A Dinuclear Au(I) Complex with a Pyrene-di-N-heterocyclic Carbene Linker: Supramolecular and Catalytic Studies

Article abstract of DOI:10.1021/acs.organomet.8b00087

A digold(I) complex with a pyrene-di-N-heterocyclic carbene ligand has been obtained and fully characterized. The host-guest properties of the complex were studied in chloroform with a series of polycylic aromatic hydrocarbons (PAHs), and the association

Full text of DOI:10.1021/acs.organomet.8b00087

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
A Dinuclear Au(I) Complex with a Pyrene-di-N-heterocyclic Carbene
Linker: Supramolecular and Catalytic Studies
Daniel Nuevo, Macarena Poyatos,* and Eduardo Peris*
Institute of Advanced Materials (INAM), Universitat Jaume I, Avda. Vicente Sos Baynat s/n, E-12071 Castellon
́
, Spain
S
* Supporting Information
ABSTRACT: A digold(I) complex with a pyrene-di-N-hetero-
cyclic carbene ligand has been obtained and fully characterized.
The host−guest properties of the complex were studied in
chloroform with a series of polycylic aromatic hydrocarbons
(PAHs), and the association constants were determined. The
affinity increases with the size of the PAH, with a maximum
binding constant of 105 M−1 for the aggregation with coronene.
The catalytic activity of the complex was tested in the
hydroamination of phenylacetylene with three different amines.
The studies showed that the addition of coronene to the reaction
medium produces an enhancement of about 20−30% in the
activity of the complex.
nature of the supramolecular interactions between our metal
complexes and pyrene, we performed some preliminary host−
guest chemistry studies, which allowed us to determine the
association constants between our complexes and pyrene (K =
12 M−1 in CD3CN). Prompted by this finding, in this new work
we describe the preparation of a one-dimensional (1D)
organometallic complex (not showing a cavity) that exhibits
interactions with a variety of PAHs. For the preparation of this
1D organometallic receptor, we envisioned that our previously
reported planar pyrene-connected di-NHC ligand15 (see
Scheme 1) could act as an effective supramolecular antenna
for interacting with aromatic guests. We decided to coordinate
this ligand to Au(I) in order to obtain a linear structure that
could minimize any undesired steric repulsions and therefore
optimize the π-stacking interactions between the complex and
the organic guests. The host−guest chemistry properties of this
new complex are described herein.
INTRODUCTION
■
Over the last two decades, the field of metallosupramolecular
chemistry has experienced great development due to its wide
range of applications in fields such as molecular recognition,1
drug delivery,2 light-emitting materials,3 and homogeneous
catalysis.4 The basis for metallosupramolecular design is the
availability of rigid bridging ligands, which in combination with
metals form well-defined architectures with a variety of shapes
and sizes.5 Most known metallosupramolecular molecules, or
supramolecular coordination compounds (SCCs),6 are based
on Werner-type polydentate ligands, and only recently have
organometallic ligands (mostly based on poly-N-heterocyclic
carbenes7) allowed the preparation of diverse types of SCCs
including discrete molecules8 and polymers.9 In line with this
field of research, our group has recently been interested in
developing a library of rigid multidentate ligands bearing
polyaromatic substituents, which have been used for the
preparation of several metallosupramolecular assemblies. While
our initial interest was to study how supramolecular
interactions influenced the catalytic performance of our
supra-organometallic complexes,10 we recently became inter-
ested in designing metal-driven self-assembly architectures for
the recognition of organic substrates11 and metal cations.12
SCCs are often called “molecular flasks”13 because they show
well-defined nanoscopic cavities. These cavities are arguably
responsible for their wide set of applications, mostly because
they facilitate the selective encapsulation and recognition of
organic substrates. We recently reported a series of Au(I)
complexes bearing N-heterocyclic carbene (NHC) ligands
fused to polycyclic aromatic hydrocarbons.14 In our work, we
found that the catalytic performance of the complexes was
significantly enhanced by the addition of a polycyclic aromatic
hydrocarbon (PAH) such as pyrene. In order to study the
RESULTS AND DISCUSSION
■
Pyrenebis(azolium) salt 1 was used as the NHC precursor for
the synthesis of di-NHC−Au(I) complex 2. As depicted in
Scheme 1, complex 2 was obtained by deprotonation of
bis(azolium) salt 1 with potassium bis(trimethylsilyl)amide
(KHMDS) and subsequent addition of [AuCl(SMe2)]. After
purification, complex 2 was obtained in 60% yield. The 1H and
13C NMR spectra of 2 are in accordance with the pseudo-D2h
symmetry of the molecule. The 13C NMR spectrum reveals the
Special Issue: In Honor of the Career of Ernesto Carmona
Received: February 9, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00087
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ordered nanostructures, such as spheres, nanoplates, fibers, or
helical ribbons.17 As can be observed in Figure 3, SEM images
of 2, prepared by slow diffusion of MeOH into a saturated
solution of the complex in chloroform, show laminar ribbons.
