Extending Stannyl Anion Chemistry to the Actinides: Synthesis and Characterization of a Uranium-Tin Bond

Uranium Chemistry: Identifying the Next Frontiers†

While uranium is the most extensively studied actinide in terms of chemical properties, there remains much to be explored about its fundamental chemistry. Organometallic and organoactinide chemistry first emerged in the 1950s with research that found inspiration from transition-metal chemistry with the synthesis and characterization of uranocene, expanding new opportunities for organoactinide chemistry. Since then, a significant amount of research has pursued many avenues characterizing the fundamental nature of the f orbitals and their modes of bonding as well as their potential in catalysis. Uranium(III/IV) arene complexes dominate much of uranium organometallic chemistry, with bonding interactions stabilized by δ-back-bonding. Recent additions to this area of chemistry include the first UI and new additions of UII organouranium compounds. Uranium-transition metal complexes are still rare and maintain UIV oxidation states, with variable bond lengths determining the transition-metal oxidation state. Resultant reactivities are discussed as synthetic complexes, and unique bonding and coordination motifs are highlighted. This Viewpoint will focus on significant developments in uranium chemistry from the last 15 years while considering key areas for future research.

Thorium(IV)-antimony complexes exhibiting single, double, and triple polar covalent metal-metal bonds

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime.

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.

Recent advances in heterometallic clusters with f-block metal-metal bonds: synthesis, reactivity and applications

Due to the heterometallic synergistic effects from different metals, heterometallic clusters are of great importance in small-molecule activation and catalysis. For example, both biological nitrogen fixation and photosynthetic splitting of water into oxygen are thought to involve multimetallic catalytic sites with d-block transition metals. Benefitting from the larger coordination numbers of f-block metals (rare-earth metals and actinide elements), heterometallic clusters containing f-block metal-metal bonds have long attracted the interest of both experimental and theoretical chemists. Therefore, a series of effective strategies or platforms have been developed in recent years for the construction of heterometallic clusters with f-block metal-metal bonds. More importantly, synergistic effects between f-block metals and transition metals have been observed in small-molecule activation and catalysis. This tutorial review highlights the recent advances in the construction of heterometallic molecular clusters with f-block metal-metal bonds and also their reactivities and applications. It is hoped that this tutorial review will persuade chemists to develop more efficient strategies to construct clusters with f-block metal-metal bonds and also further expand their applications with heterometallic synergistic effects.

Crystal structures of metallocene complexes with uranium-germanium bonds

The first structural examples of complexes with uranium-germanium bonds are presented, namely, bis-[3,5-bis-(tri-fluoro-meth-yl)phenyl-2κC 1](hydrido-2κH)(iodido-1κI)bis-[1,1(η5)-penta-methyl-cyclo-penta-dien-yl]germaniumuranium(Ge-U), [GeU(C10H15)2(C8H3F6)2HI], and bis-[3,5-bis-(tri-fluoro-meth-yl)phenyl-2κC 1](fluorido-1κI)(hydrido-2κH)bis-[1,1(η5)-penta-methyl-cyclo-penta-dien-yl]germaniumuranium(Ge-U), [GeU(C10H15)2(C8H3F6)2FH]. The two complexes both have a long U-Ge bond [distances of 3.0428 (7) and 3.0524 (7) Å].

Synthesis and Characterisation of Molecular Polarised-Covalent Thorium-Rhenium and -Ruthenium Bonds

Separate reactions of [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSi(Me)(But)(μ-CH2)]2 (1) with [Re(η5-C5H5)2(H)] (2) or [Ru(η5-C5H5)(H)(CO)2] (3) produced, by alkane elimination, [Th(TrenDMBS)Re(η5-C5H5)2] (ThRe, TrenDMBS = {N(CH2CH2NSiMe2But)3}3-), and [Th(TrenDMBS)Ru(η5-C5H5)(CO)2] (ThRu), which were isolated in crystalline yields of 71% and 62%, respectively. Complex ThRe is the first example of a molecular Th-Re bond to be structurally characterised, and ThRu is only the second example of a structurally authenticated Th-Ru bond. By comparison to isostructural U-analogues, quantum chemical calculations, which are validated by IR and Raman spectroscopic data, suggest that the Th-Re and Th-Ru bonds reported here are more ionic than the corresponding U-Re and U-Ru bonds.

f-Element silicon and heavy tetrel chemistry

The last three decades have seen a significant increase in the number of reports of f-element carbon chemistry, whilst the f-element chemistry of silicon, germanium, tin, and lead remain underdeveloped in comparison. Here, in this perspective we review complexes that contain chemical bonds between f-elements and silicon or the heavier tetrels since the birth of this field in 1985 to present day, with the intention of inspiring researchers to contribute to its development and explore the opportunities that it presents. For the purposes of this perspective, f-elements include lanthanides, actinides and group 3 metals. We focus on complexes that have been structurally authenticated by single-crystal X-ray diffraction, and horizon-scan for future opportunities and targets in the area.

