Helmut Sitzmann

Umsetzung von P4 mit (η5-C5H5)(CO)2MoMo(CO)2(η5-C5H5) zu den tetraedrischen molybdänkomplexen Pn[Mo(CO)2(η5-C5H5)]4-n (n = 2,3)

Abstract Interaction of (η 5 -C 5 H 5 ) 2 Mo 2 (CO) 4 , (I) with P 4 affords the tetrahedral molybdenum complexes P n [Mo(CO) 2 (η 5 -C 5 H 5 )] 4- n (II, n = 3 and III, n = 2). The structure of III has been elucidated by X-ray analysis.

Synthese von isopropylsubstituierten cyclopentadienylliganden

Abstract Starting from diisopropylcyclopentadiene, tri- and tetraisopropylcyclopentadienes can be synthesized by deprotonation with sodium amide and then by alkylation with 2-bromopropane. Pure tetraisopropylcyclopentadienylsodium and a 1:4 mixture of 1,2,3- and 1,2,4-triisopropylcyclopentadienylsodium are isolable. 200 MHz 1 H NMR spectra of octaisopropylferrocene were recorded at variable temperature. The activation barrier for tetraisopropylcyclopentadienyl ligand rotation was found to be 56.8 kJ/mol from the Eyring equation. The signal patterns observed in 1 H and 13 C NMR spectra of the ferrocenes [η 5 -C 5 H 5- n (i-Pr) n ] 2 Fe and the molybdenum complexes [η 5 -C 5 H 5- n (i-Pr) n ]Mo(CO) 3 CH 3 ( n = 1–4), are attributed to the presence of enantiotopic and diastereotopic methyl positions.

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).

Maximum spin cyclopentadienyl complexes of 3d transition metals

Abstract For decades maximum spin behavior of transition metal cyclopentadienyl complexes seemed to be limited to a small set of 3d central atoms in certain oxidation states such as V(II), V(III), Cr(III), Mn(II), and nickelocenes. The observation of four unpaired electrons for the Fe(II) central atoms of the tetraisopropylcyclopentadienyliron bromide dimer in 1996 erased such limitations. High spin behavior has then also been observed for chromocenes and nickel(II) half sandwich complexes. Many of these species are easily accessible, highly reactive and serve as versatile starting compounds for a broad range of follow-up products.

(E2)2-einheiten (E = P, As) als clusterbausteine

Abstract The interaction of [C p ★ 2(CO)4Mo2](MoMo) (I) with As4 gives the Mo-As clusters [C p ★ (CO)2Mo(η3-As3)] (II), [C p ★ (CO)4Mo2(μ,η2-As2)] (III) and cis-[C p ★ (CO)Mo(μ,η2-As2)]2 (IV). IV probably has the same structure as cis-[C p ★ (CO)Mo(μ,η2-P2)]2 cis-[C p ★ (CO)Mo(μ-η2-P2)]2 (V) whose derivative, cis-[C p ★ (CO)Mo(μ,η2-P2){Cr(CO)5}2]2 (VI) (C p ★ = η5-C5Me5) has been characterized by an X-ray structure analysis.

Metallocenes of Samarium, Europium, and Ytterbium with the Especially Bulky Cyclopentadienyl Ligands C5H(CHMe2)4, C5H2(CMe3)3, and C5(CHMe2)5

Octaisopropylmetallocenes of the lanthanoids Sm (1-Sm), Eu (1-Eu), and Yb (1-Yb) can be obtained easily from the diiodides of the rare earth elements. Like the hexa(tert.-butyl)metallocenes 2-Sm, 2-Eu, and 2-Yb, they show no tendency towards coordination of donor solvent molecules or alkali salts. The decaisopropylmetallocenes 3-Sm, 3-Eu und 3-Yb have been synthesized from the metal and the free pentaisopropylcyclopentadienyl radical. The three europocenes 1-Eu, 2-Eu, and 3-Eu show fluorescence in daylight or under UV irradiation (336 nm). Crystalline 1-Eu and 2-Eu are bent sandwich complexes, whereas for the decaisopropyl derivative 3-Eu axial symmetry with parallel five-membered rings has been observed.

