Coordination of 2,2‘-Bipyridyl and 1,10-Phenanthroline to Substituted Ytterbocenes: An Experimental Investigation of Spin Coupling in Lanthanide Complexes

Reduction of hexaazatrinaphthylenes by divalent lanthanocenes leads to ligand-based multiconfigurational properties

Reduction of hexaazatrinaphthylene (HAN) and its hexamethyl derivative with [Cp*2Sm(THF)2] or [Cp*2Yb(OEt2)] produces [(Cp*2Ln)3(R6HAN)] (Ln = Sm, Yb; R = H, Me), where the heterocyclic ligand forms as a trianion. The magnetism and electronic structure of these compounds reflect unusual multiconfigurational character within the reduced ligand but not the lanthanide ions.

Synthesis and Reactivity of the Rare-Earth Metal Complexes Bearing the Indol-2-yl-Based NCN Pincer Ligand

The pincer rare-earth dialkyl complexes [κ3-LRE(CH2SiMe3)2 (RE = Lu(1a), Yb(1b), Er(1c), Y(1d), Dy(1e))] with the indol-2-yl-based NCN pincer ligand were synthesized by the reactions of the proligand HL (L = 1-Me2NCH2CH2-3-(2-iPrC6H5N═CH)C8H4N) with RE(CH2SiMe3)3(THF)2. These complexes exhibited a variety of reactivities toward organic compounds such as amines, triphenylphosphine ylide, N-phenylimidazole, pyridine derivatives, and o-carborane leading to σ-bond metathesis, migration insertion, and redox reaction products. The reactions of the dialkyl rare-earth metal complexes with o-carborane afforded the novel NCN pincer-ligated carboryne-based metallacyclopropanes which reacted with diphenyl ketone to give insertion products of the RE-C2-ind and one of the RE-Ccage bonds, while the reaction of the carboryne-based metallacyclopropanes with diphenyldiazomethane produced the di-aza-metallacyclopentanes via the insertions of the N═N bond of the diphenyldiazomethane into two RE-Ccage bonds and the RE-C2-ind bond. The reactions of the dialkyl complexes with 2 equiv of 2,2'-bipyridine afforded the pincer-ligated bis(2,2'-bipyridyl monoanionic radical) complexes via the homolytic redox reaction.

Reductive Dimerization of Alkenes and Allenes Enabled by Photochemically Activated Zinc-Zinc Bonded Compounds

Metal radicals have shown versatile reactivity in modern synthetic chemistry. However, the use of zinc radicals for molecular synthesis has been barely explored. Here, we show that a transient zinc radical can be formed through photoactivation of a zinc-zinc bonded compound, which is able to mediate the selective dimerization of alkenes and allenes. Treatment of dizinc compounds [L2Zn2] [L = CH3C(2,6-iPr2C6H3N)CHC(CH3)(NCH2CH2PR2); R = Ph (LPh) or iPr (LiPr)] with a diverse array of aromatic alkenes under UV irradiation (365 nm) facilely afforded the head-to-head coupling products, i.e., 1,4-dizinciobutanes in high yields. In addition, arylallenes could also be selectively dimerized by the dizinc compound to give 2,5-dizincyl-functionalized 1,5-hexadienes under the same conditions. Control reactions of [LPh2Zn2] in the presence of UV irradiation isolated a zinc phenyl complex and a trimeric zinc phosphide complex resulting from C-P bond cleavage at the tridentate ligand. Reactions of photoactivated dizinc compounds with organic spin traps, i.e., 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 2,2'-bipyridine (2,2'-bpy), successfully isolated zinc radical trapping products [LZnOTEMP] and [LPhZn(2,2'-bpy)·-], respectively. The profile of alkene dimerization was elucidated by density functional theory calculations, which confirmed that a transient zinc radical [LZn·] was initially generated through homolytic Zn-Zn bond cleavage via photoactivation followed by single-electron transfer and radical dimerization. The unique selectivity of the current reaction was also studied computationally.

