Stabilizing a Platinum1Single-Atom Catalyst on Supported Phosphomolybdic Acid without Compromising Hydrogenation Activity

Article abstract of DOI:10.1002/anie.201602801

In coordination chemistry, catalytically active metal complexes in a zero- or low-valent state often adopt four-coordinate square-planar or tetrahedral geometry. By applying this principle, we have developed a stable Pt₁single-atom catalyst with a high Pt loading (close to 1 wt %) on phosphomolybdic acid(PMA)-modified active carbon. This was achieved by anchoring Pt on the four-fold hollow sites on PMA. Each Pt atom is stabilized by four oxygen atoms in a distorted square-planar geometry, with Pt slightly protruding from the oxygen planar surface. Pt is positively charged, absorbs hydrogen easily, and exhibits excellent performance in the hydrogenation of nitrobenzene and cyclohexanone. It is likely that the system described here can be extended to a number of stable SACs with superior catalytic activities.

Full text of DOI:10.1002/anie.201602801

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201602801
German Edition: DOI: 10.1002/ange.201602801
Single-Atom Catalysts
Stabilizing a Platinum1 Single-Atom Catalyst on Supported
Phosphomolybdic Acid without Compromising Hydrogenation
Activity
Bin Zhang, Hiroyuki Asakura, Jia Zhang, Jiaguang Zhang, Sudipta De, and Ning Yan*
Abstract: In coordination chemistry, catalytically active metal
complexes in a zero- or low-valent state often adopt four-
coordinate square-planar or tetrahedral geometry. By applying
this principle, we have developed a stable Pt1 single-atom
catalyst with a high Pt loading (close to 1 wt%) on phospho-
molybdic acid(PMA)-modified active carbon. This was
achieved by anchoring Pt on the four-fold hollow sites on
PMA. Each Pt atom is stabilized by four oxygen atoms in
a distorted square-planar geometry, with Pt slightly protruding
from the oxygen planar surface. Pt is positively charged,
absorbs hydrogen easily, and exhibits excellent performance in
the hydrogenation of nitrobenzene and cyclohexanone. It is
likely that the system described here can be extended to
a number of stable SACs with superior catalytic activities.
increased metal loading, which hinders the viability of SACs
in applications.
In classical coordination chemistry, catalytically active,
stable complexes involving zero- or low-valent noble metals
with single atomic identity are commonplace.[5] A close
inspection of these compounds reveals that many of them are
four-coordinate complexes containing a metal with a square-
planar or tetrahedral coordination geometry, states that are
intrinsically more stable. Inspired by this, we envisioned
construction or introduction of four-coordinate anchoring
sites with suitable geometry, thereby achieving a high loading
of stable SACs on
a support. Phosphomolybdic acid
(H3PMo12O40, PMA) was selected to examine the feasibility
of the approach. PMA has a classical Keggin structure with
one P atom in the center that is caged by 12 octahedral
MoO6-units linked together by 24 bridging oxygen atoms
(Obr). Another 12 corner oxygen atoms (Oc) complete the
structure, each of which is double-bonded with an additional
Mo atom. With 36 oxygen atoms exposed, PMA furnishes
a range of coordination sites (Figure 1), including the single
corner site, the bridge site (the Oc-Obr-bridge site), the three-
fold hollow site (3-H_Oc and 3-H_Obr), and the four-fold
hollow site (4-H). Thus, PMA is an ideal material with which
to differentiate the SACs stabilizing ability of various sites.
Previously, several mononuclear organometallic compounds
were loaded onto heteropolyacid-modified supports and used
as anchored homogeneous catalysts for enantioselective
hydrogenation.[6] Studies in which inorganic metal salts or
organometallic complexes are anchored to prepare small
metal nanoparticles and nanoclusters (2–4 nm) are also
described.[7] To our knowledge, there have been no reports
on highly active SACs on phosphomolybdic acid where the
accurate location and electronic state of reduced single metal
atoms has been determined.
