Gold(I) complexes of tetrathiaheterohelicene phosphanes

Article abstract of DOI:10.1021/ic4005533

New tetrathia[7]helicene-based (7-TH-based) gold(I) complexes 6 and 7 have been readily prepared by reaction of the respective phosphine ligands 2 and 3 with Au(tht)Cl in a 1:1 and 1:2 molar ratio, respectively. These complexes have been fully characterized by analytical and spectroscopic techniques as well as quantum chemical calculations. The molecular structure of dinuclear complex 7 has been determined by single-crystal X-ray diffraction, showing a gold-gold interaction of 3.1825(3) A and a significant contraction of the 7-TH total dihedral angle. Au(I) complex 7 displays luminescence emission at room and low temperature in diluted solution and in the solid state. Quantum chemical calculations show that the luminescence emission at room temperature is primarily due to slightly perturbed fluorescence emission from purely ππ* excited states of the conjugated helicene scaffold. At 77 K phosphorescence emission is displayed as well. Preliminary studies on the use of 6 and 7 as catalysts in typical Au(I)-catalyzed cycloisomerizations have demonstrated the reactivity of these systems in the intramolecular allene hydroarylations and the hydroxycarboxylation of allene-carboxylates.

Full text of DOI:10.1021/ic4005533

Article
pubs.acs.org/IC
Gold(I) Complexes of Tetrathiaheterohelicene Phosphanes
Silvia Cauteruccio,† Annette Loos,§ Alberto Bossi,‡ Maria Camila Blanco Jaimes,§ Davide Dova,†
Frank Rominger,§ Stefan Prager,∥ Andreas Dreuw,∥ Emanuela Licandro,*,† and A. Stephen K. Hashmi*,§
†Dipartimento di Chimica, Universita
̀
degli Studi di Milano, via Golgi 19, I-20133 Milan, Italy
‡Organisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
§Istituto di Scienze e Tecnologie Molecolari, CNR, via Fantoli 16/15, I-20138 Milan, Italy
∥Interdisziplinares Zentrum fur Wissenschaftliches Rechnen, Universitat Heidelberg, Im Neuenheimer Feld 368, 69120 Heidelberg,
̈
̈
̈
Germany
ABSTRACT: New tetrathia[7]helicene-based (7-TH-based) gold(I) complexes 6
and 7 have been readily prepared by reaction of the respective phosphine ligands 2
and 3 with Au(tht)Cl in a 1:1 and 1:2 molar ratio, respectively. These complexes have
been fully characterized by analytical and spectroscopic techniques as well as quantum
chemical calculations. The molecular structure of dinuclear complex 7 has been
determined by single-crystal X-ray diffraction, showing a gold−gold interaction of
3.1825(3) Å and a significant contraction of the 7-TH total dihedral angle. Au(I)
complex 7 displays luminescence emission at room and low temperature in diluted
solution and in the solid state. Quantum chemical calculations show that the luminescence emission at room temperature is
primarily due to slightly perturbed fluorescence emission from purely ππ* excited states of the conjugated helicene scaffold. At 77
K phosphorescence emission is displayed as well. Preliminary studies on the use of 6 and 7 as catalysts in typical Au(I)-catalyzed
cycloisomerizations have demonstrated the reactivity of these systems in the intramolecular allene hydroarylations and the
hydroxycarboxylation of allene-carboxylates.