Guanidinate Yttrium Complexes Containing Bipyridyl and Bis(benzimidazolyl) Radicals

Ancillary ligand scaffolds that sufficiently stabilize a metal ion to allow its coordination to an open-shell ligand are scarce, yet their development is essential for next-generation spin-based materials with topical applications in quantum information science. To this end, a synthetic challenge must be met: devising molecules that enable the binding of a redox-active ligand through facile displacement and clean removal of a weakly coordinating anion. Here, we probe the accessibility of unprecedented radical-containing rare-earth guanidinate complexes by combining our recently discovered yttrium tetraphenylborate complex [{(Me3Si)2NC(NiPr)2}2Y][(μ-η6-Ph)(BPh3)] with the redox-active ligands 2,2'-bipyridine (bpy) and 2,2'-bis(benzimidazole) (Bbim), respectively, under reductive conditions. Our endeavor resulted in the first evidence of guanidinate complexes that contain radicals, namely, a mononuclear bipyridyl radical complex, {(Me3Si)2NC(NiPr)2}2Y(bpy•) (1), and a dinuclear bis(benzimidazolyl) radical-bridged complex, [K(crypt-222)][{(Me3Si)2NC(NiPr)2}2Y]2(μ-Bbim•) (2'). The latter was achieved by an in situ reduction of [{(Me3Si)2NC(NiPr)2}2Y]2(μ-Bbim) (2), which was isolated from a salt metathesis reaction. 1 and 2 were characterized by X-ray crystallography and IR and UV-vis spectroscopy. Variable-temperature electron paramagnetic resonance spectroscopy was applied to gain insight into the distribution of unpaired spin density on 1 and 2'. Density functional theory calculations were conducted on 1 and 2' to elucidate further their electronic structures. The redox activity of 1 and 2' was also probed by electrochemical methods.

The Divalent Lanthanoid Triflates Ln(CF3SO3)2(CH3CN) (Ln=Sm, Eu): Structure, Luminescence, and Magnetism

The reaction of the trivalent lanthanoide triflates Ln(OTf)3 (Ln=Sm, Eu; OTf=CF3SO3 -) with the respective metals in acetonitrile leads to the Ln(II)-triflates Eu(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1053.54(1), b=610.28(5), c=1946.92(2) pm, β =98.611(4)) and Sm(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1054.41(4), b=612.16(2), c=1952.65(7) pm, β =98.524(2)). The isotypic strontium compound Sr(OTf)2(CH3CN) (monoclinic, P21/n, Z=4, a=1056.39(5), b=610.05(3), c=1950.1(1) pm, β =98.900(2)°) has been obtained from SrCO3 and triflic acid. The compounds have been investigated by X-ray diffraction, vibrational spectroscopy, luminescence spectroscopy, cyclic voltammetry, thermal analysis, and magnetic measurements.

Targeted Synthesis of End-On Dinitrogen-Bridged Lanthanide Metallocenes and Their Reactivity as Divalent Synthons

High-yield syntheses of the lanthanide dinitrogen complexes [(Cp2tttM)2(μ-1,2-N2)] (1M, M = Gd, Tb, Dy; Cpttt = 1,2,4-C5tBu3H2), in which the [N2]2- ligands solely adopt the rare end-on or 1,2-bridging mode, are reported. The bulk of the tert-butyl substituents and the smaller radii of gadolinium, terbium, and dysprosium preclude formation of the side-on dinitrogen bonding mode on steric grounds. Elongation of the nitrogen-nitrogen bond relative to N2 is observed in 1M, and their Raman spectra show a major absorption consistent with N═N double bonds. Computational analysis of 1Gd identifies that the local symmetry of the metallocene units lifts the degeneracy of two 5dπ orbitals, leading to differing overlap with the π* orbitals of [N2]2-, a consequence of which is that the dinitrogen ligand occupies a singlet ground state. Magnetic measurements reveal antiferromagnetic exchange in 1M and single-molecule magnet (SMM) behavior in 1Dy. Ab initio calculations show that the magnetic easy axis in the ground doublets of 1Tb and 1Dy align with the {M-N═N-M} connectivity, in contrast to the usual scenario in dysprosium metallocene SMMs, where the axis passes through the cyclopentadienyl ligands. The [N2]2- ligands in 1M allow these compounds to be regarded as two-electron reducing agents, serving as synthons for divalent gadolinium, terbium, and dysprosium. Proof of principle for this concept is obtained in the reactions of 1M with 2,2'-bipyridyl (bipy) to give [Cp2tttM(κ2-bipy)] (2M, M = Gd, Tb, Dy), in which the lanthanide is ligated by a bipy radical anion, with strong metal-ligand direct exchange coupling.

