Oxidative Addition of E-H (E = C, N) Bonds to Transient Uranium(II) Centers.

Two-electron oxidative addition is one of the most important elementary reactions for d-block transition metals but it is uncommon for f-block elements. Here, we report the first examples of intermolecular oxidative addition of E-H (E = C, N) bonds to uranium(II) centers. The transient U(II) species was formed in-situ by reducing a heterometallic cluster featuring U(IV)-Pd(0) bonds with potassium-graphite (KC8). Oxidative addition of C-H or N-H bonds to the U(II) centers was observed when this transient U(II) species was treated with benzene, carbazole or 1-adamantylamine, respectively. The U(II) centers could also react with tetracene, biphenylene or N2O, leading to the formation of arene reduced U(IV) products and uranyl(VI) species via two- or four-electron processes. This study demonstrates that the intermolecular two-electron oxidative addition reactions are viable for actinide elements.

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.

Electronic Delocalization and σ-Aromaticity in Heterometallic Cluster with Multiple Thorium-Palladium Bonds.

Research on metal-metal bonds involving f-block actinides, such as thorium, lags far behind the well-studied metal-metal bonds of d-block transition metals. The complexes with Th-TM bonds are extremely rare; all previously identified examples have only a single Th-TM bond with the Th center at an invariably +IV oxidation state. Herein, we report a series of Th2Pdn (n = 2, 3, and 6) clusters (complexes 3, 4, and 7) with multiple Th(III)-Pd bonds. Theoretical studies reveal that the Th2Pdn unit allows electronic delocalization and σ aromaticity, leading to unexpected closed-shell singlet structures for these Th(III) species. This electronic delocalization is evident in the highest occupied molecular orbital of Th(III) complexes and facilitates a 2e reduction of alkyne by complex 7, resulting in the formation of 8. Complexes 7 and 8 are distinctive in featuring a Th2Pd6 core with six and eight Th-Pd bonds, respectively, making them the largest known d-f heterometallic clusters exhibiting metal-metal bonding.

Temperature induced single-crystal to single-crystal transformation of uranium azide complexes.

The monomeric and dimeric uranium azide complexes {[(CH3)2NCH2CH2NPiPr2]2U(N3)2} (2) and {[(CH3)2NCH2CH2NPiPr2]2U(N3)2}2 (3) were synthesized by treating complex 1 with NaN3 at 60 and -20 °C, respectively. A temperature-induced single-crystal to single-crystal transformation of 3 to 2 was observed. The reduction of either 2 or 3 with KC8 yields a uranium nitride complex {[(CH3)2NCH2CH2NPiPr2]4U2(µ-N)2} (4).

Heterometallic Clusters with Cerium-Transition-Metal Bonding Supported by Nitrogen-Phosphorus Ligands.

Ligands are known to play a crucial role in the construction of complexes with metal-metal bonds. Compared with metal-metal bonds involving d-block transition metals, knowledge of the metal-metal bonds involving f-block rare-earth metals still lags far behind. Herein, we report a series of complexes with cerium-transition-metal bonds, which are supported by two kinds of nitrogen-phosphorus ligands N[CH2CH2NHPiPr2]3 (VI) and PyNHCH2PPh2 (VII). The reactions of zerovalent group 10 metal precursors, Pd(PPh3)4 and Pt(PPh3)4, with the cerium complex supported by VI generate heterometallic clusters [N{CH2CH2NPiPr2}3Ce(µ-M)]2 (M = Pd, 2 and M = Pt, 3) featuring four Ce-M bonds; meanwhile, the bimetallic species [(PyNCH2PPh2)3Ce-M] (M = Ni, 5; M = Pd, 6; and M = Pt, 7) with a single Ce-M bond were isolated from the reactions of the cerium precursor 4 supported by VII with Ni(COD)2, Pd(PPh3)4, or Pt(PPh3)4, respectively. These complexes represent the first example of species with an RE-M bond between Ce and group 10 metals, and 2 and 3 contain the largest number of RE-M donor/acceptor interactions ever to have been observed in a molecule.

