Triply Bonded Pancake π-Dimers Stabilized by Tetravalent Actinides.

Aromatic π-stacking is a weakly attractive, noncovalent interaction often found in biological macromolecules and synthetic supramolecular chemistry. The weak nondirectional nature of π-stacking can present challenges in the design of materials owing to their weak, nondirectional nature. However, when aromatic π-systems contain an unpaired electron, stronger attraction involving face-to-face π-orbital overlap is possible, resulting in covalent so-called "pancake" bonds. Two-electron, multicenter single pancake bonds are well known, whereas four-electron double pancake bonds are rare. Higher-order pancake bonds have been predicted, but experimental systems are unknown. Here, we show that six-electron triple pancake bonds can be synthesized by a 3-fold reduction of hexaazatrinaphthylene (HAN) and subsequent stacking of the [HAN]3- triradicals. Our analysis reveals a multicenter covalent triple pancake bond consisting of a σ-orbital and two equivalent π-orbitals. An electrostatic stabilizing role is established for the tetravalent thorium and uranium ions in these systems. We also show that the electronic absorption spectrum of the triple pancake bonds closely matches computational predictions, providing experimental verification of these unique interactions. The discovery of conductivity in thin films of triply bonded π-dimers presents new opportunities for the discovery of single-component molecular conductors and other spin-based molecular materials.

Syntheses and Characterization of Main Group, Transition Metal, Lanthanide, and Actinide Complexes of Bidentate Acylpyrazolone Ligands.

The synthesis of acylpyrazolone salts and their complexes of main group elements, transition metals, lanthanides, and actinides are described and characterized inter alia by means of single-crystal X-ray crystallography, NMR, and IR spectroscopies. The complexes consist of two, three, or four acylprazolone ligands bound to the metal atom, resulting in a structurally diverse set of coordination complexes with (distorted) octahedral, pentagonal-bipyramidal, or antiprismatic arrangements. Several complexes proved to be polymeric in the solid state including heterobimetallic sodium/lanthanide coordination polymers. A selection of the polymeric compounds was analyzed via TG/DTA measurements to establish their stability. The ligands, in turn, were readily synthesized in good yields from commercially available hydrazine hydrochloride salts. These findings demonstrate that acylpyrazolone ligands can form complexes with metals of varying ionic radii, highlighted by their utility in other areas such as analytical and metal organic framework chemistry.

Identification of Oxidation State +1 in a Molecular Uranium Complex.

The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η5-C5iPr5)2] and the uranium(III) metallocene [IU(η5-C5iPr5)2] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η5-C5iPr5)2]- (1) (C5iPr5 = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]+. An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f3(7s/6dz2)1(6dx2-y2/6dxy)1 configuration. The metal-ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides.

Heterometallic uranium/molybdenum nitride synthesis via partial N-atom transfer.

The reaction of a terminal Mo(II) nitride with a U(III) complex yields a heterodimetallic U-Mo nitride which is the first example of a transition metal-capped uranium nitride. The nitride is triply bonded to U(V) and singly bonded to Mo(0) and supports a U-Mo interaction. This compound shows reactivity toward CO oxidation.

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-.

Synthesis, structure, and reactivity of uranium(vi) nitrides.

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2 which are effective catalysts in the Haber-Bosch synthesis of ammonia, but the synthesis of molecular nitrides remains a challenge and studies of the reactivity and of the nature of the bonding are poorly developed. Here we report the synthesis of the first nitride bridged uranium complexes containing U(vi) and provide a unique comparison of reactivity and bonding in U(vi)/U(vi), U(vi)/U(v) and U(v)/U(v) systems. Oxidation of the U(v)/U(v) bis-nitride [K2{U(OSi(O t Bu)3)3(µ-N)}2], 1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi(O t Bu)3)3(µ-N)}2], 2 and [K2{U(OSi(O t Bu)3)3}2(µ-N)2(µ-I)], 3 while oxidation with a stronger oxidant ("magic blue") yields the U(vi)/U(vi) complex [{U(OSi(O t Bu)3)3}2(µ-N)2(µ-thf)], 4. The three complexes show very different stability and reactivity, with N2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged [K2{U(OSi(O t Bu)3)3(µ-NH)}2], 6 and rare amido bridged U(iv)/U(iv) complexes [{U(OSi(O t Bu)3)3}2(µ-NH2)2(µ-thf)], 7 while no hydrogenolysis could be observed for 4. Both complexes 2 and 4 react with H+ to yield quantitatively NH4Cl, but only complex 2 reacts with CO and H2. Differences in reactivity can be related to significant differences in the U-N bonding. Computational studies show a delocalised bond across the U-N-U for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which shows a U-N σ orbital well localised to U[triple bond, length as m-dash]N and π orbitals which partially delocalise to form the U-N single bond with the other uranium.

Stepwise Reduction of Dinitrogen by a Uranium-Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster.

