Josef Takats

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]

Synthesis and Reaction of [(TpiPr2)LnH2]3 (Ln = Y, Lu) with CO: Trinuclear Cluster-Bound Propenolate en Route to Selective Formation of Propene

The use of the Tp(iPr(2)) ligand led to the straightforward and high-yield synthesis of rare examples of trinuclear monoligand lanthanide dihydrides, [(Tp(iPr(2)))LnH(2)](3) (Ln = Y, Lu). The Y complex was found to mediate the hydrogenation and coupling of carbon monoxide with exclusive formation of propene via the intermediacy of a cluster-bound propenolate ligand.

Stabilization of a Discrete Lanthanide(II) Hydrido Complex by a Bulky Hydrotris(pyrazolyl)borate Ligand.

Divalent and solvent-free: the ytterbium hydrido complex 1 was obtained by the hydrogenolysis of [(Tp(tBu,Me))Yb(CH(2)SiMe(3))(thf)]. The steric demand of the bulky hydrotris(3-tert-butyl-5-methylpyrazolyl)borate ligand, Tp(tBu,Me), is sufficient to stabilize the dimer, yet facile room-temperature reactions with amines, alkynes, diynes, and CO indicate a rich chemistry of 1.

Synthesis of mixed sandwich complexes of the lanthanides, (h8-C8H8)Ln(h5-C5H5)

Abstract The synthesis of (h8-C8H8)Ln(h5-C5H5) is reported for Ln = Y, Nd, Sm, Ho, Er; IR evidence indicates electrostatic interaction between the lanthanide metal and the organic ligands; the Lewis acidity of the above complexes was ascertained by the isolation of (C8H8)Ho(C5H5)L, where L = NH3, pyridine, THF, CNC6H11.

Heteroleptic Tm(II) complexes: one more success for Trofimenko's scorpionates.

Reaction of TmI2(THF)x with the bulky scorpionate, KTptBu,Me, gave (TptBu,Me)TmI(THF) (1). Complex 1 proved to be a useful starting material for a select number of heteroleptic Tm(II) compounds, (TptBu,Me)TmER, including the first Tm(II)-hydrocarbyl derivative, (TptBu,Me)Tm{CH(SiMe3)2} (2).

Actinide poly(pyrazol-1-yl)borate complexes: synthesis and structure of hydrotris(3,5-dimethylpyrazol-1-yl)boratotrichlorotetrahydrofuran actinide(IV), M[HB(3,5-Me2Pz)3]Cl3(THF) (M=Th and U)

Abstract The reaction of MCl4 with K[HB(3,5-Me2Pz)3] in THF gives the monosubstituted derivatives M[HB(3,5-Me2Pz)3]Cl3(THF) (M=Th (1) and U (2)) in excellent yields. The NMR spectra of the molecules indicate symmetrical structures with equivalent pyrazolyl groups. This has been corroborated by single-crystal X-ray analysis. The uranium center in 2 is seven-coordinate and displays capped octahedral geometry. The tridentate pyrazolylborate moiety and the three Cl atoms define the two staggered triangular faces respectively, the latter is capped by the THF oxygen. The coordination geometry is close to C3v symmetry. The steric congestion imposed by the bulky pyrazolylborate ligand is evidenced by the relative low coordination number and the long U—O bond length, 2.546(4) A. Crystal data 2: monoclinic, P21/n, a=10.195(2), b=14.905(2), c=17.414(4) A, β=100.08(2)°, V= 2605.32 A3 and Z=4. Complex 1 is isomorphous with 2.

[(Tpt-Bu, Me)Yb(μ-H)]2: a fecund precursor to a host of divalent, hydrotris(pyrazolyl)borate supported f-element complexes

Abstract We present an overview of recent work from our laboratories on the chemistry of the divalent lanthanide hydride [(Tp t -Bu,Me )Yb(μ-H)] 2 ( 1 ). The first section deals with the hydrotris(3- tert -butyl, 5-methyl-pyrazolyl)borate, (Tp t -Bu,Me ), ligated divalent lanthanide complexes leading up to the discovery of 1 . The remaining sections are centered on the divalent ytterbium hydride [(Tp t -Bu,Me )Yb(μ-H)] 2 ( 1 ), its preparation, molecular structure, NMR characteristics and reactivity. Three broad reactivity classes will be described, namely reactions with Lewis bases and Lewis acids, such as the perfluoroaryl boranes, σ-bond metathesis reactions generally involving acidic H X bonds, but also an example of C Si bond cleavage, and insertion chemistry, primarily involving alkynes. The molecular structures of several representative complexes have been determined by X-ray crystallography.

Actinide—amide complexes : I. Synthesis and utility of di-η-cyclopentadienylbis(diethylamido)uranium(IV)

Abstract The reaction of two moles of cyclopentadiene with U[N(C2H5)2]4 results in the formation of (η-C5H5)2U[N(C2H5)2]2 in excellent yields. The synthetic utility of the complex as a potential starting material for other (η-C5H5)UX2 compounds is commented upon.

Hydrotris(pyrazolyl)borates. Versatile ligands for f-element complexation

The synthesis, structure and reactivity of bis- and mono-hydrotris(pyrazolyl)borate (TpR,R′) complexes of the divalent lanthanides (Ln = Sm(II) and Yb(II)) are described. The Sm(TpMe2)2 complex undergoes one electron transfer reaction with a variety of reagents yielding well-defined monomeric products. The use of the sterically very demanding TpBu,Mc ligand led to the isolation of a series of (TpBu,Mc LnER complexes that are resistant to ligand redistribution reactions.

Synthesis and structure of tris[dihydrobis(pyrazolyl)borate](tetrahydrofuran)uranium(III), U[H(μ-H)Bpz2]3 (THF): three-center BH§d U(III) interactions in the presence of coordinated THF ligand

Abstract Reaction of UI 3 (THF) 4 with three equivalents of KH 2 Bpz 2 gives the complex U[H(μ-H)Bpz 2 ] 3 (THF) ( 1 ). The molecular structure has been determined by single-crystal X-ray diffraction: monoclinic, space group P 2 1 / c , a = 10.662(2), b = 13.586(3), c = 20.868(6) A , β = 103.32(2)°, V = 2941(1) A 3 and Z = 4 . The six nitrogen donor atoms of the chelating H 2 Bpz 2 − ligands are arranged in a slightly distorted trigonal prismatic geometry. Two of the H 2 Bpz 2 − ligands span triangular edges on a single square face of the trigonal prism, while the third spans the edge common to the remaining two square faces. The oxygen atom of the coordinated THF ligand caps one of the square faces. The remaining two rectangular faces and one of the two triangular faces of the trigonal prism are capped by three-center two-electron BH⋯U bridge bonds, providing an effective ten-coordinate U(III) center. The observation of low frequency BH stretching bands is in accord with the presence of these interactions. In solution the molecule is fluxional. Although the rearrangement is nearly stopped at −100 °C, its nature could not be elucidated. Repeated cycles of dissolution of 1 in toluene and solvent removal yield the THF-free complex U[H(μ-H)Bpz 2 ] 3 ( 2 ). The molecule is also fluxional. The activation energy for equilibration of the BH 2 hydrogens is 51 kJ mol −1 .