Tuning Lanthanide Reactivity Towards Small Molecules with Electron-Rich Siloxide Ligands

Samarium- and Ytterbium-Grafted Periodic Mesoporous Silica for Carbon Dioxide Capture and Conversion

Immobilized coordination compounds of Lewis acidic metals are powerful catalytic components of systems for the cycloaddition of CO2 to epoxides that do not require sophisticated coordination frameworks to harness the metal center and modulate its activity. Surface organometallic chemistry (SOMC) is a valuable methodology to prepare well-defined and site-isolated surface complexes and coordination compounds on metal oxides, with ligand environments easily adjustable to a targeted catalytic reaction. In this work, the SOMC methodology is applied to prepare SmII, YbII, and SmIII alkoxide surface complexes on periodic mesoporous (organo)silica of distinct pore symmetry/size for application in the CO2 cycloaddition reaction. The surface complexes are readily accessible by the grafting of the bis(trimethylsilyl)amide precursors LnII[N(SiMe3)2]2(THF)2 (Ln = Sm, Yb) and SmIII[N(SiMe3)2]3, followed by ligand exchange with alcohols (ethanol and neopentanol). The use of periodic mesoporous supports led to hybrid materials with relatively high surface areas and pore sizes, affording good performance in CO2 capture and in the cycloaddition of CO2 to epoxides under mild conditions (60-80 °C, 1-10 bar). In terms of catalytic performance, recyclability, and low amount of added nucleophile TBAX (X = Br, I), the most active materials prepared in this work compare well to a variety of previously reported SOMC-derived surface complexes and to other heterogeneous Lewis acids displaying more elaborate ligand environments.

Isolation and redox reactivity of cerium complexes in four redox states

The chemistry of lanthanides is limited to one electron transfer reactions due to the difficulty of accessing multiple oxidation states. Here we report that a redox-active ligand combining three siloxides with an arene ring in a tripodal ligand can stabilize cerium complexes in four different redox states and can promote multielectron redox reactivity in cerium complexes. Ce(iii) and Ce(iv) complexes [(LO₃)Ce(THF)] (1) and [(LO₃)CeCl] (2) (LO₃ = 1,3,5-(2-OSi(O^tBu)₂C₆H₄)₃C₆H₃) were synthesized and fully characterized. Remarkably the one-electron reduction and the unprecedented two-electron reduction of the tripodal Ce(iii) complex are easily achieved to yield reduced complexes [K(2.2.2-cryptand)][(LO₃)Ce(THF)] (3) and [K₂{(LO₃)Ce(Et₂O)₃}] (5) that are formally "Ce(ii)" and "Ce(i)" analogues. Structural analysis, UV and EPR spectroscopy and computational studies indicate that in 3 the cerium oxidation state is in between +II and +III with a partially reduced arene. In 5 the arene is doubly reduced, but the removal of potassium results in a redistribution of electrons on the metal. The electrons in both 3 and 5 are stored onto δ-bonds allowing the reduced complexes to be described as masked "Ce(ii)" and "Ce(i)". Preliminary reactivity studies show that these complexes act as masked Ce(ii) and Ce(i) in redox reactions with oxidizing substrates such as Ag⁺, CO₂, I₂ and S₈ effecting both one- and two-electron transfers that are not accessible in classical cerium chemistry.

Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes

Trinuclear molecular complexes of europium (II) and ytterbium(II) [Ln₃{(Ph₂SiO)₂O}₃(THF)₆], 1-Ln₃L₃ (Ln = Eu and Yb), supported by the dianionic tetraphenyl disiloxanediolate ligand, were synthesized via protonolysis of the [Ln{N(SiMe₃)₂}₂(THF)₂] complexes. In contrast, the reaction of [Sm{N(SiMe₃)₂}₂(THF)₂] with the (Ph₂SiOH)₂O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III) complex [Sm₃{(Ph₂SiO)₂O}₃{N(SiMe₃)₂}(THF)₄], 2-Sm₃L₃, which was crystallographically characterized. The Eu(II) complex 1-Eu₃L₃ displays weak ferromagnetic coupling between the Eu(II) metal centers (J = 0.1035 cm^-1). The addition of 3 equiv of (Ph₂SiOK)₂O to 1-Eu₃L₃ resulted in the formation of the polynuclear Eu(II) dimer of dimers [K₄Eu₂{(Ph₂SiO)₂O}₄(Et₂O)₂]₂, 3-Eu₂L₄. Complexes 1-Ln₃L₃ (Ln = Eu and Yb) are stable in solution at room temperature, while 3-Eu₂L₄ shows higher reactivity and rapidly decomposes to give the mixed-valent Eu(II)/Eu(III) species [K₃Eu₂{(Ph₂SiO)₂O}₄], 4-Eu₂L₄. Complex 1-Yb₃L₃ affects the slow reductive disproportionation of carbon dioxide, but 1-Eu₃L₃ does not display any reactivity toward CO₂. However, the presence of one additional (Ph₂SiO^-)₂O per Eu(II) metal center in 3-Eu₂L₄ increases dramatically the reductive ability of the Eu(II) metal centers, affording the first example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO₂ by 3-Eu₂L₄ is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO_2.

