Tuning the Oxidation State, Nuclearity, and Chemistry of Uranium Hydrides with Phenylsilane and Temperature: The Case of the Classic Uranium(III) Hydride Complex [(C5Me5)2U(μ-H)]2

Planar Tetranuclear Uranium Hydride Cluster Supported by ansa-Bis(cyclopentadienyl) Ligands

Polynuclear metal hydride clusters play important roles in various catalytic processes, with most of the reported polynuclear metal hydride clusters adopting a polyhedral three-dimensional structure. Herein, we report the first example of a planar tetranuclear uranium hydride cluster [(CpCMe2CMe2Cp)U]4(μ2-H)4(μ3-H)4 (U4H8). It was synthesized by reacting an ansa-bis(cyclopentadienyl) ligand-supported uranium chloride precursor [(CpCMe2CMe2Cp)U]3(μ2-Cl)3(μ3-Cl)2 with NaHBEt3. The presence of hydrides in U4H8 was confirmed by NMR spectroscopy and its reactivity with phenol and carbon tetrachloride. DFT calculations also facilitated the determination of the hydrides' positions in U4H8, featuring four bridging μ2-H ligands and four face-capping μ3-H ligands, with the four U centers arranged in a rhombic geometry. The U4H8 represents not only the first example of planar polynuclear actinide metal hydride cluster but also the uranium hydride cluster with the highest nuclearity reported to date.

Synthesis and Structure of [η5-1,2,4-(Me3Si)3C5H2]2Th(bipy) and Its Reactivity toward Small Molecules

Halide exchange of (Cp3tms)2ThCl2 (1; Cp3tms = η5-1,2,4-(Me3Si)3C5H2) with Me3SiI furnishes (Cp3tms)2ThI2 (2), which is then reduced with potassium graphite (KC8) in the presence of 2,2'-bipyridine to give the thorium bipyridyl metallocene (Cp3tms)2Th(bipy) (3) in good yield. Complex 3 was fully characterized and readily reacted with various small molecules. For example, 3 may serve as a synthetic equivalent for the (Cp3tms)2Th(II) fragment when exposed to CuI, Ph2S2, organic azides, and CS2. Moreover, upon the addition of thiobenzophenone Ph2CS, p-methylbenzaldehyde (p-MeC6H4)CHO, benzophenone Ph2CO, amidate PhCONH(p-tolyl), seleno-ketone (p,p'-dimethoxy), selenobenzophenone (p-MeOPh)2CSe, di(p-tolyl)methanimine (p-tolyl)2C═NH, 1,2-di(benzylidene)hydrazine (PhCH═N)2, and nitriles PhCN, PhCH2CN, and Ph2CHCN C-C coupling results to give (Cp3tms)2Th[(bipy)(Ph2CS)] (8), (Cp3tms)2Th[(bipy)(p-MePhCHO)] (9), (Cp3tms)2Th[(bipy)(Ph2CO)] (10), (Cp3tms)2Th[(bipy){(p-tolylNH)(Ph)CO}] (11), (Cp3tms)2Th[(bipy){(p-MeOPh)2CSe}] (12), (Cp3tms)2Th[(bipy){(p-tolyl)2CNH}] (13), (Cp3tms)2Th[(bipy)(PhCHNN═CHPh)] (14), (Cp3tms)2Th[(bipy)(PhCN)] (16), (Cp3tms)2Th[(bipy)(PhCH2CN)] (17), and (Cp3tms)2Th[(bipy)(Ph2CHCN)] (18), respectively. However, when thiazole is added to 3, the dimeric sulfido complex [(Cp3tms)2Th]2[μ-(bipy)CH2NCHCHS]2 (15) can be isolated. Moreover, the addition of isonitriles such as Me3CNC and PhCH2NC to 3 results in C-N bond cleavage and C-C coupling processes to form the thorium isocyanido amido complexes (Cp3tms)2Th[4-(Me3C)bipy](NC) (19) and (Cp3tms)2Th[4-(PhCH2)bipy](NC) (20), respectively. Nevertheless, upon exposure of 3 to (trimethylsilyl)diazomethane Me3SiCHN2, the bis-amido complex (Cp3tms)2Th[5,6-(Me3SiCH)bipy] (21), concomitant with N2 release, is isolated.