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.

A 2,2'-bipyridyl calcium complex: synthesis, structure and reactivity studies

A 2,2'-bipyridyl calcium complex based on a tridentate ligand [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca(bipy)(THF) (1) was prepared by the reduction of {[CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]CaI(THF)}₂ with potassium graphite in the presence of 2,2'-bipyridine (bipy). Complex 1 is a good Ca(I)synthon, as shown by its reactivity with I₂, PhCH₂SSCH₂Ph, PhCH₂SeSeCH₂Ph and 9-fluorenone, yielding the calcium iodide complex [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]CaI(bipy) (2), calcium thiolate [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca(SCH₂Ph)(bipy) (3), calcium selenolate [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca(SeCH₂Ph)(bipy) (4), and calcium ketyl complex [CH₃C(N-2,6-ⁱPr₂C₆H₃)CHC(CH₃)NCH₂CH₂N(CH₃)₂]Ca[O-(9-C₁₃H₈˙)](bipy)·2THF (5·2THF), respectively. In addition, reactions of complex 5 with CS₂, CH₂CHCH₂Br and PhCH₂Br give the corresponding dimeric bis(thiolate) complex {[S₂CC(CMe(NAr))C(Me)NCH₂CH₂NMe₂]Ca(DME)}₂ (6), dimeric calcium bromide complex {[(9-CH₂CHCH₂-C₁₃H₈-9)-O]CaBr(THF)(bipy)}₂ (7) and {[(9-C₆H₅CH₂-C₁₃H₈-9)-O]CaBr[O-(9-C₁₃H₈)](bipy)}₂ (8). These results demonstrated that the calcium ketyl complex 5 can also be employed as a single-electron transfer reagent. All the new compounds were characterized by various spectroscopic methods, and their solid-state structures were further confirmed by single-crystal X-ray diffraction analyses.

Back to the future of organolanthanide chemistry

At the dawn of the development of structural organometallic chemistry, soon after the discovery of ferrocene, the description of the LnCp₃ complexes, featuring large and mostly trivalent lanthanide ions, was rather original and sparked curiosity. Yet, the interest in these new architectures rapidly dwindled due to the electrostatic nature of the bonding between π-aromatic ligands and 4f-elements. Almost 70 years later, it is interesting to focus on how the discipline has evolved in various directions with the reports of multiple catalytic reactivities, remarkable potential in small molecule activation, and the development of rich redox chemistry. Aside from chemical reactivity, a better understanding of their singular electronic nature - not precisely as simplistic as anticipated - has been crucial for developing tailored compounds with adapted magnetic anisotropy or high fluorescence properties that have witnessed significant popularity in recent years. Future developments shall greatly benefit from the detailed reactivity, structural and physical chemistry studies, particularly in photochemistry, electro- or photoelectrocatalysis of inert small molecules, and manipulating the spins' coherence in quantum technology.

