Gold(I) catalysis at extreme concentrations inside self-assembled nanospheres

Article abstract of DOI:10.1002/anie.201406415

Homogeneous transition-metal catalysis is a crucial technology for the sustainable preparation of valuable chemicals. The catalyst concentration is usually kept as low as possible, typically at mm or mm levels, and the effect of high catalyst concentratio

Full text of DOI:10.1002/anie.201406415

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DOI: 10.1002/anie.201406415
Homogeneous Catalysis
Gold(I) Catalysis at Extreme Concentrations Inside Self-Assembled
Nanospheres**
Rafael Gramage-Doria, Joeri Hessels, Stefan H. A. M. Leenders, Oliver Trçppner,
´
´
Maximilian Dꢀrr, Ivana Ivanovic-Burmazovic, and Joost N. H. Reek*
Abstract: Homogeneous transition-metal catalysis is a crucial
technology for the sustainable preparation of valuable chem-
icals. The catalyst concentration is usually kept as low as
possible, typically at mm or mm levels, and the effect of high
catalyst concentration is hardly exploited because of solubility
issues and the inherent unfavorable catalyst/substrate ratio.
Herein, a self-assembly strategy is reported which leads to local
catalyst concentrations ranging from 0.05m to 1.1m, inside
well-defined nanospheres, whilst the overall catalyst concen-
tration in solution remains at the conventional mm levels. We
disclose that only at this high concentration, the gold(I)
chloride is reactive and shows high selectivity in intramolecular
nation sphere around the transition-metal complex.[6] Various
examples demonstrate that the use of a second coordination
sphere can lead to activities and selectivities which are not
accessible by traditional approaches.[7] The operational modes
of these second spheres are many, including the precise
orientation of the substrate at the metal center[8] and
controlling the local pH value.[9]
What all these catalysts have in common is that they are
employed in a limited concentration window, typically
between 10À6 m and 10À3 m. At these concentrations the
catalysts generally dissolve well and a large excess of
substrate can be added to achieve high turnover numbers
without running into limitations of substrate solubility. From
a reactivity point of view, however, it is interesting to also
explore extremely high concentrations of catalyst as new
reactivity patterns may evolve through, for example, metal
cooperativity effects. We realized that if one would like to
apply molar concentrations of catalyst, this can only be
achieved by devising systems in which the “local catalyst
concentration” is high, whereas the overall catalyst concen-
À
À
C O and C C bond-forming cyclization reactions.
T
ransition-metal catalysis plays an extremely important role
in the synthesis of chemicals relevant for pharmacology,
biology, agrochemistry, materials science, and petrochemis-
try.[1] It allows the preparation of valuable molecules from
readily available bulk chemicals in an atom-economic and
sustainable manner.[2] It also enables the construction of
sophisticated molecules in more efficient manners, thus
simplifying synthetic routes towards the desired target
molecules.[3] In the past decades several tools to control the
activity and selectivity of transition-metal catalysts have been
developed, and have mostly focused on the ligands which
together with the metal form the active complex.[4,5] More
recently, enzyme-inspired approaches have been explored,
thus providing excellent tools to control the second coordi-
tration in solution is still around the commonly used 10À3
m
(Figure 1), thus still allowing high turnover numbers to be
achieved. Fujita et al. have developed appealing strategies to
form nanosized molecular spheres (M12L24) which form by
self-assembly of 24 ditopic nitrogen ligands and 12 palladium
metals.[10] Functionalization of the ditopic ligand is explored
intensively as a tool for both inner- and outer-sphere
decoration. As such, various functional groups have been
enclosed within the M12L24 spheres.[11] These systems are well
