Synthesis, characterization and luminescence of gold complexes bearing an NHC ligand based on the imidazo[1,5-a]quinolinol scaffold

Article abstract of DOI:10.1002/ejic.201300575

We report on the synthesis and characterization of an NHC ligand based on the imidazo[1,5-a]quinolinol scaffold. The benzyl-protected NHC precursor was used for the preparation of the corresponding Ag^I, Au^I and Au^III compl

Full text of DOI:10.1002/ejic.201300575

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DOI:10.1002/ejic.201300575
Synthesis, Characterization and Luminescence of Gold
Complexes Bearing an NHC Ligand Based on the
Imidazo[1,5-a]quinolinol Scaffold
Margit Kriechbaum,[a] Gerda Winterleitner,[a] Alexander Gerisch,[b]
Manuela List,[c] and Uwe Monkowius*[a]
Keywords: Nitrogen heterocycles / Carbenes / Gold / Silver / Luminescence
We report on the synthesis and characterization of an NHC
ligand based on the imidazo[1,5-a]quinolinol scaffold. The
benzyl-protected NHC precursor was used for the prepara-
tion of the corresponding AgI, AuI and AuIII complexes. The
molecular structures of the AuI and AuIII halide complexes
determined by single-crystal X-ray diffraction reveal con-
siderable distortion of the rigid NHC moiety due to steric re-
pulsion between the gold and oxygen atoms. At ambient
temperature, the complexes are weakly fluorescent from an
intraligand excited state, whereas at 77 K the emission spec-
tra are dominated by intraligand phosphorescence.
NHC.[3,11–16] For this reason, combining a carbene function
with an extended π system as the chromophoric unit should
result in luminescent AuIII complexes. We have recently re-
ported a 1,10-phenanthroline analogue NHC ligand (A)
and the emissive properties of its AuI and AuIII com-
plexes.[17] In this contribution we present a similar NHC
with an ether oxygen atom as the second binding site that
forms a potential chelating ligand (B) similar to 10-hy-
droxybenzo[h]quinoline (C, Scheme 1).
Introduction
N-Heterocyclic carbene (NHC) ligands with an extended
π system have been the focus of intensive research in recent
years due to their potential applications in catalysis and op-
toelectronics. Several metal complexes containing these
NHCs as chromophores have been used as efficient triplet
emitters in organic light-emitting devices (OLEDs) because
they are highly stable and have favourable emission proper-
ties.[1] Metal complexes containing 5d platinum group met-
als such as IrIII and PtII have mostly been investigated.[2]
An interesting alternative to these metals might be gold in
its I and III oxidation states.[3] It is well known that AuI
complexes exhibiting aurophilic interactions give rise to
emissive excited states.[4–6] Those bearing ligands with huge
π systems and energetically low-lying excited ππ* states fea-
ture intense intraligand (IL) emissions.[7–9] However, AuIII
with its low-lying dd states causes effective quenching of the
excited state, thus diminishing or even preventing lumines-
cence.[10] Therefore AuIII complexes are only emissive if
strong ligands destabilize these dd states thereby hampering
radiationless deactivation. Accordingly, these AuIII com-
plexes contain, in addition to the chromophore, strong σ-
donating ligands such as alkynyl, isocyanide or
Scheme 1. NHCs A and B, and 10-hydroxybenzo[h]quinolone (C).
Results and Discussion
Synthesis and Characterization
The NHC precursor 3 was synthesized starting from the
[a] Institute of Inorganic Chemistry, Johannes Kepler University previously reported benzyl-protected 8-benzyloxyquinoline-
Linz,
2-carbaldehyde (1), which was obtained from commercially
Altenbergerstr. 69, 4040 Linz, Austria
available 2-methyl-8-hydroxyquinoline by treatment with
sodium metal and benzyl chloride in 2-propanol and subse-
quent oxidation of the resulting 2-methyl-8-(benzyloxy)-
quinoline with SeO2.[18] Upon work-up, yellow crystals of
8-(benzyloxy)quinoline-2-carbaldehyde were formed. Con-
densation of the aldehyde with the primary amine 2,6-diiso-
propylaniline gave the Schiff base 2 (Scheme 2). Cyclization
E-mail: uwe.monkowius@jku.at
http://www.jku.at/anorganik
[b] Bruker AXS GmbH,
Östliche Rheinbrückenstrasse 49, 76187 Karlsruhe, Germany
[c] Institute of Chemical Technology of Organic Materials,
Johannes Kepler University Linz,
Altenbergerstr. 69, 4040 Linz, Austria
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300575.
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with paraformaldehyde and dry HCl yielded the carbene has an ionic form. Transmetallation of 4 with [(dms)AuCl]
precursor 3 as an off-white powder in good yield and pu- (dms = dimethyl sulfide) yielded the NHC–AuI complex 5
rity.[19] Note that a considerable amount of a side-product as a yellow solid, which was oxidized with Br2 at –50 °C to
was formed if the formaldehyde was not completely dis- the corresponding red NHC–AuIII complex 6.
solved in toluene at the beginning of the reaction. Partial
All compounds exhibit the expected 1H NMR spectra
hydrolysis of 2 proceeded easily under the acidic conditions (see the Exp. Sect.). In the 1H NMR spectra, the resonance
of the reaction and led to the regeneration of compound 1, of the imidazolium proton at δ = 10.83 ppm (in [D6]dmso)
which can function as the carbonyl component instead of vanishes upon complex formation. 13C NMR resonances
formaldehyde. A homogeneous reaction mixture favoured of the carbene carbon atoms appear at 175.6 (NHC–AuI
the formation of compound 3, thus preventing its contami- complex 5) and 148.1 ppm (NHC–AuIII complex 6), which
nation.
are very similar to the analogous complexes bearing ligand
A.[17] The corresponding carbon atom of the imidazolium
chloride 3 resonates at δ = 148.7 ppm.
