Mixed-diphosphine-protected chiral undecagold clusters Au11(S,S-DIOP)4(rac-/R-/S-BINAP): effect of the handedness of BINAP on their chiroptical responses†
In this article, we report a preference of homochiral-type ligation of BINAP that produces SS-type ligand assembly onto the Au11 clusters protected by diphosphine S,S-DIOP. The Au11 clusters synthesized and isolated are Au11(S,S-DIOP)4(rac-/R-/S-BINAP), and their optical/chiroptical responses are characterized. Absorption spectra of these Au11 clusters are almost identical to each other, but their CD profiles are dependent on the handedness of BINAP. In Au11(S,S-DIOP)4(rac-BINAP), the yield of S-BINAP or R-BINAP coordination is roughly comparable, but we found a small but distinctive preference in the S-BINAP ligation; that is, homochiral-type (SS-type) ligand assembly formation. Quantum chemical calculations for simplified model clusters suggest equal contributions of S- and R-form BINAP coordination. The experimentally-observed preference of homochiral-type ligation can then be due to that of the whole ligand structures and assemblies involving interligand interactions. Chiral sorting and amplification processes through the assembly control of homochirality or heterochirality are of primary importance for the development of enantioselective reactions, so we anticipate this finding will contribute to further understanding of such processes based on various metal clusters with chiral ligands.