Synthesis, characterization and luminescence studies of gold(I)-NHC amide complexes

Article abstract of DOI:10.3762/bjoc.9.260

A flexible, efficient and straightforward methodology for the synthesis of N-heterocyclic carbene gold(I)-amide complexes is reported. Reaction of the versatile building block [Au(OH)(IPr)] (1) (IPr = 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene) with a series of commercially available (hetero)aromatic amines leads to the synthesis of several [Au(NRR')(IPr)] complexes in good yields and with water as the sole byproduct. Interestingly, these complexes present luminescence properties. UV-vis and fluorescence measurements have allowed the identification of their excitation and emission wavelengths (λ_max). These studies revealed that by selecting the appropriate amine ligand the emission can be easily tuned to achieve a variety of colors, from violet to green.

Full text of DOI:10.3762/bjoc.9.260

Synthesis, characterization and luminescence
studies of gold(I)–NHC amide complexes
Adrián Gómez-Suárez1, David J. Nelson1, David G. Thompson2,
David B. Cordes1, Duncan Graham2, Alexandra M. Z. Slawin1
and Steven P. Nolan*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 2216–2223.
1EaStCHEM School of Chemistry, University of St Andrews, North
Haugh, St Andrews, Fife, KY16 9ST, UK and 2WestCHEM
Department of Pure and Applied Chemistry and Centre for Molecular
Nanometrology, University of Strathclyde, 295 Cathedral Street,
Glasgow, G1 1XL, UK
doi:10.3762/bjoc.9.260
Received: 15 July 2013
Accepted: 10 October 2013
Published: 28 October 2013
Email:
This article is part of the Thematic Series "Gold catalysis for organic
synthesis II".
Steven P. Nolan* - snolan@st-andrews.ac.uk
* Corresponding author
Guest Editor: F. D. Toste
Keywords:
© 2013 Gómez-Suárez et al; licensee Beilstein-Institut.
License and terms: see end of document.
fluorescence; gold; gold catalysis; N-heterocyclic carbenes; synthesis
Abstract
A flexible, efficient and straightforward methodology for the synthesis of N-heterocyclic carbene gold(I)–amide complexes is
reported. Reaction of the versatile building block [Au(OH)(IPr)] (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with
a series of commercially available (hetero)aromatic amines leads to the synthesis of several [Au(NRR’)(IPr)] complexes in good
yields and with water as the sole byproduct. Interestingly, these complexes present luminescence properties. UV–vis and fluores-
cence measurements have allowed the identification of their excitation and emission wavelengths (λmax). These studies revealed
that by selecting the appropriate amine ligand the emission can be easily tuned to achieve a variety of colors, from violet to green.
Introduction
The synthesis of organogold complexes has recently attracted have been very active in the synthesis and characterization of
wide attention due to their considerable range of applications, in new gold(I)–NHC complexes and the study of their reactivity,
areas such as materials and medicinal chemistry, as well as their with a special focus on the development of straightforward
potential role as reaction intermediates or new catalysts in gold- methodologies [11,12]. As a result of our investigations, we
catalyzed processes [1-7]. This has led several research groups have recently reported the synthesis of [AuX(NHC)] (X = Cl,
to investigate general, green and straightforward methodologies Br, I) complexes, directly from imidazolium and imidazoli-
for the synthesis of organogold complexes. We have focused on dinium salts and a suitable gold source, such as [AuCl(SMe2)],
the synthesis and study of transition metal complexes bearing using K2CO3 as a base [13]. Moreover, we have also reported
N-heterocyclic carbene (NHC) ligands [8-10]. Recently, we the synthesis of the first mononuclear gold(I) hydroxide species,
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Beilstein J. Org. Chem. 2013, 9, 2216–2223.
[Au(OH)(IPr)] (1) (IPr = 1,3-bis(2,6-diisopropylphenyl)imida- and 3 respectively, in good yields after overnight reaction at
zol-2-ylidene), using [AuCl(IPr)] and an excess of KOH in THF room temperature. These results are consistent with the known
[14,15]. This complex has proven to be an excellent synthon for pKa of [Au(OH)(IPr)] (30.3) [26]; the scope of this preparative
the preparation of a wide variety of organogold(I) species [16- route is, as with other deprotonation reactions with this building
23]. Two approaches have been developed for the synthesis of block, limited to substrates with a pKa lower than 30.