S
ingle-atom catalysts (SACs), a class of catalysts where bare
metal atoms are atomically dispersed or anchored on the
support, are a new frontier in heterogeneous catalysis.[1]
However, a key challenge that is often encountered is
stabilization of isolated metals on a support without com-
promising catalytic activity. As the surface energy of single
metal atoms is higher than corresponding metal clusters and
nanoparticles, the atoms are highly mobile and tend to form
aggregates during synthetic procedures.[2] Keeping the metal
loading at a very low level to minimize agglomeration is
a common practice employed to tackle the problem.[3]
Another strategy is to anchor single atoms on a support
with a stronger metal-support interaction.[4] Nevertheless, it
remains difficult to effectively stabilize isolated atoms at
[*] B. Zhang, Dr. J. Zhang, Dr. S. De, Prof. Dr. N. Yan
Department of Chemical and Biomolecular Engineering, National
University of Singapore, 4 Engineering Drive 4
Singapore 117585 (Singapore)
E-mail: ning.yan@nus.edu.sg
Dr. H. Asakura
Department of Molecular Engineering, Graduate School of Engi-
neering; Japan and Elements Strategy Initiative for Catalysts &
Batteries (ESICB), Kyoto University, Kyotodaigaku Katsura
Nishikyo-ku, Kyoto 615-8510; 615-8245 (Japan)
Dr. J. Zhang
Institute of High Performance Computing; Agency for Science,
Technology and Research, 1 Fusionopolis Way #16-16 Connexis
Singapore 138632 (Singapore)
Supporting information for this article, including preparative details,
characterization, catalysis, theoretical calculations, and the ORCID
identification number(s) for the author(s) of this article, are available
in the Supporting Information and on the WWW under http://dx.doi.
org/10.1002/anie.201602801.
Figure 1. Different types of surface O atoms and possible anchoring
sites for Pt atoms on Keggin-structured anion phosphomolybdic acid
(PMo12O403ꢀ). The P atom in the center is hidden inside the outer
sphere O and Mo atoms.
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The catalyst was synthesized by a simple three-step
method. PMA was first embedded on a support by wet-
impregnation. The primary screening of supports and solvents
was based on the ability of the support to adsorb PMA at
room temperature. A series of common supports (Al2O3,
SiO2, TiO2, WO3, ZSM-5, HY, HAP, CaSiO3, and activated
carbon (AC)) and solvents (H2O, HCl in H2O (0.5m), ethanol,
and acetone) were examined. AC exhibited the strongest
affinity for PMA, resulting in 100% absorption regardless of
the solvent. The exceptional affinity of AC for PMA can be
attributed to its oxygen enriched hydroxyl, ketone, and
carboxyl groups on the surface (Supporting Information,
Figure S1 (XPS spectra) and Table S1 (elemental analysis)).
Acetone was employed as the solvent, as PMA decomposes
easily in water at elevated pH[8] and can oxidize alcohols. In
the second step, readily reducible Pt(acac)2 was introduced
onto PMA/AC by impregnation. Pt(acac)2 shares a similar
size with PMA (0.7 nm vs. 1.0 nm) so that a 1:1 interaction
between the precursor and the anchoring site is expected.
Electrospray ionization mass spectrometry (ESI-MS) con-
firmed that Pt(acac)2 was physically adsorbed on PMA, rather
than undergoing proton exchange between ligands (Support-
ing Information, Figures S2 and S3). Following a final reduc-
tion step, Pt1 SAC forms on PMA modified AC. We envisaged
that our system could stabilize a large number of Pt single
atoms: 1) Pt has a choice among various sites on PMA and
may thus occupy the most stable configuration (thermody-
namic control), and 2) the kinetic barrier of agglomeration for
atoms anchored on spatially separated PMA species is
substantially higher than those on unmodified support
(kinetic control).
The reduction behavior of Pt pre-catalysts over H2 was
determined from their temperature-programmed reduction
(TPR) profiles, as shown in Figure 2a. Unreduced sample is
denoted as Pt(acac)2-PMA/AC, and reduced sample as
Pt-PMA/AC. For comparison, 1 wt% Pt on AC without
PMA (denoted as Pt(acac)2/AC before reduction, and Pt/AC
after reduction, respectively) and PMA/AC were prepared
following the same method. A reduction peak located at
approximately 1508C was observed for Pt(acac)2/AC, while
the peak shifted to about 1208C for Pt(acac)2-PMA/AC,
indicating a change in the reducibility of Pt(acac)2. Based on
TPR profiles, 1708C was selected as the unified reducing
temperature for all samples. After reduction, inductively
coupled plasma optical emission spectrometry (ICP-OES)
analysis indicated a Pt loading of 0.88 wt% on Pt/AC and
0.91 wt% on Pt-PMA/AC. FTIR spectra of reduced samples
suggested complete removal of acacꢀ ligand (Supporting
Information, Figures S4 and S5).