Revisiting Reduction of CO2 to Oxalate with First-Row Transition Metals: Irreproducibility, Ambiguous Analysis, and Conflicting Reactivity

Construction of higher C_≥2 compounds from CO₂ constitutes an attractive transformation inspired by nature's strategy to build carbohydrates. However, controlled C-C bond formation from carbon dioxide using environmentally benign reductants remains a major challenge. In this respect, reductive dimerization of CO₂ to oxalate represents an important model reaction enabling investigations on the mechanism of this simplest CO₂ coupling reaction. Herein, we present common pitfalls encountered in CO₂ reduction, especially its reductive coupling, based on established protocols for the conversion of CO₂ into oxalate. Moreover, we provide an example to systematically assess these reactions. Based on our work, we highlight the importance of utilizing suitable orthogonal analytical methods and raise awareness of oxidative reactions that can likewise result in the formation of oxalate without incorporation of CO₂. These results allow for the determination of key parameters, which can be used for tailoring of prospective catalytic systems and will promote the advancement of the entire field.

Structure, reactivity and luminescence studies of triphenylsiloxide complexes of tetravalent lanthanides

Among the 14 lanthanide elements (Ce–Lu), until recently, the tetravalent oxidation state was readily accessible in solution only for cerium while Pr(iv), Nd(iv), Dy(iv) and Tb(iv) had only been detected in the solid state. The triphenylsiloxide ligand recently allowed the isolation of molecular complexes of Tb(iv) and Pr(iv) providing an unique opportunity of investigating the luminescent properties of Ln(iv) ions. Here we have expanded the coordination studies of the triphenylsiloxide ligand with Ln(iii) and Ln(iv) ions and we report the first observed luminescence emission spectra of Pr(iv) complexes which are assigned to a ligand-based emission on the basis of the measured lifetime and computational studies. Binding of the ligand to the Pr(iv) ion leads to an unprecedented large shift of the ligand triplet state which is relevant for future applications in materials science.

The first observed luminescence emission spectra of Pr(iv) complexes are assigned to a ligand-based emission. Binding of the triphenylsiloxide ligand to the Pr(iv) ion leads to an unprecedented large red shift of its triplet state.

Reductive Reactivity of the 4f75d1 Gd(II) Ion in {GdII[N(SiMe3)2]3}-: Structural Characterization of Products of Coupling, Bond Cleavage, Insertion, and Radical Reactions

The reductive reactivity of a Ln(II) ion with a nontraditional 4fⁿ5d¹ electron configuration has been investigated by studying reactions of the {Gd^II(N(SiMe₃)₂)₃]}^- anion with a variety of reagents that survey the many reaction pathways available to this ion. The chemistry of both [K(18-c-6)₂]⁺ and [K(crypt)]⁺ salts (18-c-6 = 18-crown-6; crypt = 2.2.2-cryptand) was examined to study the effect of the countercation. CS₂ reacts with the crown salt [K(18-c-6)₂][Gd(NR₂)₃] (1) to generate the bimetallic (CS₃)^2- complex {[K(18-c-6)](μ₃-CS₃-κS,κ²S',S'')Gd(NR₂)₂]}₂, which contains two trithiocarbonate dianions that bridge Gd(III) centers and a potassium ion coordinated by 18-c-6. In contrast, the only crystalline product isolated from the reaction of CS₂ with the crypt salt [K(crypt)][Gd(NR₂)₃] (2) is [K(crypt)]{(R₂N)₂Gd[SCS(CH₂)Si(Me₂)N(SiMe₃)-κN,κS]}, which has a CS₂ unit inserted into a cyclometalated amide ligand. Complexes 1 and 2 reductively couple pyridine to form bridging dipyridyl moieties, (NC₅H₄-C₅H₄N)^2-, that generate bimetallic complexes differing only in the countercation, {[K(18-c-6)(C₅H₅N)₂]}₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]} and [K(crypt)]₂{[(R₂N)₃Gd]₂[μ-(NC₅H₄-C₅H₄N)₂]}. Complexes 1 and 2 also show similar reactivity with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the (TEMPO)^- complexes [K(18-c-6)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)] and [K(crypt)][(R₂N)₃Gd(η¹-ONC₅H₆Me₄)], respectively. The first example of a bimetallic coordination complex containing a Bi-Gd bond, [K(crypt)][(R₂N)₃Gd(BiPh₂)], was obtained by treating 2 with BiPh₃.

