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

Labile (P−P)Pd(CH 3 )(OEt 2 ) + BAr‘ 4 - complexes ( 2 ) have been prepared via protonation of (P−P)PdMe 2 ( 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-(CF 3 ) 2 C 6 H 3 . 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 CH 2 Cl 2 at −90 °C yields the (P−P)Pd(CH 3 )(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(CH 3 )(C 2 H 4 ) + ( 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(CH 2 CH 3 )(C 2 H 4 ) + ( 6a − e, g ) produced from 5a − e, g under C 2 H 4 are catalyst resting states for the dimerization of C 2 H 4 to butenes. In the case of 5f (P−P = dippp), the catalyst resting state produced is the β-agostic ethyl complex (dippp)Pd(CH 2 CH 2 -μ-H) + ( 8f ), which has been isolated. This complex exhibits two dynamic processes studied by VT-NMR: interchange of C α and C β (Δ G ⧧ = 10.3(2) kcal/mol) and rotation of the agostic methyl group (Δ G ⧧ ca. 6.4 kcal/mol). The β-agostic propyl complex 7f has been generated and identified as the n -propyl isomer (dippp)Pd(CH 2 CH 2 -μ-HCH 3 ) + .