Synthesis and characterization of silver(I) and gold(I) complexes bearing a pyrido-annelated N-heterocyclic carbene: A rare example of a cocrystal containing two different gold(I) complexes

Article abstract of DOI:10.5560/ZNB.2014-4175

The cyclohexyl-substituted imidazo[1,5-a]pyridin-2-ium hexafluorophosphate, 2a, has been prepared as precursor for the respective pyrido-annelated N-heterocyclic carbene. [(NHC)₂Ag]PF₆, 3, has been synthesized by the reaction of 2a w

Full text of DOI:10.5560/ZNB.2014-4175

Synthesis and Characterization of Silver(I) and Gold(I) Complexes
Bearing a Pyrido-annelated N-Heterocyclic Carbene: A Rare Example of
a Cocrystal Containing Two Different Gold(I) Complexes
Margit Kriechbauma, Daniela Ottea, Manuela Listb, and Uwe Monkowiusa
a
Institute of Inorganic Chemistry, Johannes Kepler University Linz, Altenbergerstr. 69, 4040
Linz, Austria
b
Institute of Chemical Technology of Organic Materials, Johannes Kepler University Linz,
Altenbergerstr. 69, 4040 Linz, Austria
Reprint requests to Dr. U. Monkowius. E-mail: uwe.monkowius@jku.at
Z. Naturforsch. 2014, 69b, 1188 – 1198 / DOI: 10.5560/ZNB.2014-4175
Received August 4, 2014
Dedicated to Professor Hubert Schmidbaur on the occasion of his 80th birthday
The cyclohexyl-substituted imidazo[1,5-a]pyridin-2-ium hexafluorophosphate, 2a, has been pre-
pared as precursor for the respective pyrido-annelated N-heterocyclic carbene. [(NHC)2Ag]PF6, 3,
has been synthesized by the reaction of 2a with AgCl/KOH in dichloromethane (DCM). Unex-
pectedly, the reaction of 3 with (tht)AuBr yielded both (NHC)AuBr and [(NHC)Au(tht)]PF6 which
formed a 1 : 1 cocrystal (4a,b). The complexes are aggregated to infinite chains, which are governed
by both π-π stacking and weak aurophilic interactions. Subsequent oxidation of the co-crystalline
material with CsBr3 gave (NHC)AuBr3, 5. All compounds were characterized by NMR spectroscopy,
mass spectrometry and elemental analysis. Additionally, compounds 2a and 5 were investigated by
electronic spectroscopy: 2a behaves like a typical aromatic compound exhibiting absorption and flu-
orescence bands attributable to π-π∗ transitions. The Au(III) complex 5 exhibits ligand-centered flu-
orescence at room temperature and both ligand-centered fluorescence and a weak phosphorescence
at 77 K.
Key words: Silver(I), Gold(I), N-Heterocyclic Carbenes, Crystal Structure, Cocrystal
Introduction
ties [32, 33]. NHC-Au(I) complexes are usually pre-
pared by the reaction of (R2S)AuX (R2S = Me2S,
Silver and gold complexes bearing N-heterocyclic tetrahydrothiophene; X = halide) with a free carbene
carbenes (NHC) have been in the focus of intense stud- or by the ‘silver salt’ method using (R2S)AuX and the
ies for over a decade [1 – 8]. Due to the fact that NHC- respective NHC-Ag complex [34].
Ag(I) complexes are versatile carbene transfer agents,
However, the ‘silver salt’ method has a serious dis-
they have been synthesized as stable intermediates in advantage: The stoichiometry of the resulting NHC-
the course of the preparation of many other transition Au(I) complex cannot be controlled by the amount
metal NHC complexes [9 – 11]. They are conveniently of the NHC-Ag(I) complex. Usually, regardless of
accessible by the reaction of an imidazolium salt and the actual amount of the silver reagent, either a neu-
Ag2O and feature a rich coordination chemistry. NHC- tral or an ionic complex of the type (NHC)AuX or
Au(I) complexes have been investigated due to numer- [(NHC)2Au]X is formed. Often, the ionic form is
ous applications ranging from homogeneous catalysts yielded in cases where the substituent at the N atoms
for unique C–C, C–O and C–N bond-forming reac- of the heterocycle are functionalized by heteroatoms.
