A homoleptic tetravalent cerium silylamide

Metal-Ligand Redox Cooperativity in Cerium Amine-/Amido-Phenolate-Type Complexes

The synthesis and characterization of two cerium complexes of redox-active amine/amido-phenolate-type ligands are reported. A tripodal framework comprising the tris(2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)amino-phenyl) amine (H6Clamp) proligand was synthesized for comparison of its cerium complex with a potassium-cerium heterobimetallic complex of the 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippap) proligand. Structural studies indicate differences in the cerium(III) cation coordination spheres, where CeIII(CH3CN)1.5(H3Clamp) (1-Ce(H3Clamp)) exhibits shorter Ce-O distances and longer Ce-N bond distances compared to the analogous distances in K3(THF)3CeIII(dippap)3 (2-Ce(ap)), due to the gross structural differences between the systems. Differences are also evident in the temperature-dependent magnetic properties, where smaller χT products were observed for 2-Ce(ap) compared to 1-Ce(H3Clamp). Solution electrochemical studies for the complexes were interpreted based on ligand- and metal-based oxidation events, and the cerium(III) oxidation of 2-Ce(ap) was observed to be more facile than that of 1-Ce(H3Clamp), behavior that was cautiously attributed to the rigidity of the encrypted 1-Ce(H3Clamp) complex compared to the heterobimetallic framework of 2-Ce(ap). These results contribute to the understanding of how ligand designs can promote facile redox cycling for cerium complexes of redox-active ligands, given the large contraction of cerium-ligand bonds upon oxidation.

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.

Imidazolin-2-iminato Cerium(IV) Chlorides: Synthesis, Structure and Electrochemical Properties

The ligand field greatly affects the redox properties of cerium. Herein, cerium(III) and cerium(IV) complexes supported by imidazolin-2-iminato ligands were synthesized and structurally characterized, and their electrochemical properties were investigated. Silylamine elimination of cerium(III) amide Ce{N(SiMe₃ )₂ }₃ with imidazolin-2-imine Im^R NH (R=Mes, tBu, iPr) provided imidazolin-2-iminato cerium(III) complexes, [(Im^Mes N)(Im^Mes NH)Ce{N(SiMe₃ )₂ }₂ ] (1), [(Im^tBu N)₂ (Im^tBu NH)Ce{N(SiMe₃ )₂ }] (2) and [(Im^iPr N)₂ (Im^iPr NH)Ce(μ-Im^iPr N)]₂ (4) in 71-85% yields. These cerium(III) complexes were successfully oxidized by Ph₃ CCl or C₂ Cl₆ to afford imidazolin-2-iminato cerium(IV) chlorides, [(Im^Mes N)₂ Ce{N(SiMe₃ )₂ }Cl] (5), [(Im^tBu N)₃ CeCl] (6) and [(Im^iPr N)₂ Ce(μ-Im^iPr N)Cl]₂ (7) in 70%-76% yields. All complexes were characterized by the single-crystal X-ray diffraction, which showed that 1, 2, 5 and 6 are monomers while 4 and 7 are dimers. The electrochemical studies indicated that the Ce(III/IV) couples for 5 and 6 are more negative than those of silylamido cerium(IV) complexes, and the Ce(III/IV) couples for 1 and 2 have a similar trend.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Photoluminescence of Pentavalent Uranyl Amide Complexes

Pentavalent uranyl species are crucial intermediates in transformations that play a key role for the nuclear industry and have recently been demonstrated to persist in reducing biotic and abiotic aqueous environments. However, due to the inherent instability of pentavalent uranyl, little is known about its electronic structure. Herein, we report the synthesis and characterization of a series of monomeric and dimeric, pentavalent uranyl amide complexes. These synthetic efforts enable the acquisition of emission spectra of well-defined pentavalent uranyl complexes using photoluminescence techniques, which establish a unique signature to characterize its electronic structure and, potentially, its role in biological and engineered environments via emission spectroscopy.

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.

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.

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.

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.

Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds

Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe₂)₃}₃ (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnI₃THF_n and 3 equiv of KC(SiHMe₂)₃. X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C₃-symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe₂)₃}₃ and DFT calculations. The Ln↼HSi and terminal SiH exchange rapidly on the NMR time scale at room temperature, but the two motifs are resolved at low temperature. Variable-temperature NMR studies provide activation parameters for the exchange process in 1a (ΔH^⧧ = 8.2(4) kcal·mol^-1; ΔS^⧧ = -1(2) cal·mol^-1K^-1) and 1a-d₉ (ΔH^⧧ = 7.7(3) kcal·mol^-1; ΔS^⧧ = -4(2) cal·mol^-1K^-1). Comparisons of lineshapes, rate constants (k_H/k_D), and slopes of ln(k/T) vs 1/T plots for 1a and 1a-d₉ reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low-energy intermediates containing five β-Si-H⇀Ln and one γ-C-H⇀Ln. The calculations also suggest the pathway for Ln↼HSi/SiH exchange involves rotation of a single C(SiHMe₂)₃ ligand that is coordinated to the Ln center through the Ln-C bond and one secondary interaction. These robust organometallic compounds persist in solution and in the solid state up to 80 °C, providing potential for their use in a range of synthetic applications. For example, reactions of Ln{C(SiHMe₂)₃}₃ and ancillary proligands, such as bis-1,1-(4,4-dimethyl-2-oxazolinyl)ethane (HMeC(Ox^Me2)₂) give {MeC(Ox^Me2)₂}Ln{C(SiHMe₂)₃}₂, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

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.

Group 2 metal (Mg, Ca, Sr) silylamides supported by a cyclen-derived macrocyclic polyamine

Heteroleptic bis(silyl)amides of magnesium and calcium [(L)M{N(SiMe₃)₂}] [M = Mg, Ca; LH = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane; (Me₃TACD)H] were previously synthesized from LH and [M{N(SiMe₃)₂}₂]. Strontium bis(silyl)amides [Sr{N(SiMe₃)₂}₂(thf)₂] and [Sr{N(SiHMe₂)₂}₂(thf)_2/3] reacted with LH to give different types of products, depending on the presence of the β-SiH function. While the former underwent protonolysis to give the amido-bridged dimer [(L)Sr{N(SiMe₃)₂}]₂ (1), the latter gave the adduct [(LH)Sr{N(SiHMe₂)₂}₂] (2) as a stable solid. 2 slowly underwent an intramolecular Si-H/H-N dehydrocoupling in solution to give [{(L)SiMe₂N(SiHMe₂)}Sr{N(SiHMe₂)₂}] (3) by liberating H₂. The results of transamination of 1 with HN(SiHMe₂)₂ depended on the relative stoichiometric ratio. A 1 : 1 mixture in n-pentane gave [{(L)SiMe₂N(SiHMe₂)}Sr{N(SiMe₃)₂}] (4) and H₂, while excess HN(SiHMe₂)₂ gave the adduct 2 under similar conditions. Compounds 2 and 3 exhibit Sr↼H-Si interactions according to X-ray crystallography, NMR, and IR spectroscopy. Lighter congeners of elusive [(L)Sr{N(SiHMe₂)₂}] were isolable for Mg (5) and Ca (6).

A [Ce21] keplerate

The solvothermal reaction between Ce(NO₃)₃·6H₂O, 2-amino-isobutyric acid, 2-hydroxy-1-naphthaldehyde and 2-amino-2-methyl-1,3-propanediol in MeOH, in the presence of base, leads to the formation of a unique [CeCe ] keplerate cage.

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.

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.

Density Functional Theory as a Predictive Tool for Cerium Redox Properties in Nonaqueous Solvents

Two methods to correlate and predict experimental redox potentials for cerium complexes were evaluated. Seventeen previously reported cerium complexes were computed using DFT methods in both the Ce^III and Ce^IV oxidation states with a dichloromethane solvent continuum. In the first computational approach, the ΔG^o(Ce^IV/Ce^III) was determined for each of the compounds and these values were correlated with the experimental E_1/2 values measured in dichloromethane, referenced to the ferrocene/ferrocenium couple. The second method involved correlating the energies of the Ce^IV LUMOs (lowest unoccupied molecular orbitals) with the experimental redox potentials, E_1/2. The predictive capabilities of these two correlative methods were tested using a new cerium hydroxylamine complex, Ce(ODiNOx)₂ (ODiNOx = bis(2-tert-butylhydroxylaminatobenzyl) ether). All 18 complexes studied in this paper were combined with the 15 complexes determined in acetonitrile from a previously published correlation by our group. These sets of data allowed us to develop two methods for predicting the redox potential of cerium complexes regardless of the solvent for the experimental measurement.

