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.

Tuning of Cu-Al Interactions in Complexes Derived from Tris(pyridonyl-6-methyl)aluminum Metalloligands

A series of bioinspired polar atrane Cu-Al complexes were studied with a combined experimental and computational approach to assess the range and nature of Cu-Al interactions in these novel species. The aluminum metalloligand [Na{Me₂Al(OPy-6-Me)₂}] (2) was furnished in excellent yield (92%) from the nucleophilic attack of Na(OPy-6-Me) to AlMe₃ and the subsequent alkane elimination reaction with 6-methyl-2-hydroxypyridine. At the same time, the metalloligand [Al(OPy-6-Me)₃] (3) was isolated in an also excellent yield (95%) via alkane elimination of AlMe₃ with 6-methyl-2-hydroxypyridine. The zwitterionic Cu-Al atranes [Cu{MeAl(OPy-6-Me)₃}] (5^Me) and [Cu{MesAl(OPy-6-Me)₃}] (5^Mes) were isolated (73 and 97% yields) from metalloligands 2 and 3, respectively. [(Cu{Al(OPy-6-Me)₄})₂(μ-Cu)]⁺ ([6⁺][B(Ar^CF3)₄]) was isolated via a reaction that involves alkane elimination and redistribution reacting from 5^Me with [H(OEt₂)₂][B(Ar^CF3)₄] in benzene solution. Alkane elimination in benzene of either 5^Me or 5^Mes with [HNEt₃][B(Ar^CF3)₄] renders [Cu{(Et₃N)Al(OPy-6-Me)₃}]⁺ (Et₃N-5⁺). The Lewis base-free cationic complex [Cu{Al(OPy-6-Me)₃}]⁺ (5⁺) was isolated in 68% yield upon reacting 3 with [Cu(COD)₂][B(Ar^CF3)₄] in benzene. Metalloligands and complexes were fully characterized with an array of spectroscopic and analytical techniques that include multinuclear NMR, ATR-IR, ESI-spectrometry, combustion microanalysis, cyclic voltammetry (CV), and, whenever feasible, SCXRD. X-ray and DFT parameters indicate that the strength of the Cu→Al transannular interaction follows the trend 5⁺ > Et₃N-5⁺ > [6⁺][B(Ar^CF3)₄], 5^Me, and 5^Mes in a smooth transition from zwitterionic species where the Cu-Al interaction is nonexistent to moderate Cu-Al Z-type interactions. CV, in conjunction with DFT calculations of Et₃N-5⁺ and 5⁺, hint at the generation in the electrochemical cell of the radical species 5^rad at -1.82 V and the anionic complex 5^- at -2.32 V vs Fc/Fc⁺, respectively. The proposed species 5^rad exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5^- a 2-center/2-electron (2c/2e) bond.

Synthesis, Structure, and Properties of Volatile Lanthanide Dialkyltriazenides

Several dialkyltriazenide complexes of the lanthanide elements neodymium, europium, and erbium have been prepared; these include the homoleptic complex Er(Bu^tN₃Bu^t)₃, the tetrahydrofuran monoadducts Ln(Bu^tN₃Bu^t)₃(THF) where Ln = Nd or Eu, and the lithium salts [Li(THF)][Ln(MeN₃Bu^t)₄] where Ln = Eu or Er. Crystal structures, nuclear magnetic resonance data, and infrared data are reported for all complexes. The di-tert-butyltriazenide complexes are thermally stable, sublime at reasonably low temperatures, and show smooth volatilization without decomposition, which make them potentially useful in lanthanide separation processes and as chemical vapor deposition precursors for lanthanide nitrides and other phases.

