Tri-, Tetra-, and Hexanuclear Copper(II) Phosphonates Containing <i>N</i>-Donor Chelating Ligands: Synthesis, Structure, Magnetic Properties, and Nuclease Activity

Reaction of Cu(ClO4)2·6H2O with cyclopentyl phosphonic acid and 2,2′-bipyridine (bpy) in presence of triethylamine afforded a trinuclear compound [Cu3(C5H9PO3)2(bpy)3(MeOH)(H2O)](ClO4)2 (2). The latter dimerizes to a hexanuclear derivative [Cu6(C5H9PO3)4(bpy)6(MeOH)4](ClO4)4 (1) under prolonged reaction conditions. Reaction of CuCl2 with cyclopentyl phosphonic acid and 2,2′-bipyridylamine (bpya) affords a tetranuclear derivative [Cu4(C5H9PO3)2(μ-Cl)2(bpya)4](Cl)2, (MeOH)2 (3). Reaction of the latter with NaClO4 also affords a trinuclear compound [Cu3(C5H9PO3)2(μ-Cl)(bpya)3(H2O)](ClO4) (4). Double and single-bridged hexanuclear species, [{Cu3(C5H9PO3)2(bpy)3(bpp)}(MeOH)2(H2O)(CH2Cl2)(ClO4)2]2 (5), [{Cu3(i-PrPO3)2(bpy)3(4.4′-bpy)(H2O)}(H2O)2(ClO4)2]2 (6), [{Cu3(C5H9PO3)2(bpya)3(4.4′-bpy)(H2O)}(MeOH)(H2O)(ClO4)2]2 (7), and [Cu6(t-BuPO3)4(phen)6(4,4′-bpy)(MeOH)4](CH2Cl2)(H2O)(ClO4)4 (8) (phen = 1,10-phenanthroline) were obtained by the reaction of an in situ generated trinuclear complex with appropriate bridging ligands 4,4′-bipyridine (4,4′-bpy) or 1,3-bis(4-pyridyl)propane (bpp). ESI-MS studies of these complexes reveal that 2−4 retain their structures in solution. Molecular structures of 2−8 were determined by X-ray crystallography. All the compounds reveal a capping coordination mode by tridentate phosphonate [RPO3]2− ligands. Detailed magnetic studies on 2 and 4−8 reveal intramolecular antiferromagnetic interactions between Cu(II) S = 1/2 spins. 2 and 4 are excellent artificial nucleases and can convert supercoiled plasmid DNA (pBR322) into its nicked form without the aid of an external oxidant.