Antimicrobial resistance is an ever-increasing problem throughout the world and has already reached severe proportions. Bacteria can develop ways to render traditional antibiotics ineffective, raising a crucial need to find new antimicrobials with novel mode of action. We demonstrate here a novel class of pyrazine functionalized Ag(I) and Au(I)-NHC complexes as antibacterial agents against human pathogens that are resistant to several antibiotics. Complete synthetic and structural studies of Au(I) and Ag(I) complexes of 2-(1- methylimidazolium) pyrimidinechloride (L-1), 2,6-bis(1-methylimidazol)pyrazinechloride (L-2) and 2,6-bis(1-methyl imidazol) pyrazinehexa-fluorophosphate (L-3) are reported herein. Chloro[2,6-bis(1-methyl imidazol)pyrazine]gold(I), 2b and chloro [2,6-bis(1- methyl imidazol)pyrazine]silver(I), 2a complexes are found to have more potent antimicrobial activity than other synthesized compounds and several conventionally used antibiotics. Complexes 2b and 2a also inhibit the biofilm formation by Gram-positive bacteria, Streptococcus mutans and Gram-negative bacteria, Escherichia coli, causing drastic damage to the bacterial cell wall and increasing membrane permeability. Complexes 2b and 2a strongly binds to both Lys and Dap-Type peptidoglycan layers, which may be the reason for damage to the bacterial cell wall. Theoretical studies of all the complexes reveal that 2b and 2a are more reactive than other complexes, and this may be the cause of differences in antibacterial activity. These findings will pave the way towards developing a new class of antibiotics against different groups of conventional antibiotic-resistant bacteria.

Keratitis treatment has become more complicated due to the emergence of bacterial or fungal pathogens with enhanced antibiotic resistance. The pharmaceutical applications of N-heterocyclic carbene complexes have received remarkable attention due to their antimicrobial properties. In this paper, the new precursor, 3,3′-(p-phenylenedimethylene) bis{1-(2- methyl-allyl)imidazolium} bromide (1a) and its analogous PF₆ salt (1b) were synthesized. Furthermore, silver(I) and gold(I) -N-heterocyclic carbene (NHC) complexes [Ag₂LBr₂/Au₂LBr₂; 2a/3a], [(Ag₂L₂)(PF₆)₂/(Au₂L₂)(PF₆)₂; 2b/3b] were developed from their corresponding ligands. All compounds were screened for their antimicrobial activities against multiple keratitis-associated human eye pathogens, including bacteria and fungi. Complexes 2a and 3a showed highest activity, and the effectiveness of 3a was also studied, focusing eradication of pathogen biofilm. Furthermore, the structures of 1a, 2a and 3b were determined using single crystal X-ray analysis, 2b and 3a were optimized theoretically. The mechanism of action of 3a was evaluated by scanning electron microscopy and docking experiments, suggesting that its target is the cell membrane. In summary, 3a may be helpful in developing antimicrobial therapies in patients suffering from keratitis-associated eye infections caused by multidrug-resistant pathogens.

Novel NCN-pincer carbene complexes of Ru(II), 1,3-bis(2-pyridylmethyl)benzimidazolineruthenium(II) bishexafluorophosphate (2), and Pt(II), 1,3-bis(2-pyridylmethyl)benzimidazolinechloroplatinum(II) hexafluorophosphate (3), complexes based on 1,3-bis(2-pyridylmethyl)-1H-benzimidazolium chloride (1) were synthesized and characterized by different spectroscopic methods. Complex 2 shows an absorption maximum at 386 nm, blue-shifted in comparison to Ru(bpy)₃ ²⁺ and Ru(tpy)₂ ²⁺, probably due to the strong σ-donor and weak π-acceptor properties of the electron-rich NHC ligand. Electrochemical studies show Ru(II)/Ru(III) and Pt(II)/Pt(IV) reversible couples at 0.67 and 0.58 eV, respectively, lower than those for the analogous complexes of ligands like bipyridine (bpy), terpyridine (tpy) and phenylbipyridine (pbpy). The solid state structure of 2 was solved by X-ray diffraction. Theoretical studies (B3LYP/LANL2DZ) of the complex show a HOMO (−0.38594 au) mainly centered on the ruthenium and benzimidazole, whereas the LUMO (−0.25130 au) is populated by pyridines. Therefore, it is assumed that the charge transfer from HOMO → LUMO is mixed ILCT (interligand charge transfer)/MLCT (metal to ligand charge transfer). The observed lower redox potentials of the Pt(II) complex compared to the Ru(II) complex is supported by theoretically calculated ionisation potentials and also electron affinity values. To the best of our knowledge, 2 is the first example of a six-membered metallacycle homoleptic chelate pincer NCN–Ru(II) N-heterocyclic carbene complex.

First principle calculation on Au ₃ (Py) ₃ and Au ₃ (IMD) ₃ systems shows that they are superalkali and exhibit good nonlinear optical property.