Metallocene derivatives of early transition metals. Part 2. Substituted cyclopentadienyl Group 4A dichloro-metallocene complexes [M(η-C5H4R)2Cl2](M = Zr or Hf; R = Me, Et, Pri, But, or SiMe3), their mono- and di-alkyl derivatives [M(η-C5H4R)2R′X](X = Cl or R′; R′= CH2SiMe3 or CH2CMe3), and their d1 reduction products

Abstract

The substituted d⁰ metallocene dichloro-complexes [M(η-C₅H₄R)₂Cl₂](M = Ti^IV, Zr^IV, or Hf^IV; R = Me, Et, Prⁱ Bu^t, or SiMe₃), (1)–(11), have been prepared from MCl₄ and 2Li[C₅H₄R] in tetrahydrofuran (thf). From the appropriate dichloride and either (i) MgR′Cl in CH₂Cl₂ there was obtained the chloro (alkyl)[M(η-C₅H₄R)₂R′Cl](R′= CH₂SiMe₃) or [Zr(η-C₅H₅)₂(CH₂CMe₃)Cl], or (ii) alkyl-lithium in OEt₂ the dialkyl [M′(η-C₅H₄R″)_2–(CH₂XMe₃)₂](M′= Zr or Hf; R″= H, Me, Et, Prⁱ, or Bu^t; X = C or Si), (13)–(16), was produced; treatment of [Hf(η-C₅H₄Bu^t)₂Cl₂] with BBr₃ in CH₂Cl₂ gave [Hf(η-C₅H₄Bu^t)₂Br₂]. The ¹H n.m.r. spectra of each of the dichlorides (1)–(11) show the ring protons of the C₅H₄R group as an AA′BB′ or A₂B₂(R = SiMe₃) signal; the dialkyls (13)–(16) show this feature as an A₂X₂ pattern, whereas in [Hf(η-C₅H₄Bu^t)₂(CH₂SiMe₃)Cl] it appears as ABCD. The ¹³C n.m.r. spectra show three signals for the corresponding carbon atoms (C₅H₄R) and the chemical-shift data are compared with results on substituted ferrocenes or benzenes, [Fe(η-C₅H₄R)₂] or C₆H₅R.

Treatment of the complex [M(η-C₅H₄R)₂R′₂] with an equimolar portion of Na[C₁₀H₈] in thf at 20 °C yields an appropriate d¹ dialkylmetal(III) complex, which is persistent for M = Ti or Zr but not for M = Hf, and is characterised by its e.s.r. spectrum [g_av., 1.984–1.993; a(¹H), 0.175–0.360 mT; a(^47/49Ti), 0.720–1.05 mT; a(⁹¹Zr), 1.000–2.410 mT] showing coupling with the α protons of the alkyl group R′. As each of these (Zr) dialkyls undergoes irreversible one-electron reduction at a Pt electrode in 0.2 mol dm^–3[NBu₄][BF₄] in thf the above d¹ complexes are formulated as [Zr(η-C₅H₄R)R′₂]. Cyclic voltammetry data are provided for several metallocene(IV) complexes: (i) dichlorides {reversible one-electron reduction leading to [M(η-C₅H₄R)₂Cl₂]^–(M = Zr), and E_½^red is nearly 1 V more negative than for Ti}; (ii) chloro(alkyls)(irreversible reduction); and (iii) dialkyls (irreversible reduction).