Figure 2. a) TPR profiles of Pt(acac)2/AC and Pt(acac)2-PMA/AC.
Reduction conditions: 5%H2 in N2, 100 mLminꢀ1, 108Cminꢀ1 heating
rate (70–2008C), catalyst (50 mg). Before heating, the sample was
stabilized in 5%H2 in N2 at 708C for 60 min. b) XRD patterns of PMA,
AC, PMA/AC, Pt/AC, and Pt-PMA/AC. The predicted diffraction peaks
for Pt metal are indicated with dashed lines. A typical TEM image of
c) Pt/AC and d) Pt-PMA/AC.
X-ray spectroscopy (EDX) analysis of the same area on the
Pt-PMA/AC material (Supporting Information, Figure S8)
confirmed the existence of Pt with an estimated content of
0.97 wt%, very close to the ICP-OES results. In the XRD
pattern of Pt/AC (Figure 2b), three peaks at 408, 478, and 688,
were observed, corresponding to the respective (111), (200),
and (220) crystal phases of Pt NPs.[9] A crystallite size of
3.8 nm was estimated from the broadening profile of the (111)
peak at 408 using the Scherrer equation. In contrast, no XRD
peak was observed for Pt-PMA/AC. As such, the size of Pt
species in Pt-PMA/AC is below the detection limit of XRD
and TEM, possibly in the single-atom regime.
Extended X-ray absorption fine structure spectroscopy
(EXAFS) provided key evidences on the dispersion of Pt
species on PMA modified AC. A Pt-Pt contribution at about
2.7 ꢀ was not observable in the k3-weighted EXAFS at the Pt
L3-edge of Pt-PMA/AC (Figure 3a), strongly indicating that
Pt exists predominantly as isolated atoms. The only prom-
inent shell, located at approximately 1.8 ꢀ, arises from a Pt-O
contribution. Unlike Pt-PMA/AC, Pt/AC exhibited a strong
Pt-Pt shell corroborating the existence of Pt NPs. A Pt-O shell
was also detected, plausibly arising from the interactions
between Pt NPs and the abundant surface oxygen-containing
groups on AC. The spectrum of Pt(acac)2-PMA/AC strongly
resembled that of pure Pt(acac)2, in good agreement with
ESI-MS analysis, where only a weak interaction between
Pt(acac)2 and PMA was identified. Combining TEM, XRD,
and in particular EXAFS analysis, it is evident that Pt SACs
could be successfully obtained on PMA modified AC with
a loading close to 1 wt%.
The reduced samples were then investigated by trans-
mission electron microscopy (TEM) and powder X-ray
diffraction (XRD) to probe the size and morphology of Pt
species. As expected, no PMA aggregates were detected from
TEM images in PMA/AC, suggesting uniform dispersion of
PMA on AC (Supporting Information, Figure S6). Pt NPs in
the size range of 1.7–3.6 nm (with an average of 2.5 nm) were
observed in Pt/AC (Figure 2c; Supporting Information,
Figure S7), whereas no NPs or clusters were detected on
Pt-PMA/AC (Figure 2d). Nevertheless, energy-dispersive
Curve-fitting was conducted based on two main peaks in
the R range of 1–3.2 ꢀ. The resulting first shell coordination
parameters, bond length, and the corresponding standard
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(move to Obr-Obr bridge site, ꢀ3.94 eV) < Oc-Obr bridge site
(ꢀ3.56 eV). Considering the fitted coordination number and
DFT calculation results, we propose that each Pt atom
interacts with four bridging oxygen atoms in the four-fold
hollow site on PMA to form a relatively stable structure
(Figure 3c,d). This coordination mode satisfies the empirical
requirements of coordination chemistry, which supports
stable zero- or low-valent Pt, and therefore explains the
capability of the system to support a high loading of Pt single
atoms without agglomeration.
