Tuning the Stability of Pd(IV) Intermediates Using a Redox Non-innocent Ligand Combined with an Organolanthanide Fragment

Preparation and Ground-State Electronic Structure of Heterobimetallic Yb-PtIV-Alkyl Complexes

This article focuses on the synthesis of heterobimetallic complexes of lanthanide and platinum. It describes the synthesis of the Cp*Yb(bipym)PtMe2 complex and its characterization, followed by its reactivity with oxidants, giving access to various Pt + IV compounds of trismethyl (PtMe3) and tetramethyl (PtMe4) fragments. Characterization of the electronic properties of the complexes by magnetic measurements demonstrated that the tetramethyl complex possesses a singlet ground state. The trismethyl fragments, on the other hand, have a ground state that evolves as a function of the ligand saturating the coordination sphere: a singlet for triflate and pyridine and a triplet for iodine, demonstrating the capacity for simple tuning of the electronic structure of these complexes. While the addition of B(C6F5)3 to the platinum + II bis methyl complex leads to FLP-like reactivity triggering THF opening, reactivity with [Ph3C]+[BPh4]- leads to oxidation of the bipym ligand. Furthermore, the light reactivity of the tetramethyl complex indicated the possible transfer of a methyl group, leading to functionalization of the bridging bipym ligand.

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

NHC-based pincers (NHC = N-heterocyclic carbene) have been broadly employed as supporting platforms, and their palladium complexes have found many synthetic applications. However, previous studies mainly focused on the NHC pincers of palladium featuring an oxidation number of +II. In contrast, oxidation of these well-defined Pd(II) species and the study of their fundamental high-valent Pd chemistry remain largely undeveloped. In addition, from a perspective of PdII/PdIV catalysis, the reactivity and degradation of NHC pincers in catalytically relevant reactions have not been well understood. In this work, a series of Pd(II) complexes supported by a well-known NHC^Aryl^NHC pincer platform have been prepared. Their reactivity towards various oxidizing reagents, including halogen surrogates, electrophilic fluorine reagents, and alkyl/aryl halides, has been examined. In some cases, ambient-characterizable high-valent Pd NHCs, which have been scarcely reported, were obtained. The carbenes incorporated into the pincer framework proved to be effective spectator donors. In contrast, the central aryl moiety exhibits versatile reactivity and collapse pathways, allowing it to function either as a spectator or a non-innocent actor.

Reversible Modulation of the Electronic and Spatial Environment around Ni(0) Centers Bearing Multifunctional Carbene Ligands with Triarylaluminum

Designing and modulating the electronic and spatial environments surrounding metal centers is a crucial issue in a wide range of chemistry fields that use organometallic compounds. Herein, we demonstrate a Lewis-acid-mediated reversible expansion, contraction, and transformation of the spatial environment surrounding nickel(0) centers that bear N-phosphine oxide-substituted N-heterocyclic carbenes (henceforth referred to as (S)PoxIms). Reaction between tetrahedral (syn-κ-C,O-(S)PoxIm)Ni(CO)₂ and Al(C₆F₅)₃ smoothly afforded heterobimetallic Ni/Al species such as trigonal-planar {κ-C-Ni(CO)₂}(μ-anti-(S)PoxIm){κ-O-Al(C₆F₅)₃} via a complexation-induced rotation of the N-phosphine oxide moieties, while the addition of 4-dimethylaminopyridine resulted in the quantitative regeneration of the former Ni complexes. The corresponding interconversion also occurred between (SPoxIm)Ni(η²:η²-diphenyldivinylsilane) and {κ-C-Ni(η²:η²-diene)}(μ-anti-SPoxIm){κ-O-Al(C₆F₅)₃} via the coordination and dissociation of Al(C₆F₅)₃. The shape and size of the space around the Ni(0) center was drastically changed through this Lewis-acid-mediated interconversion. Moreover, the multinuclear NMR, IR, and XAS analyses of the aforementioned carbonyl complexes clarified the details of the changes in the electronic states on the Ni centers; i.e., the electron delocalization was effectively enhanced among the Ni atom and CO ligands in the heterobimetallic Ni/Al species. The results presented in this work thus provide a strategy for reversibly modulating both the electronic and spatial environment of organometallic complexes, in addition to the well-accepted Lewis-base-mediated ligand-substitution methods.

