Linkage Isomerism in Silver Acylpyrazolonato Complexes and Correlation with Their Antibacterial Activity

Novel silver­(I) acylpyrazolonato coordination polymers of formula [Ag­(QR)] n (1–3) have been synthesized by interaction of silver nitrate with HQR in methanol in the presence of an equivalent quantity of KOH (in general HQR = 1-phenyl-3-methyl-4-RC­(O)-5-pyrazolone, in detail HQfb, R = -CF2CF2CF3; HQcy, R = -cyclo-C6H11; HQbe, R = -C­(H)C­(CH3)2). [Ag­(QR)] n react with 2-ethylimidazole (2EtimH), 1-methylimidazole (Meim), and triphenylphosphine (PPh3), affording the mononuclear Ag­(Qfb)­(EtimH) (4), Ag­(Qcy)­(Meim)2 (5), Ag­(Qbe)­(Meim) (6), and Ag­(QR)­(PPh3)2 (7–9). All complexes have been analytically and spectroscopically characterized, and for some of them the X-ray crystal structure has been resolved. In particular, the single crystal molecular structure determination of Ag­(Qfb)­(EtimH) and Ag­(Qbe)­(PPh3)2 has confirmed the different coordination modes of the HQfb and HQbe acylpyrazolone ligands, the former being bound to the silver­(I) ion in a monodentate fashion while the latter in the O2-chelating mode. Density functional theory computations suggest new insights about metal–ligand interactions and the observed linkage isomerism. While phosphine-containing complexes Ag­(QR)­(PPh3)2 (7–9) seem not to be able to efficiently inhibit the growth of Escherichia coli and Staphylococcus aureus, the polynuclear complexes [Ag­(QR)] n (1–3) and the mononuclear Ag­(Qfb)­(EtimH) (4), Ag­(Qcy)­(Meim)2 (5), and Ag­(Qbe)­(Meim) (6) show a high and almost steady in time antibacterial activity, comparable to that of AgNO3. This activity is likely related to the degree of saturation of the silver center and to the presence of different ancillary ligands in the diverse typologies of complexes.