Helicenes are an extremely attractive class of conjugated
molecules, which combine the electronic properties afforded by
their conjugated system with the chiroptical properties
provided by their peculiar helix-like structure, resulting from
the ortho annulation of a series of aromatic and/or
heteroaromatic rings.10
INTRODUCTION
■
Gold chemistry is currently one of the most rapidly growing
fields of chemistry because of its relevance to a large number of
topics, including catalysis,1 materials science,2 and medicine.3 In
particular, gold(I) complexes incorporating phosphines have
been widely developed during the past few years, especially for
their use as efficient and selective homogeneous catalysts in
organic transformations,4 but also for their intriguing physical
properties, including luminescence,5 as well as for their
promising anticancer activity.6
Homogeneous catalysis promoted by Au(I) complexes has
emerged as a powerful synthetic tool for organic synthesis,4
with phosphines and N-heterocyclic carbenes7 as the ligands of
choice for gold(I)-based catalysts. Moreover, asymmetric gold-
catalyzed reactions have been applied to a variety of
transformations that provide versatile routes leading to the
enantioselective formation of new carbon−carbon or carbon−
heteroatom bonds.7a,8 On the other hand, several mono- and
polynuclear organophosphane-Au(I) complexes have been
spectroscopically investigated and reviewed in the literature
due to their fascinating luminescence properties.5 So far,
different types of electronic transitions have been indicated as
the origin of their luminescence. Several parameters may
influence the photophysical behavior of Au(I) systems, but the
nature of the phosphine ligand often plays the key role.5,9
Despite a large number of gold(I) complexes bearing
different mono- or diphosphine ligands being studied, to the
best of our knowledge no example of gold(I) complexes
containing helical-based phosphines has been reported to date.
These structures display unique physicochemical properties,
which have stimulated countless studies oriented toward
chiroptical devices,11 material chemistry,12 organometallic
species,13 and organocatalysis.14 In the general context of the
growing interest in helical derivatives, we have focused our
attention on the study of tetrathiahelicene (7-TH) derivatives,
such as compound 1 (Chart 1), that represent a class of
configurationally stable heterohelicenes with intrinsic asymme-
try, potentially very interesting for applications in optoelec-
tronics15−17 as well as new chiral ligands for both organic18 and
organometallic catalysis.19 In addition, these systems can be
easily and selectively functionalized at the α positions of the
two terminal thiophene rings,20 allowing the modulation of the
chemical and physical properties by varying the nature of the
substituents.
In the course of our studies on the applications of 7-TH
derivatives in the field of the organometallic chemistry, we have
described the synthesis of organometallic derivatives of
tetrathia[7]helicenes for some ruthenium(II) and iron(II)
complexes, which have been shown to be potential interesting
Received: March 5, 2013
© XXXX American Chemical Society
A
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Chart 1. Structures of 7-TH 1 and Its Phosphane Derivatives 2 and 3
spectrometer, and the chemical shifts were reported relative to
residual protonated solvent resonances. 31P{1H} spectra were recorded
on a Bruker AC 300 MHz spectrometer, and the chemical shifts were
reported relative to external standards H3PO4 (85%). HRMS data
were recorded with the mass spectrometer Bruker Daltonics ICR-
FTMS APEX II or with a Vg Analytical 7070 EQ. Steady-state
emission and excitation spectra were measured using a Horiba
scientific FluoroLog 3. The room temperature luminescence emissions
of complex 7 were collected in dichloromethane (DCM) solution
deaerated under argon bubbling. Low temperature measurements were
carried out in 2-methyl tetrahydrofuran frozen glass at 77 K.
Monoborane Adduct (4). A solution of nBuLi (1.6 M in hexane,
0.772 mL, 1.234 mmol, 1.5 equiv) was added dropwise to a stirring
solution of 1 (400 mg, 0.823 mmol) in dry THF (20 mL) at −78 °C
under a nitrogen atmosphere. The solution was stirred for 30 min at
−78 °C. The resulting yellow solution was treated with Ph2PCl (0.221
mL, 1.234 mmol, 1.5 equiv) at −78 °C, and the progress of the
reaction was monitored by TLC (hexane/CH2Cl2, 7:3). After 5 h at
−78 °C, the yellow solution was warmed to 0 °C and treated with a
solution of BH3·THF (1 M in THF, 8.23 mL, 8.23 mmol, 10 equiv).
