Organometallic compounds of the lanthanides. Part 17. Tris[(tetramethylethylenediamine)lithium] hexamethyl derivatives of the rare earths

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.

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.

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.

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.

Element 70 – Ytterbium

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1,3-Diene Polymerization Mediated by Homoleptic Tetramethylaluminates of the Rare-Earth Metals

During the past two decades homoleptic tetramethylaluminates of the trivalent rare-earth metals, Ln(AlMe4)3, have emerged as useful components for efficient catalyst design in the field of 1,3-diene polymerization. Previous work had focused on isoprene polymerization applying Ln(AlMe4)3 precatalysts with Ln = La, Ce, Pr, Nd, Gd and Y, in the presence of Et2AlCl as an activator. Polymerizations employing Ln(AlMe4)3 with Ln = La, Y and Nd along with borate/borane co-catalysts [Ph3C][B(C6F5)4], [PhNMe2H][B(C6F5)4] and [B(C6F5)3] were mainly investigated for reasons of comparison with ancillary ligand-supported systems (cf. half-sandwich complexes). The present study investigates into a total of eleven rare-earth elements, namely Ln = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Y, Er and Lu. A full overview on the polymerization behavior of Ln(AlMe4)3 in the presence of perfluorinated borate/borane cocatalysts and R2AlCl-type activators (R = Me, Et) is provided, probing the monomers isoprene and 1,3-butadiene (and preliminary ethylene). Virtually complete cis-1,4-selectivities are obtained for several catalyst/cocatalyst combinations (e.g., Gd(AlMe4)3/Me2AlCl, >99.9%). Insights into the ‘black box’ of active species are obtained by indirect observations via screening of pre-reaction time and cocatalyst concentration. The microstructure of the polydienes is investigated by combined 1H/13C NMR and ATR-IR spectroscopies. Furthermore, the reaction of [LuMe6(Li(thf)x)3] with AlMe3 has been applied as a new strategy for the efficient synthesis of Lu(AlMe4)3. The solid-state structures of Gd(AlMe4)3 and Tb(AlMe4)3 are reported.

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).

Homoleptic organolanthanide compounds supported by the bis(dimethylsilyl)benzyl ligand

A β-SiH functionalized benzyl anion [C(SiHMe₂)₂Ph]⁻ reacts with early rare earth halides to provide homoleptic tris(alkyl)lanthanides containing secondary interactions in an efficient and high yielding route.

Highly α-regioselective neodymium-mediated allylation of diaryl ketones

The first utility of neodymium as a mediating-metal in the Barbier reaction of diaryl ketones with allyl halides is reported in this paper. This one-pot reaction was highly α-regioselective and was carried out under mild conditions.

A heterobimetallic gallyl complex containing an unsupported Ga-Y bond

The synthesis and characterization of the first unsupported Ga-Y bond in [Y{Ga(NArCH)(2)}{C(PPh(2)NSiMe(3))(2)}(THF)(2)] (Ar = 2,6-diisopropylphenyl) is described; structural and computational analyses are consistent with a highly polarized covalent Ga-Y bond.

Cationic methyl complexes of the rare-earth metals: an experimental and computational study on synthesis, structure, and reactivity

Synthesis, structure, and reactivity of two families of rare-earth metal complexes containing discrete methyl cations [LnMe(2-x)(thf)n]((1+x)+) (x = 0, 1; thf = tetrahydrofuran) have been studied. As a synthetic equivalent for the elusive trimethyl complex [LnMe3], lithium methylates of the approximate composition [Li3LnMe6(thf)n] were prepared by treating rare-earth metal trichlorides [LnCl3(thf)n] with 6 equiv of methyllithium in diethyl ether. Heteronuclear complexes of the formula [Li3Ln2Me9L(n)] (Ln = Sc, Y, Tb; L = Et2O, thf) were isolated by crystallization from diethyl ether. Single crystal X-ray diffraction studies revealed a heterometallic aggregate of composition [Li3Ln2Me9(thf)n(Et2O)m] with a [LiLn2Me9](2-) core (Ln = Sc, Y, Tb). When tris(tetramethylaluminate) [Ln(AlMe4)3] (Ln = Y, Lu) was reacted with less than 1 equiv of [NR3H][BPh4], the dimethyl cations [LnMe2(thf)n][BPh4] were obtained. The coordination number as well as cis/trans isomer preference was studied by crystallographic and computational methods. Dicationic methyl complexes of the rare-earth metals of the formula [LnMe(thf)n][BAr4]2 (Ln = Sc, Y, La-Nd, Sm, Gd-Lu; Ar = Ph, C6H4F-4) were synthesized, by protonolysis of either the ate complex [Li3LnMe6(thf)n] (Ln = Sc, Y, Gd-Lu) or the tris(tetramethylaluminate) [Ln(AlMe4)3] (Ln = La-Nd, Sm, Dy, Gd) with ammonium borates [NR3H][BAr4] in thf. The number of coordinated thf ligands varied from n = 5 (Ln = Sc, Tm) to n = 6 (Ln = La, Y, Sm, Dy, Ho). The configuration of representative examples was determined by X-ray diffraction studies and confirmed by density-functional theory calculations. The highly polarized bonding between the methyl group and the rare-earth metal center results in the reactivity pattern dominated by the carbanionic character and the pronounced Lewis acidity: The dicationic methyl complex [YMe(thf)6](2+) inserted benzophenone as an electrophile to give the alkoxy complex [Y(OCMePh2)(thf)5](2+). Nucleophilic addition of the soft anion X(-) (X(-) = I(-), BH4(-)) led to the monocationic methyl complexes [YMe(X)(thf)5](+).

