Bi-imidazole (H2bim) and bibenzimidazole η3-allylic complexes of palladium(II). Mono- and tetra-nuclear palladium(II) and heteronuclear palladium(II)–rhodium(I) complexes. Crystal structure of [Pd4(η3-C3H5)4(µ-bim)2]·CH2Cl2

Abstract

Novel cationic η³-allylic palladium(II) complexes of the type [Pd(η³-2-RC₃H₄)(H₂L)]⁺[R = H or Me; H₂L = 2,2′-bi-imidazole (H₂bim) or 2,2′-bibenzimidazole (H₂bzim)] have been obtained by reaction of [{Pd(η³-2-RC₃H₄)(µ-Cl)}₂] with AgClO₄ and subsequent addition of H₂L. Treatment of the complex [{Pd(η³-2-RC₃H₄)(µ-Cl)}₂] with H₂L (1 : 2 mol ratio) in the presence of KOH leads to neutral mononuclear η³-allylic palladium(II) complexes of general formula [Pd(η³-2-RC₃H₄)(HL)](R = H or Me; HL^–= Hbim^– or Hbzim^–). The use of a 1 : 1 mol ratio affords tetranuclear complexes of the type [Pd₄(η³-2-RC₃H₄)₄(µ-L)₂]. The neutral mono- and tetra-nuclear complexes have also been prepared by reaction of [Pd(η³-2-RC₃H₄)(acac)](Hacac = acetylacetone) with H₂bim or H₂bzim in 1 : 1 or 2 : 1 mol ratios, respectively. Reactions of acac complexes with bi-imidazolate or bibenzimidazolate derivatives containing the anion HL^– render similar heterotetranuclear η³-allylic palladium(II)–rhodium(I) compounds. The reported molecular weights, i.r. spectra, and ¹H n.m.r. studies support the tetranuclearity of the compounds, in which the bi-imidazolate or bibenzimidazolate anions are acting as tetradentate bridging ligands. The crystal structure of the compound [Pd₄(η³-C₃H₅)₄(µ-bim)₂]·CH₂Cl₂ has been determined by X-ray methods; it crystallizes in the triclinic space group P with Z= 4, a= 13.081(8), b= 20.843(13), c= 12.722(7)Å, α= 106.78(4), β= 108.56(4), γ= 84.98(3)°. The structure has been solved by direct and Fourier methods and refined by full-matrix least squares to R= 0.084 for 4 667 observed reflections. The crystals consist of two essentially identical, but crystallographically independent, complexes of [Pd₄(η³-C₃H₅)₄(µ-bim)₂] and dichloromethane molecules of crystallization. In each complex each bim^2– anion co-ordinates to the Pd metals in an unsymmetrical tetradentate manner through its four nitrogen atoms of the two imidazolate rings; chelating to one Pd atom through two N atoms and being bonded in a unidentate manner to two Pd atoms through the other two N atoms. The co-ordination at all four Pd atoms also involves an allyl ligand bonded through an η³-interaction with the three carbon atoms.