suited to explore catalysis at high local catalyst concentration,
as we estimated that enclosing 24 metal complexes within this
sphere would lead to a local 1.1m concentration. Herein we
report an approach based on the assembly of M12L24 spheres
using novel ditopic nitrogen ligands in which the local gold-
complex concentration can be tuned from 1.1m to 0.05m. We
have chosen gold(I) catalysis because of its increasing
relevance for organic synthesis[12] and the observation that
cooperativity between metals plays a role in some of the
proposed mechanisms.[13]
[*] Dr. R. Gramage-Doria, J. Hessels, S. H. A. M. Leenders,
Prof. Dr. J. N. H. Reek
Homogeneous, Supramolecular and Bio-Inspired Catalysis, Van’t
Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904, 1098XH Amsterdam (The Netherlands)
E-mail: j.n.h.reek@uva.nl
´
´
Dr. O. Trçppner, M. Dꢀrr, Prof. Dr. I. Ivanovic-Burmazovic
Lehrstuhl fꢀr Bioanorganische Chemie, Department Chemie und
Pharmazie, Friedrich-Alexander-Universitaet Erlangen
Egerlandstrasse 3, 91058 Erlangen (Germany)
To prepare novel gold-containing spheres we first pre-
pared the ditopic building blocks A and B (Figure 2). A is
utilized with a [R3PAuCl] (R = aryl) unit which is attached by
a spacer, whereas B contains an acetate functional group
instead. All building blocks were obtained according to
standard synthetic procedures and fully characterized (see the
Supporting Information). Next, we studied by nuclear mag-
netic resonance (NMR), diffusion-ordered NMR (DOSY),
and cryo-spray ionization mass spectroscopy (CSI-MS) the
typical formation of M12L24 spheres upon mixing A (or B)
[**] Financial support was provided by the University of Amsterdam,
The Netherlands Organization for Scientific Research-Chemical
Sciences (NWO-TOP to J.N.H.R.), and by the National Research
School for Catalysis. NWO is acknowledged for a Rubicon grant to
R.G.-D. I.I.-B., O.T., and M.D. gratefully acknowledge support
through the “Solar Technologies Go Hybride” initiative of the State
of Bavaria. Prof. Dr. Bas de Bruin, Dr. Jarl I. van der Vlugt, Dr.
Gunnar W. Klau, and M.Sc. Louis J. Dijkstra are also acknowledged
for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406415.
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[MÀm(OTf)]m+ (m = 6 to 12; see
Figures S39–S46 in the Supporting
Information). Also, for A24Pd12 we
obtained a clean CSI-MS spectrum
which clearly indicates the formation
of the spheres with 24 gold complexes
inside, as evidenced by peaks observed
at 2686.5668, 2403.0168, 2171.0194,
1977.6041, and 1814.0993 correspond-
ing to [MÀm((OTf)]m+ (m = 9 to 13;
see Figures S31–S36 in the Supporting
Information). The 31P{1H} NMR spec-
trum of the A24Pd12 sphere displayed
only a small downfield shift observed
upon self-assembly formation (Dd =
0.4 ppm with respect to A; see Fig-
Figure 1. New approach allowing the use of high local catalyst concentration (1m), whereas the
average catalyst concentration is in the usual 10À3–10À6 m range.
ure S25 in the Supporting Information), thus indicating that
the phosphine gold [AuCl] complex is compatible with the
self-assembly process. 19F{1H} NMR spectroscopy indicates,
as expected, the presence of noncoordinated triflate anions
(see Figures S27 and S38 in the Supporting Information).
Having established that the A24Pd12 and B24Pd12 spheres
can be prepared by simple self-assembly of the building
blocks, we were interested in the preparation of the mixed
spheres using different ratios of A and B in the presence of
palladium(II) cations (Figure 3). Since there is no clear strong
Figure 2. Building blocks A (left) and B (right) employed in this study.
PM3-Spartan-modeled supramolecular spheres derived from A and B.
Tf =trifluoromethanesulfonyl.
Figure 3. Estimated local [AuCl] concentration inside a given sphere as
a function of the ratio A and B used to form the assembly, and five
typical examples in which the number of gold atoms clearly varies
(top).
with [Pd(MeCN)4(OTf)2] as the appropriate palladium pre-
cursor (Figure 2).