Homoleptic Rare-Earth Metal(III) Tetramethylaluminates: Structural Chemistry, Reactivity, and Performance in Isoprene Polymerization

The complexes [Ln(AlMe4)3] (Ln=Y, La, Ce, Pr, Nd, Sm, Ho, Lu) have been synthesized by an amide elimination route and the structures of [Lu{(micro-Me)2AlMe2}3], [Sm{(micro-Me)2AlMe2}3], [Pr{(micro-Me)2AlMe2}3], and [La{(micro-Me)2AlMe2}2{(micro-Me)3AlMe}] determined by X-ray crystallography. These structures reveal a distinct Ln3+ cation size-dependency. A comprehensive insight into the intrinsic properties and solution coordination phenomena of [Ln(AlMe4)3] complexes has been gained from extended dynamic 1H and 13C NMR spectroscopic studies, as well as 1D 89Y, 2D 1H/89Y, and 27Al NMR spectroscopic investigations. [Ce(AlMe4)3] and [Pr(AlMe4)3] have been used as alkyl precursors for the synthesis of heterobimetallic alkylated rare-earth metal complexes. Both carboxylate and siloxide ligands can be introduced by methane elimination reactions that give the heterobimetallic complexes [Ln{(O2CAriPr)2(micro-AlMe2)}2(AlMe4)(C6H14)n] and [Ln{OSi(OtBu)3}(AlMe3)(AlMe4)2], respectively. [Pr{OSi(OtBu)3}(AlMe3)(AlMe4)2] has been characterized by X-ray structure analysis. All of the cerium and praseodymium complexes are used as precatalysts in the stereospecific polymerization of isoprene (1-3 equivalents of Et2AlCl as co-catalyst) and compared to the corresponding neodymium-based initiators reported previously. The superior catalytic performance of the homoleptic complexes leads to quantitative yields of high-cis-1,4-polyisoprene (>98%) in almost all of the polymerization experiments. In the case of the binary catalyst mixtures derived from carboxylate or siloxide precatalysts quantitative formation of polyisoprene is only observed for nLn:nCl=1:2. The influence of the metal size is illustrated for the heterobimetallic lanthanum, cerium, praseodymium, neodymium, and gadolinium carboxylate complexes, and the highest activities are observed for praseodymium as a metal center in the presence of one equivalent of Et2AlCl.

Generation of Dimethylsilylene and Allylidene Holmium Complexes from Trimethylsilylated Allyl Ligands

The reaction of K[1,3-(SiMe3)2C3H3] with partially hydrated holmium triflate leads to a dimeric complex (1) in which hydrogen abstraction from a trimethylsilyl group has occurred on two allyl ligands, forming dimethylsilylene units that bridge the holmium atoms. When the reaction time is prolonged, a different product (2) is isolated, in which in addition to two dimethylsilylene bridges, the metal centers are joined with a mu-eta1,eta3-allylidene ligand. Both crystallographic and computational studies provide evidence for delocalized bonding in the allylidene fragment.

Nonoctahedral structures

There is always a special reason when six-coordinate molecules or complexes are nonoctahedral. The Jahn-Teller distortion is the longest known example. Also well understood now is the steric activity or nonactivity of a nonbonding electron pair, for example XeF(6). In the past few years, it became obvious that six-coordinate d(0) and d(1) complexes with sigma only ligands such as Mo(CH3)(6) are far from octahedral; rather, they are trigonal prismatic or even C(3v) distorted trigonal prismatic. This phenomenon can be explained with simple molecular orbital or valence bond models.