Structural Investigations of Palladium(II) and Platinum(II) Complexes of Salicylhydroxamic Acid

Complexes of salicylhydroxamic acid (shaH) with palladium(II) and platinum(II) were investigated. The synthesis of [Pt(sha)₂] was attempted via a number of methods, and ultimately ¹H NMR investigations revealed that salicylhydroxamate would not coordinate to chloro complexes of platinum(II). However, [Pt(sha−H)(PPh₃)₂] was successfully synthesized and the crystal structure determined (orthorhombic, space group <i>Pca</i>2₁ <i>a</i> = 17.9325(19) Å, <i>b</i> = 11.3102(12) Å, <i>c</i> = 18.2829(19) Å, <i>Z</i> = 4, <i>R</i> = 0.0224). The sha binds via an [<i>O,O</i>] binding mode, in its hydroximate form. In contrast the palladium complex [Pd(sha)₂] was readily synthesized and crystallized as [Pd(sha)₂](DMF)₄ in the triclinic space group <i>P</i>1̄, a = 7.066(1) Å, b = 9.842(2) Å, c = 12.385(2) Å, α = 99.213(3)°, β = 90.669(3)°, γ = 109.767(3)°, Z = 1, R = 0.037. The unexpected [N,O‘] binding mode of the salicylhydroxamate ligand in [Pd(sha)₂] prompted investigation of the stability of a number of binding modes of salicylhydroxamic acid in [M(sha)₂] (M = Pd, Pt) by density functional theory, using the B3LYP hybrid functional at the 6-311G* level of theory. Geometry optimizations were carried out for various binding modes of the ligands and their relative energies established. It was found that the [N,O‘] mode gave the more stable complex, in accord with experimental observations. Stabilization of hydroxamate binding to platinum is evidently afforded by soft ligands lying trans to them.