Single metal four-electron reduction by U(ii) and masked "U(ii)" compounds

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U}₂(μ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η ²-C₂Ph₂){N(SiMe₃)₂}₃], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)₂{N(SiMe₃)₂}₃], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe₃)₂}₃], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N₂Ph₂){N(SiMe₃)₂}₃], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a "clear-cut" single metal four-electron transfer in f-element chemistry.

Lanthanoid Complexes as Molecular Materials: The Redox Approach

The development of molecular materials with novel functionality offers promise for technological innovation. Switchable molecules that incorporate redox-active components are enticing candidate compounds due to their potential for electronic manipulation. Lanthanoid metals are most prevalent in their trivalent state and usually redox-activity in lanthanoid complexes is restricted to the ligand. The unique electronic and physical properties of lanthanoid ions have been exploited for various applications, including in magnetic and luminescent materials as well as in catalysis. Lanthanoid complexes are also promising for applications reliant on switchability, where the physical properties can be modulated by varying the oxidation state of a coordinated ligand. Lanthanoid-based redox activity is also possible, encompassing both divalent and tetravalent metal oxidation states. Thus, utilization of redox-active lanthanoid metals offers an attractive opportunity to further expand the capabilities of molecular materials. This review surveys both ligand and lanthanoid centered redox-activity in pre-existing molecular systems, including tuning of lanthanoid magnetic and photophysical properties by modulating the redox states of coordinated ligands. Ultimately the combination of redox-activity at both ligands and metal centers in the same molecule can afford novel electronic structures and physical properties, including multiconfigurational electronic states and valence tautomerism. Further targeted exploration of these features is clearly warranted, both to enhance understanding of the underlying fundamental chemistry, and for the generation of a potentially important new class of molecular material.

Tuning the coordination sphere of octahedral Dy(iii) complexes with silanolate/stannanolate ligands: synthesis, structures and slow relaxation of the magnetization

This study reports four SMMs based on silanolate or stannanolate ligands cis-[Dy(OSiPh3)2(THF)4][BPh4]·THF·C6H6 (1), cis-[Dy(OSnPh3)2(THF)4][BPh4]·THF·C6H6·C6H14 (2), fac-[Dy(OSiPh3)3(THF)3]·THF (3) and fac-[Dy(OSiPh3)3(bipy)(THF)]·THF (4).

Alkali-metal- and halide-free dinuclear mixed-valent samarium and europium complexes

We describe the synthesis of mixed-valent (Ln(ii)/Ln(iii)) dilanthanide complexes supported by a calix[4]pyrrole ligand. The complexes are obtained by one-electron reduction of Ln(iii)/Ln(iii) complexes and are alkali-metal- and halide-free. The complexes are designed to activate small molecules by taking advantage of both the base and the one-electron reductant contained in their structure. We demonstrate a proof-of-principle concept of this mechanism by activating water and silanol.

