A: D 3h-symmetry hexaazatriphenylene-tris-N-heterocyclic carbene ligand and its coordination to iridium and gold: Preliminary catalytic studies

Article abstract of DOI:10.1039/c7cc00525c

A new D_3h-symmetry tris-N-heterocyclic carbene ligand has been prepared and coordinated to iridium and gold. The ligand contains an electron-poor hexaazatriphenylene core; thus, the resulting tris-NHC ligand is a poor electron donor. The tris-A

Full text of DOI:10.1039/c7cc00525c

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DOI: 10.1039/C7CC00525C
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A D3h symmetry hexaazatriphenylene-tris-N-heterocyclic carbene
ligand and its coordination to iridium and gold. Preliminary
catalytic studies
Received 00th January 20xx,
Accepted 00th January 20xx
S. Ibañez,a M. Poyatos*a and E. Peris*a
DOI: 10.1039/x0xx00000x
A new D3h symmetry tris-N-heterocyclic carbene ligand has been prepared and coordinated to iridium and gold. The ligand
contains an electron-poor hexaaza-triphenylene core, thus the resulting tris-NHC ligand is a poor electron-donor. The tris-
Au(I) complex was tested in the hydroamination of terminal alkynes, and in the three-component Strecker reaction.
www.rsc.org/
Hexaazatriphenylene (HAT) constitutes an attractive building main-chain organometallic polymers, therefore showing also
block for supramolecular systems and for molecular potential to afford three-dimensional structures.7 Prompted by
coordination compounds, because it combines exceptional these precedents, we became interested in merging the
topological and electronic features with a great coordination properties associated of planar and rigid tris-NHC ligands, with
ability. HAT is the smallest two-dimensional N-containing the presence of a HAT core. We thought that the introduction
polycyclic aromatic system, and has been used as building block of HAT into a tris-NHC core should provide interesting
of a myriad of supramolecular systems with a variety of properties, given the electron-deficient character of this
applications.1 Due to the presence of three geometrically molecule compared to the electron-rich triphenylene.
independent chelating coordination sites, which are useful for
the generation of self-assembled species, HAT has been
extensively used for the preparation of a large number of
molecular, supramolecular and polymer-based coordination
species.2
During the last few years we were interested in designing poly-
N-heterocyclic carbene ligands3 connected by polycyclic
aromatic hydrocarbons, aiming to facilitate the preparation of
homogeneous metal-based catalysts with enhanced properties.
In the course of our research, we observed that the use of this
type of ligands induced interesting catalytic properties that we
Chart 1
mainly attributed to substrate-ligand supramolecular
interactions,4 therefore constituting a subclass of the recently
classified as Smart NHC ligands.5 Among the NHC-based ligands
The
preparation
of
the
hexaazatriphenylene-tris-
benzoimidazolium iodide was performed according to the
method depicted in Scheme 1. The condensation of 1,3-dibutyl-
5,6-diaminobenzimidaolium iodide8 with hexaketocyclohexane
octahydrate in refluxing glacial acetic acid afforded the tris-
that we obtained, the triphenylene-tris-NHC ligand
(A)
displayed unique topological features because it allowed the
coordination of three metal fragments in a pseudo-D3h
symmetry environment,6 thus being the only tri-NHC ligand
reported to date able to facilitate this type of geometry. The
presence of the triphenylene core allowed the supramolecular
interaction of the ligand with the substrates, affording catalysts
that were superior compared to related monometallic
analogues lacking of this polycyclic aromatic core.6a The same
tris-NHC ligand was also used as scaffold for the preparation of
azolium iodide [
[Et3O](BF4) in CH2Cl2, quantitatively afforded the corresponding
tetrafluoroborate salt ](BF4)3. These two salts were
characterized by NMR spectroscopy, mass spectrometry and
elemental analysis. The elemental analysis of salts [ ](I)3 and
](BF4)3, revealed the presence of three and two molecules of
1](I)3 in 81%. Anion metathesis of [1](I)3 with
[
1
1
[
1
water of crystallization, respectively.
3+
We sought that [
1
]
should be able to bind to metals through
the nitrogen atoms of the hexaazatriphenylene core, and also
through the carbon edges of the molecule, if a tris-N-
Institute of Advanced Materials (INAM), Universitat Jaume I, Av. Vicente Sos
Baynat s/n, 12071-Castellón, Spain.
heterocyclic carbene is generated. We first reacted [1](I)3 with
[RuCl2(p-cymene)]2 aiming to obtain a compound in which the
Electronic Supplementary Information (ESI) available: NMR, spectra of new
compounds, CIF file of the molecular structure of [1][RuCl2I(p-cymene)]3 and a
summary of the crystal data, data collection and refinement details.
