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

Vinyl acetate (VA) and vinyl trifluoroacetate (VAf) react with [(N∧N)Pd(Me)(L)][X] (M = Pd, Ni,
(N∧N) = N,N‘-1,2-acenaphthylenediylidene bis(2,6-dimethyl aniline), Arf = 3,5-trifluoromethyl phenyl, L =
ArfCN, Et2O; X = B(Arf)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 VAf) 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, 92,1 and 91,2. VAf 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, 92,1, is quite stable at room temperature, but the six-membered chelate, 91,2, undergoes β-elimination at −34 °C. Treatment of the OAcf containing Pd chelate
6 with ethylene results in complete opening to the π-complex [(N∧N)Pd(κ2-CH(Et)(OAcf))(CH2CH2)]+ (OAcf
= 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 VAf in attempted copolymerizations with ethylene.