Cerium(III/IV) formamidinate chemistry, and a stable cerium(IV) diolate

Tripodal tris(siloxide) ligand supported trivalent rare-earth metal complexes and redox reactivity

Tripodal tris(siloxide) ligand supported rare-earth metal complexes LLn(III) (Ln = Ce, Pr, Tb, Y, Lu) were synthesized. The Ce(III) complex was oxidized with [N(C6H4Br)3][SbCl6] to a Ce(IV) chloride complex, which reacted with tBuONa to form a Ce(IV) tert-butoxide complex, one displaying a reduction potential cathodically shifted relative to that of Ce(IV) chloride complex.

Two-Electron Oxidations at a Single Cerium Center

Two-electron oxidations are ubiquitous and play a key role in the synthesis and catalysis. For transition metals and actinides, two-electron oxidation often takes place at a single-metal site. However, redox reactions at rare-earth metals have been limited to one-electron processes due to the lack of accessible oxidation states. Despite recent advancements in nontraditional oxidation state chemistry, the low stability of low-valent compounds and large disparity among different oxidation states prevented the implementation of two-electron processes at a single rare-earth metal center. Here we report two-electron oxidations at a cerium(II) center to yield cerium(IV) terminal oxo and imido complexes. A series of cerium(II-IV) complexes supported by a tripodal tris(amido)arene ligand were synthesized and characterized. Experimental and theoretical studies revealed that the cerium(II) complex is best described as a 4f2 ion stabilized by δ-backdonation to the anchoring arene, while the cerium(IV) oxo and imido complexes exhibit multiple bonding characters. The accomplishment of two-electron oxidations at a single cerium center brings a new facet to molecular rare-earth metal chemistry.

Manganese-Catalyzed Hydroborations with Broad Scope

Reductive transformations of easily available oxidized matter are at the heart of synthetic manipulation and chemical valorization. The applications of catalytic hydrofunctionalization benefit from the use of liquid reducing agents and operationally facile setups. Metal‐catalyzed hydroborations provide a highly prolific platform for reductive valorizations of stable C=X electrophiles. Here, we report an especially facile, broad‐scope reduction of various functions including carbonyls, carboxylates, pyridines, carbodiimides, and carbonates under very mild conditions with the inexpensive pre‐catalyst Mn(hmds)₂. The reaction could be successfully applied to depolymerizations.

Effect of Substituents of Cerium Pyrazolates and Pyrrolates on Carbon Dioxide Activation

Homoleptic ceric pyrazolates (pz) Ce(RR’pz)₄ (R = R’ = tBu; R = R’ = Ph; R = tBu, R’ = Me) were synthesized by the protonolysis reaction of Ce[N(SiHMe₂)₂]₄ with the corresponding pyrazole derivative. The resulting complexes were investigated in their reactivity toward CO₂, revealing a significant influence of the bulkiness of the substituents on the pyrazolato ligands. The efficiency of the CO₂ insertion was found to increase in the order of tBu₂pz < Ph₂pz < tBuMepz < Me₂pz. For comparison, the pyrrole-based ate complexes [Ce₂(pyr)₆(µ-pyr)₂(thf)₂][Li(thf)₄]₂ (pyr = pyrrolato) and [Ce(cbz)₄(thf)₂][Li(thf)₄] (cbz = carbazolato) were obtained via protonolysis of the cerous ate complex Ce[N(SiHMe₂)₂]₄Li(thf) with pyrrole and carbazole, respectively. Treatment of the pyrrolate/carbazolate complexes with CO₂ seemed promising, but any reversibility could not be observed.

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.

Oxidizing Cerium(IV) Alkoxide Complexes Supported by the Kläui Ligand [Co(η5-C5H5){P(O)(OEt)2}3]-: Synthesis, Structure, and Redox Reactivity

