Double Reduction of 4,4′-Bipyridine and Reductive Coupling of Pyridine by Two Thorium(III) Single-Electron Transfers

Reactivity of Strontium Hydride Supported by the Superbulky Hydrotris(pyrazolyl)borate Ligand

Hydrogenolysis of [(TpAd,iPr)Sr{CH(SiMe3)2}] (1) (TpAd,iPr = hydrotris(3-adamantyl-5-isopropyl-pyrazolyl)borate) in hexane solution under 20 atm of H2 allowed for the isolation of strontium hydride [(TpAd,iPr)Sr(μ-H)]2 (2) in good yield. Complex 2 exhibits the dimeric nature in solid state, featuring two different bond modes between the Sr center and TpAd,iPr ligand. Treatment of complex 2 with PhC(H)═NtBu or PhCH2Bpin (Bpin = pinacolateborane) afforded the strontium amide complex [(TpAd,iPr)Sr{N(CH2Ph)(tBu)}] (4) and hydroborate complex [(TpAd,iPr)Sr{μ-HBpin(CH2Ph)}] (5), respectively. Reactions of complex 2 with 2-picoline, 2-phenylquinoline, or 2-phenylpyridine led to the formation of strontium 2-pyridylmethylene/2-picoline complex [(TpAd,iPr)Sr(2-CH2-Py)(2-picoline)] (6), reductively coupling diphenyl-biquinolide complex [{(TpAd,iPr)Sr}2(2,2'-Ph2-4,4'-dihydro-4,4'-biquinolide)] (7), and diphenyl-bipyridyl radical complex [(TpAd,iPr)Sr(6,6'-Ph2-2,2'-bipyridyl)] (8), separately. All of the complexes have been well characterized, including NMR spectrum and single-crystal X-ray analysis.

Accessing a Highly Reducing Uranium(III) Complex through Cyclometalation

U(IV) cyclometalated complexes have shown rich reactivity, but their low oxidation state analogues still remain rare. Herein, we report the isolation of [K(2.2.2-cryptand)][UIII{N(SiMe3)2}2(κ2-C,N-CH2SiMe2NSiMe3)], 1, from the reduction of [UIII{N(SiMe)2}3] with KC8 and 2.2.2-cryptand at room temperature. Cyclic voltammetry studies demonstrate that 1 has a reduction potential similar to that of the previously reported [K(2.2.2-cryptand)][UII{N(SiMe)2}3] (Epc = -2.6 V versus Fc+/0 and Epc = -2.8 V versus Fc+/0, respectively). Complex 1, indeed, shows similar reducing abilities upon reactions with 4,4'-bipyridine, 2,2'-bipyridine, and 1-azidoadamantane. Interestingly, 1 was also found to be the first example of a mononuclear U(III) complex that is capable of reducing pyridine. In addition, it is shown that a wide variety of substrates can be inserted into the U-C bond, forming new U(III) metallacycles. These results highlight that cyclometalated U(III) complexes can serve as versatile precursors for a broad range of reactivity and for assembling a variety of novel chemical architectures.

Two-Electron Redox Reactivity of Thorium Supported by Redox-Active Tripodal Frameworks

The high stability of the + IVoxidation state limits thorium redox reactivity. Here we report the synthesis and the redox reactivity of two Th(IV) complexes supported by the arene-tethered tris(siloxide) tripodal ligands [(KOSiR2 Ar)3 -arene)]. The two-electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(Ot Bu)2 Ar)3 -arene)(THF)2 ] (2OtBu ) and [K(2.2.2-cryptand)][Th((OSiPh2 Ar)3 -arene)(THF)2 ](2Ph -crypt) where the formal oxidation state of Th is +II. Structural and computational studies indicate that the reduction occurred at the arene anchor of the ligand. The robust tripodal frameworks store in the arene anchor two electrons that become available at the metal center for the two-electron reduction of a broad range of substrates (N2 O, COT, CHT, Ph2 N2 , Ph3 PS and O2 ) while retaining the ligand framework. This work shows that arene-tethered tris(siloxide) tripodal ligands allow implementation of two-electron redox chemistry at the thorium center while retaining the ligand framework unchanged.