The yellow suspension was stirred for 30 min at 0 °C, and then
warmed to ambient temperature overnight. The suspension was slowly
added to a saturated aqueous NH4Cl solution (50 mL), and the
aqueous phase was extracted with CH2Cl2 (4 × 20 mL). The organic
phase was washed with water (2 × 20 mL), dried with Na2SO4, and
concentrated under reduced pressure. The crude product was purified
by chromatography on silica gel (hexane/CH2Cl2, 7:3) to give 4 as
pale yellow solid. Yield: 70% (394 mg, 0.576 mmol). The spectral
properties of this compound were in agreement with those previously
reported.19
candidates for electro-optical and nonlinear optical (NLO)
purposes.17a
More recently, we have reported the synthesis of 7-TH-based
phosphane derivatives like mono- and diphosphane 2 and 3
(Chart 1), which were isolated as air-stable borane adducts 4
and 5, respectively.19 In fact, alkyl chains in the 7- and 8-
positions of the helical system improve the solubility of
helicenes in organic solvents, but also render the phosphorus
atoms more electron-rich and then more prone to air oxidation.
The enantiopure P-(3) was employed by some of us as chiral
ligand to form a chelated Rh complex, which was used in the
asymmetric hydrogenation of the dehydroalanine achieving
almost quantitative yield but a modest 40% ee.19
In view of our interest to further investigate the coordinating
ability of phosphanes 2 and 3 toward transition metals, we
decided to study innovative complexes of gold(I) with helical
phosphines, especially with tetrathiahelicene-based phosphines
as ancillary ligands. In this Article, we describe the synthesis and
the characterization of gold(I) complexes 6 and 7 (Chart 2)
Chart 2. Structure of the Au(I) Complexes 6 and 7 under
Investigation
Monophosphine (2). A suspension of 4 (50 mg, 0.073 mmol) in a
mixture of EtOH/THF (7:4) (25 mL) was stirred and heated to reflux
(the suspension became a yellow solution when hot) under an argon
atmosphere, and the progress of the reaction was monitored by 31P
NMR analysis. After 4 h, the yellow solution was cooled to room
temperature, and the solvents along with triethyl borate were removed
under reduced pressure at 50 °C to afford 2 as pale yellow solid. Yield:
98% (48 mg, 0.071 mmol). The spectral properties of this compound
were in agreement with those previously reported.19
Diphosphine (3). A solution of 5 (120 mg, 0.136 mmol), in a
mixture of EtOH/THF (2:1) (50 mL) was stirred and heated to reflux
under an argon atmosphere, and the progress of the reaction was
monitored by 31P NMR analysis. After 3 h, the yellow solution was
cooled to room temperature, and the solvents together with triethyl
borate as byproduct were removed under reduced pressure at 50 °C to
afford 3 as yellow solid. Yield: 98% (115 mg, 0.134 mmol). The
spectral properties of this compound were in agreement with those
previously reported.19
prepared from phosphanes 2 and 3, respectively, along with the
X-ray characterization of the dinuclear complex 7. In addition, a
preliminary investigation of the luminescence properties of the
dinuclear gold(I) complex 7 has been undertaken, and the
emission data are reported. High-level quantum chemical
calculations demonstrate the emission occurs from purely ππ*
excited states of the helical ligand scaffold. Finally, we also
report a first study demonstrating the use of complexes 6 and 7
as catalysts in a set of typical Au(I) catalyzed cycloisomerization
reactions.
EXPERIMENTAL SECTION
■
General. The syntheses of gold(I) complexes 6 and 7 were
performed under an atmosphere of argon by means of standard
Schlenk and vacuum-line techniques in flame-dried glassware. To
monitor the progress of these reactions, thin-layer chromatography
(TLC) was performed with Merck silica gel 60 F254 precoated plates.
