Doping effect on the structure and properties of eight-electron silver nanoclusters

The bimetallic M₂₀ and M₂₁ compounds, {[Cu₃Ag₁₇{S₂P(OⁱPr)₂}₁₂]_0.5 [Cu₄Ag₁₆{S₂P(OⁱPr)₂}₁₂]_0.5} ({[1a]_0.5[1b]_0.5}) and [Cu₄Ag₁₇{S₂P(OⁱPr)₂}₁₂](PF₆) (2), have been structurally characterized, in which the Cu(I) ions are randomly distributed on the eight outer positions capping the eight-electron [Ag₁₃]⁵⁺ core. DFT calculations show that the statistical disorder results from the nearly neutral preference of copper to occupy any of the eight outer positions. Surprisingly, the UV-Vis absorption spectra of the M₂₀ and M₂₁ bimetallic nanoclusters display an almost identical absorption profile as that of their homometallic [Ag₂₀{S₂P(OⁱPr)₂}₁₂] and [Ag₂₁{S₂P(OⁱPr)₂}₁₂]⁺ relatives. This is rationalized by TD-DFT calculations, which show that the frontier orbitals of such eight-electron alloys are largely independent from the nature of the capping metal ions. A blue-shifted absorption is observed upon replacing by Au the central Ag atom in 2, forming the trimetallic compound [Cu₄AuAg₁₆{S₂P(OⁱPr)₂}₁₂](PF₆) (3).