Secondary Phosphine Oxide Functionalized Gold Clusters and Their Application in Photoelectrocatalytic Hydrogenation Reactions

Article abstract of DOI:10.1021/jacs.1c04048

Ligands in ligand-protected metal clusters play a crucial role, not only because of their interaction with the metal core, but also because of the functionality they provide to the cluster. Here, we report the utilization of secondary phosphine oxide (SPO), as a new family of functional ligands, for the preparation of an undecagold cluster Au11-SPO. Different from the commonly used phosphine ligand (i.e., triphenylphosphine, TPP), the SPOs inAu11-SPOwork as electron-withdrawing anionic ligands. While coordinating to gold via the phosphorus atom, the SPO ligand keeps its O atom available to act as a nucleophile. Upon photoexcitation, the clusters are found to inject holes into p-type semiconductors (here, bismuth oxide is used as a model), sensitizing the p-type semiconductor in a different way compared to the photosensitization of a n-type semiconductor. Furthermore, theAu11-SPO/Bi₂O₃photocathode exhibits a much higher activity toward the hydrogenation of benzaldehyde than a TPP-protected Au₁₁-sensitized Bi₂O₃photocathode. Control experiments and density functional theory studies point to the crucial role of the cooperation between gold and the SPO ligands on the selectivity toward the hydrogenation of the C═O group in benzaldehyde.

Full text of DOI:10.1021/jacs.1c04048

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Article
Secondary Phosphine Oxide Functionalized Gold Clusters and Their
Application in Photoelectrocatalytic Hydrogenation Reactions
Yu Wang,*,† Xiao-He Liu,† Rongbin Wang, Beatrice Cula, Zhe-Ning Chen,* Qingyun Chen,*
Norbert Koch, and Nicola Pinna*
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ABSTRACT: Ligands in ligand-protected metal clusters play a
crucial role, not only because of their interaction with the metal
core, but also because of the functionality they provide to the
cluster. Here, we report the utilization of secondary phosphine
oxide (SPO), as a new family of functional ligands, for the
preparation of an undecagold cluster Au11-SPO. Different from the
commonly used phosphine ligand (i.e., triphenylphosphine, TPP),
the SPOs in Au11-SPO work as electron-withdrawing anionic
ligands. While coordinating to gold via the phosphorus atom, the
SPO ligand keeps its O atom available to act as a nucleophile. Upon
photoexcitation, the clusters are found to inject holes into p-type
semiconductors (here, bismuth oxide is used as a model),
sensitizing the p-type semiconductor in a different way compared
to the photosensitization of a n-type semiconductor. Furthermore,
the Au11-SPO/Bi2O3 photocathode exhibits a much higher activity toward the hydrogenation of benzaldehyde than a TPP-
protected Au11-sensitized Bi2O3 photocathode. Control experiments and density functional theory studies point to the crucial role of
the cooperation between gold and the SPO ligands on the selectivity toward the hydrogenation of the CO group in benzaldehyde.
vital, as they could provide an ideal platform for a deeper
understanding of the structure−activity correlation of gold
clusters.
INTRODUCTION
■
Catalysis by gold clusters in the nanometric and subnanometric
regime has drawn enormous attention since the seminal works
by Haruta and Hutchings in the 1980s.1−4 Both bare gold
clusters formed in gas-phase and ligand-protected gold clusters
prepared by wet chemistry methods are widely utilized in
catalysis and they exhibit promising activities.5−12 For example,
the well-known Au55(PPh3)12Cl6, first reported by Schmid in
1981, exhibits good stability against oxidation, and its derived
catalyst supported on SiO2 can effectively and selectively
catalyze the oxidation of styrene by atomic oxygen.6,9 Mass-
selected Au8 clusters, deposited on MgO, can oxidize CO at a
low temperature.7 Although the application of gold catalysts in
oxidation reactions has been extensively explored, there are
only few reports on gold catalysts for hydrogenation reactions,
largely due to the inferior capacity of gold to dissociate
hydrogen, compared to other noble metals such as Pt and
Pd.13−18 Recently, secondary phosphine oxide (SPO) stabi-
lized gold nanoparticles have shown good activity and high
selectivity for aldehyde hydrogenation.19,20 The Au−P−O
moieties on the surface of the nanoparticles serve as frustrated
Lewis pairs and are proposed to have a special ability to cleave
H2.21 Such a kind of functionalized ligand has not been
described as a stabilizing agent in magic-number gold clusters.
