Hydride, chloride, and bromide show similar electronic effects in the Au9(PPh3)83+ nanocluster

Article abstract of DOI:10.1039/c9cc08009k

Recent experimental and computational results have suggested that a hydride bound to the Au₉(PPh₃)₈³⁺ gold nanocluster donates its electrons to the metal core, forming an 8-electron cluster. We present electronic absorption spectra of precisely mass-selected Au₉(PPh₃)₈³⁺ clusters featuring a surface hydride, chloride, or bromide. Comparison of these spectra shows that H^-, Cl^-, or Br^- perturb the electronic structure of the Au₉(PPh₃)₈³⁺ core in similar ways. This suggests that hydride and halides play similar roles electronically in this cluster, and thus are either both metallic or both ligand-like.

Full text of DOI:10.1039/c9cc08009k

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Hydride, Chloride, and Bromide Show Similar Elec-
tronic Effects in the Au9(PPh3)38+ Nanocluster†
Anthony Cirri, Hanna Morales Hernández, and Christopher J. Johnson∗
Recent experimental and computational results have
suggested that a hydride bound to the Au9(PPh3)38+ gold
for AunH− and Aun−+1
.
9
Classifying hydride as a ligand or metal becomes a question
over its impact on the Au9(PPh3)38+ electronic structure. As
such, comparison of electronic spectra of hydride- and halide-
containing Au9(PPh3)38+ clusters could evince the role of the hy-
dride. We hypothesized that the electronic spectrum of a metal-
lic hydride would show qualitative differences from that of a
more typical halide-containing cluster. However, purification of
Au9(PPh3)38+ clusters with unambiguous hydrides or halides has
not yet been achieved. We employed a gas-phase spectroscopy
approach that interfaces mass spectrometry with UV/Vis spec-
troscopy, yielding highly-resolved spectra of atomically-precisely
mass-selected clusters from solution-synthesized mixtures.10 This
basic approach has shown the ability to successfully isolate a va-
riety of hydride-containing species, and thus we harness it as an
alternate purification method that provides unambiguous specia-
tion of a solution-phase mixture.2,5,6,11–14
The triphenylphosphine-protected AuNCs, Au9(PPh3)38+ and
structurally-related Au8(PPh3)27+, were prepared by reduction
of chloro(triphenylphosphine)gold(I) with sodium borohydride.
Au9(PPh3)8H2+ was readily prepared by mixing the raw reaction
product containing Au9(PPh3)38+ with borane tert-butylamine, in
a similar procedure to that already reported, and electrospraying
the resulting mixture.2 The resulting mass spectra, tabulated in
the SI, match those presented by Tsukuda and co-workers.2 The
Au9(PPh3)8Cl2+ and Au9(PPh3)8Br2+ clusters were prepared by
the same approach but with the organic salts tetraphenylphospho-
nium chloride or tetrabutylamonium bromide, respectively, in-
stead of borane tert-butylamine (see SI for synthetic details). All
products were introduced into the mass spectrometer by electro-
spray ionization with no purification, an approach commonly em-
ployed in gas-phase chemistry of triphenylphosphine-protected
AuNCs.11,15 The resulting clusters were mass selected such that
UV/Vis absorption spectra could be obtained of only the cluster
of interest, as outlined in the SI.
nanocluster donates its electrons to the metal core, forming
an 8-electron cluster. We present electronic absorption
spectra of precisely mass-selected Au9(PPh3)38+ clusters
featuring a surface hydride, chloride, or bromide. Compar-
ison of these spectra shows that H−, Cl−, or Br− perturb
the electronic structure of the Au9(PPh3)38+ core in similar
ways. This suggests that hydride and halides play similar
roles electronically in this cluster, and thus are either both
metallic or both ligand-like.
Atomically-precise metal nanoclusters bearing metal-hydride
bonds are studied as homogeneous hydrogenation catalysts, as
well as intermediates in the controlled growth of gold nan-
oclusters (AuNCs) or their alloys.1–6 The reactivity of cluster-
hydrides could be expected to parallel organometallic hydride
complexes,1,7 in that the relative metal-hydride bond strength of
clusters will largely determine the reaction mechanism. Classi-
fication schemes must thus be sought to predict reactivity, sub-
strate scope, and develop analogous designer catalysts. However,
the specific nature of the hydride ligand in AuNCs is ambiguous.
Experimental and computational work provide competing expla-
nations over the hydride’s function in Au9(PPh3)8H2+ – an adduct
which exhibits remarkable flexibility as a synthetic intermediate
in the formation of alloy clusters.2 Recent synthetic work indi-
cates that the metal-hydride bond functions as an acidic metal-
hydride whereby the nucleophilic metal cage is capable of con-
trolled cluster growth.2–4 In contrast, computational results sug-
gest that the hydride participates in the gold core as a delocalizing
metal center, mimicking the role of a metal dopant in forming a
closed-shell “superatom.”2,8 Photoelectron spectra of ligand-free
gold clusters containing hydride show strikingly similar spectra
Stony Brook University, 100 Nicolls Road, Stony Brook, NY 11794, USA. Tel: 1-631-
632-7577; E-mail: chris.johnson@stonybrook.edu
† Electronic Supplementary Information (ESI) available: Synthetic protocols and
supporting mass spectra. See DOI: 00.0000/00000000.
Figure 1 compares the published UV/Vis spectra of both
Au9(PPh3)38+ and Au9(PPh3)8H2+ in solution to those recorded
1–3 | 1

