Rare-Earth Metalloligands for LowValent Cobalt Complexes: Fine Electronic Tuning via Co→RE Dative Interactions

Rare-earth metalloligand supported low-valent cobalt complexes were synthesized by utilizing a small-sized heptadentate phosphinomethylamine LsNH3 and a large-sized arene-anchored hexadentate phosphinomethylamine LlArH3 ligand precursors. The RE(III)-Co(-I)-N2 (RE = Sc, Lu, Y, Gd, La) complexes containing rare-earth metals including the smallest Sc and largest La were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The Co(-I)→RE(III) dative interactions were all polarized with major contributions from the 3dz2 orbital of the cobalt center, which was slightly affected by the identity of rare-earth metalloligands. The IR spectroscopic data and redox potentials obtained from cyclic voltammetry revealed that the electronic property of the Co(-I) center was finely tuned by the rare-earth metalloligand, which was revealed by variation of the ligand systems containing LsN, LmN, and LlAr. Unlike the direct alteration of the electronic property of metal center via an ancillary ligand, such a series of rare-earth metalloligand represents a smooth strategy to tune the electronic property of transition metals.

A scandium metalloligand supported Ni(0) complex with a heterobimetallocycle: versatile reactivity with unsaturated bonds

A N2-bridged tetranuclear Sc(III)-Ni(0) complex featuring a Ni → Sc interaction and a 4-membered [Sc-N-C-Ni] ring was synthesized and characterized. Bimetallic reactivity was demonstrated via reactions with a series of unsaturated compounds containing NC, CN, CC, CO and NN bonds.

Dinitrogen Complexes of Cobalt(-I) Supported by Rare-Earth Metal-Based Metalloligands

Sequential reactions of heptadentate phosphinoamine LH₃ with rare-earth metal tris-alkyl precursor (Me₃SiCH₂)₃Ln(THF)₂ (Ln = Sc, Lu, Yb, Y, Gd) and a low-valent cobalt complex (Ph₃P)₃CoI afforded rare-earth metal-supported cobalt iodide complexes. Reduction of these iodide complexes under N₂ allowed the isolation of the first series of dinitrogen complexes of Co(-I) featuring dative Co(-I) → Ln (Ln = Sc, Lu, Yb, Y, Gd) bonding interactions. These compounds were characterized by multinuclear NMR spectroscopy, X-ray diffraction analysis, electrochemistry, and computational studies. The correlation of N-N vibrational frequencies with the pK_a of [Ln(H₂O)₆]³⁺ showed that strongest activation of N₂ was achieved with the least Lewis acidic Gd(III) ion. Interestingly, these Ln-Co-N₂ complexes catalyzed silylation of N₂ in the presence of KC₈ and Me₃SiCl with turnover numbers (TONs) up to 16, where the lutetium-supported Co(-I) complex showed the highest activity within the series. The role of the Lewis acidic Ln(III) was crucial to achieve catalytic turnovers and tunable reactivity toward N₂ functionalization.

The crucial and multifaceted roles of main-group cations and their salts in iron-mediated cross-couplings

While a broad variety of iron-catalyzed cross-couplings involve the use of main-group organometallics R-[M] as nucleophiles, the role of the [M]ⁿ⁺ cation in the coupling process is generally disregarded. However, several beneficial effects of [M]ⁿ⁺ cations by themselves or involved in ionic salts used as additives have been observed in such procedures. At the molecular level, interaction of those [M]ⁿ⁺ cations with on-cycle organoiron intermediates can proceed in several ways. Intermolecular interactions can be observed, and also the implication of [M]ⁿ⁺ in the iron's first or second coordination sphere, e.g. by ambiphilic coordination of a [M]-X salt to an R-[Fe] bond. The use of [M]ⁿ⁺ cations in the reaction medium is also a powerful strategy enabling control of the distribution of iron oxidation states within the coupling process.

Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion

Low-valent transition metalates─anionic, electronic-rich organometallic complexes─comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M-E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations.Building on our group's long-standing interest in reactive organometallic species, a series of rational progressions in early-to-middle transition-metal chemistry ultimately led to our isolation of a rhenium(I) β-diketiminate cyclopentadienide metalate that displays exceptional reactivity. We have found this Re(I) metalate to be capable of small-molecule activation; notably, the complex reversibly binds dinitrogen in solution and can be utilized to trap N₂ for the synthesis of functionalized diazenido species. By employing isolobal analogues to N₂ (CO and RNC), we were able to thoroughly monitor the mechanism of activation and conclude that the metalate's sodium counterion plays an integral role in promoting dinitrogen activation through a novel side-on interaction. The Re(I) metalate is also used in forming a variety of M-E bonds, including a series of uncommon rhenium-tetrylene (Si, Ge, and Sn) complexes that display varying degrees of multiple bonding. These metal tetrylenes act to highlight deviations in chemical properties within the group 14 elements. Our metalate's utility also applies to metal-metal bond formation, as demonstrated through the synthesis of a heterotetrametallic rhenium-zinc dimer. In this reaction, the Re(I) metalate performs a dual role as a reductant and metalloligand to stabilize a transient Zn₂²⁺ core fragment. Finally, the metalate displays unique reactivity with uranium(III) to yield the first transition metal-actinide inverse-sandwich bonds, in this case with three rhenium fragments bound through their Cp moieties surrounding the uranium center. Notably, throughout these endeavors we demonstrate that the metalate displays reactivity at multiple locations, including directly at the rhenium metal center, at a Cp carbon, through a Cp-sandwich mode, or through reversibly bound dinitrogen.Overall, the rhenium(I) metalate described herein demonstrates utility in diverse applications: small-molecule activation, the stabilization of reduced and/or unstable species, and the formation of unconventional M-E/M-M bonds or heterometallic complexes. Moving forward, we suggest that the continued discovery of noncarbonyl, electron-rich transition-metal anions featuring new or unconventional ligands should produce additional reactive organometallic species capable of stabilizing unique structural motifs and performing novel and unusual chemical transformations.

Metal–metal bond in lanthanide single-molecule magnets

This review surveys recent critical advances in lanthanide SMMs, highlighting the influences of metal–metal bonds on the magnetization dynamics.

Phosphinoamido Ligand Supported Heterobimetallic Rare-Earth Metal-Palladium Complexes: Versatile Structures and Redox Reactivities

Heterobimetallic Ln(III)-Pd(0) complexes (Ln = Y, Sm, Gd, Yb) featuring tetranuclear structures with COD as bridges were obtained via the metallation of tris(phosphinoamido) rare-earth metal complexes [Ph2PNAd]3Ln (Ad = admantyl)...

Are lanthanide-transition metal direct bonds a route to achieving new generation {3d-4f} SMMs?

Lanthanide based single-molecule magnets are gaining wide attention due to their potential applications in emerging technologies. One of the main challenges in this area is quenching quantum tunnelling of magnetisation (QTM), which often undercuts the magnetisation reversal barrier. Among the several strategies employed, enhancing exchange coupling has been studied in detail, with large exchanges resulting in stronger quenching of QTM effects. Lanthanides, however, suffer from weak exchanges offered by the deeply buried 4f orbitals and the numerous attempts to enhance the exchange coupling in the {3d-4f} pairs have not exceeded values larger than 30 cm^-1. In this work, using a combination of DFT and the ab initio CASSCF/RASSI-SO method, we have explored lanthanide-transition metal direct bonds as a tool to quench QTM effects. In this direction, we have modelled [PyCp₂LnMCp(CO)₂] (Ln = Gd(III), Dy(III), and Er(III) and M = V(0), Mn(0), Co(0) and Fe(I) and here PyCp₂ = [2,6-(CH₂C₅H₃)₂C₅H₃N]₂^- using [PyCp₂DyFeCp(CO)₂] as an example as reported by Nippe et al. (C. P. Burns, X. Yang, J. D. Wofford, N. S. Bhuvanesh, M. B. Hall and M. Nippe, Angew. Chem., Int. Ed. 2018, 57, 8144). Bonding analysis reveals a dative Ln-TM bond with a donation of π(V/Mnd_xy-π*CO) to 5d_z² (Gd) in the case of Gd-V and Gd-Mn and 4s(Co) to 5d_xy/5d_yz (Gd) for Gd-Co with the transition metal ion being found in the low-spin S = ½ configurations in all the cases. B3LYP/TZV (Gd;CSDZ) calculations on [PyCp₂GdMCp(CO)₂] yield J_Gd-V = -46.1 cm^-1, J_Gd-Mn = -57.1 cm^-1, J_Gd-Co = +55.3 cm^-1, J_Gd-Fe⁺ = +13.9 cm^-1, J_Gd-Vhs = -162.1 cm^-1 and J_Gd-Mnhs = -343.9 cm^-1 and unveiling record-high J values for {3d-4f} complexes. The mechanism of magnetic coupling is developed, which discloses the dominating presence of strong 3d-4f orbital overlaps in most of the cases studied, leading to antiferromagnetic exchange. When these overlaps are weaker and 3d to Gd(5d_z²), charge transfer dominates, yielding a ferromagnetic coupling for the Gd-Co/Gd-Fe⁺ complexes. Calculations performed on the anisotropic Dy(III) and Er(III) complexes reveal that the ground state g_zz axis lies along the Cp-Ln-Cp axis and the Ln-TM bonds, respectively. Thus the Ln-TM bond hinders the single-ion anisotropy of Dy(III) by offering equatorial ligation and lowering the m_J = ±½ state energy, and at the same time, helping in enhancing the axiality of Er(III). When strong {3d-4f} exchange couplings are introduced, record-high barrier heights as high as 229 cm^-1 were accomplished. Furthermore, the exchange coupling annihilates the QTM effects and suggests the lanthanide-transition metal direct bond as a viable alternative to enhance exchange coupling to bring {3d-4f} complexes back in the race for high-blocking SMMs.

