Exclusive synthesis of Au11(PPh3)8Br 3 against the Cl Analogue and the Electronic Interaction between Cluster Metal Core and Surface Ligands

Article abstract of DOI:10.1002/chem.201300592

To goldly go: Exclusive Au₁₁(PPh₃)₈Br ₃ (see illustration) formation was obtained by a novel two-phase method, although the molar ratio of Br^-/Cl^- was 1:4, as identified by ESI-MS. Interestingly, the interaction of Au₁₁ core electrons with the benzene π electrons of the phosphine ligands was revealed by NMR spectroscopy and optical absorption spectroscopic analyses. Copyright

Full text of DOI:10.1002/chem.201300592

COMMUNICATION
DOI: 10.1002/chem.201300592
Exclusive synthesis of Au11ACHTNUGTRNE(GNU PPh3)8Br3 against the Cl Analogue and the
Electronic Interaction between Cluster Metal Core and Surface Ligands
Zhikun Wu*[a] and Rongchao Jin*[b]
The phosphine-protected Au11 cluster has attracted exten-
sive interest for over four decades[1–12] due to its interesting
properties and its use as a building block for large nanoclus-
can greatly influence the properties of gold nanoclusters.[24]
To address these questions, we herein start from HAuCl4
and have developed a two-phase protocol (tetraoctylammo-
nium bromide (TOAB) as the phase transfer reagent) to
synthesize Au11 clusters. Our experiments confirmed the ap-
plicability of two-phase synthesis for Au11, and revealed that
bromo-substituted Au11 is the exclusive product even if the
Br/Cl ratio was 1:4 at the start, due to the thermodynamic
ters;[11,13] phosphine-protected Au13,[14] Au9 and Au10 have
[15]
also been pursued recently. An interesting fact is that no
solely phosphine-protected Au11 has been found, other nega-
tive ions or ligands always co-exist with phosphine, and the
general formula of Au11 can be represented as Au11ACHTNUGTRNEG(UN PR3)7X3
(X=Cl, SCN, CN, SR, etc.). The structure of Au11 was re-
vealed to be an incomplete icosahedral skeleton with an ap-
proximately C3v symmetry by single crystal X-ray diffraction
by various research groups.[1,2,10,12] Recently, it was found
that the number of phosphine can be eight, too, when tri-
phenyl phosphine was employed to protect the Au11
core,[9,11] and the formula was determined to be Au11-
stability difference. Furthermore, NMR specACHTUNGERTGtNUNN rosACHTUNRTGEGUNcNN oACHTUNGTRENNUNGpy and
optical absorption spectroscopy identified the interaction
between the cluster core electrons and the ligandꢀs benzene
p electrons.
Weare et al. previously synthesized Au~101 by a two-phase
method (also using TOAB as the phase-transfer reagent)
and proposed the intermediates, AuACHTNUTRGNE(UNG PPh3)Cl and O=PPh3,
A
but without further identification.[25] Recently Goulet
et al.[26] reported that, when the phase-transfer reagent (for
instance: TOAB) was employed, the composition of the in-
termediate was different from the case without phase-trans-
fer reagent; they identified the composition of the inter-
CTHUNGTRENNUNG
ACHTUNGTRENNUNG
are constant (11 and 3, respectively) whereas the number of
phosphine is slightly varied (from 7 to 8), which can be ex-
plained in terms of the closure of the electron shells created
by spherical potentials.[16–18] Unlike another magic-size clus-
ter Au25,[19–23] the synthesis of Au11 always starts from the
AuI precursor by a one-phase process. The synthesis from
cheaper AuIII salt is not available in the literature, and the
mechanism of the synthesis is still largely unclear. Other
questions include: 1) is there some affinity competition
when different halogen ions coexist in the synthesis system,
and 2) can the metal core electrons interact with the ben-
zene p electrons in triphenyl phosphine? The latter two
issues are seemly not particularly relevant to one another,
but both of them are closely related to the surface of Au11.