The electronic interactions between reduced Pt single
atoms and PMA were investigated by X-ray absorption near-
edge structure (XANES) analysis. The Pt white-line intensity
for Pt/AC at 11568 eV[3a] is similar to Pt foil (Figure 3b). In
contrast, the white-line intensity of Pt-PMA/AC is consid-
erably higher than that of Pt foil, but lower than that of
Pt(acac)2 and Pt(acac)2-PMA/AC. This observation confirms
that Pt species in Pt-PMA/AC are positively charged by
charge transfer from Pt to PMA (even after reduction), in
excellent agreement with the coordination mode suggested by
XAFS analysis and DFT calculations, where one Pt atom is
coordinated by several oxygen atoms. Additional evidence of
Pt-PMA interaction was obtained by XPS analysis. In the
Mo 3d XPS spectrum of Pt-PMA/AC (Supporting Informa-
tion, Figure S12d), the binding energy of Mo slightly
decreased after reduction, indicating an increase of electron
density in the presence of Pt. The 31P MAS NMR spectra of
PMA/AC and Pt-PMA/AC (Supporting Information, Fig-
ure S13) display signals at ꢀ2.7 and ꢀ6.2 ppm, respectively,
suggesting an appreciable change of the electronic environ-
ment of P. The NH3 desorption peak in temperature-
programmed desorption (TPD) curves shifted from about
3908C on PMA/AC to approximately 3408C on Pt-PMA/AC,
implying that the PMA decreased in acidity upon interaction
with Pt (Supporting Information, Figure S14). Moreover,
Bader charge[10] calculations indicate that the Pt atom loses
0.83 j e j at the four-fold hollow site. On this basis, coordina-
tion of Pt by oxygen resulted in an electron flow from Pt to
PMA, producing stable, positively charged Pt single atoms.
To study the surface dispersion of Pt atoms, H2-O2
titration was applied, involving treatment of the freshly
reduced samples with air, followed by titration with H2. This
method was utilized instead of direct H2 titration, mainly to
avoid occupation of Pt atoms by H atoms introduced by H2/N2
reduction, which cannot be completely removed below about
4008C.[11] Control experiments showed that neither AC nor
PMA/AC could absorb H2 (Supporting Information, Fig-
ure S15), suggesting that any H2 uptake is due to the presence
of Pt. During titration, Pt/AC was saturated with H2 after only
two pulses, whereas Pt-PMA/AC (with the same Pt content)
was able to consume H2 equivalent to five pulses, demon-
strating a much higher percentage of exposed Pt atoms on the
latter material (Figure 4a,b). Indeed, the Pt dispersion on
Pt-PMA/AC was calculated as 86%, not far from the
predicted value of 100% for SACs. In comparison, the Pt
dispersion on Pt/AC was only 24%, which is a typical value
for supported Pt nanoparticles several nanometers in size
(Supporting Information, Table S6). The H2-O2 titration
experiment demonstrated the excellent hydrogen absorption
Figure 3. a) The k3-weighted Fourier transform spectra derived from
EXAFS for AC-supported Pt catalysts and precatalyst, Pt(acac)2, and Pt
foil; Dk=3.0–13.0 ꢀꢀ1. b) Normalized XANES spectra at the Pt L3-edge
of AC-supported Pt catalysts and precatalyst, Pt(acac)2, and Pt foil. The
spectra exhibited a decreasing trend in the intensities of white-line:
Pt(acac)2 (1.65)ꢁPt(acac)2-PMA/AC (1.66)>Pt-PMA/AC (1.53)>
Pt/AC (1.36)>Pt foil (1.20). The numbers in parentheses refer to the
normalized absorption height for different samples. c) Top view and
d) side view of the most stable configuration of Pt1 on PMA/graphene,
based on DFT calculations.
