Bond Angle Effects on the Migratory Insertion of Ethylene and Carbon Monoxide into Palladium(II)−Methyl Bonds in Complexes Bearing Bidentate Phosphine Ligands

Abstract

Labile (P−P)Pd(CH3)(OEt2)+BAr‘4- complexes (2) have been prepared via protonation of (P−P)PdMe2 (1), where P−P = cis-1,2-bis(diphenylphosphino)ethylene (a, dppee), 1,2-bis(diphenylphosphino)benzene (b, dpbz), 1,2-bis(diphenylphosphino)ethane (c, dppe), 1,2-bis(dimethylphosphino)ethane (d, dmpe), 1,3-bis(diphenylphosphino)propane (e, dppp), 1,3-bis(diisopropylphosphino)propane (f, dippp), 1,4-bis(diphenylphosphino)butane (g, dppb) and Ar‘ = 3,5-(CF3)2C6H3. Unstable complex 2d (P−P = dmpe) was generated in situ. X-ray structures are reported for 1e and 2e−g. Treatment of 2a−g with CO in CH2Cl2 at −90 °C yields the (P−P)Pd(CH3)(CO)+ complexes 3a−g. Barriers to migratory insertion in 3a−g were determined with the ordering to be: 3f (dippp) ≈ 3g (dppb) < 3e (dppp) ≪ 3a (dppee) ≈ 3b (dpbz) ≈ 3c (dppe) < 3d (dmpe). Exposure of 2a−g to ethylene at −80 °C yields the ethylene complexes (P−P)Pd(CH3)(C2H4)+ (5a−g). Barriers to migratory insertion in these complexes were determined by NMR spectroscopy to be: 5b (dpbz) ≈ 5e (dppp) ≈ 5f (dippp) ≈ 5g (dppb) < 5a (dppee) ≈ 5c (dppe) < 5d (dmpe). Complexes (P−P)Pd(CH2CH3)(C2H4)+ (6a−e,g) produced from 5a−e,g under C2H4 are catalyst resting states for the dimerization of C2H4 to butenes. In the case of 5f (P−P = dippp), the catalyst resting state produced is the β-agostic ethyl complex (dippp)Pd(CH2CH2-μ-H)+ (8f), which has been isolated. This complex exhibits two dynamic processes studied by VT-NMR: interchange of Cα and Cβ (ΔG⧧ = 10.3(2) kcal/mol) and rotation of the agostic methyl group (ΔG⧧ ca. 6.4 kcal/mol). The β-agostic propyl complex 7f has been generated and identified as the n-propyl isomer (dippp)Pd(CH2CH2-μ-HCH3)+.