Homoleptic Rare-Earth Metal Complexes Containing Ln−C σ-Bonds

Tris-Silanide f-Block Complexes: Insights into Paramagnetic Influence on NMR Chemical Shifts

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

Reactivity of a Neutral Yttrium(III) Trimethyl Complex toward Brønsted and Lewis Acids

The present study corroborates that the neutral tridentate N-ligand 1,4,7-trimethyl-triazacyclononane (Me3TACN) qualifies as a versatile platform to study selective ligand exchange with rare-earth-metal alkyl complexes, herein [(Me3TACN)YMe3]. Treatment with Brønsted-acidic bis(dimethylsilyl)amine, HN(SiHMe2)2, gave selectively the mono-exchanged heteroleptic complex [(Me3TACN)YMe2{N(SiHMe2)2}]. Depending on the molecular ratio employed, the reaction of [(Me3TACN)YMe3] with AlMe3 resulted in the isolation/crystallization of [(Me3TACN)YMe2(AlMe4)] [1 : 1] or ion-separated [(Me3TACN)YMe(AlMe4)][AlMe4] [1 : 2] and [(Me3TACN)YMe(AlMe4)][Al2Me7] [1 : 3]. Analogous reactions with the heavier group 13 methyls GaMe3 and InMe3 generated mixed methyl/tetramethylgallato complex [(Me3TACN)YMe2(GaMe4)] and ion-separated [{(Me3TACN)YMe2}2{μ-Me}][InMe4]. Finally, dimethylalane, HAlMe2, converted [(Me3TACN)YMe3] into heteroaluminate [(Me3TACN)Y(HAlMe3)3], representing an AlMe3-supported, molecular yttrium trihydride complex. All compounds were investigated by single crystal X-ray diffraction (SC-XRD), homo- and heteronuclear (13C, 27Al, 89Y, 115In) NMR as well as IR spectroscopies and elemental analyses.

Rare-Earth-Metal Methyl and Methylidene Complexes Stabilized by TpR,R'-Scorpionato Ligands─Size Matters

Homoleptic tetramethylaluminates Ln(AlMe4)3 react with KTptBu,Me (TptBu,Me = tris(3-tBu-5-Me-pyrazolyl)borato) to yield rare-earth-metal methylidene complexes (TptBu,Me)Ln(μ3-CH2)[(μ-Me)AlMe2]2 (Ln = La, Ce, Nd). The lanthanum reaction is prone to additional C-H- and B-N-bond activation, affording coproducts La[HB(pzMe,tBu)(pzCMe2,Me)2][(μ-CH2)(μ-Me)AlMe2]2 and [La(μ-pztBu,Me)(AlMe4)2]2 (pztBu,Me = 3-tBu-5-Me-pyrazolato). The protonolysis reaction of Ln(AlMe4)3 and HpztBu,Me provides more efficient access to [Ln(μ-pztBu,Me)(AlMe4)2]2 (Ln = La, Nd). Treatment of Ln(AlMe4)3 with KTpMe,Me led to methylidene complexes (TpMe,Me)Ln(μ3-CH2)[(μ-Me)AlMe2]2 (Ln = Nd, Sm) or bis(tetramethylaluminate) complexes (TpMe,Me)Ln(AlMe4)2 (Ln = Y, Lu). The neodymium reaction generated methine derivative (TpMe,Me)Nd[(μ4-CH)(AlMe2)2(μ-pz,Me,Me)][(μ-Me)AlMe2] as a minor coproduct. The reaction of Ln(GaMe4)3 (Ln = Y, La, Ce, Nd, Sm, Ho) with HTptBu,Me gave methylidene complexes (TptBu,Me)Ln(μ3-CH2)[(μ-Me)GaMe2]2 (Ln = La, Ce, Nd, Sm) and alkyl complexes (TptBu,Me)LnMe[(μ-Me)GaMe3] (Ln = Y, Ho), while competing B-N bond activation reactions produced GaMe2[BH(Me)(μ-pztBu,Me)2] and (TptBu,Me)Ln(η2-pztBu,Me)[(μ-Me)GaMe3] (Ln = Y, Ho). The steric impact of the TpR,Me ligands was examined by cone angle calculations. Rare-earth-metal methylidene complexes (TptBu,Me)Ln(μ3-CH2)[(μ-Me)EMe2]2 (E = Al, Ga) successfully promote carbonyl methylenation reactions upon addition of ketone.