Altering the spectroscopy, electronic structure, and bonding of organometallic curium(III) upon coordination of 4,4'-bipyridine

Structural and electronic characterization of (Cp'₃Cm)₂(μ-4,4'-bpy) (Cp' = trimethylsilylcyclopentadienyl, 4,4'-bpy = 4,4'-bipyridine) is reported and provides a rare example of curium-carbon bonding. Cp'₃Cm displays unexpectedly low energy emission that is quenched upon coordination by 4,4'-bipyridine. Electronic structure calculations on Cp'₃Cm and (Cp'₃Cm)₂(μ-4,4'-bpy) rule out significant differences in the emissive state, rendering 4,4'-bipyridine as the primary quenching agent. Comparisons of (Cp'₃Cm)₂(μ-4,4'-bpy) with its samarium and gadolinium analogues reveal atypical bonding patterns and electronic features that offer insights into bonding between carbon with f-block metal ions. Here we show the structural characterization of a curium-carbon bond, in addition to the unique electronic properties never before observed in a curium compound.

Bonding and Reactivity in Terminal versus Bridging Arenide Complexes of Thorium Acting as ThII Synthons

Thorium redox chemistry is extremely scarce due to the high stability of Th^IV . Here we report two unique examples of thorium arenide complexes prepared by reduction of a Th^IV -siloxide complex in presence of naphthalene, the mononuclear arenide complex [K(OSi(O^t Bu)₃ )₃ Th(η⁶ -C₁₀ H₈ )] (1) and the inverse-sandwich complex [K(OSi(O^t Bu)₃ )₃ Th]₂ (μ-η⁶ ,η⁶ -C₁₀ H₈ )] (2). The electrons stored in these complexes allow the reduction of a broad range of substrates (N₂ O, AdN₃ , CO₂ , HBBN). Higher reactivity was found for the complex 1 which reacts with the diazoolefin IDipp=CN₂ to yield the unexpected Th^IV amidoalkynyl complex 5 via a terminal N-heterocyclic vinylidene intermediate. This work showed that arenides can act as convenient redox-active ligands for implementing thorium-ligand cooperative multielectron transfer and that the reactivity can be tuned by the arenide binding mode.

A photoinduced mixed valence photoswitch

The ground state and photoinduced mixed valence states (GSMV and PIMV, respectively) of a dinuclear (Dp⁴⁺) ruthenium(II) complex bearing 2,2'-bipyridine ancillary ligands and a 2,2':4',4'':2'',2'''-quaterpyridine (Lp) bridging ligand were investigated using femtosecond and nanosecond transient absorption spectroscopy, electrochemistry and density functional theory. It was shown that the electronic coupling between the transiently light-generated Ru(II) and Ru(III) centers is H_DA ∼ 450 cm^-1 in the PIMV state, whereas the electrochemically generated GSMV state showed H_DA ∼ 0 cm^-1, despite virtually identical Ru-Ru distances. This stemmed from the changes in dihedral angles between the two bpy moieties of Lp, estimated at 30° and 4° for the GSMV and PIMV states, respectively, consistent with a through-bond rather than a through-space mechanism. Electronic coupling can be turned on by using visible light excitation, making Dp⁴⁺ a competitive candidate for photoswitching applications. A novel strategy to design photoinduced charge transfer molecular switches is proposed.

Cyclopentadienyl coordination induces unexpected ionic Am-N bonding in an americium bipyridyl complex

Variations in bonding between trivalent lanthanides and actinides is critical for reprocessing spent nuclear fuel. The ability to tune bonding and the coordination environment in these trivalent systems is a key factor in identifying a solution for separating lanthanides and actinides. Coordination of 4,4'-bipyridine (4,4'-bpy) and trimethylsilylcyclopentadienide (Cp') to americium introduces unexpectedly ionic Am-N bonding character and unique spectroscopic properties. Here we report the structural characterization of (Cp'₃Am)₂(μ - 4,4'-bpy) and its lanthanide analogue, (Cp'₃Nd)₂(μ - 4,4'-bpy), by single-crystal X-ray diffraction. Spectroscopic techniques in both solid and solution phase are performed in conjunction with theoretical calculations to probe the effects the unique coordination environment has on the electronic structure.

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.

