Synthesis and solution behaviour of stable mono-, di- and trinuclear Pd(II) complexes of 2,5-pyridinedihydroxamic acid: X-ray crystal structure of a novel Pd(II) hydroxamato complex

Mono-, di- and trinuclear Pd(II) complexes of 2,5-pyridinedihydroxamic acid (2,5-pyha) are reported. The X-ray crystal structure of the mononuclear complex, [Pd(en)(2,5-pyha-1H)]Cl (en = ethylenediamine), was determined exhibiting an (Npy,N) binding mode of 2,5-pyha where (Npy,N) is through a pyridine N and deprotonated hydroxamate N. Such a coordination mode has never previously been observed for a Pd(II) hydroxamato complex. The first examples of di- and trinuclear Pd(II) complexes of a hydroxamic acid are described where one 2,5-pyha ligand acts as a bridge for two and three Pd(en) moieties, respectively. The stability of potential binding modes of 2,5-pyha for a proposed dinuclear complex were investigated by density functional theory at the RI-BP86/def2-TZVP level, in the Turbomole package. It was found that (Npy,N), (O,O′)2 gave the more stable complex where (Npy,N′) is through a pyridine N and deprotonated hydroximate N and (O,O′)2 is via the carbonyl oxygen atom and the deprotonated hydroxyl group of the hydroxamic acid at the 2-position on the pyridine ring. A trinuclear complex, [{Pd(en)}3(2,5-pyha-4H)](PF6)2 was isolated and fully characterised. Spectroscopic studies indicate that the coordination modes are (Npy,N), (O,O′)2 and (O,O′)5 where (Npy,N′) is through a pyridine N and deprotonated hydroximate N and (O,O′) is via doubly deprotonated hydroximate groups at positions 2 and 5 on the pyridine ring. Stability constants for the mono-, di- and trinuclear [Pd(en)]2+ complexes of 2,5-pyridinedihydroxamic acid are also reported. In solution at equimolar concentrations, 2,5-pyha effectively binds one equivalent of Pd(II) to give a mononuclear Pd(II) 2,5-pyha complex and under strongly acidic conditions. In addition formation of stable trinuclear Pd(II) 2,5-pyha complexes are favoured at an excess of Pd(II).