Unsupported Lanthanide-Transition Metal Bonds: Ionic vs Polar Covalent?

Lanthanide-transition metal complexes continue to be of interest, not only because of their synthetic challenge but also of their promising magnetic properties. Computational work examining the chemical bonding between lanthanides and transition metals in PyCp₂Ln-TMCp(CO)₂ (DyPyCp₂^2- = [2,6-(CH₂C₅H₃)₂C₅H₃N]^2-) reveals strong Ln-TM dative bonds. Gas-phase optimized geometries are in good agreement with experimental structures at the density functional theory (DFT) level with large-core pseudopotentials. From La to Lu, there is a small increase in the bond dissociation energy, as well as a decrease in Ln-Fe bond lengths. Energy decomposition analyses attribute this trend to an increase in the electrostatic contribution from the decreasing bond length and a modest increase in the orbital contribution. The natural bond orbital analysis clearly indicates that 3d⁶ "lone pairs" in the [FeCp(CO)₂]^- fragment act as a Lewis bases donating nearly 0.5 electron to Ln virtual orbitals of mainly d character. The interfragment bonding was also quantified by the quantum theory of atoms in molecules, which indicates that the Ln-Fe bond is more covalent than the Ca-Fe bond in the hypothetical CpCa-FeCp(CO)₂ but less covalent than the Zn-Fe bond in the hypothetical CpZn-FeCp(CO)₂. Further comparisons suggest that to the [PyCp₂Ln]⁺ cation the [FeCp(CO)₂]^- anion appears much like a halide. Overall, these Ln-TM dative bonds appear to have strong electrostatic contributions as well as significant orbital mixing and dispersion contributions.

Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions

Metalation of β-diketiminato rare-earth metal complexes L^nacnacLn(PhNCH₂PPh₂)₂ (Ln = Y, Yb, Lu) with (COD)Pd(CH₂SiMe₃)₂ afforded three-coordinate Pd(0) complexes supported by two sterically less bulky phosphines and a Pd → Ln dative interaction. The Pd(0) center is prone to ligation with isonitrile and CO; in the latter case, the insertion of a second CO with the Y-N bond was assisted via a precoordination of CO on the Pd(0) center, which led to the formation of an anionic Pd(0) carbamoyl. The reaction of the Pd-Y complex with iodobenzene showed a remarkable double P-C bond cleavage-formation pathway within the heterobimetallic Pd-Y core to afford (Ph₃P)₂PdI(Ph), imine PhNCH₂, and a β-diketiminato yttrium diiodide. In the related reaction of L^nacnacY(PhNCH₂PPh₂)₂ with (Ph₃P)₂PdI(Ph), the P-C bond cleavage following with a N-C bond formation was observed. Computational studies revealed a synergetic bimetallic mechanism for these reactions.

Heterotrimetallic {LnOVPt} complexes with antiferromagnetic Ln-V coupling and magnetic memory

The new PtVO(SOCR)4 lantern complexes, 1 (R = CH3) and 2 (R = Ph) behave as neutral O-donor ligands to Ln(OR)3 with Ln = Ce, Nd. Four heterotrimetallic complexes with linear {LnOVPt} units were prepared: [Ln(ODtbp)3{PtVO(SOCR)4}] (Ln = Ce, 3Ce (R = CH3), 4Ce (R = Ph); Nd, 3Nd (R = CH3), 4Nd (R = Ph); ODtbp = 2,6-ditertbutylphenolate). Magnetic characterization confirms slow magnetic relaxation behaviour and suggests antiferromagnetic coupling across {Ln-O[double bond, length as m-dash]V} in all four complexes, with variations tunable as a function of Ln and R.