Due to ligand-exchange reactions, the mass spectra of
the NHC–M complexes (M = Ag, Au) are usually domi-
nated by the ionic species [(NHC)2M]+ irrespective of the
actual composition in the solid state.[22,23] Interestingly, in
the ESI mass spectrum of NHC–AuI complex 5, not only
the signal of [(NHC)Au]+ but also of the chloronium spe-
cies [{(NHC)Au}2Cl]+ can be detected. The chloronium cat-
ion can be formed by facile halide elimination with the for-
mation of [(NHC)Au]+ and its subsequent reaction with 5.
The formation of chloronium and bromonium species
[{(NHC)Au}2Cl]+ and [{(NHC)Au}2Br]+ can also be ob-
served for 6. This is a result of the easy reductive elimi-
nation of halogens from AuIII-halides to yield (NHC)AuCl/
Br. The mass spectroscopic detection of halonium ions
[(LAu)2X]+ (X = Cl, Br) is common for phosphane gold(I)
complexes, but relatively seldom reported for L = NHC.[24]
Note that the corresponding complexes [(LAu)2X]Y (L =
R3P, Y = weakly coordinating anion) have previously been
prepared and structurally characterized.[25]
Structural Studies
Compounds 1–3, 5 and 6 were characterized by single-
crystal X-ray diffraction. Yellow needles of 8-benzyloxy-
quinoline-2-carbaldehyde (1) were obtained upon work-up.
The compound crystallizes in the monoclinic space group
Scheme 2. Synthesis of NHC precursor 3 and its Ag and Au com-
plexes 4–6.
Reaction of the imidazolium chloride 3 with an excess of P21/c with one molecule in the asymmetric unit (Figure 1,
Ag2O under ambient conditions yielded the AgI–carbene top). All geometric parameters are in the range expected.[26]
complex 4. As no crystals of this compound could be The aldehyde group is coplanar with the aromatic ring
grown, its constitution is not completely certain. It is plane and the oxygen atom is in a trans position relative to
known that NHC–silver halides undergo ligand-scrambling the nitrogen atom of the quinoline moiety. Yellow needles
reactions to form either neutral or ionic complexes of the of 2 were obtained upon recrystallization in ethanol. The
form NHC–Ag–X and [(NHC)2Ag][AgX2], respectively. compound crystallizes in the orthorhombic space group
However, the 13C NMR shifts of the carbene carbon atoms Pbca with one molecule of the E isomer in the asymmetric
of the two forms are too similar to allow the isomers to be unit showing the expected structural features (Figure 1, bot-
distinguished.[20] A thorough investigation of 4 was further tom).
hampered by the instability of the compound in solution:
Compound 3 crystallizes in the monoclinic space group
no 13C NMR spectrum could be recorded because of de- P21/c. The bond lengths and angles are as expected. Despite
composition during the measurement. Therefore we com- their common charges and bulky substituents, the cations
pared the molar conductivity of 4 with the molar conduc- aggregate to form dimers through π–π stacking interactions
tivity of a previously reported ionic complex [(NHC)2Ag]- with relatively short distances between the planes of about
[AgX2] in dichloromethane (dcm).[21] Because of the similar 3.4 Å (Figure 2, bottom). As previously mentioned, the
values
[4:
1.4 Ω–1 cm2 mol–1;
reference
sample: AgI–carbene is not stable in solution. During slow gas-
1.1 Ω–1 cm2 mol–1 (see the Exp. Sect.)] we can infer that 4 phase diffusion of diethyl ether into a dilute chloroform
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Figure 2. Molecular structure of the imidazolium cation in crystals
of 3 (top). Structure of the aggregate of cations in 3 formed through
π–π stacking interactions (bottom). Selected bond lengths [Å] and
angles [°]: C1–N1 1.333(3), C1–N2 1.353(3), N2–C1–N1 108.1(2).
Figure 1. Molecular structures of 1 (top) and 2 (bottom; ellipsoids
drawn at the 50% probability level, H atoms have been omitted for
clarity). Selected bond lengths [Å] and angles [°] for 1: C1–O1
1.206(2), C2–N1 1.324(2), C3–N1 1.366(2), C4–O2 1.364(2), O1–
C1–C2 123.4(2); for 2: N2–C1 1.267(1), C2–N1 1.324(1), C3–N1
1.365(1), C4–O1 1.362(1), N2–C1–C2 121.0(1).
solution of 4, the complex decomposes. As a result, very
few colourless crystals of the imidazolium salt with
[AgCl2]– as the counter ion suitable for X-ray diffraction
were obtained. The imidazolium dichloroargentate crys-
tallizes in the monoclinic space group P21/n (Figure 3). No
aggregation of the anions through argentophilic interac-
tions was observed. The parameters of the cation are very
similar to those of the chloride salt 3.