Impact of 1,10-Phenanthroline-Induced Intermediate Valence on the Luminesence of Divalent Europium Halides

Starting from EuX₂ (X = Cl, Br, I), we systematically investigated a variety of divalent europium complexes containing bidentate 1,10-phenanthroline (Phen) ligands. Depending on the Eu/Phen ratio, mono-, di-, and polynuclear complexes are formed, with the latter yielding one-dimensional _∞¹[EuBr₂(phen)] chains. Seven new divalent europium complexes, [Eu(phen)₄(H₂O)]Br₂·2MeCN, [Eu(phen)₄]I₂·1.7Tol, [EuBr(phen)₃]₂Br₂·4MeCN, [EuCl₂(phen)₂]₂·2MeCN, [EuBr₂(phen)₂]₂, [EuI₂(phen)₂]₂, and [EuBr₂(phen)]_x, are presented in this work. All species show remarkable optical properties based on a partial electron transfer from the Eu^II center to the Phen ligand. The photophysical characterization is further supported by electrochemistry studies in order to describe the intermediate valence of the Eu center.

Synthesis and isolation of dinuclear N,C-chelate organoboron compounds bridged by neutral, anionic, and dianionic 4,4'-bipyridine via reductive coupling of pyridines

The reaction of Bppy(Mes)2 (BN1; ppy = 2-phenylpyridine) and BCH₂ppy(Mes)₂ (BN3) with the reducing reagent KC₈ resulted in C-C bond formation via intermolecular radical coupling to generate the 4,4'-bipyridyl ligand compounds BN2 and BN4. Adding 1 equivalent of KC₈ to a THF solution of BN2 and BN4 generated the 4,4'-bipyridyl radical anions BN2K and BN4K. The dianion species BN2K2 and BN4K2 could be obtained by adding 2 equivalents of KC₈ to the THF solution of BN2 and BN4. In the presence of 2,2,2-cryptand or 18-crown-6, the radical anion salt BN2K(crypt) and the dianion salt BN2K2(18c6)2 were isolated for single-crystal X-ray diffraction analysis. Structural, spectroscopic, and computational studies were performed on the three species of BN2 derivatives (neutral, radical anion, and dianion species). BN2 and BN4 were stable and did not undergo photoisomerization or photoelimination under UV light irradiation.

Reactivity of a Sterical Flexible Pentabenzylcyclopentadienyl Samarocene

Reactivity studies of the classical divalent lanthanide compound [CpBz52Sm] (CpBz5 = pentabenzylcyclopentadienyl-anion) towards diphenyl dichalcogenides and d-element carbonyl complexes led to remarkable results. In the compounds obtained, a different number of Sm-C(phenyl) interactions and differently oriented benzyl groups were observed, suggesting—despite the preference of these interactions in [CpBz52Sm] described in previous studies—a flexible orientation of the benzyl groups and thus a variable steric shielding of the metal center by the ligand. The obtained compounds are either present as monometallic complexes (reduction of the dichalcogenides) or tetrametallic bridged compounds in the case of the d/f-element carbonyl complexes.

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.

Divalent Ytterbium Hydrido Complex Supported by a β-Diketiminato-Based Tetradentate Ligand: Synthesis, Structure, and Reactivity

While the chemistry of trivalent rare-earth metal hydrido complexes has been well developed in the past 40 years, that of the divalent rare-earth metal hydrido complexes remains in its infancy because of the synthetic challenge of such complexes. In this paper, we report the synthesis and structural characterization of a divalent ytterbium hydrido complex supported by a bulky β-diketiminato-based tetradentate ligand. This hydrido complex is a dimer containing two μ-hydrogen ligands, and it easily undergoes a hydrido shift reaction to form a new divalent ytterbium hydrido complex that contains only one hydrido bridge. Furthermore, this hydrido complex reacts with pyridine and pyridine derivatives, showing versatile reactivity [Yb-H addition to pyridine, hydrido shift to ancillary ligand, and ytterbium(II)-center-induced redox reaction with bipyridine]. This hydrido complex reacts with Ph₃P═O, resulting in a P-C^Ph cleavage of Ph₃P═O and an elimination of C₆H₆; on the other hand, the reaction with Ph₃P═S is a hydrido coupling-based redox reaction. The reactions of this hydrido complex with 1 and 2 equiv of PhSSPh clearly indicate that the hydrido coupling-based redox reaction is prior to the ytterbium(II) oxidation-based redox reaction.

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.

