Mechanistic Studies of Platinum(II)-Catalyzed Ethylene Dimerization: Determination of Barriers to Migratory Insertion in Diimine Pt(II) Hydrido Ethylene and Ethyl Ethylene Intermediates

The diimine platinum(II) ethylene hydride complex [(N∧N)Pt(H)(ethylene)][BAr‘4] (1, N∧N = [(2,6-Me2C6H3)NC(An)−C(An)N(2,6-Me2C6H3)], An = 1,8-naphthalenediyl, Ar‘ = 3,5-(CF3)2C6H3) was prepared
by protonation of the diethyl complex (N∧N)PtEt2 with [H(OEt2)2][BAr‘4]. The energy barrier to interchange
of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation
transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N∧N)Pt(Et)(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a
second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed β-agostic
ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to
migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The
1-butene hydride complex, (N∧N)Pt(H)(1-butene)+ (3), is a key intermediate in the dimerization cycle and
has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of
free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of
free nitrile with bound nitrile in (N∧N)Pt(Et)(CH3CN)+ are reported.