driving force for various coordination motifs. Pauli repulsion is similarly determined as a significant interaction in AuI -alkyne species, corroborating this revised understanding of AuI bonding.

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

A method to explore head‐to‐head ϕ back‐bonding from uranium f‐orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single‐crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N‐((dimethyl)prop‐2‐enylsilyl)‐2,6‐diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI₂(L1)₂ (1), which was synthesized by using the mono‐deprotonated ligand [K][N‐((dimethyl)prop‐2‐enyl)silyl)‐2,6‐diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2]₂ (2) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligand [K]₂[N‐((dimethyl)propenidesilyl)‐2,6‐diisopropylanilide] (L2). This allyl‐based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back‐bonding. The bonding between U^IV and the allyl fragments is best described as ligand‐to‐metal electron donation from a two carbon fragment‐localized electron density into empty f‐orbitals.

Reactivity of Multimetallic Thorium Nitrides Generated by Reduction of Thorium Azides

Thorium nitrides are likely intermediates in the reported cleavage and functionalization of dinitrogen by molecular thorium complexes and are attractive compounds for the study of multiple bond formation in f-element chemistry, but only one example of thorium nitride isolable from solution was reported. Here, we show that stable multimetallic azide/nitride thorium complexes can be generated by reduction of thorium azide precursors─a route that has failed so far to produce Th nitrides. Once isolated, the thorium azide/nitride clusters, M₃Th═N═Th (M = K or Cs), are stable in solutions probably due to the presence of alkali ions capping the nitride, but their synthesis requires a careful control of the reaction conditions (solvent, temperature, nature of precursor, and alkali ion). The nature of the cation plays an important role in generating a nitride product and results in large structural differences with a bent Th═N═Th moiety found in the K-bound nitride as a result of a strong K-nitride interaction and a linear arrangement in the Cs-bound nitride. Reactivity studies demonstrated the ability of Th nitrides to cleave CO in ambient conditions yielding CN^-.

A crystalline tri-thorium cluster with σ-aromatic metal-metal bonding

Metal-metal bonding is a widely studied area of chemistry^1-3, and has become a mature field spanning numerous d transition metal and main group complexes^4-7. By contrast, actinide-actinide bonding, which is predicted to be weak⁸, is currently restricted to spectroscopically detected gas-phase U₂ and Th₂ (refs. ^9,10), U₂H₂ and U₂H₄ in frozen matrices at 6-7 K (refs. ^11,12), or fullerene-encapsulated U₂ (ref. ¹³). Furthermore, attempts to prepare thorium-thorium bonds in frozen matrices have produced only ThH_n (n = 1-4)¹⁴. Thus, there are no isolable actinide-actinide bonds under normal conditions. Computational investigations have explored the probable nature of actinide-actinide bonding¹⁵, concentrating on localized σ-, π-, and δ-bonding models paralleling d transition metal analogues, but predictions in relativistic regimes are challenging and have remained experimentally unverified. Here, we report thorium-thorium bonding in a crystalline cluster, prepared and isolated under normal experimental conditions. The cluster exhibits a diamagnetic, closed-shell singlet ground state with a valence-delocalized three-centre-two-electron σ-aromatic bond^16,17 that is counter to the focus of previous theoretical predictions. The experimental discovery of actinide σ-aromatic bonding adds to main group and d transition metal analogues, extending delocalized σ-aromatic bonding to the heaviest elements in the periodic table and to principal quantum number six, and constitutes a new approach to elaborate actinide-actinide bonding.

Synthesis, bonding properties and ether activation reactivity of cyclobutadienyl-ligated hybrid uranocenes

A series of hybrid uranocenes consisting of uranium(iv) sandwiched between cyclobutadienyl (Cb) and cyclo-octatetraenyl (COT) ligands has been synthesized, structurally characterized and studied computationally. The dimetallic species [(η4-Cb'''')(η8-COT)U(μ:η2:η8-COT)U(THF)(η4-Cb'''')] (1) forms concomitantly with, and can be separated from, monometallic [(η4-Cb'''')U(THF)(η8-COT)] (2) (Cb'''' = 1,2,3,4-tetrakis(trimethylsilyl)cyclobutadienyl, COT = cyclo-octatetraenyl). In toluene solution at room temperature, 1 dissociates into 2 and the unsolvated uranocene [(η4-Cb'''')U(η8-COT)] (3). By applying a high vacuum, both 1 and 2 can be converted directly into 3. Using bulky silyl substituents on the COT ligand allowed isolation of base-free [(η4-Cb'''')U{η8-1,4-(iPr3Si)2C8H6}] (4), with compounds 3 and 4 being new members of the bis(annulene) family of actinocenes and the first to contain a cyclobutadienyl ligand. Computational studies show that the bonding in the hybrid uranocenes 3 and 4 has non-negligible covalency. New insight into actinocene bonding is provided by the complementary interactions of the different ligands with uranium, whereby the 6d orbitals interact most strongly with the cyclobutadienyl ligand and the 5f orbitals do so with the COT ligands. The redox-neutral activation of diethyl ether by [(η4-Cb'''')U(η8-C8H8)] is also described and represents a uranium-cyclobutadienyl cooperative process, potentially forming the basis of further small-molecule activation chemistry.