Synthesis and Structure of Uranium-Silylene Complexes

While carbene complexes of uranium have been known for over a decade, there are no reported examples of complexes between an actinide and a "heavy carbene." Herein, we report the syntheses and structures of the first uranium-heavy tetrylene complexes: (CpSiMe₃ )₃ U-Si[PhC(NR)₂ ]R' (R=tBu, R'=NMe₂ 1; R=iPr, R'=PhC(NiPr)₂ 2). Complex 1 features a kinetically robust uranium-silicon bonding interaction, while the uranium-silicon bond in 2 is easily disrupted thermally or by competing ligands in solution. Calculations reveal polarized σ bonds, but depending on the substituents at silicon a substantial π-bonding interaction is also present. The complexes possess relatively high bond orders which suggests primarily covalent bonding between uranium and silicon. These results comprise a new frontier in actinide-heavy main-group bonding.

Polarised covalent thorium(iv)– and uranium(iv)–silicon bonds

We report thorium- and uranium–silicon bonds in structurally analogous complexes with surprisingly similar actinide–silicon bonding regimes.

Preparation of Heterobimetallic Ketimido-Actinide-Molybdenum Complexes

During our attempts to prepare paddlewheel actinide-molybdenum complexes of the type [(X)An(MesNPR₂)₃Mo(CO)₃] (Mes= 2,4,6-trimethylphenyl; X = Cl or I; An = U or Th; R = ⁱPr or Ph) we have found that under certain conditions acetonitrile insertion reactions occur to give the heterobimetallic bridging ketimido species [ClAn(μ-MesNPⁱPr₂)₂(μ-MesNPⁱPr₂{μ-NCMe})Mo(CO)₃] (An = U, 1; Th, 2), [ClAn(μ-MesNPPh₂)₂(μ-MesNPPh₂{μ-NCMe})Mo(CO)₃] (An = U, 3; An = Th, 4), and [IAn(η²-MesNPⁱPr₂)(μ-MesNPⁱPr₂){μ-NC(Me)N(Mes)PⁱPr₂}Mo(CO)₃] (An = U, 5; Th, 6). Structural and spectroscopic data confirm the assignment of a ketimido ligand bridging An(IV) and Mo(0) centers. The isolation of 1-6 is in contrast to our previously reported preparations of [(X)An(MesNPPh₂)₃Mo(CO)₃] (An = U or Th; X= Cl or I; Chem. Commun. 2018, 54, 13515-13518) with the difference in reactivity being attributable to a combination of ancillary phosphino-amide, reaction solvent, and temperature variation. Complexes 1-5 represent the first examples of structurally characterized ketimido-bridged actinide-transition metal linkages and demonstrate the profound differences in reaction outcomes that can occur from relatively minor experimental changes.

Inverse Trans Influence in Low-Valence Actinide-Group 10 Metal Complexes of Phosphinoaryl Oxides: A Theoretical Study via Tuning Metals and Donor Ligands

The recognition and in-depth understanding of inverse trans influence (ITI) have successfully guided the synthesis of novel actinide complexes and enriched actinide chemistry. Those complexes, however, are mainly limited to the involvement of high-valence actinide and/or metal-ligand multiple bonds. Examples containing both low oxidation state actinide and metal-metal single bond remain rare. Herein, more than 20 actinide-transition metal (An-TM) complexes of phosphinoaryl oxide ligands have been designed in accordance with several experimentally known analogs, by changing the metal atoms (An = Th, Pa, U, Np, and Pu; and TM = Ni, Pd, and Pt), actinide oxidation states (IV and III) and metal-metal axial donor ligands (X = Me₃SiO, F, Cl, Br, and I). The relativistic density functional theory study of structural (trans-An-X and cis-An-O toward An-TM), bonding (topological electron/energy density), and electronic properties reveals the order of the ITI stabilizing actinide-metal bond. Computed electron affinity (EA) values, related to the electrochemical reduction, linearly correlate with experimentally measured reduction potentials. Although the same ITI order for the ligand donors was shown as in a previous study, the correlation between electrochemical reduction and the ITI was found to be weak when the actinide atoms were changed. For most complexes, the reduction is primarily of an actinide-based mechanism with minor participation of transition metal and phosphinoaryl oxide, whereas that of thorium-nickel complexes is different.