The molecular structure of 5 is depicted in Figure 4.
Interestingly, the aromatic moiety deviates considerably
from planarity (Figure 4, bottom). This deformation is
clearly a result of the steric repulsion between the oxygen
and gold atoms. Due to the geometrical constraint of the
ligand, there is an Au···O contact of 2.84(1) Å, which is well
below the sum of the van der Waals radii [r(Au) + r(O) =
3.18 Å].[27] However, its classification as a T-coordinated
complex, although not unknown in AuI complexes,[28] is not
justified in this case. We therefore infer that the AuI atom
is linearly coordinated to the carbene carbon atom and the
Figure 3. Molecular structure of the imidazolium cation of imid-
azolium dichlororargentate 3·AgCl. Selected bond lengths [Å] and
angles [°]: C1–N1 1.33(1), C1–N2 1.35(1), N2–C1–N1 108.2(5),
chloride [C1–Au1–Cl1 177.0(1)°] with Au1–C1 and Au1– Ag1–Cl1 2.308(2), Ag1–Cl2 2.318(2), Cl2–Ag1–Cl1 170.8(1).
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Cl1 distances of 1.99(1) and 2.285(2) Å, respectively, which
is within the range of reported values. The shortest Au–Au
distance is around 8.8 Å, longer than the aurophilicity limit
of approximately 3.5 Å. Thus, no π–π interactions of the
extended π system of the NHC moiety analogous to 3 are
present in the crystals.
Figure 5. Molecular structure of 6 (top) and the distorted coordina-
tion environment around the gold atom (bottom). The chloroform
molecule has been omitted for clarity. Selected bond lengths [Å]
and angles [°]: Au1–C1 2.010(6), Au1–Cl1 2.331(2), Au1–Br1
2.427(1), Au1–Br2 2.417(1), C1–N1 1.39(1), C1–N2 1.34(1). C1–
Au1–Cl1 175.7(2), C1–Au1–Br2 92.5(2), Cl1–Au1–Br2 90.40(4),
Br2–Au1–Br1 177.63(3), C1–N1–C2 111.8(5), N1–C1–Au1
122.3(5), N2–C1–Au1 129.8(5), O1–Au1–C1 73.92(4).
Figure 4. Molecular structure of 5. Illustration of the strong defor-
mation of the aromatic moiety (bottom). Selected bond lengths [Å]
and angles [°]: Au1–C1 1.99(1), Au1–Cl1 2.285(2), C1–N2 1.37(1),
C1–N1 1.34(1), C1–Au1–Cl1 177.0(1), N1–C1–N2 104.8(6), N1–
C1–Au1 123.1(5), N2–C1–Au1 130.9(6), C1–Au1–O1 70.8(5), Au1–
O1 2.84(1).
atoms in square-planar positions and the oxygen atom of
AuIII complex 6 crystallizes in the monoclinic space the ether group in an axial position (Figure 5). As observed
group P21/n and contains one complex and one chloroform for other mixed (NHC)AuXY2 (X, Y = halide) complexes,
molecule in the asymmetric unit. The gold atom is sur- the more electronegative chloride is in a trans position rela-
rounded by the carbene carbon atom C1, the three halogen tive to the carbene carbon atom.[21,23] All Au–C and Au–
Table 1. UV/Vis and emission spectroscopic data of solutions of compounds 3, 5, and 6.
Absorption: λmax [nm]
[log(ε/lmol–1 cm–1)]
Excitation: λmax [nm]
Emission: λmax [nm]
298 K
Quantum
yield [%]
298 K
77 K
77 K
3
233 [4.22], 262 [4.33], 270
[4.37], 280 [4.15], 291 [3.98],
303 [4.04], 318 [3.96], 331
[3.48][a]
332, 317, 303, 289, 329, 320, 315, 305,
337, 351, 368, 390 330, 346, 368,
(sh)[a,c] 385[b]
19[a]
272[a]
292, 279, 268, 264[b]
5
6
270 [4.00], 282 [4.13], 292
331, 317, 303, 289, 348, 330, 318, 294,
268[b] 292, 282[b]
353, 369, 393 (sh), 346, 363, 384, 521,
0.2[a]
[4.24], 321 [3.97][a]
490[a]
568, 621, 691[b]
346, 363, 420, 522,
568, 621, 690[b]
330 [3.99], 316 [4.08], 304
334, 318, 303, 293, 348, 330, 316, 304,
354, 369[b]
0.05[a]
[4.02], 285 [4.17], 275 [4.24][a] 290, 281, 268,
262[a]
293, 282[b]
[a] Dichloromethane. [b] Ethanol. [c] sh: shoulder.
