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

Metal-metal bonding is a widely studied area of chemistry1-3, and has become a mature field spanning numerous d transition metal and main group complexes4-7. 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 ThHn (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 bond16,17 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.

Carbene Complexes of Plutonium: Structure, Bonding, and Divergent Reactivity to Lanthanide Analogs.

Organoplutonium chemistry was established in 1965, yet structurally authenticated plutonium-carbon bonds remain rare being limited to π-bonded carbocycle and σ-bonded isonitrile and hydrocarbyl derivatives. Thus, plutonium-carbenes, including alkylidenes and N-heterocyclic carbenes (NHCs), are unknown. Here, we report the preparation and characterization of the diphosphoniomethanide-plutonium complex [Pu(BIPMTMSH)(I)(µ-I)]2 (1Pu, BIPMTMSH = (Me3SiNPPh2)2CH) and the diphosphonioalkylidene-plutonium complexes [Pu(BIPMTMS)(I)(DME)] (2Pu, BIPMTMS = (Me3SiNPPh2)2C) and [Pu(BIPMTMS)(I)(IMe4)2] (3Pu, IMe4 = C(NMeCMe)2), thus disclosing non-actinyl transneptunium multiple bonds and transneptunium NHC complexes. These Pu-C double and dative bonds, along with cerium, praseodymium, samarium, uranium, and neptunium congeners, enable lanthanide-actinide and actinide-actinide comparisons between metals with similar ionic radii and isoelectronic 4f5 vs 5f5 electron-counts within conserved ligand fields over 12 complexes. Quantum chemical calculations reveal that the orbital-energy and spatial-overlap terms increase from uranium to neptunium; however, on moving to plutonium the orbital-energy matching improves but the spatial overlap decreases. The bonding picture that emerges is more complex than the traditional picture of the bonding of lanthanides being ionic and early actinides being more covalent but becoming more ionic left to right. Multiconfigurational calculations on 2M and 3M (M = Pu, Sm) account for the considerably more complex UV/vis/NIR spectra for 5f5 2Pu and 3Pu compared to 4f5 2Sm and 3Sm. Supporting the presence of Pu═C double bonds in 2Pu and 3Pu, 2Pu exhibits metallo-Wittig bond metathesis involving the highest atomic number element to date, reacting with benzaldehyde to produce the alkene PhC(H)═C(PPh2NSiMe3)2 (4) and "PuOI". In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4.

f-Element Zintl Chemistry: Actinide-Mediated Dehydrocoupling of H2Sb1- Affords the Trithorium and Triuranium Undeca-Antimontriide Zintl Clusters [{An(TrenTIPS)}3(µ3-Sb11)] (An = Th, U; TrenTIPS = {N(CH2CH2NSiiPr3)3}3-).

Reaction of the cesium antimonide complex [Cs(18C6)2][SbH2] (1, 18C6 = 18-crown-6 ether) with the triamidoamine actinide separated ion pairs [An(TrenTIPS)(L)][BPh4] (TrenTIPS = {N(CH2CH2NSiiPr3)3}3-; An/L = Th/DME (2Th); U/THF (2U)) affords the triactinide undeca-antimontriide Zintl clusters [{An(TrenTIPS)}3(µ3-Sb11)] (An = Th (3Th), U (3U)) by dehydrocoupling. Clusters 3Th and 3U provide two new examples of the Sb113- Zintl trianion and are unprecedented examples of molecular Sb113- being coordinated to anything since all previous reports featured isolated Sb113- Zintl trianions in separated ion quadruple formulations with noncoordinating cations. Quantum chemical calculations describe dominant ionic An-Sb interactions in 3Th and 3U, though the data suggest that the latter exhibits slightly more covalent An-Sb linkages than the former. Complexes 3Th and 3U have been characterized by single crystal X-ray diffraction, NMR, IR, and UV/vis/NIR spectroscopies, elemental analysis, and quantum chemical calculations.

Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes.

We introduce the boryloxide ligand {(HCNDipp)2BO}- (NBODipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe3)2}3] with 3 equiv of NBODippH produced the uranium(III) tris(boryloxide) complex [U(NBODipp)3] (1). In contrast, treatment of UCl4 with 3 equiv of NBODippK in THF at room temperature or reflux conditions produced only [U(NBODipp)2(Cl)2(THF)2] (2) with 1 equiv of NBODippK remaining unreacted. However, refluxing the mixture of 2 and unreacted NBODippK in toluene instead of THF afforded the target complex [U(NBODipp)3(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN3 (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBODipp)3(NAd)] (4). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene δ-back-bonding with approximately equal donation into the arene π4 and π5 δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |ml,ms⟩ states and mj projections. Complexes 1-4 have been variously characterized by single-crystal X-ray diffraction, 1H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.