Tetravalent cerium alkoxide complexes supported by the Kläui tripodal ligand [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^- (L_OEt^-) have been synthesized, and their nucleophilic and redox reactivity have been studied. Treatment of the Ce(IV) oxo complex [Ce^IV(L_OEt)₂(O)(H₂O)]·MeCONH₂ (1) with ⁱPrOH or reaction of [Ce^IV(L_OEt)₂Cl₂] (2) with Ag₂O in ⁱPrOH afforded the Ce(IV) dialkoxide complex [Ce^IV(L_OEt)₂(OⁱPr)₂] (3-iPr). The methoxide and ethoxide analogues [Ce^IV(L_OEt)₂(OR)₂] (R = Me (3-Me), Et (3-Et)) have been prepared similarly from 2 and Ag₂O in ROH. Reaction of 3-iPr with an equimolar amount of 2 yielded a new Ce(IV) complex that was formulated as the chloro-alkoxide complex [Ce^IV(L_OEt)₂(OⁱPr)Cl] (4). Treatment of 3-iPr with HX and methyl triflate (MeOTf) afforded [Ce(L_OEt)₂X₂] (X^- = Cl^-, NO₃^-, PhO^-) and [Ce^IV(L_OEt)₂(OTf)₂], respectively, whereas treatment with excess CO₂ in hexane led to isolation of the Ce(IV) carbonate [Ce^IV(L_OEt)₂(CO₃)]. 3-iPr reacted with water in hexane to give a Ce(III) complex and a Ce(IV) species, presumably the reported tetranuclear oxo cluster [Ce^IV₄(L_OEt)₄(O)₅(OH)₂]. The Ce(IV) alkoxide complexes are capable of oxidizing substituted phenols, possibly via a proton-coupled electron transfer pathway. Treatment of 3-iPr with ArOH afforded the Ce(III) aryloxide complexes [Ce^III(L_OEt)₂(OAr)] (Ar = 2,4,6-tri-tert-butylphenyl (5), 2,6-diphenylphenyl (6)). On the other hand, a Ce(III) complex containing a monodeprotonated 2,2'-biphenol ligand, [Ce^III(L_OEt)₂(^tBu₄C₁₂H₄O₂H)] (7) (^tBu₄C₁₂H₄O₂H₂ = 4,4',6,6'-tetra-tert-butyl-2,2'-biphenol), was isolated from the reaction of 3-iPr with 2,4-di-tert-butylphenol. The crystal structures of complexes 3-iPr, 3-Me, 3-Et, and 5-7 have been determined.

Video colorimetry of single-chromophore systems based on vector analysis in the 3D color space: Unexpected hysteresis loops in oscillating chemical reactions

Colorimetry, the quantitative determination of color, usually of a digital image, has useful applications in diverse areas of research. Many methods have been proposed for translating the RGB data of an image to obtain concentration information. Among the many methods for RGB analysis, we focus on the vector projection method (VP), which is based on a vector analysis in 3D RGB color space. This method has the major advantages of being conceptually intelligible and generalizable to various systems. For solutions with variable concentrations of one chromophore, we will show that the analysis of the trace in RGB color space allows for a judgment about the reliability of the linear concentration dependence of the chromapostasi parameter. We discuss the theoretical underpinnings of the method in two test cases, a simple dye solution and a titration of an organic acid with phenolphthalein indicator. The VP method was then applied to the Ce-catalyzed Belousov-Zhabotinsky reaction with the expectation that the colorimetry would quantify [Ce⁴⁺] oscillations. Surprisingly, the 3D color space analysis revealed hysteresis loops and the origin and implications of this observation are discussed.

Cerium-quinone redox couples put under scrutiny

Homoleptic cerous complexes Ce[N(SiMe₃)₂]₃, [Ce{OSi(OtBu)₃}₃]₂ and [Ce{OSiⁱPr₃}₃]₂ were employed as thermally robust, weakly nucleophilic precursors to assess their reactivity towards 1,4-quinones in non-aqueous solution. The strongly oxidizing quinones 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) or tetrachloro-1,4-benzoquinone (Cl₄BQ) readily form hydroquinolato-bridged ceric complexes of the composition [(Ce^IVL₃)₂(μ₂-O₂C₆R₄)]. Less oxidising quinones like 2,5-di-tert-butyl-1,4-benzoquinone (tBu₂BQ) tend to engage in redox equilibria with the ceric hydroquinolato-bridged form being stable only in the solid state. Even less oxidising quinones such as tetramethyl-1,4-benzoquinone (Me₄BQ) afford cerous semiquinolates of the type [(Ce^IIIL₂(thf)₂)(μ₂-O₂C₆Me₄)]₂. All complexes were characterised by X-ray diffraction, ¹H, ¹³C{¹H} and ²⁹Si NMR spectroscopy, DRIFT spectroscopy, UV-Vis spectroscopy and CV measurements. The species putatively formed during the electrochemical reduction of [Ce^IV{N(SiMe₃)₂}₃]₂(μ₂-O₂C₆H₄) could be mimicked by chemical reduction with Co^IICp₂ yielding [(Ce^III{N(SiMe₃)₂}₃)₂(μ₂-O₂C₆H₄)][Co^IIICp₂]₂.