such species using hydroxide 1: a) via transmetallation from
either boronic acids or siloxides, or b) via deprotonation reac- Encouraged by these exploratory reactions, a series of
tions of suitable substrates. We have applied the latter method- (hetero)aromatic amines were employed to prepare the corres-
ology to the synthesis of organogold–NHC complexes bearing ponding gold(I)–amide complexes. The desired complexes were
several trans-ligands, such as acetylene, phenyl and phenol obtained by reaction of [Au(OH)(IPr)] (1) with each
derivatives. These reactions can be carried out using reagent- (hetero)aromatic amine in THF at room temperature for 20 h. A
grade materials, without the requirement to exclude light, air or range of aromatic amines were employed, including aniline,
moisture; providing an easy, straightforward 3-step route for the diphenylamine, pyridines, a pyrimidine and one isoquinoline.
synthesis of organogold–NHC complexes from readily avail- The corresponding complexes were obtained in analytically
able gold sources, such as [Au(SMe2)Cl] (Scheme 1).
pure form and in good yields as yellow or white microcrys-
talline powders after a simple work-up (Scheme 2). All
Toste and Bergman have recently reported the synthesis, complexes were characterized by 1H and 13C{1H} NMR
characterization and reactivity studies of a series of spectroscopy and elemental analysis (see Supporting Informa-
[Au(NRR’)(NHC)] complexes [24]. This study represents the tion File 1). The new species are bench stable in the solid
first reported synthesis of gold(I)–NHC amide complexes. The state, and do not require special handling. However, some do
procedure employed for the synthesis of these species required decompose slowly in solution over the course of a number of
the use of lithium amide reagents that are not stable towards air hours.
or moisture [24]. Therefore, the development of a more robust
approach would be desirable. In addition to the interesting re- A selected number of these complexes were characterized by
activity shown by Toste and Bergman, Hoffman and Viseux X-ray crystallography, as further support to confirm atom
have probed the anticancer properties of gold amide complexes, connectivity and molecular geometry (Figure 1) [27].
reacting modified triflamide compounds with phosphine-
bearing gold chloride species [25]. Taking into account the All complexes were found to display the expected two-
interest in these type of complexes, we sought to use hydroxide coordinate linear geometry around the metal center, with all
1 to develop an easy, green (in such deprotonation reactions, C–Au–N angles in the range 173–179°. There was no evidence
water is the sole byproduct) and straightforward methodology of any interactions between the gold center and the heteroatoms
for the synthesis of gold(I)–amide complexes.
in complexes 3, 7, 8, 10 or 11. However, for 3, 7, 8, 10 and 11
there is an intermolecular contact between the aromatic nitrogen
atom and the NHC ligand backbone proton (dN-H =
2.27 Å–2.34 Å). All gold–carbon bond lengths were in the
range 1.95 Å (8) to 2.02 Å (2), while gold–nitrogen bond
lengths varied from 1.98 Å (8) to 2.06 Å (2).
Results and Discussion
Synthesis and characterization of
gold(I)–amide complexes
We began our studies by exposing hydroxide 1 to a series of
alkyl- and arylamines. While no reaction was observed with
either morpholine or isopropylamine, the use of 1 equiv of
aniline or 2-aminopyridine led to the isolation of complexes 2
During the characterization of the gold–NHC amide complexes
we observed that some of the complexes possessed luminescent
Scheme 1: Straightforward synthesis of organogold complexes via deprotonation reactions, using 1.
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Scheme 2: Scope of the reaction between 1 and several (hetero)aromatic amines. Reaction conditions: 1 (1 equiv), amine (1 equiv), THF (0.5–2 mL),
rt, 20 h.
properties when exposed to ultraviolet light (λ = 366 nm) in the decided to explore the potential of these gold amide species as
solid state (Figure 2).
luminescent materials.
A number of luminescent organogold complexes have been We began our luminescence studies by recording the UV–vis
reported in the literature, often demonstrating interesting prop- spectra of the aforementioned gold–amide complexes using a
erties that may lead to their application in materials science and dilute (ca. 0.2 mmol/L) CH2Cl2 solution. The wavelengths of
the preparation of optical materials [28-34]. Amongst these, the absorption maxima on the UV–vis spectra were in the range
NHC-bearing gold complexes have been shown by several of 250–350 nm for most complexes, with the exception of
research groups to be very useful luminescent materials [19,35- 2-amino-5-nitropyridine derived 9 and 3-aminoisoquinoline-
39], and have attracted industrial interest [40-42]. Thus, we derived 10, which exhibited absorption maxima at ca. 430 nm
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Figure 1: X-ray crystal structures of complexes 2, 3, 7, 8, 10, 11 and 12. Hydrogen atoms are omitted for clarity.
Figure 2: Selected examples of gold–NHC amide complexes under UV light (λ = 366 nm).
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Beilstein J. Org. Chem. 2013, 9, 2216–2223.