tions [12 – 22] to pharmaceutical uses as anticancer, The polarity of the solvent also plays a role as has been
antiarthritis and antibacterial agents [23 – 31]. In ad- proven by very elegant NMR-spectroscopic experi-
dition, NHC-Au(I) and -Au(III) complexes which con- ments [35, 36]. A further shortcoming of the method
tain an NHC ligand with an extended π system fea- is the unknown composition of the NHC-Ag complex
ture interesting photophysical and -chemical proper- if no further structural information is available. This is
© 2014 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
1189
a critical issue as the NHC-Ag complex is frequently be ideally suited for the formation of cocrystals. In-
used merely as an intermediate and is not isolated in deed, aurophilic interactions have been utilized to syn-
many cases.
thezise homo- and heterometallic clusters from two
In a recent publication we described this prob- different metal complexes [59]. However, the ‘molec-
lem for the related pyrido-annelated NHC with ular entities’ are usually completely merged into the
a diisopropyl-phenyl- and a dimethyl-phenyl-substitu- clusters. Likewise, salts containing complex cations
ted imidazo[1,5-a]pyridine-3-ylidene [37]: For the and anions formed from neutral complexes by ligand
methyl derivative, we isolated the neutral complex scrambling reactions are no cocrystals in a narrow def-
(NHC)AuBr, whereas for the isopropyl derivative the inition.
ionic complex [(NHC)2Au]PF6 was obtained. In this
contribution, we present the very unusual formation Results and Discussion
of a cocrystal containing the two discrete metal com-
plexes (NHC)AuBr and [(NHC)Au(tht)]PF6 (tht = Synthesis
tetrahydrothiophene).
The definition of a ‘cocrystal’ is still under de-
The synthesis of the NHC precursor 2a started with
bate [38 – 44]. In general, it denotes a crystalline ma- the condensation reaction of 2-pyridyl-carbaldehyde
terial formed by two or more different molecular enti- and cyclohexyl-amine to give the imine 1 [60]. Its
ties. Hence, this definition also includes many types of condensation with CH2O/HCl in dry toluene yielded
compounds like hydrates, solvates and clatherates, etc. 2-cyclohexyl-imidazo[1,5-a]pyridine-2-ium chloride.
It was suggested to use ‘cocrystal’ as a synonym for Precipitation from aqueous solution as hexafluo-
a ‘multi-component molecular crystal’, a denotation rophosphate gave pure 2a. If some formaldehyde re-
which is somewhat inconvenient. However, the ques- mained un-dissolved in the reaction mixture before
tion is still open whether an ionic material which has HCl was added, considerable amounts of 2b were
been formed by proton transfer is a cocrystal, a salt formed. Upon cyclization, water is formed, which
or both. There are several nuances between fully pro- can hydrolyze 1. Either the imine 1 or the recov-
tolyzed on one side and completely separated, neutral ered 2-pyridyl-carbaldehyde can efficiently substitute
components which are ‘just’ connected by hydrogen the formaldehyde in the cyclization reaction and fa-
bonds on the other side [45]. Nevertheless, ‘crystal cilitate the formation of the side-product 2b. The
engineers’ frequently use the combination of organic ionic NHC-silver complex [(NHC)2Ag]PF6 is acces-
acids and bases as a versatile method for the prepara- sible via the reaction of in situ generated Ag2O from
tion of new materials and call them ‘cocrystals’ even KOH/AgCl with 2a. Stoichiometric reaction of 2a
though they are actually salts [46]. There are numer- with (tht)AuBr gives a mixture of two complexes,
ous examples of such salts/cocrystals formed by the [(NHC)Au(tht)]PF6 and [(NHC)AuBr] (4a and 4b),
(de)protonation of active pharmaceutical ingredients from which cocrystals are obtained upon recrystalliza-
to alter their bioavailability, which might be advanta- tion from DCM/Et2O. Finally, the oxidation of 4a,b
geous from the point of intellectual property [39].
with CsBr3 yields (NHC)AuBr3, 5. The reaction se-
By contrast, cocrystals composed of two or more quence is depicted in Scheme 1.
1
metal complexes are rare [47 – 58]. Similar to cocrys-
The identity of all compounds is supported by H,
tals of purely organic compounds, weak interactions 13C NMR spectroscopy, mass spectrometry and ele-
like hydrogen bonds can also be utilized to im- mental analysis. Upon complex formation, the signal
pel cocrystallization of different complex molecules. of the acidic NC(H)N proton at 9.78 ppm vanishes.