Magnesium complexes supported by a dianionic double layer nitrogen-phosphorus ligand: a synthesis and reactivity study.

A heterobimetallic complex [MeN(CH2CH2NPiPr)2MgLiCl(THF)]2 (1) supported by a dianionic double layer nitrogen-phosphorus ligand was synthesized by the reaction of H2L1 (H2L1 = MeN(CH2CH2NHPiPr)2) with MgCl2 in the presence of n-BuLi. Reactions of complex 1 with 2 equivalents of CuI, AgI and AuCl·SMe2 led to the formation of heterobimetallic clusters [MeN(CH2CH2NPiPr)2MgCuI]2 (2), [MeN(CH2CH2NPiPr)2MgAgI]2 (3) and [MeN(CH2CH2NPiPr)2MgAuCl]2 (4), respectively. X-ray single-crystal diffraction analysis revealed that these complexes are dimers, which are composed of two [MeN(CH2CH2NPiPr)2Mg] units connected by coinage metals (i.e., Cu, Ag, and Au). The reactivity of 1 was further investigated and it was found that complex 1 could react with 4 equivalents of MeI, giving a complex [CH3N(CH2CH2NPiPr2Me)2MgI]+[I]- (5), which can be viewed as a magnesium complex supported by a neutral double layer nitrogen-phosphorus ligand (CH3N(CH2CH2NPiPr2Me)2). Complex 1 could also react with 2 equivalents of NaNH2, leading to the isolation of an amido anion bridged magnesium-sodium heterobimetallic cluster [MeN(CH2CH2NPiPr)2MgNaNH2]2 (6).

Dinitrogen cleavage and hydrogenation to ammonia with a uranium complex.

The Haber-Bosch process produces ammonia (NH3) from dinitrogen (N2) and dihydrogen (H2), but requires high temperature and pressure. Before iron-based catalysts were exploited in the current industrial Haber-Bosch process, uranium-based materials served as effective catalysts for production of NH3 from N2. Although some molecular uranium complexes are known to be capable of combining with N2, further hydrogenation with H2 forming NH3 has not been reported to date. Here, we describe the first example of N2 cleavage and hydrogenation with H2 to NH3 with a molecular uranium complex. The N2 cleavage product contains three uranium centers that are bridged by three imido µ 2-NH ligands and one nitrido µ 3-N ligand. Labeling experiments with 15N demonstrate that the nitrido ligand in the product originates from N2. Reaction of the N2-cleaved complex with H2 or H+ forms NH3 under mild conditions. A synthetic cycle has been established by the reaction of the N2-cleaved complex with trimethylsilyl chloride. The isolation of this trinuclear imido-nitrido product implies that a multi-metallic uranium assembly plays an important role in the activation of N2.

Complexes Featuring a cis-[M → → ${{\rm{ \mathbin{{\stackrel{\textstyle\rightarrow} { {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ U ← ← ${{\rm{ \mathbin{{\stackrel{\textstyle\leftarrow} { {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ M] Core (M=Rh, Ir): A New Route to Uranium-Metal Multiple Bonds.

Although examples of multiple bonds between actinide elements and main-group elements are quite common, studies of the multiple bonds between actinide elements and transition metals are extremely rare owing to difficulties associated with their synthesis. Here we report the first example of molecular uranium complexes featuring a cis-[M → → ${{\rm{ \mathbin{{\stackrel{\textstyle\rightarrow} { {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ U ← ← ${{\rm{ \mathbin{{\stackrel{\textstyle\leftarrow} { {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ M] core (M=Rh, Ir), which exhibits an unprecedented arrangement of two M → → ${{\rm{ \mathbin{{\stackrel{\textstyle\rightarrow} { {\smash{\rightarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ U double dative bond linkages to a single U center. These complexes were prepared by the reactions of chlorine-bridged heterometallic complexes [{U{N(CH3 )(CH2 CH2 NPi Pr2 )2 }(Cl)2 [(µ-Cl)M(COD)]2 }] (M=Rh, Ir) with MeMgBr or MeLi, a new method for the construction of species with U-M multiple bonds. Theoretical calculations including dispersion confirmed the presence of two U ← ← ${{\rm{ \mathbin{{\stackrel{\textstyle\leftarrow} { {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ M double dative bonds in these complexes. This study not only enriches the U ← ← ${{\rm{ \mathbin{{\stackrel{\textstyle\leftarrow} { {\smash{\leftarrow}\vphantom{_{\vbox to.5ex{\vss}}}} } }} }}}$ M multiple bond chemistry, but also provides a new opportunity to explore the bonding of actinide elements.