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halide distances are in the expected range. Compared with
5, the Au···O distance decreases significantly to 2.717(1) Å,
presumably due to the higher charge of the coordinated
metal. Again, the orientation of the ligand towards the
metal indicates the low oxophilicity of gold: the aromatic
moiety is less twisted than in 5, but the angle between the
NHC moiety plane and the vector Au1–C1 is only around
160°. Unlike in non-distorted systems, the Au1–C1 bond
does not lie on the line bisecting the N1–C1–N2 angle in
the case of compound 6. It is clear that the geometry and
rigidity of the ligand imposes a square-pyramidal coordina-
tion environment, and the gold and oxygen atoms try to
avoid close contact. This is in contrast to a related amine-
functionalized NHC ligand in which a square-pyramidal
coordination was observed although the pendant amine was
not forced due to the rigidity of the ligand framework.[21]
Other examples of a square-pyramidal coordination envi-
ronment are documented for diimine-type ligands such as
phen and 2,2Ј-bipyridine in [(NʝN)AuCl3] complexes.[29]
This coordination geometry was also found for the corre-
sponding AuIII complex bearing ligand A.[17]
Figure 6. Electronic spectra of 3: (a) absorption, (b) emission and
(c) excitation spectra recorded at 298 K (c = 5.0ϫ10–5 m in dcm,
λexc. = 300 nm, λdet. = 390 nm) and (d) emission spectrum recorded
at 77 K and (e) excitation spectrum recorded at 77 K (c =
2.5ϫ10–6 m in ethanol, λexc. = 300 nm, λdet. = 380 nm).
clear: neither π–π stacking nor aurophilic interactions were
observed in the solid-state structure, although these may
exist transiently in solution. The solid-state structure of the
Electronic Spectra
The absorption spectrum of the imidazolium salt 3 shows imidazolium salt 3 shows that close contact of the π systems
a structured band with maxima at 270, 291, 303, 318 and of two NHC ligands is possible despite the bulky substitu-
331 nm, and two shoulders at 262 and 280 nm (Table 1). ents, whereas contact between the well-shielded gold atoms
Due to their high extinction coefficients, the signals can be appears unlikely. In the emission spectrum of a thoroughly
assigned to π–π* transitions, possibly masking additional degassed solution of 5, no further luminescence peaks
n–π* transitions. The emission spectrum at room tempera- evolved.
ture shows a well-resolved vibronic structure with three
At 77 K in ethanol glass, the luminescence spectrum of
main peaks at 337, 351 and 368 nm with a shoulder at 5 is dominated by structured long-wavelength emissions at
390 nm. A small Stokes shift of 540 cm–1 and an energy 521, 568 and 621 nm. The average spacing between the
E(0–0) of 30000 cm–1 can be derived for the first excited state. peaks is around 1545 cm–1, which roughly concurs with the
The emission quantum yield is 19%. As expected, the vib- tabulated ring-stretching frequencies of the aromatic mole-
ronic structure is much better resolved and slightly nar- cules. At higher energy, less intense emission bands from IL
rowed in the low-temperature luminescence spectra re- fluorescence and a broad peak (presumably of excimer na-
corded at 77 K, but no additional emission band evolves ture) are observed. Considering the huge Stokes shift (ca.
(Figure 6). This behaviour is typical of aromatic chromo- 9540 cm–1) and the structure of the LE band, we assign this
phores with an extended π system.
emission to an 3IL excited state. The very long emission
Because of its light sensitivity, no photophysical mea- decay time of the LE band of 1.90 ms at 77 K also supports
surements were performed on complex 4. The absorption this interpretation. A similar emission spectrum was ob-
spectrum of the NHC–AuI complex 5 shows a structured tained for a sample of neat 5 at 298 K (see Figure S1 in
band with maxima at 282, 292 and 321 nm with a shoulder the Supporting Information), which indicates the efficient
at 270 nm (Figure 7 and Table 1). As with 3, these bands quenching of the triplet state in liquid solution.
can be assigned to π–π* transitions that are in good agree-
At 77 K, dual emission or dominance of the phosphores-
ment with their high extinction coefficients; possible n–π* cence is not unusual for gold complexes bearing chromo-
transitions might be masked. The emission spectrum at phores with an extended π system, and has frequently been
room temperature displays a structured high-energy (HE) reported in recent years.[7,17,30,31] Despite the high atomic
band comparable to that of the imidazolium salt at 353, number of gold, the spin–orbit coupling promoted by the
369 and 393 (sh) nm, and a broad lower-energy (LE) emis- gold atom is comparably low and leads to relatively long
sion band at around 490 nm. Because the HE bands of 3 phosphorescence lifetimes at 77 K, but it is high enough to
and 5 nearly coincide, this emission can be assigned to IL allow for efficient S1–T1 intersystem crossing. This behav-
fluorescence. Interestingly, the ratio of the intensities of the iour can be traced back to the low-lying d orbitals, which
low- and high-energy bands decreases with decreasing con- are not involved in low-energy electronic transitions. As the
centration (Figure 8), which indicates an excimer character metal d orbitals do not directly participate in these transi-
of the LE band. The nature of the excimer is not completely tions, the spin–orbit coupling is moderate and comparable
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Figure 8. Concentration-dependent emission of 5 in ethanol at
298 K (not degassed, λexc. = 300 nm, normalized to the same inten-
sity at 353 nm).
