Facile oxidation of NHC-Au(i) to NHC-Au(iii) complexes by CsBr3

Article abstract of DOI:10.1039/c4dt00695j

CsBr₃ was investigated as a new and convenient oxidant for NHC-Au(i) complexes (NHC = imidazo[1,5-a]pyridin-3-ylidene) for the preparation of the respective Au(iii) complexes. The Au(i) complexes were synthesized by the silver salt method using [(NHC)₂Ag]PF₆ and (tht)AuBr. Unexpectedly, the reactions yielded both neutral (NHC)AuBr and ionic [(NHC) ₂Au]PF₆, depending on the N-substituent of the NHC ligand. Oxidation with CsBr₃ gave the complexes (NHC)AuBr₃ and [(NHC)₂AuBr₂]PF₆ in high yields and purity, which proves the suitability of this reagent. The complexes were further characterised by X-ray diffraction and electronic absorption and emission spectroscopy. The Au(i) complexes exhibit a dual emission attributable to intraligand fluorescence and phosphorescence at both room temperature and 77 K. Upon irradiation with polychromatic light (λ > 305 nm), the Au(iii) complexes are cleanly photo-reduced to the Au(i) congener.

Full text of DOI:10.1039/c4dt00695j

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Facile oxidation of NHC-Au(I) to NHC-Au(III)
complexes by CsBr3†
Margit Kriechbaum,a Daniela Otte,a Manuela Listb and Uwe Monkowius*a
Cite this: Dalton Trans., 2014, 43,
8781
CsBr3 was investigated as a new and convenient oxidant for NHC-Au(I) complexes (NHC = imidazo[1,5-a]-
pyridin-3-ylidene) for the preparation of the respective Au(III) complexes. The Au(I) complexes were syn-
thesized by the silver salt method using [(NHC)2Ag]PF6 and (tht)AuBr. Unexpectedly, the reactions yielded
both neutral (NHC)AuBr and ionic [(NHC)2Au]PF6, depending on the N-substituent of the NHC ligand.
Oxidation with CsBr3 gave the complexes (NHC)AuBr3 and [(NHC)2AuBr2]PF6 in high yields and purity,
which proves the suitability of this reagent. The complexes were further characterised by X-ray diffraction
and electronic absorption and emission spectroscopy. The Au(I) complexes exhibit a dual emission
attributable to intraligand fluorescence and phosphorescence at both room temperature and 77 K. Upon
irradiation with polychromatic light (λ > 305 nm), the Au(III) complexes are cleanly photo-reduced to the
Au(I) congener.
Received 7th March 2014,
Accepted 16th April 2014
DOI: 10.1039/c4dt00695j
www.rsc.org/dalton
oxidation with a halogen. Working with gaseous Cl2 or liquid
Br2 is inconvenient, particularly when precise and small
Introduction
In recent years, the interest in Au(III) compounds has increased amounts are required. The only easy-to-handle halogen is I2,
significantly. For example, Au(III) complexes can be used for but several Au(III)-iodides are unstable and in equilibrium with
silver-salt-free gold catalysis1 or as a catalyst in oxidative coup- their Au(I) congeners and I2.7
ling reactions,2 and there are also several reports on lumine-
An attractive alternative for gaseous Cl2 is iodobenzene
scent Au(III) complexes.3–5 However, gold chemistry is still dichloride, PhICl2, which has already been used for the syn-
dominated by compounds of the oxidation state +1. Complexes thesis of several Au(III) chlorides.8 This compound can be syn-
of Au(III) are not particularly rare, but the number of published thesized from iodobenzene and chlorine but cannot be stored
compounds is remarkably lower than that of their Au(I) conge- for a long time.9 Br2 can be ‘protected’ as its tribromide in
ners. There are several reasons for this, such as the interest in CsBr3. This substance is a solid and can be weighed con-
catalytically or pharmaceutically active Au(I) complexes and veniently under air. This reagent has in fact already been used
their unique solid-state structures and luminescence behav- by Bellemin-Laponnaz and Gade for the oxidation of Pt- and
iour.6 A more trivial reason might be that Au(I) compounds are Rh-complexes.10 It is not commercially available, but easily
more easily accessible: they are commonly synthesized by the accessible from CsBr and Br2.
reaction of (R2S)AuX (R2S = tetrahydrothiophene, Me2S) and
To demonstrate this approach for gold, we present the use
the ligand of interest under ambient conditions. No compar- of CsBr3 for the synthesis of (NHC)AuBr3 (NHC – N-hetero-
able general synthon is known for Au(III) complexes. For direct cyclic carbene) from (NHC)AuBr. The NHC used was
preparation of Au(III) complexes bearing a hard-to-oxidize imidazo[1,5-a]pyridin-3-ylidene, which possesses an extended
ligand (e.g., pyridines, amines), the precursor of choice is π-system. Further, we studied the basic photophysical and
HAuCl4 or its alkali salt. For other ligands, the standard prepa- photochemical properties of the prepared Au(I) and Au(III)
ration is via the respective Au(I) complex and successive compounds.
aInstitute of Inorganic Chemistry, Johannes Kepler University Linz, Altenbergerstr.
Result and discussion
Synthesis
69, 4040 Linz, Austria. E-mail: uwe.monkowius@jku.at; http://www.jku.at/anorganik;
Fax: +43 732 2468 9681; Tel: +43 732 2468 8814
bInstitute for Chemical Technology of Organic Materials, Johannes Kepler University
Synthesis of the NHC precursors 2a,b started with the conden-
sation reaction of 2-pyridyl-carbaldehyde and the respective
Linz, Altenbergerstr. 69, 4040 Linz, Austria
†Electronic supplementary information (ESI) available. CCDC 985732–985740.
aniline derivative to give the imines 1a,b (Scheme 1). Their
cyclisation with CH2O–HCl in dry toluene yielded the imidazo-
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt00695j
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Scheme 1 (a) Synthesis of the NHC precursors (2a,b) and (b) of the corresponding silver and gold complexes.
