[Pd(en)(H2O)2]2+ and [Pd(pic)(H2O)2]2+ complexation by monohydroxamic acids: A solution equilibrium and solid state approach

Complex formation between [Pd(en/pic)(H2O)2]2+ (en = ethylenediamine, pic = pyridine-2-methylamine) and primary hydroxamic acids (ahaH = acetohydroxamic, bhaH = benzohydroxamic acid) or secondary hydroxamic acids (meahaH = N-methyl-acetohydroxamic, pheahaH = N-phenylacetohydroxamic and phebhaH = N-phenylbenzohydroxamic acid) was studied in aqueous solution by pH-potentiometry, 1H NMR and ESI-TOF-MS. Secondary hydroxamates form 1:1 species with the [Pd(N,N)(H2O)2]2+ ions via the [O,O] chelating sets. Unexpectedly, in the primary ligands deprotonation and coordination of the hydroxamate-NH starts as low as pH <2, where the RCC(O)N−O− ion is capable of linking two [Pd(diamine)]2+ units via the coordination through the [O,O] chelate to one unit and through the monodentate hydroxymato-N− atom to the another one. As a consequence, primary ligands can bind an excess of metal ion too. A trinuclear complex predominates in a wide pH-range (5–10 for en, 3–10 for pic) and the hydroxide ion starts to compete with the hydroxymato ligand only above pH 10. In the trinuclear species two [O,O] chelated Pd(II) units are bridged via a third palladium core that binds to the hydroximato-N donors of the two ligands. This binding mode was also proved by MS studies in solution and by revealing the molecular structure of [(Pd(en))3(bhaH−1)2](BF4)2·2H2O (2), the first reported structure with a Pd(II)-hydroxymate-N− monodentate coordination, characterized with single crystal X-ray diffraction in the solid state.

Graphical abstractWhile secondary hydroxamates form mononuclear [O,O] chelated complexes with [Pd(en/pic)(H2O)2]2+ primary hydroxamates are capable of binding an excess of metal ion too with the involvement of the hydroxymate-N− donor(s) yielding oligonuclear complexes with high stability.Figure optionsDownload full-size imageDownload as PowerPoint slide