It has been revealed in our previous work that the surface
mediate to be [TOA]ACHTNUGTRNEUNG[AuX2] (X: halogen) by NMR spectro-
scopy in the Brust–Schiffrin synthesis.[26] Given that the size
and dispersity of products of these syntheses are rather de-
pendent upon the reaction conditions[20,21,27–29] and thus im-
plicitly upon the reaction intermediates, it is important to
establish clearly the composition of precursors to the final
Au11 clusters during the reaction process. In the present
work, the Au11 clusters are synthesized following a previous-
[22]
ly reported method of synthesizing Au25
with modifica-
tions (for details, see the Experimental Section). First, the
stoichiometry of the reduction of AuIII to AuI by phosphine
was established by direct observation of the solution color
fading: when 1 equiv of PPh3 (per mole of Au) was added,
there was no sudden change of the solution color; after
adding 1.87 equiv of PPh3, the solution became turbid and
the color faded obviously; further addition of 0.13 equiv of
PPh3 resulted in complete fading of the color of AuIII salt.
UV/Vis monitoring also showed that the peak at 336 nm
almost disappeared (Figure 1). The color change of the
entire process is shown in Figure S1 in the Supporting Infor-
mation.
The above titration experiments clearly show that the re-
duction process of AuIII to AuI requires 2 equiv of PPh3. To
analyze the composition of the reaction intermediates, the
precipitate and the solution were separated and analyzed.
The precipitate cannot dissolve well in toluene, indicating
[a] Prof. Dr. Z. Wu
Key Laboratory of Materials Physics
Anhui Key Laboratory of Nanomaterials and Nanotechnology
Institute of Solid State Physics, Chinese Academy of Sciences
Hefei 230031 (P.R. China)
E-mail: zkwu@issp.ac.cn
[b] Prof. Dr. R. Jin
Department of Chemistry, Carnegie Mellon University
4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213 (USA)
E-mail: rongchao@andrew.cmu.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300592.
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(LDI) mass spectrometry confirms this (Figure S3 in the
Supporting Information). On the other hand, NMR spectro-
scopy reveals that the main compounds left in the solution
of the intermediate product are OPPh3 and [NACHTNUGRTNEUNG
(C8H17)4]+
(i.e., TOA+). The peaks in the range of 7.4–7.8 ppm are the
typical benzene proton signals of OPPh3, and the proton
number ratio in o-, m-, p-positions of benzene is 2:2:1 as ex-
pected (Figure S4 in the Supporting Information); for com-
parison, the standard NMR spectrum of OPPh3 is also
shown in Figure S4 in the Supporting Information. In the
high-field region (0.8–3.5 ppm), all peaks can be assigned to
TOA+; specifically, the single peak at approximately
0.9 ppm is from the methyl group of TOA+; the peak at
about 3.3 ppm is from the methylene protons close to nitro-
Figure 1. The UV/Vis spectra of AuIII solution after addition of 2 equiv of
PPh3 (the spectra were recorded immediately after the 0.3 mL reaction
solution was diluted in 600 mL toluene).
ꢀ
ꢀ
gen ( N
ACHTUNGTRENN(UGN CH2) ); the peaks at approximately 1.7 ppm are
assigned to the b-methylene protons (the position relative to
that it may be the AuI complex without the TOA+ unit;
NMR spectroscopy indeed confirmed this (i.e., no TOA+
signals were found in the high-field region, Figure 2).
N, NCH
2ACHTUNGTERN(NUNG CH2) ); the multiple peaks centered at about
ꢀ
ꢀ
1.3 ppm are assigned to the other methylene protons
ꢀ
ꢀ
AHCTNUERTG(GNNUN NCH2CH2(CH2CH2CH2CH2CH2) ), and their number
ratio based on the integrated areas is 4.72:2.79:2.96:15.53
(equal to 3.0:1.8:1.9:10.0), very close to the theoretical value
3:2:2:10. ESI-MS analysis further confirmed this: the peak
at m/z 279.2 was assigned to [OPPh3 +H]+ (C18H16OP, ex-
pected: 279.1), and the dominant peak at m/z 466.6 was as-
signed to [TOA]+ (C32H68N, expected: 466.5); the isotope
patterns match with the simulations well (Figure S5 in the
Supporting Information).
