Substitution reactions between bis-chelate ligands in palladium(II) alkenyl complexes: an unusual way to form unstable trans-P complexes. A study on the isomerization mechanism

The substitution reactions between asymmetric bis-chelate ligands and alkenyl chloro derivatives of palladium(II) of the type [Pd(L–L¢)(Rx)Cl] (L–L¢ = 2-phenylsulfanylmethyl-pyridine (HN–SPh), 2-methyl-6-phenylsulfanylmethyl-pyridine (MeN–SPh), 2,2¢-bipyridinyl (BiPy), Rx = –CCOOMe=CMeCOOMe (Ra), –CCOOEt=CMeCOOEt (Rb), –CCOOt-Bu=CMeCOOt-Bu (Rc), –(CCOOMe=CCOOMe)2Me (Rd)) with phosphoquinoline moieties (8-diphenylphosphanyl-quinoline (DPPQ), 8-diphenylphosphanyl-2-methyl-quinoline (DPPQ-Me)) usually leads to the formation of the stable geometrical isomer bearing these groups in the cis position thanks to the mutual trans influence of the alkenyl and phosphine groups. However, when the leaving group MeN–SPh and the entering ligand DPPQ are involved, the fast and quantitative substitution reaction leads to the formation of a couple of geometrical isomers [Pd(DPPQ)(Rx)Cl]-trans P and [Pd(DPPQ)(Rx)Cl]-cis P (Rx = Ra, Rb, Rc, Rd) in which the alkenyl and the phosphine groups are in mutual trans or cis position. The substrate [Pd(DPPQ)(Rx)Cl]-trans P (Rx = Ra, Rb, Rc) slowly interconverts into its thermodynamically stable -cis P counterpart while the bulky [Pd(DPPQ)(Rd)Cl]-trans P displays no tendency to isomerize, thereby allowing separation of the two geometrical forms. Also, the ligand DPPQ-Me induces the formation of the -trans P geometrical isomer which is only detectable at low temperature since it rapidly interconverts into the -cis P derivative at RT. The kinetics of the interconversion process, a reasonable explanation of the observed phenomenon based on theoretical calculations, and eventually an unequivocal structure determination of the stable [Pd(DPPQ)(Rx)Cl]-cis P substrate are reported in the present paper