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Ru(p-cymene) fragment could be bound to the ligand through three N,N’-dibutylimidazolium fragments linked by
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DOI: 10.1039/C7CC00525C
the chelating nitrogen donors. However, instead of obtaining hexaazatriphenylene core. The disposition of the ten fused
our desired compound, we observed that the reaction afforded cycles is essentially coplanar, as can be observed from the lower
[1][RuCl2I(p-cymene)]3, a species in which the iodide from the image of the structure shown in Figure 1. An interesting feature
starting triazolium salt is coordinated to the metal fragment of the structure is that the distance between the hydrogen
generating the anionic complex [RuCl2I(p-cymene)]-. All other atoms of the NCHN fragments of the imidazoliums is 15.4 Å. This
attempts to achieve the coordination of other metal fragments distance is assumed to be similar to the distance that should be
3+
to the nitrogen atoms of [
1
]
were also unsuccessful. We established between the metals of a potential trimetallic
attributed this behavior to the deactivation of the donor complex with a tris-NHC ligand derived from this trisazolium
electron pairs at the nitrogens due to the three positive charges salt.
of the molecule, and to the high steric strain produced by the
presence of the N-butyl groups at the adjacent imidazolium
moieties.
Scheme 1. Preparation of [1](I)3
Figure 1. Two perspectives of the molecular structure of [1][RuCl2I(p-cymene)]3.
Ellipsoids at 50% probability. Hydrogen atoms, and solvent (CH3OH and CH2Cl2) are
omitted for clarity of the representation. The crystal contains a mixture of
[RuCl3(p-cymene)] and [RuCl2I(p-cymene)] counterions, which were refined with a
40% and 60% occupancy, respectively. Nitrogen atoms represented in blue.
3+
Although all our attempts to coordinate [
1
]
through the
nitrogen atoms were unsuccessful, we were able to find a way
to use this tricationic compound as a scaffold for the
preparation of several planar tris-NHC metal complexes. In all
the reactions, the trisazolium salt [1](I)3 was deprotonated by
NaOtBu in THF, in the presence of catalytic substoichiometric
amounts of NaH at -78ºC. Then the metal source was added and
the reaction mixture was allowed to reach room temperature.
This method allowed us to obtain complexes
[IrCl(COD)]2, [AuCl(SMe)2] and [AuCl(CNC)] (CNC
2
,
4
and
5
, by using
2,6-
=
diphenylpyridine), respectively, in low yields ranging from 23-
30%. We ascribed these low yields to the formation of the
hexaazotriphenylene-tris-benzoimidazolidone
consequence of the deprotonation of the trisimidazolium [
6
,
as
]
3+
1
Scheme 2. Metal complexes obtained in this work
and subsequent oxidation of the resulting N-heterocyclic
carbene. It is important to mention, that the procedures used
for the preparation of metal complexes using other known tris-
The molecular structure of [1][RuCl2I(p-cymene)]3 was further
confirmed by single crystal X-ray diffraction studies (Figure 1).
The determination of the structure of this molecule allowed us
to determine the structural parameters of the
hexaazatriphenylene-tris-benzoimidazolium salt, which may be
related to the structure of a potential tris-NHC ligand that could
be derived from its deprotonation. The structure consists of
NHC ligands, such as
(yields < 50 %), so we did not find surprising the low yields that
we found for the coordination of [ ](I)3. The oxidation of NHCs
A (Scheme 1), are also very low yielding
1
to yield a cyclic urea derivative has been observed by other
authors.9 This oxidation process is difficult to prevent, because
2 | J. Name., 2012, 00, 1-3
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the tricationic salt [
ARTICLE
1
](I)3 is accompanied by a number of water used as model for testing the effects of modifyiVniegwtAhrteicleLOenwlinies
DOI: 10.1039/C7CC00525C
molecules, which come from the hexaketocyclohexane acidity of the gold center on its catalytic activity, and the studies
octahydrate used in the preparation of the salt. In fact, the demonstrated that increasing the Lewis acid character of the
coordination complexes
the reactions were carried out in the presence of molecular summarizes the results that we obtained for the
sieves, otherwise the hexaazatriphenylene-tris- hydroamination of phenylacetylene with five different primary
benzoimidazolidone was the only product formed. Compound amines. The reactions were carried out in acetonitrile at 90ºC,
can be isolated from the reaction of [ ](I)3 with Ag2O (Scheme using a catalyst loading of 0.33 mol% (1 mol% based on the
3). The identity of was unequivocally established by single amount of active gold centres). AgBF4 was added as a chloride
2, 4 and 5
could be obtained only when metal results in an enhancement of the activity.11, 14f, 14h Table 1
6
6
1
6
crystal X-ray diffraction, although the quality of the crystal did scavenger, in order to activate the catalyst. We also tested the
not allow us to obtain a molecular structure with the accuracy catalytic activity of the complex [AuCl(1,3-di-nbutyl-
needed for its publication (see the Supporting Information for benzoimidazolylidene)], 7 15
preliminary data and an ORTEP diagram).
