Homoleptic Organolanthanum-Catalyzed Carbohalogenation

Metal-halogen exchange reactions are fundamental processes in chemistry that transform organic halides into organometallic reagents. However, using these reactions to build intricate structures in a cascade manner, especially in a catalytic mode, has been a challenge. In this study, we introduce a homoleptic organolanthanum catalyst to initiate lanthanum-halogen exchange and intramolecular carbohalogenation. The catalytic pathway can be achieved through metal-halogen exchange and carbometalation, followed by the extraction of halogen atoms from starting materials. Our approach offers a flexible and sustainable way to create a variety of useful compounds, showcasing its potential in chemical synthesis.

Studies on the Coordination Patterns of Diamine-Bridged Tetraphenoxy Ligands with Group 3 and 4 Metals

Alkane elimination reactions between the diamino- and dianilino-bridged tetrakis(phenolate) proligands 1a,b-H4 and precursors M(CH2SiMe3)3(THF)2, M(CH2C6H4-o-NMe2)3 (M = Sc and Y), and Hf(CH2Ph)4 were investigated. The diamino-bridged 1a-H4 afforded nonsymmetric complex 2a-Y2 incorporating two metal centers in different coordination environments. This one and other dinuclear compounds 2b-Sc2, 2a-Hf2, and 2b-Hf2 were characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction study (for 2a-Y2 and 2b-Sc2) and turned out to be symmetric in solution. Compound 2a-Y2, upon treatment with 2 equiv of 2-phenylpyridine, afforded symmetric bis(aryl) product 3a-Y2, which was authenticated by NMR spectroscopy and X-ray crystallography. The mechanism of its formation was studied by DFT computations and presumably involves a cooperative reorganization process within the nonsymmetric parent 2a-Y2 to afford a symmetric isomer prior to its reaction with 2-phenylpyridine.

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.

Monodentate Alkoxy/Aryloxy Anchored Yttrium Dihydrides; Anticipated Outcomes and Surprises

The synthesis, characterization, and solid-state structure of bulky alkoxy- and aryloxy-supported yttrium polynuclear hydrides are reported. Hydrogenolysis of the supertrityl alkoxy anchored yttrium dialkyl, Y(OTr*)(CH₂SiMe₃)₂(THF)₂ (1) (Tr* = tris(3,5-di-tert-butylphenyl)methyl), resulted in the clean conversion to the tetranuclear dihydride, [Y(OTr*)H₂(THF)]₄ (1a). X-ray analysis revealed a highly symmetrical structure (4̅ site symmetry) with the four Y atoms located on the corners of a compressed tetrahedron, each bonded to an OTr* and tetrahydrofuran (THF) ligand and the cluster held together by four face-capping, μ₃-H, and four edge-bridging, μ₂-H, hydrides. DFT calculations on the full system with and without THF, but also on model systems, clearly show that the structural preference for complex 1a is controlled by the presence and coordination of THF molecules. Contrary to the exclusive formation of the tetranuclear dihydride, hydrogenolysis of the bulky aryloxy yttrium dialkyl, Y(OAr*)(CH₂SiMe₃)₂(THF)₂ (2) (Ar* = 3,5-di-tert-butylphenyl) gave a mixture of the analogous tetranuclear 2a and trinuclear, [Y₃(OAr*)₄H₅(THF)₄], polyhydride, 2b. Similar results, i.e., a mixture of tetra-/tri-nuclear products, were obtained from hydrogenolysis of the even bulkier Y(OAr^Ad2,Me)(CH₂SiMe₃)₂(THF)₂ compound. Experimental conditions were established to optimize the production of either the tetra- or trinuclear products. X-ray structure of 2b revealed a triangular array of three yttrium atoms with two face-capping μ₃-H and three edge-bridging μ₂-H hydrides, with one yttrium bonded to two aryloxy ligands while the other two have a complement of one aryloxy and two THF ligands; the solid-state structure is close to being C₂ symmetric, with the C₂ axis running through the unique Y and unique μ₂-H hydride. As opposed to 2a, which shows distinct ¹H NMR resonances for μ₃/ μ₂-H (δ = 5.83/6.35 ppm, respectively), no hydride signals for 2b were observed at room temperature, indicating hydride exchange on the NMR time scale. Their presence and assignment were secured at -40 °C from ¹H SST (spin saturation) experiment.

