SYNTHESIS BY LIGAND-EXCHANGE, STRUCTURAL CHARACTERIZATION, AND AQUEOUS CHEMISTRY OF ORTHO-PALLADATED OXIMES

Dimeric chloro-bridged ortho-palladated complexes of general formula [PdCl{C6H4(CR=NOH)-2}]2, where R = H, Me, Et, and Ph, were prepared via the ligand-exchange reaction starting from aryl oximes and cyclopalladated complexes of NN-dimethylbenzylamine, ((dimethylamino)methyl)ferrocene, or benzophenone oxime. By reaction with various pyridines (Zpy), the dimers were converted into the corresponding monomers [PdCl{C6H4(CR=NOH)2}(Zpy)] (4). Four acetophenone oxime (R = Me) derivatives with Z = H (4b), 4-Me2N (4c), 2-Me (4d), and 2,4,6-Me3 (4e) were characterized by X-ray crystallography. Compound 4b: C13H13N2OPdCl, orthorhombic, space group Pcab, a = 15.385 (4) angstrom, b = 20.980 (i) angstrom, c = 8.140 (1) angstrom, z = 8. Compound 4c: C15H18N3OPdCl, triclinic, space group P1BAR, a = 8.307 (5) angstrom, b = 8.772 (6) angstrom, c = 11.949 (8) angstrom, alpha = 82.15 (5)-degrees, beta = 77.36 (5)-degrees, gamma = 68.00 (5)-degrees, Z = 2. Compound 4d: C14H15N2OPdCl.1/2C6H6, monoclinic, space group C2/c, a = 21.978 (8) angstrom, b = 9.702 (2) angstrom, c = 21.728 (8) angstrom, beta = 132.99 (2)-degrees, Z = 8. Compound 4e: C16H19N2OPdCl, monoclinic, space group P2(1)/n, a = 9.488 (2) angstrom, b = 15.664 (4) angstrom, c = 11.221 (3) angstrom, beta = 91.12 (1)-degrees, Z = 4. All have a square planar coordination at palladium with a slight tetrahedral distortion and trans nitrogens. The pyridine and palladium planes are practically perpendicular only in the case of 4d,e; an interaction of the pyridine 2- and 2,6-methyl C-H bonds with the palladium axial sites is proposed. The Pd-Cl bond length is large in all four complexes, of ca. 2.4 angstrom, and the chloro ligand is readily hydrolyzed in aqueous solution to afford the aqua/hydroxo species [Pd{C6H4(CR=NOH)-2}(Zpy)OHn](n-1)+ (n = 1, 2). In the pH range 3-9 the complexes are involved in two acid-base equilibria with the values of pK(a1) and pK(a2) of ca. 7 and <5 ascribed to deprotonation of the oxime hydroxyl and the aqua ligand, respectively. The very low values of pK(a2) were confirmed by the study of several related monomeric palladacycles lacking the oxime group.