Preparation of chelate bis(imine)nickel allyl systems by reaction of their corresponding butadiene complexes with electrophiles

The chelate 1,2-bis(imine)nickel(butadiene) complex 4a (chelate ligand derived from condensation of biacetyl with 2,6-diisopropylaniline) adds the strong Lewis acid B(C6F5)3 at the terminal carbon atom of the butadiene ligand to yield the dipolar substituted π-allyl-type betaine complex (lig)Ni[η3-C3H4-CH2B(C6F5)3] (Z-6a). At 90 °C the kinetically formed product equilibrated with its E-6a isomer. Similarly, 4a adds the boron Lewis acid (pyrrolyl)B(C6F5)2 to yield the corresponding neutral dipolar π-allyl betaine complex Z-7a, that slowly equilibrated with E-7a over several hours at ambient temperature. Protonation of the butadiene ligand of complex 4a was achieved by treatment with the neutral Brønsted acid (2H-pyrrol)B(C6F5)3 to yield the [(lig)Ni(η3-crotyl)+][(pyrrolyl)B(C6F5)3−] salt 9a (Z-/E-9a ratio=90:10 upon preparation). At 298 K this salt rearranged to a 5:95 mixture of Z-9a/E-9a with a Gibbs activation energy of ΔG‡ (298 K)=22.3±0.2 kcal mol−1. Complex 4a added [Ph3C+] to the butadiene ligand to yield the salt [(lig)Ni(η3-C3H4-CH2CPh3)+][B(C6F5)4−] (Z-12a), that proved isomerically stable under the applied reaction conditions. Similar reactions were carried out starting from the acenaphthylene 1,2-dione derived chelate bis(imine)Ni(butadiene) complex 4b. The systems 6, 7, 9 and 12 were found to be active ethene polymerization catalysts in the presence of Al(i-Bu)3.