Enhanced Control of Isoprene Polymerization with Trialkyl Rare Earth Metal Complexes through Neutral Donor Support

The development of catalysts for stereospecific polymerization of 1,3-dienes is an area of interest due to the robust nature of poly(1,3-diene)s' physical and mechanical properties, as well as the material's versatility in many applications. Dialkyl rare earth metal complexes supported by a diverse cast of ligand frameworks are selective for the polymerization of 1,3-dienes and are an exciting option for examination. However, development in this area has been hampered by the focus on complex catalyst systems that are costly to make. In this study, we synthesize a series of simple homoleptic trialkyl rare earth metal precatalysts and highlight their efficacy for isoprene polymerization using 1 or 2 equiv of [Ph3C][B(C6F5)4] activator. We investigated the addition of commercially available in situ donors, leading to the identification of triphenylphosphine as an ideal support to enhance the dispersity control and prevent loss of catalyst activity. We demonstrated how the activation and reaction conditions, including the order/time of reagent addition and donor electronics, had a major impact on the rate, control, and selectivity for the polymerization of 1,3-dienes. Further interrogation of the catalyst system signals the crucial role of triphenylphosphine in providing enhanced stability and control in this living catalyst system.

Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

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.

Rare-earth metal ethylene and ethyne complexes

Guanidinate homometallic rare-earth ethyl complexes [LLn(μ2-η1:η2-Et)(Et)]2 (Ln = Y(1-Y), Lu(1-Lu)) and heterobimetallic rare-earth ethyl complexes LLn(Et)(μ2-η1:η2-Et)(μ2-η1-Et)(AlEt2) (Ln = Y(2-Y), Lu(2-Lu)) have been synthesized by the treatment of LLn(CH2C6H4NMe2-o)2 (L = (PhCH2)2NC(NC6H3iPr2-2,6)2) with different equivalents of AlEt3 in toluene at ambient temperature. Interestingly, the unprecedented rare-earth ethyne complex [LY(μ2-η1-Et)2(AlEt)]2(μ4-η1:η1:η2:η2-C2H2) (3-Y) containing a [C2H2]4- unit was afforded from 2-Y. The formation mechanism study on 3-Y was carried out by DFT calculations. Furthermore, the nature of the bonding of 3-Y was also revealed by NBO analysis. The reactions of LLn(CH2 C6H4NMe2-o)2 (Ln = Y, Lu) with AlEt3 (4 equiv.) in toluene at 50 °C produced firstly the non-Cp rare-earth ethylene complex LY(μ3-η1:η1:η2-C2H4)[(μ2-η1-Et)(AlEt2)(μ2-η1-Et)2(AlEt)] (4-Y), and the Y/Al ethyl complex LY[(μ2-η1-Et)2(AlEt2)]2 (5-Y) as an intermediate of 4-Y was isolated from the reaction of LY(CH2C6H4NMe2-o)2 with AlEt3 (4 equiv.) in toluene at -10 °C.

Infrared spectroscopic study of solvation and size effects on reactions between water molecules and neutral rare-earth metals

Elucidating the solvation and size effects on the reactions between water and neutral metals is crucial for understanding the microscopic mechanism of the catalytic processes but has been proven to be a challenging experimental target due to the difficulty in size selection. Here, MO4H6 and M2O6H7 (M = Sc, Y, La) complexes were synthesized using a laser-vaporization cluster source and characterized by size-specific infrared-vacuum ultraviolet spectroscopy combined with quantum chemical calculations. The MO4H6 and M2O6H7 complexes were found to have H˙M(OH)3(H2O) and M2(μ2-OH)2(η1-OH)3(η1-OH2) structures, respectively. A combination of experiments and theory revealed that the formation of H˙M(OH)3(H2O) and M2(μ2-OH)2(η1-OH)3(η1-OH2) is both thermodynamically exothermic and kinetically facile in the gas phase. The results indicated that upon the addition of water to H˙M(OH)3, the feature of the hydrogen radical is retained. In the processes from mononuclear H˙M(OH)3 to binuclear M2(μ2-OH)2(η1-OH)3(η1-OH2), the active hydrogen atom undergoes the evolution from hydrogen radical → bridging hydrogen → metal hydride → hydrogen bond, which is indicative of a reduced reactivity. The present system serves as a model for clarifying the solvation and size effects on the reactions between water and neutral rare-earth metals and offers a general paradigm for systematic studies on a broad class of the reactions between small molecules and metals at the nanoscale.