Scheme 1. Synthesis of Complex 2
characteristic singlet assigned to the resonance of the two
equivalent carbene carbons at 188.9 ppm.
Single crystals suitable for X-ray diffraction were obtained by
slow diffusion of methanol into a solution of 2 in chloroform.
The structure consists of a pyrenebis(imidazolylidene) ligand
bridging two AuI units (Figure 1). The iodide and NHC
Figure 3. SEM images of 2, prepared by slow diffusion of MeOH into
a saturated solution of the complex in chloroform, at (a) lower and (b)
higher magnifications.
We thought that the structural and electronic features of 2
should make it a good candidate for exploring its host−guest
chemistry properties, despite the obvious fact that the complex
does not exhibit any well-defined cavity for the encapsulation of
organic guests. The recognition abilities of 2 were studied by
1H NMR titration experiments in CDCl3 by monitoring the
variation of the chemical shifts of the signals of the complex
upon addition of solutions containing the different polyar-
omatic guests. We first confirmed that the self-association of 2
in CDCl3 was negligible by measuring a series of spectra of the
complex in CDCl3 at concentrations ranging from 0.2 to 20
mM. All of the titrations with guests were performed at a
constant concentration of 2. In general, the addition of the
solution of the guest induced important perturbations in the 1H
NMR spectra, indicating the formation of host−guest
aggregates that showed fast kinetics on the NMR time scale.
Figure 4 shows an example of the titration of 2 with coronene.
As can be observed, the addition of incremental amounts of
coronene induces an upfield shift of the signals due to the
Figure 1. Molecular structure of 2. Hydrogen atoms and solvent have
been omitted for clarity. Thermal ellipsoids are shown at the 50% level
of probability. Selected bond distances (Å) and angles (deg): Au1−C1
2.003(6), Au1−I1 2.5416(4), C1−Au1−I1 177.15(16), N1−C1−N2
107.5(5).
ligands are coordinated to the Au(I) centers in a quasi-linear
geometry, as shown by the I−Au−CNHC angle of 177.15°. The
Au−CNHC bond distance is 2.003 Å, and the through-space
distance between the two Au centers is 13.15 Å, which is very
similar to the M−M distance provided by the same ligand in
our previously reported complexes of rhodium, iridium,15a
ruthenium,15c and platinum.16
Given that the packing properties of complex 2 are highly
influenced by π-stacking interactions (Figure 2), we decided to
study the morphology of the bulk solid by scanning electron
microscopy (SEM). Previous studies have shown that the
rational design of supramolecular interactions may determine
the morphology of the bulk material by the formation of highly
Figure 4. Representative region of the 1H NMR (400 MHz) spectra of
the titration of 2 with coronene in CDCl3. The spectra were recorded
at a constant concentration of 2 (0.4 mM).
Figure 2. Detail of the crystal packing of 2 showing the π-stacking
interactions along the a axis.
B
DOI: 10.1021/acs.organomet.8b00087
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Article
a
proton of the host pyrene linker and the protons of the NCH2
groups of the n-butyl wingtips. This observation is a clear
indication that a π-stacking interaction between coronene and 2
occurs through the pyrene linker of the host. The analysis of
the binding isotherm generated by this titration is consistent
with a 1:1 host:guest stoichiometry, and an association constant
Table 2. Hydroamination of Phenylacetylene
of 105
1 M−1 was determined by global nonlinear
b
b
c
entry cat. (mol %)
Ar
conv. (%)
yield (%)
TON
regression.18 A Job plot was also made in order to confirm
the formation of the 1:1 host−guest complex (see the
Supporting Information for details).
1
2
3
4
5
6
7
8
9
0.5
Ph
98
93
99
34
58
41
45
45
51
66
65
86
26
50
33
40
36
44
196
186
0.5
4-MeC6H4
2,4,6-Me3C6H2
Ph
0.5
198
By means of a similar procedure, the association constants of
2 with a series of PAHs were also calculated. As can be
observed from the data shown in Table 1, the binding affinities
0.05
0.05
0.05
0.05
0.05
0.05
680
d
d
d
Ph
1160
820
4-MeC6H4
4-MeC6H4
2,4,6-Me3C6H2
2,4,6-Me3C6H2
900
Table 1. Association Constants K for the Complexation of 2
900
a
with PAHs
1110
a
K (M−1
)
Reaction conditions: 0.5 mmol of phenylacetylene, 0.55 mmol of
entry
guest
amine, 0.2 or 2 mol % AgBF4 (depending on the catalyst loading), 1
1
2
3
4
5
phenanthrene
pyrene
−
4
b
mL of MeCN, 90 °C, 6 h. Conversions and yields were determined
1
1
1
1
c
by GC using anisole (0.5 mmol) as an internal standard. Turnovers
triphenylene
perylene
10
d
calculated as a function of converted substrates. Upon the addition of
17
0.05 mmol of coronene.