[1,5-a]pyridin-2-ium chlorides. Precipitation from aqueous 134.6 (2a) and 145.0 ppm (2b) to 172.8 (3a) and 172.8 ppm
solution as hexafluorophosphate salts gave pure 2a,b. Accord- (3b), respectively. The signal of the carbene carbon atom in
−
ing to our standard procedure for PF6 salts, the reaction with complex 3b is split into a doublet of doublets, with a coupling
AgCl–KOH produces the [(NHC)2Ag]PF6 complexes 3a,b.11 between the carbon and silver atom of 1J(107/109Ag–13C) of
Their transmetallation with (tht)AuBr (tht = tetrahydrothio- 216 Hz and 187 Hz. Complex 3a exhibits a broad doublet at
phene) yielded the respective NHC-Au complexes. Interestingly, 172.8 ppm, with an average silver–carbon coupling of 193 Hz.
for R = Me the neutral complex (NHC)AuBr is isolated, whereas The 13C-NMR signals of the carbene carbon atom for the gold
for R = iPr the cationic complex [(NHC)2Au]PF6 prevails. complexes are found at: 164.9 (4a), 176.6 (4b), 135.1 (5a), 144.7
However, the crude product 4b always contains an impurity in (5b) ppm.
the form of a small amount of the neutral complex (NHC)
AuBr. This result clearly demonstrates the general problem
Structural studies
when using the silver salt method for preparing NHC-Au(I)
complexes: depending on the N-substituent and solvent, but
usually irrespective of the actual stoichiometry, neutral or
ionic Au complexes can be formed. However, in this case the
finding is somewhat surprising as the N-substituents differ
only slightly and the reaction conditions are identical. The
most obvious difference is the increased level of shielding of
the Au+ atom in 4b. Steric factors can have a huge influence on
the stoichiometry of gold complexes. We have previously
reported on the dominance of the ionic form [L2Au][AuCl2]
over the neutral form LAuCl bearing a very bulky phosphane
ligand Ad2BnP, which we explained by efficient shielding (and
additional Au+–C(π) interactions).12 Upon successive oxidation
of the Au(I) complexes with CsBr3, the corresponding NHC-Au
(III) complexes 5a,b formed in high yields in a clean reaction,
which proves the suitability of the preparative approach.
The molecular structures of all compounds were determined
by single-crystal X-ray diffraction. The diimine 1a crystallizes in
the monoclinic space group P21/c with two molecules of the
E-isomer in the asymmetric unit. 1b crystallizes in the tetragonal
ˉ
space group P421/c. The asymmetric unit contains one mole-
cule of the E-isomer. The NvC–CvN unit is almost planar
with torsion angles of 177.51°/172.30° (N2–C6–C5–N1/N4–
C20–C19–N3) (1a) and 176.71° (1b), respectively. The bond
lengths and angles are as expected (Fig. S1†).
The two carbene precursors 2a and 2b are shown in Fig. 1.
Compound 2a crystallizes in the monoclinic space group C2/c,
compound 2b, on the other hand, in the orthorhombic space
group Pna21, which indicates the influence of the substituent
at the phenyl group on the arrangement in the crystal lattice.
The asymmetric unit consists of one half of a formula unit.
Hence, the imidazo[1,5-a]pyridyl moiety is disordered, and the
positions labelled as N2 and C3 are occupied by both N and C
atoms at a ratio of 1 : 1. An identical disorder has previously
been found for the mesityl derivate.13 Also for 2b a comparable
disorder of the atoms N1 and C3 atoms was observed. In both
The identities of all compounds were proved by 1H-,
13C-NMR spectroscopy and elemental analysis. Upon complex
formation, the signals of the acidic NC(H)N proton vanishes
(2a: 9.96; 2b: 10.01 ppm). The 13C resonances are shifted from
8782 | Dalton Trans., 2014, 43, 8781–8791
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−
Fig. 1 Molecular structures of 2a (left) and 2b (right, ellipsoids drawn at the 50% probability level, H atoms and PF6 anions omitted for clarity).
Selected bond lengths [Å] and angles [°] for 2a: N1–C1 1.347(6), C1–N2 1.337(6), N2–C1–N1 108.2(5); and for 2b: N1–C1 1.315(7), C1–N2 1.341(6),
N2–C1–N1 108.5(4).
−
Fig. 2 Molecular structures of 3b (left) and 4b (right, ellipsoids drawn at the 50% probability level, H atoms and PF6 anions omitted for clarity).
Selected bond lengths [Å] and angles [°]: 3b: C1–Ag1 2.065(5), N1–C1 1.363(6), N2–C1 1.355(5), C1–Ag1–C1i 180, N2–C1–N1 103.2(4); 4b: C1–Au1
2.014(5), C1i–Au1 2.014(5), C1–N1 1.362(6), C1–N2 1.350(6), C1–Au1–C1i 180.0, N1–C1–N2 104.0(4), N1–C1–Au1 131.1(3), N2–C1–Au1 124.8(3).
molecules, the phenyl group is perpendicular to the plane of the imidazo-pyridyl moieties. The cationic gold carbene
the imidazo[1,5-a]pyridyl moiety.
complex 4b is isostructural to 3b. The bond length C1–Au1 of
The silver complex 3b crystallizes as the cationic carbene 2.014(5) Å is similar to the C1–Ag1 (2.065(5) Å) bond in 3b,
−
complex with the PF6 counterion in the triclinic space group illustrating the comparable radii of Ag(I) and Au(I) atoms
ˉ
P1. The cation has inversion symmetry, with the silver atom as (Fig. 2).
the inversion centre. Hence, the silver atom is ideally linearly
coordinated by two carbene carbon atoms with a C1–Ag1–C1i ordinated by the carbene carbon and three bromine atoms in
angle of 180° and a Ag1–C1 bond length of 2.065(5) Å (Fig. 2). square-planar geometry. The C1–Au1 bond length is 2.05(1) Å.
In the neutral Au(III) complex 5a, the gold atom is co-
Complex 4a crystallizes in the monoclinic space group P21/n The NHC ligand induces a trans effect with a lengthening of the
with one molecule of the neutral gold carbene complex in the Au1–Br3 bond compared to both cis Au–Br bonds (2.446(1) Å
asymmetric unit (Fig. 3). The gold atom is almost linearly co- vs. ∼2.42 Å). These distances are very similar to reported
ordinated by the carbene carbon and the bromine atoms data.4,7c,14 Due to the higher steric demand of the bromide
with a C1–Au1–Br1 angle of 177.1(2)°. The bond lengths are ligands compared to the carbene carbon atom, the Br1 and
1.997(8) Å for C1–Au1 and 2.401(1) Å for Au1–Br1. Although Br2 are bent towards the carbene ligand [Br1–Au1–Br2 171.14
the arrangement of the complexes seems to support dimeriza- (4)°], whereas the C1–Au1–Br3 bonds are almost linear with an
tion of the complexes via aurophilic interactions, the shortest angle of 176.0(3)°.The angle between the coordination and the
Au–Au distance [3.783(1) Å] is well over the aurophilicity limit imidazo-pyridyl plane is about 80°. It is interesting to note
of ∼3.5 Å. Hence, the intermolecular arrangement of the com- that in several reported (NHC)Au(III)Br3 complexes bearing
plexes in the crystal is dominated by other weak interactions, both symmetrical and unsymmetrical substituted NHC ligands
particularly by ππ interactions among the phenyl moieties and with small substituents at the N atom, the NHC and the
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Fig. 3 Molecular structure of 4a (left) and 5a (right, ellipsoids drawn at the 50% probability level). Selected bond lengths [Å] and angles [°]: 4a: C1–
Au1 1.997(8), Au1–Br1 2.401(1), C1–N1 1.33(1), C1–N2 1.36(1), Au1–Au1i 3.783(1), C1–Au1–Br1 177.1(2), N1–C1–N2 104.7(7), N1–C1–Au1 129.9(6),
N2–C1–Au1 125.4(6). 5a: Au1–C1 2.05(1), Au1–Br1 2.426(1), Au1–Br2 2.422(1), Au1–Br3 2.446(1), N2–C1 1.34(1), N1–C1 1.33(1), C1–Au1–Br3
176.0(3), Br2–Au1–Br1 171.14(4), C1–Au1–Br2 88.3(3), C1–Au1–Br1 88.7(3), N1–C1–Au1 125.7(7), N2–C1–Au1 126.5(8).