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

Heterobimetallic scandium–group 10 metal complexes with LM → Sc (LM = Ni, Pd, Pt) dative bonds

Heterobimetallic scandium–group 10 metal complexes featuring notable LM → Sc (LM = Ni, Pd, Pt) dative bonding interactions.

Heterobimetallic Pd(0) complexes with Pd→Ln (Ln = Sc, Y, Yb, Lu) dative bonds: rare-earth metal-dominated frustrated Lewis pair-like reactivity

A series of Pd–Ln complexes with Pd→Ln (Ln = Sc, Y, Yb, Lu) dative bonds exhibited notable dynamic structural features and unexpected frustrated Lewis pair-like reactivity.

3d–4f heterometallic complexes by the reduction of transition metal carbonyls with bulky LnII amidinates

Heterometallic lanthanide-transition metal carbonyl complexes [Sm₂–Co₂], [Yb–Co], and [Sm₂–Fe₃] have been synthesized by redox reactions between bulky amidinate stabilized divalent Ln and TM carbonyl complexes.

Bimetallic nickel-lutetium complexes: tuning the properties and catalytic hydrogenation activity of the Ni site by varying the Lu coordination environment

We present three heterobimetallic complexes containing an isostructural nickel center and a lutetium ion in varying coordination environments. The bidentate ⁱPr₂PCH₂NHPh and nonadentate (ⁱPr₂PCH₂NHAr)₃tacn ligands were used to prepare the Lu metalloligands, Lu(ⁱPr₂PCH₂NPh)₃ (1) and Lu{(ⁱPr₂PCH₂NAr)₃tacn} (2), respectively. Reaction of Ni(COD)₂ (where COD is 1,5-cyclooctadiene) and 1 afforded NiLu(ⁱPr₂PCH₂NPh)₃ (3), with a Lu coordination number (CN) of 4 and a Ni-Lu distance, d(Ni-Lu), of 2.4644(2) Å. Complex 3 can further bind THF to form 3-THF, increasing both the Lu CN to 5 and d(Ni-Lu) to 2.5989(4) Å. On the other hand, incorporation of Ni(0) into 2 provides NiLu{(ⁱPr₂PCH₂NAr)₃tacn} (4), in which the Lu coordination environment is more saturated (CN = 6), and the d(Ni-Lu) is substantially elongated at 2.9771(5) Å. Cyclic voltammetry of the three Ni-Lu complexes shows an overall ∼410 mV shift in the Ni(0/I) redox couple, suggesting tunability of the Ni electronics across the series. Computational studies reveal polarized bonding interactions between the Ni 3d _{z ²} (major) and the Lu 5d _{z ²} (minor) orbitals, where the percentage of Lu character increases in the order: 4 (6.0% Lu 5d _{z ²} ) < 3-THF (8.5%) < 3 (9.3%). All three Ni-Lu complexes bind H₂ at low temperatures (-30 to -80 °C) and are competent catalysts for styrene hydrogenation. Complex 3 outperforms 4 with a four-fold faster rate. Additionally, adding increasing THF equivalents to 3, which would favor build-up of 3-THF, decreases the rate. We propose that altering the coordination sphere of the Lu support can influence the resulting properties and catalytic activity of the active Ni(0) metal center.

A new bis-phenolate mesoionic carbene ligand for early transition metal chemistry

A new chelating mesoionic carbene ligand, derived from 1,2,3-triazoles, with two redox-active tert-butyl-phenolate linkers has been synthesized and explored towards its reactivity and electrochemical properties in early transition metal chemistry.

Tuning the molecular antenna effect using donor and acceptor substituents on the optical properties of the [(C5F5)2ThMCp2]2+ and [(C5F5)2ThMCpL2]+ complexes, where M = Fe, Ru and Os and L = CO and C5H5N

In the present work, a theoretical methodology based on DFT is used to establish the effects of the electron rearrangements on the optical properties in a series of Th-Transtion metal complexes.

Towards understanding of lanthanide–transition metal bonding: investigations of the first Ce–Fe bonded complex

The syntheses, structural, and magnetic characterization of three new organometallic Ce complexes stabilized by PyCp₂²⁻ (PyCp₂²⁻ = [2,6-(CH₂C₅H₃)₂C₅H₃N]²⁻) are reported.

Frustrated Lewis pair behavior of a neutral scandium complex

A neutral scandium complex featuring a Sc/P combination underwent typical FLP-type addition reactions to yield Sc/Rh heterobimetallic and scandium phosphazine complexes, respectively.

