Synthesis, Structures, and Ethylene Polymerization Behavior of Bis(pyrazolyl)borate Zirconium and Hafnium Benzyl Complexes

The alkyl hydrido bis(pyrazolyl)borate reagent Tl(MeHB(3-mesitylpyrazolyl)2) (Tl(MeBpMs), 1) has been prepared
and used to generate group 4 metal MeBpMs complexes. The
reaction of 1 with ZrCl4 affords (MeBpMs*)2ZrCl2 (2, MeBpMs* = MeHB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)−). The reaction of 1 with M(CH2Ph)4 yields (MeBpMs)M(CH2Ph)3 (M = Zr
(3), Hf (4)), bibenzyl, and Tl0, via initial MeBpMs/benzyl exchange to produce 3 or 4 and Tl(CH2Ph), followed by
thermal decomposition of Tl(CH2Ph). The reaction of 3 with [Ph3C][B(C6F5)4] proceeds by one-electron oxidation of a zirconium benzyl
bond to yield [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] (5) and
a series of organic products derived from coupling and H atom transfer
reactions of trityl and benzyl radicals. The reaction of 4 with [Ph3C][B(C6F5)4] yields [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] (6) along with Ph3CCH2Ph (86%) as the major organic product. The
high yield of Ph3CCH2Ph in this reaction suggests
that electrophilic benzyl abstraction competes with oxidative M−CH2Ph bond cleavage in this case. The reaction of 3 or 4 with 1 equiv of [H(OEt2)2][B(C6F5)4] affords [(MeBpMs)M(CH2Ph)2(OEt2)][B(C6F5)4] (M = Zr (7), Hf (8)). Zirconium complexes 3, 5, and 7 exhibit a stronger tendency for η2-benzyl
bonding than hafnium analogues 4, 6, and 8. The cationic species 5 and 6 polymerize
ethylene.