Antibacterial action of 4,4′-bipyrazolyl-based silver(I) coordination polymers embedded in PE disks

Coupling the rigid spacer 4,4′-bipyrazole (H2BPZ), in its anionic or neutral form, to different silver(I) salts allowed isolation of the novel coordination polymers [Ag2(BPZ)] (1) and [Ag(H2BPZ)(X)] (X = NO3, 2; ClO4, 3; BF4, 4; PF6, 5; CH3SO3, 6; CF3SO3, 7), which were fully characterized by infrared and emission spectroscopies, thermal analysis, and X-ray powder diffraction. The crystal structure of 1 consists of 2-D layers containing 1-D chains of Ag(I) ions bridged by exotetradentate bipyrazolato moieties. The crystal structures of the [Ag(H2BPZ)(X)] species 2−7 feature 1-D chains of [Ag(H2BPZ)] stoichiometry, along which the metal centers are bridged by exo-bidentate bipyrazolyl spacers. Contacts among adjacent chains are mediated by the counterions through nonbonding interactions involving the Ag(I) ions and the pyrazolyl N−H groups. Thermogravimetric analyses disclosed the good thermal stability of these materials, decomposing in the range 200− 300 °C. Under UV irradiation at room temperature, all the species showed a yellow-green emission centered in the range 520− 522 nm. When embedded into polyethylene disks, 1, 2, and 4−7 demonstrated their activity as topical antibacterial agents against suspensions of E. coli, P. aeruginosa, and S. aureus: complete reduction of the three bacterial strains was achieved in 24 h, reduction of S. aureus reaching ca. 90% in only 2 h. Biocidal action was expressed also by contact susceptibility tests.