Homoleptic cerium tris(dialkylamido)imidophosphorane guanidinate complexes

We report the synthesis of a new potassium tris(piperdino)imidophosphorane N,N'-dicyclohexylguanidinate, K[CyGNP(pip)3], and describe the synthesis and characterization of the tris-homoleptic compounds, [Ce(CyGNP(pip)3)3], 1-Ce, and [Ce(CyGNP(pip)3)3][BPh4], 2-Ce. The latter is an unusual cationic tetravalent cerium complex. Cyclic voltammetry studies of 1-Ce and 2-Ce revealed Epc potentials of -1.56 V and -1.81 V, and Epa potentials of -0.78 V and -0.66 V (200 mV s-1; THF, vs. Fc0/+), respectively. Compounds 1-Ce and 2-Ce were studied by L3-edge X-ray absorption near-edge spectroscopy (XANES), and the fit of the spectrum of 2-Ce revealed a white-line multiplet with an nf value of 0.50(2).

Series of Tetravalent Actinide Amidinates: Structure Determination and Bonding Analysis

Two series of isostructural tetravalent actinide amidinates [AnX((S)-PEBA)₃] (An = Th, U, Np; X = Cl, N₃) bearing the chiral (S,S)-N,N'-bis(1-phenylethyl)benzamidinate ((S)-PEBA) ligand have been synthesized and thoroughly characterized in solid and in solution. This study expands the already reported tetravalent neptunium complexes to the lighter actinides thorium and uranium. Furthermore, a rare Ce(IV) amidinate [CeCl((S)-PEBA)₃] was synthesized to compare its properties to those of the analogous tetravalent actinide complexes. All compounds were characterized in the solid state using single-crystal XRD and infrared spectroscopy and in solution using NMR spectroscopy. Quantum chemical bonding analysis including also the isostructural Pa and Pu complexes was used to characterize the covalent contributions to any bond involving the metal cation. Th shows the least covalent character throughout the series, even substantially smaller than for the Ce complex. For U, Np, and Pu, similar covalent bonding contributions are found, but a natural population analysis reveals different origins. The 6d participation is the highest for U and decreases afterward, whereas the 5f participation increases continuously from Pa to Pu.

Emergence of a New [NNN] Pincer Ligand via Si-H Bond Activation and β-Hydride Abstraction at Tetravalent Cerium

The cerium(IV) pyrazolate complexes [Ce(Me2 pz)4 ]2 and [Ce(Me2 pz)4 (thf)] initiate β-hydride abstraction of the bis(dimethylsilyl)amido moiety, to afford a heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me2 pz)3 (bpsa)] (bpsa=bis((3,5-dimethylpyrazol-1-yl)dimethylsilyl)amido; Me2 pz =3,5-dimethylpyrazolato), along with some cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom and concomitant Si-H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me2 pz)4 ]2 is treated with [Li{N(SiMe3 )2 }], that is, in the absence of the SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe2 (Me2 pz)9 ] and the stable mixed-ligand ceric species [Ce(Me2 pz)2 {N(SiMe3 )2 }2 ] were obtained.

Dimerized p-Semiquinone Radical Anions Stabilized by a Pair of Rare-Earth Metal Ions

Here we report stable p-quinone-radical-bridged rare-earth complexes involving the ligand tetramethylquinone (QMe₄^•-). The complexes, {Y[(QMe₄)^•-Cl₂(THF)₃]}₂ (1) and {Gd[(QMe₄)^•-Cl₂(THF)₃]}₂ (2), where THF = tetrahydrofuran, are sufficiently stable that we can measure the single-crystal structures and perform magnetic and electron paramagnetic resonance measurements. These studies show the presence of a semiquinone form and that the magnetic interaction between the radicals in the dimer is strong and antiferromagnetic.

