C-centred chiral metal alkyls,

The chloro(alkyl)metallocenes [MR*(Cl)(η-C₅H₅)₂] [R* = ^-CH(SiMe₃)C₆H₄Me-o; M = Zr 1 or Hf 2] have been prepared from [MCl₂(η-C₅H₅)₂] and LiR*(tmen) (tmen = N,N,N′,N′-tetramethylethane-1,2-diamine); further alkylation occurred only for M = Zr, affording rac-3a and meso-3b, [ZrR*₂(η-C₅H₅)₂]. In contrast, reaction of 2 equivalents of Li[CH(SiMe₃)₂](tmen) with [ZrCl₂(η-C₅H₅)₂] yielded an alkyl elimination product possessing μ-η¹:η⁵-C⁵H₄^2- ligands. Compound 1 was reversibly reduced (E_1/2^red = -1.72 V vs. saturated calomel electrode, SCE) whereas 2, 3a or 3b were irreversibly reduced (E^red = -2.12, -2.08, -2.00 V respectively vs. SCE). Thermolysis of 3b in toluene gave 3a (80°C) or 'Zr(C₁₀H₈)' (120°C). Photolysis of 3a or 3b at 20°C in tetrahydrofuran (thf) yielded a 1:1 mixture of 3a and 3b; a d¹ intermediate has been unambiguously identified as [ZrR*(η-C₅H₅)₂]. Reduction of 1 (Na-Hg), or 3a and 3b (Na[C₁₀H₈]), in thf also gave [ZrR*(η-C₅H₅)₂] but this slowly transformed into [ZrR*(η-C₅H₅)₂(thf)], and 1 with Na[C₁₀H₈] and PPh₃ gave [ZrR*(η-C₅H₅)₂(PPh₃)]. Reduction of 1, 3a or 3b, or photolysis of 3a or 3b, in the presence of PMe₃ gave [ZrR*(η-C₅H₅)₂(PMe₃)]. Crystal structure determinations showed a slightly more crowded metal environment in 2 than in 1, consistent with the view that dialkylation in 2 is limited on steric grounds; M-σ-C 2.359(4) (1), 2.322(8) (2), metal-centroid 2.22 (1), 2.21 (2), M-Cl 2.444(1) (1), 2.418(3) (2) Å. The structure of 3a confirms the rac assignment, M-σ-C 2.37₄ Å, metal-centroid 2.23, 2.24 Å.