Based on the above results, the reaction from AuIII to AuI
can be represented as follows [Eq. (1)]:
HAuCl4 þ 2 PPh3 þ TOAB þ H2O !
ð1Þ
AuðPPh3ÞBr þ OPPh3 þ TOAC þ 3 HCl
Our result is contrary to the case of AuIII/thiol system,[26]
also different from the previous assumption of Au-
AHCTUNGTRENNUNG
(PPh3)Cl.[25]
Figure 2. Comparison of the NMR spectra of the AuI intermediate
(bottom) and AuACHTUNGTRENNUNG(PPh3)Cl (top; CDCl2 as the solvent). The asterisks indi-
cate the residual water in the NMR samples. The spike at 5.35 ppm is a
residual solvent (CD2Cl2) peak. Insets: the enlargements of the portions
corresponding to the benzene proton signals.
In the present two-phase synthesis of Au11, an interesting
issue is the potential competition between Brꢀ and Clꢀ. As
mentioned above, the AuI intermediate is mainly the bro-
mide complex, rather than the chloride complex as expect-
ed, although Clꢀ is far more in excess than Brꢀ under our re-
action conditions. Furthermore, we found that Brꢀ is exclu-
sively present in the final Au11 clusters as shown in the ESI
mass spectrometric analysis (Figure 3). The dominant peak
at m/z 4424.15 corresponds to the formula of [Au11-
1H NMR spectroscopy also showed the intermediate to be
very similar to the standard spectrum of Au
98%, from Strem Chemicals). Considering that the ligand in
Au
(PPh3)X may be Brꢀ (introduced by the phase-transfer
ACHTUNGRTENN(UNG PPh3)Cl (>
U
(PPh3)8Br2]+ (Au11Br2C144H120P8, expected m/z 4424.21). In
ACHTUNGTRENNUNG
reagent, TOAB), we performed thermogravimetric analysis
(TGA) to find out whether Br or Cl is associated with the
AuI intermediate. We conducted three measurements and
the losses of weight were 62.3, 62.3, and 62.2%, respectively
(see Figure S2 for a typical TGA profile). The average loss
of weight (62.3%) actually does not match the composition
of AuPPh3Cl (expected loss: 60.2%), but is closer to the
the lower m/z region, no obvious Cl substitute(s) was found
in the ESI spectrum (note: for monosubstituted Au11-
AHCTUNGTRENNUNG
CTHUNGTRENNUNG
molecular weight would be 4335.30). Since the ESI spectrum
was obtained in the positive mode, the full molecular formu-
la of the dominant species should be [Au11ACHTUNGTRENUNG(PPh3)8Br3] (coun-
composition of Au
TGA result indicates that the AuI intermediate in our case
is mainly Au
(PPh3)Br (expected loss: 63.5%). The
terion: Brꢀ). TGA also confirmed this formula (Figure S6 in
the Supporting Information): the observed weight loss
(52.5%) is very close to the theoretical value of 51.9%.
ACHTUNGTRENNUNG
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Electronic Interaction in Well-Defined Gold Nanoclusters
COMMUNICATION
chirality[22] and fluorescence[24a] imply this possibility. Herein
NMR spectroscopy was employed for further investigation.
NMR spectra revealed that the benzene protons in Au11
were shifted to high field (6.6 ppm), and the three subgroups
(o-, m-, p-positions) of benzene signals were separated clear-
ly (from 6.6 to 7.5 ppm) compared with the free ligands (es-
sentially one broad peak centered at 7.37 ppm; Figure 4). In
Figure 3. ESI mass spectrum of Au11ACHTUNGRTNEG(UN PPh3)8Br3. Top inset: the spectral
section in the m/z range of 4334–4434; bottom inset: comparison of the
simulated and experimental isotope patterns.