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.

Photochemical Synthesis of Transition Metal-Stabilized Uranium(VI) Nitride Complexes.

Uranium nitrides play important roles in dinitrogen activation and functionalization and in chemistry for nuclear fuels, but the synthesis and isolation of the highly reactive uranium(VI) nitrides remains challenging. Here, we report an example of transition metal (TM) stabilized U(VI) nitride complexes, which are generated by the photolysis of azide-bridged U(IV)-TM (TM = Rh, Ir) precursors. The U(V) nitride intermediates with bridged azide ligands are isolated successfully by careful control of the irradiation time, suggesting that the photolysis of azide-bridged U(IV)-TM precursors is a stepwise process. The presence of two U(VI) nitrides stabilized by three TMs is clearly demonstrated by an X-ray crystallographic study. These TM stabilized U(V) nitride intermediates and U(VI) nitride products exhibit excellent stability both in the solid-state and in THF solution under ambient light. Density functional theory calculations show that the photolysis necessary to break the N-N bond of the azide ligands implies excitation from uranium f-orbital to the lowest unoccupied molecular orbital (LUMO), as suggested by the strong antibonding N-(N2) character present in the latter.

Heterometallic Clusters with Uranium-Metal Bonds Supported by Double-Layer Nitrogen-Phosphorus Ligands.

ConspectusHeterometallic clusters with M-M bonds have significantly interested chemists because of their attractive structures and synergistic effects in small-molecule activation and catalysis. However, reports of the isolation of heterometallic clusters with uranium-transition metal (U-TM) bonds remain very limited. In this Account, we describe our research in the construction of heterometallic molecular clusters with multiple U-TM single or multiple bonds supported by novel double-layer N-P ligands. Multimetallic synergistic catalysis and small-molecule activation with these species are also summarized.First, according to the hard-soft acid-base theory, we employed a three-armed N-P ligand, which can be used to construct heterometallic clusters with four or six U-Ni bonds. This strategy was also effective in the construction of complexes with direct rare earth metal-TM bonding. The similar two-armed N-P ligands also are effective platforms for the synthesis of heterometallic complexes with U-Ni, U-Pd, and U-Pt bonds.Second, a set of heterometallic clusters featuring U≡Rh, U≡Co, and U≡Fe triple bonds were constructed under routine experimental conditions. X-ray diffraction analysis of these clusters exhibits the shortest U-TM bond distance (1.9693(4) Å for the U≡Fe triple bond) in these complexes. Theoretical studies reveal that the nature of the triple bond is one covalent σ bond and two TM → U dative π bonds. A large Wiberg bond index (WBI) of 2.93 and a significant degree of covalency for the U≡TM triple bonds were also found in these complexes.Third, these uranium complexes supported by the double-layer N-P ligands exhibit great potential in small-molecule activation. For instance, N2 cleavage without an external reducing agent was achieved by a U(III)-P(III) synergistic six-electron reduction. The synergism between U(III) and P(III) enables the activation of other small molecules, such as O2, P4, and As0(nano), and highlights the importance of the P atom in the double-layer N-P ligand for the activation of small molecules. A heterometallic cluster with U-Rh bonds can break the strong N≡N triple bond in N2 in the presence of potassium graphite, suggesting a synergistic effect between U and Rh. This multimetallic synergistic effect was also observed in catalytic processes. A heterometallic cluster with U≡Co triple bonds shows excellent selectivity and activity in the hydroboration of a series of alkynes under mild conditions. These results lead to effective methods for the construction of heterometallic molecular clusters with U-TM single or multiple bonds and could promote the application of heterometallic clusters with U-TM bonds in catalysis and the activation of small molecules.