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N3-), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K2{[UV(OSi(OtBu)3)3]2(µ-O)(µ-η2:η2-N2)}], B, with KC8, yielding the tetranuclear tetranitride cluster [K6{(OSi(OtBu)3)2UIV}3{(OSi(OtBu)3)2UVI}(µ4-N)3(µ3-N)(µ3-O)2], 1, a novel example of N2 cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-N═UVI═N core analogue of UO22+. Experimental and computational studies indicate that the formation of the U(IV)/U(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N24- ligand in complex B, resulting in N2 cleavage and the formation of the putative diuranium(V) bis-nitride [K4{[UV(OSi(OtBu)3)3]2(µ-O)(µ-N)2}], X. Additionally, cooperative potassium binding to the U-bound N24- ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93-97% yield and with excess 13CO to yield K13CN and KN13CO. The structures of two tetranuclear U(IV)/U(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Photochemical Synthesis of a Stable Terminal Uranium(VI) Nitride.

Terminal uranium nitrides have so far proven impossible to isolate by photolysis of azides. Here we report the second ever example of an isolated terminal uranium(VI) nitride. We show that the terminal nitride [NBu4][U(OSi(OtBu)3)4(N)], 3, can be prepared upon photolysis with UV light of the U(IV) azide analogue. This is achieved by careful tailoring of the azide precursor and of the reaction conditions. Complex 3 is stable under ambient conditions but reacts readily with electrophiles (H+ and CO).

Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands.

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U[double bond, length as m-dash]N[double bond, length as m-dash]U bonding imposed by the supporting ligand. The U4+-U4+ nitride complexes, [NBu4][{(( t BuO)3SiO)3U}2(µ-N)], [NBu4]-1, and [NBu4][((Me3Si)2N)3U}2(µ-N)], 2, were synthesised by reacting NBu4N3 with the U3+ complexes, [U(OSi(O t Bu)3)2(µ-OSi(O t Bu)3)]2 and [U(N(SiMe3)2)3], respectively. Oxidation of 2 with AgBPh4 gave the U4+-U5+ analogue, [((Me3Si)2N)3U}2(µ-N)], 4. The previously reported methylene-bridged U4+-U4+ nitride [Na(dme)3][((Me3Si)2)2U(µ-N)(µ-κ2-C,N-CH2SiMe2NSiMe3)U(N(SiMe3)2)2] (dme = 1,2-dimethoxyethane), [Na(dme)3]-3, provided a versatile precursor for the synthesis of the mixed-ligand U4+-U4+ nitride complex, [Na(dme)3][((Me3Si)2N)3U(µ-N)U(N(SiMe3)2)(OSi(O t Bu)3)], 5. The reactivity of the 1-5 complexes was assessed with CO2, CO, and H2. Complex [NBu4]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{(( t BuO)3SiO)3U}2(µ-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO2 but not H2. The nitride complexes [NBu4]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu4]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the χ versus T plot of 90 K and 55 K). Computational analysis indicates that the U(µ-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.

Small molecule activation by multimetallic uranium complexes supported by siloxide ligands.

The synthesis and reactivity of uranium compounds supported by the tris-tert-butoxysiloxide ligand is surveyed. The multiple binding modes of the tert-butoxysiloxide ligand have proven very well suited to stabilize highly reactive homo- and heteropolymetallic complexes of uranium that have shown an unusual high reactivity towards small molecules such as CO2, CS2, chalcogens and azides. Moreover, these ligands have allowed the isolation of dinuclear nitride and oxide bridged complexes of uranium in various oxidation states. The ability of the tris-tert-butoxysiloxide ligands to trap alkali ions in these nitride or oxide complexes leads to unprecedented ligand based and metal based reduction and functionalization of N2, CO, CO2 and H2.

Correction: Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex.

[This corrects the article DOI: 10.1039/C8SC05721D.].

Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex.

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(v) bis-nitride complex [K2{[U(OSi(O t Bu)3)3]2(µ-N)2}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(µ-N)(µ-N3)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS2 and 1 equiv. of CO2 resulting in N-C bond formation to afford the diuranium(v) complexes [K2{[U(OSi(O t Bu)3)3]2(µ-N)(µ-S)(µ-NCS)}], 5 and [K2{[U(OSi(O t Bu)3)3]2(µ-N)(µ-O)(µ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(µ-CN)(µ-O)(µ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H2 in mild conditions to afford the bis-imido bridged diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(µ-NH)2}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber-Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(v) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium complexes.

The role of bridging ligands in dinitrogen reduction and functionalization by uranium multimetallic complexes.

Cooperativity between metal centres is identified as a crucial step in dinitrogen reduction both for the industrial Haber-Bosch process and for the natural fixation of nitrogen by nitrogenase enzymes, but the mechanism of N2 reduction remains poorly understood. This is in large part because multimetallic complexes that reduce and functionalize dinitrogen in the absence of strong alkali reducing agents are crucial to establish a structure-activity relationship, but remain extremely rare. Recently, we reported a multimetallic nitride-bridged diuranium(III) complex capable of reducing and functionalizing dinitrogen. Here we show that an analogous complex assembled with an oxo instead of a nitride linker also effects the four-electron reduction of dinitrogen, but the reactivity of the resulting oxo-(N2) complex differs significantly from that of the nitride-(N2). Computational studies show a different bonding scheme for the dinitrogen where the bridging nitride does participate in the binding and consequent activation of N2, while the oxide does not.