Effects of Different Substituents on the Crystal Structures and Antimicrobial Activities of Six Ag(I) Quinoline Compounds

The syntheses and single crystal X-ray structures of [Ag­(5-nitroquinoline)₂]­NO₃ (<b>1</b>), [Ag­(8-nitroquinoline)₂]­NO₃·H₂O (<b>2</b>), [Ag­(6-methoxy-8-nitroquinoline)­(NO₃)]_<i>n</i> (<b>3</b>), [Ag­(3-quinolinecarbonitrile)­(NO₃)]_<i>n</i> (<b>4</b>), [Ag­(3-quinolinecarbonitrile)₂]­NO₃ (<b>5</b>), and [Ag­(6-quinolinecarboxylic acid)₂]­NO₃ (<b>6</b>) are described. As an alternative to solution chemistry, solid-state grinding could be used to prepare compounds <b>1</b> and <b>3</b>, but the preparation of <b>4</b> and <b>5</b> in this way failed. The Ag­(I) ions in the monomeric compounds <b>1</b>, <b>2</b>, <b>5</b>, and <b>6</b> are coordinated to two ligands via the nitrogen atoms of the quinoline rings, thereby forming a distorted linear coordination geometry with Ag–N bond distances of 2.142(2)–2.336(2) Å and N–Ag–N bond angles of 163.62(13)°–172.25(13)°. The 1D coordination polymers <b>3</b> and <b>4</b> contain Ag­(I) centers coordinating one ligand and two bridging nitrate groups, thereby forming a distorted trigonal planar coordination geometry with Ag–N bond distances of 2.2700(14) and 2.224(5) Å, Ag–O bond distances of 2.261(4)–2.536(5) Å, and N–Ag–O bond angles of 115.23(5)°–155.56(5)°. Hirshfeld surface analyses of compounds <b>1</b>–<b>6</b> are presented as <i>d</i>_norm and curvedness maps. The <i>d</i>_norm maps show different interaction sites around the Ag­(I) ions, i.e., Ag···Ag interactions and possible O–H···O, C–H···O, C–H···N, and C–H···C hydrogen bonds. Curvedness maps are a good way of visualizing π–π stacking interactions between molecules. The antimicrobial activities of compounds <b>1</b>, <b>2</b>, and <b>6</b> were screened against 15 different multidrug-resistant strains of bacteria isolated from diabetic foot ulcers and compared to the antimicrobial activities of the clinically used silver sulfadiazine (SS). Compound <b>2</b> showed activity similar to SS against this set of test organisms, being active against all strains and having slightly better average silver efficiency than SS (5 vs 6 μg Ag/mL). Against the standard nonresistant bacterial strains of Staphylococcus aureus, Pseudomonas aeruginosa, Proteus mirabilis, and Streptococcus pyogenes, compound <b>1</b> performed better than silver nitrate, with an average MIC of 6 μg Ag/mL versus 18 μg Ag/mL for the reference AgNO₃. Electrospray ionization mass spectrometry (ESI-MS) analyses of compounds <b>3</b> and <b>6</b> in DMSO/MeOH confirm the two-coordinated Ag⁺ complexes in solution, and the results of the ¹H NMR titrations of DMSO solutions of 5-nitroquinoline and 8-nitroquinoline with AgNO₃ in DMSO suggest that 5-nitroquinoline is more strongly coordinated to the silver ion