In this report, we evaluate the present state of the rapidly emerging field of monolayer-protected cluster (MPC) molecules with regard to their synthesis and monolayer functionalization, their core and monolayer structure, their composition, and their properties. Finally, we canvass some of the important remaining research opportunities involving MPCs.
The mean size of the gold (Au) core in the synthesis of
dodecanethiolate-stabilized Au cluster compounds
can be finely adjusted by choice of the Au:dodecanethiolate ratio and
the temperature and rate at which
the reduction is conducted. The Au clusters have been examined
with a large number of independent
analytical tools, producing a remarkably consistent picture of these
materials. Average cluster and core
dimensions, as ascertained by 1H NMR line broadening,
high-resolution transmission electron microscopy,
small-angle X-ray scattering, and thermogravimetric analysis, vary
between diameters of 1.5 and 5.2 nm
(∼110−4800 Au atoms/core). The electronic properties of the Au
core were examined by UV/vis and X-ray
photoelectron spectroscopy; the core appears to remain largely metallic
in nature even at the smallest core
sizes examined. The alkanethiolate monolayer stabilizing the Au
core ranges with core size from ∼53 to
nearly 520 ligands/core, and was probed by Fourier transform infrared
spectroscopy, differential scanning
calorimetry, contact-angle measurements, and thermal desorption mass
spectrometry. The dodecanethiolate
monolayer on small and large core clusters exhibits discernable
differences; the line dividing “3-dimensional”
monolayers and those resembling self-assembled monolayers on flat Au
(2-dimensional monolayers) occurs
at clusters with ∼4.4 nm core diameters.
We report the crystal structure of the thiolate gold nanoparticle [TOA+][Au25(SCH2CH2Ph)18-], where TOA+ = N(C8H17)4+. The crystal structure reveals three types of gold atoms: (a) one central gold atom whose coordination number is 12 (12 bonds to gold atoms); (b) 12 gold atoms that form the vertices of an icosahedron around the central atom, whose coordination number is 6 (five bonds to gold atoms and one to a sulfur atom), and (c) 12 gold atoms that are stellated on 12 of the 20 faces of the Au13 icosahedron. The arrangement of the latter gold atoms may be influenced by aurophilic bonding. Together they form six orthogonal semirings, or staples, of -Au2(SCH2CH2Ph)3- in an octahedral arrangement around the Au13 core.
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