N-Heterocyclic carbene–silver complexes: A new class of antibiotics

N-Heterocyclic carbene (NHC)–silver complexes were synthesized from pyridine linked pincer ligands and methylated caffeine. Pincer NHC–silver complexes were found to have more potent antimicrobial activity than the conventionally used silver antimicrobials. Encapsulation of a gem-diol pincer NHC–silver complex in a polymer mat demonstrated a promising method for the sustained delivery of silver ions in wound care applications. An NHC precursor derived from caffeine was found to have a low toxicity and the resulting silver complex showed encouraging antimicrobial activity against numerous pathogens including resistant organisms isolated from the lungs of patients with cystic fibrosis (CF). Bacteria studied included members of the Burkholderia cepacia complex, which cause significant morbidity and mortality in infected CF patients. This review explores this newly growing area, focusing on the synthesis from pincer and xanthine ligands of new silver–NHC complexes and their antimicrobial activities.