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.

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.

Homoleptic tris(6,6′-dimethyl-2,2′-bipyridine) rare earth metal complexes

Homoleptic tris(bipy) rare earth metal complexes were synthesized and structurally characterized. While two parallel bipy radical anions were strongly antiferromagnetically coupled, the remaining bipy radical anion hosted most spin densities.

Distinct electronic structures and bonding interactions in inverse-sandwich samarium and ytterbium biphenyl complexes

Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized by the reduction of their trivalent halide precursors with potassium graphite in the presence of biphenyl. While the samarium complex had a similar structure as previously reported rare earth metal biphenyl complexes, with the two samarium ions bound to the same phenyl ring, the ytterbium counterpart adopted a different structure, with the two ytterbium ions bound to different phenyl rings. Upon the addition of crown ether to encapsulate the potassium ions, the inverse-sandwich samarium biphenyl structure remained intact; however, the ytterbium biphenyl structure fell apart with the concomitant formation of a divalent ytterbium crown ether complex and potassium biphenylide. Spectroscopic and computational studies were performed to gain insight into the electronic structures and bonding interactions of these samarium and ytterbium biphenyl complexes. While the ytterbium ions were found to be divalent with a 4f¹⁴ electron configuration and form a primarily ionic bonding interaction with biphenyl dianion, the samarium ions were in the trivalent state with a 4f⁵ electron configuration and mainly utilized the 5d orbitals to form a δ-type bonding interaction with the π* orbitals of the biphenyl tetraanion, showing covalent character.

Inverse-sandwich samarium and ytterbium biphenyl complexes were synthesized and characterized by X-ray crystallography. Combined experimental and computational studies indicated that they have distinct electronic structures and bonding interactions.

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.

Syntheses of Dianionic α-Iminopyridine Rare-Earth Metal Complexes and Their Catalytic Acitivities toward Dehydrogenative Coupling of Amines with Hydrosilanes

Reactions of [(Me₃Si)₂N]₃RE(μ-Cl)Li(THF)₃ with aminomethylene-substituted pyridine 2-[O(CH₂CH₂)₂NCH₂CH₂NCH₂]C₅H₄N (1) gave the dianionic α-iminopyridine rare-earth metal amido complexes {μ-η²:σ¹:κ¹:κ¹-2-[O(CH₂CH₂)₂NCH₂CH₂NCH]C₅H₄N}₂RE₂[N(SiMe₃)₂]₂ (RE = Y(2a), La(2b), Pr(2c), Nd(2d), Sm(2e), Dy(2f), Er(2g), and Lu (2h)). However, reaction of [(Me₃Si)₂N]₃Y(μ-Cl)Li(THF)₃ with pyridin-2-ylmethyl-substituted amines such as 2-(RNHCH₂)C₅H₄N (R = ^tBu (3a) and 2,6-ⁱPr₂Ph (3b)) or benzyl-substituted amine O(CH₂CH₂)₂NCH₂CH₂NHCH₂C₆H₅ (5) afforded the corresponding yttrium complexes containing monoanionic ligands [2-(RNCH₂)C₅H₄N]₂YN(SiMe₃)₂ (R = ^tBu (4a) and 2,6-ⁱPr₂Ph (4b)) or [O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅][(Me₃Si)₂N)]Y(μ-Cl)(μ-η³-O(CH₂CH₂)₂NCH₂CH₂NCH₂C₆H₅)Li(THF) (6). Dianionic α-iminopyridine rare-earth metal amido complexes showed high catalytic activities for the dehydrogenation coupling reaction of hydrosilanes and amines providing a variety of silylamines in high yields.

2.2.2-Cryptand as a bidentate ligand in rare-earth metal chemistry

Crystal structures demonstrate that 2.2.2-cryptand can function as a κ²-O,O′ bidentate ligand to rare-earth metal ions.