Organolanthanide Complexes Containing Ln-CH3 σ-bonds: Unexpectedly Similar Hydrolysis Rates for Trivalent and Tetravalent Organocerium

We report the gas-phase preparation, isolation, and reactivity of a series of organolanthanides featuring the Ln-CH3 bond. The complexes are formed by decarboxylating anionic lanthanide acetates to form trivalent [LnIII(CH3)(CH3CO2)3]- (Ln = La, Ce, Pr, Nd, Sm, Tb, Tm, Yb, Lu), divalent [EuII(CH3)(CH3CO2)2]-, and the first examples of tetravalent organocerium complexes featuring CeIV-Calkyl σ-bonds: [CeIV(O)(CH3)(CH3CO2)2]- and [CeIV(O)(CH3)(NO3)2]-. Attempts to isolate PrIV-CH3 and TbIV-CH3 were unsuccessful; however, fragmentation patterns reveal that the oxidation of LnIII to a LnIV-oxo-acetate complex is more favorable for Ln = Pr than for Ln = Tb. The rate of Ln-CH3 hydrolysis is a measure of bond stability, and it decreases from LaIII-CH3 to LuIII-CH3, with increasing steric crowding for smaller Ln stabilizing the harder Ln-CH3 bond against hydrolysis. [EuII(CH3)(CH3CO2)2]- engages in a much faster hydrolysis versus LnIII-CH3. The surprising observation of similar hydrolysis rates for CeIV-CH3 and CeIII-CH3 is discussed with respect to sterics, the oxo ligand, and bond covalency in σ-bonded organolanthanides.

Aryl C-H bond functionalization with diphenyldiazomethane induced by rare-earth metal alkyl complexes

The first examples of regioselective aryl ortho-C-H functionalization with diphenyldiazomethane for the construction of Caryl-Nhydrazinato bonds were accomplished via the activation of C-H bonds and the subsequent reaction of diphenyldiazomethane with the RE-Caryl bond. The reactions of rare-earth metal monoalkyl complexes LRE(CH2SiMe3)(THF)2 (L = 2,5-[(2-pyrrolyl)CPh2]2(N-Me-pyrrole)) supported by a neutral N-methylpyrrole anchored dipyrrolyl ligand with 2 equiv. of Ph2CN2 gave irreversibly unprecedented hydrazonato-functionalized imino rare-earth metal complexes LRE(Ph2CNNC6H4-(o-CNHPh) (RE = Y (2a), Lu (2a')) in good yields involving a rather complex process including the interaction of a diazo unit with a RE-Calkyl bond, a β-H elimination, a N-N cleavage, 1,4-hydrogen transfer and the subsequent C-N coupling with another diphenyldiazomethane. More important is that regioselective aryl C-H bond functionalization with diphenyldiazomethane to construct the Caryl-Nhydrazinato bonds can be easily achieved by three-component reactions of rare-earth metal monoalkyl complexes, a wide range of substituted imines (including aldimines, ketimines or analogous 2-phenylpyridine) and diphenyldiazomethane, affording various hydrazonato-functionalized phenyl, thienyl imino or pyridyl rare-earth metal complexes 2b-2j at room temperature. A further study indicated that the substituents on the phenyl ring have a great effect on the reaction pathway and governed the Caryl-Nhydrazinato bond construction. Moreover, the experimental studies show that the formation of the Caryl-Nhydrazinato bonds is thermodynamically facile, which could be realized at room temperature easily.