The structural determination of SPO-protected gold clusters is
In addition to the catalytic activities, gold clusters exhibit
good light absorption abilities due to their discrete energy
levels.22−24 Thiolate-protected gold clusters have been
employed as photosensitizers in photocatalytic systems.25−33
The sensitization of large band gap semiconductor, such as
TiO2, with gold clusters has extended the photo response of
TiO2, enabling visible-light-driven hydrogen evolution.27,34
These seminal studies mainly focus on the injection of
electrons from the excited states of the clusters to the n-type
semiconductors. On the other hand, it is unclear that would
happen if the clusters are deposited on a p-type semiconductor.
Moreover, mechanistic studies devoted to the elucidation of
the role of gold clusters in photocatalysis are still scarce and are
needed especially because of the dual role, as catalyst and
photosensitizer, they can cover.
Received: April 17, 2021
Published: June 15, 2021
https://doi.org/10.1021/jacs.1c04048
J. Am. Chem. Soc. 2021, 143, 9595−9600
© 2021 American Chemical Society
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Herein, we report the first example of SPO-stabilized gold
cluster. [Au11(PPh2O)10]+ (denoted as Au11-SPO) clusters
with ten functional SPO ligands have been prepared, and
structurally characterized. Different from the common used
phosphine ligands, the SPOs serve as anionic ligands. The
deposition of Au11-SPO onto p-type bismuth oxide extends
the photoresponse of bismuth oxide in the visible region,
suggesting the successful photosensitization. Furthermore, the
as-prepared photocathode can readily reduce benzaldehyde to
benzyl alcohol without additional H2 supply, which further
verifies the catalytic role of Au11-SPO clusters.
(Figure S2).35 The mean Au−Au bond length amounts to
2.8103(8) Å, which is in good agreement with the report data
in literature for similar clusters (Table S1).36−38 Except the
sharing vertex gold, all the other ten gold atoms in Au11-SPO
bind to ten SPO ligands, forming Au-PPh2O coordination
modes. The average length of P−O bond is 1.55 Å, indicating
the double bond between P and O atoms.39 Both attenuated
total reflection-infrared (ATR-IR) and 1H NMR study showed
that the P−H bond in free SPO was absent in Au11-SPO
(Figure 2a and Figure S3). Thus, the SPO here works as
RESULTS AND DISCUSSION
The preparation of Au11-SPO started with the reduction of
gold precursor by NaBH4 in THF solution (Scheme 1).
■
Scheme 1. Synthetic Scheme for Au11-SPO
Figure 2. (a) ATR-IR spectra of the free SPO and Au11-SPO. (b)
Experimental and simulated UV−vis absorption spectra of Au11-
SPO. The slight shifts in the calculated UV−vis absorption spectra
were corrected for better comparison. See the Supporting Information
for the uncorrected data.
Instead of the commonly used HAuCl4 and AuPPh3Cl,
AuSMe2Cl was used as gold precursor because of the lower
affinity of the SMe2 group to Au (see the Supporting
Information for details). X-ray crystallographic analysis
revealed that Au11-SPO was crystallized in the monoclinic
space groups P21 (Figure S1 and the CIF files). As illustrated
in Figure 1, the core of Au11-SPO can be described as a
anionic ligand, which is different from the phosphine and N-
heterocyclic carbene ligands in previously reported gold
nanoclusters.19,40−45 The O atom on SPO can readily serve
as nucleophile, making Au11-SPO a functionalized cluster.46
The accessibility of the cluster in catalytic reaction was
evaluated by the hydrogenation of 4-nitrophenol, and the
results indicate that small molecules can readily access the
cluster (Figure S4).
The comparison of the powder diffractogram of the
precipitate with the powder diffraction data generated from
the single-crystal XRD indicates the phase purity of the Au11-
SPO (Figure S5). This is consistent with the 31P NMR data,
where one single peak was found (Figure S6). The silent
electron paramagnetic resonance (EPR) spectrum of Au11-
SPO indicates its closed-shell electronic structure (Figure S7).
The charge state of Au11-SPO was confirmed by comparing
the UV−vis spectra to time-dependent density functional
theory (TD-DFT) calculations. Three closed-shell Au11-SPO
models that carry +1, +3, and −1 charges, respectively, were
considered. As shown in Figure 2b, the experimental UV−vis
spectrum of Au11-SPO shows two peaks at 416 and 557 nm.
