Synthesis, structure, magnetism and nuclease activity of tetranuclear copper(ii) phosphonates containing ancillary 2,2′-bipyridine or 1,10-phenanthroline ligands

Abstract

The reaction of cyclohexylphosphonic acid (C₆H₁₁PO₃H₂), anhydrous CuCl₂ and 2,2′-bipyridine (bpy) in the presence of triethylamine followed by a metathesis reaction with KNO₃ afforded [Cu₄(µ-Cl)₂(µ₃-C₆H₁₁PO₃)₂(bpy)₄](NO₃)₂ (1). In an analogous reaction involving Cu(OAc)₂·H₂O, the complex [Cu₄(µ-CH₃COO)₂(µ₃-C₆H₁₁PO₃)₂(2,2′-bpy)₄](CH₃COO)₂ (2) has been isolated. The three-component reaction involving Cu(NO₃)₂·3H₂O, cyclohexylphosphonic acid and 2,2′-bipyridine in the presence of triethylamine afforded the tetranuclear assembly [Cu₄(µ-OH)(µ₃-C₆H₁₁PO₃)₂(2,2′-bpy)₄ (H₂O)₂](NO₃)₃ (3). Replacing 2,2′-bipyridine with 1,10-phenanthroline (phen) in the above reaction resulted in [Cu₄(µ-OH)(µ₃-C₆H₁₁PO₃)₂(phen)₄(H₂O)₂](NO₃)₃ (4). In all the copper(II) phosphonates (1–4) the two phosphonate ions bridge the four copper(II) ions in a capping coordination action. Each phosphonate ion bridges four copper(II) ions in a µ₄, η³ coordination mode or 4.211 of the Harris notation. Variable-temperature magnetic studies on 1–4 reveal that all four complexes exhibit moderately strong intramolecular antiferromagnetic coupling. The DNA cleavage activity of complexes 1–4 is also described. Compounds 1 and 3 were able to completely convert the supercoiled pBR322 DNA form I to nick form II without any co-oxidant. In contrast, 50% conversion occurred with 2 and 40% with 4. In the presence of magnesium monoperoxyphthalate all four compounds achieved rapid conversion of form I to form II.