Uranium-sulfilimine chemistry. Hydrolysis of Cp*2UCl2 with HNSPh2·H2O and the crystal structure of Cp*2UCl(OH)(HNSPh2), a metallocene terminal hydroxy complex of tetravalent uranium

Molecular Models of Atomically Dispersed Uranium at MoS2 Surfaces Reveal Cooperative Mechanism of Water Reduction

Single atoms of uranium supported on molybdenum sulfide surfaces (U@MoS2) have been recently demonstrated to facilitate the hydrogen evolution reaction (HER) through electrocatalysis. Theoretical calculations have predicted uranium hydroxide moieties bound to edge-sulfur atoms of MoS2 as a proposed transition state involved in the HER process. However, the isolation of relevant intermediates involved in this process remains a challenge, rendering mechanistic hypotheses unverified. The present work describes the isolation and characterization of a uranium-hydroxide intermediate on molybdenum sulfide surfaces using [(Cp*3Mo3S4)UCp*], a molecular model of a reduced uranium center supported at MoS2. Mechanistic investigations highlight the metalloligand cooperativity with uranium involved in the water activation pathway. The corresponding uranium-oxo analogue, [(Cp*3Mo3S4)Cp*U(═O)], was also accessed from the hydroxide cluster via hydrogen atom transfer and from [(Cp*3Mo3S4)UCp*] through an alternative direct oxygen atom transfer. These results provide an atomistic perspective on the reactivity of low-valent uranium at molybdenum sulfide surfaces toward water, modeling key intermediates associated with the HER of U@MoS2 catalysts.

Unprecedented pairs of uranium (iv/v) hydroxido and (iv/v/vi) oxido complexes supported by a seven-coordinate cyclen-anchored tris-aryloxide ligand

We present the synthesis and reactivity of a newly developed, cyclen-based tris-aryloxide ligand precursor, namely cyclen(Me)( t-Bu,t-BuArOH)3, and its coordination chemistry to uranium. The corresponding uranium(iii) complex [UIII((OAr t-Bu,t-Bu)3(Me)cyclen)] (1) was characterized by 1H NMR analysis, CHN elemental analysis and UV/vis/NIR electronic absorption spectroscopy. Since no single-crystals suitable for X-ray diffraction analysis could be obtained from this precursor, 1 was oxidized with methylene chloride or silver fluoride to yield [(cyclen(Me)( t-Bu,t-BuArO)3)UIV(X)] (X = Cl (2), F (3)), which were unambiguously characterized and successfully crystallized to gain insight into the molecular structure by single-crystal X-ray diffraction analysis (SC-XRD). Further, the activation of H2O and N2O by 1 is presented, resulting in the U(iv) complex [(cyclen(Me)( t-Bu,t-BuArO)3)UIV(OH)] (4) and the U(v) complex [(cyclen(Me)( t-Bu,t-BuArO)3)UV(O)] (6). Complexes 2, 3, 4, and 6 were characterized by 1H NMR analysis, CHN elemental analysis, UV/vis/NIR electronic absorption spectroscopy, IR vibrational spectroscopy, and SQUID magnetization measurements as well as cyclic voltammetry. Furthermore, chemical oxidation of 3, 4, and 6 with AgF or AgSbF6 was achieved leading to complexes [(cyclen(Me)( t-Bu,t-BuArO)3)UV(F)2] (5), [(cyclen(Me)( t-Bu,t-BuArO)3)UV(OH)][SbF6] (7), and [(cyclen(Me)( t-Bu,t-BuArO)3)UVI(O)][SbF6] (8). Finally, reduction of 7 with KC8 yielded a U(iv) complex, spectroscopically and magnetochemically identified as K[(cyclen(Me)( t-Bu,t-BuArO)3)UIV(O)].

Stabilization of U(III) to Oxidation and Hydrolysis by Encapsulation Using 2.2.2-Cryptand

The electrochemical properties of U(III)-in-crypt (crypt = 2.2.2-cryptand) were examined in dimethylformamide (DMF) and acetonitrile (MeCN) to determine the oxidative stability offered by crypt as a ligand. Cyclic voltammetry revealed a U(III)/U(IV) irreversible oxidation at E_PA= -0.49 V (vs Fe(C₅H₅)₂^+/0) in DMF and at E_PA= -0.31 V (vs Fe(C₅H₅)₂^+/0) in MeCN. The electrochemistry of U(III)-in-crypt complexes in the presence of water was also examined. These studies are supported by crystallographically characterized examples of U(III)-in-crypt complexes as DMF, MeCN, and water adducts.