Divalent metallocenes of the lanthanides - a guideline to properties and reactivity

Since the discovery in the early 1980s, the soluble divalent metallocenes of lanthanides have become a steadily growing field in organometallic chemistry. The predominant part of the investigation has been performed with samarium, europium, and ytterbium, whereas only a few reports dealing with other rare earth elements were disclosed. Reactions of these metallocenes can be divided into two major categories: (1) formation of Lewis acid-base complexes, in which the oxidation state remains +II; and (2) single electron transfer (SET) reductions with the ultimate formation of Ln(III) complexes. Due to the increasing reducing character from Eu(II) over Yb(II) to Sm(II), the plethora of literature concerning redox reactions revolves around the metallocenes of Sm and Yb. In addition, a few reactivity studies on Nd(II), Dy(II) and mainly Tm(II) metallocenes were published. These compounds are even stronger reducing agents but significantly more difficult to handle. In most cases, the metals are ligated by the versatile pentamethylcyclopentadienyl ligand: (C₅Me₅). Other cyclopentadienyl ligands are fully covered but only discussed in detail, if the ligand causes differences in synthesis or reactivity. Thus, the focus lays on three compounds: [(C₅Me₅)₂Sm], [(C₅Me₅)₂Eu] and [(C₅Me₅)₂Yb] and their solvates. We discuss the synthesis and physical properties of divalent lanthanide metallocenes first, followed by an overview of the reactivity rendering the full potential of these versatile reactants.

Structure and bonding patterns in heterometallic organometallics with linear Ln-Pd-Ln motifs

Complexes with short intermetallic distances between transition metal fragments and lanthanide (Ln) fragments are fascinating objects of study, owing to the ambiguity of the nature of the interaction. The addition of the divalent lanthanide fragments Cp*₂Ln(OEt₂) (Ln = Sm or Yb) to a Pd(ii) complex bearing the deprotonated form of the redox-active, non-symmetrical ligand, 2-pyrimidin-2-yl-1H-benzimidazole (Hbimpm), leads to two isostructural complexes, of the general formula (Cp*₂Ln)₂[μ-Pd(pyridyl)₂] (Ln = Sm (4) and Yb (5)). These adducts have interesting features, such as unique linear Ln-Pd-Ln arrangements and short Ln-Pd distances, which deviate from the expected lanthanide contraction. A mixed computational and spectroscopic study into the formation of these adducts gathers important clues as to their formation. At the same time, thorough characterization of these complexes establishes the +3 oxidation state of all the involved Ln centers. Detailed theoretical computations demonstrate that the apparent deviation from lanthanide contraction is not due to any difference in the intermetallic interaction between the Pd and the Ln, but that the fragments are joined together by electrostatic interactions and dispersive forces. This conclusion contrasts with the findings about a third complex, Cp*₂Yb(μ-Me)₂PdCp* (6), formed during the reaction, which also possesses a short Yb-Pd distance. Studies at the CASSCF level of theory on this complex show several orbitals containing significant interactions between the 4f and 4d manifolds of the metals. This demonstrates the need for methodical and careful analyses in gauging the intermetallic interaction and the inadequacy of empirical metrics in describing such phenomena.

Electron Shuttle in N-Heteroaromatic Ni Catalysts for Alkene Isomerization

Simple N-heteroaromatic Ni(II) precatalysts, (L)NiMe2 (L = bipy, bipym), were used for alkene isomerization. With an original reduction method using a simple borane (HB(Cat)), a low-valent Ni center was formed readily and showed good conversion when a reducing divalent lanthanide fragment, Cp*2Yb, was coordinated to the (bipym)NiMe2 complex, a performance not achieved by the monometallic (bipy)NiMe2 analogue. Experimental mechanistic investigations and computational studies revealed that the redox non-innocence of the L ligand triggered an electron shuttle process, allowing the elusive formation of Ni(I) species that were central to the isomerization process. Additionally, the reaction occurred with a preference for mono-isomerization rather than chain-walking isomerization. The presence of the low-valent ytterbium fragment, which contributed to the formation of the electron shuttle, strongly stabilized the catalysts, allowing catalytic loading as low as 0.5%. A series of alkenes with various architectures have been tested. The possibility to easily tune the various components of the heterobimetallic catalyst reported here, the ligand L and the divalent lanthanide fragment, opens perspectives for further applications in catalysis induced by Ni(I) species.