deviations are summarized in Table S5 (Supporting Informa-
tion). As expected, a Pt-O coordination number of around
four was found for both Pt(acac)2 and Pt(acac)2-PMA/AC
samples. Meanwhile, the Pt-O coordination number remained
3.4 in Pt-PMA/AC, although all oxygen-containing ligands
were removed during reduction. Considering the fitting error
of XAFS analysis (ca. 20%), the Pt-O coordination number
can be either three or four. To understand the exact bonding
state of Pt on PMA/AC, DFT calculations were carried out to
investigate the adsorption behavior of Pt atoms on the PMA
system. A graphene layer was used as the support in the
modelling because it possesses a simple and unified structure
(see the Supporting Information, Figure S9 for the optimized
geometry of PMA on the support). Subsequently, four sites
were considered for Pt adsorption, including the four-fold
hollow site (4-H), the three-fold hollow site surrounded by
three Obr atoms (3-H_Obr), the three-fold hollow site formed
by one Oc atom and two Obr atoms (3-H_Oc), as well as the
Oc-Obr bridge site (Figure 3c,d; Supporting Information,
Figure S10). The structures were fully optimized, revealing
that the Pt atom does indeed prefer to adsorb on the four-fold
hollow site with an adsorption energy of ꢀ5.72 eV. In this
coordination mode, Pt and surrounding O atoms adopt quasi
ꢀ
square-planar geometry, with Pt O bond lengths ranging
from 2.00 to 2.02 ꢀ. The Pt atom protrudes slightly from the
oxygen planar surface. It is interesting to note that three-fold
hollow sites were not stable for Pt adsorption. After full
structural relaxation, Pt atoms move from the initial site to
ꢀ
the Obr-Obr_bridge site with a Pt O bond length of 1.95 ꢀ.
The adsorption energies may be ranked in increasing order:
four-fold hollow site (ꢀ5.72 eV) < three-fold hollow sites
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them into aniline. Recycling of Pt-PMA/AC was performed,
and the catalyst exhibited high activity and excellent selec-
tivity over five cycles (Supporting Information, Figure S17).
Longer term stability of Pt-PMA/AC was also tested with
a nitrobenzene:Pt ratio of 8000:1. The catalyst maintained an
average TOF of about 800 hꢀ1, enabling quantitative substrate
conversion in 10 h.
Subsequently, we extended the application of Pt SACs to
=
the hydrogenation of the -C O group in cyclohexanone
(Figure 4d). Pt-PMA/AC exhibited a TOF six times higher
than that of Pt/AC under the same conditions, confirming that
the positively charged, four-oxygen coordinated single Pt
atoms on Pt-PMA/AC are superior active sites when it comes
to activating polar functional groups. As we are aware, this is
the first example where SACs are reported with a significantly
higher activity in carbonyl group hydrogenation compared to
non-SACs with a similar metal loading. Pt-PMA/AC was also
more active than Pt/AC in the hydrogenation of phenyl-
=
acetylene (Figure 4 f), but it was less effective in C C bond
hydrogenation (Figure 4e). The difference in catalytic activity
could be rationalized by different substrate adsorptions, as
well as different hydrogen activation on Pt NPs and on
isolated, charged Pt atoms. It is worth mentioning that PMA
may not be an “innocent” support for catalysis because of its
acidic nature. 3 wt% and 10 wt% PMA were intentionally
added on Pt/AC; the latter induced complete deactivation of
Pt/AC, whereas the former increased selectivity in nitro-
benzene hydrogenation, although with reduced activity
(Supporting Information, Tables S7–S10).
Figure 4. H2 pulse titration profiles of samples after treatment with
air: a) Pt/AC, b) Pt-PMA/AC. Titration conditions: N2-carrier gas,
100 mLminꢀ1; 1008C; H2-titration gas, 159 mL/pulse; catalyst
(120 mg). Before titration the sample was treated in air at 1008C for
30 min. c–f) Catalytic performance of PMA/AC, Pt/AC, and Pt-PMA/AC
in hydrogenation reactions. Reaction conditions: catalyst (5 mg),
1.0 MPa H2 (2.0 MPa for cyclohexanone), ethyl acetate (2 mL), 1 h,
room temperature (508C for cyclohexanone). For (c) nitrobenzene
(53.6 mL), Pt:nitrobenzene=1:2000 (molmolꢀ1); for (d) cyclohexanone
(2.7 mL), Pt:cyclohexanone=1:100 (molmolꢀ1); for (e) styrene
(57.3 mL), Pt:styrene=1:2000 (molmolꢀ1); for (f) phenylacetylene
(27.4 mL), Pt:phenylacetylene=1:1000 (molmolꢀ1).
