Water-soluble phosphine-protected Au9 clusters: Electronic structures and nuclearity conversion via phase transfer

- Isolation of water-soluble phosphine-protected Au9 clusters is successful.
- UV-vis absorption and magnetic circular dichroism (MCD) spectra are characterized.
- MCD demonstrates the presence of very weak electronic transitions in the cluster.
- DFT calculations suggest the cluster's possible skeletal structure in solution.
- A new nuclearity conversion route from Au9 to Au8 is found for the cluster.

In this article, isolation, exploration of electronic structures, and nuclearity conversion of water-soluble triphenylphosphine monosulfonate (TPPS)-protected nonagold (Au9) clusters are outlined. The Au9 clusters are obtained by the reduction of solutions containing TPPS and HAuCl4 and subsequent electrophoretic fractionation. Mass spectrometry and elemental analysis reveal the formation of [Au9(TPPS)8]5− nonagold cluster. UV-vis absorption and magnetic circular dichroism (MCD) spectra of aqueous [Au9(TPPS)8]5− are quite similar to those of [Au9(PPh3)8]3+ in organic solvent, so the solution-phase structures are likely similar for both systems. Simultaneous deconvolution analysis of absorption and MCD spectra demonstrates the presence of some weak electronic transitions that are essentially unresolved in the UV-vis absorption. Quantum chemical calculations for a model compound [Au9(pH3)8]3+ show that the possible (solution-phase) skeletal structure of the nonagold cluster has D2h core symmetry rather than C4-symmetrical centered crown conformation, which is known as the crystal form of the Au9 compound. Moreover, we find a new nuclearity conversion route from Au9 to Au8; that is, phase transfer of aqueous [Au9(TPPS)8]5− into chloroform using tetraoctylammonium bromide yields [Au8(TPPS)8]6− clusters in the absence of excess phosphine.

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