Widely contrasting outcomes from the use of tris(pentafluorophenyl)bismuth or pentafluorophenylsilver as oxidants in the reactions of lanthanoid metals with N,N′-diarylformamidines

Reactions of lanthanoid metals with Bi(C₆F₅)₃ or AgC₆F₅ and two widely disparate formamidines, have been investigated.

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.

Structure and properties of [(4,6-tBu2C6H2O)2Se]2An(THF)2, An = U, Np, and their reaction with p-benzoquinone

The synthesis and characterization of U(iv) and Np(iv) selenium bis(phenolate) complexes are reported. The reaction of two equivalents of the U(iv) complex with p-benzoquinone results in the formation of a U(v)-U(v) species with a bridging reduced quinone. This represents a rare example of high-valent uranium chemistry as well as a rare example of a neptunium aryloxide complex.

The difficult search for organocerium(iv) compounds

This Tutorial Review provides an overview of the historic and current development of the organometallic chemistry of cerium in its oxidation state 4+. Among the tetravalent lanthanide ions, only Ce⁴⁺ forms stable coordination compounds (e.g. (NH₄)₂[Ce(NO₃)₆]). Important fields of applications for cerium(iv) compounds include organic synthesis, bioinorganic chemistry, materials science, and industrial catalysis. In sharp contrast, organometallic cerium(iv) compounds are still exceedingly rare. The history of organocerium(iv) compounds is an exciting story of ups and downs. The so-called cerocene (= bis(η⁸-cyclooctatetraenyl) cerium) has been known since 1976. Other early reports e.g. about Cp₄Ce (Cp = η⁵-cyclopentadienyl), were later disproven. However, significant progress in this field has been made in recent years through the use of carefully designed ligands and more sophisticated synthesis protocols. Taking the case of organocerium(iv) chemistry, this Tutorial Review also tries to exemplarily show how difficult synthetic and theoretical problems can eventually be solved through newly designed synthesis strategies (e.g. as accomplished for cyclopentadienyl and carbene derivatives) and a rewarding collaboration between synthetic and theoretical chemists (cf. the cerocene problem).

Pyrazolates advance cerium chemistry: a CeIII/CeIV redox equilibrium with benzoquinone

Two stable cerium(iv) 3,5-dialkylpyrazolate complexes are presented, namely dimeric [Ce(Me₂pz)₄]₂ (Me₂pz = 3,5-dimethylpyrazolate) and monomeric Ce(tBu₂pz)₄ (tBu₂pz = 3,5-di-tert-butylpyrazolate) along with their trivalent counterparts [Ce(Me₂pz)₃] and [Ce(tBu₂pz)₃]₂. All complexes were obtained from protonolysis reactions employing the silylamide precursors Ce[N(SiHMe₂)₂]₄ and Ce[N(SiMe₃)₂]₃. Treatment of homoleptic Ce^IV and Ce^III Me₂pz complexes with 1,4-hydroquinone (H₂hq) or 1,4-benzoquinone (bq), respectively, ultimately gave the same trimetallic Ce^III species via a cerium redox equilibrium. The Ce^III complex Ce₃(Me₂pz)₅(pchd)₂(L) (pchd = 1,4-bis(3,5-dimethylpyrazol-1-yl)cyclohex-2,5-diene-1,4-diolato; L = Me₂pzH or (thf)₂) results from a di-1,4-pyrazolyl attack on pre-coordinated bq. The reduction of bq by [Ce(Me₂pz)₃(thf)]₂, and re-oxidation by the resulting Ce^IV species was supported by UV-vis spectroscopic investigations. Comparisons with the redox-innocent complexes [Ln(Me₂pz)₃(thf)]₂ (Ln = La and Pr) revealed far less selective reactions with bq, giving hexametallic and octametallic rare-earth metal side products containing 2-Me₂pz substituted hq ligands.

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.

