Synthesis and Reactivity of a Sterically Crowded Tris(pyrazolyl)borate Hafnium Benzyl Complex

The reaction of Hf(CH2Ph)4 and Tl[TpMs*] (TpMs* = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)−) yields TpMs*Hf(CH2Ph)3 (1), bibenzyl, and Tl0 via ligand exchange followed by thermal decomposition of Tl[CH2Ph]. Compound 1 reacts with [H(OEt2)2][B(C6F5)4] to form TpMs*Hf(CH2Ph)2(OEt2)+ (2, 100%) and with [Ph3C][B(C6F5)4] or B(C6F5)3 to form an unstable species characterized by NMR as TpMs*Hf(CH2Ph)2+. 1/[Ph3C][B(C6F5)4] behaves similarly to TpMs*HfCl3/MAO in olefin polymerization, producing high molecular weight, linear PE, and ethylene/1-hexene copolymer with good hexene incorporation. These results are consistent with the proposal that similar cationic TpMs*Hf(X)R+ species (X = Cl or R) are active species in these catalysts. Ethylene polymerization by 1/[Ph3C][B(C6F5)4] at low temperature proceeds without significant chain transfer, enabling the synthesis of bromo benzyl double-end-capped linear PE (PhCH2(CH2CH2)nBr).