Dicyclopentadienyl complexes of titanium,niobium, and tungsten in the controlled synthesis of poly(methyl methacrylate)

Organometallic compounds Cp₂TiCl₂, (EtC₅H₄)₂NbCl₂, and (PrⁱC₅H₄)₂WCl₂ were assessed as additives that control polymer chain growth in the polymerization of methyl methacrylate. In the presence of compounds mentioned in amounts comparable with that of the initiator, a uniform process with no gel-effect occured and respective linear increase in the molecular weight of the polymer up to high degrees of the monomer conversion was observed.     

Stereoselective formation of ansa-metallocenes in reactions of radical-anionic salts of acenaphthylene with lanthanide and calcium halides

ansa-Metallocenes (⁵:⁵-C₂₄H₁₆)M(THF)₂ (M = Sm (1), Yb (2), Ca (3)) and (⁵:⁵-C₂₄H₁₆)MI(THF) (M = Dy (8), Er (9), Tm (10), Lu (11)) were prepared in 50—90% yields by the in situ reactions of two equivalents of potassium acenaphthylenide K⁺C₁₂H₈ ^– with MI₂ or MI₃, respectively. Complexes 2 and 3 were also obtained by direct reduction of acenaphthylene with ytterbium and calcium naphthalenides, respectively. An ESR signal of the acenaphthylene radical anion, which was observed upon dissolution of compound 2 in THF, indicates that the [C₂₄H₁₆]^2– ansa-ligand dissociated into two [C₁₂H₈]^·– radical anions. Hydrolysis of complex 2 in benzene afforded 1,1",3,3"-tetrahydro-3,3"-biacenaphthylene (4) and 3,3",4,4"-tetrahydro-3,3"-biacenaphthylene (5). The reaction of complex 2 with ZrCl₄ and the reaction of compound 3 with Me₃SiCl proceeded with the cleavage of the C—C bond between two acenaphthylene fragments of the [C₂₄H₁₆]^2– ansa-ligand to produce (²-C₁₂H₈)ZrCl₂(THF)₃ (6) and bis(trimethylsilyl)acenaphthene (Me₃Si)₂C₁₂H₈ (7), respectively. Compounds 1—3, 6, 7, and 11 were characterized by ¹H and ¹³C NMR spectroscopy. The temperature dependence of the ¹H NMR spectrum of compound 11 in tetrahydrofuran is indicative of the dynamic exchange of the solvent molecules in the coordination sphere of the Lu atom. After cooling of the solution to 210 K, the dynamic process was terminated as evidenced by the nonequivalence of the ¹H signals of two acenaphthylene fragments. According to the X-ray diffraction data for complex 11, dimerization of two acenaphthylene radical anions at the Lu atom gave rise to the rac-ansa-metallocene structure. In compound 11, the Lu atom is ⁵-coordinated by two five-membered rings of the acenaphthylene ligands and also by the I atom and the THF molecule. The coordination environment about the Lu atom is a distorted tetrahedron. The average distance between the lutetium atom and the carbon atoms of the five-membered rings is 2.623 .     

Reduction of 2,5-di-tert-butylcyclopentadienone and pyridine with thulium diiodide. Structures of the complexes TmI2(THF)2[η5-But2C5H2O]TmI2(THF)3 and [TmI2(C5H5N)4]2(μ2-N2C10H10)

Reduction of 2,5-di-tert-butylcyclopentadienone with two equivalents of thulium diiodide in tetrahydrofuran afforded the binuclear thulium(iii) complex with the cyclopentadienyl oxide ligand, viz., TmI₂(THF)₂[⁵-Bu^t ₂C₅H₂O]TmI₂(THF)₃ (1). Shielding of the carbonyl carbon atom with two tert-butyl substituents prevents pinacolization of the ketyl radical anions that formed upon one-electron reduction of cyclopentadienone. The reaction of thulium diiodide with an excess of pyridine in tetrahydrofuran gave a product of reductive coupling of two pyridine radical anions, viz., [TmI₂(C₅H₅N)₄]₂(₂-N₂C₁₀H₁₀) (2). The structures of complexes 1 and 2 were established by single-crystal X-ray diffraction analysis.     

