Reactions of Vinyl Acetate and Vinyl Trifluoroacetate with Cationic Diimine Pd(II) and Ni(II) Alkyl Complexes: Identification of Problems Connected with Copolymerizations of These Monomers with Ethylene

Abstract

Vinyl acetate (VA) and vinyl trifluoroacetate (VA_f) react with [(N∧N)Pd(Me)(L)][X] (M = Pd, Ni, (N∧N) = N,N‘-1,2-acenaphthylenediylidene bis(2,6-dimethyl aniline), Ar_f = 3,5-trifluoromethyl phenyl, L = Ar_fCN, Et₂O; X = B(Ar_f)₄^-, SbF₆^-) to form π-adducts 3 and 5 at −40 °C. Binding affinities relative to ethylene have been determined. Migratory insertion occurs in a 2,1 fashion (ΔG^⧧ = 19.4 kcal/mol, 0 °C for VA, and 17.4 kcal/mol, −40 °C for VA_f) to yield five-membered chelate complexes [(N∧N)Pd(κ²-CH(Et)(OC(O)CH₃))]⁺, 4, and [(N∧N)Pd(κ²-CH(Et)(OC(O)CF₃))]⁺, 6. When VA is added to [(N∧N)Ni(CH₃)]⁺, an equilibrium mixture of an η² olefin complex, 8c, and a κ-oxygen complex, 8o, results. Insertion occurs from the η² olefin complex, 8c (ΔG^⧧ = 15.5 kcal/mol, −51 °C), in both a 2,1 and a 1,2 fashion to generate a mixture of five- and six-membered chelates, 9 _2,1 and 9 _1,2. VA_f inserts into the Ni−CH₃ bond (−80 °C) to form a five-membered chelate with no detectable intermediate. Thermolysis of the Pd chelates results in β-acetate elimination from 4 (ΔG^⧧ = 25.5 kcal/mol, 60 °C) and β-trifluoroacetate elimination from 6 (ΔG^⧧ = 20.5 kcal/mol, 10 °C). The five-membered Ni chelate, 9 _2,1, is quite stable at room temperature, but the six-membered chelate, 9 _1,2, undergoes β-elimination at −34 °C. Treatment of the OAc_f containing Pd chelate 6 with ethylene results in complete opening to the π-complex [(N∧N)Pd(κ²-CH(Et)(OAc_f))(CH₂CH₂)]⁺ (OAc_f = OC(O)CF₃), 18, while reaction of the OAc containing Pd chelate 4 with ethylene establishes an equilibrium between 4 and the open form 16, strongly favoring the closed chelate 4 (ΔH = −4.1 kcal/mol, ΔS = −23 eu, K = 0.009 M^-1 at 25 °C). The open chelates undergo migratory insertion at much slower rates relative to those of the simple (N∧N)Pd(CH₃)(CH₂CH₂)⁺ analogue. These quantitative studies provide an explanation for the behavior of VA and VA_f in attempted copolymerizations with ethylene.