coordination planes are not perpendicular to each other and
frequently feature angles of ∼80° and even below.4b,7c,14
Reaction of the cationic Au(I) complex 4b with CsBr3
yielded the cationic Au(III) complex 5b. Slow gas-phase
diffusion of diethyl ether into a dilute DCM solution gave
small bright red platelets. The crystals were suitable for X-ray
diffraction, and a structure was solved in space group P21/n.
The structure refinement was somewhat complicated by
solvent residue electron densities, and no satisfactory atom
positions could be found for the solvent molecule. Also the
−
PF6 counterion is highly disordered. The anisotropic refine-
ment was not stable, and therefore the structure was refined
isotropically. The structure solution verifies the cationic nature
of the Au(III) complex, but a discussion of bond lengths and
angles is not meaningful (see Fig. S2 in ESI†). When the crude
product of 4b is directly reacted with CsBr3 without purifi-
Fig. 4 Molecular structure of the neutral complex 5b*. Selected bond
lengths [Å] and angles [°]: Au1–C1 2.02(1), Au1–Br1 2.410(1), Au1–Br2
2.431(1), Au1–Br3 2.439(1), N2–C1 1.36(1), N1–C1 1.35(1), C1–Au1–Br3
176.0(2), Br2–Au1–Br1 176.84(3), C1–Au1–Br2 88.6(2), C1–Au1–Br1
cation, two kinds of crystals are identified by single-crystal
X-ray diffraction (see Fig. S3 ESI†): The majority of crystals are
bright red and were identified as the cationic Au(III) complex
5b. A second crop of very few dark red crystals appear to be the
neutral Au(III) complex (NHC)AuBr3 (5b*). However, if the
pure Au(I)-complex 4b is treated with CsBr3, the sole product
is complex 5b. The structural parameters are very similar to
those of complex 5a (Fig. 4).
90.7(2), N1–C1–Au1 127.0(4), N2–C1–Au1 126.8(4).
544, and 592 nm emerge – most probably due to phosphor-
escence. The maxima in the excitation spectra are superimpo-
sable with the absorption maxima (Fig. 5). The electronic
spectra of compound 2a are very similar, as shown in Fig. S4
of the ESI† (Table 1).
Due to their light sensitivity, the silver complexes were not
investigated. The low-energy onset of the absorption of the
Au(I) complex is somewhat bathochromically shifted com-
pared to the NHC precursor: 4b starts to absorb in the UV
range below 360 nm. At room temperature the emission spec-
trum of a degassed ethanolic solution of compound 4b is
dominated by a broad, intense emission with maxima at 375
and 394 nm and two shoulders at 354 and 423 nm. This emis-
sion is comparable to the room-temperature emission of the
imidazolium salt and can be attributed to IL fluorescence.
Further peaks, with very low intensity, arise in the low-energy
(LE) range from 530 to 650 nm. At 77 K the low-energy emis-
Photophysical and photochemical characterisation
The absorption spectrum of the carbene precursor 2b shows a
structured band in the UV range with maxima at 322, 308, 297,
284, 273, and 263 nm and shoulders at 244 and 235 nm.
Because of their high extinction coefficients, the absorptions
can be attributed to π–π* transitions, possibly masking
additional n–π* transitions. At room temperature the com-
pound exhibits a broad emission with maxima at 332, 350,
and 365 nm and a shoulder at 386 nm. As expected, the vibro-
nic structure in the emission spectrum is much better resolved
at 77 K, and in the low-energy range low-intensity peaks at 503,
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sion is more intense, showing three distinct maxima at 524,
569, and 622 nm. At higher energy, emission bands of lower
intensity are observed. On the basis of the structure of the LE
band and the huge Stokes shift (approx. 17 200 cm−1), we attri-
3
bute this emission to an IL excited state. The long emission
decay time of the LE band of 0.73 ms at 77 K further supports
this interpretation (Fig. S8 ESI†). Complex 4a exhibits compar-
able absorption, emission, and excitation spectra. The HE
bands are not as structured as for 4b, and the emission life-
time is somewhat shorter (τ = 0.28 ms, see Fig. S5 and S7
ESI†).
In principle, the Au(III) complexes 5a,b exhibit similar elec-
tronic spectra: Fig. 6 shows the electronic spectra of the gold
(III) complex 5b in ethanolic solution. The HE absorption is
structured in the UV range below 350 nm, whereas the LE
absorption is weak and very broad at ∼415 nm. The latter
band, which is unusually strongly red-shifted and is compar-
able to that of K[AuBr4], can be attributed to n(Br) → 5dx2−y2
and π(Br) → 5dx2−y2 ligand-to-metal charge transfer (LMCT)
states.15 The overwhelming majority of compounds of type
LAuX3 (L = phosphanes, HNCs; X = Cl, Br) exhibit a conspicu-
ous signal between 330 and 400 nm.7c,16,17 The bathochromic
shift is due to the lower σ-donor ability of the NHC ligand
used,18 which reduces the electron density at the gold atom
and stabilizes the d-orbitals. The emission at room tempera-
ture has a maximum at 368 nm. At 77 K a second emission at
lower energy evolves with maxima at 539 and 584 nm and two
shoulders at 570 and 524 nm. The similarity of the bands
allows the HE emission to be attributed to IL fluorescence; the
3
LE emission is the result of an IL excited state. The electronic
spectra of 5a are very similar (see Fig. S6).
