Reiner Anwander

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

ion using borate reagents [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4], respectively, results in highly active polymerization catalysts (Table 4). Scandium bis(alkyl) complex 51Sc shows excellent activity for the syndiospecific styrene homopolymerization (activity, 1.36 × 104 (kg PS)/ (mol Sc h); Mw/Mn ) 1.37), and complexes 51, 53, and 54 proved suitable for the coand terpolymerization of a series of monomers.204,206,210-225 8.2.3. Constraint Geometry Complexes Incorporation of the cyclopentadienyl ancillary ligand into a chelate array of (pendant) donor functionalities gives access to prominent Cp derivatives. Since the original introduction by the Bercaw group, the linked amido-cyclopentadienyl (Cp) ligand has advanced to be one of the most versatile ligands for group 4 metal polymerization catalysts.235 Catalysts based on this type of ligand provide a constrained ligand environment but are anticipated to be more active toward sterically demanding monomers than metallocenes. Aminecyclopentadiene proligands react with Ln(CH2SiMe3)3(thf)x (Jthf) according to an alkane-elimination reaction in a similar manner as shown in Scheme 30. Because of the dianionic nature of the resulting ligand, only one [CH2SiMe3] ligand is retained in the resulting compounds, which allows for further derivatization (Chart 11). In the presence of Ph3SiH or H2, complexes 66-73 form dimeric hydrido complexes,176,177,204,236 showing high potential in the catalytic hydrosilylation of olefins.226,237,238 Catalytic activities and stereoselectivities are hereby influenced by the length of the linker between the cyclopentadienyl and the amido-functionality and the substituents at the amidonitrogen.237 Remarkable catalytic activity was observed for complexes 68cyclohexyl. Upon activation with equimolar amounts of [Ph3C][B(C6F5)4], such compounds polymerized ethylene and isoprene, regiospecifically yielding 3,4-polyisoprene with isotactic-rich stereo microstructures and relatively narrow molecular weight distribution (Mw/Mn ) 1.8).222 Complex 67Y was found to initiate the polymerization of the polar monomers tert-butyl acrylate and acrylonitrile, however, yielding atactic polymeric products (see Table 5).204 8.2.4. Complexes with Neutral Nitrogenand Oxygen-Based Ligands While early work in organorare-earth metal chemistry was dominated by complexes supported by cyclopentadienyl-type ligands of varying substitution and modification, the limitations inherent to these ligand sets triggered the development of alternative ancillary ligands. Particularly in the past 15 years, advanced ligand design gave access to a wide variety of rare-earth metal complexes supported by noncyclopentadienyl ligand environments. Because of the Lewis acidic nature of the rare-earth metal ions, ligands based on the hard donor elements oxygen and nitrogen are most commonly used, while some notable exceptions have been reported. To avoid ligand redistribution, multidentate ligands are generally favored. Since rare-earth metal cations are invariable in the +3 oxidation state (except Eu(II), Sm(II), Yb(II), and Ce(IV)), neutral, monoanionic, or dianionic ligand sets are the most desirable. Table 4. Further Applications of Half-Sandwich Complexes (Cp)Ln(CH2SiMe3)2(donor)x compound further application ref 50 [CH2SiMe3] exchange reactions 205-209 formation of mono(cations) alternating ethylene-norbornene copolymerization

Rare-Earth Metals and Aluminum Getting Close in Ziegler-Type Organometallics

A prolific and synergetic interplay of rare-earth metal components and organoaluminum reagents is of fundamental importance in Ziegler-type catalysts used in diene polymerization. The present article surveys organoaluminum-promoted alkylation/cationization activation pathways which have been elaborated for various lanthanide compounds, i.e., halides, carboxylates, alkoxides, aryloxides, (silyl)amides, and hydrides. Special emphasis is put on the identification of stable discrete Ln/Al heterobimetallic complexes and the interpretation of their solution and solid-state structure (LnX n Al coordination modes). Surpassing the pure model character of many structurally evidenced Ln/Al heterobimetallics their applicability and performance in the manufacture of polydienes will be highlighted and emerging structure-reactivity relationships will be addressed. Consideration is also given to various heterogenized variants involving organopolymeric and inorganic support materials.