Figure 7. Top: electronic spectra of 5 in ethanol: (a) absorption,
(b) emission and (c) excitation spectra recorded at 298 K (c =
8ϫ10–5 m, λexc. = 300 nm, λdet. = 370 nm) and (d) emission and
(e) excitation spectra recorded at 77 K (c = 2 10–4 m, λexc. = 300 nm,
λdet. = 570 nm). Bottom: electronic spectra of 6 in ethanol: (a) ab-
sorption, (b) emission and (c) excitation specttra recorded at 298 K
(c = 5ϫ10–5 m, λexc. = 300 nm, λdet. = 370 nm) and (d) emission
Figure 9. Irradiation of
a methanolic solution of 6 (c =
4.1ϫ10–5 m) with polychromatic light (λ Ͼ 305 nm). Dotted curve
shows the absorption of a methanolic solution of 5.
and (e) excitation spectra recorded at 77 K (c = 5ϫ10–5 m, λexc.
300 nm, λdet. = 520 nm).
=
maxima at 354 and 369 nm can be detected. Again, at 77 K
the spectrum is dominated by an LE emission with three
well-defined maxima at 522, 568 and 621 nm (spacing ca.
to the “external heavy atom effect” observed for solvents 1570 cm–1), which is comparable to the LE emission of 5
containing heavy elements.[7] It should be mentioned that and can be assigned to IL phosphorescence. The emission
Gray and co-workers postulated a non-emissive excimer decay time of the LE band is 1.51 ms at 77 K and supports
state to explain the concentration-dependent phosphores- this assignment. As expected for an AuIII halide, complex 6
cence intensities of some aryl–gold(I) complexes.[31–33] For is sensitive to light and is photoreduced to the AuI conge-
5, IL fluorescence, excimer emission and IL phosphores- ner. As a further deactivation path of the excited state ex-
cence can be observed at 77 K.
ists, the photoreactivity is also accompanied by a significant
NHC–AuIII bromide complexes usually feature a weak decrease in the emission quantum yield to 0.05%.[34] The
LMCT band at around 335 nm. However, this band is ob- photochemical reactivity of 6 was tested by irradiation of a
scured by the more intense π–π* transitions of the annu- methanolic solution with polychromatic light (λ Ͼ 305 nm).
lated NHC moiety in complex 6. The (pseudo-)square-py- As reported recently, NHC–AuIII bromides undergo photo-
ramidal coordination environment of 6 destabilizes the dz2 reductive elimination of Br2 to form the corresponding
orbital leading to a dd excited state as the lowest-energy NHC–AuI bromide.[17,23] Figure 9 illustrates the spectral
transition,[21] as indicated by a long absorption tail above changes of complex 6 upon irradiation for 10 min. Initially,
350 nm. At 298 K, only a weak emission band with two the spectrum shows LE bands at 330, 315, 304, 285 and
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spectra, the samples were degassed by at least three freeze–pump–
thaw cycles. The photoluminescence quantum yields were deter-
mined relative to quinine sulfate as standard. Luminescence life-
times were measured by using a Fluorolog FL-1040 phosphorim-
eter accessory. The estimated experimental errors are 5% in the
molar absorption coefficients and fluorescence lifetimes, and 15%
in the fluorescence quantum yields.[37] The irradiation experiments
were performed with an HBO 100 W lamp using polychromatic
light with λ Ͼ 305 nm (WG305 filter). Conductivities were mea-
sured with a four-electrode conductivity cell, model TetraCon 96
274 nm, HE absorption bands at 325 and 228 nm, and a
long dd absorption tail above 340 nm. The photoreductive
elimination of bromine occurs during the first seconds of
irradiation, and after 10 min the AuIII complex is com-
pletely photoreduced. The absorption spectrum of the re-
duced species features bands at 321, 391 and 281 nm, and
a shoulder at 325 nm. The absorption bands can be super-
imposed on those of the AuI complex 5. Likewise, the long
absorption tail vanishes upon irradiation, which is in ac-
cordance with the colour change of the solution from yel- (WTW). The reference sample [(NHC)2Ag][AgCl2] {NHC = 1-[2-
(diisopropylamino)ethyl]-3-methylimidazol-2-ylidene} was synthe-
sized in a previous study and the structure was verified by single-
crystal X-ray diffraction.[21] Concentrations of the reference and
complex 4 were identical.
low to colourless. Four isosbestic points, at 288, 297, 336
and 350 nm, are indicative of clean photoreduction.[35]
Single-crystal structure analysis of compounds 1, 3, 3·AgCl, 5 and
6 was carried out with a Bruker Smart X2S diffractometer and
single-crystal structure analysis of compound 2 was performed with
a Bruker D8 Quest diffractometer using Mo-Kα radiation (λ =
0.71073 Å). Further crystallographic and refinement data can be
found in Table 2. The structures were solved by direct methods
(SHELXS-97)[38] and refined by full-matrix least-squares on F2
(SHELXL-97).[39] The H atoms were calculated geometrically and
a riding model was applied during the refinement process. The
chloride anion of the imidazolium chloride 3 was found to be disor-
dered and no unequivocal atomic position could be determined.
Therefore this disordered atom was treated as a diffuse contri-
bution by using the SQUEEZE routine in the PLATON software
package.[40] The sqf file produced by PLATON, which contains the
SQUEEZE results, is appended to the cif file.