Redox-induced reversible P-P coupling in a uranium complex.

A synthesized redox-active multidentate N-P ligand reacted with UCl4 in the presence of KHMDS or nBuLi, where two novel U(IV) complexes with or without P-P coupling were formed, respectively. The reversible P-P coupling in these complexes was observed in redox-induced reactions.

Heterometallic Clusters with Multiple Rare Earth Metal-Transition Metal Bonding.

Although a series of complexes with rare earth (RE) metal-metal bonds have been reported, complexes which have multiple RE-Rh bonds are unknown. Here we present the identification of the first example of a molecule containing multiple RE-Rh bonds. The complex with multiple Ce-Rh bonds was synthesized by the reduction of a d-f heterometallic molecular cluster Ce{N[(CH2CH2NPiPr2)RhCl(COD)]3} with excess potassium-graphite. The oxidation state of Ce in 3a appears to be a mixture of Ce(III) and Ce(IV), which was confirmed by X-ray photoelectron spectroscopy, magnetism, and theoretical investigations (DFT and CASSCF). For comparison, the analogous species with multiple La(III)-Rh and Nd(III)-Rh bonds were also constructed. This study provides a possible route for the construction of complexes with multiple RE metal-metal bonds and an investigation of their potential properties and applications.

Facile Dinitrogen and Dioxygen Cleavage by a Uranium(III) Complex: Cooperativity Between the Non-Innocent Ligand and the Uranium Center.

Activation of dinitrogen (N2 , 78 %) and dioxygen (O2 , 21 %) has fascinated chemists and biochemists for decades. The industrial conversion of N2 into ammonia requires extremely high temperatures and pressures. Herein we report the first example of N2 and O2 cleavage by a uranium complex, [N(CH2 CH2 NPi Pr2 )3 U]2 (TMEDA), under ambient conditions without an external reducing agent. The N2 triple bond breaking implies a UIII -PIII six-electron reduction. The hydrolysis of the N2 reduction product allows the formation of ammonia or nitrogen-containing organic compounds. The interaction between UIII and PIII in this molecule allows an eight-electron reduction of two O2 molecules. This study establishes that the combination of uranium and a low-valent nonmetal is a promising strategy to achieve a full N2 and O2 cleavage under ambient conditions, which may aid the design of new systems for small molecules activation.

Chemoselectivity for B-O and B-H Bond Cleavage by Pincer-Type Phosphorus Compounds: Theoretical and Experimental Studies.

Selective cleavage of the B-O bond or B-H bond in HBpin can be achieved by adjusting the pincer ligand of a phosphorus(III) compound guided by a combination of theoretical prediction and experimental verification. Theoretical calculations reveal that a pincer-type phosphorus compound with an [ONO]3- ligand reacts with HBpin, leading to cleavage of the stronger B-O bonds (ΔG°â§§ = 23.2 kcal mol-1) rather than the weaker B-H bond (ΔG°â§§ = 26.4 kcal mol-1). A pincer-type phosphorus compound with a [NNN]3- ligand reacts with HBpin, leading to the weaker B-H bond cleavage (ΔG°â§§ = 16.2 kcal mol-1) rather than cleavage of the stronger B-O bond (ΔG°â§§ = 33.0 kcal mol-1). The theoretical prediction for B-O bond cleavage was verified experimentally, and the final products were characterized by NMR, HRMS, and single-crystal X-ray diffraction. The chemoselectivity of B-O bond cleavage was also observed in the presence of B-C or B-B bonds in borane substrates.