Fig. 5 Top: Electronic spectra of 2b (c = 7.7 × 10−5 M in ethanol): (a)
absorption, (b) emission and (e) excitation spectra recorded at 298 K
(λexc. = 280 nm, λdet. = 400 nm) and (c) emission spectrum (asterisk:
second order of excitation light) and (d) excitation spectrum recorded at
77 K (λexc. = 280 nm, λdet. = 400 nm); bottom: electronic spectra of 4b
(c = 8.5 × 10−5 M in ethanol): (a) absorption, (b) emission and (e) excitation
spectra recorded at 298 K (λexc. = 300 nm, λdet. = 400 nm) and (c) emis-
As expected for NHC-Au(III) halides, complexes 5a,b are
light-sensitive and can be photoreduced to the Au(I) conge-
ners.7c,15 The photochemical reactivity of 5b was investigated
by irradiation of an ethanolic solution with polychromatic
light (λ > 305 nm). As previously reported, NHC-Au(III) bro-
mides undergo photo-reductive elimination of Br2 and for-
mation of the respective NHC-Au(I) bromide (no Br2 could be
detected because of the fast oxidation of the solvent).7c Fig. 7
sion spectrum and (d) excitation spectrum recorded at 77 K (λexc.
300 nm, λdet. = 580 nm).
=
Table 1 UV/Vis and emission spectroscopic data of compounds 2–5 in ethanol
Emission/λmax [nm]
Excitation/λmax [nm]
Absorption
λmax [nm] (log ε/l mol−1 cm−1
)
298 K
77 K
298 K
77 K
2a 323 (3.24), 308 (3.58), 297 (3.65), 283 (3.92),
272 (3.94), 262 (3.79), 235 (3.83)
4a 350 (3.30), 333 (3.51), 318 (3.61), 297 (4.03),
286 (4.07), 275 (3.92), 258 (3.76), 249 (3.79)
5a 337 (3.01), 321 (3.13), 307 (3.11), 284 (3.41),
272 (3.51), 262 (3.55), 236 (3.91)
360 (broad)
392
330, 346, 364, 385, 407,
(503, 544, 592)
380, 400, 531, 576, 630
323, 308, 285,
272, 260
358, 326, 290,
271
324, 310, 299,
281, 269, 258
268, 322
377, 394
366, 384, 404, 433, 470,
530, 546, 576, 630
272, 325, 359
348, 327, 317,
270
2b 322 (3.26), 308 (3.58), 297 (3.67), 284 (3.96),
273 (3.97), 263 (3.83), 244 (3.58), 235 (3.74)
4b 348 (3.23), 329 (3.96), 315 (4.09), 305 (4.06),
289 (3.88), 274 (3.63), 259 (3.63)
5b 338 (3.57), 321 (3.77), 308 (3.80), 286 (4.00),
275 (4.01), 265 (3.91), 232 (4.27)
332, 350, 365, 386
329, 345, 362, 381, 405,
432, (506, 548, 596)
350, 368, 388, 411, 436,
524, 569, 622
347, 364, 381, 524, 539,
570, 585, 621, (638)
321, 307, 296,
282, 272, 260
341, 322, 285,
272, 264, 250
322, 307, 282,
270, 260, 250
323, 309, 297,
282, 270, 261
382, 345, 328,
302, 310, 268
356, 340, 318,
304
328, 354, 375, 394, 423,
533, 573, 606, 629
331, 350, 369
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Conclusion
We have demonstrated that CsBr3 can be used as an easy-to-
handle oxidant for NHC-Au(I) complexes (NHC = imidazo-
[1,5-a]pyridin-3-ylidene). The gold complexes were further
investigated by electronic absorption and emission spectro-
scopy. The Au(I) complexes exhibit a dual emission at both
room temperature and 77 K that can be attributed to intrali-
gand fluorescence and phosphorescence. In ethanolic solu-
tions, the Au(III) complexes are cleanly photo-reduced to the Au
(I) congeners upon irradiation with polychromic light.
Experimental section
General
Fig. 6 Electronic spectra of 5b (c = 8.0 × 10−5 M in ethanol): (a) absorp-
tion, (b) emission and (e) excitation spectra recorded at 298 K (λexc.
300 nm, λdet. = 400 nm) and (c) emission spectrum and (d) excitation
spectrum recorded at 77 K (λexc. = 310 nm, λdet. = 580 nm).
=
All reactions and manipulations of air-sensitive and/or moist-
ure-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 from gold,
bromine and tetrahydrothiophene following a published pro-
cedure.20,21 Pyridine-2-carboxaldehyde is commercially avail-
able and was used as received.
Elemental analyses were carried out by the Institute for
Chemical Technology of Organic Materials at the University Linz.
NMR spectra were recorded on a Bruker Avance III (300 MHz)
spectrometer. 1H and 13C NMR shifts are reported in ppm rela-
tive to Si(CH3)4 and were referenced internally with respect to
the residual signal of the deuterated solvent. Mass spectra
were collected on a Finnigan LCQ DecaXPPlus Ion Trap Mass
spectrometer with an ESI ion source.
For photophysical characterisation, spectroscopic-grade sol-
vents were used throughout all measurements. Absorption
spectra were recorded with a Varian Cary 300 double-beam
spectrometer. Emission spectra at 300 and at 77 K were
measured with a steady-state fluorescence spectrometer (Jobin
Yvon Fluorolog 3). Before recording the emission and exci-
tation spectra, the samples were degassed by at least three
freeze–pump–thaw cycles. Luminescence lifetimes were
measured using the Fluorolog’s FL-1040 phosphorimeter
accessory. The estimated experimental errors for the molar
absorption coefficient and fluorescence lifetime are 5%.22 The
irradiation experiments were performed with an HBO 100 W
lamp using polychromatic light with λ > 305 nm (WG305
filter).
Single-crystal structure analyses were carried out on a
Bruker Smart X2S diffractometer operating with Mo-Kα radi-
ation (λ = 0.71073 Å). Further crystallographic and refinement
data can be found in Tables 2 and 3. The structures were
solved by direct methods (SHELXS-97)23 and refined by full-
matrix least squares on F2 (SHELXL-97).24 The H atoms were
calculated geometrically, and a riding model was applied in
the refinement process. The Flack parameter for the crystal
structure of compound 2b is 0.1(2). CCDC 985732–985740
Fig. 7 Irradiation of an ethanolic solution of 5b (c = 9 × 10−5 M) with
polychromatic light (λ > 305 nm). The lower, black graph shows the
absorption of an ethanolic solution of 4b.
and S9† plot the spectral changes of the complexes 5a,b upon
irradiation for 5 minutes. Initially, the spectrum of 5b shows
low-energy bands at 337, 321, 308, 286, 274, and 264 nm, a
high-energy absorption at 233 nm, and the broad LMCT
absorption at ∼415 nm. Photo-reductive elimination of
bromine already takes place during the first seconds of
irradiation, and after 5 minutes the Au(III) complex is comple-
tely photo-reduced. The absorption spectrum of the reduced
species is superimposable with the spectrum of the Au(I) con-
gener 4b. Likewise, the broad LMCT absorption vanishes upon
irradiation, which is in accordance with the colour change of
the solution from yellow to colourless. Two isosbestic points at
293 and 355 nm are indicative of a clean photoreduction.19
A
similar behaviour is observed upon irradiation of a methanolic
solution of Au(III) compound 5a.