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for the same clusters in the gas phase. Notable similarities be-
tween the gas- and solution-phase spectra of each cluster are ap-
parent, particularly in the position and width of the broad peaks
found in Au9(PPh3)8H2+ and the position of the intense, sharp
feature near 430 nm in Au9(PPh3)83+
. This similarity provides
strong evidence that the solution-phase species is faithfully trans-
mitted to the gas phase via electrospray. Figure S3 similarly com-
pares the solution phase UV/Vis spectra of the samples used in
these experiments to the same spectra reported in the literature,
again finding a close match.
Fig. 2 Response of the electronic absorption spectrum of Au9(PPh3)83+
(red) to compositional modulation. Au8(PPh3)2+ (purple) shows several
electronic transitions are preserved between 7clusters. Au9(PPh3)8H2+
(blue), Au9(PPh3)8Cl2+ (green), and Au9(PPh3)8Br2+ (orange) are pre-
sented below.
Fig. 1 Comparison of previously-reported solution-phase UV/Vis spec-
tra for Au9(PPh3)38+ and Au9(PPh3)8H2+ to those recorded in this study.2
Both spectra of Au9(PPh3)3+ feature a strong, narrow absorbance near
430 nm and additional feat8ures near 370 and 500 nm that are less well-
resolved in the solution phase. For Au9(PPh3)8H2+, a broad, bimodal
feature spanning 400-500 nm is found in both spectra. The close corre-
spondence between the solution- and gas-phase spectra indicates that
the electronic structure of these clusters is preserved upon introduction
to the mass spectrometer. Adapted with permission from Reference [2].
Copyright 2018 American Chemical Society.
ter’s electronic structure. Subtle features across the spectrum in-
dicate the presence of several unresolved, overlapping transitions
with both broader and narrower bands. The dramatic change
in the UV/Vis spectrum suggests that addition of a hydride lig-
and is more perturbative to the electronic structure than loss of
a [Au(I)PPh3]+ fragment, where both the MLCT and intracore
transitions notably respond to hydride addition. Comparison to
the Au9(PPh3)8Cl2+ and Au9(PPh3)8Br2+ spectra (green and or-
ange traces, respectively) shows that the spectra closely resemble
the Au9(PPh3)8H2+ spectrum. Surprisingly, the spectral profile
spanning 2.25 – 3.40 eV remains essentially intact (highlighted in
gray), despite the substantial electronic differences between these
two halides.
Figure 2 presents the mass-selective UV/Vis spectra of all five
ligand-protected AuNCs measured at 3.8 K. The Au8(PPh3)72+
(purple) and Au9(PPh3)38+ (red) spectra highlight the the fact that
changing the overall cluster composition need not qualitatively
change the electronic spectrum of this cluster. Previous crystallo-
graphic characterization of Au9(PPh3)38+ and Au8(PPh3)72+ sug-
gests that the two clusters are geometrically-related (D2h and
Cs, respectively), each sharing an analogous mirror plane, with
Au8(PPh3)27+ bearing a lower symmetry structure via “loss" of
a closed-shell [Au(I)PPh3]+ fragment.16,17 We have previously
shown that there is essentially a one-to-one correspondence be-
tween the peaks in these spectra, suggesting that the electronic
structure of the two clusters are very similar.10
We can consider the following interpretations:
1. The hydride ion participates in the metal core as a delocaliz-
ing dopant, overall increasing the electron count of the clus-
ter core. If this is true, then it follows that both chloride and
bromide also participate in the clusters as metal dopants.
2. Ligating the Au9(PPh3)38+ with H−, Cl−, or Br− similarly iso-
merize the core geometric structure, thus making structure
the strongest factor governing the electronic structure.
3. H−, Cl−, or Br− interface with the cluster as X-type ligands
with electrons relatively localized on the ligand.
Addition of the hydride ion to the Au9(PPh3)38+ cluster yields
a dramatic, qualitative change to the spectrum, as the blue trace
in Figure 2 shows no apparent preservation of the parent clus-
We have previously reported the gas-phase electronic absorp-
tion spectrum of Au9(P(p-OCH3-Ph)3)38+, which features a sim-
2 |
1–3