The 1H NMR spectra clearly show the typical shift of the
pyridine protons, thus indicating coordination to palladium,
and the simplicity of the spectra indicate the formation of
highly symmetric structures consistent with A24Pd12 and
B24Pd12 spheres (see Figures S22 and S37 in the Supporting
Information). DOSY studies revealed that the structures
formed in solution are about 5 nm in diameter (see Figure S30
in the Supporting Information), which is in line with the data
reported by Fujita et al. for these types of structures.[11a] Final
evidence for the formation of well-defined sphere structures
came from CSI-MS analysis. For B24Pd12 a clean spectrum was
obtained with peaks displaying the expected isotopic pattern
at 2013.4220, 1704.5132, 1472.8300, 1292.6317, 1148.3730,
1030.4342, and 932.1523 belonging respectively to
driving force for the self-sorted formation of spheres, the use
of mixtures leads to a local dilution of the gold concentration.
We estimated that the local gold concentration of gold
complexes by dilution in these spheres could be controlled
between within the range between 0.05m and 1.1m, by just
mixing different ratios of building blocks A and B during the
assembly formation (Figure 3). The formation of a statistical
mixture of spheres should follow a binomial distribution of
(AnB24Àn)Pd12(OTf)24 (see Section S8 in the Supporting Infor-
mation), and for the various ratios of A and B these
distributions are displayed in Figure 4. For example, at the
12:12 ratio of A/B, the predicted distribution with a maximum
at the 1:1 ratio is shown in green.
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concentration in the hydroalkoxylation of the g-allenol 1,
a typical gold-mediated transformation, in which 1 can react
in an intramolecular fashion to give either the product 2 as
a five-membered ring or 3 as a six-membered ring (Table 1).
Table 1: Hydroalkoxylation of the allenol 1.[a]
Entry Catalyst
mol%
Overall Local Yield
[AuCl] 2 [%][d]
[mm][b] [m][c]
TON
AABBPd12(OTf)24 (mol% A) [AuCl]
(A/B ratio)
Figure 4. Top: Illustrative peaks in the CSI-MS spectra [measured
(top)spectrum versus simulated spectrum (bottom)] of selected
spheres (AnB24Àn)Pd12(OTf)24 present in the statistical mixture. Bottom:
Binomial distribution of spheres (AnB24Àn)Pd12(OTf)24 formed at differ-
ent A/B ratios: 1:23 (blue, 6:18 (orange), 12:12 (green), 18:6 (violet),
21:3 (red) with a palladium(II) precursor.
1
2
3
4
5
6
1:23
6:18
12:12
18:6
21:3
24:0
48 (48)
8 (48)
4 (48)
2.7 (48)
2.3 (48)
2 (48)
5
5
5
5
5
5
0.05
0.27
0.54
0.80
0.94
1.07
0 (–)
10 (–)
37 (–)
55 (50) 1.13
82 (80) 1.69
90 (88) 1.86
0
0.21
0.76
[a] [1]=10.4 mm, (see the Supporting Information for details).
[b] Overall concentration of [AuCl] in the reaction mixture. [c] Average
concentration of [AuCl] within the spheres according to Figure 3.
[d] Calculated yield (after 12 h) as determined by 1H NMR spectroscopy.
Value within parentheses is that of the yield of the isolated product after
column chromatography; reactions performed 2–3 times.
The formation of spheres based on mixtures of the
building blocks A and B (different ratios 1:23, 6:18, 12:12,
18:6 and 21:3) in the presence of [Pd(MeCN)4(OTf)2] was
studied by CSI-MS and NMR spectroscopy. Indeed, 1H NMR
spectra clearly show that all pyridine protons were involved in
palladium coordination and that the formed structures are
symmetric, which is in line with sphere formation. The
simplicity of the 1H NMR spectra indicates that the presence
of different amounts of gold inside the sphere does not change
the NMR signature of the outer framework. Also, in these
experiments the phosphine coordination to the [AuCl]
complex remains intact, as is confirmed by 31P{1H} NMR
spectroscopy. As expected from statistical mixtures of spheres
in which the number of gold complexes varies, the CSI-MS
spectra of the solutions are more complex than the parent
A24Pd12 and B24Pd12 spheres, but the expected peaks of the
various spheres in the mixtures could be identified in all
experiments with various ratios of A and B [(peaks at m/z
[MÀm((OTf)]m+ (m = 7 to 13)]. The expected isotopic
profiles belonging to the main spheres that should form
according to the mathematical model were most prominent,
and typical examples are displayed in Figure 4 (for more
details see the Supporting Information). For example, the
CSI-MS spectrum of the mixture formed with a ratio of A/B =
1:23 clearly shows the peaks with the expected isotopic
profiles at m/z values for [MÀm((OTf)]m+ (m = 7 to 13)
belonging to (A0B24)Pd12(OTf)24, (A1B23)Pd12(OTf)24, and
(A2B22)Pd12(OTf)24. The same features were found when
mixing A and B at ratios 6:18, 12:12, 18:6, and 21:3 with the
palladium(II) precursor (see Tables S1–S5 in the Supporting
Information), and illustrative peaks including the calculated
isotope patterns are displayed in Figure 4. Importantly, with
this strategy in hand we can create spheres in which we
control the local concentration of gold complexes (Figure 3
and section S9 in the Supporting Information), which allows,
for the first time, a study of the effect of extremely high
concentrations of catalyst on its reactivity.