8786 | Dalton Trans., 2014, 43, 8781–8791
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Table 2 Crystal data and data collection and structure refinement details for compounds 1a, 2a, 4a, 5a
Crystal data
1a
2a
4a
5a
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
C
14H14N2
C15H15N2PF6
368.26
0.42 × 0.26 × 0.05
Monoclinic
C2/c
9.498 (5)
17.132 (8)
10.148 (5)
90
C15H14AuBrN2
499.16
0.57 × 0.17 × 0.18
Monoclinic
P21/n
10.646 (2)
10.8148 (19)
13.179 (3)
90
C15H14AuBr3N2
658.98
0.63 × 0.24 × 0.05
Monoclinic
P21/n
9.591 (2)
14.252 (3)
12.728 (3)
90
210.27
0.41 × 0.37 × 0.32
Monoclinic
P21/c
10.2806 (13)
11.5990 (14)
19.760 (2)
90
β (°)
97.596 (4)
90
2335.6 (5)
1.196
107.178 (15)
90
1577.6 (13)
1.550
107.979 (7)
90
1443.3 (5)
2.297
99.907 (7)
90
1713.9 (6)
2.554
γ (°)
V (Å3)
Dcalcd (g cm−1
)
Z
8
0.07
200
4
0.24
200
4
4
μ (mm−1
T (K)
)
12.95
200
15.57
200
θ range (°)
2.0–23.1
21 615
3251
293/0
0.055
2.4–20.7
5480
800
120/0
0.104
2.5–25.0
8914
2539
174/0
0.065
2.2–25.0
10 864
3021
192/0
0.095
No. of reflections measured
No. of independent reflections
Parameters refined/restraints
Rint
Absorption correction
Multi scan
0.97, 0.98
0.21/−0.19
R1 = 0.0453
wR2 = 0.1123
R1 = 0.0714
wR2 = 0.1282
985732
Multi scan
0.66, 0.99
0.25/−0.24
R1 = 0.0518
wR2 = 0.1166
R1 = 0.0759
wR2 = 0.1263
985734
Multi scan
0.05, 0.42
3.84/−1.87
R1 = 0.0391
wR2 = 0.1022
R1 = 0.0499
wR2 = 0.1105
985737
Multi scan
0.04, 0.51
1.02/−0.96
R1 = 0.0412
wR2 = 0.0804
R1 = 0.0725
wR2 = 0.0926
985739
Tmin, Tmax
Largest diff. peak and hole (e Å−3
Final R indices [I ≥ 2σ(I)]
)
R indices (all data)
CCDC no.
Table 3 Crystal data and data collection and structure refinement details for compounds 1b, 2b, 3b, 4b, 5b*
Crystal data
1b
2b
3b
4b
5b*
Empirical formula
Formula weight
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C18H22N2
266.38
C19H23N2PF6
424.36
0.40 × 0.37 × 0.13
Orthorhombic
Pna21
16.3670 (16)
13.090 (2)
9.463 (3)
90
C38H44N4AgPF6
809.61
0.37 × 0.16 × 0.14
Triclinic
ˉ
P1
8.527 (2)
10.424 (3)
11.059 (3)
84.065 (8)
73.767 (8)
80.064 (9)
928.1 (4)
1.449
C38H44N4AuPF6
898.71
0.60 × 0.40 × 0.05
Triclinic
ˉ
P1
8.6606 (9)
10.3644 (11)
11.0776 (13)
83.862 (4)
74.229 (3)
80.572 (4)
941.95 (18)
1.584
C19H22AuBr3N2
715.08
0.45 × 0.44 × 0.37
Monoclinic
P21/n
10.005 (4)
13.609 (5)
16.219 (6)
90
100.229 (14)
90
0.40 × 0.32 × 0.29
Tetragonal
ˉ
P421c
15.3674 (11)
15.3674 (11)
13.9783 (11)
90
90
90
90
90
γ (°)
V (Å3)
3301.1 (4)
1.072
2027.4 (7)
1.390
2173.2 (14)
2.186
Dcalcd (g cm−1
)
Z
8
0.06
200
4
0.20
200
1
0.65
200
1
4.01
200
4
μ (mm−1
T (K)
)
12.29
300
θ range (°)
2.7–24.3
34 152
1507
186/0
0.066
2.5–21.8
27 566
2382
261/5
0.071
1.9–25.1
17 396
3254
233/0
0.134
2.5–27.1
75 271
4071
233/0
0.050
2.2–25.1
23 730
3826
230/0
0.072
No. of reflections measured
No. of independent reflections
Parameters refined/restraints
Rint
Absorption correction
Multi scan
0.98, 0.98
0.13/−0.11
R1 = 0.0348
wR2 = 0.0851
R1 = 0.0434
wR2 = 0.0916
985733
Multi scan
0.82, 0.97
0.40/−0.33
R1 = 0.0553
wR2 = 0.1498
R1 = 0.0642
wR2 = 0.1576
985735
Multi scan
0.80, 0.91
0.77/−0.75
R1 = 0.0537
wR2 = 0.1100
R1 = 0.0689
wR2 = 0.1300
985736
Multi scan
0.49, 0.82
3.89/−1.05
R1 = 0.0375
wR2 = 0.0933
R1 = 0.0376
wR2 = 0.0935
985738
Multi scan
0.07, 0.09
0.93/−1.14
R1 = 0.0296
wR2 = 0.0619
R1 = 0.0439
wR2 = 0.0671
985740
Tmin, Tmax
Largest diff. peak and hole (e Å−3
Final R indices [I ≥ 2σ(I)]
)
R indices (all data)
CCDC no.
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contain the supplementary crystallographic data for com- 4 M HCl in 1,4-dioxane (7.1 ml mol, 1 equiv.) was added drop-
pounds 1a–2a, 4a–5a, 1b–4b, and 5b*.
wise. After stirring for 15 h at ambient temperature, the
solvent was separated and the oily residue washed with diethyl
ether. To remove non-reacted paraformaldehyde, the residue
Caesium tribromide CsBr3
CsBr3 was synthesized following a somewhat vague procedure was dissolved in methanol and filtered. The solvent was
from the literature.25 Caesium bromide CsBr (10.0 g, 47 mmol) removed under reduced pressure, and an oil was obtained. For
was dissolved in a small amount of water (15 mL). Bromine purification the residue was dissolved in a small amount of
Br2 (6.76 g, 42 mmol, 0.9 equiv.) was added under stirring, water, and a solution of KPF6 was added to precipitate com-
whereupon an orange precipitate formed immediately. The pound 2a as a yellow solid. Yield: 2.75 g (43.6%). Slow gas-
reaction mixture was stirred for another 15 minutes at room phase diffusion of diethyl ether into a diluted DCM solution
1
temperature. The precipitate was filtered, washed with a very gave colourless crystals suitable for X-ray diffraction. H-NMR
3
small amount of cold pentane and dried. Caesium tribromide (300 MHz, DMSO, δ [ppm]): 9.96 (s, 1H), 8.60 (d, 1H, JHH
=
3
was yielded as an orange, hygroscopic solid, which can be 7 Hz), 8.43 (s, 1H), 7.94 (d, 2H, JHH = 9 Hz), 7.54–7.48 (m,
stored at 5 °C for months. Yield: 7.8 g (21 mmol, 50%). 1H), 7.41–7.36 (m, 3H), 7.32–7.27 (m, 1H), 2.06 (s, 6H).
