Unexpected structural preference with metallophilic Ag-Au contacts in silver(I)-N-heterocyclic carbene cluster; experimental and theoretical approach.

A novel synthetic donor-atom-selective approach has been adopted for the synthesis of a heterobimetallic cluster of a new NCN-pincer, 1,3-bis-(1-methyl-1H-benzo[d]imidazol-2-yl-methyl)-1H-imidazol-3-ium hexafluorophosphate (1·HPF6). The complex [Ag3(1)3][PF6]3 (2) has been prepared via the Ag2O route; which undergoes transmetallation to yield a cluster that seems to be the first example of the heterobimetallic trinuclear system [Au-Ag2(1)2Cl][PF6]2, 3. Finally, the trinuclear cluster geometries of 2 and 3 were confirmed via SCXRD studies. Interestingly, Au(I) binds preferentially with soft donor Ccarbene, which transmetallated from the cluster of 2. In both the cyclic trinuclear clusters, the M-M interactions were further inspected using gauge independent atomic orbital (GIAO) computations. Both 2 and 3 are luminescent and possess σ-aromaticity; the NICS values indicate that 3 is more aromatic than 2.

Design, Synthesis and Bioactivity Evaluation of Ag(I)-, Au(I)- and Au(III)-Quinoxaline-Wingtip N-Heterocyclic Carbene Complexes Against Antibiotic Resistant Bacterial Pathogens.

Intending to homogenize the biological activities of both quinoxaline and imidazole moieties, the proligand, 1-methyl-3-quinoxaline-imidazolium hexaflurophosphate (1.HPF6), and [Ag(1)2][PF6], (2); [Au(1)2][PF6], (3); and [Au(1)Cl3], (4) NHC complexes were synthesized. All the synthesized compounds were characterized by elemental analysis, NMR, and UV-Vis spectroscopy. Finally, single crystal X-ray structures revealed a linear geometry for complex 2 whereas a square planar geometry for complex 4. The formation of complex 3 was confirmed and supported by its MS spectra. The antibacterial activities of all the synthesized complexes were investigated against gram-positive bacteria and gram-negative bacteria. The Au(III)-NHC complex, 4 showed the highest antibacterial activity with extremely low MIC values against both the bacterial strains (0.24 µg mL-1). Monitoring of zeta potential supports the higher activity of complex 4 compared to 2 and 3. ROS production by complex 4 has also been measured in vitro in the CT26 cancer cell lines, which is directly responsible for targetting and killing the bacterial pathogens. Cell cytotoxicity assay using 293T cell lines has been performed to investigate the biocompatibility nature of complex 4. Also, an excellent hemocompatibility was assigned to it from its hemolytic studies, which provide valuable insights into the design of novel antibacterial agents.