The selectivity of this reaction is determined by the attack of
the hydroxy nucleophile group on one of the carbon atoms of
the allene group.[13d] In the first experiments, different sets of
self-assembled spheres with local gold concentrations varying
between 0.05m and 1.1m were used as catalysts, while keeping
the overall gold concentration in solution at 5 mm (Table 1).
When the local gold concentration was higher than 0.27m
(ratio A/B > 6:18) product formation was observed, which
increases with the local concentration (Table 1, entries 2–
6).[15] With the highest local concentration the yield of the
isolated product was 88%. Remarkably, usually the AuCl
complexes become active only after abstraction of the
chloride, but at these high concentrations in the spheres the
AuCl complexes are already active. Indeed, control experi-
ments employing free AuCl complexes do not show any
conversion under these reaction conditions. This is the case
for gold complexes based on building block A as well as for
benchmark Ph3PAuCl, that is no conversion even with
24 mol% of Ph3PAuCl at 1.1m concentration (see Table S12
in the Supporting Information). Next to the unusual activity
of the AuCl complexes, we also found that all active catalysts
were selectively forming the small cyclic compound 2.
The formation of 2 was monitored over time by 1H NMR
spectroscopy (see Figure S122 in the Supporting Informa-
tion), from which the turnover numbers (TON) and initial
turnover frequencies (TOFini) were calculated (Table 1 and
Figure 5). These results clearly indicate that the catalyst
activity increases with the local gold concentration (overall
gold concentration was the same in all experiments), that is, at
high A/B ratios, with the highest activity observed for
A24Pd12(OTf)24 ([AuCl] ca. 1.1m; Figures 3 and 5). The
TOFini and TON values were increased by further optimiza-
tion of the parameters (see Table S10 in the Supporting
With these gold(I)-containing spheres[14] (AnB24Àn)Pd12-
(OTf)24 in hand we explored the effect of local catalyst
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local gold concentration, other factors play a role. We
hypothesized that the limited freedom of the individual gold
complexes inside the A24Pd12(OTf)24 sphere leads to a further
preorganization which may be required for the formation of
the final active species. Interestingly, UV/Vis measurements
performed on A24Pd12(OTf)24 show a broad shoulder at about
l = 315 nm (Figure 6 and section S7 in the Supporting Infor-
mation), thus suggesting the formation of higher-order
Figure 5. Plot of TOFini calculated at 15% conversion versus the local
gold concentration within each sphere; the overall gold concentration
is 5 mm in all experiments. Data are obtained by fitting the initial part
of the reaction profiles of the different reactions [A/B=1:23 (blue),
6:18 (orange), 12:12 (green), 18:6 (violet), 21:3 (red), 24:0 (black);
see the Supporting Information for details].
Information), that is, application of higher temperatures
(908C in toluene as solvent) and decreasing catalyst loading.
Under these reaction conditions a TON of 67 and TOFini of
55 hÀ1 were reached using A24Pd12(OTf)24 at 0.03125 mol%
(0.75 mol% of AuCl; see Figure S125 in the Supporting
Information).