1
Furthermore, metal atoms offer second-coordination In the H NMR spectrum of 4a,b two sets of signals
sphere interactions or metallophilic interactions as ad- in the aromatic region are partly resolved. By com-
ditional driving forces for the self-organization in the parison with the known NMR data of (NHC)AuBr
solid state. Metallophilic interactions are particularly complexes bearing homologous imidazo-pyridine-2-
important for low-coordinate heavy metal atoms. Be- yl ligands reported just recently by us [37], the
cause metallophilic interactions reach their maximum set with the sharp signals can be assigned to 4a,
for linearly coordinated Au(I) complexes and are in the whereas the broad peaks are aromatic protons of 4b.
range of the strength of hydrogen bonds, they should The 13C resonances are shifted from 125.3 (2a) to
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1190
M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
Scheme 1.
170.4 ppm (3). The 13C NMR signals of the car- plex 2a crystallizes in the monoclinic space group
bene carbon atoms are found at 173.5 (4a), 162.6 P21/n with two formula units in the asymmetric unit
(4b) and 121.9 ppm (5). In the ESI mass spectra of (Fig. 1). Despite the Coulombic repulsion, the cations
the imidazo-pyridinium salts, the dominating species are aggregated to columns via relatively short π-π
are the cations with m/z = 201 (2a) and 278 (2b). interactions of about 3.3 Å. Complex 2b crystallizes
For the silver complex the ions [(NHC)2Ag]+ (507) in the triclinic spa−ce group P1 with Z = 2. In both
and [(NHC)Ag]+ (307) are detected. The mass spec- structures, the PF6 anions are disordered. The bond
trum of 4a,b shows a peak representing the cation lengths and angles are as expected.
[(NHC)2Au]+ (597) which results from ligand ex-
Complex 3 crystallizes in the monoclinic space
change reactions. The cation [(NHC)Au(tht)]+ could group P21/c with Z = 4 (Fig. 2). The Ag atom is lin-
not be detected, presumably because of the facile dis- early coordinated by the carbon atoms of the NHC
sociation of the tht ligand. The MS spectrum of 5 ligands [C1–Ag1–C14 174.4(3)◦]. With an angle of
shows signals for the cations [(NHC)2Au]+ and the ∼ 11◦ between the two NHC planes, the ligands are
bromonium ion [{(NHC)Au}2Br]+ (875). The mass not completely coplanar. Again, π-π stacking interac-
spectroscopic detection of halonium ions [(LAu)2X]+ tions are present. The Ag–C bond lengths are 2.063(9)
(X = Cl, Br) is common for phosphane gold(I) com- and 2.075(9) Å and comparable to other reported Ag–
plexes, but is relatively rare for L = NHC [61 – 63]. It C bond lengths [64]. There are no close argentophilic
should be noted that for neutral complexes of the form contacts between the silver atoms.
(NHC)AuX and (NHC)AuXY2 (X, Y = Cl, Br), the
Cocrystals of 4a and 4b crystallize in the mono-
cation [(NHC)2Au]+ is the dominant signal in the ESI clinic space group Cc with Z = 4 (Fig. 3). The gold
mass spectra, whereas the signal for [(NHC)AuBr2]+ atoms are linearly coordinated with angles of 179.4(4)
is never and the one for [(NHC)Au]+ is not very of- and 174.3(4)◦ for C1–Au1–Br1 and C14–Au2–S1, re-
ten observed or very weak. The reason for the ab- spectively. Obviously, the second ligand at the gold
sence of an Au(III) signal is the easy reductive elimi- atom does not strongly affect the Au–C bond lengths
nation of Br2 and the high stability of the formed Au(I) as they are very similar [2.03(1) vs. 2.00(1) Å] and
species [34, 37, 62].
comparable to reported values [34]. The Au–S bond
length [2.304(4) Å] is almost identical to the bond
length found in the only published example of an
NHC-Au complex bearing a tht ligand (2.304 Å) [65]
Structural studies
The molecular structures of compounds 2–4 were but slightly shorter than in analogous phosphane com-
determined by single-crystal X-ray diffraction. Com- plexes (2.33 – 2.34 Å) [61, 66 – 69]. The cation 4a and
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
1191
Fig. 1. (a) One of the two cations in crystals of 2a and (b) of 2b; (c) aggregation of the cations in 2a via π–π stacking
(ellipsoids drawn at the 50% probability level). Selected bond lengths (Å) and angles (deg) for 2a: C1–N1 1.331(4), C1–N2
1.356(4); N1–C1–N2 107.7(2), parameters of the second cation (not shown): C16–N3 1.346(4), C16–N4 1.324(4); N3–C16–
N4 108.0(3); 2b: C1–N1 1.368(5), C1–N2 1.352(5); N1–C1–N2 106.3(3); N1–C1–C14–N3 132(1).