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.

Rare-Earth Metal Complexes Supported by 1,3-Functionalized Indolyl-Based Ligands for Efficient Hydrosilylation of Alkenes

Two different 1,3-functionalized indolyl-based proligands 1-(2-C₄H₇O)CH₂-3-(2-^tBuC₆H₅N═CH)C₈H₅N (HL¹) and 1-Me₂NCH₂CH₂-3-(2-ⁱPrC₆H₅N═CH)C₈H₅N (HL²) were designed, prepared in high yields, and successfully applied to rare-earth metal chemistry showing different reactivities and different bondings with the central metals. The reactions of HL¹ with RE(CH₂SiMe₃)₃(THF)₂ provided two types of rare-earth metal complexes: the pincer type mononuclear complexes κ³-(L¹)RE(CH₂SiMe₃)₂ [L¹ = 1-(2-C₄H₇O)CH₂-3-(2-^tBuC₆H₅N═CH)C₈H₄N, RE = Lu(1), Yb(2)], and the dinuclear rare-earth metal alkyl (per alkyl/per metal) complexes having the ligand in novel coordination modes {(η¹:(μ-η²:η¹):η¹-1-(2-C₄H₇O)CH₂-3-[2-^tBuC₆H₅NCH-(CH₂SiMe₃)]C₈H₄N)RECH₂SiMe₃}₂ [RE = Er(3), Y(4), Dy(5), and Gd(6)]. Meanwhile, the reactions of HL² with RE(CH₂SiMe₃)₃(THF)₂ led to the isolation and characterization of only the mononuclear rare-earth metal dialkyl complexes κ³-(L²)RE(CH₂SiMe₃)₂ [L² = 1-Me₂NCH₂CH₂-3-(2-ⁱPrC₆H₅N═CH)C₈H₄N, RE = Lu(7), Gd(8)] bearing the ligand in the pincer chelate form. The mononuclear complexes were formed through the sp² C-H activation of the 2-indolyl moiety, while the dinuclear complexes were produced unexpectedly through the tandem 2-indolyl sp² C-H activation and C═N insertion into the RE-CH₂SiMe₃ bond. These complexes were fully characterized by spectroscopic methods, elemental analyses, and single-crystal X-ray crystallography. The applications of the synthesized complexes as catalysts for the hydrosilylation of terminal alkenes with phenylsilane are described. Anti-Markovnikov addition products were produced by the hydrosilylation of aliphatic olefins, and Markovnikov addition products were isolated with aromatic olefins with high selectivity in the absence of cocatalysts. It is found that the dinuclear rare-earth alkyl complexes exhibited the best catalytic activity with the advantages of mild reaction conditions, short reaction time, low catalyst loading, and wide substrate applicability in comparison with the synthesized mononuclear complexes and the reported catalysts.

Sc and Y bis(alkyl) complexes supported by bidentate and tridentate amidinate ligands. Synthesis, structure and catalytic activity in polymerization of isoprene and 1-heptene