Conclusions
We have presented the synthesis and characterization of
an annulated NHC ligand that is based on the imidazo[1,5-
a]quinolinol scaffold. The benzyl-protected NHC precursor
was used to prepare the corresponding AgI, AuI and AuIII
complexes. The molecular structures of the AuI and AuIII
complexes determined by single-crystal X-ray diffraction re-
veal a considerable distortion of the rigid NHC moiety due
to steric repulsion of the gold and oxygen atoms. The emis-
sion properties of the gold complexes depend significantly
on temperature: at ambient temperature the complexes are
1
weakly emissive from an IL excited state, whereas at 77 K
the emission spectra are dominated by IL phosphorescence.
For the NHC–AuI complex, we observed concentration-de-
pendent emission spectra, indicative of excimer formation
in solution. Preparative work to deprotect the phenolic oxy-
gen and synthesize a potential mono-anionic carbene ligand
is underway.
CCDC-921406 (for 1), -921407 (for 2), -921408 (for 3), -921409 (for
3·AgCl), -921410 (for 5) and 932544 (for 6) contain the supplemen-
tary crystallographic data. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
N-(2Ј,6Ј-Diisopropylphenyl)-8-(benzyloxy)quinolin-2-ylmethylen-
imine (2): 8-(Benzyloxy)quinoline-2-carbaldehyde (1; 1.60 g,
6.1 mmol) was dissolved in ethanol (5 mL) upon heating and 2,6-
diisopropylaniline (purity 92%, 2 mL, 9.7 mmol) was added. Sub-
sequently a yellow precipitate formed, which was purified by
Experimental Section
General: All reactions and manipulations of air- and/or moisture-
sensitive compounds were carried out in dry nitrogen using stan-
dard Schlenk techniques. Toluene was dried and distilled from Na.
All other solvents and reagents were commercially available and
used as received. [(dms)AuCl] (dms = Me2S) was synthesized from
gold, HCl, and dmso following a published procedure.[36] 8-Benzyl-
oxyquinoline-2-carbaldehyde (1) was prepared from 2-methyl-8-
benzyloxyquinoline by oxidation with SeO2 following a published
procedure.[18]
recrystallization in ethanol, yield 2.17 g (83%). 1H NMR
3
(300 MHz, CDCl3): δ = 8.52 (s, 1 H, HC=N), 8.39 (d, JHH
=
3
8.6 Hz, 1 H), 8.16 (d, JHH = 8.6 Hz, 1 H), 7.48–7.45 (m, 2 H),
7.37 (d, JHH = 4.8 Hz, 2 H), 7.29–7.21 (m, 2 H), 7.16–7.02 (m, 2
H), 5.28 (s, 2 H, O-CH2), 2.95 [sept, JHH = 6.8 Hz, 2 H, CH-
(CH3)2], 1.10 [d, JHH = 6.8 Hz, 12 H, CH(CH3)2] ppm. 13C NMR
(75 MHz, CDCl3, 30 °C): δ = 163.8, 154.8, 153.6, 148.5, 140.3,
137.1, 136.9, 136.7, 130.4, 128.6, 128.0, 127.8, 127.0, 124.4, 123.0,
120.1, 118.7, 111.0, 71.1, 28.0, 23.4 ppm. MS (ESI pos): m/z = 445
[C29H30N2ONa]+, 423 [C29H31N2O]+. C29H30N2O (422.57): calcd.
C 82.43, H 7.16, N 6.63; found C 82.07, H 7.13, N 6.65.
Elemental analyses were carried out by the Institute for Chemical
Technology of Organic Materials, Johannes Kepler University
Linz. NMR spectra were recorded with a Bruker Digital Avance
DPX 200 (200 MHz), Avance III (300 MHz) or Avance DRX 500
(500 MHz) spectrometer. 1H and 13C NMR shifts are reported
in ppm relative to Si(CH3)4 and are referenced internally to the
residual signal of the deuteriated solvent. Mass spectra were re-
corded with a Finnigan LCQ DecaXPlus ion-trap mass spectrome-
ter with an ESI ion source.
Iimidazolium Chloride Derivative 3: The reaction was carried out in
dry nitrogen. Paraformaldehyde (0.35 g, 11.2 mmol) was com-
pletely dissolved in hot toluene (30 mL) and N-(2Ј,6Ј-diiso-
propylphenyl)-8-benzyloxyquinolin-2-ylmethylenimine
(4.72 g,
11.2 mmol) was added. A 4 m HCl solution in 1,4-dioxane (2.8 mL,
11.2 mmol) was added dropwise and a red precipitate formed im-
mediately. After stirring for 15 h at ambient temperature, the pre-
cipitate was filtered and washed with diethyl ether (5 mL). To re-
move unreacted paraformaldehyde, the residue was dissolved in
dichloromethane and filtered. The solvent was removed under re-
Spectroscopic-grade solvents were used for all photophysical char-
acterizations. Absorption spectra were recorded with a Varian Cary
300 double-beam spectrometer. Emission spectra at 300 and 77 K
were recorded with a Jobin–Yvon Fluorolog 3 steady-state fluores-
cence spectrometer. Before recording the emission and excitation
duced pressure, and
a pale-yellow residue was obtained.
Eur. J. Inorg. Chem. 0000, 0–0
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FULL PAPER
Table 2. Crystal data and data collection and structure refinement details for compounds 1, 2, 3, 3·AgCl, 5 and 6..
