Chiroptical Activity in BINAP- and DIOP-Stabilized Octa- and Undecagold Clusters

In order to learn more about the origin of chirality in chiral organometallic complexes and to contribute to the understanding of the differences in chiroptical activity of metal clusters stabilized by various phosphine ligands, we examined the optical properties of undecagold (Au-11(3+)) and octagold (Au-8(2+)) clusters protected by two types of bisphosphine ligands [BINAP (2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) and DIOP (o-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane)]. The chirality of pairs of clusters [Au-11(BINAP)(4)Cl-2](+), [Au-11(DIOP)(4)Cl-2] (+) and [Au-8(BINAP)(3)(PPh3)(2)](2+), [Au-8(DIOP)(3)(PPh3)(2)](2+) are investigated with density functional theory (DFT) and time-dependent density functional theory (TDDFT). The geo- metries of the octa- and undecagold cores in the model clusters are similar to the gold cores of the crystal structures. Theoretical optical absorption and CD spectra of the model clusters are in good agreement with experimental data. Three main hypotheses to explain the different chiroptical activity of the clusters were suggested: (i) the CD activity originates from core deformation due to ligation, (ii) the nature of the chiral ligands can play a crucial role in the optical activity of the core, and (iii) the Cl atom positions can affect the CD intensity. It was shown that the gold core geometry deformation due to ligation and the nature of the ligand play the most important roles in the chiroptical activity of the gold clusters. In addition, the ligands determine the gold core structural deformation and affect the high-energy region of the CD spectra, whereas the gold core itself exhibits a significant effect on the shape and sign of the CD spectra in the low energy region with wavelengths above similar to 350 nm.