Ceric Cyclopentadienides Bearing Alkoxy, Aryloxy, Chlorido, or Iodido Co-Ligands

A series of heteroleptic tris(cyclopentadienyl) Ce^IV complexes has been isolated and crystallographically characterized, revealing the broad accessibility of such organocerium(IV) compounds. The oxidation of complexes Cp^Me₃ Ce(thf) and Cp'₃ Ce(thf) (Cp^Me =C₅ H₄ Me, Cp'=C₅ H₄ SiMe₃ ), bearing monosubstituted electron-poor cyclopentadienyl ligands, with 0.5 equivalents of 1,4-benzoquinone or C₂ Cl₆ gave the cerium(IV) hydroquinolate complexes Cp^Me₃ Ce(OC₆ H₄ O)CeCp^Me₃ and Cp'₃ Ce(OC₆ H₄ O)CeCp'₃ , or the chloride complexes Cp^Me₃ CeCl and Cp'₃ CeCl, respectively; the iodide complex Cp'₃ CeI was obtained from the reaction of Cp'₃ Ce(thf) with elemental iodine. The behavior of Cp'₃ CeCl in salt metathesis protocols employing alkali metal amides or alkyl, alkoxide, and aryloxide reagents was investigated, which gave rise to the robust and isolable cerium(IV) alkoxide Cp'₃ Ce(OtBu). Trivalent [Cp'₂ CeCl]₂ was synthesized by AlMe₄ →Cl exchange utilizing Cp'₂ Ce(AlMe₄ ) and AlMe₂ Cl. The reactivity of [Cp'₂ CeCl]₂ towards the oxidants Ph₃ CCl, C₂ Cl₆ , 1,4-benzoquinone, and I₂ has been assessed, and has provided useful information on Ce^III /Ce^IV redox deactivation pathways. In addition to X-ray structure analysis, all the complexes were characterized by NMR, DRIFT (diffuse reflectance IR Fourier transform), and UV/Vis spectroscopy as well as elemental analysis. The tetravalent compounds were further analyzed for their magnetic susceptibility by using Evans' method.

1,3-N,O-Complexes of late transition metals. Ligands with flexible bonding modes and reaction profiles

1,3-N,O-Chelating ligands are ubiquitous in nature owing to their occurrence as α-chiral amino acids in metalloproteins.

Synthesis and structural diversity of trivalent rare-earth metal diisopropylamide complexes

A series of rare-earth metal diisopropylamide complexes has been obtained via salt metathesis employing LnCl3(THF)x and lithium (LDA) or sodium diisopropylamide (NDA) in n-hexane. Reactions with AM : Ln ratios ≥3 gave ate complexes (AM)Ln(NiPr2)4(THF)n (n = 1, 2; Ln = Sc, Y, La, Lu; AM = Li, Na) in good yields. For smaller rare-earth metal centres such as scandium and lutetium, a Li : Ln ratio = 2.5 accomplished ate-free tris(amido) complexes Ln(NiPr2)3(THF). The chloro-bridged dimeric derivatives [Ln(NiPr2)2(μ-Cl)(THF)]2 (Ln = Sc, Y, La, Lu) could be obtained in high yields for Li : Ln = 1.6-2. The product resulting from the Li : La = 1 : 1.6 reaction revealed a crystal structure containing two different molecules in the crystal lattice, [La(NiPr2)2(THF)(μ-Cl)]2·La(NiPr2)3(THF)2. Recrystallization of the chloro-bridged dimers led to the formation of the monomeric species Ln(NiPr2)2Cl(THF)2 (Ln = Sc, Lu) and La(NiPr2)3(THF)2. The reaction of YCl3 and LDA with Li : Y = 2 in the absence of THF gave a bimetallic ate complex LiY(NiPr2)4 with a chain-like structure. For scandium, the equimolar reactions with LDA or NDA yielded crystals of tetrametallic mono(amido) species, {[Sc(NiPr2)Cl2(THF)]2(LiCl)}2 and [Sc(NiPr2)Cl2(THF)]4, respectively. Depending on the Ln(iii) size, AM, and presence of a donor solvent, ate complexes (AM)Ln(NiPr2)4(THF)n show distinct dynamic behaviour as revealed by variable temperature NMR spectroscopy. The presence of weak LnCH(iPr) β-agostic interactions, as indicated by Ln-N-C angles <105°, is corroborated by DFT calculations and NBO analysis.