Anhydrous THF and CH2Cl2 (Aldrich) were purchased and purged
with argon prior to use. Reagents obtained from commercial sources
were used without further purification. Diborane adduct 5 was
prepared as previously reported.19 Au(tht)Cl21 was prepared according
Mononuclear Gold(I) Complex (6). To a solution of 2 (40.0 mg,
0.059 mmol, 1 equiv) in dry and degassed CH2Cl2 (3 mL) was added
Au(tht)Cl (18.7 mg, 64.6 μmol, 1 equiv) under an argon atmosphere.
The resulting yellow solution was stirred at room temperature for 1 h,
and then the solvent was removed under reduced pressure to afford 6
as pale yellow powder. Yield: 99% (52 mg, 0.059 mmol). Mp 150 °C
1
(dec). H NMR (CDCl3, 600 MHz): δ = 1.16 (m, 6 H), 1.85 (m, 4
H), 3.07 (m, 2 H), 3.13 (m, 2 H), 6.72 (d, J = 5.5 Hz, 1 H), 7.00 (d, J
= 5.5 Hz, 1 H), 7.19 (d, J = 10.1 Hz, 1 H), 7.25 (m, 2 H), 7.33 (m, 6
H), 7.43 (m, 1 H), 7.48 (m, 1 H), 7.94 (m, 2 H), 8.06 (m, 2 H) ppm.
13C{1H} NMR (CDCl3, 150 MHz): δ = 14.67 (s, CH3), 14.71 (s,
to literature procedure. Melting points were determined with a Buchi
̈
1
510 apparatus and are uncorrected. The H, 13C{1H} spectra were
recorded on the Bruker Avance 600 MHz Ultra Shield Plus
B
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CH3), 23.2 (s, CH2, 2 C), 34.35 (s, CH2), 34.38 (s, CH2), 119.0 (s,
CH), 120.2 (s, CH), 121.2 (s, CH), 122.4 (s, CH), 124.5 (s, CH),
125.2 (s, CH), 127.4 (s, Cq), 127.9 (d, JC,P = 62.8 Hz, Cq), 128.2 (s,
Table 1. Crystallographic Data for Compound 7
1
formula
C52.50H41Au2Cl3P2S4 (7)
1
1
Cq), 128.8 (d, JC,P = 64.1 Hz, Cq), 129.00 (d, JC,P = 64.8 Hz, Cq),
mol wt
1362.32
200(2)
3
3
129.01 (d, JC,P = 12.3 Hz, CH), 129.04 (d, JC,P = 12.3 Hz, CH),
130.6 (s, Cq), 131.86 (d, JC,P = 2.3 Hz, Cq), 131.9927 (s, Cq),
T, K
4
crystal syst
space group
a, Å
monoclinic
P21/c
4
131.9944 (d, JC,P = 2.3 Hz, Cq), 132.09 (s, Cq), 133.0 (s, Cq), 133.5
(d, 2JC,P = 14.4 Hz, CH), 133.7 (d, 2JC,P = 14.3 Hz, CH), 135.2 (s, Cq),
135.5 (d, 3JC,P = 13.3 Hz, Cq), 136.5 (s, Cq), 136.7 (s, Cq), 136.8 (d,
2JC,P = 14.0 Hz, CH), 136.9 (s, Cq), 140.0 (s, Cq), 140.1 (s, Cq), 141.7
12.3030(1)
30.9040(1)
12.8265(1)
90
b, Å
c, Å
(d, JC,P = 5.1 Hz, Cq) ppm. 31P NMR (CDCl3, 121 MHz): δ = 22.3
4
α, deg
β, deg
γ, deg
V, Å3
(s) ppm. MS (FAB+): m/z (%) = 902 (20) [M]+, 867 (100) [M −
Cl]+. HRMS (EI): calcd for C60H52S4PClAu 902.0400; found
902.0434.
