Ln(II)/Pb(II)–Ln(III)/Pb(0) Redox Approach toward Rare-Earth-Metal Half-Sandwich Complexes

Divalent Lanthanide Tetraisobutylaluminates: Reactivity and Living Isoprene Polymerization

Lanthanide (Ln) tetraisobutylaluminates constitute key components in commercial 1,3-diene polymerization catalysts, and likewise are the homogeneous rare-earth-metal catalysts of prime industrial importance. Discrete divalent rare-earth-metal complexes [Ln(AliBu₄ )₂ ] (Ln=Sm, Eu, Yb) reported here display the first structurally characterized homoleptic metal tetraisobutylaluminates. Treatment of [Ln(AliBu₄ )₂ ] with C₂ Cl₆ gives access to Sm^II /Sm^III mixed-valence cluster [Sm₆ Cl₈ (AliBu₄ )₆ ] and the Yb^II cluster [Yb₄ Cl₄ (AliBu₄ )₄ ], respectively. Reaction with B(C₆ F₅ )₃ leads to hydride abstraction and formation of arene-coordinated hydroborates such as [Sm{HB(C₆ F₅ )₃ }₂ (toluene)₂ ]. Complexes [Ln(AliBu₄ )₂ ] engage in single-component isoprene polymerization, affording high cis-1,4 polyisoprenes with narrow molecular weight distributions. Binary [Yb(AliBu₄ )₂ ]/[HNPhMe₂ ][B(C₆ F₅ )₄ ] fabricates polyisoprene in a perfectly living manner. The catalytically active species are scrutinized by NMR spectroscopy.

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 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.

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}].

Rare-earth-metallocene alkylaluminates trigger distinct tetrahydrofuran activation

Thermal treatment of Cp*2YMe(thf) (Cp* = C5Me5), obtained from Cp*2Y(AlMe4) via donor-induced AlMe3 cleavage, in THF resulted in the concomitant formation of vinyl oxide Cp*2Y(OC2H3)(thf) and 2-ethylene-tetrahydrofuranyl complex Cp*2Y(2-C2H4-OC4H7) via the release of methane. In stark contrast, dissolving Cp*2La(AlMe4) in THF/n-hexane led to the quantitative formation of AlMe3-stabilized 2-tetrahydrofuranyl complex Cp*2La(2-AlMe3-OC4H7), with methane elimination.

Ln(ii) alkyl complexes: from elusive exotics to catalytic applications

The synthesis, structures and reactivity of isolable Ln^II (Ln = Sm, Eu, Yb) alkyl complexes are discussed. The application of Ln^II alkyl derivatives in a variety of catalytic reactions is considered as well.

Emergence of a New [NNN] Pincer Ligand via Si-H Bond Activation and β-Hydride Abstraction at Tetravalent Cerium

The cerium(IV) pyrazolate complexes [Ce(Me2 pz)4 ]2 and [Ce(Me2 pz)4 (thf)] initiate β-hydride abstraction of the bis(dimethylsilyl)amido moiety, to afford a heteroleptic cerium(IV) species containing a dimethylpyrazolyl-substituted silylamido ligand, namely [Ce(Me2 pz)3 (bpsa)] (bpsa=bis((3,5-dimethylpyrazol-1-yl)dimethylsilyl)amido; Me2 pz =3,5-dimethylpyrazolato), along with some cerium(III) species. Remarkably, the nucleophilic attack of the pyrazolyl at the silicon atom and concomitant Si-H-bond cleavage is restricted to the tetravalent cerium oxidation state and appears to proceed via the formation of a transient cerium(IV) hydride, which engages in immediate redox chemistry. When [Ce(Me2 pz)4 ]2 is treated with [Li{N(SiMe3 )2 }], that is, in the absence of the SiH functionality, any redox chemistry did not occur. Instead, the ceric ate complex [LiCe2 (Me2 pz)9 ] and the stable mixed-ligand ceric species [Ce(Me2 pz)2 {N(SiMe3 )2 }2 ] were obtained.

