Generation and Reactivity of Electron-Rich Carbenes on the Surface of Catalytic Gold Nanoparticles

Article abstract of DOI:10.1021/jacs.7b13696

The reactive nature of carbenes can be modulated, and ultimately reversed, by receiving additional electron density from a metal. Here, it is shown that Au nanoparticles (NPs) generate an electron-rich carbene on surface after transferring electron density to the carbonyl group of an in situ activated diazoacetate, as assessed by Fourier transformed infrared (FT-IR) spectroscopy, magic angle spinning nuclear magnetic resonance (MAS NMR), and Raman spectroscopy. Density functional theory (DFT) calculations support the observed experimental values and unveil the participation of at least three different Au atoms during carbene stabilization. The surface stabilized carbene shows an extraordinary stability against nucleophiles and reacts with electrophiles to give new products. These findings showcase the ability of catalytic Au NPs to inject electron density in energetically high but symmetrically allowed valence orbitals of sluggish molecules.

Full text of DOI:10.1021/jacs.7b13696

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Communication
Generation and reactivity of electron–rich carbenes
on the surface of catalytic gold nanoparticles
Judit Oliver-Meseguer, Mercedes Boronat, Alejandro Vidal-Moya, Patricia
Concepción, Miguel Ángel Rivero-Crespo, Antonio Leyva-Perez, and Avelino Corma
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13696 • Publication Date (Web): 20 Feb 2018
Downloaded from http://pubs.acs.org on February 20, 2018
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Generation and reactivity of electron–rich carbenes on the
surface of catalytic gold nanoparticles
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Judit Oliver–Meseguer,† Mercedes Boronat, † Alejandro Vidal–Moya, † Patricia Concepción, † Miguel
Ángel Rivero–Crespo, † Antonio Leyva–Pérez,*,† and Avelino Corma.*,†
†Instituto de Tecnología Química ꢀUPV‐CSICꢁ, Universitat Politècnica de València‐Consejo Superior de Investigaciones
Científicas, Avda. de los Naranjos s/n, 46022 Valencia, Spain.
Supporting Information Placeholder
from the NP bulk. The electron–rich intermediates are stable
ABSTRACT: The reactive nature of carbenes can be modulat‐
enough to be detected8 and used in productive catalytic pro‐
ed, and ultimately reversed, by receiving additional electron
cesses.6‐9
density from a metal. Here, it is shown that Au nanoparticles
ꢀNPsꢁ generate an electron–rich carbene on surface after
transferring electron density to the carbonyl group of an in–
situ activated diazoacetate, as assessed by Fourier trans‐
formed infrared ꢀFT–IRꢁ spectroscopy, magic angle spinning
nuclear magnetic resonance ꢀMAS NMRꢁ and Raman spectros‐
copy. Density functional theory ꢀDFTꢁ calculations support
the observed experimental values and unveil the participation
of at least three different Au atoms during carbene stabiliza‐
tion. The surface stabilized carbene shows an extraordinary
stability against nucleophiles, and rather react with electro‐
philes to give new products. These findings showcase the
ability of catalytic Au NPs to inject electron density in energet‐
ically high but symmetrically allowed valence orbitals of slug‐
gish molecules.
Figure 1 shows the temperature programmed FT–IR spectrum
of a sample of commercially–available, homogeneously dis‐
persed 3 nm Au NPs on TiO2 ꢀ1 wt%ꢁ, degassed at 200 ºC
under vacuum, and treated with ethyl diazoacetate ꢀEDA, 1ꢁ at
25 ºC. According to blank experiments with 1 adsorbed on
TiO2 ꢀFig S1‐3ꢁ, the peaks at 2115 and 1640 cm‐1 corresponds
to unreacted 1, and the peaks at 1736, 1679 and 1568 cm‐1
correspond to the hydration product ethyl glycolate 2, as also
assessed by the disappearance of hydroxyl groups of Au–TiO2
after adsorption of 1. However, new peaks at 1704 and 1549
cm‐1 rise in intensity after 1 dosing, remaining quite stable to
evacuation. These peaks can be ascribed to the strongly blue–
shifted CꢃO of an electron–rich carbene that receives electron
density from Au ꢀ1549 cm‐1, 3Aꢁ10 and the free CꢃO of a clas‐
sical Au carbene ꢀ1704 cm‐1, 3Bꢁ.11 Increasing temperature to
150 ºC triggers formation of ethyl fumarate and ethyl maleate
4, as assessed by the peaks at 1711, 1667 and 1524 cm‐1 ꢀsee
Fig S2ꢁ.1b Benzyl diazoacetate gives also blue–shifted CꢃO
signals in the presence of Au–TiO2 ꢀFig S3ꢁ.