90.411(1)
90
4876.66(6)
4
Dinuclear Gold(I) Complex (7). To a solution of 3 (110 mg, 0.129
mmol, 1 equiv) in dry and degassed CH2Cl2 (3 mL) was added
Au(tht)Cl (82.73 mg, 0.258 mmol, 2 equiv) under an argon
atmosphere. The resulting yellow solution was stirred at room
temperature for 1 h, and then the solvent was removed under reduced
pressure to afford 7 as yellow crystals. Yield: 99% (168 mg, 0.128
Z
D
calcd, g cm−3
1.86
F(000)
2636.0
cryst size (mm3)
μ, mm−1
0.11 × 0.10 × 0.08
6.45
1
mmol). Mp 310−311 °C (dec). H NMR (CDCl3, 600 MHz): δ =
reflns collected
reflns unique
reflns obsd [I > 2σ(I)]
params
48 434
1.15 (t, J = 7.3 Hz, 6 H), 1.85 (m, 4 H), 3.08 (m, 2 H), 3.14 (m, 2 H),
7.10 (m, 4 H), 7.15 (d, J = 10.2 Hz, 2 H), 7.21 (m, 4 H), 7.36 (m, 2
H), 7.42 (m, 8 H), 7.55 (m, 2 H), 8.05 (d, J = 8.6 Hz, 2 H), 8.09 (d, J
= 8.6 Hz, 2 H) ppm. 13C{1H} NMR (CDCl3, 150 MHz): δ = 14.7 (s,
CH3, 2 C), 23.3 (s, CH2, 2 C), 34.4 (s, CH2, 2 C), 121.2 (s, CH, 2 C),
11131 (Rint = 0.0575)
8717
596
a
R indices [I > 2σ(I)]
R1 = 0.039, wR2 = 0.073
1
R indices (all data)
R1 = 0.0610, wR2 = 0.0798
122.3 (s, CH, 2 C), 127.4 (s, Cq, 2C), 128.2 (d, JC,P = 61 Hz, Cq, 2
C), 128.6 (d, 1JC,P = 63 Hz, Cq, 2 C), 128.9 (d, 1JC,P = 63 Hz, Cq, 2 C),
a
R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. wR2 = [Σ[w(Fo2 − Fc2)2]/Σ[w (Fo )2]]1/2
.
2
3
3
129.0 (d, JC,P = 12.1 Hz, CH, 4 C), 129.4 (d, JC,P = 12.2 Hz, CH, 4
4
C), 130.8 (s, Cq, 2 C), 131.8 (d, JC,P = 2.4 Hz, CH, 2 C), 132.4 (d,
Quantum Chemical Calculations. The equilibrium structures of
4JC,P = 2.3 Hz, CH, 2 C), 132.9 (s, Cq, 2 C), 133.2 (d, 2JC,P = 14.6 Hz,
the ground states of compounds 3, 5, and 7 have been optimized at the
theoretical level of density functional theory (DFT)23 using the long-
range separated exchange-correlation (xc) functional ωB97X,24 the 6-
31G* basis set for the light atoms, and the LANL2DZ effective core
potential for gold as implemented into Gaussian09 Rev. A02.25 The
lowest excited states of these compounds have been computed with
time-dependent DFT (TDDFT)26 employing the same xc-functional
basis set combination. An evaluation of xc-functionals with varying
amounts of nonlocal Hartree−Fock (HF) exchange has demonstrated
that at least 50% are required to avoid the occurrence of spurious low-
lying charge-transfer excited states and to achieve a reasonable
agreement with the experimental spectra. Here, ωB97X has turned out
to yield the most reliable results and to be required for reliable excited
state geometry optimizations.27 However, the high amount of HF
exchange leads to an essentially constant shift of the computed spectra
to higher excitation energies as compared to the experimental ones.28
For the interpretation of the fluorescence spectrum of 7 the
equilibrium structures of the two lowest singlet S1 and S2 states
have been optimized. The properties of the compounds 3 and 5 were
computed for the gas phase only, while for 7, the calculations were
performed in combination with a polarizable continuum model (C-
PCM) for dichloromethane solution.29 While all optimizations have
been performed using equilibrium solvation, electronic transitions are
computed with its nonequilibrium counterpart. In general it turned out
that modeling DCM solvation has only a negligible effect on structures
and spectra of compound 7.