Reductive Coupling of (Fluoro)pyridines by Linear 3d-Metal(I) Silylamides of Cr-Co: A Tale of C-C Bond Formation, C-F Bond Cleavage and a Pyridyl Radical Anion

Herein, we disclose the facile reduction of pyridine (and its derivatives) by linear 3d-metal(I) silylamides (M=Cr-Co). This reaction resulted in intermolecular C-C coupling to give dinuclear metal(II) complexes bearing a bridging 4,4'-dihydrobipyridyl ligand. For iron, we demonstrated that the C-C coupling is reversible in solution, either directly or by reaction with substrates, via a presumed monomeric metal(II) complex bearing a pyridyl radical anion. In the course of this investigation, we also observed that the dinuclear metal(II) complex incorporating iron facilitated the isomerisation of 1,4-cyclohexadiene to 1,3-cyclohexadiene as well as equimolar amounts of benzene and cyclohexene. Furthermore, we synthesised and structurally characterised a non-3d-metal-bound pyridyl radical anion. The reactions of the silylamides with perfluoropyridine led to C-F bond cleavage with the formation of metal(II) fluoride complexes of manganese, iron and cobalt along with the homocoupling or reductive degradation of the substrate. In the case of cobalt, the use of lesser fluorinated pyridines led to C-F bond cleavage but no homocoupling. Overall, in this paper we provide insights into the multifaceted behaviour of simple (fluoro)pyridines in the presence of moderately to highly reducing metal complexes.

Delivery of a Masked Uranium(II) by an Oxide-Bridged Diuranium(III) Complex

Oxide is an attractive linker for building polymetallic complexes that provide molecular models for metal oxide activity, but studies of these systems are limited to metals in high oxidation states. Herein, we synthesized and characterized the molecular and electronic structure of diuranium bridged U^III /U^IV and U^III /U^III complexes. Reactivity studies of these complexes revealed that the U-O bond is easily broken upon addition of N-heterocycles resulting in the delivery of a formal equivalent of U^III and U^II , respectively, along with the uranium(IV) terminal-oxo coproduct. In particular, the U^III /U^III oxide complex effects the reductive coupling of pyridine and two-electron reduction of 4,4'-bipyridine affording unique examples of diuranium(III) complexes bridged by N-heterocyclic redox-active ligands. These results provide insight into the chemistry of low oxidation state metal oxides and demonstrate the use of oxo-bridged U^III /U^III complexes as a strategy to explore U^II reactivity.

The experimental determination of Th(iv)/Th(iii) redox potentials in organometallic thorium complexes

The first Th^IV/Th^III redox couple values have been determined experimentally using cyclic voltammetry (CV), which has been facilitated by the use of [ⁿBu₄N][BPh₄] as a supporting electrolyte in THF. Th(iv) and Th(iii) metallocene compounds have been studied and their redox couple values are in the range of -2.96 V to -3.32 V vs FeCp₂^+/0.

Actinide Organometallic Complexes with π-Ligands

Recent developments and results from the organometallic chemistry of the actinides are reviewed. In the last one and a half years the structural data of about 15 organometallic complexes of transuranium actinides (Np or Pu) have been published, all involving π-ligands in the coordination sphere of the metal ion. On the basis of these data, a comparison of these molecules is presented. Depending on the steric demands of the ligands, effects like the actinide contraction seem to be stronger or weaker in the structural features. This indicates that the interplay between the actinide ion and the π-ligand is rather flexible, enabling the formation of stable bonds over a broad range of actinide ion oxidation states.

Elucidation of the inverse trans influence in uranyl and its imido and carbene analogues via quantum chemical simulation

The inverse trans influence (ITI) is investigated in uranyl, UO22+, and its isoelectronic imido (U(NH)22+) and carbene (U(CH2)22+) analogues at the density functional and complete active space self consistent field levels of theory. The quantum theory of atoms in molecules is employed to quantify, for the first time, the effect of the ITI on covalent bond character and its relationship to bond lengths and complex stability.

Quantification of f-element covalency through analysis of the electron density: insights from simulation

The electronic structure of f-element compounds is complex due to a combination of relativistic effects, strong electron correlation and weak crystal field environments. However, a quantitative understanding of bonding in these compounds is becoming increasingly technologically relevant. Recently, bonding interpretations based on analyses of the physically observable electronic density have gained popularity and, in this Feature Article, the utility of such density-based approaches is demonstrated. Application of Bader's Quantum Theory of Atoms in Molecules (QTAIM) is shown to elucidate many properties including bonding trends, orbital overlap and energy degeneracy-driven covalency, oxidation state identification and bond stability, demonstrating the increasingly important role that simulation and analysis play in the area of f-element bond characterisation.