MOLECULAR GOLD NANOCLUSTERS CAPPED BY N-HETEROCYCLIC CARBENE LIGANDS

Gold nanoparticles (AuNPs) and gold complexes are among the most studied chemical entities for their luminescence behavior and catalytic applications. However, other gold species started to attract worldwide scientific interest in the course of the last years: molecular gold nanoclusters (AuNCs). These are generally composed by several gold atoms forming a metallic core, in which part of the gold atoms are formally zerovalent. Despite their particle-like structure, AuNCs show molecular proprieties, such as a discrete orbital structure and a precise stoichiometric formula. So far, AuNCs stabilization has been mainly reached exploiting thiolate, alkynyl and phosphine ligands, which form a layer saturating and protecting the cluster core. However, the steric and electronic properties of such ligands cannot be independently varied. For this reason, N-heterocyclic carbenes (NHCs) are currently studied as alternative ligands to achieve core protection, also considering the better stabilization obtained with these. Hitherto, AuNCs synthesis has been performed through gold complex reduction using NaBH4, although the reduction mechanism is still unknown and consequently this synthesis is complicate to control. Exploited this methodology, in this PhD thesis the ole of different NHCs has been investigated, showing as such ligands generally provide superatomic (SA) clusters, characterized by [Au11]3+ or [Au13]5+ metallic cores, presenting therefore a complete SA electronic configuration providing further stabilization of the cluster. The role of reducing agent and solvent has been also evaluated, underlining as it is possible to tune the reduction pathways, changing therefore the reduction products. For instance, it is possible to obtain CH3 anionic ligands bind on clusters core when the reduction promoted by NaBH4 is performed in presence of CH2Cl2, used as solvent. Using instead hydrazine as reducing agent [Au6(C)]2+ clusters can be obtained. Notwithstanding the interesting products derived from these syntheses, these reductions are generally not selective, limiting therefore their use. Considering these limitations, in this thesis we have developed another method to isolate such molecular species, called stepwise approach, in which the obtained AuNCs are stabilized by triphenylphosphine (PPh3) and di-NHCs. With this latter method a reaction between a pre-formed PPh3-stabilized [Au11(PPh3)8Cl2]+ cluster and [(di-NHC)Au2Cl2] gold(I) complexes occurs. Depending on starting complex and reaction conditions, a metathesis cluster [Au11(di-NHC)(PPh3)6Cl2]+ is produced first, followed by formation of [Au13(di-NHC)2(PPh3)4Cl4]+ via a subsequent metal complex addition. In some cases, a further metathesis takes place, providing [Au13(di-NHC)3(PPh3)3Cl3]2+. We have tested these clusters as anticancer pro-drugs, demonstrating as these can accumulate in cancer cells thanks to enhanced permeation and retention (EPR) effect, affording high anticancer efficiency too. Likely the antitumor activity derived from AuNCs degradation in biological environment, providing gold complexes acting like anticancer drugs. Finally, thanks to a collaboration with Prof. Dominik Munz at Saarland University(Germany), we have extended our studies towards the synthesis of an unsaturated and cubic palladium cluster and started also a study regarding the stabilization of gold nanowires using NHC ligands.