Carbenes are divalent C–atoms, often generated in–situ with
catalytic metals to program their reactivity towards nucleo‐
philes.1 However, carbenes can reverse their reactivity if the
catalytic metal transfers a significant amount of electron den‐
sity to unoccupied bonding orbitals. This has been achieved so
far by spontaneous one electron oxidation of soluble Co2ꢂ and
Fe2ꢂ/0 carbene complexes.2 The latter feature suitable quasi–
planar ligands that not only furnish an appropriate chemical
environment for the electron–rich carbene, but also provide
the energetically and spatially suitable valence orbitals to
engage the empty anti–bonding orbitals of the carbene, since a
direct electron transfer from the metal to the carbene is se‐
verely restricted. It would be of interest to have metals able to
do so on solid surfaces, thus enabling heterogeneous catalysis
and avoiding ligands.
1711
1667
0.1
1524
150 ºC
1640
2115
1736
1549
25 ºC
Au is able to bind carbenes as a metal complex in solution
and also as NPs.3‐5 The latter reacts in a classical way, proba‐
bly on unsaturated Au atoms present in the boundaries, cor‐
ners and vertexes of the NP.5 It may occur that bulk Au atoms
would inject electron density into the symmetrically matching
unoccupied valence orbitals of a suitable carbene, if efficiently
formed on the NP surface. This is not more what occurs dur‐
ing the activation on Au NPs of relatively inert molecules such
as H2,6 O2,7 HCl8 and benzenes and alkynes,9 which coordinate
on unsaturated Au atoms and then receive electron density
2400
2200
2000
1800
1600
Wavenumber (cm-1)
Figure 1 Temperature programmed FT–IR spectra of 1 ad‐
sorbed on Au–TiO2 at increasing dosing ꢀblack linesꢁ, after
evacuation at 10‐6 mbar ꢀblue linesꢁ at 25 ºC, and after in‐
creasing temperature to 150 ºC ꢀred lineꢁ.
Figure 2A shows the 13C CP/MAS NMR spectra of isotopically
labelled EtOOC13CHN2 ꢀ1–13Cꢁ,12a adsorbed on Au–TiO2 ꢀsur‐
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face Au atoms/1 ꢃ1ꢁ, and sealed in an ampule ꢀsee Fig S4 for
bene C atom in structure A, 1578, 407 and 305 cm‐1, and 21
ppm, respectively, are close to those observed experimentally
ꢀ1537, 445, 302 cm‐1 and 41 ppmꢁ, much closer than any
other computed structure ꢀsee Table S1 and also Table S2 for
related reported valuesꢁ.4,10 The CꢃO bond length changes
from 1.22 to 1.24 ꢄ in structure A, with a ca. 20% increase of
neat negative charge at the O atom and a similar increase of
neat positive charge at the C atom, which unambiguously
indicates the weakening of the CꢃO bond. Finally, the net and
total charges summarized in Fig. 3 and Fig. S5 show that there
is a transfer of ⁓ 0.5e‐ from Au to the adsorbed carbene, caus‐
ing an increase in the net negative charge on the carbene atom
of 0.7–0.8 e ꢀsee Table S3 for validation with B3PW91 and
M06 methodsꢁ.3b,4 These results, together, strongly support
the electronic transfer from the Au surface to the antibonding
orbitals of the CꢃO bond to generate the electron–rich car‐
bene 3A.
full spectraꢁ. The mixture at RT shows the original signal of 1–
13C centered at 46 ppm ꢀFig S4ꢁ together with three new sig‐
nals centered at 41, 77 and 132 ppm ꢀbroadꢁ, the two latter
corresponding to 2 and 4, respectively. Notice that, under
these conditions, the dimerization reaction has been complet‐
ed. After heating at 80 ºC during 30 min, the signal of 1–13C
completely disappears and the signal ranging from ~30 to
~50 ppm, with a maximum at 41 ppm, persists, without fur‐
ther changes in the spectrum for longer heating times. Notice
that surface heterogeneity broadens signals in the spectrum. A
very recent work4 reports a nucleophilic Au carbene complex
resonating at 18.2 ppm in 13C liquid NMR, which strongly
suggests that the signal at 41 ppm may correspond to an elec‐
tron–rich C carbene atom. Please note that a carbene can
persist after heating since the experiment is made in the gas
phase with very low covering level, thus bimolecular reactions
are hampered. 1–15N was also prepared12b and co‐adsorbed
with 1–13C, and the combined 15N and 13C CP/MAS NMR spec‐
tra confirm that the peak at 41 ppm only forms after N2 re‐
lease ꢀFig S5ꢁ. Please notice the difficulties associated in char‐
acterizing nucleophilic metal carbenes by NMR, since they
usually feature paramagnetic metals.2,12c Figure 2B shows the
corresponding time–resolved Raman spectra, and two new
peaks at ~290‐320 and 445 cm‐1 appear, which could be as‐
signed to Au‐C and Au‐O vibrations of carbenes 3, respective‐
ly, beside the Raman bands at 197, 396, 514 and 639 cm‐1
corresponding to TiO2.
