Reactions of palladium(II) allyl dimers with palladium(II) complexes containing σ-bonded 1,2-bis(imino)alkyl groups. Crystal and molecular structure of the zwitterionic binuclear compound [Pd(η3-2-MeC3H4){R1NC(PdCl2L)–CMeNR2}](L = PPh3, R1= R2= C6H4OMe-p)

Abstract

The 1,2-bis(imino)alkylpalladium derivatives (A), R¹NCL′–CR³NR²[R¹= R²= C₆H₄OMe-p, R³= Me, L′=trans-PdCIL₂(L =para-substituted triarylphosphine or AsPh₃); R¹= R²= C₆H₄OMe-p, R³= H or Ph, L′=trans-PdCl(PPh₃)₂; R¹= C₆H₄OMe-p, R²= R³= Me, L′=trans-PdCl(PPh₃)₂] react with [{PdCl(η³-2-R⁴C₃H₄)}₂](R⁴= H or Me) in the presence of NaClO₄ to yield the cationic binuclear complexes (B), [Pd(η³-2-R⁴C₃H₄)(R¹NCL′–CR³NR²)][ClO₄], where the 1,2.bis(imino)alkyl group acts as σσ′-NN′ chelating ligand. In the absence of NaClO₄, the reaction leads initially to the formation of the ionic compounds (C), [Pd(η³-2-R⁴C₃H₄)(R¹NCL′–CR³NR²)][PdCl₂(η³-2-R⁴C₃H₄)], which in a subsequent slower stage reacts further with exchange of ancillary ligands between the cationic and anionic species, to give the zwitterionic binuclear complexes (D), [Pd(η³-2- R⁴C₃H₄)(R¹NCL″–CR³NR²)](L″=cis-PdCl₂L), and [PdCl(η³-2-R⁴C₃H₄)L]. A complex of type (C)[R¹= R²= C₆H₄OMe-p, R³= R⁴= Me, L′= PdCl(dppe), dppe = 1,2-bis(diphenylphosphino)ethane] can be isolated from the reaction of the corresponding derivative (A) with [{PdCl(η³-2-MeC₃H₄)}₂]. The rates of ligand exchange depend markedly on the substituents R¹, R², and R³, on the nature of L, and on the solvent. Based on these effects and on ¹H n.m.r. spectral data, a mechanism is proposed, which essentially involves opening the five-membered 1,2-bis(imino) ring of the cation promoted by interaction with the anionic species in the intermediate (C). The ¹H n.m.r. spectra of (D) indicate the presence of diastereoisomers which interconvert more or less rapidly at room temperature depending on the substituents R² or R³ and on the ligand L. The crystal and molecular structure of a typical complex (D)(R¹= R²= C₆H₄OMe-p, R³= R⁴= Me, L = PPh₃) has been determined by X-ray diffraction analysis. Crystals are Orthorhombic, a= 20.224(5), b= 20.073(8),c= 19.829(9)Å, space group Pbca, and Z= 8. The structure has been solved by the heavy-atom method and refined dy full-matrix least squares to R= 0.086 for 1 795 diffractometer data (Mo-K_α radiation). The structural data show that the η³-allyl group is almost symmetrically bound to palladium and its plane forms a dihedral angle of 107.4° with the planar five-membered ring. The 1,2-bis(imino)propyl group is σσ′-NN′ chelated to the Pd(η³-2-MeC₃H₄) unit and the mean plane of the cycle makes a dihedral angle of 81.9° with the co-ordination mean plane of the cis-PdCl₂(PPh₃) group. The major features of the structure are represented by a short Pd–C(imino) bond [1.92(3)Å], indicating a relevant π contribution, and by a close approach of the R³ substituent to the metal centre of the cis-PdC1₂(PPh₃) unit [Pd ⋯ C(methyl) 3.19 Å].