.
Complex
one of the gold-containing branches of
electron-poor pyrazine ring. This makes
comparison, because it bears a ligand with the same steric
properties as the triscarbene ligand in , but with a stronger
electron-donating character.12b As seen from the results shown
in Table 1, the activity of was excellent for all the combinations
of substrates used, with product yields ranging from 81-98 %.
On the contrary, the activity of the monometallic complex was
only moderate, affording product yields 14-20 % points below
those given by . This result illustrates how the reduction of the
7
, can be considered as
, but lacking of the
a perfect pattern for
4
7
4
4
7
4
electron-donating character of the NHC ligand produced by the
presence of the HAT core has a positive impact on the catalytic
Scheme 3. Preparation of compound 6
activity of 4. All in all compound 4 is less active than the best
All new complexes were characterized by NMR spectroscopy,
mass spectrometry and elemental analysis. Unfortunately, we
were unable to obtain single crystals of the quality required to
allow the determination of any of their molecular structures by
X-ray diffraction. As a common feature, the 1H NMR spectra, of
Au(I) catalysts reported to date for this reaction.14j
Table 1. Hydroamination of phenylacetylene.[a]
2, 4 and 5 (and also 6) were consistent with the threefold
symmetry of their predicted geometries. Their 13C NMR spectra
revealed the signals due to the metallated carbene carbons at
205.6, 197.2 and 172.3 ppm, for 2, 4 and 5, respectively. These
signals are significantly downfield shifted compared to those of
other related NHC-based complexes of iridium (I), gold (I) and
gold (III), an observation that may be related to the low
electron-donating character of the tris-NHC ligand.10
Entry
1
2
3
4
5
6
7
8
Catalyst
Ar
Ph
Ph
Yield[b]
92
72
98
80
91
72
82
68
4
7
4
7
4
7
4
7
4
7
2-MeC6H4
2-MeC6H4
4-MeC6H4
4-MeC6H4
2,4,6-Me3C6H2
2,4,6-Me3C6H2
2,6-iPr2C6H3
2,6-iPr2C6H3
The reaction of
hexacarbonylated complex
2
with carbon monoxide in CH2Cl2 afforded the
. This compound displayed two IR
3
bands at 2069 and 1992 cm-1, assigned to the CO stretching
vibrations. The comparison of these values with those given by
related NHC-IrI(CO)211 and NHC-IrCl(CO)212 complexes indicates
9
10
81
64
[a] Reaction conditions: 0.5 mmol phenylacetylene, 0.55 mmol amine, 1% mol
catalyst (based on concentration of Au active sites), 2 % mol AgBF4, 1 mL of MeCN
at 90 ºC, 6 h. [b] Yields determined by GC using anisole as internal standard.
that the electrondonating character of the tris-NHC ligand in
3
is significantly reduced, as expected by the presence of the
electron-poor hexaazatriphenylene core.
We also tested
the synthesis of
aldehyde (or ketone) and trimethylsilyl cyanide.16 For
comparative purposes we also tested the activity of the
benzimidazolylidene-Au(I) complex 7. To our knowledge, there
is only one example of a Au(I) complex used to promote this
process.7 This reaction is normally catalyzed by complexes with
4
in the three-component Strecker reaction for
-aminonitriles, by reaction of an amine, an
In order to determine if the new tris-NHC ligand may have
applications in homogeneous catalysis, we decided to test the

tri-Au(I) complex
4 in two benchmark reactions. We chose this
compound for our preliminary studies because the activity of
gold-based catalysts have often been related to the ability of
the Au center to behave as an electrophile. Given the low
electron-donating character of the tris-NHC ligand in
4, we
Lewis acidic functions, so we thought that
4 may be able to
though that this complex should yield good catalytic
promote this process. The reactions were carried out at room
temperature in CH2Cl2, using a catalyst loading of 4 mol%
(based on the amount of Au(I) active centers). As can be seen in
efficiencies. We first tested complex 4 in the hydroamination of
terminal alkynes,13 a reaction for which several Au(I) complexes
have shown good activity.6a, 14 In fact, this reaction has been
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2 S. Kitagawa and S. Masaoka, Coord. Chem. Rev., 2003, 246, 73-
Table 2, the results are highly dependent on the combination of
the substrates used. For the reactions carried out with
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3 M. Poyatos, J. A. Mata and E. Peris, ChemD. OReI:v1.0,.21003099/C, 17C0C90, 30562757C-
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2013, 52, 7009-7013; b) S. Gonell, R. G. Alabau, M. Poyatos
and E. Peris, Chem. Commun., 2013, 49, 7126-7128.
