A trigonal-prismatic Ag(I) complex and an octahedral Cu(II) complex incorporating with oxamide Ni(II) complex ligand: Syntheses, crystal structures and oxidative nuclease activities

A mononuclear macrocyclic complex NiIIL3a (L3a = dianion of 2,3-dioxo-5,6:13,14-dibenzo-9,10-cyclohexyl-7,12-bis(methoxycarbonyl)-1,4,8,11-tetraazacyclotetradeca-7,11-diene), which shows high DNA cleavage activity in the presence of H2O2, was reported in our previous work. Considering that many systems for natural enzyme-mediated DNA cleavage contain two or more metal active sites, two new trinuclear complexes [Cu(NiL3a)2(dca)2]·2CH3OH (abbreviated as Cu(NiL3a)2) and [Ag(NiL3a)2(NO3)]·2CH3OH·0.5H2O (abbreviated as Ag(NiL3a)2) were synthesized in this work, where dca is the dicyanamide. The complexes were structurally characterized by single crystal X-ray analysis. The central Cu(II) or Ag(I) atom is linked to two [NiL3a] ligands by oxamido bridges forming a trinuclear structure. In Cu(NiL3a)2, the central Cu(II) ion is in an octahedral coordination geometry. Whereas in Ag(NiL3a)2, the central Ag(I) ion is in a rarely reported trigonal-prismatic coordination geometry. The DNA cleavage behavior of the complexes in the presence of H2O2 was studied in detail. Comparing with the NiL3a, the trinuclear complex Ag(NiL3a)2 nearly has no ability to cleave DNA, whereas Cu(NiL3a)2 is a much better DNA cleavage agent. Cu(NiL3a)2 can efficiently convert supercoiled DNA to nicked DNA with a rate constant of 0.074 ± 0.002 min−1 when 40 μM Cu(NiL3a)2 and 0.6 mM H2O2 are used. The cleavage mechanism between the complex Cu(NiL3a)2 and plasmid DNA is likely to involve singlet oxygen as reactive oxygen species. Circular dichroism (CD) and fluorescence spectroscopy indicate that both Cu(NiL3a)2 and NiL3a bind to DNA by a groove binding mode, and the binding between Cu(NiL3a)2 and DNA is much stronger than that between NiL3a and DNA. The present results may provide some information for the design of efficient multinuclear artificial nucleases.