coronene
150
a
The association constants were calculated by global nonlinear
(entries 5, 7 and 9), thus 20−30% higher than the results
provided by the same catalyst in the absence of coronene. This
result is in clear agreement with our previous published
findings,14 which we interpret to be a consequence of the
formation of π-stacking aggregates of coronene and 2, which
prevents the formation of nonactive dimers formed by the self-
association of 2.
regression analysis18 using the HypNMR2008 program. Titrations
were carried out using constant concentrations of 2 (0.3−0.4 mM) in
CDCl3 at 298 K. Errors refer to the nonlinear regression fits.
of the PAH guests increase in the order phenanthrene < pyrene
< triphenylene < perylene < coronene (entries 1−5). This
trend is in accordance with previous reports when receptors
with large portals were used11b,c,19 and indicates that the host−
guest interaction is dominated by the π surface area of the
guests rather than by their degree of aromaticity.
CONCLUSIONS
■
We prepared and characterized a dimetallic complex of Au(I)
with a pyrenebis(imidazolylidene) ligand. The supramolecular
properties of the complex were studied with regard to the
association with several polycyclic aromatic hydrocarbons. Our
studies demonstrate that despite not having a cavity that would
allow the formation of inclusion host−guest aggregates, the
dimetallic Au(I) complex is able to associate to large PAHs,
achieving association constants as high as 105 M−1 in
chloroform for the case of coronene. The catalytic activity of
the complex was tested in the hydroamination of phenyl-
acetylene, where we observed that the addition of coronene to
the reaction medium produced an enhancement in the activity
of the catalyst of about 20−30%. In line with our previous
findings in this regard, this new work demonstrates how
supramolecular interactions may be used to fine-tune the
catalytic activity of catalysts bearing polyaromatic function-
alities.
Given the relatively high affinity of 2 to associate with
coronene, we wanted to test whether this would have
consequences for the catalytic performance of the complex.
We recently showed how the addition of π-stacking additives
increased the catalytic activity of a series of monometallic Au(I)
complexes bearing NHC ligands decorated with polyaromatic
functionalities in the hydroamination of terminal alkynes.14
Initially, the reactions were carried out at 90 °C in acetonitrile
using a 0.5 mol % catalyst loading with the addition of AgBF4 as
a halide scavenger. The results given in Table 2 show that full
conversion was achieved after 6 h of reaction, although the
yields of the desired product were lower, thus indicating that
the reaction was not fully selective. However, we were not able
to identify any of the possible byproducts of the reaction. In
any case, under the same reaction conditions, complex 2
outperformed the related pyrene-imidazolylidene−Au(I) com-
plex that we recently described,14 although it shows lower
activity than some other previous Au−NHC catalysts reported
by us20 and others.21 In order to test whether the addition of a
π-stacking additive would modify the activity of the complex,
we decided to perform a series of reactions at a lower catalyst
loading (0.05 mol %), aiming to find significant differences at
lower conversion values. As observed from the data shown in
Table 2, this low catalyst loading still afforded moderate yields
of the final products with turnovers ranging from 680 to 900
depending on the substrate used (entries 4, 6, and 8). The
addition of a substoichiometric amount of coronene (10 mol %
with respect to substrate) produces a clear enhancement of the
activity of the catalyst, with turnovers rising to 900−1160
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00087.
■
S
Description of synthetic procedures and characterization
details, catalytic experiments, spectra of the new complex,
1
crystallographic data, and H NMR spectra of host−
guest titrations (PDF)
Accession Codes
CCDC 1811341 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
C
DOI: 10.1021/acs.organomet.8b00087
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: eperis@uji.es (E. Peris).
*E-mail: poyatosd@uji.ess (M. Poyatos).
ORCID
Eduardo Peris: 0000-0001-9022-2392
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from MINECO of
Spain (CTQ2014-51999-P) and Universitat Jaume I (UJI-
B2017-07, P11B2015-24 and P11B2014-02). The authors are
(e) Wurthner, F.; You, C.-C.; Saha-Moller, C. R. Metallosupramo-
̈
̈
́
́
grateful to the Serveis Centrals d’Instrumentacio Cientıfica
(SCIC) of Universitat Jaume I for providing spectroscopic and
X-ray facilities.
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DEDICATION
■
We dedicate this article to Professor Ernesto Carmona on the
occasion of his 70th birthday. Professor Carmona has been one
of the strongest contributors to the development of organo-
metallic chemistry in Spain, a good friend, and a reference for
many chemistry researchers.
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DOI: 10.1021/acs.organomet.8b00087
Organometallics XXXX, XXX, XXX−XXX