A Rare‐Earth Metal Variant of the Tebbe Reagent

Tebbe reagent [Cp2Ti{(m-CH2)(m-Cl)Al(CH3)2}] (A ; Cp = cyclopentadienyl) belongs to the most enigmatic organometallic compounds. Its successful synthesis, resulting from the careful investigation of the reaction of [Cp2TiCl2] with two equivalents Al(CH3)3, was triggered by important discoveries in two fundamentally different areas of homogeneous catalysis. Indeed, the initial studies of methane (and methylidene) formation from [Cp2TiCl2]/Al(CH3)3 mixtures were conducted in the context of Ziegler–Natta polymerization catalysis, but the methylene unit was structurally characterized by X-ray crystallography for the first time in tantalum alkylidene complexes, such as [Cp2Ta(CH2)(CH3)], [3] and tungsten methylene compounds, related to proposed catalysts for olefin metathesis. Although catalytically active in olefin metathesis, the Tebbe reagent is currently used for efficient carbonyl methylenation reactions. In his initial studies, Tebbe also commented on the synthesis of the structurally similar all-methyl derivative [Cp2Ti{(m-CH2)(m-CH3)Al(CH3)2}] (B) from the labile [Cp2Ti(CH3)2] and Al(CH3)3, suggesting [Cp2Ti(CH3)2{Al(CH3)3}] (B ) as a stabilized intermediate. Although the structures of the bis(neopentyl) derivative [Cp2Ti{(m-CH2)(m-Cl)Al(CH2C(CH3)3)2}] [7] and a zirconium analogue [Cp2Zr{(m-CHCH2C(CH3)3)(m-Cl)Al(CH2CH(CH3)2)2}] [8] have been reported, there are no X-ray structures of the Tebbe reagent nor of discrete metallacycles of the type [M(m-CH2)(m-R)Al(CH3)2] (R = Cl, CH3). [9, 10] Previous studies from our laboratories on rare-earthmetal(III) tetramethylaluminate complexes [LxLn{Al(CH3)4}y] (y = 1, 2, 3; x + y = 3, L = monovalent ancillary ligand, Ln = lanthanides and Sc, Y, La) as polymerization catalysts 12] led to the isolation of Ln clusters with methylene, 14] methine, and carbide functionalities. We also found that complex [Cp*3Y3(m-Cl)3(m3-Cl)(m3-CH2)(thf)3] (Cp* = C5(CH3)5) displayed Tebbe-like reactivity. [13]

SELF-ASSEMBLY IN ORGANOLANTHANIDE CHEMISTRY : FORMATION OF RINGS AND CLUSTERS

Highly symmetrical arrangements of the lanthanide metals Ln including the S6 -symmetrical chair conformations of the [Sm(CN)]6 ring in 1 or an icosahedron as in 2 (see picture) are found in novel multinuclear cyclopentadienyl complexes. The formation of different structural motifs is directed by ionic bonding criteria such as the nature of the bridging heteroligands and the Cp/Ln ratio. [{(C5 Me5 )2 Sm(µ-CN)}6 ] 1 [(C5 Me5 )12 Sm12 (µ3 -Cl)24 ] 2.

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.

Half‐Sandwich Bis(tetramethylaluminate) Complexes of the Rare‐Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization

The protonolysis reaction of [Ln(AlMe(4))(3)] with various substituted cyclopentadienyl derivatives HCp(R) gives access to a series of half-sandwich complexes [Ln(AlMe(4))(2)(Cp(R))]. Whereas bis(tetramethylaluminate) complexes with [1,3-(Me(3)Si)(2)C(5)H(3)] and [C(5)Me(4)SiMe(3)] ancillary ligands form easily at ambient temperature for the entire Ln(III) cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4-(Me(3)C)(3)C(5)H(3)] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X-ray structure analyses of seven representative complexes of the type [Ln(AlMe(4))(2)(Cp(R))] reveal a similar distinct [AlMe(4)] coordination (one eta(2), one bent eta(2)). Treatment with Me(2)AlCl leads to [AlMe(4)] --> [Cl] exchange and, depending on the Al/Ln ratio and the Cp(R) ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe(4))(mu-Cl)(Cp(R))}(2)] and [{Ln(mu-Cl)(2)(Cp(R))}(n)], respectively, have been identified. Complexes [{Y(AlMe(4))(mu-Cl)(C(5)Me(4)SiMe(3))}(2)] and [{Nd(AlMe(4))(mu-Cl){1,2,4-(Me(3)C)(3)C(5)H(2)}}(2)] have been characterized by X-ray structure analysis. All of the chlorinated half-sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe(4))(2)(Cp(R))] with boron-containing cocatalysts, such as [Ph(3)C][B(C(6)F(5))(4)], [PhNMe(2)H][B(C(6)F(5))(4)], or B(C(6)F(5))(3), produces initiators for the fabrication of trans-1,4-polyisoprene. The choice of rare-earth metal cation size, Cp(R) ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe(4))(2)(C(5)Me(4)SiMe(3))]/B(C(6)F(5))(3) (trans-1,4 content: 95.6 %, M(w)/M(n)=1.26) and [La(AlMe(4))(2)(C(5)Me(5))]/B(C(6)F(5))(3) (trans-1,4 content: 99.5 %, M(w)/M(n)=1.18).