Here, an interesting question naturally arises—what
makes Au11ACHTUNGTRENNUNG
(PPh3)8Br3 the exclusive product even if the Brꢀ/
Clꢀ atom number ratio is 1:4 (see the Experimental Sec-
tion)? Is it due to the kinetic control or thermodynamic sta-
bility?[30] As is known, Brꢀ is larger than Clꢀ and the con-
centration of Brꢀ is much lower than that of Clꢀ, so the ki-
netic consideration is not favored for the exclusive forma-
tion of Au11ACHTUNGTRENNUNG(PPh3)8Br3. Thus, the possible reason may be
due to the thermodynamic stability difference; our further
experiments confirmed this.
For comparison, we prepared Au11ACHTNUTRGEN(UNG PPh3)8Cl3 clusters (see
the Experimental Section). To compare the stability, we
monitored the intensity change of the 421 nm absorption
peak of the Au11ACHTUNGTRENNUNG(PPh3)8Br3 and Au11CAHUTNTGREN(NUGN PPh3)8Cl3 under the
same conditions (Figure S7 in the Supporting Information).
At the start, both of them have the same absorption at
421 nm (0.22 OD, Figure S8 in the Supporting Information).
After 13 min exposure to air and room light, the 421 nm ab-
sorption of Au
0.14 OD, whereas for Au
crease was slower (to 0.19 OD only). After 4 h, the absorp-
tion of Au11A(PPh3)8Cl3 solution at 421 nm decreased to
0.037 OD; in contrast, it took about 7 h for Au11A(PPh3)8Br3
to decrease to the same absorption intensity. The decrease
of absorption is due to the irreversible aggregation of Au11
clusters, which can be visually seen on the tube wall (Fig-
ure S9 in the Supporting Information). The slower aggrega-
(PPh3)8Cl3 solution decreased rapidly to
CTHUNGTRENNUNG
CHTUNGTRENNUNG
CTHUNGTRENNUNG
Figure 4. Comparison of the NMR spectra of the four materials. The as-
terisks indicate the residual water in the NMR samples. The spike at
5.35 ppm is a residual solvent (CD2Cl2) peak.
contrast, in phenylethanethiol-capped Au25, not only the
chemical shifts but also the spacings of different subgroup
proton signals are similar to free ligand (Figure S10 in the
Supporting Information). Therefore, the benzene protons in
tion rate of Au
the enhanced stability of Au
Au11A(PPh3)8Cl3.
It is known that some nanoclusters can be regarded as su-
peratoms or analogues;[16–18] for Au
11A(PPh3)8Br3, it is an 8-
(PPh3)8Br3 than Au11CAHTNUGTREN(NUNG PPh3)8Br3 indicates
CTHUNGTRENNUNG
CHTUNGTRENNUNG
Au11ACHTUNGTRENUNG(PPh3)8Br3 are more intimately coupled to the metal
N
core than in [Au25(SCH2CH2Ph)18]ꢀ nanoclusters. It should
be noted that Au25 is also an 8-electron shell closing struc-
electron (n*=NvAꢀMꢀZ=11ꢀ2ꢀ1=8; see ref. [17]) shell
closing structure; that is, eight free-valence electrons of
Au 6s are contained in the core consisting of eleven gold
atoms. An essential question is: can the Au11 metal core
electrons interact with the adjacent benzene p electrons?