4b are synergistically connected to linear chains by and of the mixture 4a,b were found to be too light-
both π-π interactions and relatively long Au···Au sensitive, and no reproducible emission spectra could
contacts, the latter is indicative of rather weak au- be obtained. Not unexpectedly, the spectra of 2a and
rophilic interactions [Au1···Au2 3.498(1), Au1i···Au2 5 are very similar to the ones containing the same
3.591(1) Å]. The torsion angle C1–Au1–Au2–C14 of imidazo-pyridine chromophore reported by us just re-
72.7◦ leads to a ‘crossed swords’ motif. The re- cently [37].
ported cocrystal [AuL2]+[LAuCl][AuCl2]− (L = 2-
The NHC precursor 2a features intense and struc-
amionopyridine) is similar to 4a,b in the sense that tured absorption bands which can be attributed to π-
it is also composed of a neutral complex and a com- π∗ transitions. At room temperature the compound ex-
plex salt which is stabilized via cooperative action of hibits a broad, unstructured emission band at 367 nm.
aurophilic and π-π interactions (and N–H···Cl hydro- The vibronic structure is much better resolved at
gen bonds) [57]. It should be noted that to our knowl- 77 K. The maxima in both excitation spectra (r. t. and
edge only one example of a cocrystal with two different 77 K) are superimposable with the absorption spec-
NHC-gold complexes is known [58].
trum (Fig. 4, Table 1). Fig. 5 shows the electronic spec-
tra of the Au(III) complex 5 in ethanolic solution.
The high-energy absorption is structured in the UV
range below 350 nm similar to that of the NHC precur-
Photophysical characterization
Compounds 2a and 5 were investigated by elec- sor, whereas the low-energy absorption band is weak
tronic spectroscopy. Solutions of the silver complex 3 and long-tailing. The latter band can be attributed
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
Fig. 3. Aggregation of−the gold complexes in crystals of
Fig. 2. Molecular structure of the cation in crystals of 3
(ellipsoids drawn at the 50% probability level). Selected
bond lengths (Å) and angles (deg): C1–Ag1 2.063(9), C14–
Ag1 2.075(9), N1–C1 1.374(10), N2–C1 1.382(10), N3–C14
1.356(9), N4–C14 1.365(10); C1–Ag1–C14 174.4(3), N2–
C1–N1 102.3(7), N3–C14–N4 102.7(7).
4a,b (H atoms and PF6 anion omitted for clarity, ellipsoids
drawn at the 50% probability level). Selected bond lengths
(Å) and angles (deg): Au1–C1 2.03(1), Au2–C14 2.00(1),
Au1–Br1 2.389(2), Au2–S1 2.304(4), Au1–Au2 3.498(1),
Au1i–Au2 3.591(1); C1–Au1–Br1 179.4(4), C14–Au2–S1
174.3(4), C1–Au1–Au2–C14 65.2(5).
(d) Excitation,
77K
λdet.= 420 nm
(c) Emission, 77K
λexc.= 280 nm
(e) Excitation,
298 K
λdet.=
420 nm
(b) Emission, 298 K
λexc.= 280 nm
(a) Absorption
250
300
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 4 (color online). Electronic spectra of 2a (c = 3.8×10−5 M in ethanol): (a) absorption, (b) emission and (e) excitation
spectra recorded at 298 K (λexc. = 280 nm, λdet. = 420 nm) and (c) emission and (d) excitation spectra recorded at 77 K
(λexc. = 280 nm, λdet. = 420 nm).
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
1193
Table 1. UV/Vis and emis-
sion spectral data of com-
pounds 2a and 5.