Cooperative dihydrogen activation with unsupported uranium-metal bonds and characterization of a terminal U(iv) hydride

Cooperative chemistry between two or more metal centres can show enhanced reactivity compared to the monometallic fragments. Given the paucity of actinide-metal bonds, especially those with group 13, we targeted uranium(iii)-aluminum(i) and -gallium(i) complexes as we envisioned the low-valent oxidation state of both metals would lead to novel, cooperative reactivity. Herein, we report the molecular structure of [(C5Me5)2(MesO)U-E(C5Me5)], E = Al, Ga, Mes = 2,4,6-Me3C6H2, and their reactivity with dihydrogen. The reaction of H2 with the U(iii)-Al(i) complex affords a trihydroaluminate complex, [(C5Me5)2(MesO)U(μ2-(H)3)-Al(C5Me5)] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(iv) hydride, [(C5Me5)2(MesO)U(H)]. Noteworthy is that neither U(iii) complexes nor [(C5Me5)Al]4 are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C5Me5)2(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C5Me5)2(MesO)U(CH2CH3)] or treatment of [(C5Me5)2U(i)(Me)] with KOMes forms [(C5Me5)2(MesO)U(CH3)], which followed by hydrogenation with either complex cleanly affords [(C5Me5)2(MesO)U(H)]. All complexes have been characterized by spectroscopic and structural methods and are rare examples of cooperative chemistry in f element chemistry, dihydrogen activation, and stable, terminal ethyl and hydride compounds with an f element.

Four-Electron Oxidative Addition of an N=N Double Bond at a Chromium Metallocyclopropene

This report describes the synthesis of a pseudo-tetrahedral chromium alkyne complex supported by a bidentate phosphinimide ligand and its reactivity with an azobenzene derivative. Characterization of the former by structural and computational methods reveals an unprecedented extent of alkyne activation by a formal chromium(II) center, suggesting that this complex is best described as a chromium(IV)-metallocyclopropene. Exposure of this compound to 4,4'-difluoroazobenzene results in the formation of a chromium(VI) diimido complex, which constitutes a rare 4-electron oxidative addition of an N=N double bond. The isolation of a chromium(IV)-hydrazido intermediate enabled mechanistic investigations of this challenging bond cleavage process. This work substantiates the notion that terminal phosphinimide ligands can engender first-row transition metal ions with exceptional reactivity.

Intrinsic reactivity of [η5-1,3-(Me3Si)2C5H3]2U(η4-C4Ph2) in small molecule activation

The uranium metallacyclocumulene, [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(η⁴-C₄Ph₂) (3) was isolated from the reaction mixture containing [η⁵-1,3-(Me₃Si)₂C₅H₃]₂UCl₂ (1), potassium graphite (KC₈) and 1,4-diphenylbutadiyne (PhCC-CCPh) in good yield. The reactivity of 3 towards various small organic molecules was evaluated. For example, while complex 3 shows no reactivity towards alkynes and 2,2'-bipyridine, it may deliver the [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(II) fragment in the presence of Ph₂E₂ (E = S, Se) and Ph₃CN₃, or react as a nucleophile in the presence of carbodiimides, isothiocyanates, aldehydes, ketones, and pyridine derivatives, forming five-, seven- or nine-membered heterometallacycles. On the contrary, addition of Ph₂CS to 3 induces CS bond cleavage yielding the dithiolate complex [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[S₂(C₁₂H₅Ph₅)] (14). In contrast, the closely related, but sterically more encumbered uranium metallacyclocumulene [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(η⁴-C₄Ph₂) (4) features a more limited reactivity which is restricted to mono- and double insertions with small unsaturated organic molecules such as isothiocyanates, ketones and nitriles.

Small-Molecule Activation Mediated by [η5-1,3-(Me3Si)2C5H3]2U(bipy)

The uranium bipyridyl metallocene, [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(bipy) (2), is readily accessible in good yield by adding potassium graphite (KC₈) to a mixture of [η⁵-1,3-(Me₃Si)₂C₅H₃]₂UCl₂ (1) and 2,2'-bipyridine. Compound 2 was fully characterized and employed for small-molecule activation. It has been demonstrated that 2 may serve as a synthon for [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(II) fragment in the presence of Ph₂E₂ (E = S, Se), alkynes, and a variety of hetero-unsaturated molecules such as diazabutadienes, azine (Ph₂C═N)₂, o-benzoquinone, pyridine N-oxide, CS₂, isothiocyanates, and organic azides. However, upon exposure of 2 to thio-ketone Ph₂CS, aldehyde p-MePhCHO, ketone Ph₂CO, imine PhCH═NPh, azine (PhCH═N)₂, and nitrile PhCN, it may also promote C-C coupling reactions forming [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(Ph₂CS)] (16), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(p-MePhCHO)] (17), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(Ph₂CO)] (18), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(PhCHNPh)] (19), [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(bipy)(PhCHNN═CHPh)] (20), and [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U[(N₂C₁₀H₇C(Ph)NH)] (22), respectively, in quantitative conversion. Furthermore, in the presence of CuI, a single-electron transfer (SET) process is observed to yield the uranium(III) iodide complex [η⁵-1,3-(Me₃Si)₂C₅H₃]₂U(I)(bipy) (15).