The catalytic mechanisms operating over Pt-PMA/AC
and Pt/AC could be fundamentally different. On Pt/AC,
hydrogen and substrate activation are achieved on a NP
surface, followed by surface reaction among adsorbed species.
On the contrary, both hydrogen and substrate activation occur
on a single Pt atom on Pt-PMA/AC. Based on DFT
calculations, the substrate adsorption pulls the Pt atom
away from the most stable 4-H site (coordination number
four) to an off-4-H site leading to reduced oxygen coordina-
tion, but the total coordination number remains four (Sup-
porting Information, Table S11 and Figure S18b–e). Co-
adsorption of hydrogen and cyclohexanone resulted in
a much higher adsorption energy (ꢀ2.96 eV) than that for
pure cyclohexanone adsorption (ꢀ0.50 eV), suggesting the
feasibility of a co-adsorption strategy (Supporting Informa-
tion, Table S11 and Figure S18 f). Interestingly, removing the
adsorbed molecules and fully optimizing the structure caused
the Pt atom to shift from the off-4-H site back to the four-
coordinate 4-H site, thereby recovering the catalyst. This
mechanism is consistent with a recent study, where single Pt
atoms coordinated with four sulfur atoms in the resting state,
but only coordinated with two sulfur atoms during the
catalytic cycle.[13] A detailed kinetic study, in situ spectro-
scopic analysis, and DFT calculations are needed to reveal the
exact working mechanism of the Pt-PMA/AC catalyst, and to
establish the concrete role of PMA in the reaction.
property of Pt-PMA/AC, implying that the material has
potential in hydrogenation reactions.
=
=
Consequently, hydrogenations of -NO2, -C O, -C C and
ꢂ
-C C groups were conducted. Initially, catalytic hydrogena-
tion of nitrobenzene was performed under mild conditions,
given that substantial activity was previously observed on Pt
SACs supported on FeOx.[12] Nitrobenzene conversion was
maintained below 30% in all cases to ensure that the reaction
remained under kinetic control. Little product was detected in
Pt-free (Figure 4c), Pt(acac)2-PMA/AC, or Pt(acac)2 control
catalysts (Supporting Information, Table S7, Entry 1,2), but
both Pt/AC and Pt-PMA/AC were very active. Although the
estimated TOFs for the two catalysts were comparable (ca.
900 hꢀ1 for Pt/AC, ca. 800 hꢀ1 for Pt-PMA/AC; calibrated by
Pt dispersion), drastically different product distributions were
observed. Over Pt/AC, five different products were detected,
including aniline, nitrosobenzene, phenylhydroxylamine, azo-
benzene, and azoxybenzene; the overall selectivity towards
aniline was only 50%. In sharp contrast, aniline was the only
product obtained over Pt-PMA/AC. Considering that the side
products are all intermediates in nitrobenzene hydrogenation,
the exceptional selectivity may, at least partially, be due to the
presence of Pt single atoms on the PMA-modified catalyst,
which adsorb and polarize the intermediates and transform
In summary, we have shown that a high loading of
atomically dispersed Pt catalyst is achieved by stabilizing Pt
on PMA with a classical four-coordinate quasi square-planar
geometry. The stability of Pt single atoms is promoted by an
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Acknowledgements
We thank the National University of Singapore Young
Investigator Award (WBS: R-279-000-464-133) for financial
support. XAS measurements at SPring-8 were carried out
with the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) (Proposal No. 2014B1029).
Keywords: four-fold hollow sites · heterogeneous catalysis ·
hydrogenation · phosphomolybdic acid · single-atom catalysts
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Received: March 20, 2016
Revised: April 18, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Single-Atom Catalysts
An atomically dispersed Pt1 catalyst has
been developed with a high catalyst
loading where each Pt atom is anchored
on supported phosphomolybdic acid with
distorted square-planar coordination
geometry. The catalyst is highly active for
nitrobenzene and cyclohexanone hydro-
genation.
B. Zhang, H. Asakura, J. Zhang, J. Zhang,
S. De, N. Yan*
&&&& — &&&&
Stabilizing a Platinum1 Single-Atom
Catalyst on Supported Phosphomolybdic
Acid without Compromising
Hydrogenation Activity
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!