A series of bis(alkyl) complexes {(tBu)C[N(2,6-Me₂C₆H₃)]₂}Ln(CH₂SiMe₃)₂(THF)_n (Ln = Y, n = 1 (1); Ln = Sc, n = 1 (2)), {2-[Ph₂P(O)]C₆H₄NC(tBu)N(2,6-Me₂C₆H₃)}Sc(CH₂SiMe₃)₂ (3), {2-[Ph₂P(NPh)]C₆H₄NC(tBu)N(2,6-Me₂C₆H₃)}Sc(CH₂SiMe₃)₂ (4) coordinated by bidentate (N,N) and tridentate (N,N,O; N,N,N) amidinate ligands are synthesized using an alkane elimination approach. Yttrium complex 1 demonstrated a half-life of ∼2.5 days at room temperature in benzene-D6 (C₆D₆) solution, whereas scandium complexes proved to be much more stable (25 d (2), 30 d (3) and 42 d (4)). Complexes 1-4 as a part of ternary catalytic systems 1-4/TB, HNB/AlR₃ (AlR₃ = AliBu₃, AliBu₂H; TB = [Ph₃C][B(C₆F₅)₄], HNB = [PhNHMe₂][B(C₆F₅)₄]) demonstrated high catalytic activity in isoprene polymerization and enable 80%-100% conversion of 1000 equivalents of monomer into polymer at 25 °C within 3-180 min. The isolated polyisoprenes feature predominantly cis-1,4-regularity (69.2%-87.3%) and polydispersities M_w/M_n = 2.26-8.92. Moreover, the binary (2/TB) and ternary (1-4/TB/10 AliBu₃) systems initiate 1-heptene polymerization providing 40%-100% conversion of 500 equivalents of monomer in 24 h at 25 °C giving polymer samples with M_n = 1.55-190.2 × 10³ and M_w/M_n = 1.55-3.87.

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.

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.

Potential Precursors for Terminal Methylidene Rare-Earth-Metal Complexes Supported by a Superbulky Tris(pyrazolyl)borato Ligand

A series of solvent-free heteroleptic terminal rare-earth-metal alkyl complexes stabilized by a superbulky tris(pyrazolyl)borato ligand with the general formula [TptBu,Me LnMeR] have been synthesized and fully characterized. Treatment of the heterobimetallic mixed methyl/tetramethylaluminate compounds [TptBu,Me LnMe(AlMe4 )] (Ln=Y, Lu) with two equivalents of the mild halogenido transfer reagents SiMe3 X (X=Cl, I) gave [TptBu,Me LnX2 ] in high yields. The addition of only one equivalent of SiMe3 Cl to [TptBu,Me LuMe(AlMe4 )] selectively afforded the desired mixed methyl/chloride complex [TptBu,Me LuMeCl]. Further reactivity studies of [TptBu,Me LuMeCl] with LiR or KR (R=CH2 Ph, CH2 SiMe3 ) through salt metathesis led to the monomeric mixed-alkyl derivatives [TptBu,Me LuMe(CH2 SiMe3 )] and [TptBu,Me LuMe(CH2 Ph)], respectively, in good yields. The SiMe4 elimination protocols were also applicable when using SiMe3 X featuring more weakly coordinating moieties (here X=OTf, NTf2 ). X-ray structure analyses of this diverse set of new [TptBu,Me LnMeR/X] compounds were performed to reveal any electronic and steric effects of the varying monoanionic ligands R and X, including exact cone-angle calculations of the tridentate tris(pyrazolyl)borato ligand. Deeper insights into the reactivity of these potential precursors for terminal alkylidene rare-earth-metal complexes were gained through NMR spectroscopic studies.

Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds

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

Pd-Catalyzed C(sp3)-C(sp2) cross-coupling of Y(CH2SiMe3)3(THF)2 with vinyl bromides and triflates

Pd-Catalyzed C(sp³)-C(sp²) cross-coupling of Y(CH₂SiMe₃)₃(THF)₂ with vinyl bromides and triflates has been developed for efficient synthesis of various allyltrimethylsilanes. The cross-coupling reaction was conducted at room temperature with low catalyst loading of either Pd(PPh₃)₄ or Pd(PPh₃)₂Cl₂, and exhibited high efficiency and a broad substrate scope. In combination with the cross-coupling by the Lewis-acid catalyzed Hosomi-Sakurai reaction, a novel three-component one-pot cascade reaction was then accomplished to deliver homoallylic alcohols and ethers with high regioselectivity and diastereoselectivity. The three-component reaction defined the yttrium complex as a novel one-carbon synthon, which could either trigger bifunctionalization of alkenes or link two electrophiles and would find applications in organic synthesis.