Single metal four-electron reduction by U(ii) and masked "U(ii)" compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η ²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a "clear-cut" single metal four-electron transfer in f-element chemistry.

Uranium versus Thorium: Synthesis and Reactivity of [η5 -1,2,4-(Me3 C)3 C5 H2 ]2 U[η2 -C2 Ph2 ]

The synthesis, electronic structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Addition of diphenylacetylene (PhC≡CPh) to the uranium phosphinidene metallocene [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U=P‐2,4,6‐tBu₃C₆H₂ (1) yields the stable uranium metallacyclopropene, [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U[η²‐C₂Ph₂] (2). Based on density functional theory (DFT) results the 5f orbital contributions to the bonding within the metallacyclopropene U‐(η²‐C=C) moiety increases significantly compared to the related Th^IV compound [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂Th[η²‐C₂Ph₂], which also results in more covalent bonds between the [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U²⁺ and [η²‐C₂Ph₂]²⁻ fragments. Although the thorium and uranium complexes are structurally closely related, different reaction patterns are therefore observed. For example, 2 reacts as a masked synthon for the low‐valent uranium(II) metallocene [η⁵‐1,2,4‐(Me₃C)₃C₅H₂]₂U^II when reacted with Ph₂E₂ (E=S, Se), alkynes and a variety of hetero‐unsaturated molecules such as imines, ketazine, bipy, nitriles, organic azides, and azo derivatives. In contrast, five‐membered metallaheterocycles are accessible when 2 is treated with isothiocyanate, aldehydes, and ketones.

Incorporation of Boron into Uranium Metallacycles: Synthesis, Structure, and Reactivity of Boron-Containing Uranacycles Derived from Bis(alkynyl)boranes

The reaction of uranacyclopropene complex (C₅ Me₅ )₂ U[η² -1,2-C₂ (SiMe₃ )₂ ] with B-aryl bis(alkynyl)borane PhB(C≡CPh)₂ led to the first six-membered uranium metallaboracycle, while the reaction with B-amino bis(alkynyl)borane (Me₃ Si)₂ NB(C≡CPh)₂ afforded an unexpected uranaborabicyclo[2.2.0] complex via [2+2] cycloaddition. The reaction with CuCl revealed the non-innocent property of the rearranged bis(alkynyl)boron species towards oxidant. The reactions with isocyanide DippNC: (Dipp=2,6-iPr₂ -C₆ H₃ ) and isocyanate tBuNCO afforded the novel uranaborabicyclo[3.2.0] complexes. All new complexes have been structurally characterized. DFT calculations were performed to provide more insights into the electronic structures and the reaction mechanism.

C-H Bond Activation by an Isolated Dinuclear U(III)/U(IV) Nitride

Synthetic studies of bimetallic uranium nitride complexes with the N(SiMe₃)₂ ligand have generated a new nitride complex of U(III), which is highly reactive toward C-H bonds and H₂. Treatment of the previously reported U(IV)/U(IV) nitride complex [Na(DME)₃][((Me₃Si)₂N)₂U(μ-N)(μ-κ²:CN̵-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂)₂] (DME = 1,2-dimethoxyethane), 1, with 2 equiv of HNEt₃BPh₃ yielded the cationic U(IV)/U(IV) nitride complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][BPh₄] (THF = tetrahydrofuran), 3, by successive protonolysis of one N(SiMe₃)₂ ligand and the uranium-methylene bond. Reduction of 3 with KC₈ afforded a rare example of a U(III) nitride, namely, the U(III)/U(IV) complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)], 4. Complex 4 is highly reactive and undergoes 1,2-addition of the C-H bond of the N(SiMe₃)₂ ligand across the uranium-nitride moiety to give the U(III)/U(IV) imide cyclometalate complex, [((Me₃Si)₂N)₂(THF)U(μ-NH)(μ-κ²:C,N̵-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂))(THF)], 5. Complex 4 also reacts with toluene at -80 °C to yield an inverse sandwich imide complex arising from C-H bond activation of toluene, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][{((Me₃Si)₂N)₃U(μ-NH)U(N(SiMe₃)₂)}₂(C₇H₈)], 6. Complex 4 effects the heterolytic cleavage of the C-H of phenylacetylene to yield the imide acetylide [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][((Me₃Si)₂N)₂U(η¹-CCPh)(μ₂-NH)(μ₂-η²:η¹-CCPh)U(N(SiMe₃)₂)₂], 7. Complex 4 also reacts with H₂ to produce an imide hydride U(III)/U(IV) complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-NH)(μ-H)], 9. These data demonstrate that nitride complexes of U(III) are accessible with amide ligands and show the high reactivity of molecular U(III) nitrides in C-H bond activation.

