Alkyl-bridged complexes of the d- and f-block elements. Part 1. Di-µ-alkyl-bis(η-cyclopentadienyl)metal(III)dialkylaluminium(III) complexes and the crystal and molecular structure of the ytterbium methyl species

Abstract

Reaction of [{M(η-C₅H₅)₂Cl}₂] with Li[AlR₄], or in some cases Mg[AlR₄]₂, affords the novel crystalline complexes [(η-C₅H₅)₂M(µ-R)₂AlR₂](M = Sc, Y, Gd, Dy, Ho, Er, Tm, or Yb, R = Me; or M = Sc, Y, or Ho, R = Et). The yttrium, unlike the scandium, complexes are fluxional at 40 °C, but at –40 °C bridging and terminal alkyl groups give distinct n.m.r. signals; ΔG^‡ for site exchange in [(η-C₅H₅)₂Y(µ-Me)₂AlMe₂] is 15.9 kcal mol^–1 at 392 K. A di-µ-alkyl-bridged structure is confirmed by i.r. (bridging CH₃ band at 1 250 and 1 235 cm^–1), variable-temperature ¹H and ¹³C n.m.r. (M = Sc or Y), and X-ray studies (M = Yb). Additional data are given on the less stable [(η-C₅H₅)₂Ti(µ-Me)₂AlMe₂], [Ti(η-C₅H₅)₂(AlMe₃Cl)], and [(η-C₅H₅)₂Ti(µ-H)₂AlMe₂](structure deduced in part from e.s.r. spectra). A single-crystal X-ray analysis of [(η-C₅H₅)₂Yb(µ-Me)₂AlMe₂] has been carried out to R 0.036 and R′ 0.042; the complex has an approximately tetrahedral Yb and Al environment (space group Pna2₁) with the YbMe₂Al unit strikingly similar to AlMe₂Al in [Al₂Me₆]. Important bond lengths (t = terminal, b = bridge) and angles are: Yb–C (cyclopentadienyl, average) 2.61(3), Yb–C_b 2.59(3), Al–C_b 2.18(5), and Al–C_t 2.00(1)Å; Yb–C–Al 79.9(1.6) and C–Al–C 113.3(8)°.