Probing the Chemical Bond between Lanthanides and Carbon: CeC, PrC, NdC, LuC, and TmC2

Resonant two-photon ionization experiments have been conducted to probe the bond dissociation energy (BDE) of the lanthanide-carbon bond, allowing the BDEs of CeC, PrC, NdC, LuC, and Tm-C₂ to be measured to high precision. Values of D₀(CeC) = 4.893(3) eV, D₀(PrC) = 4.052(3) eV, D₀(NdC) = 3.596(3) eV, D₀(LuC) = 3.685(4) eV, and D₀(Tm-C₂) = 4.797(6) eV are obtained. Additionally, the adiabatic ionization energy of LuC was measured, giving IE(LuC) = 7.05(3) eV. The electronic structure of these species, along with the previously measured LaC, has been further investigated using quantum chemical calculations. Despite LaC, CeC, PrC, and NdC having ground electronic configurations that differ only in the number of 4f electrons present and have virtually identical bond orders, bond lengths, fundamental stretching frequencies, and metallic oxidation states, a peculiar 1.30 eV range in bond dissociation energies exists for these molecules. A natural bond orbital analysis shows that the metal atoms in these molecules have a natural charge of +1 with a 5d² 4fⁿ 6s⁰ configuration while the carbon atom has a natural charge of -1 and a 2p³ configuration. The diabatic bond dissociation energies, calculated with respect to the lowest energy level of this separated ion configuration, show a greatly reduced energy range of 0.32 eV, with the diabatic BDE decreasing as the amount of 4f character in the σ-bond increases. Thus, the wide range of measured BDEs for these molecules is a consequence of the variation in atomic promotion energies at the separated ion limit. TmC₂ has a smaller BDE than the other LnC₂ molecules, due to the tiny amount of 5d participation in the valence molecular orbitals.

Regioselective C-H alkylation of anisoles with olefins by cationic imidazolin-2-iminato scandium(iii) alkyl complexes

A new type of rare-earth alkyl complexes supported by monoanionic imidazolin-2-iminato ligands were synthesised and structurally characterised by X-ray diffraction and NMR analyses. The utility of these imidazolin-2-iminato rare-earth alkyl complexes in organic synthesis was demonstrated by their performance in highly regioselective C-H alkylation of anisoles with olefins. With as low as 0.5 mol% catalyst loading, various anisole derivatives without ortho-substitution or 2-methyl substituted anisoles reacted with several alkenes under mild conditions, producing the corresponding ortho-Csp2-H and benzylic Csp3-H alkylation products in high yield (56 examples, 16-99% yields). Control experiments revealed that rare-earth ions, ancillary imidazolin-2-iminato ligands, and basic ligands were crucial for the above transformations. Based on deuterium-labelling experiments, reaction kinetic studies, and theoretical calculations, a possible catalytic cycle was provided to elucidate the reaction mechanism.

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.

Distinct Reactivity of Trimethylytterbium toward AlMe3 and GaMe3 : Synthesis of Donor-Stabilized Dimethylytterbium

Me₃ TACN (1,4,7-trimethyl-1,4,7-triazacyclononane)-stabilized trimethylytterbium was obtained via a salt-metathesis protocol employing [(Me₃ TACN)YbCl₃ ] and methyllithium. Complex [(Me₃ TACN)YbMe₃ ] seems not to engage in redox chemistry with potassium graphite and is thermally quite stable in the solid state. Treatment of trivalent [(Me₃ TACN)YbMe₃ ] with 3 equiv. of AlMe₃ afforded divalent tetramethylaluminate complex [(Me₃ TACN)Yb(AlMe₄ )₂ ]. The reaction of [(Me₃ TACN)YbMe₃ ] with GaMe₃ in THF gave trivalent ion pair [(Me₃ TACN)YbMe₂ (thf)][GaMe₄ ], which is susceptible to reduction with KC₈ . The thermally very labile divalent [(Me₃ TACN)YbMe(μ-Me)]₂ is the first discrete donor adduct of a divalent dimethyl rare-earth-metal complex.