A Dimeric Chromium(II) Pincer as an Electron Shuttle for N=N Bond Scission

Reduction of the bis-pyrazolyl pyridine complex [CrL]₂ with 4 KC₈ , followed by addition of one azobenzene (overall mole ratio 1:4:1), PhNNPh, transfers reducing equivalents to three azobenzenes, to form [K₃ Cr(PhNNPh)₃ ]. This has three κ² PhNNPh^2- ligands and K⁺ bound to nitrogen atoms of azobenzene. When the stoichiometry is modified to 1:4:3, the product is changed to [K₂ CrL(PhNNPh)₂ ], which has C₂ symmetry except for the intimate ion pairing of two K⁺ ions to reduced azobenzene nitrogen atoms, and to pyrazolate and phenyl rings. The origin of the observed delivery of reducing equivalents to several, not to a single N=N bond, is traced to the resistance of the one-electron-reduced substrate to receiving a second electron, and is thus a general phenomenon. [CrL]₂ alone is shown to be a two-electron reductant towards benzo[c]cinnoline (BCC) resulting in a product of formula [Cr₂ L₂ (BCC)], in which the reducing equivalents originate purely from Cr^II . An analogous study of the reaction of [CrL]₂ with azobenzene yields [Cr₂ L₂ (PhNNPh)(THF)], an adduct in which one THF has displaced one of four hydrazide nitrogen/Cr bonds. Together these illustrate different modes for the Cr₂ L₂ unit to bind and reduce the N=N bond. Collectively, these results show that two divalent Cr, without added K⁰ , have the ability to reduce the N=N bond. Further KC₈ reduction of preformed Cr₂ L₂ (RNNR) inevitably gives products in which K⁺ stabilizes the charge in the increasingly electron-rich nitrogen atoms, in a phenomenon which mimics proton coupled electron transfer: K⁺ performs the role of H⁺ . A least-squares fit of the two singly reduced DFT structures shows that the only major change is a re-orientation of one of the two phenyl rings in order to avoid repulsion with potassium but to still allow interaction of that phenyl π system with K⁺ . This shows both the impact of K⁺ , being modest to nitrogen/chromium interactions, but nevertheless accommodating some π donation of phenyl to potassium. Finally, delivering increasing equivalents of KC₈ leads to complete cleavage of the N=N bond, and both N bind to three Cr^II . The varied impacts of the K⁺ electrophile on NN multiple bond reduction is discussed.

Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

The homoleptic pyrazolate complexes [CeIII 4 (Me2 pz)12 ] and [CeIV (Me2 pz)4 ]2 quantitatively insert CO2 to give [CeIII 4 (Me2 pz⋅CO2 )12 ] and [CeIV (Me2 pz⋅CO2 )4 ], respectively (Me2 pz=3,5-dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO2 per [CeIV (Me2 pz)4 ] complex could be inserted to give trimetallic [Ce3 (Me2 pz)9 (Me2 pz⋅CO2 )3 (thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO2 under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co-catalyst tetra-n-butylammonium bromide (TBAB).

Structure and small molecule activation reactivity of a metallasilsesquioxane of divalent ytterbium

The first metallasilsesquioxane of a divalent lanthanide was synthetized and structurally characterized. The dinuclear Yb(ii) complex effects the two electrons reduction of azobenzene, and the selective CO₂ reduction to CO and carbonate.

Carbon dioxide reduction by lanthanide(iii) complexes supported by redox-active Schiff base ligands

The reduction of Ln(iii)-trensal complexes allows to store electrons, that become available for CO₂ reduction, trough the formation of new C–C bonds.

Carbonyl group and carbon dioxide activation by rare-earth-metal complexes

The rare-earth elements (Ln = Sc, Y, La-Lu) are widely used in stoichiometric and catalytic carbonyl group transformations. Sufficient availability, non-toxicity, high oxophilicity, tunable ion size/Lewis acidity and enhanced ligand exchangeability have been major driving factors for their successful implementation. Routinely employed reagents for stoichiometric carbonyl group transformations are divalent ytterbium and samarium compounds (e.g., ketone reduction), bimetallic CeCl3/LiR (C-C coupling), or ceric ammonium nitrate CAN (cyclic ketone oxidation). Rare-earth-metal triflates, and in particular Sc(OTf)3, are prominent examples of Lewis acid catalysts for versatile use in organic synthesis (e.g., Aldol and Michael reactions). Moreover, Ln(ii) and Ln(iii) complexes efficiently catalyze the (co)polymerization of carbonyl group-containing monomers including lactones, lactides, acrylates, and carbon dioxide. Featuring the most notorious greenhouse gas, CO2 is currently assessed as a cheap, abundant, and non-toxic C1 building block. Ln(iii) complexes are not only capable of efficient CO2 capture via reversible insertion but also of CO2 activation for catalytic conversions (copolymerization/cycloaddition with epoxides). This perspective focuses on structurally elucidated Ln complexes resulting from ketone or carbonyl derivative activation/insertion as well as carbon dioxide insertion products. The respective compounds will be sorted by structural motifs and, if applicable, details on reactivity and feasibility of catalytic reactions are presented. The article is subdivided in three parts: (i) donor and insertion products of ketones and aldehydes, (ii) redox-enhanced activation of carbonyl derivatives, and (iii) CO2 insertion/redox products and homogeneous catalytic conversion.