7 S. Gonell, M. Poyatos and E. Peris, Chem. Eur. J., 2014, 20, 5746-
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8 D. Tapu, Z. McCarty, L. Hutchinson, C. Ghattas, M. Chowdhury,
J. Salerno and D. VanDerveer, J. Organomet. Chem., 2014,
749, 134-141.
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Organomet. Chem., 2014, 751, 620-627; b) M. Slivarichova, R.
Ahmad, Y. Y. Kuo, J. Nunn, M. F. Haddow, H. Othman and G.
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acetophenone,
4-bromoacetophenone
and
4-
methoxiacetophenone,
4
provided excellent yields, ranging
from 78-99 %, with the highest activity shown for the reaction
of 4-bromoacetophenone. This observation indicates that the
catalyst tolerates the presence of halides in the substrate, thus
affording a clear advantage over palladium-based catalysts,17
for which the dehalogenation of the halide-substituted ketone
may constitute an important side reaction. The product yield
was very low for the reaction carried our using p-
nitroacetophenone (22%), but it has to be taken into account
that this substrate is highly deactivated for the coupling with
aniline, thus rarely providing yields over 20%. As seen from the
data shown in Table 2, the activity of the monometallic complex
7
is significantly lower, thus indicating that the presence of the
HAT core in the trimetallic complex
the complex.
7 improves the activity of
10 a) Q. Teng and H. Han Vinh, Inorg. Chem., 2014, 53, 10964-
10973; b) J. C. Bernhammer and H. V. Huynh, Dalton Trans.,
2012, 41, 8600-8608.
11 S. Ibañez, M. Poyatos, L. N. Dawe, D. Gusev and E. Peris,
Organometallics, 2016, 35, 2747-2758.
Table 2. Three-component Strecker reaction[a]
12 a) H. Valdes, M. Poyatos and E. Peris, Organometallics, 2014,
33, 394-401; b) H. Valdes, M. Poyatos and E. Peris, Inorg.
Chem., 2015, 54, 3654-3659; c) A. R. Chianese, X. W. Li, M. C.
Janzen, J. W. Faller and R. H. Crabtree, Organometallics, 2003,
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Scott, E. D. Stevens, J. Bordner, I. Samardjiev, C. D. Hoff, L.
Cavallo and S. P. Nolan, Organometallics, 2008, 27, 202-210;
Entry
Catalyst
R
H
H
Br
Yield[b]
77
39
99
59
22
5
79
52
1
2
3
4
5
6
7
8
4
7
4
7
4
7
4
7
e) D. J. Nelson and S. P. Nolan, Chem. Soc. Rev., 2013, 42
,
Br
6723-6753.
NO2
NO2
MeO
MeO
13 L. Huang, M. Arndt, K. Goossen, H. Heydt and L. J. Goossen,
Chem. Rev., 2015, 115, 2596-2697.
14 a) M. J. Pouy, S. A. Delp, J. Uddin, V. M. Ramdeen, N. A.
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and W. H. Myers, ACS Catal., 2012, 2, 2182-2193; b) M. Katari,
[a] Reaction conditions: 0.5 mmol ketone, 0.55 mmol aniline, 1 mmol TMSCN, 4%
mol catalyst loading (based on the concentration of Au active sites), 2 mL of CH2Cl2,
24 h at room temperature. [b] GC yields using anisole as internal standard.
M. N. Rao, G. Rajaraman and P. Ghosh, Inorg. Chem., 2012, 51
,
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Badaj, T. G. Larocque and G. G. Lavoie, Chem. Eur. J., 2012, 18
,
In summary, we prepared a new hexaazatriphenylene-tris-
benzoimidazolium salt,
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a
precursor of
a
planar
hexaazatriphenylene-tris-benzoimidazolilydene ligand. The
ligand was coordinated to several metal fragments, allowing the
preparation of two iridium and two gold star-shaped tri-NHC
complexes. The catalytic activity of the tri-NHC-Au(I) complex
was tested in the hydroamination of terminal alkynes, and in
the three-component Strecker reaction, where it displayed
excellent activities due to the presence of the
hexaazatriphenylene linker in the ligand, which enhances
electrophilic character of the gold centers in the catalyst.
We gratefully acknowledge financial support from MINECO of
Nat. Commun., 2014, 5, 3470; j) D. Malhotra, M. S. Mashuta,
G. B. Hammond and B. Xu, Angew. Chem. Int. Ed., 2014, 53
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,
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45.
Spain (CTQ2014-51999-P) and the Universitat Jaume
I
(P11B2014-02 and P11B2015-24). We are grateful to the Serveis
Centrals d’Instrumentació Científica (SCIC-UJI) for providing
with spectroscopic facilities.
17 a) Q. Q. Teng and H. V. Huynh, Inorg. Chem., 2014, 53, 10964-
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Notes and references
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2015, 44, 6850-6885.
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