Synthesis and crystal structure of a hexanuclear silver(I) cluster [Ag(Hmna)]6·4H2O (H2mna = 2-mercaptonicotinic acid) and a supramolecular gold(I) complex H[Au(Hmna)2] in the solid state, and their antimicrobial activities

Abstract

2-Mercaptonicotinic acid (H₂mna = 2-HS(C₅H₃N)CO₂H) gave two types of silver(I) complexes by a reaction with Ag₂O suspended in aqueous media; one was a water-insoluble, DMSO-soluble yellow powder [Ag(Hmna)]₆·4H₂O 1 formed under acidic conditions, and the other was a water-soluble, yellow powder {Na[Ag(mna)]·H₂O}_n 2 formed under alkaline conditions. These two complexes were interconverted on changing the acidity of the solution. Crystallization by vapor diffusion of the DMSO and aqueous solutions with external acetone gave pale yellow cubic crystals for 1 and pale yellow plate crystals for 2, but structure determination was successful only for 1. The molecular structure of 1 consists of a discrete, hexanuclear silver(I) cluster with the two silver(I) triangles linked by the mercaptonicotinate anions, the geometry around each silver(I) atom being constructed by one aromatic nitrogen atom and two μ-S atoms and the two weak silver(I)–silver(I) interactions. The corresponding gold(I) complex as a water-insoluble, DMSO-soluble yellow powder, H[Au(Hmna)₂] 3, was prepared by treatment with aqueous HCl of Na₃[Au(mna)₂]·2H₂O 4 which was obtained by a 1∶4∶8 molar ratio reaction of Na[AuCl₄]·2H₂O∶H₂mna∶NaOH in aqueous solution. Complex 3 crystallized as yellow needle crystals by vapor diffusion of the DMSO solution with external acetone. Its crystal structure with a monomeric 2-co-ordinate AuS₂ core shows a supramolecular arrangement by a π–π stacking interaction between two pyridine rings (face-to-face separation 3.42(2)–3.52(1) Å) and, also, by two different hydrogen-bonding interactions; one formed between the carboxyl proton and aromatic nitrogen, and the other an unusual single-mode hydrogen bond between two twisted carboxyl groups. The complexes were also characterized by elemental analysis, TG/DTA, FT-IR, ¹H and ¹³C NMR for 1–4, as well as ¹⁰⁹Ag NMR for 2. Noteworthy antimicrobial activities for 1–4, evaluated by minimum inhibitory concentration, were observed.