2,6-Dimethyl-N-(2-pyridinylmethylene)phenylamine (1a). 13C-NMR (75 MHz, DMSO, δ [ppm]): 134.6, 134.0, 130.8, 129.8,
Although these Schiff bases were synthesized before,26 we 128.8, 127.5, 125.3, 124.7, 118.3, 118.0, 114.4, 16.9. MS (ESI
report here a simpler method for their synthesis. Under stir- pos): m/z 223 [C15H15N2]+. MS (ESI neg): m/z 145 [PF6]−. Anal.
ring, 6.5 ml (6.4 g, 0.053 mol) of 2,6-dimethylaniline were Calcd for C15H15N2PF6 (368.26) C 48.92; H 4.11, N 7.61. Found:
added to a solution of 5.0 ml pyridine-2-carboxaldehyde (5.6 g, C 48.65, H 3.53, N 7.53.
0.053 mol) in 10 ml toluene. Then a molecular sieve (4 Å) was
2-(2′,6′-Diisopropylphenyl)imidazo[1,5-a]pyridin-2-ium hexa-
added and the reaction mixture allowed to stand for several fluorophosphate (2b). Compound 2b was prepared analo-
hours. Afterwards, the molecular sieve was separated from the gously to 2a. Starting materials: 1b (9.19 g, 0.0346 mol),
reaction mixture by filtration and the solvent removed paraformaldehyde (1.3 g, 0.0433 mol), HCl 4 M in 1,4-dioxane
in vacuo. For purification, the oily residue was layered with (8.6 ml, g mol). Subsequently, metathesis with KPF6 as
ethanol, separated, and dried in vacuo. Yield: 9.0 g (81.0%) of described for 2a was performed. Yield: 6.3 g (58.4%) of an
a yellow oil. Yellow crystals suitable for X-ray diffraction were off-white powder. Slow gas-phase diffusion of diethyl ether
obtained from ethanol. Analytical data are in accordance with into a diluted DCM solution gave colourless crystals suitable
the literature.25 1H-NMR (300 MHz, CDCl3, δ [ppm]): 8.72 (d, for X-ray diffraction. 1H-NMR (300 MHz, DMSO, δ [ppm]):
3
3
3
1H, JHH = 4.8 Hz), 8.37 (s, 1H, CHvN), 8.30 (d, 1H, JHH
=
10.01 (s, 1H), 8.61 (d, 1H, JHH = 7 Hz), 8.52 (s, 1H), 7.94 (d,
3
4
3
8 Hz), 7.83 (td, 1H JHH = 8 Hz, JHH = 1.5 Hz), 7.42 (ddd, 1H, 1H, JHH = 9 Hz), 7.70–7.65 (m, 1H), 7.51–7.48 (m, 2H,),
3JHH = 7.6 Hz, JHH = 4.8 Hz, JHH = 1.2 Hz), 7.11–7.08 (m, 2H), 7.44–7.38 (m, 1H), 7.34–7.29 (m, 1H), 2.16 (sept, 2H, J = 7 Hz),
7.01–6.95 (m, 1H), 2.17 (s, 6H, CH3). 13C-NMR (75 MHz, CDCl3, 1.15–1.11 (m, 12H). 13C-NMR (75 MHz, DSMO, δ [ppm]): 145.0,
δ [ppm]): 163.2, 154.0, 150.0, 149.2, 136.2, 127.8, 126.3, 124.9, 131.7, 130.9, 129.8, 127.9, 125.6, 124.8, 124.4, 118.3, 118.1,
3
3
123.7, 120.7, 17.9.
115.7, 27.8, 24.0, 23.8. MS (ESI pos): m/z 279 [C19H23N2]+.
2,6-Diisopropyl-N-(2-pyridinylmethylene)phenylamine (1b). MS (ESI neg): m/z 145 [PF6]−. Anal. Calcd for C19H23N2PF6
Under stirring, 9.9 ml (9.3 g 0.053 mol) of 2,6-diisopropylani- (424.36) C 53.78; H 5.46, N 6.60. Found: C 53.62, H 5.19, N 6.48.
line were added to a solution of 5.0 ml pyridine-2-carboxalde-
Chlorido-{2-(2′,6′-dimethylphenyl)imidazo[1,5-a]pyridin-3-
hyde (5.6 g, 0.053 mol) in 5 ml ethanol. Then a molecular ylidene}silver(I) (3a). In a flask covered with aluminium foil,
sieve (4 Å) was added, and the reaction mixture was allowed to 2a (1.50 g, 4.07 mmol) was dissolved in 10 ml DCM. AgCl
stand for several hours. Subsequently, the molecular sieve was (0.88 g, 6.14 mmol) and powdered KOH (0.35 g, 6.06 mmol)
separated from the reaction mixture and the solvent removed were added, forming a brown suspension. After stirring for 3 h
in vacuo. Recrystallization of the residue in ethanol yielded a at ambient temperature the reaction mixture was filtered over
yellow crystalline solid. Yield: 9.2 g (65.7%). The spectro- Celite. The solvent was partially removed in vacuo, and the
scopic data correspond to the reported data.27 1H-NMR product was precipitated with diethyl ether. Yield: 1.16 g
3
1
(300 MHz, CDCl3 δ [ppm]): 8.76 (d, 1H, JHH = 4.9 Hz), 8.38 (s, (77.6%) of a yellow solid. H-NMR (300 MHz, DMSO, δ [ppm]):
1H), 8.32 (d, 1H, JHH = 7.8 Hz), 7.86 (t, 1H, JHH = 7.8 Hz, 8.69 (d, 1H, 3JHH = 7 Hz), 7.93 (s, 1H), 7.65 (d, 1H, 3JHH = 9 Hz),
3
3
4JHH = 1.2 Hz), 7.42 (ddd, 1H, JHH = 7.6 Hz, JHH = 4.9 Hz, 7.44–7.39 (m, 1H), 7.27–7.24 (m, 2H), 7.09 (dd, 1H 3JHH = 7 Hz,
4JHH = 1.2 Hz), 7.24–7.14 (m, 3H), 3.03 (sept, 2H, 3JHH = 6.9 Hz), 3JHH = 9 Hz), 6.95–6.90 (m, 1H), 1.74 (s, 6H). 13C-NMR
1.22 (d, 12H, 3JHH = 6.9 Hz). 13C-NMR (75 MHz, DMSO, δ [ppm]): (125 MHz, DMSO, δ [ppm]): 172.8, 138.2, 134.2, 130.6, 129.5,
163.2, 153.5, 149.7, 148.1, 137.2, 136.5, 126.0, 124.3, 122.9, 128.8, 128.5, 123.5, 118.0, 114.7, 113.7, 17.0. MS (ESI pos): m/z
3
3
120.8, 27.5, 23.0.
