Evidence for Internal Ion Pair Formation upon Insertion of Reactive Alkene in the Zirconium–Carbon Bond of the cp2Zr(μ‐C4h6)b(c6f5)3 Metallocene‐Boron‐Betaine Ziegler Catalyst System

Abstract

AbstractTreatment of the (butadiene)ML2 complexes 1 [ML2 = Cp2Zr (a), Cp2Hf (b), and (.‐C5H4CH3)2Zr (c)] with B(C6F5)3 gives the 1:1 addition products (CH2CHCHCH2–B(C6F5_3)ML2 (3a–c). At –40°C the betaine complex 3a inserts one equivalent of methylenecyclopropane to give the regioisomeric insertion products 5a and 6a in a 60:40 ratio. These products exhibit the cyclopropylidene moiety in the α‐ and β‐positions, respectively, relative to zirconium. The corresponding hafnocene complexes 5b and 6b are obtained in a 70:30 ratio starting from 3b. The reaction of 3(a–c) with allene gives a single insertion product (7a–c) in each case where the exo‐methylene group is in the α‐position to the metal center ([2,1]‐insertion). The complexes 5–7 are chiral. They all exhibit a pronounced ·‐interaction of the internal –C4H=C5H− double bond of the s̀‐ligand chain with the metal center in addition to a metallocene/–C6H2–[B] ion pair interaction. The relative contributions of the cationic metallocene end of the dipolar complexes 5–7 are quite dependent on the steric and electronic properties of the respective metallocene units involved. This is revealed by a comparison to typical 13C‐NMR parameters of the complexes 5–7 with a pair of suitable model complexes, namely the ethylene insertion product 4 into the betaine system 3a and its THF adduct 4.THF.