Synthesis and Characterization of Yttrium Methanediide Silanide Complexes

The salt metathesis reactions of the yttrium methanediide iodide complex [Y(BIPM)(I)(THF)₂] (BIPM = {C(PPh₂NSiMe₃)₂}) with the group 1 silanide ligand-transfer reagents MSiR₃ (M = Na, R₃ = ^tBu₂Me or ^tBu₃; M = K, R₃ = (SiMe₃)₃) gave the yttrium methanediide silanide complexes [Y(BIPM)(Si^tBu₂Me)(THF)] (1), [Y(BIPM)(Si^tBu₃)(THF)] (2), and [Y(BIPM){Si(SiMe₃)₃}(THF)] (3). Complexes 1-3 provide rare examples of structurally authenticated rare earth metal-silicon bonds and were characterized by single-crystal X-ray diffraction, multinuclear NMR and ATR-IR spectroscopies, and elemental analysis. Density functional theory calculations were performed on 1-3 to probe their electronic structures further, revealing predominantly ionic Y-Si bonding. The computed Y-Si bonds show lower covalency than Y═C bonds, which are in turn best represented by Y⁺-C^- dipolar forms due to the strong σ-donor properties of the silanide ligands investigated; these observations are in accord with experimentally obtained ¹³C{¹H} and ²⁹Si{¹H} NMR data for 1-3 and related Y(III) BIPM alkyl complexes in the literature. Preliminary reactivity studies were performed, with complex 1 treated separately with benzophenone, azobenzene, and N,N'-dicyclohexyl-carbodiimide. ²⁹Si{¹H} and ³¹P{¹H} NMR spectra of these reaction mixtures indicated that 1,2-migratory insertion of the unsaturated substrate into the Y-Si bond is favored, while for the latter substrate, a [2 + 2]-cycloaddition reaction also occurs at the Y═C bond to afford [Y{C(PPh₂NSiMe₃)₂[C(NCy)₂]-κ⁴C,N,N',N'}{C(NCy)₂(Si^tBu₂Me)-κ²N,N'}] (4); these reactivity profiles complement and contrast with those of Y(III) BIPM alkyl complexes.

Homoleptic 1-norbornyl complexes of group 4 and manganese: synthesis, structures, volatility, and thermal stability

Detailed syntheses are reported for the previously reported homoleptic 1-norbornyl complexes [M(norb)4] {M = Ti (1), Zr (3), Hf (4), and Mn (5)}. These compounds were characterized by combustion elemental analysis, NMR spectroscopy, single crystal X-ray diffraction (for 1 and 3), and SQUID magnetometry (for 5). The reaction by-product [{(norb)3Ti}2(μ-O)] (2) was also spectroscopically and structurally characterized. To assess the potential viability of 1 and 3–5 as precursors for thin film deposition by CVD or ALD, their volatility and thermal stability were evaluated by sublimation, thermogravimetric analysis, and in the case of 4, bulk thermolysis.

π-Carbazolyl supported bis(alkyl) complexes of Sc, Y and La for α-olefin polymerization and hydrogenation

A series of new half-sandwich bis(alkyl) rare-earth metal complexes coordinated by a sterically demanding 1,3,6,8-tetra-tert-butyl-carbazol-9-yl ligand [tBu₄Carb]La(CH₂C₆H₅)₂(THF) (1-La), [tBu₄Carb]Ln(o-NMe₂C₆H₄CH₂)₂ (Ln = Sc (2-Sc), Y (2-Y), La (2-La), [tBu₄Carb]Ln(CH₂SiMe₃)₂(THF) (Ln = Sc (3-Sc), Y (3-Y)), were synthesized. 1-La, 2-La, and 2-Y were prepared by an alkane elimination protocol, while 2-Sc, 3-Sc, and 3-Y became accessible only when salt metathesis reactions of tBu₄CarbK with R₂Ln(THF)_n⁺[BPh₄]^- were employed. X-ray analysis revealed that in all complexes the carbazolyl ligand exhibits π-coordination with metal ions. 2-Sc and 3-Sc when activated with [Ph₃C][B(C₆F₅)₄] demonstrate excellent activity in α-olefin (octene-1, nonene-1, decene-1 and 1,1-diphenyl-but-1-ene) polymerization. When H₂ was used as a chain transfer agent (1 bar, rt) in the presence of 3-Sc/[Ph₃C][B(C₆F₅)₄] or 2-Y, 2-La olefin hydrogenation occurred with quantitative conversion.

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.