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.

Homo- and heterodehydrocoupling of phosphines mediated by alkali metal catalysts

Catalytic chemistry that involves the activation and transformation of main group substrates is relatively undeveloped and current examples are generally mediated by expensive transition metal species. Herein, we describe the use of inexpensive and readily available tBuOK as a catalyst for P-P and P-E (E = O, S, or N) bond formation. Catalytic quantities of tBuOK in the presence of imine, azobenzene hydrogen acceptors, or a stoichiometric amount of tBuOK with hydrazobenzene, allow efficient homodehydrocoupling of phosphines under mild conditions (e.g. 25 °C and < 5 min). Further studies demonstrate that the hydrogen acceptors play an intimate mechanistic role. We also show that our tBuOK catalysed methodology is general for the heterodehydrocoupling of phosphines with alcohols, thiols and amines to generate a range of potentially useful products containing P-O, P-S, or P-N bonds.

Rare-earth metal and actinide organoimide chemistry

Elaborate synthesis schemes pave the way to f-element and group 3 complexes with multiply bonded imido ligands displaying intriguing reactivity.

Carbon dioxide reduction by dinuclear Yb(ii) and Sm(ii) complexes supported by siloxide ligands

Low-coordinate dinuclear lanthanide complexes supported by siloxides effect the reduction of carbon dioxide to both carbonate and oxalate, but the cooperative binding of CO2 to the two Ln(ii) cations in the dimer favours oxalate formation.

Unveiling the Takai Olefination Reagent via Tris( tert-butoxy)siloxy Variants

The elusive Takai olefination reagent, namely, the iodo-methylidene Cr(III) complex [Cr₂Cl₄(CHI)(thf)₄], has been isolated by careful handling of the reaction between CrCl₂ and CHI₃ in THF at -35 °C. Alternatively, treatment of [Cr(OSi(O tBu)₃)₂] with CHI₃ gave the mixed-valent dihalido-methanide complex [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)], featuring a Cr(III)-CHI₂ moiety. In the presence of TMEDA nucleophilic attack at CHI₂ occurred generating the zwitterionic species [Cr^III(OSi(O tBu)₃)₂(tmeda-CHI)][I]. Complexes [Cr₂Cl₄(CHI)(thf)₄] and [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)] were screened for their ability to induce monohalido olefination of benzaldehyde. Remarkably, both complexes promote olefination, with [Cr₂Cl₄(CHI)(thf)₄] accomplishing the same E selectivity as Takai 's original mixture. Complex [Cr^II/III₂I₂(OSi(O tBu)₃)₂(CHI₂)], however, appeared to give preferentially Z isomer, corroborating the monoiodo-methylidene species Cr(III)-CHI-Cr(III) as the active olefination component of the original in situ generated Takai reagent mixture.

Phenylacetylene and Carbon Dioxide Activation by an Organometallic Samarium Complex

Small molecule activation is a topic of growing importance and the use of low-valent f-elements to perform these reactions is nowadays well established. The complex Cptt2Sm(thf) (1, Cptt = 1,3-(tBu)2Cp) is shown to activate the alkyne C–H bond of phenylacetylene to form the Cptt2Sm(C≡C–Ph)(thf) complex. The subsequent reaction of this Sm(III) complex with CO2 leads to the CO2 insertion, yielding a dimeric [Cptt2Sm(O2C–C≡C–Ph)]2 complex (2), in which the carbon dioxide has been inserted in the Sm–C bond. Along with the experimental chemical structure analysis, theoretical calculations have been performed in order to rationalize the formation of 1 and 2.

A tetranuclear samarium(ii) inverse sandwich from direct reduction of toluene by a samarium(ii) siloxide

The dinuclear Sm^II complex [Sm₂L₄(dme)] (L = OSi(OtBu)₃) reacts slowly with toluene, resulting in the isolation of the triple decker arene-bridged Sm^II complex [{Sm₂L₃}₂(μ-η⁶:η⁶-C₇H₈)] in 44% yield. This reactivity provides the first example of an unambiguous arene reduction by an isolated Sm^II species.