1
2
3
3·AgCl
5
6
Formula
Mw
C17H13NO2
263.28
C29H30N2O
422.55
C30H31N2O
435.57
C30H31AgCl2N2O C30H30AuClN2O
C31H31AuBr2Cl4N2O
946.16
614.34
666.98
Crystal size
[mm]
0.37ϫ0.43ϫ0.71
0.118ϫ0.263ϫ1.759
0.13ϫ0.40ϫ0.63
0.05ϫ0.18ϫ0.50
0.13ϫ0.31ϫ0.53
0.05ϫ0.07ϫ0.51
Crystal system
Space group
a [Å]
b [Å]
c [Å]
monoclinic
P21/c
8.1562(8)
18.3255(18)
9.1463(8)
90
orthorhombic
Pbca
16.667(1)
12.114(1)
23.607(1)
90
monoclinic
P21/c
12.4346(10)
12.4728(9)
17.1958(15)
90
monoclinic
P21/n
14.672(2)
11.7643(15)
16.271(2)
90
monoclinic
P21/n
10.4332(11)
18.705(2)
13.7404(14)
90
monoclinic
P21/n
10.1530(9)
23.547(3)
13.9529(13)
90
α [°]
β [°]
102.173(3)
90
1336.3(2)
1.309
4
0.09
90
90
4766.3(3)
1.178
8
96.396(3)
90
2650.4(4)
1.092
4
0.07
101.422(4)
90
2752.8(7)
1.482
4
0.95
94.888(4)
90
2671.7(5)
1.658
4
5.63
90.492(3)
90
3335.6(6)
1.884
4
7.15
γ [°]
V [Å3]
ρcalcd. [gcm–1]
Z
μ [mm–1]
T [K]
0.07
100
200
201
200
200
200
θ range [°]
Reflections col- 8478
lected
3.2–25.0
2.1–25.0
101156
1.7–24.7
16287
1.7–20.1
32604
3.0–25.0
17049
2.2–25.3
64151
Unique reflec-
tions
2339
4189
4488
2579
4704
5955
Observed reflec- 1785
3710
2996
1962
3456
4382
tions [IϾ2σ(I)]
Parameters re-
fined/restrained
181/0
294/0
303/0
329/0
320/0
374/0
Absorption cor- multi-scan
rection
multi-scan
multi-scan
multi-scan
multi-scan
multi-scan
Tmin, Tmax
σfin (max/min)
[eÅ–3]
0.94, 0.97
0.19/–0.16
0.7160, 0.7459
0.24/–0.19
0.96, 0.99
0.31/–0.15
0.65/0.95
0.73/–0.32
0.15, 0.53
1.09/–0.99
0.12, 0.72
0.82/–0.84
R1 [IՆ2σ(I)]
wR2
0.040
0.108
0.032
0.084
0.061
0.166
0.037
0.095
0.041
0.103
0.036
0.084
Recrystallization in ethanol gave 3 as a white solid, yield 0.33 g
Gold(I) Complex 5: Compound 4 (70 mg, 0.12 mmol) was dissolved
in dcm (5 mL). Solid [(dms)AuCl] (35 mg, 0.12 mmol) was added
to the stirred solution, and a precipitate of AgCl formed immedi-
(61%). Slow gas-phase diffusion of diethyl ether into a diluted dcm
1
solution gave colourless needles suitable for X-ray diffraction. H
NMR (300 MHz, [D6]dmso, 30 °C): δ = 10.83 (s, 1 H, N-CH-N), ately. After stirring for 60 min at ambient temperature, the AgCl
8.61 (s, 1 H), 7.79–7.50 (m, 10 H), 7.33–7.23 (m, 3 H), 5.66 (s, 2
H, O-CH2), 2.33 [sept, JHH = 6.8 Hz, 2 H, CH(CH3)2], 1.19–1.15
[m, 12 H, CH(CH3)2] ppm. 13C NMR (75 MHz, [D6]dmso, 30 °C):
δ = 148.7, 145.1, 136.0, 132.6, 131.5, 131.2, 129.7, 128.8, 128.4,
128.1, 127.8, 127.1, 126.9, 124.3, 121.0, 120.0, 116.9, 115.9, 114.0,
69.7, 27.9, 23.9 ppm. HRMS (ESI): calcd. for C30H31N2O 435.2431
[C30H31N2O]+; found 435.2424.
was filtered off. The product was precipitated with diethyl ether as
a yellow solid. Slow gas-phase diffusion of pentane into a diluted
dcm solution gave yellow crystals of 5 suitable for X-ray diffraction,
1
yield 40 mg (50%). H NMR (300 MHz, CDCl3, 30 °C): δ = 7.46–
7.29 (m, 4 H), 7.22–7.07 (m, 10 H), 5.38 (s, 2 H, O-CH2), 2.23
[sept, JHH = 6.8 Hz, 2 H, CH(CH3)2], 1.12 [d, JHH = 6.8 Hz, 6 H,
CH(CH3)2], 1.02 [d, JHH = 6.8 Hz, 6 H, CH(CH3)2] ppm. 13C
NMR (75 MHz, CDCl3, 30 °C): δ = 175.6, 150.5, 145.6, 135.9,
135.5, 131.1, 130.6, 128.6, 128.4, 128.3, 128.0, 127.4, 126.0, 124.1,
123.1, 120.4, 115.4, 114.2, 114.0, 70.4, 28.5, 24.4, 24.0 ppm. 1H
NMR (500 MHz, [D6]dmso, 30 °C): δ = 8.21 (s, 1 H), 7.54–7.25
(m, 13 H), 5.37 (s, 2 H, O-CH2), 2.16 [sept, JHH = 6.8 Hz, 2 H,
CH(CH3)2], 1.09–1.06 [m, 12 H, CH(CH3)2] ppm. MS (ESI pos):
m/z = 1297 [C60H60N4O2Au2Cl]+, 668 [C30H31N2OAuCl]+, 631
[C30H30N2OAu]+, 435 [C30H31N2O]+. C30H30AuClN2O (667.00):
calcd. C 54.82, H 4.53, N 4.02; found C 54.86, H 4.41, N 3.88.