Rare-Earth-Metal-Complex-Catalyzed Hydroalkoxylation and Tandem Hydroalkoxylation/Cyclohydroamination of Isocyanates: Synthesis of Carbamates and Oxazolidinones

Novel N,N,N-tridentate β-diketiminato rare-earth-metal dialkyl complexes LRE(CH₂SiMe₃)₂ [RE = Y (1a), Gd (1b), Yb (1c), Lu (1d); L = MeC(NDipp)CHC(Me)N(CH₂)₂NC₄H₈, where Dipp = 2,6-ⁱPr₂C₆H₃] have been conveniently synthesized by one step from reactions of the rare-earth-metal trialkyl complexes RE(CH₂SiMe₃)₃(THF)₂ (THF = tetrahydrofuran) with a pyrrolidine-functionalized β-diketiminate HL, and their catalytic behaviors toward hydroalkoxylation and tandem hydroalkoxylation/cyclohydroamination of isocyanates have been described. These rare-earth-metal catalysts exhibited high efficiency in the hydroalkoxylation of isocyanates, providing a variety of N-alkyl and N-aryl carbamate derivatives under mild reaction conditions with a rather low catalyst loading (0.04 mol %). More significantly, they can promote a tandem hydroalkoxylation/cyclohydroamination reaction between terminal and internal propargylic alcohols with substituted arylisocyanates, leading to the efficient synthesis of methylene and (Z)-selective arylidene oxazolidinones in good-to-high yields via consecutive C-O and C-N bond formation. The stoichiometric reaction of 1a with p-tolylisocyanate generated an unusual dinuclear yttrium complex, {[η²-(4-MePhNCO)(CH₂SiMe₃)]Y[μ-η²:η¹:η¹-(4-MePhNCO)CC(Me)(NDipp)C(Me)N(CH₂)₂NC₄H₈]}₂ (7a), with two different amidate units, which underwent an sp² C-H bond activation of the β-diketiminato backbone, followed by the insertion of isocyanate.

Reactivity of Mixed Methyl-Aminobenzyl Guanidinate Lutetium Complex towards iPrN=C=NiPr, CS2 and Ph2PH

A heteroleptic terminal alkyl lutetium complex stabilized by a bulky guanidinato ligand, LLu(CH2C6H4NMe2-o)(Me)(THF) (1) (L = (PhCH2)2NC(NC6H3iPr2-2,6)2) has been synthesized by treatment of LLu(CH2C6H4NMe2-o)2 with AlMe3 (1 equiv) via an...

Commercially available palladium salts as practical and green single-component catalysts in the coordination polymerization of 1-chloro-2-phenylacetylenes in air

Commercially available Pd salts serve as single-component catalysts for coordination polymerization of 1-chloro-2-phenylacetylenes, which affords a new design concept of metal catalysts for coordination polymerization of disubstituted alkynes.

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)...

Yttrium tris(trimethylsilylmethyl) complexes grafted onto MCM-48 mesoporous silica nanoparticles

A series of tris(trimethylsilylmethyl) yttrium donor adduct complexes was synthesized and fully characterized by X-ray diffraction, ¹H/¹³C/²⁹Si/³¹P/⁸⁹Y heteronuclear NMR and FTIR spectroscopies as well as elemental analyses. Treatment of Y(CH₂SiMe₃)₃(thf)_x with various donors Do led to complete (Do = TMEDA, DMAP) and partial displacement of THF (Do = NHC^iPr, DMPE). Exceptionally large ⁸⁹Y NMR shifts to low field were observed for the new complexes. Complexes Y(CH₂SiMe₃)₃(tmeda) and Y(CH₂SiMe₃)₃(dmpe)(thf) were chosen to perform surface organometallic chemistry, due to a comparatively higher thermal stability and the availability of the ³¹P nucleus as a spectroscopic probe, respectively. Mesoporous nanoparticles of the MCM-48-type were synthesized and used as a 3^rd generation silica support. The parent and hybrid materials were characterized using X-ray powder diffraction, solid-state-NMR spectroscopy, DRIFTS, elemental analyses, N₂-physisorption, and scanning electron microscopy (SEM). The presence of surface-bound yttrium alkyl moieties was further proven by the reaction with carbon dioxide. Quantification of the surface silanol population by means of HN(SiHMe₂)₂-promoted surface silylation is shown to be superior to titration with lithium alkyl LiCH₂SiMe₃.