Small molecule activation with divalent samarium triflate: a synergistic effort to cleave O2

The reaction of divalent samarium triflate with O₂ leads to the entire reductive cleavage of O₂, highlighting a synergistic effort since four electrons, and therefore four samarium centers, are necessary.

Redox-enhanced hemilability of a tris(tert-butoxy)siloxy ligand at cerium

Combined structural/electrochemical/computational studies of ceric Ce[OSi(OtBu)₃]₄ and cerous [Ce{OSi(OtBu)₃}₄][K(2.2.2-crypt)] suggest a redox-modulated coordination switch of a tris(tert-butoxy)siloxy ligand.

Evidence for single metal two electron oxidative addition and reductive elimination at uranium

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.

Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple-Decker Arene-Bridged Cerium(II) Sandwich

Organometallic multi-decker sandwich complexes containing f-elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the Ce^III siloxide complex, [KCeL₄ ] (1; L=OSi(OtBu)₃ ), with excess potassium in a THF/toluene mixture afforded a quadruple-decker arene-bridged complex, [K(2.2.2-crypt)]₂ [{(KL₃ Ce)(μ-η⁶ :η⁶ -C₇ H₈ )}₂ Ce] (3). The structure of 3 features a [Ce(C₇ H₈ )₂ ] sandwich capped by [KL₃ Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of Ce^II ions involved in δ bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi-decker complex and the first containing non-classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf=O₃ SCF₃ ) yielded the Ce^IV complex, [CeL₄ ] (2), showing that siloxide ligands can stabilize Ce in three oxidation states.

Terminal Uranium(V/VI) Nitride Activation of Carbon Dioxide and Carbon Disulfide: Factors Governing Diverse and Well-Defined Cleavage and Redox Reactions

The reactivity of terminal uranium(V/VI) nitrides with CE₂ (E=O, S) is presented. Well-defined C=E cleavage followed by zero-, one-, and two-electron redox events is observed. The uranium(V) nitride [U(Tren^TIPS )(N)][K(B15C5)₂ ] (1, Tren^TIPS =N(CH₂ CH₂ NSiiPr₃ )₃ ; B15C5=benzo-15-crown-5) reacts with CO₂ to give [U(Tren^TIPS )(O)(NCO)][K(B15C5)₂ ] (3), whereas the uranium(VI) nitride [U(Tren^TIPS )(N)] (2) reacts with CO₂ to give isolable [U(Tren^TIPS )(O)(NCO)] (4); complex 4 rapidly decomposes to known [U(Tren^TIPS )(O)] (5) with concomitant formation of N₂ and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS₂ to give [U(Tren^TIPS )(κ² -CS₃ )][K(B15C5)₂ ] (6), 2, and [K(B15C5)₂ ][NCS] (7), whereas 2 reacts with CS₂ to give [U(Tren^TIPS )(NCS)] (8) and "S", with the latter trapped as Ph₃ PS. Calculated reaction profiles reveal outer-sphere reactivity for uranium(V) but inner-sphere mechanisms for uranium(VI); despite the wide divergence of products the initial activation of CE₂ follows mechanistically related pathways, providing insight into the factors of uranium oxidation state, chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include common one-electron reactions and a less-common two-electron redox event. Caution, we suggest, is warranted when utilising CS₂ as a reactivity surrogate for CO₂ .

Synthesis and derivatisation of ceric tris(tert-butoxy)siloxides

Heteroleptic ceric Ce[OSi(OtBu)₃]₃Cl(thf) is straightforwardly obtained from cerous Ce[OSi(OtBu)₃]₃(thf)₂ and trityl chloride and gives access to the terminal siloxy/methoxy derivative Ce[OSi(OtBu)₃]₃(OCH₃)(thf)₂.

