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

The reaction of cyclohexylphosphonic acid (C<SUB>6</SUB>H<SUB>11</SUB>PO<SUB>3</SUB>H<SUB>2</SUB>), anhydrous CuCl<SUB>2</SUB> and 2,2'-bipyridine (bpy) in the presence of triethylamine followed by a metathesis reaction with KNO<SUB>3</SUB> afforded [Cu<SUB>4</SUB>(μ-Cl)<SUB>2</SUB>(μ<SUB>3</SUB>-C<SUB>6</SUB>H<SUB>11</SUB>PO<SUB>3</SUB>)<SUB>2</SUB>(bpy)<SUB>4</SUB>](NO<SUB>3</SUB>)<SUB>2</SUB> (1). In an analogous reaction involving Cu(OAc)<SUB>2</SUB>·H<SUB>2</SUB>O, the complex [Cu<SUB>4</SUB>(μ-CH<SUB>3</SUB>COO)<SUB>2</SUB>(μ<SUB>3</SUB>-C<SUB>6</SUB>H<SUB>11</SUB>PO<SUB>3</SUB>)<SUB>2</SUB>(2,2'-bpy)<SUB>4</SUB>](CH<SUB>3</SUB>COO)<SUB>2</SUB> (2) has been isolated. The three-component reaction involving Cu(NO<SUB>3</SUB>)<SUB>2</SUB>·3H<SUB>2</SUB>O, cyclohexylphosphonic acid and 2,2'-bipyridine in the presence of triethylamine afforded the tetranuclear assembly [Cu<SUB>4</SUB>(μ-OH)(μ<SUB>3</SUB>-C<SUB>6</SUB>H<SUB>11</SUB>PO<SUB>3</SUB>)<SUB>2</SUB>(2,2'-bpy)<SUB>4</SUB> (H<SUB>2</SUB>O)<SUB>2</SUB>](NO<SUB>3</SUB>)<SUB>3</SUB> (3). Replacing 2,2'-bipyridine with 1,10-phenanthroline (phen) in the above reaction resulted in [Cu<SUB>4</SUB>(μ-OH)(μ<SUB>3</SUB>-C<SUB>6</SUB>H<SUB>11</SUB>PO<SUB>3</SUB>)<SUB>2</SUB>(phen)<SUB>4</SUB>(H<SUB>2</SUB>O)<SUB>2</SUB>](NO<SUB>3</SUB>)<SUB>3</SUB> (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 μ<SUB>4</SUB>, η<SUP>3</SUP> 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