Water-soluble silver nanoparticles stabilized by amino acid-derived N-heterocyclic carbenes: synthesis, properties and theoretical study of the nucleation process

Abstract

Silver nanoparticles stabilized by amino acid-derived N-heterocyclic carbenes, denoted as Ag(NHC^R)-NPs (R = H, 3a; Me, 3b; ⁱPr, 3c; and ⁱBu, 3d), were synthesized by reducing the parent complexes Na₃[Ag(NHC^R)₂] (2a–d) with NaBH₄ under appropriate reaction conditions. The stability of the aqueous AgNP solutions was found to depend strongly on the presence of the NHC ligand, the solution concentration, and the nature of the R substituent. In particular, the stability of the nanoparticles decreases as the steric bulk of R increases. Among the series, 3a (R = H) exhibits remarkable stability in water and can be isolated by ultracentrifugation and lyophilization. Notably, solid Ag(NHC^H)-NPs (3a) can be redissolved in water to regenerate a stable AgNP solution. The Ag(NHC^R)-NPs were characterized by infrared (IR) and ultraviolet-visible (UV-Vis) spectroscopies, polarimetry, dynamic light scattering (DLS), and transmission electron microscopy (TEM). 3a behaves as an active and versatile nanocatalyst in water, efficiently promoting both the model reduction of 4-nitrophenol to 4-aminophenol and the catalytic hydrolysis of NaBH₄ to generate H₂ under basic conditions. From a theoretical perspective, the nucleation and growth of the Ag(NHC^R)-NPs were modelled using density functional theory (DFT) at the PBE-D3/def2-TZVP level, considering systems of the type [Ag_n(NHC^R)]²⁻ (with R = H, Me and ⁱPr and n = 2, 3, 4, 20, 30). The Quantum Theory of Atoms in Molecules (QTAIM) was employed to analyze the bonding characteristics within the nanoparticles, with particular attention to the Ag–Ag and Ag–C(carbene) interactions. It is noteworthy that the bond dissociation energy (BDE) of the Ag–C(carbene) bond decreases with increasing steric bulk of R, consistent with the experimental observations. Based on experimental data, the Ag : NHC ratio is approximately 30 : 1 and the calculated IR spectrum of [Ag₃₀(NHC^H)]²⁻ model (corresponding to 3a) provides a satisfactory match with the experimental spectrum.