Comparison of the computed spectra for Au11-SPO carrying
+1, +3, and −1 charges to the experimental spectrum reveals
that the calculated spectrum for the monocationic Au11-SPO
is in good agreement with the experimental one, even though
slight shifts in the peak positions are observed (Figure 2b and
Figure S8). Furthermore, the average Au−Au distance in the
optimized monocationic Au11-SPO corresponds much better
to the experimental value as compared to the other two (Table
S2). The results indicate that the stable Au11-SPO cluster
should carry a +1 charge.
Figure 1. (a) Crystal structure, (b) the gold core and (c) the Au-SPO
moiety of Au11-SPO. No counterion is found in the crystallographic
data. Color legend: orange sphere, Au; green sphere, capping Au;
purple sphere, P; red sphere, O; gray sphere, C. All hydrogen atoms
are omitted for clarity.
bicapped, vertex-sharing, bitriangular-bipyramid. For clarity,
triangular-bipyramids and capped gold atoms are highlighted
in orange and green, respectively. The arrangement of gold
atoms in Au11-SPO is different from that in the
triphenylphosphine-protected Au11(PPh3)8Cl3 (denoted as
Au11-TPP), which is a substructure of icosahedral Au13
α-Bi2O3 was used as a model support for the deposition of
Au11-SPO (Figure S9). The good light-absorption ability and
p-type property make Bi2O3 a promising photocathode
material, where photoexcited electrons are available for
reduction reactions.47 The UV−vis spectrum of Au11-SPO/
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Bi2O3 showed enhanced absorption in the visible region,
indicating the successful deposition of Au11-SPO (Figure
S10). Photoelectron spectroscopy was conducted to assess the
electronic properties of the Au11-SPO/Bi2O3 composite
(Figure S11). The secondary electron cutoffs (SECOs), before
and after the deposition of Au11-SPO, were determined as 4.2
and 3.6 eV, respectively. The valence band maximum (VBM)
of Bi2O3 and the highest occupied molecular orbital (HOMO)
of Au11-SPO are 1.23 and 1.86 eV, respectively, with respect
to the Fermi level (EF).48 The energy level diagram indicates a
staggered structure at the interface (Figure 3a). Our previous
Figure 3. (a) Schematic energy level diagram after the deposition of
Au11-SPO onto Bi2O3. (b) Schematic illustration of the charge
transfer pathway upon illumination.
study demonstrated that atomically precise metal nanoclusters
with distinct HOMO−LUMO (lowest unoccupied molecular
orbital) gap can be treated as small band gap semi-
conductors.49 Here, a type II photosystem was built upon
the deposition of Au11-SPO onto Bi2O3. As shown in Figure
3b and Figure S12, when both Bi2O3 and Au11-SPO are
excited, the photoinduced electrons in the conduction band
(CB) of Bi2O3 will transfer to the LUMO of Au11-SPO and,
simultaneously, holes in the HOMO of Au11-SPO will inject
into the valence band (VB) of Bi2O3. This process is expected
to reduce the resistance to charge transfer and improve the
efficiency of the charge separation.
Photoelectrochemical measurements were carried out to
verify the charge transfers at the interface (Figures S13 and
14). The photocurrent response of Au11-SPO/Bi2O3 under
on−off cycles of 565 nm light illumination is shown in Figure
S15. As pristine Bi2O3 does not respond to the most part of
visible light due to its relatively large band gap, the
photoresponse can be attributed to the Au11-SPO only
(Figure S10b). This suggests that Au11-SPO can act as the
photosensitizer in a p-type semiconductor based system. It
should be noted that the charge transfer pathway here is
different from that of the previously reported n-type semi-
conductor based systems (Figure S16).27,49 Under visible light,
only the electron−hole pairs in Au11-SPO are excited. As the
energy level of the HOMO is lower than the VB of Bi2O3, the
holes in the clusters are transferred to Bi2O3, collected, and
transferred through the external circuit to the counter
electrode to induce the oxidation reactions. The photoinduced
electrons, at the same time, assemble at the photocathode for
reduction reactions.
Figure 4. (a) Reaction scheme for photoelectrochemical hydro-
genation of benzaldehyde. (b) Catalytic activities of Bi2O3 (blue),
Au11-TPP/Bi2O3 (magenta) and Au11-SPO/Bi2O3 (red) photo-
cathodes for the hydrogenation of benzaldehyde at varying bias
potentials. Condition: 0.1 mM benzaldehyde in phosphate saline
buffer (pH 7); simulated solar light; 0.5 h duration. (c) Recyclability
of Au11-TPP/Bi2O3 (magenta) and Au11-SPO/Bi2O3 (red).
