Reversible addition of protic molecules to co-ordinated di-2-pyridyl ketone in palladium(II), platinum(II), and gold(III) complexes. X-Ray crystal structures of dichloro(dihydroxy-di-2-pyridylmethane)palladium(II) and dichloro(dihydroxy-di-2-pyridylmethane)gold(III) chloride

Abstract

Palladium(II), platinum(II), and gold(III) species react with di-2-pyridyl ketone (dpk) to give complexes in which one or two ligand molecules in the original keto- or in a newly formed diolic form are co-ordinated to the metal cation, namely: [PdCl₂(dpk·HX)][X = OH (1), OMe (2), or OEt (3)]; [PdCl₂(dpk)], (4); [Pd(dpk·H₂O)₂][ClO₄]₂, (5); [PtCl₂(dpk)], (6); [AuCl₂(dpk·H₂O)]Cl, (7); and [Au(dpk·H₂O)₂][ClO₄]₃, (8). In solution, all species undergo either addition or release of the protic molecules depending upon the HX concentration; the rates of reversible hydrolysis and the stability of the hydrated species increase going from Pt^II to Pd^II to Au^III thus paralleling both the electron-withdrawing properties of the metal cations and their ability to fill the apical position of a tetragonal-pyramidal co-ordination shell. The crystal and molecular structures of (1) and (7) have been determined by X-ray diffraction: (1) crystallizes in the tetragonal space group P4₁ with Z= 4, a= 10.597(8), and c= 11.468(9)Å; (7) crystallizes in the monoclinic space group P2₁/c with Z= 4, a= 10.398(10), b= 7.249(8), c= 18.406(12)Å, and β= 91.6(8)°. The structures were solved by Patterson and Fourier methods and refined by full-matrix least squares to R= 0.018 and 0.063 for (1) and (7) respectively. Neutral [PdCl₂(dpk·H₂O)] and cationic [AuCl₂(dpk·H₂O)]⁺ are present in the crystals of (1) and (7) respectively; the co-ordination geometry of the metal atom is square planar in both cases and involves two Cl and two N atoms, the dpk·H₂O molecule acting as a chelating N-donor ligand. The ligand conformation is very similar in both complexes, the six-membered chelate being in a boat form with a dihedral angle between the pyridyl rings of 114.54 and 116.63° respectively; this allows a very favourable co-ordination angle of 87.1(2) and 86.4(6)°, and a short contact between the oxygen atom of a hydroxyl group and the metal cation of 2.824(6) and 2.77(1)Å for (1) and (7) respectively.