Synthesis of uranium-in-cryptand complexes

The facile encapsulation of U(iii) and La(iii) by 2.2.2-cryptand (crypt) using simple starting materials is described. Addition of crypt to UI3 and LaCl3 forms the crystallographically-characterizable complexes, [U(crypt)I2]I and [La(crypt)Cl2]Cl. In the presence of water, the U(iii)-aquo adducts, [U(crypt)I(OH2)][I]2 and [U(crypt)I(OH2)][I][BPh4], can be isolated.

Uranium-mediated electrocatalytic dihydrogen production from water

Depleted uranium is a mildly radioactive waste product that is stockpiled worldwide. The chemical reactivity of uranium complexes is well documented, including the stoichiometric activation of small molecules of biological and industrial interest such as H2O, CO2, CO, or N2 (refs 1 - 11), but catalytic transformations with actinides remain underexplored in comparison to transition-metal catalysis. For reduction of water to H2, complexes of low-valent uranium show the highest potential, but are known to react violently and uncontrollably forming stable bridging oxo or uranyl species. As a result, only a few oxidations of uranium with water have been reported so far; all stoichiometric. Catalytic H2 production, however, requires the reductive recovery of the catalyst via a challenging cleavage of the uranium-bound oxygen-containing ligand. Here we report the electrocatalytic water reduction observed with a trisaryloxide U(III) complex [(((Ad,Me)ArO)3mes)U] (refs 18 and 19)--the first homogeneous uranium catalyst for H2 production from H2O. The catalytic cycle involves rare terminal U(IV)-OH and U(V)=O complexes, which have been isolated, characterized, and proven to be integral parts of the catalytic mechanism. The recognition of uranium compounds as potentially useful catalysts suggests new applications for such light actinides. The development of uranium-based catalysts provides new perspectives on nuclear waste management strategies, by suggesting that mildly radioactive depleted uranium--an abundant waste product of the nuclear power industry--could be a valuable resource.

Analysis of Uranium Azide and Nitride Complexes by Atmospheric Pressure Chemical Ionization Mass Spectrometry

Atmospheric pressure chemical ionization mass spectrometry (APCI-MS) has been used to characterize the air-sensitive paramagnetic organouranium azide and nitride complexes [(C5Me5)2UN3(mu-N3)]3 and [(C5Me5)U(mu-I)2]3N, respectively. The trimetallic complex [(C5Me5)U(mu-I)2]3E had been identified by X-ray crystallography, but the data did not definitively identify E as N3- versus O2- or (OH)-, a common problem in heavy-element nitride complexes involving metals with variable oxidation states. A comparison of the 250 degrees C APCI-MS spectra of products made from NaN3 and Na15NNN showed mixed [M]+ and [M + H]+ envelopes at expected ion intensities for the 14N and 15N isotopomers. A compilation of U-C(C5Me5) and U-I bond distance data for U3+ and U4+ is also reported that shows that the ranges for the two oxidation states have significant overlap.

Unique σ-Bond Metathesis of Silylalkynes Promoted by an ansa-Dimethylsilyl and Oxo-Bridged Uranium Metallocene

The tetrachloride salt of uranium reacts with 1 equiv of the lithium ligand Li2[(C5Me4)2SiMe2] in DME to form the complex [eta5-(C5Me4)2SiMe2]UCl2.2LiCl.2DME (1), which undergoes a rapid hydrolysis in toluene to yield the dimeric bridged monochloride, monooxide complex [{[eta5-(C5Me4)2SiMe2]UCl}2(mu-O)(mu-Cl)*Li*1/2DME]2 (2). Metathesis of 2 with BuLi in DME gives the mono-bridged dibutyl complex {[eta5-(C5Me4)2SiMe2]UBu}2(mu-O) (3). Complex 2 was characterized by solid-state X-ray analysis. Complex 3 was found to be an active catalyst for the disproportionation metathesis of TMSCCH (TMS = SiMe3) and the cross-metathesis of TMSCCH or TMSCCTMS with various terminal alkynes. The metathesis of TMSCCH gives TMSCCTMS and HCCH, whereas the cross-metathesis of TMSCCH or TMSCCTMS with terminal alkynes (RCCH) yields TMSCCTMS, TMSCCR, and HCCH. In addition, TMSCCCH3 also was found to react with tBuCCH, yielding TMSCCBut and CH3CCH. A plausible mechanism for the catalytic process is presented.