551 [C30H28N4Ag]+, 361 [C15H14N2Ag
+
CH3OH]+, 347
2-(2′,6′-Dimethylphenyl)imidazo[1,5-a]pyridin-2-ium hexa- [C15H14N2Ag + H2O]+, 329 [C15H14N2Ag]+, 223 [C15H15N2]+.
fluorophosphate (2a). The reaction was carried out in an Anal. Calcd for C30H28N4AgPF6 (697.41) C 51.67; H 4.05,
atmosphere of dry nitrogen: paraformaldehyde (1.07 g, N 8.03. Found: C 51.40, H 3.55, N 7.96.
0.036 mol, 1.2 equiv.) was completely dissolved in 250 ml hot
Chlorido-{2-(2′,6′-diisopropylphenyl)imidazo[1,5-a]pyridin-3-
toluene. After adding imine 1a (6.0 g, 0.029 mol, 1 equiv), ylidene}silver(I) (3b). Compound 3b was prepared analogously
8788 | Dalton Trans., 2014, 43, 8781–8791
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to 3a. Starting materials: 2b (5.03 g, 12.58 mmol), AgCl (2.55 g, 0.9 Hz), 7.56–7.48 (m, 2H), 7.29–7.23 (m, 3H), 7.09 (ddd, 1H,
3
4
3
17.79 mmol), KOH (1.04 g, 18.54 mmol). Yield: 2.22 g (42.9%) 3JHH = 9 Hz, JHH = 7 Hz, JHH = 0.7 Hz), 6.94 (td, 1H, JHH
=
4
3
of a yellow powder. Slow gas-phase diffusion of diethyl ether 7 Hz, JHH = 1 Hz), 2.07 (sept, 2 H, JHH = 7 Hz), 1.09 (d, 6H,
into a dilute DCM solution gave colourless crystals of 3b suit- 3JHH = 7 Hz), 0.97 (d, 6H, JHH = 7 Hz). 13C-NMR (75 MHz,
3
1
able for X-ray diffraction. H-NMR (300 MHz, DMSO, δ [ppm]): CDCl3, δ [ppm]): 176.6, 145.7, 134.4, 131.0, 130.9, 126.7, 125.0,
8.45 (d, 1H, 3JHH = 7 Hz), 8.06 (s, 1H,), 7.67 (d, 1H, 3JHH= 9 Hz), 124.2, 118.0, 116.2, 114.2, 28.4, 27.4, 24.3. Anal. Calcd for
7.53–7.48 (m, 1H), 7.29–7.27 (m, 2H), 7.12 (dd, 1H 3JHH = 7 Hz, C38H44N4AuPF6 (898.72) C 50.79; H 4.93, N 6.23. Found:
3JHH = 9 Hz), 6.97–6.93 (m, 1H), 1.99 (sept, 2H, JHH = 7 Hz), C 50.49, H 5.02, N 6.19. MS (ESI pos): m/z 753 [C38H44N4Au]+.
3
3
3
1.06 (d, 6H, JHH = 7 Hz), 0.82 (d, 6H, JHH = 7 Hz). 13C-NMR
Tribromido-{2-(2′,6′-dimethylphenyl)imidazo[1,5-a]pyridin-
(75 MHz, DMSO, δ [ppm]): 172.8, 144.7, 134.8, 130.4, 130.3, 3-ylidene}gold(III) (5a). Compound 4a (0.300 g, 0.60 mmol)
128.1, 123.9, 123.8, 118.1, 115.5, 115.4, 115.1, 27.7, 24.5, 23.6. was dissolved in 10 ml DCM and cooled with ice bath.
MS (ESI pos): m/z 665 [C38H44N4Ag]+, 417 [C19H22N2Ag + Solid CsBr3 (0.230 g, 0.61 mmol) was added, and the
CH3OH]+, 403 [C19H22N2Ag + H2O]+, 385 [C19H22N2Ag]+. Anal. reaction mixture stirred for 15 min. on ice bath and for a
Calcd for C38H44N4AgPF6 (802.63) C 56.37; H 5.48, N 6.92. further 30 min. at ambient temperature, during which the
Found: C 56.29, H 5.36, N 6.93.
colour of the reaction mixture changed from light brown to
Bromido-{2-(2′,6′-dimethylphenyl)imidazo[1,5-a]pyridin-3- orange. To remove any residues, the reaction mixture was
ylidene}gold(I) (4a). Compound 3a (0.501 g, 1.37 mmol) passed through a filter, and the solvent was partly removed
was dissolved in 10 ml DCM. To the stirred solution, solid under reduced pressure. Precipitation with pentane yielded
(tht)AuBr (0.500 g, 1.37 mmol) was added, whereupon complex 5a as a rust-brown powder. Yield: 0.232 g (58.6%).
AgCl precipitate formed. After stirring for 30 min. at ambient Slow gas-phase diffusion of pentane into a dilute DCM solu-
temperature, the AgCl was filtered off and the solvent partly tion gave red crystals of 5a suitable for X-ray diffraction.
removed under reduced pressure. Precipitation with diethyl 1H-NMR (300 MHz, DMSO, δ [ppm]): 8.82 (d, 1H, 3JHH = 7 Hz),
3
ether yielded complex 4a as a light-brown powder. Yield: 8.45 (s, 1H), 7.84 (d, 1H, JHH = 9 Hz), 7.51–7.46 (m, 1H),
0.614 g (89.8%). Slow gas-phase diffusion of diethyl ether into 7.36–7.26 (m, 3H), 7.21–7.16 (m, 1H), 2.14 (s, 6H). 13C-NMR
a dilute DCM solution gave colourless platelets of 4a suitable (75 MHz, DMSO,
δ [ppm]): 135.1, 131.4, 130.7, 128.9,
1
for X-ray diffraction. H-NMR (300 MHz, CDCl3, δ [ppm]): 8.62 128.7, 125.7, 125.2, 124.9, 118.5, 117.2, 116.2, 18.7. MS (ESI
3
3
(dd, 1H, JHH = 7 Hz), 7.44 (d, 1H, JHH = 9 Hz), 7.36–7.31 (m, pos): m/z 801 [C30H28N4AuBr2]+, 722 [C30H28N4AuBr]+,
1H), 7.21–7.17 (m, 3H), 7.00 (dd, 1H, 3JHH = 7 Hz, 3JHH = 9 Hz), 641 [C30H28N4Au]+, 301 [C15H14N2Br]+. Anal. Calcd for
6.79 (td, 1H, JHH = 7 Hz, JHH = 1 Hz), 2.02 (s, 6 H). 13C-NMR C15H14N2AuBr3 (658.97) C 27.34; H 2.14, N 4.25. Found: C
3
4
(75 MHz, CDCl3, δ [ppm]): 176.6, 137.6, 134.9, 130.4, 130.1, 27.51, H 1.87, N 4.27.
