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(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)(2)C6H3. 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 degreesC 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) approximate to 3g (dppb) < 3e (dppp) much less than 3a (dppee) approximate to 3b (dpbz) approximate to 3c (dppe) < 3d (dmpe). Exposure of 2a-g to ethylene at -80 degreesC 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) approximate to 5e (dppp) approximate to 5f (dippp) approximate to 5g (dppb) < 5a (dppee) approximate to 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 beta -agostic ethyl complex (dippp)Pd(CH2CH2-mu -H)(+) (8f), which has been isolated. This complex exhibits two dynamic processes studied by VT-NMR: interchange of C-mu and C-beta (DeltaG(double dagger) = 10.3(2) kcal/mol) and rotation of the agostic methyl group (DeltaG(double dagger) ca. 6.4 kcal/mol). The beta -agostic propyl complex 7f has been generated and identified as the n-propyl isomer (dippp)Pd(CH2CH2-mu -HCH3)(+).