Duration: 0.5 h for Au11-TPP/Bi2O3 and 0.25 h for Au11-SPO/
Bi2O3. (d) DFT calculated reaction kinetics for benzaldehyde
hydrogenation as well as hydrogen evolution from different
hydrogenated Au11-SPO intermediates.
depicts the conversion of benzaldehyde by pristine Bi2O3
photocathode (blue curve) and Au11-SPO/Bi2O3(red curve)
with a bias range from 0 to −0.6 V versus Ag/AgCl. Although
pristine Bi2O3 showed poor activity, the Au11-SPO/Bi2O3
photocathode showed very high activities at all bias employed,
indicating the importance of the Au11-SPO clusters.
The role of Au11-SPO in the catalytic hydrogenation of
benzaldehyde was then investigated by DFT calculations.
Hydrogenated species are generally considered as the
intermediate in photoelectrochemical hydrogen evolution.50
The hydrogenated Au11-SPO species were thus used as the
starting reagent for the benzaldehyde hydrogenation, as well as
the hydrogen evolution. As shown in Figure 4d and Figure S17,
two different hydrogenated Au11-SPO intermediates were
considered, where two heterolytic hydrogens bonded to Au
and O atoms, respectively (LM1), or two homolytic hydrogens
both bonded to Au atoms (LM2). Our calculations showed
that the heterolytic hydrogenated Au11-SPO species is much
Given the good catalytic properties of gold nanoparticles and
the accessibility of Au11-SPO, we further explored the
possibility of using Au11-SPO/Bi2O3 photocathode in a
photoelectrochemical cell for the coupling of water oxidation
and benzaldehyde hydrogenation (Figure 4a). Figure 4b
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more favorable than the homolytic hydrogenated one (LM1 vs
LM2: 0.0 vs 34.6 kcal/mol), suggesting that the heterolytic
hydrogenated Au11-SPO species should exist as the
predominant hydrogenated intermediate. The heterolytic
hydrogenated Au11-SPO species showed a kinetic selectivity
toward benzaldehyde hydrogenation against hydrogen evolu-
tion (TS1 vs TS1′: 22.0 vs 25.6 kcal/mol), indicating the high
activity for a couple of heterolytic hydrogens in polar CO
bond reduction. In contrast, the homolytic hydrogenated
Au11-SPO species displayed an obvious kinetic advantage
toward hydrogen evolution (free energy barriers of TS2 vs
TS2′: 2.2 vs 26.5 kcal/mol). Moreover, the DFT investigation
on Au11-TPP species indicated that, in absence of the SPO
ligand, only the homolytic hydrogenated species could be
generated, resulting in the selectivity toward hydrogen
evolution (Figure S18). These findings clearly indicates that
the cooperation between the Au and SPO ligand plays a key
role for the generation of the heterolytic hydrogenated species,
promoting the selectivity toward polar CO bond reduction.
To verify the important role of the Au−PO moiety, we
conducted several control experiments. First, Bi2O3 photo-
cathodes sensitized by free SPO ligand and [Ph2P(OH)AuCl]2
were prepared and tested, respectively. As shown in Table S3
(Entries 2 and 3), both SPO/Bi2O3 and [Ph2P(OH)AuCl]2/
Bi2O3 showed neglectable activities. This was attributed to the
lower light absorption and sensitization abilities of [Ph2P-
(OH)AuCl]2 and SPO ligand. Next, Au11-TPP/Bi2O3 was
prepared and tested. Because the sensitization abilities of
Au11-TPP and Au11-SPO are comparable (Figure S19), the
influence of different catalytic processes on the performances
of Au11-TPP/Bi2O3 and Au11-SPO/Bi2O3 could be inves-
tigated. As shown in Figures 4b (magenta curve and Table S3,
Entry 2), Au11-TPP/Bi2O3 showed a much lower activity for
benzaldehyde hydrogenation than Au11-SPO/Bi2O3 at all bias
employed. Instead of catalyzing benzaldehyde hydrogenation,
Au11-TPP/Bi2O3 exhibited good hydrogen production ability
(Figure S20). This is consistent with the results of DFT
calculations (Figure S18). As there was no big difference on
the sensitization ability and stability between Au11-SPO and
Au11-TPP (Figures 4c), the higher selectivity toward
benzaldehyde hydrogenation by Au11-SPO/Bi2O3 was attrib-
uted to the functional SPO ligand and its cooperation with Au.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c04048.
