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

Abstract

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 beta-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.

Synthesis, Characterization, and Protonation Reactions of Ar‐BIAN and Ar‐BICAT Diimine Platinum Diphenyl Complexes

“…The closely related Ar-BIAN [Ar-BIAN = bis(arylimino)-acenaphthene, see Scheme 1] ligand system, pioneered by the Elsevier group, [33][34][35][36][37][38][39][40] has been explored with respect to many catalytic processes, including olefin oligomerization and polymerization [41][42][43][44][45] and olefin/CO copolymerization. [46] The Ar-BIAN system is a highly stable, rigid bidentate spectator ligand with interesting electronic properties.…”

“…[47,48] Elsevier and co-workers suggested that Ar-BIAN ligands may act as stronger σ-donors toward the metal when compared to bpy, and also proposed that the rigid Ar-BIAN ligands are rather comparable to open chain R-DAB analogues in electronic properties, which would be the case if the diimine system in the Ar-BIAN ligands is electronically isolated from and has no conjugation with the naphthalene backbone. [35] The capacity of this ligand type to support many oxidation states has been amply demonstrated; for example, Pt Ar-BIAN complexes have been reported in oxidation states ranging from Pt 0 [35] via Pt II [45] to Pt IV . [38] Ar-BIAN Pt complexes have found uses in catalytic hydrosilylation of styrene [49] and some complexes have interesting photophysical properties.…”

Mechanism of Ethylene Migratory Insertion and Dimerization Catalyzed by δ -Alkyl Platinum Complexes-A DFT Study

“…From π-complex IMF-1 to γ-agostic butyl complex IMF-2 via transition state TSF-1, the insertion barrier is 118.37 kJ/mol (relative to the π-complex IMF-1), which is in agreement with the value measured by Brookhart of 124.68 kJ/mol. 16 The endothermic amount for the migratory insertion process, IMF(IMH-2) ＋ C 2 H 4 → IMF-2, is 39.08 kJ/mol. Next the complex IMF-2 can easily rotate around the C 5 -C 6 bond (TSF-2) with a barrier of 35.15 kJ/mol to give the more stable product IMF-3 than IMF-2 (36.61 kJ/mol).…”

“…16 Starting from metalolefin-hydride (IMH-1), there are two possible pathways. As shown in Figure 13, the pathway I can be overall written as: IMH-1→TSH→IMH-2(IMF)→IMF-1.…”

“…Very recently, an experimental investigation of platinum(II)-catalyzed ethylene dimerization has been reported by Brookhart et al 16 However, for the forma-tion of ethyl ethylene complex, they cannot distinguish between pathways involving either ethylene trapping of a reversibly formed β-agostic ethyl intermediate or migratory insertion of a reversibly formed five-coordinate bis-ethylene hydride. In this paper, we have systematically carried out a theoretical investigation of the mechanism of ethylene insertion reactions and ethylene dimerization catalyzed by cationic σ-alkyl platinum complexes.…”