C-Centred chiral metal alkyls, [MR*(Cl)(η-C5H5)2], rac- and meso-[ZrR*2(η-C5H5 )2] [R* = CH(SiMe3)C6H4 Me-o], and their one-electron reductions; reaction of Li[CH(SiMe3)2](Me2NCH2CH 2NMe2) with [MCl2(η-C5H5)2] (M = Zr or Hf )

Abstract

The chloro(alkyl)metallocenes [MR*(Cl)(η-C 5 H 5 ) 2 ] [R* = - CH(SiMe 3 )C 6 H 4 Me-o; M = Zr 1 or Hf 2] have been prepared from [MCl 2 (η-C 5 H 5 ) 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-C 5 H 5 ) 2 ]. In contrast, reaction of 2 equivalents of Li[CH(SiMe 3 ) 2 ](tmen) with [ZrCl 2 (η-C 5 H 5 ) 2 ] yielded an alkyl elimination product possessing µ-η 1 ∶η 5 -C 5 H 4 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(C 10 H 8 )’ (120 °C). Photolysis of 3a or 3b at 20 °C in tetrahydrofuran (thf) yielded a 1∶1 mixture of 3a and 3b; a d 1 intermediate has been unambiguously identified as [ZrR*(η-C 5 H 5 ) 2 ]. Reduction of 1 (Na–Hg), or 3a and 3b (Na[C 10 H 8 ]), in thf also gave [ZrR*(η-C 5 H 5 ) 2 ] but this slowly transformed into [ZrR*(η-C 5 H 5 ) 2 (thf)], and 1 with Na[C 10 H 8 ] and PPh 3 gave [ZrR*(η-C 5 H 5 ) 2 (PPh 3 )]. Reduction of 1, 3a or 3b, or photolysis of 3a or 3b, in the presence of PMe 3 gave [ZrR*(η-C 5 H 5 ) 2 (PMe 3 )]. 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) A. The structure of 3a confirms the rac assignment, M–σ-C 2.37 4 A, metal–centroid 2.23, 2.24 A.