■
sı
Physical measurements, synthesis details, character-
ization data, computational details, DFT computed
Cartesian coordinates of important structures, support-
ing figures, and the CIF of Au11-SPO (PDF)
Accession Codes
CCDC 2068288 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Nicola Pinna − Institut fur Chemie and IRIS Adlershof,
̈
Humboldt-Universität zu Berlin, Berlin 12489, Germany;
orcid.org/0000-0003-1273-803X; Email: nicola.pinna@
hu-berlin.de
Qingyun Chen − International Research Center for Renewable
Energy (IRCRE), State Key Laboratory of Multiphase Flow
in Power Engineering School of Energy and Power
Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R.
China; Email: qychen@mail.xjtu.edu.cn
Zhe-Ning Chen − State Key Laboratory of Structural
Chemistry, Fujian Institute of Research on the Structure of
Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R.
China; orcid.org/0000-0003-3821-5843;
Email: znchen@fjirsm.ac.cn
Yu Wang − Institut fur Chemie and IRIS Adlershof,
̈
Humboldt-Universität zu Berlin, Berlin 12489, Germany;
orcid.org/0000-0001-8625-5415; Email: wangyuxx@hu-
berlin.de
Authors
Xiao-He Liu − International Research Center for Renewable
Energy (IRCRE), State Key Laboratory of Multiphase Flow
in Power Engineering School of Energy and Power
Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R.
China; College of Geology and Environment, Xi’an University
of Science and Technology, Xi’an 710054, P. R. China
CONCLUSION
■
In summary, secondary phosphine oxide ligands are success-
fully employed as anionic protecting agents for gold nano-
clusters. The obtained Au11-SPO is structurally characterized
and a bicapped, vertex-sharing, bitriangular-bipyramid metal
core is revealed. Upon photoexcitation, Au11-SPO injects
holes to the VB of Bi2O3, sensitizing the p-type semiconductor
in a different way compared to n-type semiconductors. The
photoinduced electrons stay on the cluster and are available for
reduction reactions. Instead of catalyzing hydrogen evolution,
Au11-SPO/Bi2O3 exhibits excellent performance for benzalde-
hyde-hydrogenation under photoelectrochemical condition.
On the basis of delicate control experiments and DFT
calculations, the superior performance is attributed to the
Au−PO moieties. These observations suggest that SPOs are
promising functional ligands for cluster stabilization and
catalytic application. We expect that this work will stimulate
more research interests on the preparation of functionalized
metal nanoclusters, the study of catalytic cooperative effects,
and their applications in photoelectrocatalysis.
Rongbin Wang − Institut fu
Humboldt-Universität zu Berlin, Berlin 12489, Germany
r Chemie and IRIS Adlershof,
̈
r Physik and IRIS Adlershof,
Beatrice Cula − Institut fu
̈
Humboldt-Universität zu Berlin, Berlin 12489, Germany;
orcid.org/0000-0001-6262-4925
Norbert Koch − Institut fu
Humboldt-Universität zu Berlin, Berlin 12489, Germany;
Helmholtz-Zentrum Berlin fur Materialien und Energie,
̈
r Physik and IRIS Adlershof,
̈
Berlin 12489, Germany; orcid.org/0000-0002-6042-
6447
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c04048
Author Contributions
†Y.W. and X.-H.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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(19) Cano, I.; Chapman, A. M.; Urakawa, A.; van Leeuwen, P. W. N.
M. Air-Stable Gold Nanoparticles Ligated by Secondary Phosphine
Oxides for the Chemoselective Hydrogenation of Aldehydes: Crucial
Role of the Ligand. J. Am. Chem. Soc. 2014, 136, 2520.
(20) Cano, I.; Huertos, M. A.; Chapman, A. M.; Buntkowsky, G.;
Gutmann, T.; Groszewicz, P. B.; van Leeuwen, P. W. N. M. Air-Stable
Gold Nanoparticles Ligated by Secondary Phosphine Oxides as
Catalyst for the Chemoselective Hydrogenation of Substituted
Aldehydes: a Remarkable Ligand Effect. J. Am. Chem. Soc. 2015,
137, 7718.
ACKNOWLEDGMENTS
■
We are grateful to the German Research Foundation (DFG,
project 456075917, DFG, 182087777-SFB951) and the
National Natural Science Foundation of China (21878242,
21828802, and 21973094) for financial support.
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