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77.2
B)
41.2
46.5
A)
TiO2
132.5
Figure 3 DFT calculations of the most probable configurations
of carbene 3 on Au NPs, including bond lengths ꢀblackꢁ and
net atomic charges ꢀblueꢁ.
1
RT
The Au–TiO2 ꢀ0.1 mol%ꢁ catalyzed reaction of 1, in toluene at
70 ºC, gives an equimolecular mixture of dimers 4, up to 90%
yield.13 Au–ZnO and Au–Al2O3 were also effective, and Au–
TiO2 could be reused up to 10 times without depletion in the
final yield of 4. For all Au NPs, the equation rate for the for‐
mation of 4 was r0ꢃ kexpꢅ1ꢆꢅAuꢆ, which is the expected equa‐
tion rate for a classical Lewis–acid catalyzed activation and
dimerization of 1 ꢀFig S6ꢁ.14a The Lewis base Bu3N ꢀ0.01
mol%ꢁ severely stopped the formation of 4, while a similar
amount of NaI had no effect. Besides, the cross dimerization
between ethyl ꢀ1ꢁ and tert–butyl diazoacetate, which possess
a bulky group that impedes a good coordination on the bulk
Au atoms and thus hampers metal catalysis on planar surfaces
ꢀTaft effectꢁ,14b proceeds very well ꢀFig S7ꢁ, which supports
that 1 transforms to 3B at unsaturated Au atoms. This process
may run in parallel and be co–operative with the formation of
3A on bulk Au atoms, with an electronic flow from the unsatu‐
rated to the bulk ꢀFig S8ꢁ.14c‐e Notice that products in solution
differ from species observed by in–situ IR, NMR and Raman
techniques, since the latter mainly correspond to species
remaining adsorbed on surface ꢀFig S9ꢁ.
302
2
3A
445
4
80 ºC
140 120 100 80 60 40 20
0
200
400
Raman shift (cm-1
600
 (ppm)
)
Figure 2 Aꢁ 13C CP/MAS NMR spectra of Au–TiO2 loaded with 1
ꢀAu/1ꢃ 5ꢁ at RT ꢀtopꢁ and after heating at 80 ºC for 30 min
ꢀbottomꢁ. Spectrometer frequency is 100.6 MHz and spinning
rate is 5 kHz. Bꢁ Raman spectra of 1 adsorbed on Au–TiO2 at
RT and evolution with time ꢀ24, 35, 46, 74 and 86 minꢁ.
Figure 3 shows the calculated interaction energies, net atomic
charges, bond lengths, CO and RAMAN frequencies, and NMR
values of carbene 3 adsorbed on a Au10 model, with either the
oxygen atom of the carbonyl ꢀAꢁ or ester ꢀBꢁ groups attached
to the surface Au atoms. Remarkably, the carbene atom bonds
to two Au atoms ꢀAu‐C‐Auꢁ in all configurations, regardless
different coordination modes or Au NP arrangements ꢀsee
Table S1 for additional computed structuresꢁ. A further stabi‐
lization occurs if an O atom coordinates Au, in such a way that
the highest interaction energy ꢀ‐37 kcal mol–1ꢁ is achieved
when the O atom of CꢃO coordinates to the Au surface at the
relatively small distance of 2.36 ꢄ ꢀstructure Aꢁ. The calculat‐
ed CO and Raman frequencies, and NMR shifts for the car‐
The reactivity of 3A and other potential electron–rich Au
carbenes was then studied. Table 1 shows the results with
Au–TiO2, Auꢂ complexes and some representative catalysts. In
general, the reactivity of 1 drastically decreases in the pres‐
ence of Au–TiO2. For instance, toluene, n–hexane and ethanol
ꢀentry 1ꢁ can be used as a reaction solvent since they do not
react in the presence of Au–TiO2, in clear contrast with Auꢂ
complexes15,16 and classical metal species.17–23 The electron–
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Table 1 Reactivity of carbene–forming substrates in the presence of different catalysts.