Reduction of 2,2′-Bipyridine by Quasi-Linear 3d-Metal(I) Silylamides—A Structural and Spectroscopic Study

Quasi-linear anionic 3d-metal(I) silylamides are a new and promising class of molecules. Due to their highly negative reduction potential we wanted to test their capability to reduce substrates under coordination of their monoanionic radicaloid form. In a proof of principle study, we present the results of the reaction of metal(I) silylamides of chromium to cobalt with 2,2′-bipyridine (bipy), the redox non-innocence and reducibility of which was already established. In the course of these studies complexes of the type K{18-crown-6}[M(hmds)2(bipy)] (hmds = –N(SiMe3)2) were obtained. These compounds were isolated and thoroughly characterized to confirm the electron transfer onto the bipyridine ligand, which now acts as a radical monoanion. For comparison of the structural changes of the bipyridine ligand, the analogous zinc complexes were also synthesized. Overall our results indicate that anionic metal(I) silylamides are capable of reducing and ligate substrates, even when the electrochemical reduction potential of the latter is by up to 1 V higher.

Steric control in the metal–ligand electron transfer of iminopyridine–ytterbocene complexes

A systematic study of reactions between Cp*₂Yb(THF) and iminopyridine ligands featuring similar electron accepting properties but variable denticity and steric demand, has provided a new example of steric control on the redox chemistry of ytterbocenes.

Synthesis and molecular structure of pentadienyl complexes of the rare-earth metals

In combination with small and difficult to reduce rare-earth metals pdl′ undergoes CH-bond activations instead of sterically induced reductions to form dimeric complexes with a unique bridging six-membered metallacycle as the central structural motif.

Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism

The reaction of (dpp-Bian)Eu^II(dme)₂ (3) (dpp-Bian is dianion of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; dme is 1,2-dimethoxyethane) with 2,2'-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium(II) complex (dpp-Bian)Eu^II(bipy)₂ (9). In contrast the reaction of related ytterbium complex (dpp-Bian)Yb^II(dme)₂ (4) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)Yb^III(bipy)(bipy^-̇) (10) - ytterbium(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex 3 reacts with N,N'-bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)Eu^II(mes-dad)(dme) (11) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-di-tert-butyl-4-(3,6-di-tert-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6-di-tert-butylphenoxy radical) with (dpp-Bian)Eu^II(dme)₂ (3) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)(ArO)Eu^II(dme) (ArO = OC₆H₂-3,6-tBu₂-2-OEt) (12). The molecular structures of complexes 9-12 have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2-310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium(II) complexes (11 and 12). The best-fit isotropic exchange parameters are in good agreement with their density functional theory-computed counterparts.

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

The unique combination of a divalent organolanthanide fragment, Cp*₂Yb, with bipyrimidine (bipym) and a palladium bis-alkyl fragment, PdMe₂, allows the rapid formation and stabilization of a Pd^IV tris-alkyl moiety after oxidative addition with MeI. The crucial role of the organolanthanide fragment is demonstrated by the substitution of bipym by the 4,5,9,10-tetraazaphenanthrene ligand, which drastically modifies the electronic structure and tunes the stability of the Pd^IV species.

Control of Ligand pKa Values Tunes the Electrocatalytic Dihydrogen Evolution Mechanism in a Redox-Active Aluminum(III) Complex

Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pK_a values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis(pyrazolyl)pyridine ligand, ^iPrPz₂P, reacts with 2 equiv of AlCl₃ to afford [(^iPrPz₂P)AlCl₂(THF)][AlCl₄] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(^iPrHPz₂P^-)AlCl₂] (2), where each of the electron- and proton-transfer events is ligand-centered. Protonation of 2 would formally close a catalytic cycle for dihydrogen production. At -1.26 V versus SCE, in a 0.3 M Bu₄NPF₆/tetrahydrofuran solution with salicylic acid or (HNEt₃)⁺ as the source of H⁺, 1 produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electron-transfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis(imino)pyridine system (^PhI₂P) reveals that the pK_a values of the N-donor atoms in ^iPrPz₂P are lower, which facilitates reduction before ligand protonation. In contrast, the ^PhI₂P ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.