CO reductive oligomerization by a divalent thulium complex and CO2-induced functionalization

The divalent thulium complex [Tm(Cpttt)2] (Cpttt = 1,2,4-tris(tert-butyl)cyclopentadienyl) reacts with CO to afford selective CO reductive dimerization and trimerization into ethynediolate (C2) and ketenecarboxylate (C3) complexes, respectively. DFT calculations were performed to shed light on the elementary steps of CO homologation and support a stepwise chain growth. The attempted decoordination of the ethynediolate fragment by treatment with Me3SiI led to dimerization and rearrangement into a 3,4-dihydroxyfuran-2-one complex. Investigation of the reactivity of the C2 and C3 complexes towards other electrophiles led to unusual functionalization reactions: while the reaction of the ketenecarboxylate C3 complex with electrophiles yielded new multicarbon oxygenated complexes, the addition of CO2 to the ethynediolate C2 complex resulted in the formation of a very reactive intermediate, allowing C-H activation of aromatic solvents. This original intermolecular reactivity corresponds to an unprecedented functionalization of CO-derived ligands, which is induced by CO2.

Assembling Diuranium Complexes in Different States of Charge with a Bridging Redox-Active Ligand

Radical-bridged diuranium complexes are desirable for their potential high exchange coupling and single molecule magnet (SMM) behavior, but remain rare. Here we report for the first time radical-bridged diuranium(iv) and diuranium(iii) complexes. Reaction of [U{N(SiMe₃)₂}₃] with 2,2′-bipyrimidine (bpym) resulted in the formation of the bpym-bridged diuranium(iv) complex [{((Me₃Si)₂N)₃U^IV}₂(μ-bpym²⁻)], 1. Reduction with 1 equiv. KC₈ reduces the complex, affording [K(2.2.2-cryptand)][{((Me₃Si)₂N)₃U}₂(μ-bpym)], 2, which is best described as a radical-bridged U^III–bpym˙⁻–U^III complex. Further reduction of 1 with 2 equiv. KC₈, affords [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U^III}₂(μ-bpym²⁻)], 3. Addition of AgBPh₄ to complex 1 resulted in the oxidation of the ligand, yielding the radical-bridged complex [{((Me₃Si)₂N)₃U^IV}₂(μ-bpym˙⁻)][BPh₄], 4. X-ray crystallography, electrochemistry, susceptibility data, EPR and DFT/CASSCF calculations are in line with their assignments. In complexes 2 and 4 the presence of the radical-bridge leads to slow magnetic relaxation.

Convenient routes to dinuclear complexes of uranium where two uranium centers are bridged by the redox-active ligand bpym were identified resulting in unique and stable radical-bridged dimetallic complexes of U(iii) and U(iv) showing SMM behaviour.

Intermediate Valence States in Lanthanide Compounds

Over more than 50 years, intermediate valence states in lanthanide compounds have often resulted in unexpected or puzzling spectroscopic and magnetic properties. Such experimental singularities could not be rationalised until new theoretical models involving multiconfigurational electronic ground states were established. In this minireview, the different singularities that have been observed among lanthanide complexes are highlighted, the models used to rationalise them are detailed and how such electronic effects may be adjusted depending on energy and symmetry considerations is considered. Understanding and tuning the ground-state multiconfigurational behaviour in lanthanide complexes may open new doors to modular and unusual reactivities.

Redox activity of a dissymmetric ligand bridging divalent ytter-bium and reactive nickel fragments

The reaction of a reactive nickel dimethyl 1 bearing a redox-active, dissymmetric ligand, which is obtained by deprotonation of 2-pyrimidin-2-yl-1H-benzimidazole (Hbimpm) with a divalent lanthanide complex, Cp*₂Yb(OEt₂), affords an unprecedented, trimeric 2 with C(sp³)-C(sp³) bond formation between two ligands in an exo position. Meanwhile, the transient, dimeric species 3 can be isolated with the same ligand coupling fashion, however, with a drastic distorsion angle of the bimpm ligand and reactive NiMe₂ fragment, revealing the possible mechanism of this rearrangement. A much more stable dimeric congener, 5, with an exo ligand coupling, is synthesized in the presence of 18-crown-6, which captures the potassium counter ion. The C-C coupling formation between two bimpm ligands results from the effective electron transfer from divalent lanthanide fragments. Without the divalent lanthanide, the reductive coupling occurs on a different carbon of the ligand, nicely showing the modulation of the spin density induced by the presence of the lanthanide ion. The electronic structures of these complexes are investigated by magnetic study (SQUID), indicating a ²F_7/2 ground state for each ytterbium in all the heterometallics. This work firstly reports ligand coupling reactivity in a redox-active, yet dissymmetric system with divalent organolanthanides, and the reactive nickel moiety can impact the intriguing transition towards a stable homoleptic, trinulear lanthanide species.