2
3
CH, 4 C), 133.9 (d, JC,P = 14.4 Hz, CH, 4 C), 135.0 (d, JC,P = 13.9
Hz, Cq, 2 C), 136.2 (d, 2JC,P = 14.7 Hz, CH, 2 C), 137.1 (s, Cq, 2 C),
140.3 (s, Cq, 2 C), 141.8 (d, JC,P = 4.5 Hz, Cq, 2 C) ppm. 31P NMR
4
(CDCl3, 121 MHz): δ = 22.4 (s) ppm. MS (FAB+): m/z (%) = 1283
(100) [M − Cl]+, 1051 (48) [M − AuCl2]+. HRMS (ESI): calcd for
C52H40S4P2Cl2Au2Na (M+) 1341.0088; found 1341.0097. Anal. Calcd
for C52H40S4P2Cl2Au2 (Mw 1318.1): C, 47.32; H, 3.05. Found: C,
47.04; H, 3.29. Yellow crystals of 7 for X-ray diffraction were grown
from CH2Cl2/hexane mixture at room temperature over a period of
several hours.
General Procedure for Gold-Catalyzed Reactions. The specific
substrate and the internal standard were dissolved in deuterated
solvent, and an NMR spectrum was measured. Catalyst 6 or 7 together
with AgNTf2 or AgOTf was added, and the tube was shaken vigorously
at room temperature. The yield of the products was determined by
integrating the NMR signals against the internal standard.
For entries 1−3 of Table 5, the following conditions were used: 8
(30.0 mg, 200 μmol), tri-tert-butylbenzene (2 mg, internal standard),
CDCl3 (500 μL), 6 or 7, and AgNTf2 (0.5−4 μmol, 0.25−2 mol %, a 1
wt % solution in CDCl3). Yield of 9: 4−23%.
For entries 4−5 of Table 5, the following conditions were used: 10
(42.0 mg, 200 μmol), tri-tert-butylbenzene (2 mg, internal standard),
CD2Cl2 (500 μL), 6 or 7, and AgNTf2 (4 μmol, 2 mol %, a 1 wt %
solution in CD2Cl2). Yield of 11: 15−30%. Yield of 12: 5−20%.
For entries 6−8 of Table 5, the following conditions were used: 13
(41.6 mg, 125 μmol), hexamethylbenzene (2 mg, internal standard),
CD2Cl2 (500 μL), 6 or 7, and AgNTf2 (1.25−6.25 μmol, 2 mol %, a 1
wt % solution in CD2Cl2). Yield of 14: 65−99%.
For entry 9 of Table 5, the following conditions were used: 15 (12.8
mg, 100 μmol), CHCl3 (internal standard), C6D6 (500 μL), 7 (2.5 mol
%), and AgOTf (5 mol %). Yield of 16: 99%.
X-ray Crystallographic Analysis for compound 7. Data for the
X-ray crystallographic analysis of 7 were collected on a Bruker Smart
CCD diffractometer (Mo Kα radiation, λ = 0.710 73 Å). Crystallo-
graphic and experimental details are summarized in Table 1. The
structure was solved by direct methods and refined against F2 with full-
matrix least-squares algorithm using the software package SHELXTL
2008/1 for structure solution and refinement.22
RESULT AND DISCUSSION
■
Improvement on the Selective Synthesis of Mono
Adduct 4. Since diphosphane 3 is easily oxidized by the
oxygen of air, we developed an efficient one-pot procedure to
prepare the corresponding air-stable borane adduct 5, which
could be readily converted into the free phosphane 3 (Chart
3).19
On the contrary, the first studies aimed at preparing
monoborane adduct 4, from which the free monophosphane
2 can be recovered, gave unsatisfactory results in terms of yield
and selectivity. In fact, the reaction between helicene 1 with 2
C
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the structure of dinuclear complex 7 was also fully elucidated by
X-ray crystal structure analysis, while we have been unable to
obtain suitable single crystals for the mononuclear complex 6,
which was significantly less soluble in the organic solvents than
complex 7. Finally, taking into account that diphosphine 3 was
able to generate chelate monodentate complexes with Rh(I)
salts,19 many efforts have been carried out to synthesize a
chelate monodentate gold(I) complex by reacting 3 with
Au(tht)Cl in 1:1 molar ratio. Unlike Rh(I) salts, Au(tht)Cl
afforded no chelate complex with 2. Presumably, this is due to
the too large P−P distance (up to 5 Å) between the two PPh2
groups in the helical structure of 3, in combination with the
great tendency of gold(I) centers to form linear two-coordinate
complexes.
