Strong chiroptical activity in Au25 clusters protected by mixed ligands of chiral phosphine and achiral thiolate

Abstract

We report the successful synthesis of a chiroptically active Au₂₅ cluster protected by mixed ligands of chiral bidentate S-BINAP and achiral dodecanethiol (DDT), which can be formulated as [Au₂₅(S-BINAP)₄(DDT)₅X₄] (X = Cl or Br). The UV-vis absorption spectral pattern is similar to that of the well-known bi-icosahedral cluster [Au₂₅(PPh₃)₁₀(SR)₅X₂]²⁺, so the obtained cluster should also have a similar bi-icosahedral structure assembled from two vertex-sharing icosahedral Au₁₃ units. With a closer inspection of the optical absorption, interestingly, the lowest-energy peak is red-shifted as compared to that of [Au₂₅(PPh₃)₁₀(SR)₅X₂]²⁺. Quantum chemical calculations for model bi-icosahedral Au₂₅ structures suggest the reason of the red shift. On the other hand, the obtained Au₂₅ cluster exhibits a weak CD signature in the lowest-energy transition region, whereas higher-energy transitions have very large chiroptical responses with a maximum g-factor of 1.7 × 10⁻³. The calculations also give implications for the origin of the CD response in the Au₂₅ cluster. We then believe that bi-icosahedral Au₂₅ clusters with chirality will be a good prototype for understanding the influence of constituent Au₁₃ units on the chiroptical activity of their assembling structures.