Double and triple stranded mesocates containing the bis(bidentate) bridging ligand 1,3-bis(pyridine-2-carboxamide)benzene. Structure, properties and DNA interaction

The double stranded Cu^II₂ metallacyclic complex of formula [Mn(hfac)₂(H₂O)₂][Cu₂(mbpb)₂(CH₃CN)₂] (1) and the triple stranded Ni^II₂, Zn^II₂ and Co^III₂ metallacyclic complexes of formula [M₂(Hmbpb)₃]X·nH₂O (M = Zn^II, X = NO₃⁻ (2), n = 17; M = Ni^II, and X = ClO₄⁻ (3), n = 15) and [Co₂(mbpb)₃]·19H₂O (4) (H₂mbpb is the bisbidentate dinucleating bridging ligand, 1,3-bis(pyridine-2-carboxamide)benzene) have been prepared and structurally characterised. Their X-ray structures show that inside the dinuclear molecules metal ions are bridged by either fully or semideprotonated bisbidentate ligands, which are coordinated through the pyridine and amidato nitrogen donor atoms. In 1, the neutral metallamacrocycle dinuclear entities [Cu₂(mbpb)₂(CH₃CN)₂] and the [Mn(hfac)₂(H₂O)₂] molecules are connected by hydrogen bonds to afford a 1D system. These intermolecular interactions overcome the expected intradinuclear weak ferromagnetic interaction leading to an overall weak antiferromagnetic interaction. The Ni₂ complex exhibit ferromagnetic coupling between the metal ions through the bridging ligand with J_Ni–Ni = 3.1 cm⁻¹. DFT calculations were performed to estimate the value of the exchange magnetic coupling inside the dinuclear unit in 1 and to confirm that the spin polarisation mechanism is responsible for the ferromagnetic coupling. AFM studies show that the Zn^II₂ and Ni^II₂ cationic complexes interact with pBR322 DNA producing supercoiled forms in higher extension as well as kinks and cross linking.