Dysprosium complexes bearing unsupported DyIII-GeII/SnII metal-metal bonds as single-ion magnets

Two dysprosium complexes bearing unsupported Dy-Ge/Sn metal-metal bonds are reported here, wherein the Dy-Ge and Dy-Sn bonds both contain relatively large covalency. The complexes exhibit slow relaxation of magnetization at zero field with energy barriers of 485 and 620 K, respectively, and the blocking temperature of 6 K.

Actinide-transition metal bonding in heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes

We report the preparation of four heterobimetallic uranium- and thorium-molybdenum paddlewheel complexes. The characterisation data suggest the presence of Mo → An σ-interactions in all cases. These complexes represent unprecedented actinide-group 6 metal-metal bonds, where before heterobimetallic uranium-metal bonds were restricted to group 7-11 metals.

A Very Short Uranium(IV)-Rhodium(I) Bond with Net Double-Dative Bonding Character

Reaction of [U{C(SiMe3 )(PPh2 )}(BIPM)(μ-Cl)Li(TMEDA)(μ-TMEDA)0.5 ]2 (BIPM=C(PPh2 NSiMe3 )2 ; TMEDA=Me2 NCH2 CH2 NMe2 ) with [Rh(μ-Cl)(COD)]2 (COD=cyclooctadiene) affords the heterotrimetallic UIV -RhI2 complex [U(Cl)2 {C(PPh2 NSiMe3 )(PPh[C6 H4 ]NSiMe3 )}{Rh(COD)}{Rh(CH(SiMe3 )(PPh2 )}]. This complex has a very short uranium-rhodium distance, the shortest uranium-rhodium bond on record and the shortest actinide-transition metal bond in terms of formal shortness ratio. Quantum-chemical calculations reveal a remarkable RhI→→ UIV net double dative bond interaction, involving RhI 4dz2 - and 4dxy/xz -type donation into vacant UIV 5f orbitals, resulting in a Wiberg/Nalewajski-Mrozek U-Rh bond order of 1.30/1.44, respectively. Despite being, formally, purely dative, the uranium-rhodium bonding interaction is the most substantial actinide-metal multiple bond yet prepared under conventional experimental conditions, as confirmed by structural, magnetic, and computational analyses.

Formate complexes of titanium(iv) supported by a triamido-amine ligand

The terminal formate complex [(OCHO)Ti(N₃N)] (3) containing the trianionic triamido-amine ligand (Me₃SiNCH₂CH₂)₃N^3- (N₃N) was prepared via salt metathesis of [ClTi(N₃N)] (1) with sodium formate or alternatively by treatment of the alkyl complex [ⁿBuTi(N₃N)] (2) with ammonium formate [HNEt₃][OCHO]. Deprotonation of 3 with potassium hexamethyldisilazide gave a polymeric helical chain of the oxo complex {K[OTi(N₃N)]}_n (4). Reaction of 2 with the trityl salt [Ph₃C][B(3,5-Cl₂C₆H₃)₄] or the Brønsted acid [HNEt₃][B(C₆F₅)₄] gave [(Et₂O)Ti(N₃N)][BR₄] (6[BR₄]·Et₂O) with R = 3,5-Cl₂C₆H₃ or C₆F₅. The diethyl ether ligand was easily replaced by other L-type donor ligands such as tetrahydrofuran, pyridine, and 4-dimethylaminopyridine to give 6[BR₄]·L with L = thf, py, and dmap. Reaction of 6[BR₄]·Et₂O with a stoichiometric amount of CO₂ gave the dimeric, dicationic bis(carbamate)-bridged complexes [Ti{N(CH₂CH₂NSiMe₃)₂(CH₂CH₂NSiMe₃(μ-CO₂-ηO:ηO'))}]₂[BR₄]₂ (7[BR₄]₂) through insertion of one CO₂ into one of the titanium-amido bonds. Addition of pyridine to 7[B(C₆F₅)₄]₂ formed the monomeric carbamate complex [(py)Ti{((O₂C-κ²O,O')NSiMe₃CH₂CH₂)N(CH₂CH₂NSiMe₃)₂}][B(C₆F₅)₄] (8[B(C₆F₅)₄]·py). The cationic formate-bridged species [(Ti(N₃N))₂(μ-OCHO-ηO:ηO')][BR₄] (10[BR₄]) readily formed when the terminal formate complex 3 was reacted with the cationic 6[BR₄]. The reactivity of triamido-amine stabilized titanium(iv) complexes is shown to differ considerably from that of related titanium tris(anilide) complexes.