Cyclopentadienylderivate von Aluminium(I)

Zusammenfassung 27Al-NMR-Untersuchungen zur Bildung neuer Cyclopentadienylaluminium(I)-Derivate ausgehend von AlX (X=Cl, Br, Cp* (=C5Me5)) und MRn (n=1: M=Alkalimetall; n=2: M=Mg; R=Cyclopentadienylderivat (CpD) bzw. N(SiMe3)2): Bildung monomerer und tetramerer Al(I)-Spezies sowie rontgenstrukturanalytische Charakterisierung einer neuen Aluminium(I)-Verbindung mit Al4-tetraedergerust.

SYNTHESIS OF SYMMETRIC METALLOCENES FROM METALLIC CALCIUM, STRONTIUM, OR BARIUM AND PENTAISOPROPYLCYCLOPENTADIENYL RADICALS

The largest interplanar distance known to date between adjacent parallel rings of any sandwich compound (5.497(3) Å) is displayed by decaisopropylbarocene, the first heavy alkaline earth metal sandwich compound to possess axial symmetry. A new efficient metallocene formation [Eq. (1)] utilizes the free cyclopentadienyl radical [C5 R5 ]. (R=CHMe2 ) as an oxidizing agent for elemental Ca, Sr, and Ba (M).

Cyclopentadienylmolybdenum(VI) and Molybdenum(V) Oxo Chemistry: New Synthetic and Structural Features

Convenient syntheses for Cp, Cp*, and related cyclopentadienyl derivatives (4Cp = C5HiPr4; Cp = C5H2tBu31,2,4) of formula [(Ring)2Mo2O5] are described. Compound [Cp2Mo2O5] was produced in good yields by the rapid oxidation of red [CpMoO2]4 with PhIO in CH2Cl2. [Cp*2Mo2O5] was obtained by CH3COOH acidification of aqueous solutions of [Cp*MoO3]Na, the latter being generated in a single step from [Cp*MoCl4] and more than 5 equiv. of aqueous NaOH in air. Minor quantities of [Cp*MoO2]2 were also isolated from this reaction, although the formation of this by-

Cation size dependent reactivity of lanthanide trihalides with bulky alkylcyclopentadienyl anions

Reactions of NdCl3 and PrCl3 with two equivalents of sodium tri-tert-butylcyclopentadienide furnished base- and salt-free [Cp′2NdCl] and [Cp′2PrCl] (Cp′ = 1,2,4-(Me3C)3C5H2) in good yield. Trimethylaluminium has been added to the neodymium complex to form [Cp′2NdClAlMe3]. With LaCl3 or CeCl3 base-free bis(ring) complexes were not obtained, but in the latter case the salt adduct [(4Cp2Ce)(μ-Cl)2Na(tmeda)2]∞ (4Cp = (Me2CH)4C5H) could be extracted from the product mixture with tetramethylethylenediamine and crystallized as a zigzag chain polymer. [4Cp2SmCl2Na(dme)2] retained the coordinated sodium chloride even when dissolved in non-polar solvents. Attempted preparation of [Cp′2YbCl] gave the mono(ring) complex [Cp′YbCl(μ-OCH2CH2OCH3)]2 from cleavage of the dimethoxyethane solvent and with lutetium trichloride the hexanuclear complex [(4CpLu)5LuCl13(OEt2)5] was prepared in low yield. For lanthanum and thulium use of the triiodide as a starting compound enabled synthesis of the corresponding bis(tetraisopropylcyclopentadienyl)metal iodide, bis{tri-tert-butylcyclopentadienyl}lanthanum iodide was also prepared from LnI3. [4Cp2TmI] shows a unique conformation of one of the tetraisopropylcyclopentadienyl ligands with two isopropyl neighbours rotated towards each other indicating extreme steric congestion. Oxidation of [Cp′2Sm] with copper(I) iodide gave [Cp′2SmI] in high yield. Mono(ring) complexes are readily available from trichlorides of thulium, ytterbium, and lutetium. Apart from the donor solvent adducts [4CpTmCl2(dme)], [Cp′YbCl2(thf)2], and [4CpLuCl2(dme)], which were isolated from solutions in the corresponding donor solvent, the salt- and donor-free dihalides [Cp′TmCl2]n and [Cp′YbCl2]n were obtained as oligomers from pentane or petroleum ether extracts. The thulium compound gave [Cp′Tm{N(SiMe3)2}2] with two equivalents of Na[N(SiMe3)2] and the ytterbium complex underwent ring exchange with lithium tert-butylcyclopentadienide and formation of the bis(tert-butylcyclopentadienyl)ytterbium complex [(Me3CC5H4)2Yb(μ-Cl)]2.