Compound
Absorption, λmax (nm)
Excitation, λmax (nm)
Emission, λmax (nm)
298 K 77 K
367 (broad) 334, 350,
366, 387,
(log{ε/l mol−1 cm−1})
298 K
77 K
2a
226 (3.58), 233
(3.52), 241 (3.35),
260 (3.67), 271
(3.87), 281 (3.87),
298 (3.61), 310
(3.55), 323 (3.29)
235 (4.56), 259
(4.08), 270 (4.05),
281 (3.94), 310
(3.66), 325 (3.68),
341 (3.65)
255, 258,
270, 280,
297, 310,
325
260, 268,
279, 302
314, 326
411 (sh)
5
275, 297,
310
318, 342
(sh)
390 (broad) 360, 375,
395, 507,
528, 549,
576, 597,
631
(d) Excitation, 77 K
λdet.= 420 nm
(c) Emission, 77 K
λexc.= 300 nm
x 10
(e) Excitation,
298 K
λdet.= 420 nm
(b) Emission, 298 K
λexc.= 300 nm
(a) Absorption
300
400
Wavelength (nm)
500
600
Fig. 5 (color online). Electronic spectra of 5: (a) absorption (c = 6.4×10−5 M in ethanol), (b) emission and (e) excitation
spectra recorded at 298 K (c = 1.0×10−4 M in degassed ethanol, λexc. = 300 nm, λdet. = 420 nm) and (c) emission and (d)
excitation spectra recorded at 77 K (c = 1.0×10−4 M in ethanol glass, λexc. = 300 nm, λdet. = 420 nm).
to n(Br) → 5dx2−y2 and π(Br) → 5dx2−y2 ligand-to- Conclusion
metal charge transfer (LMCT) states. The emission at
room temperature has a maximum at 390 nm. At 77 K
a second, very weak emission at lower energy evolves
In this contribution we have presented the un-
with a maximum centered around 550 nm. The sim- expected cocrystallization of (NHC)AuBr and
ilarity between the band shapes of 2a and 5 allows [(NHC)Au(tht)]PF6 (4a,b) with NHC = imidazo[1,5-
the high-energy emission to be attributed to ligand- a]pyridin-3-ylidene. The aggregation of the complexes
centered fluorescence whereas the low-energy emis- in the crystal is governed by both π-π stacking
sion is ligand-centered phosphorescence. A compara- and weak aurophilic interactions. The Au(III) com-
ble behavior is frequently found for Au(I) and Au(III) plex 5 exhibits ligand-centered fluorescence at r. t.
complexes bearing ligands with an extended π chro- and both ligand-centered fluorescence and a weak
mophore [32, 37, 62, 70].
phosphorescence at 77 K.
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
Experimental Section
in a small amount of water. A solution of KPF6 is added to
precipitate a bright-orange solid, which is purified by crys-
tallization from dichloromethane (DCM) and diethyl ether.
Yield: 2.25 g (20%). Slow gas-phase diffusion of diethyl
ether into a DCM solution gives yellow crystals of 2a suit-
able for X-ray diffraction. – 1H NMR (300 MHz, DMSO):
General
All reactions and manipulations of air- and/or moisture-
sensitive compounds were carried out in an atmosphere of
dry nitrogen using standard Schlenk techniques. Toluene
was dried and distilled from Na. All other solvents and
reagents were commercially available and used as received.
(tht)AuBr (tht = tetrahydrothiophene) was synthesized ac-
cording to a published procedure from gold, bromine and
tetrahydrothiophene [71, 72]. Pyridine-2-carboxaldehyde is
commercially available and used as received.
3
δ[ppm] = 9.78 (s, 1H), 8.53 (d, 1H, JHH = 7 Hz), 8.35
3
(s, 1H), 7.84 (d, 1H, JHH = 9 Hz), 7.30 – 7.24 (m, 1H),
7.20 – 7.15 (m, 1H), 4.66 – 4.56 (m, 1H), 2.20 – 2.16 (m, 2H),
1.91 – 1.69 (m, 5H), 1.52 – 1.24 (m, 3H). – 13C{1H} NMR
(75 MHz, DSMO): δ[ppm] = 129.3, 125.4, 124.6, 124.3,
118.1, 117.3, 111.7, 60.0, 32.9, 24.6, 24.4. – MS ((+)-
ESI): m/z = 201 [C13H17N2]+. – MS ((–)-ESI): m/z = 145
[PF6]−. – C13H17N2PF6 (346.26): calcd. C 45.09; H 4.95, N
8.09; found C 45.61, H 4.81, N 8.35.
Elemental analyses were carried out by the Institute of
Chemical Technology of Organic Materials at the Univer-
sity Linz. NMR spectra were recorded on a Bruker Avance
III (300 MHz) spectrometer. H and 13C shifts are reported
1
2-Cyclohexyl-3-(pyridin-2-yl)imidazo[1,5-a]pyridin-2-ium
hexafluorophosphate, 2b
in ppm relative to Si(CH3)4 and are referred internally with
respect to the residual signal of the deuterated solvent. Mass
spectra were collected on a Finnigan LCQ DecaXPlus ion
trap mass spectrometer with ESI ion source.