Influence of a Lanthanide Ion on the Ni Site of a Heterobimetallic 3d-4f Mabiq Complex

This work presents the synthesis and characterization of a 3d-4f bimetallic complex based on the redox-active macrocyclic biquinazoline ligand, Mabiq. The mixed Yb-Ni complex, [(Cp*)₂Yb(Mabiq)Ni]BArF (3), was synthesized upon reaction of [Ni^II(Mabiq)]BArF (2) with (Cp*)₂Yb^II(OEt₂). The molecular structures of 3 and its sister complex, [(Cp*)₂Yb(Mabiq)Ni][(Cp*)₂Yb(OTf)₂] (1), confirmed the presence of a Yb(III) center and a reduced Ni-Mabiq unit. Spectroscopy (absorption and NMR), cyclic voltammetry, and magnetic susceptibility studies were employed to analyze the electronic structure of 3, which is best described by the [(Cp*)₂Yb^III(Mabiq^•)Ni^II]⁺ formulation. Notably, the ligand-centered radical is delocalized over both the diketiminate and bipyrimidine units of the Mabiq ligand. The magnetic susceptibility and variable temperature NMR studies for 3 denote coupling between the Ni-Mabiq site and the peripheral Yb center-previously unobserved in 3d-3d Mabiq complexes. The complex nature of the exchange interactions is highlighted by the multiconfigurational ground state for 3, comprising nearly degenerate singlet and triplet states.

Chemical engineering of quasicrystal approximants in lanthanide-based coordination solids

Tessellation of self-assembling molecular building blocks is a promising strategy to design metal-organic materials exhibiting geometrical frustration and ensuing frustrated physical properties. Appearing in two-dimensional quasiperiodic phases, tilings consisting of five-vertex nodes are regarded as approximants for quasicrystals. Unfortunately, these structural motifs are exceedingly rare due to the complications of acquiring five-fold coordination confined to the plane. Lanthanide ions display the sufficient coordinative plasticity, and large ionic radii, to allow their incorporation into irregular molecule-based arrays. We herein present the use of ytterbium(II) as a five-vertex node in a two-dimensional coordination solid, YbI₂(4,4'-bipyridine)_2.5. The semi-regular Archimedean tessellation structure verges on quasicrystallinity and paves the way for lanthanide-based metal-organic materials with interesting photonic and magnetic properties.

Synthesis of Well-Defined High-Valent Palladium Complexes by Oxidation of Their Palladium(II) Precursors

The last decade has witnessed the rapid development of high-valent Pd-involved organic transformations. This has also led to a steadily growing number of publications concerning the preparation of isolable and characterizable palladium(III) and palladium(IV) complexes. A variety of one-electron and two-electron oxidants have been employed to give access to high-oxidation-state Pd compounds. Undoubtedly, the study of these stoichiometric reactions has great implications for relevant Pd-mediated catalysis. In this minireview, the focus is on the synthetic approaches to structurally determined Pd^III/IV complexes starting from their Pd^II precursors, and the advances in this research area from early 2010 to late 2019 will be highlighted. Chemical oxidations exploiting various oxidizing agents including 1) hypervalent iodine reagents; 2) halogens; 3) electrophilic fluorination reagents; 4) alkyl/aryl halides; 5) ferrocenium salts; 6) peroxides/O₂ ; 7) sulfonyl chlorides; and 8) others are covered. A "greener" electrooxidation manner has also been reviewed.

Bis(imino)pyrazine-Supported Iron Complexes: Ligand-Based Redox Chemistry, Dearomatization, and Reversible C-C Bond Formation

Iron complexes supported by novel π-acidic bis(imino)pyrazine (P^PzDI) ligands can be functionalized at the nonligated nitrogen atom, and this has a marked effect on the redox properties of the resulting complexes. Dearomatization is observed in the presence of cobaltocene, which reversibly reduces the pyrazine core and not the imine functionality, as observed in the case of the pyridinediimine-ligated iron analogues. The resulting ligand-based radical is prone to dimerization through the formation of a long carbon-carbon bond, which can be subsequently cleaved under mild oxidative conditions.

