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){R1NC(PdCl2L)–CMeNR2}](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)A, 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-Kα 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)A], 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 A].