Dinitrogen Cleavage by a Heterometallic Cluster Featuring Multiple Uranium-Rhodium Bonds.

Reduction of dinitrogen (N2) is a major challenge for chemists. Cooperation of multiple metal centers to break the strong N2 triple bond has been identified as a crucial step in both the industrial and the natural ammonia syntheses. However, reports of the cleavage of N2 by a multimetallic uranium complex remain extremely rare, although uranium species were used as catalyst in the early Harber-Bosch process. Here we report the cleavage of N2 to two nitrides by a multimetallic uranium-rhodium cluster at ambient temperature and pressure. The nitride product further reacts with acid to give substantial yields of ammonium. The presence of uranium-rhodium bond in this multimetallic cluster was revealed by X-ray crystallographic and computational studies. This study demonstrates that the multimetallic clusters containing uranium and transition metals are promising materials for N2 fixation and reduction.

[3+2] cycloaddition reaction of metallacyclopropene with nitrosonium ion: isolation of aromatic metallaisoxazole.

The synthesis of metallaisoxazole by the [3+2] cycloaddition reaction of metallacyclopropene with nitrosonium tetra-fluoroborate has been achieved under mild conditions. Nuclear magnetic resonance spectra, X-ray crystallographic analysis, and density functional theory calculations all suggest that the metallaisoxazole exhibits an aromatic character.

Triple Frustrated Lewis Pair-Type Reactivity on a Single Rare-Earth Metal Center.

Rare-earth metal cations have been used rarely as Lewis-acidic components in the chemistry of frustrated Lewis pairs (FLPs). Herein, we report the first cerium/phosphorus system (2) employing a heptadentate N4 P3 ligand, which exhibits triple FLP-type reactivity towards a series of organic substrates, including isocyanates, isothiocyanates, diazomethane, and azides on a single rare-earth Lewis acidic Ce center. This result shows that the Ce center and three P atoms in 2 could simultaneously activate three equivalents of small molecules under mild conditions. This study broadens the diversity of FLPs and demonstrates that rare earth based FLP exhibit unique properties compared with other FLP systems.

Construction of heterometallic clusters with multiple uranium-metal bonds by using dianionic nitrogen-phosphorus ligands.

Compared with the prevalent metal-metal bond in transition metals, examples of the actinide-metal bond in heterometallic clusters are rare. Herein, a series of heterometallic clusters with multiple uranium-metal bonds has been prepared based on two newly synthesized nitrogen-phosphorus ligands L1 {O[(CH2)2NHP(iPr)2]2} and L2 {[CH2O(CH2)2NHP(iPr)2]2}. Different P-P distances, 6.069 and 4.464 Å, are observed in the corresponding uranium complexes 1 {O[(CH2)2NP(iPr)2]2UCl2} and 2 {[CH2O(CH2)2NP(iPr)2]2UCl2}, respectively, and lead to the different coordination modes with transition metals. The reactions of zero-valent group 10 metal compounds with complex 1 generate heterometallic clusters (3-U2Ni2 and 4-U2Pd2) featuring four uranium-metal bonds; whereas reactions with 2 afford one-dimensional metal-chain 5-(UNi)n, bimetallic species 6-UPd, and a tri-platinum bridged diuranium molecular cluster 7-U2Pt3. Complex 5-(UNi)n represents the first infinite chain containing the U-M bond and 7-U2Pt3 is the first species with multiple U-Pt bonds. This study further highlights the important role of ligands in the construction of multiple uranium-metal bonds and may allow the synthesis of novel d-f heterometallic clusters and the investigation of their applications in catalysis and small-molecule activation.

Isolation of heterometallic cerium(iii) complexes with a multidentate nitrogen-phosphorus ligand.

Heterometallic complexes play an important role in catalysis and activation of small molecules due to the synergistic effects from different metals. Here we report a straightforward strategy to construct a series of heterobimetallic complexes of bromine bridged cerium(iii)-alkali metal or group 9 metals using a multidentate nitrogen-phosphorus ligand.