New silver (thio)semicarbazide derivatives: synthesis, structural features, and antimicrobial activity

Abstract

Two novel semicarbazide [HL¹, 1, 1-((4-nitrophenyl)(phenyl)methylene)semicarbazide] and thiosemicarbazide [HL², 2, 1-((4-nitrophenyl)(phenyl)methylene)thiosemicarbazide] ligands were generated from 4-nitrobenzophenone and the corresponding (thio)semicarbazide precursors. Compounds 1 and 2 were fully characterized and their X-ray crystal structures were established. These organic derivatives were further applied as bioactive building blocks for the self-assembly synthesis of silver(I) coordination compounds. The reactions of silver(I) nitrate with HL¹/HL² in the presence of aqueous NH₄OH resulted in new silver(I) coordination compounds formulated as [Ag(HL¹)(NO₃)]·H₂O (3) and [Ag₆(μ₃-L²)₆] (4). These complexes were characterized by standard methods (FT-IR, NMR, ESI-MS, and elemental analysis) as well as single-crystal X-ray diffraction. The structure of 3 features a two-coordinate silver(I) center with a distorted linear {AgNO} geometry, wherein an unusual coordination of HL¹ through the nitrogen atom of the amine group is realized. The structure of 4 reveals a discrete Ag₆ cluster composed of two joined trisilver(I) rings, wherein the three-coordinate Ag(I) centers adopt an {AgNS₂} environment. The antimicrobial activity of 1–4 was screened against Gram-positive (S. epidermidis and S. aureus) and Gram-negative (P. aeruginosa and E. coli) bacteria. Compound 4 presented antimicrobial activity against Gram-positive bacteria, including the ability to impair biofilm formation. This study thus contributes to a still limited research area on the development of new silver coordination compounds with potential antibiofilm activity.