Employing Aryl-Linked Bis-mesoionic Carbenes as a Pincer-Type Platform to Access Ambient-Stable Palladium(IV) Complexes

The study of palladium(IV) species has great implications for Pd^II /Pd^IV -mediated catalysis. However, most of the Pd^IV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize Pd^IV species are dominated by chelating N-donors such as bipyridines. In this work, we present two Pd^IV complexes with scarcely used C-donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC'C]-type pincer framework to access a series of ambient-stable Pd^IV tris(halido) complexes. Their synthesis, solid-state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated Pd^IV -MIC as well as the first Pd^IV carbene-based aryl pincer.

Reactive Heterobimetallic Complex Combining Divalent Ytterbium and Dimethyl Nickel Fragments

This article presented the synthesis and characterization of original heterobimetallic species combining a divalent lanthanide fragment and a divalent nickel center bridged by the bipyrimidine ligand, a redox-active ligand. X-ray crystal structures were obtained for the Ni monomer (bipym)NiMe₂, 1, as well as the heterobimetallic dimer compounds, Cp*₂Yb(bipym)NiMe₂, 2, along with ¹H solution NMR, solid-state magnetic data, and DFT calculations only for 1. The reactivity with CO was investigated on both compounds and the stoichiometric acetone formation is discussed based on kinetic and mechanistic studies. The key role of the lanthanide fragment was demonstrated by the relatively slow CO migratory insertion step, which indicated the stability of the intermediate.

Reversible electron transfer in organolanthanide chemistry

This article relates the synthesis and characterization of novel heterobimetallic complexes containing a low-valent lanthanide, a tetradentate redox non-innocent ligand, viz. the 4,5,9,10-tetraazaphenanthrene, taphen ligand and transition metal fragments of PdMe2 and PtMe2. The experimental results are supported by a theoretical study. Investigation of their reduction properties allowed the formation of isostructural original heterotrimetallic complexes containing two Cp*2Yb fragments and the (taphen)MMe2 (M = Pd and Pt) motifs. These complexes are stable in non-coordinating solvent such as toluene but decompose in coordinating solvents such as thf. Investigation of the internal electron transfer shows that the taphen ligand behaves as a two-electrons reservoir but is capable of transferring back only one electron in thf. This reversible electron(s) transfer is rare in organolanthanide chemistry and show the potential interest of these complexes in reductive chemistry. Additionally, the trinuclear complexes feature odd X-ray crystal structures in which a deviation of symmetry is observed. The latter observation was studied in depth using quantum chemistry calculations highlighting the role of non-covalent weak interactions.

Cerium Tetrakis(tropolonate) and Cerium Tetrakis(acetylacetonate) Are Not Diamagnetic but Temperature-Independent Paramagnets

A new synthesis of cerium tetrakis(tropolonate), Ce(trop)₄, where trop is deprotonated 2-hydroxy-2,4,6-cycloheptatrienone) or Ce(O₂C₇H₅)₄, is developed that results in dark-purple crystals whose X-ray crystal structure shows that the geometry of the eight-coordinate compound closely resembles a D_{2 d} dodecahedron, based on shape parameters. The magnetic susceptibility as a function of the temperature (4-300 K) shows that it is a temperature-independent paramagnet, χ = 1.2(3) × 10^-4 emu/mol, and the L_III-edge X-ray absorption near-edge structure spectrum shows that the molecule is multiconfigurational, comprised of a f¹:f⁰ configuration mixture in a 50:50 ratio. Ce(acac)₄ and Ce(tmtaa)₂ (where acac is acetylacetonate and tmtaaH₂ is tetramethyldibenzotetraaza[14]annulene) have similar physical properties, as does the solid-state compound CeO₂. The concept is advanced that trop^-, acac^-, tmtaa^2-, cot^2-, and O^2- are redox-active ligands that function as electron donors, rendering the classification of these compounds according to their oxidation numbers misleading because their magnetic susceptibilities, χ, are positive and their effective magnetic moments, μ_eff, lie in the range of 0.1-0.7 μ_B at 300 K.