Our recent findings that ligands greatly influence the cluster
ture. In the NMR spectrum of Au11ACHTNUGTRENG(UN PPh3)8Br3, the benzene
proton signals shifting to high field generally means an in-
crease in delocalized electron density in benzene due to con-
jugation with the Au11 core; for example, the strong elec-
tron-donating group NH2 in aniline can cause the benzene
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12261

Z. Wu and R. Jin
proton signals to shift to higher field (6.7–7.2 ppm); in con-
trast, in OPPh3 with an electron-withdrawing group (i.e., the
oxygen atom), the benzene proton signals shift to lower
field (7.5–7.7 ppm; Figure 4). In Au11ACHTUNGTRENUNG(PPh3)8Br3, since the
benzene ring is close to the metal core and the core is elec-
tron-rich, it is possible for the metal core electrons to conju-
aniline, in which the o-position protons are located in the
highest field (i.e., 6.7 ppm), and the m-position protons are
located in the lowest field (Figure 4 and Figure 5), which
can be understood since the amine group can influence the
o-position electrons more than the p-position electrons. This
difference (i.e., the relative chemical shifts in o- and m-posi-
gate with the benzene p elec
G
tions in substituted benzene) between Au11ACTHUNGTRE(UNNG PPh3)8Br3 and
It is surprising to find that in Au
G
aniline may be due to the different conjugation mechanism;
p-position proton signals of the benzene ring shift to high
field, whereas the o-position protons are not much influ-
enced compared with that of free PPh3 ligands (thus, result-
ing in enlargement of the spacings of three subgroup proton
signals). This observation is distinctively different from the
common aromatic compounds with electron-donating
group(s), like aniline. The o-, m-, p-position proton assign-
ment can be assisted with 2D COSY (Figure 5). To simplify
in aniline, it is a p–p-type conjugation, whereas in Au11-
AHCTUNGTREN(GNUN PPh3)8Br3 it is a conjugation between the benzene p elec-
trons and metal core electrons, which has not been reported
previously. UV/Vis spectra provide further evidence for the
conjugation between the metal core electrons and the ben-
zene p electrons (Figure 6): the absorption peak associated
with the PPh3 unit red-shifts from approximately 265 nm in
free PPh3 to about 300 nm in Au11ACHTNUTGRENUNG(PPh3)8Br3.
Figure 6. Comparison of the UV/Vis spectra of PPh3 and Au
(both dissolved in CH2Cl2).
11ACHTUNGTRENNUNG(PPh3)8Br3
In summary, we have developed a two-phase synthesis of
monodisperse Au11 clusters with the precise composition de-
termined to be Au11ACTHUNTRGNEUNG(PPh3)8Br3, and we have investigated the
reaction intermediates by NMR spectroscopy, MS, TGA,
etc. We found that Brꢀ, instead of Clꢀ, was the dominant
halogen in both the AuI intermediate and the final Au11
product, although Clꢀ was in large excess (Brꢀ/Clꢀ =1:4,
molar ratio) and would kinetically favor binding with gold.
Our result is explained by the finding that Brꢀ can enhance
the stability of the cluster. Another interesting finding is the
interaction of Au11 metal core electrons with benzeneꢀs
Figure 5. Comparison of the 1H,1H correlation spectra (COSY) of Au11-
ACHTUNGTRENNUNG(PPh3)8Br3 and C6H5NH2.
the interpretation, the three subgroups of protons in Au11-
(PPh3)8Br3 are designated as A, B and C from low field to
ACHTUNGTRENNUNG
high field, respectively; for aniline, the three subgroups of
protons are represented with D, E and F. From 1D spectra
(Figure 5), based on the integrated area, the proton number
ratio A/B/C is 2:1:2; thus it is clear that B represents the p-
position proton since there is only one p-position proton. In
2D COSY (Figure 5), it can be found that C protons couple
with A and B protons, so C protons should be the m-posi-
tion protons since the m-position protons are adjacent to
both o- and p-position protons. As a result, the remaining A
should represent the o-position protons. Herein, the m-posi-
p elecACTHNGUTERtNNUG rons probed by NMR and UV/Vis spectroscopy. We
hope this work stimulates more research on the important
issues related to nanocluster surface properties.