128.9, 128.0, 124.1, 117.7, 114.6, 111.7, 18.0. 1H-NMR
Dibromido-bis{2-(2′,6′-diisopropylphenyl)imidazo[1,5-a]-
3
(300 MHz, DMSO, δ [ppm]): 8.61 (d, 1H, JHH = 7 Hz), 8.01 (s, pyridin-3-ylidene}gold(III) hexafluorophosphate (5b). Com-
1H), 7.69 (d, 1H, 3JHH = 9 Hz), 7.46–7.41 (m, 1H), 7.33–7.31 (m, pound 4b (0.100 g, 0.11 mmol) was dissolved in 20 ml
3
3
3
2H), 7.12 (dd, 1H, JHH = 7 Hz, JHH = 9 Hz), 6.97 (t, 1H, JHH
=
DCM and cooled with ice bath. Solid CsBr3 (0.070 g,
7 Hz), 1.98 (s, 6H). 13C-NMR (75 MHz, DMSO, δ [ppm]): 164.9, 0.19 mmol, 1.7 equiv.) was added, and the reaction mixture
137.7, 134.5, 129.9, 129.8, 128.5, 127.0, 123.9, 118.3, 115.5, stirred for 15 min. on ice bath and for a further 30 min. at
113.4, 17.2. MS (ESI pos): m/z 641 [C30H28N4Au]+. Anal. Calcd ambient temperature, during which the colour of the reaction
for C15H14N2AuBr (499.16) C 36.09; H 2.83, N 5.61. Found: C mixture changed from light brown to red. To remove any resi-
36.15, H 2.66, N 5.56.
dues, the reaction mixture was passed through a filter, then
Bis{2-(2′,6′-diisopropylphenyl)imidazo[1,5-a]pyridin-3-ylidene}- the solvent was partly removed under reduced pressure. Pre-
gold(I) hexafluorophosphate (4b). Compound 3b (0.231 g, cipitation with diethyl ether yielded complex 5b as a rust-
0.55 mmol) was dissolved in 10 ml DCM. To the stirred brown powder. Yield: 0.112
g (95.5%). Slow gas-phase
solution, solid (tht)AuBr (0.200 g, 0.55 mmol) was added, diffusion of diethyl ether into a dilute DCM solution gave
1
whereupon AgCl precipitate formed. After stirring for bright red crystals. H-NMR (300 MHz, DMSO, δ [ppm]): 9.00
3
3
30 min. at ambient temperature, the AgCl was filtered off and (d, 1H, JHH = 7 Hz), 8.48 (s, 1H,), 7.82 (d, 1H, JHH = 9 Hz),
3
the solvent partly removed under reduced pressure. Precipi- 7.62–7.57 (m, 1H), 7.35–7.21 (m, 4H), 2.17 (sept, 2H, JHH
=
3
3
tation with diethyl ether yielded a light-brown powder. The 7 Hz), 0.95 (d, 6H, JHH = 7 Hz), 0.87 (d, 6H, JHH = 7 Hz).
1H-NMR of the crude compound is not clean; it possibly shows 13C-NMR (125 MHz, DMSO, δ [ppm]): 144.7, 137.7, 132.1,
a mixture of ionic and neutral gold(I) compound. Dissolving 131.6, 131.0, 126.2, 125.0, 124.4, 120.2, 118.5, 117.1, 28.2, 26.3,
the crude compound in DCM, filtering over celite, and precipi- 22.2. MS (ESI pos): m/z 913 [C38H44N4AuBr2]+, 834
tating with diethyl ether yielded the clean product (as a yellow [C38H44N4AuBr]+, 753 [C38H44N4Au]+. Anal. Calcd for
solid), which appears to be the cationic gold(I) carbene. Yield: C38H44N4AuBr2PF6 (1058.53) C 43.12; H 4.19, N 5.29. Found:
0.283 g (93.1%) of a pale yellow powder. Slow gas-phase C 43.05, H 4.07, N 5.76.
diffusion of diethyl ether into a dilute DCM solution gave
Tribromido-{2-(2′,6′-diisopropylphenyl)imidazo[1,5-a]pyridin-3-
yellow platelets of 4b suitable for X-ray diffraction. 1H-NMR ylidene}gold(III) (5b*). 1H-NMR (300 MHz, DMSO, δ [ppm]):
3
4
(300 MHz, CDCl3, δ [ppm]): 8.11 (dd, 1H, JHH = 7 Hz, JHH
=
8.83 (d, 1H, JHH = 7 Hz), 8.58 (s, 1H), 7.69–7.63 (m, 1H),
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3
7.47–7.44 (m, 4H), 7.32–7.25 (m, 1H), 2.05 (sept, 2H, JHH
7 Hz), 1.13 (d, 6H, 3JHH = 7 Hz), 1.06 (d, 6H, 3JHH = 7 Hz).
=
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Acknowledgements
We thank Prof. Benno Bildstein (University Innsbruck) and
Dr I. Abfalter (JKU) for valuable suggestions. The authors
thank Prof. G. Knör and the JKU for continuous and generous
support of the experimental work. The NMR spectrometers
were acquired in collaboration with the University of South 10 (a) N. Schneider, S. Bellemin-Laponnaz, H. Wadepohl and
Bohemia (CZ) with financial support from the European
Union (EU) through the EFRE INTERREG IV ETC-AT-CZ pro-
gramme (project number M00146, “RERI-uasb”).
L. H. Gade, Eur. J. Inorg. Chem., 2008, 5587–5598;
(b) L. H. Gade, G. Marconi, C. Dro, B. D. Ward, M. Poyatos,
S. Bellemin-Laponnaz, H. Wadepohl, L. Sorace and
G. Poneti, Chem. – Eur. J., 2007, 13, 3058–3075.
11 M. Kriechbaum, J. Hölbling, H.-G. Stammler, M. List,
R. J. F. Berger and U. Monkowius, Organometallics, 2013,
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