Synthetic, Structural, and Computational Studies on Heavier Tetragen and Chalcogen Triazenide Complexes

The syntheses of the triazenide complexes [{N(N^Dipp)₂}₂M] (Dipp = 2,6-di-isopropylphenyl; M = Ge(II) (1), Sn(II) (2), Pb(II) (3), and Te(II) (5)) are described for the first time. These compounds have been characterized by single-crystal X-ray diffraction and heteronuclear NMR spectroscopy. Density functional theory calculations were employed to confirm the presence and nature of the stereochemically active lone pairs in 1-5, alongside the Gibbs energy changes for their general synthesis, which enable the rationalization of observed reactivities.

Kinetic stabilization of low-oxidation state and terminal hydrido main group metal complexes by a sterically demanding N,N′-bis(2,6-terphenyl)triazenide

Thermally robust main group metal complexes featuring terminal hydride ligands are achieved by deploying a sterically demanding N,N′-bis(2,6-terphenyl)triazenide ligand.

Luminescent liquid crystalline hybrid materials by embedding octahedral molybdenum cluster anions with soft organic shells derived from tribenzo[18]crown-6

Crown ethers and their derivatives are versatile building blocks for the design of supramolecular materials. They can be functionalized at will and are well known for their abilities to complex with alkali cations. Here, we show that emissive lanthanide free hybrid materials can be generated by using such building blocks. The organic tribenzo[18]crown-6 central core was functionalized via six-fold Suzuki cross-coupling as a key reaction with three o-terphenyl units which could be converted into their corresponding triphenylenes by the Scholl reaction, leading to novel liquid-crystalline columnar materials. Selected tribenzo[18]crown-6 o-terphenyls could interact with emissive ternary metal cluster compound salts to generate hybrid materials combining the properties of both moieties. Due to synergistic effects and despite the anisometry of the cluster compounds, individual properties such as liquid-crystalline phase stability of the organic part and emission abilities of its inorganic counter-part are enhanced in the hybrid compounds.

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.

A displacement-type fluorescent probe reveals active species in the coordinative polymerization of olefins

The correct identification of active species is an important prerequisite to study the mechanism of coordinative polymerization of olefins, which can afford important theoretical guidance for the design and synthesis of new organometallic catalysts and high-performance polyolefin materials.

Synthesis and stability of homoleptic metal(III) tetramethylaluminates

Whereas a number of homoleptic metal(III) tetramethylaluminates M(AlMe(4))(3) of the rare earth metals have proven accessible, the stability of these compounds varies strongly among the metals, with some even escaping preparation altogether. The differences in stability may seem puzzling given that this class of metals usually is considered to be relatively uniform with respect to properties. On the basis of quantum chemically obtained relative energies and atomic and molecular descriptors of homoleptic tris(tetramethylaluminate) and related compounds of rare earth metals, transition metals, p-block metals, and actinides, multivariate modeling has identified the importance of ionic metal-methylaluminate bonding and small steric repulsion between the methylaluminate ligands for obtaining stable homoleptic compounds. Low electronegativity and a sufficiently large ionic radius are thus essential properties for the central metal atom. Whereas scandium and many transition metals are too small and too electronegative for this task, all lanthanides and actinides covered in this study are predicted to give homoleptic compounds stable toward loss of trimethylaluminum, the expected main decomposition reaction. Three of the predicted lanthanide-based compounds Ln(AlMe(4))(3) (Ln = Ce, Tm, Yb) have been prepared and fully characterized in the present work, in addition to Ln(OCH(2)tBu)(3)(AlMe(3))(3) (Ln = Sc, Nd) and [Eu(AlEt(4))(2)](n). At ambient temperature, donor-free hexane solutions of Ln(AlMe(4))(3) of the Ln(3+)/Ln(2+) redox-active metal centers display enhanced reduction to [Ln(AlMe(4))(2)](n) with decreasing negative redox potential, in the order Eu ≫ Yb ≫ Sm. Whereas Eu(AlMe(4))(3) could not be identified, Yb(AlMe(4))(3) turned out to be isolable in low yield. All attempts to prepare the putative Sc(AlMe(4))(3), featuring the smallest rare earth metal center, failed.