Amplification of the Optical Activity of Gold Clusters by the Proximity of BINAP

Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au₁₁(R/S-DIOP)₄Cl₂]⁺ (DIOP = 1,4-bis(diphenylphosphino)-2,3-o-isopropylidene-2,3-butanediol) and [Au₈(R/S-BINAP)₃(PPh₃)₂]²⁺ (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl). [Au₈(BINAP)₃(PPh₃)₂]²⁺ showed stronger rotatory strengths than [Au₁₁(DIOP)₄Cl₂]⁺ in the visible region, while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than that of the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core.