When the formaldehyde in the above experiment is not
dissolved completely, two differently shaped types of crys-
tals are obtained upon slow gas-phase diffusion of diethyl
ether into the DCM solution of the crude reaction product.
X-Ray diffraction of a pale-yellow needle gives the struc-
ture of 2b, with might result from the reaction of 1 with
itself or the hydrolysis product pyridine-2-carboxaldehyde.
The pale-yellow needles are separated by hand under the
microscope and characterized by elemental analysis. Due to
some contamination by 2a, no satisfactory elemental anal-
ysis can be obtained. The ESI mass spectrum of the crude
product shows peaks of both 2a and 2b with m/z = 201
[C13H17N2]+ and 278 [C18H20N3]+. The peak at m/z =
278 [C18H20N3]+ is not detectable in the ESI mass spec-
trum of the purified compound 2a. – MS ((+)-ESI, crude
product): m/z = 201 [C13H17N2]+, 278 [C18H20N3]+, 196
[C18H20N3–C6H10]+. – MS ((−)-ESI): m/z = 145 [PF6]−.
– C18H20N3PF6 (423.34): calcd. C 51.07; H 4.76, N 9.93;
found C 52.68, H 5.34, N 9.01.
N-Cyclohexyl-(2-pyridyl)methanimine, 1
Cyclohexylamine (6.1 mL, 53 mmol) is added to a stirred
solution of pyridine-2-carboxaldehyde (5.0 mL, 53 mmol) in
10 mL toluene. Molecular sieve (4 Å) is added, and the reac-
tion mixture is allowed to stand for several hours. The molec-
ular sieve is separated from the reaction mixture by filtration
and the solvent removed in vacuo. For purification, the oily
residue was washed with ethanol and dried in vacuo. Yield:
8.4 g (85%) of a brown oil. Analytical data are in accor-
dance with the literature values [60]. – 1H NMR (300 MHz,
CDCl3): δ[ppm] = 8.55 (d, 1H, 3JHH = 4.8 Hz), 8.32 (s, 1H),
3
4
7.91 – 7.88 (m, 1H), 7.65 (td, 1H, JHH = 7.6 Hz, JHH
=
3
3
1.3 Hz), 7.19 (ddd, 1H, JHH = 7.5 Hz, JHH = 4.9 Hz,
4JHH = 1.1 Hz), 3.26 – 3.16 (m, 1H), 1.78 – 1.45 (m, 7H),
1.37 – 1.14 (m, 3H). – 13C{1H} NMR (75 MHz, DSMO):
δ[ppm] = 159.2, 154.5, 149.0, 136.2, 124.5, 120.2, 68.4,
33.8, 25.2, 24.0.
Bis{2-cyclohexylimidazo[1,5-a]pyridin-3-ylidene}silver(I)
hexafluorophosphate, 3
2-Cyclohexyl-imidazo[1,5-a]pyridin-2-ium
hexafluorphosphate, 2a
In a flask covered with aluminum foil, 2a (1.50 g,
The reaction is carried out in an atmosphere of dry ni- 4.33 mmol) is dissolved in 10 mL of DCM. AgCl (0.94 g,
trogen: Paraformaldehyde (1.20 g, 0.040 mol) is completely 6.6 mmol) and powdered KOH (0.38 g, 6.8 mmol) are added,
dissolved in 250 mL of hot toluene. After the addition of and a brown suspension is formed. After stirring for 3 h
imine 1 (6.30 g, 0.033 mol), 4 M HCl in 1,4-dioxane (8.4 mL, at ambient temperature the reaction mixture is filtered over
0.034 mol) is added dropwise. After stirring for 15 h at Celite. The solvent is partially removed in vacuo, and the
ambient temperature, the solvent is removed and the oily product is precipitated with diethyl ether. Yield: 1.29 g