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ilar core geometry as that predicted by density functional the-
ory calculations for Au9(PPh3)8H2+ 2,10,18
There is no obvious
7
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correspondence between the two spectra (Fig. S1); indeed
the Au9(P(p-OCH3-Ph)3)38+ spectrum bears more resemblance to
those of Au9(PPh3)83+ and Au8(PPh3)27+. Thus, it is unlikely that
the core reorganization alone drives this spectral change, casting
doubt on interpretation 2. Further, recent experimental evidence
suggests that the Au9(PPh3)38+ core may take on a more anal-
ogous core structure to Au9(P(p-OCH3-Ph)3)38+ than previously
shown.19 The new features between 2.50 and 3.40 eV arise in a
region of the spectrum previously predicted (for Au9(PPh3)38+) to
show metal-to-ligand charge transfer (MLCT) character by time-
dependent density functional theory calculations,20 suggesting
that they may involve final states with substantial contribution
from the hydride or halide. Given dissimilar electronic struc-
tures amongst these ions, it is unlikely that their MLCT transi-
tions would coincidentally overlap – arguing against interpreta-
tion 3. We then converge on the realization that there is no spec-
troscopic evidence that is inconsistent with interpretation 1; in
this case, the new features could be ascribed to particularly in-
tense transitions between well-separated superatomic P and D or-
bitals in the case of a formally 8-electron core. Recent theoretical
work has suggested that 8-electron gold nanoclusters with dif-
fering metal dopants and hydride numbers show similar frontier
orbitals,8 which would likely lead to similarities in their spectra.
10 A. Cirri, H. Morales Hernández, C. Kmiotek and C. J. Johnson, Angew. Chem.
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This analysis suggests that, if hydride behaves as a metal center,
then chloride and bromide should also be investigated for metallic
character. If chloride and bromide are to be considered as X-type
ligands, then it follows that hydride likely behaves as an X-type
ligand as well. Lacking a clear framework by which to interpret
these complex spectra, we are limited to such qualitative argu-
ments. Detailed comparisons of the reactivity of chloride- and
bromide-containing clusters to hydride-containing clusters will
likely be clarifying. Quantum chemical support will be required
to draw a definitive conclusion, and the relatively high resolu-
tion, atomic precision, and minimal environmental broadening of
these spectra make them ideal benchmarks for such efforts.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Air Force Office of Scientific Re-
search under grant FA9550-17-1-0373. The authors acknowledge
assistance from Jonathan Fagan in the collection of solution-phase
UV/Vis spectra presented here.
Notes and references
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1–3 | 3

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Hydride and halide ligands in gold nanoclusters
exhibit an unexpected similar electronic relationship,
suggesting an underlying chemical linkage between
them.