A simple one-pot route to stable formamidinatoiodidolanthanoid(III) complexes from lanthanoid metals

A number of formamidinatoiodidolanthanoid(III) complexes, [Ln(DFForm)₂I(thf)₂] and [Ln(DFForm)I₂(thf)₃] (DFFormH = N,N'-bis(2,6-difluorophenyl)formamidine) and [Ln(DippForm)I₂(thf)₃] (DippFormH = N,N'-bis(2,6-diisopropylphenyl)formamidine) have been synthesized in good yields by one-pot direct reactions of the corresponding free metals with iodine and DFFormH or DippFormH in suitable ratios and are stable to rearrangement.

The Alkylaluminate/Gallate Trap: Metalation of Benzene by Heterobimetallic Yttrocene Complexes [Cp*2Y(MMe3R)] (M = Al, Ga)

Yttrocene derivatives [Cp*₂Y(MMe₄)] (Cp* = C₅Me₅; M = Al, Ga) and Cp*₂Y[Me₃Al{B(NDippCH)₂}] (Dipp = C₆H₃iPr₂-2,6) deprotonate benzene at elevated temperatures via the release of methane. The formation of [Cp*₂Y(Me₂MPh₂)] (M = Al, Ga), Cp*₂Y(MPh₄) (M = Al, Ga), Cp*₂Y[Me₂AlPh{B(NDippCH)₂}], and Cp*₂Y[AlPh₃{B(NDippCH)₂}] can be controlled via the temperature applied. The activation temperature and formation of the coordinatively unsaturated "reactive" [Cp*₂YMe] strongly depend on the coordination strength of the displaceable Lewis acids [AlMe₃]₂, GaMe₃, and [Me₂Al{B(NDippCH)₂}]₂. Hence, [Cp*₂Y(AlMe₄)] requires temperatures above 100 °C to metalate benzene, while Cp*₂Y[AlMe₃{B(NDippCH)₂}] undergoes C-H-bond activation even at ambient temperatures. A kinetic deuterium isotope effect was observed for the reactions in C₆D₆ solutions. Distinct differences in the stabilities of the bulky Group 13 anions ([Me₂MPh₂]^-, [MPh₄]^-, [Me₃Al{B(NDippCH)₂}]^-, [Me₂AlPh{B(NDippCH)₂}]^-, and [AlPh₃{B(NDippCH)₂}]^-) are assessed by detailed studies of the coordination chemistry with tetrahydrofuran (THF) and by variable-temperature ¹H NMR spectroscopy. Thus, increased steric bulk or a reduced Lewis acidity of the Group 13 metal center promote temperature-sensitive dissociation of trivalent Group 13 alkyl entities. Consequently, compound Cp*₂Y[AlPh₃{B(NDippCH)₂}] was found to engage in a dissociation equilibrium with [Cp*₂YPh] and AlPh₂{B(NDippCH)₂} in a C₆D₆ solution at ambient temperature. The reaction of Cp*₂Y[AlPh₃{B(NDippCH)₂}] with THF results in the concomitant formation of monometallic Cp*₂YPh(THF) and the solvent-separated ion pair [Cp*₂Y(THF)₂][AlPh₃{B(NDippCH)₂}].

Ytterbium (II) Hydride as a Powerful Multielectron Reductant

A dimeric β-diketiminato ytterbium(II) hydride affects both the two-electron aromatization of 1,3,5,7-cyclooctatetraene (COT) and the more challenging two-electron reduction of polyaromatic hydrocarbons, including naphthalene (E⁰ =-2.60 V). Confirmed by Density Functional Theory calculations, these reactions proceed via consecutive polarized Yb-H/C=C insertion and deprotonation steps to provide the respective ytterbium (II) inverse sandwich complexes and hydrogen gas. These observations highlight the ability of a simple ytterbium(II) hydride to act as a powerful two-electron reductant at room temperature without the necessity of an external electron to initiate the reaction and avoiding radicaloid intermediates.

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.