Entry
Reactiona
Catalyst / products (catalyst loading, product yield and reference)b
Au–TiO2 Other representative catalysts
Au+ complexes
7
1
1–20 mol%, no reaction
8
9
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2c
5 mol%; 7a (10%), 7b (27%), 7c-f
(35-24%)
3
10-25 mol%,
4
a Reaction conditions for Au–TiO2 catalyst, Au–ZnO and Au–Al2O3 catalysts gave similar results in most cases. Blank experiments with Au–TiO2
show only marginal yields of the nucleophile carbene products 7a‐d, 9, 13 and 14. b See detailed reaction conditions in references. IPr: 1,3–
bisꢀdiisopropylphenylꢁimidazole–2–ylideneꢆ, BAr´4: tetrakis–ꢀ3,5–bisꢀtrifluoromethylꢁphenylꢁborate, PyrPy: 2,2′–pyridylpyrrolideꢇligand, TpMs
hydrotrisꢀ3,5– ꢀ2,4,6–trimethylphenylꢁpyrazolylꢁborate, ba: benzylideneacetone. c R1ꢃ n‐But, R2ꢃ H 6a; R1ꢃ Ph, R2ꢃ Me 6b; R1ꢃ 4‐R‐Ph ꢀRꢃCF3,
Br, H, Meꢁ, R2ꢃ H 6c‐f. Mass balances account for ꢈ95% in entries 1–3 and c.a. 80% in entry 4
:
rich nature of 3A is also seen during the cyclopropanation of
alkenes ꢀentry 2ꢁ. In contrast to classical cyclopropanation
reactions where electron–rich alkenes are much more reac‐
tive,16,23,24 3A reacts worse with electron–rich 1–hexene 6a
than with electron–poor styrenes 6b–f, the similar reactivity
Ortho–nitro phenylacetylene 12 ꢀentry 4ꢁ generates oxo–
carbenes with metal catalysts, including products 1532a and
16.32b When Au–TiO2 was used as a catalyst, neither 15 nor 16
were formed, but just isatin 13 and indigo dye 14. These
products come from rearrangements and radical oxidation of
carbene atoms, as previously observed with Cuꢂ,32c which
suggests the formation of electron–rich carbenes of 12.32d
trend found with Co2ꢂ porphyrin radical carbene catalysts.25
A
Hammett plot confirms this tendency ꢀFig S10ꢁ. The higher
reactivity of the Co2ꢂ complex respect Au–TiO2 catalyst corre‐
lates with the more electron density transferred by the former
ꢀca. 1e‐ꢁ than by the latter ꢀ0.2e‐ according to DFTꢁ. Different
Au–TiO2 samples with average sizes of 7, 12, 17 and 21 nm,
respectively, were prepared, characterized,26 and tested as
catalysts for the cyclopropanation of 6d ꢀFig S11–15ꢁ, and the
results clearly showed that the cyclopropanation rate increas‐
es linearly with the amount of exposed bulk Au atoms in the
NP, and not with the amount of unsaturated Au atoms.
In conclusion, combined experimental and theoretical evi‐
dences strongly support that Au NPs generate and stabilize
electron–rich carbenes on surface, after electronic transfer to
the anti–bonding valence orbitals of the CꢃO group of
RCOCHN2 molecules. This carbene umpolung avoids participa‐
tion in insertion reactions but rather enables the carbene to
act as a nucleophile in addition reactions.33
ASSOCIATED CONTENT
Supporting Information
Other representative carbene‐forming substrates were
tested. Dimedone 827 ꢀentry 3ꢁ is a typical reagent for Wolff
rearrangements or alcohol insertion reactions, which gives
cyclopentane 1014d,28–29 and ether 1130 after intra– or inter–
molecular nucleophilic attack to the carbene of 8 in ethanol
solvent, respectively, under metal or microwave catalyzed
conditions. In clear contrast, coupled bis–dione 9 was the only
product obtained with Au–TiO2 catalyst, in up to 82% yield,
irrespective of the presence of light or not, and without any
trace of Wolff rearrangement or alcohol insertion products.
An electron–rich carbene of 8 on Au–TiO2 can be invoked as
intermediate of the reaction,31a,b since blank experiments
showed that Au–TiO2 does not oxidize ethanol to acetaldehyde
in the absence of 8 under identical reaction conditions, and a
very active aerobic oxidation catalyst of alcohols such as Au–
CeO2 gives a similar yield of 9 ꢀ57%ꢁ.31c
Experimental section, compound characterization and addi‐
tional Figures S1–15 and Table S1–3. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
E‐mail: anleyva@itq.upv.es, acorma@itq.upv.es; phone:
ꢂ34963877800; fax: ꢂ349638 77809.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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Financial support by MINECO through the Severo Ochoa pro‐
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Cajal Program ꢀA. L.–P.ꢁ is acknowledged. J. O.–M. thanks ITQ
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