(see Supporting Information File 1). These data are consistent λmax = 516 nm). In each case, there is a considerable Stokes
with the physical appearance of complexes 2–12 in the solution shift (27 to 166 nm). In terms of the intensity of emission, a
state, i.e. complexes 9 and 10 are yellow.
wide range of values were recorded, although complexes could
be considered as part of one of three distinct groups. Phenyl-
The excitation and emission maxima were then determined. bearing complexes (such as 2 and 12), 4-aminopyridine-derived
These measurements were conducted on more concentrated 5 and electron-poor 9 showed relatively weak emission
(ca. 4 mmol/L) CH2Cl2 solutions, compared to those used for (<10 AU). Electron-poor 2-aminopyridine derivatives,
UV–vis spectroscopy. In each case, the relevant maxima could 3-aminopyridine-derived 4 and 2-amino-5-chloropyrimidine-
be identified. An example dataset is presented in Figure 3. Life- derived 11 exhibited moderate emission intensity (ca.
times for all luminescence measurements were ≤1 μs (at the 10–50 AU). The most intense fluorescence was observed with
limit of the apparatus), suggesting that the luminescence was complexes 3, 6 and 10: 2-pyridine derivatives without electron-
due to fluorescence, rather than phosphorescence.
withdrawing substituents (ca. 120–290 AU) (Table 1).
The wavelengths of emission maxima spans the range 390 to DFT calculations were used to probe the nature of the frontier
516 nm, demonstrating that by selecting the appropriate amine orbitals of complex 3 (at the M06-L/SDD level of theory) [43]
ligand the emission can be easily tuned to achieve a variety of using Gaussian 09 [44]. The LUMO, HOMO and HOMO-1 are
colors, i.e. from violet (3, λmax = 409 nm) to green (4, pictured in Figure 4. While the HOMO and HOMO-1 are
Figure 3: Excitation (blue) and emission (pink) data for complex 3, bearing a 2-pyridine ligand (see inset).
Figure 4: (a) LUMO, (b) HOMO and (c) HOMO-1 of complex 3.
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Table 1: Fluorescence measurement data.a
entry
substituent
λmax/excitation (nm)
λmax/emission (nm)
intensitymax (AU)
3.8
1
396
423
470
390
2
12
5
2
3
410
352
4.61
7.25
4
5
6
7
8
350
398
368
395
385
516
454
406
428
435
1.09
21.2
12.8
45.8
36.0
9
4
8
7
11
3
9
379
389
471
409
420
505
126
119
293
10
6
11
10
aAt ca. 4 mmol/L in CH2Cl2 solution.
centered predominantly on the amide ligand, the LUMO is Conclusion
localized on the aryl ring of the NHC ligand; we propose that A novel series of NHC-bearing gold(I)–amide complexes have
the fluorescence behavior is due to a HOMO-to-LUMO tran- been prepared using a simple, straightforward synthetic route
sition. Therefore, the use of different NHC ligands should also that can be conducted using reagent-grade materials on the
allow access to different fluorescence behavior.
laboratory bench in air. The resulting species have been charac-
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Beilstein J. Org. Chem. 2013, 9, 2216–2223.
12.Nolan, S. P. Acc. Chem. Res. 2011, 44, 91–100.
terized using a number of methods, including NMR spec-
troscopy and X-ray crystal diffraction. These new species have
been shown to be fluorescent, and their absorbance and emis-
sion maxima have been determined. Notably, there are key
trends in the fluorescence behavior of these materials, with
more electron-rich 2-pyridine derivatives showing strong emis-
sion, and isoquinoline-derived complexes showing the strongest
fluorescence. While the present study has been conducted using
commercial reagent-grade amines, there is significant scope to
prepare a much wider range of gold(I)–amide complexes,
including those prepared using designed aromatic amines.
Further work is underway in our group to explore both the
potential of gold(I)–NHC amide complexes, and further appli-
cations of gold(I) hydroxides as building blocks and catalysts.
doi:10.1021/ar1000764
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Supporting Information
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Supporting Information File 1
Experimental part.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-260-S1.pdf]
23.Egbert, J. D.; Slawin, A. M. Z.; Nolan, S. P. Organometallics 2013, 32,
2271–2274. doi:10.1021/om301187a
Acknowledgements
24.Johnson, M. W.; Shevick, S. L.; Toste, F. D.; Bergman, R. G.
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SPN thanks the ERC (Advanced Investigator Award-
FUNCAT), EPSRC and Syngenta for support, and Umicore AG
for their generous gift of materials. D.G. and S.P.N. are Royal
Society Wolfson Merit Award holders. We thank EaStCHEM
for computational support through the EaStCHEM Research
Computing Facility.
26.Boogaerts, I. I. F.; Nolan, S. P. J. Am. Chem. Soc. 2010, 132,
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27.CCDC 949310 (2), 949311 (3), 949312 (7), 949313 (8), 949314 (10),
949315 (11) and 949316 (12) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre (CCDC) via
http://www.ccdc.cam.ac.uk/data_request/cif.
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