[Au18(dppm)6Cl4]4+: a phosphine-protected gold nanocluster with rich charge states

Abstract

A diphosphine-protected 18-gold-atom nanocluster was isolated via a facile reduction of an Au^I precursor by NaBH₄. Its composition was identified as {[Au₁₈(dppm)₆Cl₄]·C₆H₆·3Cl·PF₆} (SD/Au18, SD = SunDi; dppm = bis-(diphenylphosphino)methane) by X-ray single crystal structural analysis. This nanocluster possesses a prolate shape and is built from an Au₁₀ kernel (bi-octahedral Au₆ units sharing one edge) fused with two Au₇ caps via sharing six gold atoms. The identity of the Au₁₈ cluster is further demonstrated by ESI-MS. The number of valence electrons of [Au₁₈(dppm)₆Cl₄]⁴⁺ is 10 (n* = 18–4–4), which does not match with the known magic numbers according to the spherical jellium model, and elongated models must be considered. The special stability of the Au₁₈ cluster likely arises from geometrical factors in the metallic core. Two charge states are reported for this system. This work not only presents the structure elucidation of a diphosphine-protected Au₁₈ nanocluster, but also provides an important insight into the growth pattern of gold nanoclusters and the charge states they can achieve.