Silver(I) Complex 4: In a flask covered with aluminium foil, 3
(0.210 g, 0.45 mmol) was dissolved in dcm (5 mL). Ag2O (0.110 g,
0.47 mmol) was added whilst stirring, and a precipitate of AgCl
formed immediately. After stirring for 3 h at ambient temperature,
the reaction mixture was filtered. The solvent was removed in
vacuo, the residue was dissolved in a small amount of dcm, and
diethyl ether was added to precipitate 4 as a light-brown solid, yield
1
0.190 g (74%). H NMR (300 MHz, [D6]dmso, 30 °C): δ = 8.28 (s,
1 H), 7.65–7.42 (m, 11 H), 7.31–7.29 (m, 3 H), 5.64 (s, 2 H, O-
CH2), 2.27 [sept, JHH = 6.7 Hz, 2 H, CH(CH3)2], 1.21 [d, JHH
=
Gold(III) Complex 6: Compound 5 (100 mg, 0.15 mmol) was dis-
6.7 Hz, 6 H, CH(CH3)2], 1.16 [d, JHH = 6.7 Hz, 6 H, CH- solved in dcm (5 mL) and the solution cooled to –50 °C (2-prop-
(CH3)2] ppm. 13C NMR (75 MHz, [D6]dmso, 30 °C): decomposi-
tion during measurement. MS (ESI pos): m/z 977
anol/liquid nitrogen). A solution of Br2 in dcm (0.25 mL of a
=
0.623 m Br2 solution, 0.16 mmol) was added whilst stirring. Sub-
[C60H60N4O2Ag]+, 543 [C30H30N2OAg]+, 435 [C30H31N2O]+. sequently the reaction mixture turned red. The solution was slowly
C30H30AgClN2O (577.90): calcd. C 62.35, H 5.23, N 4.85; found C
62.63, H 5.24, N 4.85.
allowed to warm to room temperature over 4 h. The solvent was
removed under reduced pressure and the residue re-crystallized
Eur. J. Inorg. Chem. 0000, 0–0
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/KAP1
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FULL PAPER
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from dcm/diethyl ether to yield a red powder. Slow gas-phase dif-
fusion of diethyl ether into a dilute chloroform solution gave
bright-red crystals suitable for X-ray diffraction, yield 100 mg
(80%). 1H NMR (300 MHz, CDCl3, 30 °C): δ = 7.52–7.47 (m, 2
H), 7.38–7.29 (m, 4 H), 7.24 (m, 2 H), 7.21–7.14 (m, 6 H), 5.71 (s,
2 H, O-CH2), 2.59 [sept, JHH = 6.7 Hz, 2 H, CH(CH3)2], 1.38 [d,
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126.7, 126.3, 123.5, 122.2, 120.4, 117.9, 114.9, 113.7, 69.8, 28.0,
25.7, 22.1 ppm. MS (ESI pos): m/z = 1343 [(LAu)2Br]+, 1297
[(LAu)2Cl]+, 631 [C30H30N2OAu]+. C30H30AuBr2ClN2O·CHCl3
(946.19): calcd. C 39.35, H 3.30, N 2.96; found C 39.73, H 3.08, N
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were acquired in collaboration with the University of South Bohe-
mia (CZ) with financial support from the European Union (EU)
through the EFRE INTERREG IV ETC-AT-CZ programme (pro-
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Published Online: ᭿
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9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Job/Unit: I30575
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Date: 18-09-13 18:08:28
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FULL PAPER
NHC Gold Complexes
M. Kriechbaum, G. Winterleitner,
A. Gerisch, M. List,
U. Monkowius* ............................... 1–10
Silver(I), gold(I) and gold(III) complexes
bearing an NHC ligand based on imid-
azo[1,5-a]quinolinol have been synthesized.
The molecular structures of the gold com-
plexes reveal distortion of the rigid NHC
moiety due to steric repulsion between the
gold and oxygen atoms. At room tempera-
ture, the complexes are weakly fluorescent,
whereas at 77 K the emission spectra are
dominated by phosphorescence.
Synthesis, Characterization and Lumin-
escence of Gold Complexes Bearing an
NHC Ligand Based on the Imidazo[1,5-a]-
quinolinol Scaffold
Keywords: Nitrogen heterocycles / Carb-
enes / Gold / Silver / Luminescence
Eur. J. Inorg. Chem. 0000, 0–0
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim