Toward Controlled Syntheses of Diphosphine-Protected Homochiral Gold Nanoclusters through Precursor Engineering

Controllable
syntheses of Au nanoclusters (NCs) with different
nuclearities are of great significance due to the kernel-dependent
physicochemical properties. Herein, two pairs of enantiomeric Au NCs
[Au19­(R/S-BINAP)4­(PhCC)­Cl4] (SD/Au19) and [Au11­(R/S-BINAP)4­(PhCC)2]·Cl (SD/Au11), both with atropos (rigid axial
chirality) diphosphine BINAP (2,2′-bis­(diphenylphosphino)-1,1′-binaphthalene)
as the predominant organic ligands, were controllably synthesized
through precursor engineering. The former was obtained by direct reduction
of HAuCl4·4H2O, while the latter was obtained
by reduction of [Au­(SMe2)­Cl] instead. Intriguingly, the
kernel of SD/Au19 contains an Au7 pentagonal
bipyramid capped by two boat-like Au6 rings, which represents
another type of Au19 kernel, making SD/Au19 a good candidate for comparative study with other Au19 NCs to get more insight into the distinct structural evolution of
phosphine-protected Au NCs. Despite the previous chiroptical studies
on some other chiral undecagold NCs, the successful attainment of
the X-ray crystal structures for SD/Au11 not only provides
a step forward toward better correlating the chiroptical activities
with their structural details but also reveals that even the auxiliary
protecting ligands also play a nontrivial role in tuning the geometrical
structures of the metal NCs. The chiroptical activities of both SD/Au19 and SD/Au11 were found to originate from
the chiral ligands and core distortions; the extended π-electron
systems in the BINAP ligands have proved to positively contribute
to the electronic absorptions and thus disturb the corresponding circular
dichroism (CD) responses.