Elucidating the Mechanism of Uranium Mediated Diazene N═N Bond Cleavage

Investigation into the reactivity of reduced uranium species toward diazenes has revealed key intermediates in the four-electron cleavage of azobenzene. Trivalent Tp*₂U(CH₂Ph) (1a) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) and Tp*₂U(2,2'-bpy) (1b) both perform the two-electron reduction of diazenes affording η²-hydrazido complexes Tp*₂U(AzBz) (2-AzBz) (AzBz = azobenzene) and Tp*₂U(BCC) (2-BCC) (BCC = benzo[c]cinnoline) in contrast to precursors of the bis(Cp*) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide) ligand framework. The four-electron cleavage of diazenes to give trans-bis(imido) species was possible by using Cp*U(^MesPDI^Me)(THF) (3) (^MesPDI^Me = 2,6-((Mes)N═CMe)₂-C₅H₃N, Mes = 2,4,6-trimethylphenyl), which is supported by a highly reduced trianionic chelate that undergoes electron transfer. This proceeds via concerted addition at a single uranium center supported by both a crossover experiment and through addition of an asymmetrically substituted diazene, Ph-N═N-Tol. Further investigation of 3 and its substituted analogue, Cp*U(^tBu-^MesPDI^Me)(THF) (3-^tBu) (^tBu-^MesPDI^Me = 2,6-((Mes)N═CMe)₂-p-C(CH₃)₃-C₅H₂N), with benzo[c]cinnoline, revealed that the four-electron cleavage occurs first by a single electron reduction of the diazene with the redox chemistry performed solely at the redox-active pyridine(diimine) to form dimeric [Cp*U(BCC)(^MesHPDI^Me)]₂ (5) and Cp*U(BCC)(^tBu-^MesPDI^Me) (6). While a transient pyridine(diimine) triplet diradical in the formation of 5 results in H atom abstraction and p-pyridine coupling, the tert-butyl moiety in 6 allows for electronic rearrangement to occur, precluding deleterious pyridine-radical coupling. The monomeric analogue of 5, Cp*U(BCC)(^MesPDI^Me) (7), was synthesized via salt metathesis from Cp*UI(^MesPDI^Me) (3-I). All complexes have been characterized by ¹H NMR and electronic absorption spectroscopies, X-ray diffraction, and, where pertinent, EPR spectroscopy. Further, the electronic structures of 3-I, 5, and 7 have been investigated by SQUID magnetometry.

A Mononuclear Uranium(IV) Single-Molecule Magnet with an Azobenzene Radical Ligand

A tetravalent uranium compound with a radical azobenzene ligand, namely, [{(SiMe2 NPh)3 -tacn}U(IV) (η(2) -N2 Ph2 (.) )] (2), was obtained by one-electron reduction of azobenzene by the trivalent uranium compound [U(III) {(SiMe2 NPh)3 -tacn}] (1). Compound 2 was characterized by single-crystal X-ray diffraction and (1) H NMR, IR, and UV/Vis/NIR spectroscopy. The magnetic properties of 2 and precursor 1 were studied by static magnetization and ac susceptibility measurements, which for the former revealed single-molecule magnet behaviour for the first time in a mononuclear U(IV) compound, whereas trivalent uranium compound 1 does not exhibit slow relaxation of the magnetization at low temperatures. A first approximation to the magnetic behaviour of these compounds was attempted by combining an effective electrostatic model with a phenomenological approach using the full single-ion Hamiltonian.

A Dimetalloxycarbene Bonding Mode and Reductive Coupling Mechanism for Oxalate Formation from CO2

We describe the stable and isolable dimetalloxycarbene [(TiX3 )2 (μ2 -CO2 -κ(2) C,O:κO')] 5, where X=N-(tert-butyl)-3,5-dimethylanilide, which is stabilized by fluctuating μ2 -κ(2) C,O:κ(1) O' coordination of the carbene carbon to both titanium centers of the dinuclear complex 5, as shown by variable-temperature NMR studies. Quantum chemical calculations on the unmodified molecule indicated a higher energy of only +10.5 kJ mol(-1) for the μ2 -κ(1) O:κ(1) O' bonding mode of the free dimetalloxycarbene compared to the μ2 -κ(2) C,O:κ(1) O' bonding mode of the masked dimetalloxycarbene. The parent cationic bridging formate complex [(TiX3 )2 (μ2 -OCHO-κO:κO')][B(C6 F5)4], 4[B(C6 F5)4], was simply deprotonated with the strong base K(N(SiMe3 )2 ) to give 5. Complex 5 reacts smoothly with CO2 to generate the bridging oxalate complex [(TiX3 )2 (μ2 -C2 O4 -κO:κO'')], 6, in a C-C bond formation reaction commonly anticipated for oxalate formation by reductive coupling of CO2 on low-valent transition-metal complexes.