residue washed with diethyl ether. To remove non-reacted (91%) of a light-brown powder. Slow gas-phase diffu-
paraformaldehyde, the residue is dissolved in methanol and sion of diethyl ether into a DCM solution gives colorless
filtered. The solvent is removed under reduced pressure, and needles of 3 suitable for X-ray diffraction. – 1H NMR
3
an oil is obtained. For purification the residue is dissolved (300 MHz, DMSO): δ[ppm] = 8.64 (d, 1H, JHH = 7 Hz),
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
1195
8.07 (s, 1H), 7.60 (d, 1H, 3JHH = 9 Hz), 7.05 – 7.00 (m, 1H),
–
1H NMR (300 MHz, DMSO): δ[ppm] = 8.77 (d, 1H,
6.88 – 6.83(m, 1H) , 4.65 – 4.57 (m, 1H), 2.15 – 2.11 (m, 2H), 3JHH = 7 Hz), 8.64 – 8.47 (m, broad, 1H), 8.16 (s, 1H),
2.03 – 1.71 (m, 5H), 1.52 – 1.23 (m, 3H). – 13C{1H} NMR 8.10 (s, broad, 1H), 7.67 (d, 1H, JHH = 9 Hz), 7.61 (d,
3
3
(75 MHz, DMSO): δ[ppm] = 170.4, 130.3, 128.7, 122.9, broad, JHH ∼ 8 Hz), 7.11 – 7.02 (m, 2H, sharp and broad
117.8, 114.1, 110.1, 62.2, 34.1, 25.1, 24.6. – MS ((+)- signals overlap), 6.96 – 6.89 (m, 2H, sharp and broad sig-
ESI): m/z = 507 [C26H32N4Ag]+, 307 [C13H16N2Ag]+, nals overlap), 4.87 – 4.76 (m, 1H), 4.70 – 4.62 (m, broad,
201 [C13H17N2]+. – C26H32N4AgPF6 (653.40): calcd. C 1H), 2.19 – 1.16 (m, 18H), 1.74 – 1.70(m, 2H), 1.53 – 1.41
47.49; H 4.94, N 8.57; found C 47.62, H 4.70, N 8.49.
(m, 4H), 1.32 – 1.23 (m, 4H). – 13C{1H} NMR (75 MHz,
DMSO): δ[ppm] = 173.5, 162.6, 130.0, 129.7, 127.3, 126.7,
123.4, 123.3, 118.0, 114.9, 110.4, 109.8, 61.9, 61.8, 33.8,
33.5, 30.5, 25.2, 25.0, 24.8, 24.5. – MS ((+)-ESI): m/z = 597
[C26H32N4Au]+, 515 [C26H32N4Au–C6H10]+. – MS ((–)-
ESI): m/z = 145 [PF6]−. – C30H40N4SBrAu2PF6 (1107.54):
calcd. C 32.53; H 3.64, N 5.06; found C 32.33, H 3.46, N
5.01.
Cocrystals of bromido-{2-(cyclohexyl)imidazo
[1,5-a]pyridin-3-ylidene}gold(I), 4b, and
{2-(cyclohexyl)imidazo[1,5-a]pyridin-3-ylidene}(tetra-
hydrothiophene)gold(I) hexafluorophosphate, 4a
Compound 3 (0.47 g, 0.72 mmol) is dissolved in 10 mL
DCM. To the stirred solution solid (tht)AuBr (0.50 g,
1.4 mmol) is added, whereupon a precipitation of AgBr is
formed immediately. After stirring for 30 min at ambient
temperature, the AgBr is filtered off and the solvent partly
removed under reduced pressure. Precipitation with diethyl
Tribromido-{2-(cyclohexyl)imidazo[1,5-a]pyridin-
3-ylidene}gold(III), 5
Compound 4a,b (0.30 g, 0.27 mmol) is dissolved in
ether yields a light-brown powder. Yield: 0.63 g (83%). 15 mL DCM and cooled with an ice bath. Solid CsBr3
Slow gas-phase diffusion of diethyl ether into a DCM so- (0.24 g, 0.64 mmol) is added and the reaction mixture stirred
lution gives yellow needles suitable for X-ray diffraction. for 15 min and for another 30 min at ambient temperature.
Table 2. Crystal data, data collection and structure refinement details.