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.

An Alkali Metal-Capped Cerium(IV) Imido Complex

Structurally authenticated, terminal lanthanide-ligand multiple bonds are rare and expected to be highly reactive. Even capped with an alkali metal cation, poor orbital energy matching and overlap of metal and ligand valence orbitals should result in strong charge polarization within such bonds. We expand on a new strategy for isolating terminal lanthanide-ligand multiple bonds using cerium(IV) complexes. In the current case, our tailored tris(hydroxylaminato) ligand framework, TriNOx(3-), provides steric protection against ligand scrambling and metal complex oligomerization and electronic protection against reduction. This strategy culminates in isolation of the first formal Ce═N bonded moiety in the complex [K(DME)2][Ce═N(3,5-(CF3)2C6H3)(TriNOx)], whose Ce═N bond is the shortest known at 2.119(3) Å.

Emergence of comparable covalency in isostructural cerium(iv)- and uranium(iv)-carbon multiple bonds

We report comparable levels of covalency in cerium- and uranium-carbon multiple bonds in the iso-structural carbene complexes [M(BIPMTMS)(ODipp)2] [M = Ce (1), U (2), Th (3); BIPMTMS = C(PPh2NSiMe3)2; Dipp = C6H3-2,6-iPr2] whereas for M = Th the M[double bond, length as m-dash]C bond interaction is much more ionic. On the basis of single crystal X-ray diffraction, NMR, IR, EPR, and XANES spectroscopies, and SQUID magnetometry complexes 1-3 are confirmed formally as bona fide metal(iv) complexes. In order to avoid the deficiencies of orbital-based theoretical analysis approaches we probed the bonding of 1-3 via analysis of RASSCF- and CASSCF-derived densities that explicitly treats the orbital energy near-degeneracy and overlap contributions to covalency. For these complexes similar levels of covalency are found for cerium(iv) and uranium(iv), whereas thorium(iv) is found to be more ionic, and this trend is independently found in all computational methods employed. The computationally determined trends in covalency of these systems of Ce ∼ U > Th are also reproduced in experimental exchange reactions of 1-3 with MCl4 salts where 1 and 2 do not exchange with ThCl4, but 3 does exchange with MCl4 (M = Ce, U) and 1 and 2 react with UCl4 and CeCl4, respectively, to establish equilibria. This study therefore provides complementary theoretical and experimental evidence that contrasts to the accepted description that generally lanthanide-ligand bonding in non-zero oxidation state complexes is overwhelmingly ionic but that of uranium is more covalent.

The role of dynamic ligand exchange in the oxidation chemistry of cerium(iii)

We report the exchange processes and reactivity of 1-Ce(het), which establish the effects of ligand redistribution and ligand reorganization in Ce^III oxidation chemistry.

The Ce^III/IV couple is useful for many applications in organic, inorganic, and materials chemistry. However, attaining a general method to access both oxidations states through reversible solution redox chemistry remains challenging. Herein we report the synthesis, characterization, and oxidation chemistry of the novel Ce/Li REMB heterochiral diastereomer, 1-Ce(het). The solution exchange processes of 1-RE(het) (RE = Ce and Yb) were investigated to estimate rates of ligand and cation exchange relevant in homochiral and heterochiral frameworks. A detailed mechanistic investigation following the solution dynamics of 1-Ce(het) revealed reactivity controlled both by ligand reorganization and redistribution processes. Ligand reorganization was responsible for the kinetics associated with the chemical oxidation reaction, whereas ligand redistribution and exchange dictated the isolated products.

Control of cerium oxidation state through metal complex secondary structures

A series of alkali metal cerium diphenylhydrazido complexes, M _x (py) _y [Ce(PhNNPh)₄], M = Li, Na, and K, x = 4 (Li and Na) or 5 (K), and y = 4 (Li), 8 (Na), or 7 (K), were synthesized to probe how a secondary coordination sphere would modulate electronic structures at a cerium cation. The resulting electronic structures of the heterobimetallic cerium diphenylhydrazido complexes were found to be strongly dependent on the identity of the alkali metal cations. When M = Li⁺ or Na⁺, the cerium(iii) starting material was oxidized with concomitant reduction of 1,2-diphenylhydrazine to aniline. Reduction of 1,2-diphenylhydrazine was not observed when M = K⁺, and the complex remained in the cerium(iii) oxidation state. Oxidation of the cerium(iii) diphenylhydrazido complex to the Ce(iv) diphenylhydrazido one was achieved through a simple cation exchange reaction of the alkali metals. UV-Vis spectroscopy, FTIR spectroscopy, electrochemistry, magnetic susceptibility, and DFT studies were used to probe the oxidation state and the electronic changes that occurred at the metal centre.

Variation of electronic transitions and reduction potentials of cerium(IV) complexes

The trivalent compound K[Ce[N(SiHMe2)2]4] was synthesized and oxidized, providing a convenient route to the reported cerium(IV) compound Ce[N(SiHMe2)2]4. Protonolysis reactions of Ce[N(SiHMe2)2]4 with tert-butanol, substituted benzyl alcohols, and 2,6-diphenylphenol yielded the neutral tetravalent compounds Ce(O(t)Bu)4(py)2, Ce2(OCH2C6R5)8(thf)2 (R = Me, F), and Ce(Odpp)4 (dpp = 2,6-(C6H5)2-C6H3). Spectroscopic and electrochemical characterization of the monometallic cerium(IV) silylamide, alkoxide, and aryloxide compounds revealed variable ligand-to-metal charge transfer transitions and metal-based reduction potentials. Computational bonding analyses were performed to complement the physical characterization of the complexes.

Cerium tetrakis(diisopropylamide)--a useful precursor for cerium(IV) chemistry

Homoleptic cerium(IV) diisopropylamide was synthesized via oxidation of ate complex Ce(NiPr2)4Li(thf) with trityl chloride or hexachloroethane. Due to its ready accessibility, and high pKa value of the proligand HNiPr2, Ce(NiPr2)4 features a promising candidate for protonolysis reactions, as shown for the synthesis of dimeric cerium(IV) tetrakis(1,1,3,3-N,N,N',N'-tetramethylguanidinate).

Europium bis(dimethylsilyl)amides including mixed-valent Eu3[N(SiHMe2)2]6[μ-N(SiHMe2)2]2

Trivalent Eu[N(SiHMe2)2]3(THF)2 can easily be synthesized by applying a routine salt metathesis protocol (EuCl3(THF)2 and 3 equiv. of Li[N(SiHMe2)2] in n-hexane) which crystallizes isotypically to its analogues of the rare-earth metal series (space group P21/c). Transsilylamination of Eu[N(SiMe3)2]2(THF)2 with a slight excess of HN(SiHMe2)2 in n-hexane-THF yields the divalent trinuclear compound Eu{[μ-N(SiHMe2)2]2Eu[N(SiHMe2)2](THF)}2, the solid-state structure of which differs significantly from the samarium and ytterbium analogues by showing three unique molecules in the asymmetric unit of which one is related to the two others by an inversion. Using crude Eu[N(SiMe3)2]3 in transsilylamination reactions with HN(SiHMe2)2 in n-hexane afforded n-hexane-insoluble trivalent ate complexes {MEu[N(SiHMe2)2]4}n (M = Na, K) depending on the synthesis conditions of Eu[N(SiMe3)2]3. Performing the transsilylamination of Eu[N(SiMe3)2]3 with a large excess of HN(SiHMe2)2 at elevated temperatures gave reproducibly the donor-free, mixed-valent, trinuclear compound Eu(II){[μ-N(SiHMe2)2]Eu(III)[N(SiHMe2)2]3}2 in good yield.

Synthesis and analysis of a family of cerium(IV) halide and pseudohalide compounds

The first complete series of isostructural cerium(IV) halide complexes in a conserved ligand framework was isolated by halogen-exchange reactions of CeF[N(SiMe3)2]3 with Me3SiX (X = Cl(-), Br(-), I(-)). The use of Me3SiX reagents represents a useful method for obtaining cerium(IV) complexes. Spectroscopic, electrochemical, and computational analyses were used to describe the effects of halide coordination on the cerium(IV) metal center. Cerium(IV) complexes of the pseudohalide ligands: N3(-) and NCS(-) were also synthesized and evaluated in comparison to the halide congeners. The results showed that the complexes exhibited reduction potentials and electronic absorption energies that varied with the identity of the halide or pseudohalide ligand.

Silylamide complexes of chromium(II), manganese(II), and cobalt(II) bearing the ligands N(SiHMe2)2 and N(SiPhMe2)2

Bis(dimethylsilyl)amide and bis(dimethylphenylsilyl)amide complexes of the divalent transition metals chromium, manganese, and cobalt were synthesized. Dimeric, donor-free {Mn[N(SiHMe2)2]2}2 could be obtained via two different pathways, a salt metathesis route (utilizing MnCl2(thf)1.5 and LiN(SiHMe2)2) and a transsilylamination protocol (utilizing Mn[N(SiMe3)2]2(thf) and HN(SiHMe2)2). Addition of 1,1,3,3-tetramethylethylendiamine (tmeda) to {Mn[N(SiHMe2)2]2}2 yielded the monomeric adduct Mn[N(SiHMe2)2]2(tmeda). The syntheses of Cr[N(SiHMe2)2]2(tmeda), Co[N(SiMe3)2][N(SiHMe2)2](tmeda), and Co[N(SiHMe2)2]2(tmeda) were achieved by transsilylamination from Cr[N(SiMe3)2]2(tmeda) and {Co[N(SiMe3)2]2}2(μ-tmeda), respectively. Bis(dimethylphenylsilyl)amide complexes Mn[N(SiMe2Ph)2]2, Cr[N(SiMe2Ph)2]2, and Co[N(SiMe2Ph)2]2(thf) were obtained via salt metathesis employing MCl2(thf)x (M = Cr, Mn, Co) with equimolar amounts of LiN(SiMe2Ph)2 in n-hexane. Treatment of CrCl2 with LiN(SiMe2Ph)2 in thf gave Cr[N(SiMe2Ph)2]2(thf)2, featuring an almost square planar trans-coordination. All complexes were examined by elemental analyses, DRIFT and UV-vis spectroscopy, as well as X-ray structure analysis, paying particular attention to secondary M---SiH β-agostic and M---π(arene) interactions. Magnetic moments were determined by Evans' method.

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

Four new cerium(III) formamidinate complexes comprising [Ce(p-TolForm)3 ], [Ce(DFForm)3 (thf)2 ], [Ce(DFForm)3 ], and [Ce(EtForm)3 ] were synthesized by protonolysis reactions using [Ce{N(SiMe3 )2 }3 ] and formamidines of varying functionality, namely N,N'-bis(4-methylphenyl)formamidine (p-TolFormH), N,N'-bis(2,6-difluorophenyl)formamidine (DFFormH), and the sterically more demanding N,N'-bis(2,6-diethylphenyl)formamidine (EtFormH). The bimetallic cerium lithium complex [LiCe(DFForm)4 ] was synthesized by treating a mixture of [Ce{N(SiHMe2 )2 }3 (thf)2 ] and [Li{N(SiHMe2 )2 }] with four equivalents of DFFormH in toluene. Oxidation of the trivalent cerium(III) formamidinate complexes by trityl chloride (Ph3 CCl) caused dramatic color changes, although the cerium(IV) species appeared transient and reformed cerium(III) complexes and N'-trityl-N,N'-diarylformamidines shortly after oxidation. The first structurally characterized homoleptic cerium(IV) formamidinate complex [Ce(p-TolForm)4 ] was obtained through a protonolysis reaction between [Ce{N(SiHMe2 )2 }4 ] and four equivalents of p-TolFormH. [Ce{N(SiHMe2 )2 }4 ] was also treated with DFFormH and EtFormH, but the resulting cerium(IV) complexes decomposed before isolation was possible. The new cerium(IV) silylamide complex [Ce{N(SiMe3 )2 }3 (bda)0.5 ]2 (bda=1,4-benzenediolato) was synthesized by treatment of [Ce{N(SiMe3 )2 }3 ] with half an equivalent of 1,4-benzoquinone, and showed remarkable resistance towards protonolysis or reduction.

Synthesis, bonding, and reactivity of a cerium(IV) fluoride complex

Oxidation of Ce[N(SiMe3)2]3 in the presence of PF6(-) or BF4(-) afforded isolation of CeF[N(SiMe3)2]3. Structural and electrochemical characterization shows that this compound is in its tetravalent oxidation state and contains a terminal fluoride ligand. Spectroscopy and density functional theory have been used to characterize the Ce-F bond as ionic, which is reinforced by an initial reactivity study that demonstrates the nucleophilicity of the fluoride ligand.