Zirconium and hafnium diene and dienyl half-sandwich complexes: synthesis, polymerization catalysis and deactivation pathways. The molecular structures of [M(η3-C3H5)(2,3-Me2 C4H4){η-C5H 3(SiMe3)2-1,3}] (M = Zr or Hf ) and [Hf(η3-C3H5){ η3-CH2CMeCMeCH2B(C6 F5)3}{η-C5 H3(SiMe2)-1,3}]

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Abstract
The reduction of [MCl 3 Cp″] [M = Zr or Hf; Cp″ = η-C 5 H 3 (SiMe 3 ) 2 -1,3] with sodium amalgam in the presence of dienes gave the compounds [MCl(diene)Cp″] which are alkylated with MeMgBr or RMgCl (R = allyl) to give [MX(diene)Cp″] (X = CH 3 or η 3 -C 3 H 5 ; diene = 2,3-dimethylbuta-1,3-diene or isoprene). The reduction of [ZrCl(CHCMeCMeCH)Cp″] with an excess of Na–Hg leads to the binuclear η 4 (5e)-butadienyl complex [Zr(µ-η 1 ∶η 4 -C 4 H 3 Me 2 -2,3)Cp″] 2 , also formed from [ZrMe(C 4 H 4 Me 2 )Cp″] by methane elimination. The butadiene complex [Zr(η 3 -CH 2 CMeCHCH 2 )(η 4 -C 4 H 6 )Cp″] is obtained directly from [ZrCl 3 Cp″] and MeCHCHCH 2 MgCl. The complexes [M(allyl)(diene)Cp″] react with B(C 6 F 5 ) 3 to give the zwitterionic complexes [Cp″M + (η 3 -C 3 H 5 ) {η 3 -C 4 H 4 R 1 R 2 B - (C 6 F 5 ) 3 }] which contain a 14-electron [CpM(allyl) 2 ] + core stabilised by agostic bonding of the B–CH 2 methylene hydrogens. These zwitterions catalyse the polymerisation of ethene to high molecular weight polyethene. Catalysts with similar activities are obtained by the activation of [M(allyl)(diene)Cp″] with [CPh 3 ][B(C 6 F 5 ) 4 ]. The thermal stability of the zwitterionic active species depends strongly on the steric requirements of the dienyl ligands and decreases sharply in the order R 1 = R 2 = Me > R 1 = Me, R 2 = H R 1 = H, R 2 = H; i.e. the dimethylbutadiene derivatives are stable at room temperature, while in the latter case decomposition is significant even at -60 °C. The complexes [Zr(η 3 -CH 2 CHCHR 1 ){η 3 -CH 2 CR 2 MeCHCH 2 B(C 6 F 5 ) 3 }Cp″] (R 1 = H, R 2 = Me, 9a; R 1 = Me, R 2 = H, 10) decompose via an unusual C–H activation pathway, with alkene elimination and concomitant migration of a C 6 F 5 substituent from boron to zirconium, to give the catalytically inactive boryldiene complexes [Zr(C 6 F 5 ){η 4 -CH 2 CR 1 CHCHB(C 6 F 5 ) 2 }Cp ″]. The crystal structures of [M(C 3 H 5 )(Me 2 C 4 H 4 )Cp″] (M = Zr or Hf) and [Hf(η 3 -C 3 H 5 ){η 3 -CH 2 CMeCMeCH 2 B(C 6 F 5 ) 3 }Cp″] are reported.

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