A sulphur and uranium fiesta! Synthesis, structure, and characterization of neutral terminal uranium(vi) monosulphide, uranium(vi) η2-disulphide, and uranium(iv) phosphine sulphide complexes

Three new uranium species, (C₅Me₅)₂U(N-2,6-ⁱPr₂-C₆H₃)(S), (C₅Me₅)₂U(N-2,6-ⁱPr₂-C₆H₃)(η²-S₂), and (C₅Me₅)₂U(N-2,6-ⁱPr₂-C₆H₃)(SPMe₃) have been prepared.

Recent developments in actinide metallacycles

All-carbon metallacycles of the d-transition metals have received widespread attention over the past three decades because of their exceptional intrinsic reactivity. However, in recent years, significant progress has also been made in the synthesis and characterization of actinide metallacyclopropenes, metallacyclopentadienes, and metallacyclocumulenes (metallacyclopentatrienes). Such actinide metallacycles are of interest because of their unique structural properties, their potential application in novel group transfer reactions and catalysis, as well as their ability to engage the 5f orbitals in metal-ligand bonding. This short review summarizes the latest developments in this area focusing on all-carbon actinide metallacycles, i.e., metallacyclopropenes, metallacyclopentadienes, and metallacyclocumulenes (metallacyclopentatrienes).

Metallacyclopentadienes: synthesis, structure and reactivity

Metallacyclopentadienes, which possess two M-C(sp²) bonds and feature the structure of M(C[upper bond 1 start]R¹[double bond, length as m-dash]CR²-CR³[double bond, length as m-dash]C[upper bond 1 end]R⁴), are an important class of five-membered metallacycles. They are considered as both reactive intermediates in the stoichiometric and catalytic transformations of organic molecules and useful precursors to main group element compounds, and have received considerable attention in organometallic chemistry, coordination chemistry and synthetic organic chemistry over the past six decades because of their unique metallacyclic structure. This review comprehensively presents the synthesis, structure and reactivity of the s-, p-, d- and f-block metallacyclopentadienes distributed in the whole periodic table. In addition, their application in synthetic organic chemistry and polymer chemistry is summarized. This review aims to be beneficial for the design and synthesis of novel metallacyclopentadienes, and for promoting the rapid development of metallacyclic chemistry.

Preparation of a uranium metallacyclocumulene and its reactivity towards unsaturated organic molecules

Steric and electronic properties of the coordinated ligands exert a pronounced influence on the reactivity of the uranium metallacyclocumulene complexes.

Investigation of N-Heterocyclic Carbene-Supported Group 12 Triflates as Pre-catalysts for Hydrosilylation/Borylation

N-Heterocyclic carbene (NHC) complexes of Cd and Hg triflates (OTf) were prepared and their attempted conversion into rare cadmium and mercury hydrides was explored. In contrast to zinc, which forms stable [ZnH]⁺ complexes with NHCs, the heavier Cd and Hg congeners could not be formed; the increased instability of Cd-H and Hg-H units was rationalized with the aid of computations. It was also discovered that the dimeric adduct [IPr⋅Cd(μ-OTf)₂ ]₂ (IPr=[(HCNDipp)₂ C:]; Dipp=2,6-iPr₂ C₆ H₃ ) is an active precatalyst for the hydrosilylation and hydroborylation of hindered aldehydes and ketones. The related zinc congener was inactive as a catalyst highlighting a distinct advantage of using heavy Group 12 metals to promote catalytic hydrosilylation/borylation.

Influence of the 5f Orbitals on the Bonding and Reactivity in Organoactinides: Experimental and Computational Studies on a Uranium Metallacyclopropene

The synthesis, structure, and reactivity of a uranium metallacyclopropene were comprehensively studied. Reduction of (η(5)-C5Me5)2UCl2 (1) with potassium graphite (KC8) in the presence of bis(trimethylsilyl)acetylene (Me3SiC≡CSiMe3) allows the first stable uranium metallacyclopropene (η(5)-C5Me5)2U[η(2)-C2(SiMe3)2] (2) to be isolated. Magnetic susceptibility data confirm that 2 is a U(IV) complex, and density functional theory (DFT) studies indicate substantial 5f orbital contributions to the bonding of the metallacyclopropene U-(η(2)-C═C) moiety, leading to more covalent bonds between the (η(5)-C5Me5)2U(2+) and [η(2)-C2(SiMe3)2](2-) fragments than those in the related Th(IV) compound. Consequently, very different reactivity patterns emerge, e.g., 2 can act as a source for the (η(5)-C5Me5)2U(II) fragment when reacted with alkynes and a variety of heterounsaturated molecules such as imines, bipy, carbodiimide, organic azides, hydrazine, and azo derivatives.

Intrinsic reactivity of a uranium metallacyclopropene toward unsaturated organic molecules

A uranium metallacyclopropene shows a rich chemistry in the activation of small molecules by two electron transfer processes.