Metal-Metal Bonding in Uranium-Group 10 Complexes

Heterobimetallic complexes containing short uranium–group 10 metal bonds have been prepared from monometallic IU^IV(OAr^P-κ²O,P)₃ (2) {[Ar^PO]⁻ = 2-tert-butyl-4-methyl-6-(diphenylphosphino)phenolate}. The U–M bond in IU^IV(μ-OAr^P-1κ¹O,2κ¹P)₃M⁰, M = Ni (3–Ni), Pd (3–Pd), and Pt (3–Pt), has been investigated by experimental and DFT computational methods. Comparisons of 3–Ni with two further U–Ni complexes XU^IV(μ-OAr^P-1κ¹O,2κ¹P)₃Ni⁰, X = Me₃SiO (4) and F (5), was also possible via iodide substitution. All complexes were characterized by variable-temperature NMR spectroscopy, electrochemistry, and single crystal X-ray diffraction. The U–M bonds are significantly shorter than any other crystallographically characterized d–f-block bimetallic, even though the ligand flexes to allow a variable U–M separation. Excellent agreement is found between the experimental and computed structures for 3–Ni and 3–Pd. Natural population analysis and natural localized molecular orbital (NLMO) compositions indicate that U employs both 5f and 6d orbitals in covalent bonding to a significant extent. Quantum theory of atoms-in-molecules analysis reveals U–M bond critical point properties typical of metallic bonding and a larger delocalization index (bond order) for the less polar U–Ni bond than U–Pd. Electrochemical studies agree with the computational analyses and the X-ray structural data for the U–X adducts 3–Ni, 4, and 5. The data show a trend in uranium–metal bond strength that decreases from 3–Ni down to 3–Pt and suggest that exchanging the iodide for a fluoride strengthens the metal–metal bond. Despite short U–TM (transition metal) distances, four other computational approaches also suggest low U–TM bond orders, reflecting highly transition metal localized valence NLMOs. These are more so for 3–Pd than 3–Ni, consistent with slightly larger U–TM bond orders in the latter. Computational studies of the model systems (PH₃)₃MU(OH)₃I (M = Ni, Pd) reveal longer and weaker unsupported U–TM bonds vs 3.

Bimetallic rare-earth/platinum complexes ligated by phosphinoamides

The heterometallic early-late 5d/4f binuclear phosphinoamido Ln/Pt(0) complexes [(Ph₂PNHPh)Pt{μ-(Ph₂PNPh)}₃Ln(μ-Cl)Li(THF)₃] (Ln = Y, Lu) and [(Ph₂PNHPh)Pt{μ-(Ph₂PNPh)}₃Ln{η²-(Ph₂PNPh)}][Li(THF)₄] (Ln = Y, Lu) are reported.

Isolation and structural characterization of a mainly ligand-based dimetallic radical

The isolation and crystal structure of a mainly ligand-based dimetallic radical of ruthenium together with induced sp³ C–H bond activation were described.

A structural investigation of heteroleptic lanthanide substituted cyclopentadienyl complexes

The synthesis and structural authentication of novel heteroleptic lanthanide complexes supported by bulky cyclopentadienyl ligands is herein presented. Steric effects play a fundamental role in the coordination motifs.

On the reaction mechanism of redox transmetallation of elemental Yb with Hg(C6F5)2 and subsequent reactivity of Yb(C6F5)2 with pyrazole: a DFT investigation

DFT investigations of the redox transmetallation reaction of the diorganomercurial (Hg(C₆F₅)₂) with Yb metal, yielding Yb(C₆F₅)₂, allowed us to define a very low energy reaction mechanism via a C₆F₅Yb–HgC₆F₅ intermediate.

Bi- and trimetallic rare-earth–palladium complexes ligated by phosphinoamides

Heterometallic early–late 4d/4f bi- and trinuclear phosphinoamido Ln–Pd(0) complexes [(Ph₂PNHPh)Pd{μ-(Ph₂PNPh)}₃Ln(μ-Cl)Li(THF)₃] (Ln = Y, Lu) and [Li(THF)₄][{(Ph₂PNHPh)Pd}₂{μ-(Ph₂PNPh)}₄Ln] (Ln = Y, Lu) are described.

Rare earth–metal bonding in molecular compounds: recent advances, challenges, and perspectives

In this review, all structurally authenticated molecular compounds with direct bonds between rare earth metals and transition or main group metals are summarized. Novel aspects of their syntheses, properties and reactivities are highlighted.

Bis(phenolate) N-heterocyclic carbene rare earth metal complexes: synthesis, characterization and applications in the polymerization of n-hexyl isocyanate

Polyhexyl isocyanantes catalyzed by N-heterocyclic carbene rare earth metal complexes show high molecular weight with narrow molecular weight distribution.