2a
2b
3
4a, b
Empirical formula
Mr
Size, mm3
Crystal system
Space group
a, Å
C26H33F12N4P2
691.50
0.48 × 0.45 × 0.40
monoclinic
P21/n
6.5805(6)
27.709(3)
16.772(2)
90
95.389(4)
90
3044.7(6)
1.51
C18H20F6N3P
423.34
0.51 × 0.28 × 0.27
triclinic
P1
8.668(5)
10.323(6)
11.424(7)
68.073(18)
83.92(2)
86.98(2)
942.8(10)
1.49
C26H32AgF6N4P
653.40
0.41 × 0.07 × 0.05
monoclinic
P21/c
14.589(3)
9.986(2)
19.243(4)
90
90.170(7)
90
2803.4(10)
1.55
C30H40Au2BrF6N4PS
1107.53
0.40 × 0.08 × 0.05
monoclinic
Cc
6.8926(11)
21.898(4)
22.405(3)
90
90.780(6)
90
3381.4(9)
2.18
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
ρ
calcd., g cm−1
Z
4
0.2
205
2
0.2
205
4
0.8
200
4
10.0
200
µ(MoKα ), mm−1
T, K
θ
max, deg
29.2
42617
7874
0.059
23.7
25240
2770
0.121
18.7
28032
2133
0.145
25.1
18613
5006
0.106
Measured reflections
Independent reflections
Rint
Reflections with I > 2 σ(I)
Absorption correction
5312
2046
1646
4269
multi-scan
0.77/0.91
0/427
0.077
0.251
multi-scan
0.71/0.95
0/291
0.067
0.214
multi-scan
0.72/0.96
0/343
0.038
0.096
multi-scan
0.38/0.63
2/406
0.044
0.097
Tmin/Tmax
Restraints/refined param.
R1 (I > 2 σ(I))
wR2
x (Flack)
∆ρfin (max/min), e Å−3
CCDC no.
–
–
–
0.03(1)
2.08/−1.42
1017395
0.85/−0.59
0.38/−0.41
0.41/−0.40
1017392
1017393
1017394
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M. Kriechbaum et al. · Silver(I) and Gold(I) Complexes Bearing a Pyrido-annelated N-Heterocyclic Carbene
– C13H16N2AuBr3 (636.96): calcd. C 24.51; H 2.53, N 4.40;
found C 24.82, H 2.43, N 4.41.
Crystal structure determinations
Single-crystal structure analyses were carried out on
a Bruker Smart X2S diffractometer operating with MoKα
radiation (λ = 0.71073 Å). Further crystallographic and re-
finement data are listed in Table 2. The structures were solved
by Direct Methods (SHELXS-97) [73, 74] and refined by full-
matrix least-squares on F2 (SHELXL-97) [75, 76]. The H
atoms were calculated geometrically, and a riding model was
applied during the refinement process. No higher symmetry
could be found for 4a,b using the program PLATON [77, 78].
CCDC 1017392 – 1017395 contain the supplementary
crystallographic data for 2a, 2b, 3, and 4a,b. These data can
be obtained free of charge from The Cambrige Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request.
cif.
Thereby, the color of the reaction mixture changes from
light-brown to orange. The reaction mixture is filtered, and
the solvent is partly removed under reduced pressure. Precip-
itation with n-pentane yields complex 5 as a red-brown pow-
der. Yield: 0.32 g. (93%). – 1H NMR (300 MHz, DMSO):
3
δ[ppm] = 8.70 (d, 1H, Ha, Jab = 7 Hz), 8.49 (s, 1H, He),
7.73 (d, 1H, Hd, 3Jcd = 9 Hz), 7.18 (dd, 1H, Hc, 3Jbc = 7 Hz,
3Jcd = 9 Hz), 7.09 – 7.04 (t, 1H, Hb), 4.72 – 4.65 (m, 1H),
2.12 – 2.09 (m, 2H), 1.94 – 1.88 (m, 4H), 1.55 – 1.42 (m, 2H)
1.31 – 1.23 (m, 2H). – 13C{1H} NMR (75 MHz, DMSO):
δ[ppm] = 131.8, 124.8 (Ca), 124.2 (Cc), 121.9 (C1), 118.4
(Cd), 116.7 (Cb), 113.3 (Ce), 61.7, 32.5, 24.7, 24.2 (aro-
Acknowledgement
The authors thank Prof. G. Knör and the JKU for generous
support of the experimental work. The NMR spectrometers
were acquired in collaboration with the University of South
Bohemia (CZ) with financial support from the European
Union (EU) through the EFRE INTERREG IV ETC-AT-CZ
programme (project number M00146, “RERI-uasb”).
1
matic hydrogen and carbon atoms were assigned by H-1H
COSY, 1H-13C HSQC and HMBC experiments). – MS ((+)-
ESI): m/z = 597 [C26H32N4Au]+, 875 [C26H32N4Au2Br]+.
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