Synthesis and Characterization of Yttrium Methanediide Silanide Complexes.

The salt metathesis reactions of the yttrium methanediide iodide complex [Y(BIPM)(I)(THF)2] (BIPM = {C(PPh2NSiMe3)2}) with the group 1 silanide ligand-transfer reagents MSiR3 (M = Na, R3 = tBu2Me or tBu3; M = K, R3 = (SiMe3)3) gave the yttrium methanediide silanide complexes [Y(BIPM)(SitBu2Me)(THF)] (1), [Y(BIPM)(SitBu3)(THF)] (2), and [Y(BIPM){Si(SiMe3)3}(THF)] (3). Complexes 1-3 provide rare examples of structurally authenticated rare earth metal-silicon bonds and were characterized by single-crystal X-ray diffraction, multinuclear NMR and ATR-IR spectroscopies, and elemental analysis. Density functional theory calculations were performed on 1-3 to probe their electronic structures further, revealing predominantly ionic Y-Si bonding. The computed Y-Si bonds show lower covalency than Y═C bonds, which are in turn best represented by Y+-C- dipolar forms due to the strong σ-donor properties of the silanide ligands investigated; these observations are in accord with experimentally obtained 13C{1H} and 29Si{1H} NMR data for 1-3 and related Y(III) BIPM alkyl complexes in the literature. Preliminary reactivity studies were performed, with complex 1 treated separately with benzophenone, azobenzene, and N,N'-dicyclohexyl-carbodiimide. 29Si{1H} and 31P{1H} NMR spectra of these reaction mixtures indicated that 1,2-migratory insertion of the unsaturated substrate into the Y-Si bond is favored, while for the latter substrate, a [2 + 2]-cycloaddition reaction also occurs at the Y═C bond to afford [Y{C(PPh2NSiMe3)2[C(NCy)2]-κ4C,N,N',N'}{C(NCy)2(SitBu2Me)-κ2N,N'}] (4); these reactivity profiles complement and contrast with those of Y(III) BIPM alkyl complexes.

Carbene Complexes of Neptunium.

Since the advent of organotransuranium chemistry six decades ago, structurally verified complexes remain restricted to π-bonded carbocycle and σ-bonded hydrocarbyl derivatives. Thus, transuranium-carbon multiple or dative bonds are yet to be reported. Here, utilizing diphosphoniomethanide precursors we report the synthesis and characterization of transuranium-carbene derivatives, namely, diphosphonio-alkylidene- and N-heterocyclic carbene-neptunium(III) complexes that exhibit polarized-covalent σ2π2 multiple and dative σ2 single transuranium-carbon bond interactions, respectively. The reaction of [NpIIII3(THF)4] with [Rb(BIPMTMSH)] (BIPMTMSH = {HC(PPh2NSiMe3)2}1-) affords [(BIPMTMSH)NpIII(I)2(THF)] (3Np) in situ, and subsequent treatment with the N-heterocyclic carbene {C(NMeCMe)2} (IMe4) allows isolation of [(BIPMTMSH)NpIII(I)2(IMe4)] (4Np). Separate treatment of in situ prepared 3Np with benzyl potassium in 1,2-dimethoxyethane (DME) affords [(BIPMTMS)NpIII(I)(DME)] (5Np, BIPMTMS = {C(PPh2NSiMe3)2}2-). Analogously, addition of benzyl potassium and IMe4 to 4Np gives [(BIPMTMS)NpIII(I)(IMe4)2] (6Np). The synthesis of 3Np-6Np was facilitated by adopting a scaled-down prechoreographed approach using cerium synthetic surrogates. The thorium(III) and uranium(III) analogues of these neptunium(III) complexes are currently unavailable, meaning that the synthesis of 4Np-6Np provides an example of experimental grounding of 5f- vs 5f- and 5f- vs 4f-element bonding and reactivity comparisons being led by nonaqueous transuranium chemistry rather than thorium and uranium congeners. Computational analysis suggests that these NpIII═C bonds are more covalent than UIII═C, CeIII═C, and PmIII═C congeners but comparable to analogous UIV═C bonds in terms of bond orders and total metal contributions to the M═C bonds. A preliminary assessment of NpIII═C reactivity has introduced multiple bond metathesis to transuranium chemistry, extending the range of known metallo-Wittig reactions to encompass actinide oxidation states III-VI.

A Series of Rare-Earth Mesoionic Carbene Complexes.

We report the synthesis and characterisation of a series of rare-earth mesoionic carbene complexes, [RE{N(SiMe3 )2 }3 {CN(Me)C(Me)N(Me)CH}] (3RE, RE=Sc, Ce, Pr, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), greatly expanding the limited library of f-block mesoionic carbene complexes. These complexes were prepared by treatment of the parent RE-triamides with an N-heterocyclic olefin (NHO), where an NHO backbone proton undergoes a formal 1,4-proton migration to the NHO-methylene group. For all RE(III) metals, as expected, quantum chemical calculations suggest only a σ-component to the metal-carbene bonding, in contrast to a previously reported uranium(III) congener where the 5f3 metal engages in a weak π-back-bond to the MIC. All complexes were characterised by static variable-temperature magnetic measurements, and dynamic magnetic measurements reveal that 3Dy and 3Er are field-induced single-molecule magnets (SMMs), with Ueff energy barriers of 35 and 128 K, respectively. Complex 3Dy is, as expected, a poorly performing SMM, but conversely 3Er performs unexpectedly well.

Actinide Pnictinidene Chemistry: A Terminal Thorium Parent-Arsinidene Complex Stabilised by a Super-Bulky Triamidoamine Ligand.

We report the direct synthesis of the terminal pnictidenes [An(TrenTCHS )(PnH)][M(2,2,2-cryptand)] (TrenTCHS ={N(CH2 CH2 NSiCy3 )3 }3- ; An/Pn/M=Th/P/Na 5, Th/As/K 6, U/P/Na 7, U/As/K 8) and their conversion to the pnictides [An(TrenTCHS )(PnH2 )] (An/Pn=Th/P 9, Th/As 10, U/P 11, U/As 12). Use of the super-bulky TrenTCHS ligand was essential to accessing complete families, and 6 is an unprecedented example of a terminal thorium-arsinidene complex and only the second structurally authenticated parent terminal arsinidene complex of any metal. Comparison of the terminal Th=AsH unit of 6 to the bridging ThAs(H)K linkage in structurally analogous [Th(TrenTIPS ){µ-As(H)K(15-crown-5)}] (TrenTIPS ={N(CH2 CH2 NSiPri 3 )3 }3- ) reveals a stronger Th-As bond in the former compared to the latter, and a large response overall to the nature of the Th=AsH bonding upon removal of the electrostatically-bound K-ion; the σ-bond changes little but the π-bond is significantly perturbed.

Dipnictogen f-Element Chemistry: A Diphosphorus Uranium Complex.

The first isolation and structural characterization of an f-element dinitrogen complex was reported in 1988, but an f-element complex with the first heavier group 15 homologue diphosphorus has to date remained unknown. Here, we report the synthesis of a side-on bound diphosphorus complex of uranium(IV) using a 7λ3-(dimethylamino)phosphadibenzonorbornadiene-mediated P atom transfer approach. Experimental and computational characterization reveals that the diphosphorus ligand is activated to its dianionic (P2)2- form and that in-plane U-P π-bonding dominates the bonding of the U(µ-η2:η2-P2)U unit, which is supplemented by a weak U-P interaction of δ symmetry. A preliminary reactivity study demonstrates conversion of this diphosphorus complex to unprecedented uranium cyclo-P3 complexes, suggesting in situ generation of transient, reactive phosphido species.

29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency.

We report the use of 29Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(SitBu3)2(THF)2(THF)x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(SitBu2Me)2(THF)2(THF)x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and 29Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe3)3}--, {Si(SiMe2H)3}--, and {SiPh3}--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29Si NMR isotropic chemical shifts, δSi, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δSi and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δSi is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

The "Hidden" Reductive [2+2+1]-Cycloaddition Chemistry of 2-Phosphaethynolate Revealed by Reduction of a Th-OCP Linkage.

The reduction chemistry of the newly emerging 2-phosphaethynolate (OCP)- is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(TrenTIPS )(OCP)] (2, TrenTIPS =[N(CH2 CH2 NSiPri 3 )]3- ), with RbC8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(TrenTIPS )}6 (µ-OC2 P3 )2 (µ-OC2 P3 H)2 Rb4 ] (5) featuring four five-membered [C2 P3 ] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(TrenTIPS )(µ-ORb)}2 ] (6) and the known cyclometallated complex [Th{N(CH2 CH2 NSiPri 3 )2 (CH2 CH2 SiPri 2 CHMeCH2 )}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2-phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)- , rather than cycloaddition products that then decompose with [Th(TrenTIPS )O]- essentially acting as a protecting then leaving group. In contrast, when KC8 or CsC8 were used the phosphinidiide C-H bond activation product [{Th(TrenTIPS )}Th{N(CH2 CH2 NSiPri 3 )2 [CH2 CH2 SiPri 2 CH(Me)CH2 C(O)µ-P]}] (3) and the oxo complex [{Th(TrenTIPS )(µ-OCs)}2 ] (7) were isolated.

A Uranium(VI)-Oxo-Imido Dimer Complex Derived from a Sterically Demanding Triamidoamine.

The reaction of [UO2(µ-Cl)4{K(18-crown-6)}2] with [{N(CH2CH2NSiPri3)3}Li3] gives [{UO(µ-NCH2CH2N[CH2CH2NSiPri3]2)}2] (1), [{(LiCl)(KCl)(18-crown-6)}2] (2), and [LiOSiPri3] (3) in a 1:2:2 ratio. The formation of the oxo-imido 1 involves the cleavage of a N-Si bond and the activation of one of the usually robust U═O bonds of uranyl(VI), resulting in the formation of uranium(VI)-imido and siloxide linkages. Notably, the uranium oxidation state remains unchanged at +6 in the starting material and product. Structural characterization suggests the dominance of a core RN═U═O group, and the dimeric formulation of 1 is supported by bridging imido linkages in a highly asymmetric U2N2 ring. Density functional theory analyses find a σ > π orbital energy ordering for the U═N and U═O bonds in 1, which is uranyl-like in nature. Complexes 1-3 were characterized variously by single crystal X-ray diffraction, multinuclear NMR, IR, Raman, and optical spectroscopies; cyclic voltammetry; and density functional theory.

f-Element Half-Sandwich Complexes: A Tetrasilylcyclobutadienyl-Uranium(IV)-Tris(tetrahydroborate) Anion Pianostool Complex.

Despite there being numerous examples of f-element compounds supported by cyclopentadienyl, arene, cycloheptatrienyl, and cyclooctatetraenyl ligands (C5-8 ), cyclobutadienyl (C4 ) complexes remain exceedingly rare. Here, we report that reaction of [Li2 {C4 (SiMe3 )4 }(THF)2 ] (1) with [U(BH4 )3 (THF)2 ] (2) gives the pianostool complex [U{C4 (SiMe3 )4 }(BH4 )3 ][Li(THF)4 ] (3), where use of a borohydride and preformed C4 -unit circumvents difficulties in product isolation and closing a C4 -ring at uranium. Complex 3 is an unprecedented example of an f-element half-sandwich cyclobutadienyl complex, and it is only the second example of an actinide-cyclobutadienyl complex, the other being an inverse-sandwich. The U-C distances are short (av. 2.513 Å), reflecting the formal 2- charge of the C4 -unit, and the SiMe3 groups are displaced from the C4 -plane, which we propose maximises U-C4 orbital overlap. DFT calculations identify two quasi-degenerate U-C4 π-bonds utilising the ψ2 and ψ3 molecular orbitals of the C4 -unit, but the potential δ-bond using the ψ4 orbital is vacant.

Nature of the Arsonium-Ylide Ph3 As=CH2 and a Uranium(IV) Arsonium-Carbene Complex.

Treatment of [Ph3 EMe][I] with [Na{N(SiMe3 )2 }] affords the ylides [Ph3 E=CH2 ] (E=As, 1As; P, 1P). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P, confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E+ -C- dipolar resonance form (sp3 -C) over the E=C ene π-bonded form (sp2 -C), as group 15 is descended. The uranium(IV)-cyclometallate complex [U{N(CH2 CH2 NSiPri 3 )2 (CH2 CH2 SiPri 2 CH(Me)CH2 )}] reacts with 1As and 1P by α-proton abstraction to give [U(TrenTIPS )(CHEPh3 )] (TrenTIPS =N(CH2 CH2 NSiPri 3 )3 ; E=As, 2As; P, 2P), where 2As is an unprecedented structurally characterised arsonium-carbene complex. The short U-C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U-C distance is found for 2As than 2P, consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations.

Photolytic and Reductive Activations of 2-Arsaethynolate in a Uranium-Triamidoamine Complex: Decarbonylative Arsenic-Group Transfer Reactions and Trapping of a Highly Bent and Reduced Form.

Little is known about the chemistry of the 2-arsaethynolate anion, but to date it has exclusively undergone fragmentation reactions when reduced. Herein, we report the synthesis of [U(TrenTIPS )(OCAs)] (2, TrenTIPS =N(CH2 CH2 NSiiPr3 )3 ), which is the first isolable actinide-2-arsaethynolate linkage. UV-photolysis of 2 results in decarbonylation, but the putative [U(TrenTIPS )(As)] product was not isolated and instead only [{U(TrenTIPS )}2 (µ-η2 :η2 -As2 H2 )] (3) was formed. In contrast, reduction of 2 with [U(TrenTIPS )] gave the mixed-valence arsenido [{U(TrenTIPS )}2 (µ-As)] (4) in very low yield. Complex 4 is unstable which precluded full characterisation, but these photolytic and reductive reactions testify to the tendency of 2-arsaethynolate to fragment with CO release and As transfer. However, addition of 2 to an electride mixture of potassium-graphite and 2,2,2-cryptand gives [{U(TrenTIPS )}2 {µ-η2 (OAs):η2 (CAs)-OCAs}][K(2,2,2-cryptand)] (5). The coordination mode of the trapped 2-arsaethynolate in 5 is unique, and derives from a new highly reduced and bent form of this ligand with the most acute O-C-As angle in any complex to date (O-C-As ∠ ≈128°). The trapping rather than fragmentation of this highly reduced O-C-As unit is unprecedented, and quantum chemical calculations reveal that reduction confers donor-acceptor character to the O-C-As unit.

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.

Bimetallic Cooperative Cleavage of Dinitrogen to Nitride and Tandem Frustrated Lewis Pair Hydrogenation to Ammonia.

Although reductive cleavage of dinitrogen (N2 ) to nitride (N3- ) and hydrogenation with dihydrogen (H2 ) to yield ammonia (NH3 ) is accomplished in heterogeneous Haber-Bosch industrial processes on a vast scale, sequentially coupling these elementary reactions together with a single metal complex remains a major challenge for homogeneous molecular complexes. Herein, we report that the reaction of a chloro titanium triamidoamine complex with magnesium effects complete reductive cleavage of N2 to give a dinitride dititanium dimagnesium ditriamidoamine complex. Tandem H2 splitting by a phosphine-borane frustrated Lewis pair (FLP) shuttles H atoms to the N3- , evolving NH3 . Isotope labelling experiments confirmed N2 and H2 fixation. Though not yet catalytic, these results give unprecedented insight into coupling N2 and H2 cleavage and N-H bond formation steps together, highlight the importance of heterobimetallic cooperativity in N2 activation, and establish FLPs in NH3 synthesis.

Trapping of a Highly Bent and Reduced Form of 2-Phosphaethynolate in a Mixed-Valence Diuranium-Triamidoamine Complex.

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-O fragmentations or couplings. Here, we report that reduction of [U(TrenTIPS )(OCP)] (TrenTIPS =N(CH2 CH2 NSiPri 3 )3 ) with KC8 /2,2,2-cryptand gives [{U(TrenTIPS )}2 {µ-η2 (OP):η2 (CP)-OCP}][K(2,2,2-cryptand)]. The coordination mode of this trapped 2-phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P-C-O angle in any complex to date (P-C-O ∡ ≈127°). The characterisation data support a mixed-valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P-C-O unit that is perhaps best described as a uranium-stabilised OCP2-. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P-C-O unit, which engages in a weak donor-acceptor interaction with one of the uranium ions.

Uranyl-tri- bis(silyl)amide Alkali Metal Contact and Separated Ion Pair Complexes.

We report the preparation of a range of alkali metal uranyl(VI) tri- bis(silyl)amide complexes [{M(THF) x}{(µ-O)U(O)(N″)3}] (1M) (N″ = {N(SiMe3)2}-, M = Li, Na, x = 2; M = K, x = 3; M = K, Rb, Cs, x = 0) containing electrostatic alkali metal uranyl-oxo interactions. Reaction of 1M with 2,2,2-cryptand or 2 equiv of the appropriate crown ether resulted in the isolation of the separated ion pair species [U(O)2(N″)3][M(2,2,2-cryptand)] (3M, M = Li-Cs) and [U(O)2(N″)3][M(crown)2] (4M, M = Li, crown = 12-crown-4 ether; M = Na-Cs, crown = 15-crown-5 ether). A combination of crystallographic studies and IR, Raman and UV-vis spectroscopies has revealed that the 1M series adopts contact ion pair motifs in the solid state where the alkali metal caps one of the uranyl-oxo groups. Upon dissolution in THF solution, this contact is lost, and instead, separated ion pair motifs are observed, which is confirmed by the isolation of [U(O)2(N″)3][M(THF) n] (2M) (M = Li, n = 4; M = Na, K, n = 6). The compounds have been characterized by single crystal X-ray diffraction, multinuclear NMR spectroscopy, IR, Raman, and UV-vis spectroscopies, and elemental analyses.