CeCl3 /n-BuLi: Unraveling Imamoto's Organocerium Reagent

CeCl₃(thf) reacts at low temperatures with MeLi, t‐BuLi, and n‐BuLi to isolable organocerium complexes. Solvent‐dependent extensive n‐BuLi dissociation is revealed by ⁷Li NMR spectroscopy, suggesting “Ce(n‐Bu)₃(thf)_x” or solvent‐separated ion pairs like “[Li(thf)₄][Ce(n‐Bu)₄(thf)_y]” as the dominant species of the Imamoto reagent. The stability of complexes Li₃Ln(n‐Bu)₆(thf)₄ increases markedly with decreasing Ln^III size. Closer inspection of the solution behavior of crystalline Li₃Lu(n‐Bu)₆(thf)₄ and mixtures of LuCl₃(thf)₂/n‐BuLi in THF indicates occurring n‐BuLi dissociation only at molar ratios of <1:3. n‐BuLi‐depleted complex LiLu(n‐Bu)₃Cl(tmeda)₂ was obtained by treatment of Li₂Lu(n‐Bu)₅(tmeda)₂ with ClSiMe₃, at the expense of LiCl incorporation. Imamoto's ketone/tertiary alcohol transformation was examined with 1,3‐diphenylpropan‐2‐one, affording 99 % of alcohol.

Renaissance of Homogeneous Cerium Catalysts with Unique Ce(IV/III) Couple: Redox-Mediated Organic Transformations Involving Homolysis of Ce(IV)-Ligand Covalent Bonds

Recent advances in the catalytic application of cerium complexes were achieved through controlling the Ce(IV/III) redox couple. Although Ce(IV) complexes have been extensively investigated as stoichiometric oxidants in organic synthesis on the basis of their highly positive redox potentials, these complexes can be used as catalysts, not only by introducing supporting ligands around the coordination sphere of cerium, but also by taking advantage of the photoresponsive properties of Ce(IV) and Ce(III) species. Cerium is highly abundant, comparable to that of some first-row transition metals such as copper, nickel, and zinc. Cerium complexes are new and promising homogeneous catalyst candidates for a variety of organic transformations under mild reaction conditions. They are typically used to activate dioxygen to oxidize organic compounds and applied for organic radical generation using the photoresponsive character of Ce(IV) carboxylates and alkoxides as well as electronic transition of Ce(III), in which homolysis of Ce(IV)-ligand covalent bonds is an important step for the overall catalytic cycle. In this Perspective, we first review the early discovery of Ce(OAc)₄-mediated oxidative transformations to emphasize the importance of Ce(IV)-OAc bond homolysis in various C-C bond-forming reactions and its relation to recent developments. We then focus on the fundamental importance of Ce(IV) reactivity involving thermal and photoassisted homolysis of the Ce(IV)-ligand covalent bond and the developments regarding Ce(IV/III) redox changes in catalytic reactions together with our recent findings on cerium-based catalysis.

Selective Coupling of Lanthanide Metallacyclopropenes and Nitriles via Azametallacyclopentadiene and η2-Pyrimidine Metallacycle

Exploring new lanthanide metallacycles and finding their unique chemistry different from the analogues of transition metals are of great interest and importance. In this work, we reported the synthesis, characterization, and reactivity toward nitriles of two lanthanide metallacyclopropenes: lutetacyclopropene 2a and dysprosacyclopropene 2b. The selective coupling of 2a and three molecules of PhCN was found for the first time to provide the unexpected fused lutetacycle 3a with one 1,6-dihydropyrimidine ring. Mechanistic studies by DFT calculations reveal that the triple insertion of PhCN into 2a proceeds through four key steps: the insertion of the first PhCN into 2a giving azalutetacyclopentadiene IM1, the insertion of the second PhCN into the Lu-N bond of IM1, the intramolecular electrocyclization providing a highly strained η²-pyrimidine metallacycle, and the insertion of the third PhCN into the Lu-C_sp3 bond. Isolation and characterization of two active intermediates, azalutetacyclopentadiene IM1 and η²-pyrimidine dysprosacycle, provide critical evidence for the formation of 3a. Furthermore, IM1 was also reported to react with TMSCN, isocyanides, or W(CO)₆ to furnish the fused [4,5] lutetacycles. The chemistry of two lanthanide metallacyclopropenes with nitriles is significantly different from these metallacyclopropenes of scandium and other metals. Most notably, the azalutetacyclopentadienes, η²-pyrimidine complex, and other metallacycles all represent the first examples in rare-earth organometallic chemistry; the formation of these new lutetacycles provides concrete evidence for understanding the mechanism of transition metal promoted or catalyzed [2+2+2] cycloaddition between alkynes and nitriles.