Experimental Section
Synthesis of Au11 clusters: Gold salt (HAuCl4·3H2O, 224.9 mg,
0.57 mmol) was first dissolved in ethanol (0.5 mL), then tetraoctylammo-
nium bromide (TOAB, 312.0 mg, 0.57 mmol) was added, followed by
slow addition of toluene (9.6 mL) into the ethanol solution. The resulting
mixture was stirred at a medium speed for about 15 min until the solution
became unturbid, then the solution was cooled to approximately 08C in
tion protons in benzene of Au
11CAHTUNGTRNEUN(NG PPh3)8Br3 are located in the
highest field (i.e., 6.6 ppm), which is contrary to the case of
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Electronic Interaction in Well-Defined Gold Nanoclusters
COMMUNICATION
an ice-bath over a period of 30 min. Triphenylphosphine (PPh3, 300.2 mg,
1.14 mmol, dissolved in 4.7 mL toluene) was slowly added to the reaction
mixture under slow stirring (~60 rpm). After the addition of PPh3, the
solution color faded quickly; further stirring for 2 h ensured the comple-
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(260.5 mg, 6.9 mmol, dissolved in 7.1 mL ice-cold H2O) was added rapidly
into the reaction mixture under vigorous stirring. The solution was con-
tinuously stirred for approximately 40 h (note: in our previous work,[21,22]
we found the aging time markedly influenced the purity and yield of clus-
ters). For postsynthetic treatment, the water layer was first removed, and
the remaining mixture was washed with Nanopure water twice; then the
black precipitate was collected and thoroughly washed with toluene, re-
crystallized in CH2Cl2/hexane system several times, and UV/Vis and
NMR spectroscopy were used to monitor the purity of the product. A
small fraction of Au11 can also be isolated from the combined washing
toluene solution. The as-prepared cluster was identified to be Au11-
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same except that TOAB was replaced by TOAC (286.0 mg, 0.57 mmol).
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For the investigation of the reaction intermediates the protocol was the
same as above, except that the triphenylphosphine was added in three
portions successively (the amounts were 1.0, 0.87 and 0.13 equiv, respec-
tively; see illustration in Figure S1) with 1 h intervals to ensure the reac-
tion was completed. After the addition of 2 equiv PPh3 (total) and further
stirring for two more hours, the solution was concentrated, and the white
precipitate (mainly AuACHTUNGTRENNUNG(PPh3)Br, see the above discussion) was collected,
thoroughly washed with toluene and MeOH, then dried for characteriza-
tion. The remaining toluene solution was combined with all the filtrate
and was dried under reduced pressure for further purification.
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Characterization: UV/Vis absorption spectra were recorded in the range
190–1100 nm by using a Hewlett–Packard (HP) 8543 diode array spectro-
photometer. Laser desorption ionization (i.e., no matrix) mass spectrom-
etry analyses were performed with a PerSeptive Biosystems Voyager DE
super-STR time-of-flight (TOF) mass spectrometer. Electrospray ioniza-
tion mass spectra were acquired by using a quadrupole field ion trap
mass spectrometer (for OPPh3 and TOA+ identification, MeOH as sol-
vent) or a Waters Q-TOF mass spectrometer equipped with Z-spray
source (for Au11 identification, 1:1 v/v CH2Cl2/MeOH as solvent). NMR
analysis was conducted on a Bruker Avanceꢂ DMX 500 spectrometer
by using standard Bruker software. The data were collected with samples
dissolved in CD2Cl2. The NMR peak assignments were made by the fol-
lowing experiments: 1D 1H NMR spectroscopy, 1H,1H correlation spec-
troscopy (COSY). Thermogravimetric analysis (TGA) was carried out on
a TG/DTA6300 thermogravimetric analyzer (Seiko Instruments, Inc.)
with a 58Cminꢀ1 heating rate and an approximately 50 mLminꢀ1 N2
flow-rate. About 2.0 mg of fine solid samples were used in TGA.
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Acknowledgements
Z.W. thanks the support by “Hundred Talents Program” of the Chinese
Academy of Sciences, National Basic Research Program of China (Grant
No. 2013CB934302), the Natural Science Foundation of China (Grant
No. 21222301, 21171170) and startup from the Ministry of Human Re-
sources and Social Security of China. R.J. acknowledges the support by
Air Force Office of Scientific Research under AFOSR Award No.
FA9550-11-1-9999 (FA9550-11-1-0147) and the Camille Dreyfus Teacher-
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Received: February 15, 2013
Revised: June 5, 2013
Published online: August 13, 2013
Keywords: conjugation · gold · nanoclusters · phosphane
ligands · synthetic methods
Chem. Eur. J. 2013, 19, 12259 – 12263
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
12263