Coordination Diversity in Mono- and Oligonuclear Copper(II) Complexes of Pyridine-2-Hydroxamic and Pyridine-2,6-Dihydroxamic Acids

Solution and solid state studies on Cu­(II) complexes of pyridine-2-hydroxamic acid (HPicHA) and pyridine-2,6-dihydroxamic acid (H₂PyDHA) were carried out. The use of methanol/water solvent allowed us to investigate the Cu­(II)–HPicHA equilibria under homogeneous conditions between pH 1 and 11. In agreement with ESI-MS indication, the potentiometric data fitted very well with the model usually reported for copper­(II) complexes of α-aminohydroxamate complexes ([CuL]⁺, [Cu₅(LH_–1)₄]²⁺, [CuL₂], [CuL₂H_–1]⁻), however with much higher stability of the 12-MC-4 species. A series of copper­(II) complexes has been isolated in the solid state and characterized by a variety of spectroscopic methods, X-ray structure analysis, and magnetic susceptibility measurements. The ligands show the tendency to form bi- and trinuclear species with copper­(II) ions due to the {(N,N′); (O,O′)} bis-(bidentate) chelating-and-bridging mode involving (O,O′)-hydroxamate chelate formation combined with (N,N′) chelating with participation of the pyridine and hydroxamic nitrogen atoms, so that the hydroxamate groups play a μ₂-(N,O)-bridging role. Molecular and crystal structures of three synthesized complexes [Cu₃(PicHA-H)₂(dipy)₂]­(ClO₄)₂·4/3DMSO·2/3H₂O (1), [Cu₂(PyDHA)­(dipy)₂(ClO₄)₂]·DMF·H₂O (4), and [Cu₃(PyDHA-2H)­(tmeda)₃]­(ClO₄)₂ (5) (dipy, 2,2′-dipyridyl; tmeda, N,N,N′,N′-tetramethyl-1,2-diaminoethane) have been determined by single crystal X-ray analysis. In 1, two trans-situated doubly deprotonated hydroxamic ligands play a {(O,O′)­(N,N′)}–(bis)­bidentate-bridging function forming bridges between the medial, Cu(2) (CuN₄), and the terminal, Cu(1) and Cu(3) (CuN₂O₂), copper­(II) ions; the chelating dipy ligands are coordinated to the latter. In 4, the ligand is coordinated in a classical (O,O′)-hydroxamate chelating mode with the help of two separate hydroxamic groups while the central tridentate donor compartment remains vacant. In 5, the hydroxamate ligand is coordinated by the {(O,O′);(N,N′,N″);(O″,O‴)}-tridentate-(bis)­bidentate mode, bridging three copper­(II) ions, while the chelating tmeda ligands are coordinated to all three copper­(II) ions. Magnetic susceptibility measurements (1.7–300 K) of powdered samples of the trinuclear complexes 1 and 5 revealed strong antiferromagnetic coupling between the copper­(II) ions mediated by the hydroxamate bridges.