After having established the unusual reactivity of these
spheres, we explored the activity of A24Pd12(OTf)24 in the
gold(I)-catalyzed intramolecular cycloisomerization of the
less reactive substrate 1,6-enyne 4 [Eq. (1)], which typically
produces two products when traditional cationic gold com-
Figure 6. Normalized UV/Vis spectra of solutions of AnB24ÀnPd12-
(OTf)24 in acetonitrile at room temperature showing an increased band
at l=315 nm, indicative of d10–d10 aurophilic interactions at high local
concentrations.
complexes which experience d10–d10 aurophilic
interactions.[18] Upon diluting the local gold-
chloride concentration within the spheres, by
using lower A/B ratios during the sphere
formation, the shoulder at about l = 315 nm
decreases in intensity (Figure 6; see Fig-
ure S121 in the Supporting Information).
plexes are applied.[16] As expected, the reaction proceeds
more slowly than the reaction shown in Table 1, and at room
temperature in the presence of 2 mol% A24Pd12(OTf)24
(48 mol% AuCl) no conversion was obtained. Interestingly
at 408C, 45% conversion was reached, and at 908C full
conversion was obtained after 12 hours (95% yield). Under
reaction conditions in which the overall gold concentration
was 1.19 ꢀ 10À4 m (sphere concentration 1.25 ꢀ 10À6 m) the
reaction performed smoothly at 908C, leading to 19.3%
conversion and implying a TON of 26 with respect to the gold
concentration (see Table S11 in the Supporting Information).
Interestingly, A24Pd12(OTf)24 produces only 5 as the product,
which is unusual as there are only limited examples with
in situ generated cationic gold(I) species complexes which are
selective.[16c,17]
This change shows that the concentration of complexes that
display d10–d10 aurophilic interactions increases the high local
gold concentration, which correlates with the activity
observed in catalysis (see Table S9 in the Supporting Infor-
mation). In line with the inactivity of the Ph3PAuCl complex,
we did not observe the shoulders that typically indicate d10–d10
aurophilic interactions, even at 1.1m (see Figure S120 in the
Supporting Information).[19]
Next to these findings, we observed only the peaks
belonging to active spheres AnB24ÀnPd12(OTf)24 and inactive
building block A in the 31P{1H} NMR spectra during the
catalytic reactions.[15] Most interestingly, if a lot of product is
formed, the sphere disassembled completely, with the
remaining building block A (the gold(I) chloride complex)
as the only species, thus supporting the formation of the
catalytically active species only at high concentration within
the sphere. This outcome suggests that either a) the chloride
dissociation to generate an active cationic gold species,
facilitated by Au…Au interactions is reversible and too fast
to be detected on NMR time scale or b) the chloride does not
The origin of the activity at high gold-chloride concen-
tration inside the sphere is puzzling, and control experiments
using Ph3PAuCl or A applied at the same high concentration
[1.1m, the concentration within sphere A24Pd12(OTf)24] did
not show conversion, thus suggesting that next to the high
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dissociate and Au···Au interactions occur to form a multi-
nuclear (P-Au-Cl)n complex that is responsible for the
activity.
In conclusion, we report here a self-assembly strategy to
prepare nanosized molecular spheres with an extremely high
metal-complex concentration inside. The local gold-chloride
concentration can be controlled between 0.05 and 1.1m by just
mixing the various bifunctional building blocks used for
sphere formation in different ratios. Importantly, the overall
catalyst concentration in solution can be kept identical and at
the usual 10À6–10À3 m range. This control allows, for the first
time, a thorough investigation of the reactivity at catalyst
concentrations at the molar level, a dimension under-
explored in homogeneous catalysis. The high local catalyst
concentration turns an inactive AuCl complex into an active
system. Next to this unusual reactivity displayed by the gold
chloride complexes, the nanospheres also displays high
selectivities in intramolecular cycloisomerization reactions.
As the self-assembly strategy is simple and based on
accessible building blocks, the approach is likely widely
applicable and new reactivity can be uncovered by exploring
high concentration catalysis.
Received: June 27, 2014
Revised: July 18, 2014
Published online: September 12, 2014
[15] Control experiments in which the isolated product 2 was added
to a solution of the sphere A24Pd12 show the appearance of peaks
belonging to the building block A, presumably because of the
coordination of 2 to palladium.
Keywords: gold · homogeneous catalysis · metal–
metal interactions · nanostructures · supramolecular chemistry
.
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