C-H-Bond Activation and Isoprene Polymerization Studies Applying Pentamethylcyclopentadienyl-Supported Rare-Earth-Metal Bis(Tetramethylaluminate) and Dimethyl Complexes

As previously shown for lutetium and yttrium, 1,2,3,4,5-pentamethylcyclopentadienyl (C₅Me₅ = Cp*)-bearing rare-earth metal dimethyl half-sandwich complexes [Cp*LnMe₂]₃ are now also accessible for holmium, dysprosium, and terbium via tetramethylaluminato cleavage of [Cp*Ln(AlMe₄)₂] with diethyl ether (Ho, Dy) and tert-butyl methyl ether (TBME) (Tb). C–H-bond activation and ligand redistribution reactions are observed in case of terbium and are dominant for the next larger-sized gadolinium, as evidenced by the formation of mixed methyl/methylidene clusters [(Cp*Ln)₅(CH₂)(Me)₈] and metallocene dimers [Cp*₂Ln(AlMe₄)]₂ (Ln = Tb, Gd). Applying TBME as a “cleaving” reagent can result in both TBME deprotonation and ether cleavage, as shown for the formation of the 24-membered macrocycle [(Cp*Gd)₂(Me)(CH₂OtBu)₂(AlMe₄)]₄ or monolanthanum complex [Cp*La(AlMe₄){Me₃Al(CH₂)OtBu}] and monoyttrium complex [Cp*Y(AlMe₄)(Me₃AlOtBu)], respectively. Complexes [Cp*Ln(AlMe₄)₂] (Ln = Ho, Dy, Tb, Gd) and [Cp*LnMe₂]₃ (Ln = Ho, Dy) are applied in isoprene and 1,3-butadiene polymerization, upon activation with borates [Ph₃C][B(C₆F₅)₄] and [PhNHMe₂][B(C₆F₅)₄], as well as borane B(C₆F₅)₃. The trans-directing effect of AlMe₃ in the binary systems [Cp*Ln(AlMe₄)₂]/borate is revealed and further corroborated by the fabrication of high-cis-1,4 polybutadiene (97%) with “aluminum-free” [Cp*DyMe₂]₃/[Ph₃C][B(C₆F₅)₄]. The formation of multimetallic active species is supported by the polymerization activity of pre-isolated cluster [(Cp*Ho)₃Me₄(CH₂)(thf)₂].

Trimethylscandium

The hitherto unknown homoleptic tetramethylaluminate complex [Sc(AlMe₄)₃] could be obtained by reacting the ate complex [Li₃ScMe₆(thf)_1.2] with AlMe₃ in the cold. It cocrystallizes with AlMe₃ as [Sc(AlMe₄)₃(Al₂Me₆)_0.5] and decomposes at ambient temperature in n-pentane via multiple C-H bond activations to the mixed methyl/methylidene complex [Sc₃(μ₃-CH₂)₂(μ₂-CH₃)₃(AlMe₄)₂(AlMe₃)₂]. Donor-induced methylaluminate cleavage of [Sc(AlMe₄)₃(Al₂Me₆)_0.5] produced [ScMe₃]_n in good yield, which could be derivatized with trimethyltriazacyclononane (Me₃TACN) to form the structurally characterizable [(Me₃TACN)ScMe₃]. Additionally, half-sandwich complex [Cp*Sc(AlMe₄)₂] and sandwich complex [Cp*₂Sc(AlMe₄)] were accessible by salt metathesis reactions from [Sc(AlMe₄)₃(Al₂Me₆)_0.5] and KCp* (Cp* = C₅Me₅). ⁴⁵Sc NMR spectroscopy was applied as a significant probe to evidence the existence of [ScMe₃]_n. Compounds [(Me₃TACN)ScMe₃] (+624.6 ppm) and [ScMe₃(thf)_x] (+601.7 ppm) gave large ⁴⁵Sc NMR shifts, revealing the strong deshielding effect of the σ-bonded alkyl ligands on the scandium nuclei. Ultimately, cationized [Sc(AlMe₄)₃(Al₂Me₆)_0.5] was employed in isoprene polymerization, leading to polymers in high yields (>95%) and with high (>90%) cis-1,4-polyisoprene content.