Ligand influence on intramolecular cyclometalation in bis(phosphinimine) rare earth alkyl complexes

The synthesis and reactivity of two new bis(phosphinimine)carbazole ligands (PippN=PMe2)2DMC (HLA, 3) and (PippN=P(C4H8))2DMC (HLB, 10), where Pipp = para-isopropylphenyl and DMC = 3,6-dimethylcarbazole, are reported. Dialkyl lutetium complexes of 3 and 10 were prepared in the presence of DMAP and THF by reaction of the proteo ligands with the new trialkyl reagent, Lu(CH2SiMe3)3(DMAP)2 (4) as well as Lu(CH2SiMe3)3(THF)2. For both ligands 3 and 10, the resulting lutetium complexes were prone to intramolecular cyclometalative alkane elimination reactions whereby the location of cyclometalation was influenced by the identity of the ancillary ligand coordinated to the metal. For ligand 3, cyclometalation of two PMe2 groups generated the complex (LA-κ3N,κ2C)Lu(DMAP)2 (5), whereas ligand 10 resulted in the single ortho-metalation of a para-isopropylphenyl ring to afford (LB-κ3N,κC)Lu(CH2SiMe3) (12). When complexed with scandium, ligand 10 behaved differently; double cyclometalation of two phospholane moieties resulted in the species (LB-κ3N,κ2C)Sc (15). The nature of the cyclometalation reactivity of ligands 3 and 10 is supported by X-ray crystallography and kinetic analysis, respectively.

Simple entry into N-tert-butyl-iminophosphonamide rare-earth metal alkyl and chlorido complexes

New series of rare-earth metal dialkyls [(NPN^tBu)Ln(CH₂SiMe₃)₂(THF)_n] (Ln = Sc,n= 0 (2), Ln = Y,n= 1 (3)) and monoalkyls [(NPN^tBu)₂Ln(CH₂SiMe₃)] (Ln = Y (4), Nd (6), Sm (7)), [(NPN^tBu)₂Sc(THF)CH₃] (5) are achieved by protolysis of highly basic ligand Ph₂P(N-tBu)(NH-tBu) = (NPN^tBu)H (1). Further alkyl abstraction, alkyl/Cl exchange reactions and X-ray structure analyses of thus obtained new complexes are presented.

Palladium-catalyzed C(sp3)–C(sp2) cross-coupling of homoleptic rare-earth metal trialkyl complexes with aryl bromides: efficient synthesis of functionalized benzyltrimethylsilanes

The first C(sp³)–C(sp²) cross-coupling of rare-earth metal alkyl complexes with aryl bromides has been developed for an efficient synthesis of benzyltrimethylsilanes with diverse functional groups.

Bis(alkyl) rare-earth complexes coordinated by bulky tridentate amidinate ligands bearing pendant Ph2PO and Ph2PNR groups. Synthesis, structures and catalytic activity in stereospecific isoprene polymerization

Replacement of Ph₂PO group by Ph₂PNPh leads to a switch of stereoselectivity from cis-1,4 to trans-1,4.

Bis(alkyl) rare-earth complexes supported by a new tridentate amidinate ligand with a pendant diphenylphosphine oxide group. Synthesis, structures and catalytic activity in isoprene polymerization

The introduction of a pendant Ph₂P(O) group into an amidinate ligand resulted in high 1,4-cis selectivity (96.6%) while maintaining very high activity.

Differences in the cyclometalation reactivity of bisphosphinimine-supported organo-rare earth complexes

A novel yttrium complex [L_nY(CH₂SiMe₃)₂] is resistant to cyclometalation, while samarium variants undergo C–H activation, forming unique cyclometalated motifs.

A cascade reaction: ring-opening insertion of dioxaphospholane into lutetium alkyl bonds

Although an organolutetium complex of a new dioxaphospholane-substituted carbazole-based pincer ligand appears immune to cyclometalation, an unusual ring-opening insertion reaction was observed.

Tetrahydropentalenyl-phosphazene constrained geometry complexes of rare-earth metal alkyls

A new sterically demanding rigid cyclopentadienyl chelate ligand facilitates NMR spectroscopic investigations and signal assignment of paramagnetic rare earth organometallics.