Complexes of lanthanum with radical anions of 2,2-bipyridyl and 3,6-di-tert-butyl-o-benzoquinone

Complexes of a rare-earth element containing only one radical-anion ligand have been synthesized and isolated in pure states for the first time. The LaI₂(bpy)(THF)₃ complex has been prepared from [LaI₂(THF)₃]₂(C₁₀H₈) and 2,2-bipyridyl in DME. The semiquinone complex LaI₂(SQ)(THF)₃ has been obtained by reaction of lanthanum iodide with 3,6-di-tert-butyl-o-benzoquinone in THF in the presence of lanthanum powder. ESR spectra of the complexes have been studied.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2278–2280, November, 1995.We thank Mr. A. V. Protchenko for magnetic measurements and Dr. L. G. Abakumova for recording IR spectra and helpful discussion.This work was supported by the Russian Foundation for Basic Research (Project No. 95-03-08443a).     

New sterically hindered di-o-quinones of the biphenyl series

New di-o-quinones of the biphenyl series, namely, 2,2′-dialkyl-5,5′-di-tert-butylbiphenyl-3,4,3′,4′-diquinones, were synthesized. Their structures were established by IR and NMR spectroscopy. The molecular structure of 2,2′-dimethyl-5,5′-di-tert-butylbiphenyl-3,4,3′,4′-diquinone was established by X-ray structural analysis. The structure is characterized by orthogonal (the torsion angle is 82.9°) mutual arrangement ofo-benzoquinone fragments. ESR studies demonstrated that chemical reduction of diquinone proceeds in four oneelectron stages to form paramagnetic mono- and trianions as intermediates. Quinopyrocatechols, which are intermediates in the synthesis of di-o-quinones, were isolated and characterized. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 804–809, April, 1997.     

Synthesis and structure of 1,1′-bis(pentafluorophenyl)tetrahydrodigermane

Dehydrogenation of pentafluorophenylgermane in the presence of catalytic amounts of a Wilkinson complex, certain Rh^I ortho-benzosemiquinolate complexes, or (Ph₃P)₄Pt leads to tetrahydrodigermane, C₆F₅H₂GeGeH₂C₆F₅. X-ray analysis has confirmed thetrans-arrangement of the C₆F₅-cycles at the Ge atoms. The Ge-Ge bond length is 2.394(1) Å. The doublet pattern of the band due to the Ge-H bonds (v 2090 and 2104 cm^–1) in the IR spectrum is probably associated with nonequivalence of the hydrogen atoms in the molecule.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 315–318, February, 1994.     

Anionic bipyridyl complexes of lanthanides. Structure of [Yb(bipy)3][Li(THF)4]

Reactions of Sm^II, Tb^III, Tm^II, Yb^II, and Lu^III iodides with 2,2′-bipyridyllithium in THF afford [Li(THF)₄][Ln(bipy) _n ] complexes (n=3 or 4) containing trivalent lanthanides. X-ray structural analysis demonstrated that in the crystalline state, the Yb derivative has the ionic structure, [Li(THF)₄]⁺[Yb(bipy)₃]^?. In THF solutions, the reversible ligand exchange between metal atoms occurs to yield neutral compounds [Ln(bipy) _n?1(THF) _x ] and [Li(bipy)(THF) _y ]. A decrease in the temperature shifts the equilibrium to ionic pairs.     

Formation and structures of 2,5-di-<Emphasis Type="Italic">tert</Emphasis>-butylcyclopentadienone dimers

The structures of dimers formed from 2,5-di-tert-butylcyclopentadienone in the reaction with alkaline metals and in the Diels-Alder reaction were studied. A photochemical rearrangement with ring contraction was found for the second dimer. Spectral features of the dimers related to steric hindrance were studied by 1D and 2D NMR procedures.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2179–2183, October, 2004.