β-SiH agostic bonding in sterically crowded lanthanidocene silylamide complexes

Abstract The synthesis as well as the spectroscopic and structural characterization of mononuclear metallocene complexes of the trivalent rare earth elements yttrium and lanthanum is described. Lanthanidocene silylamide complexes were obtained ate-complex free and in high yields according to a silylamine elimination reaction from complexes Ln[N(SiHMe 2 ) 2 ] 3 (THF) 2 (Ln=Y, La) and highly substituted cyclopentadiene derivatives (CH 3 ) 5 C 5 H, (CH 3 ) 4 C 5 H 2 and (C 6 H 5 ) 4 C 5 H 2 . Deprotonation of tetraphenylcyclopentadiene was accomplished only for the lanthanum derivative indicating steric constraints due to the size of the metal cation. IR and 1 H-NMR spectroscopy reveal the presence of asymmetrically bonded silylamide ligands featuring a strong agostic interaction between the electron-deficient metal centers and the SiH moiety of the bis(dimethylsilyl)amide ligand: SiH stretching vibrations as low as 1827 cm −1 are indicative of a distinct weakening of the SiH bonding. X-ray structure analyses of complexes [(CH 3 ) 4 C 5 H] 2 YN(SiHMe 2 ) 2 , [(CH 3 ) 5 C 5 ] 2 YN(SiHMe 2 ) 2 and [(C 6 H 5 ) 4 C 5 H] 2 LaN(SiHMe 2 ) 2 show that the structural features of the agostically bonded bis(dimethylsilyl)amide moiety depend on the steric crowding of the ancillary cyclopentadienyl ligand: Y⋯Si and Y⋯H contacts as close as 3.0506(7) and 2.40(3) A, respectively, are detected, forcing Ln–N–Si angles as low as 99.7(1)°.

Intramolecular Hydroamination/Cyclization of Aminoalkenes Catalyzed by Ln(N(SiMe3)2)3 Grafted onto Periodic Mesoporous Silicas

Homoleptic rare-earth metal silylamide complexes Ln[N(SiMe(3))(2)](3) (Ln = Y, La, Nd) were grafted onto a series of partially dehydroxylated periodic mesoporous silica (PMS) supports, SBA-15(-500) (d(p) = 7.9 nm), SBA-15LP(-500) (d(p) = 16.6 nm), and MCM-41(-500) (d(p) = 4.1 nm). The hybrid materials Ln[N(SiMe(3))(2)](3)@PMS efficiently catalyze the intramolecular hydroamination/cyclization reaction of 2,2-dimethyl-4-penten-1-amine. Under the prevailing slurry conditions the metal size (Y > La > Nd), the pore size, and the particle morphology affect the catalytic performance. Material Y[N(SiMe(3))(2)](3)@SBA-15LP(-500) displayed the highest activity (TOF = up to 420 h(-1) at 60 °C), with the extralarge pores minimizing restrictive product inhibition and substrate diffusion effects. The catalytic activity of Y[N(SiMe(3))(2)](3)@SBA-15LP(-500) is found to be much higher than that of the molecular counterpart (TOF = up to 54 h(-1)), and its recyclability is demonstrated.

Synthesis of highly soluble yttrium–salen complexes and the X-ray structure of N,N′-bis(3,5-di-tert-butylsalicylidene)ethylenediamine-[bis(dimethylsily)amido]yttrium

N,N′-Bis(3,5-di-tert-butylsalicylidene)ethylenediamine H2L reacts with [Y{N(SiHMe2)2}3(thf)2] in thf–hexane at ambient conditions to afford monomeric [YL{N(SiHMe2)2}(thf)], the X-ray structure determination of which reveals a strongly bent coordination mode of the anionic salen derived ligand.

Facile Access to Tetravalent Cerium Compounds: One-Electron Oxidation Using Iodine(III) Reagents

Readily accessible and easy-to-use phenyliodine(III) dichloride, PhICl(2), has been established as an innovative and superior reagent for the one-electron oxidation of cerium(III) complexes, comprising amide, amidinate, and cyclopentadienyl derivatives. Its use allowed the successful synthesis and structural characterization of the first members of three new classes of chloro-functionalized (organo)cerium(IV) compounds, including the long sought-after Cp(3)CeCl.