Crystal Structure of Dinuclear Complex 7. Polyhedron
yellow crystals of dinuclear complex 7 were grown from layered
CH2Cl2/hexane, and its solid-state structure was determined by
X-ray crystallography. The structure of 7 consists of a pair of
AuCl bridged by one bis(phosphine)ligand as shown in Figure
1. Binuclear gold(I) complexes are commonly found with
bidentate phosphine ligands. The gold(I) centers do not
interact with the thiophene moieties, demonstrating their
strong preference for phosphine donors.31 The high values of
intra- and intermolecular distances between gold(I) atoms and
the thiophene sulfur atoms indicate no bonding interaction. On
the other hand, theoretical investigations32 along with
experimental studies33 on the interactions between thiophene
rings and gold(I) centers are in agreement with our results.
Compound 7 crystallizes in space group P21/c with one-half
of a molecule of CH2Cl2 in the asymmetric unit, and both
enantiomers are present in the crystal.
Chart 3
equiv of nBuLi and Ph2PCl, followed by the addition of an
excess of BH3·THF, provided the monoborane 4 in 39% yield
together with the diborane 5 in 49% yield.19 We found that the
selectivity toward the formation of monoborane 4 can be much
improved by using the proper amount of base and
chlorodiphenylphosphine. In particular, the use of 1.5 equiv
of nBuLi and 1.5 equiv of Ph2PCl provided the mono adduct 4
in 70% yield, together with only a 10% of unreacted helicene 1,
and no disubstituted borane adduct 5 was observed in the crude
reaction mixture (Scheme 1).
Both monoborane and diborane adduct 4 and 5 can be
quantitatively converted into the corresponding free mono- and
diphosphane 2 and 3, respectively, by heating to reflux in a
mixture of MeOH or EtOH and THF under an inert
atmosphere.19,30
Synthesis of Mono- and Dinuclear Au(I) Complexes 6
and 7. Exploiting this general and reliable synthetic procedure
to prepare 7-TH mono- and diphosphane ligands for transition
metals, we focused our attention on the synthesis of the new
mono- and dinuclear gold(I) complexes 6 and 7 in view of their
potential applications in optoelectronics as well as in
homogeneous catalysis.
In Table 2 selected bond lengths and angles for 7 are
reported, and there are no unusual features of the gold
coordination geometry while some distortions are found in the
helical system. The Au−Cl bond lengths [2.30 and 2.28 Å] are
longer than the P−Au bonds [2.24 and 2.23 Å], and fall within
the expected range for other related phosphinogold(I) halide
complexes. The P−Au−Cl fragments are nearly linear with
angles of 175.4° and 172.4°, and they are almost perpendicular
to each other, with absolute value of Cl−Au···Au−Cl torsion
angle of 93°. The most striking feature of this complex is the
Au···Au distance of 3.18 Å, which indicates a moderate
intramolecular aurophilic interaction. Such interactions are
common in the gold(I) chemistry, and, in this case, could be
accountable for the unusual small value of 45° of the total
dihedral angle between the two terminal thiophene rings. In
fact, unsubstituted helicene 1 shows a total dihedral angle of
53°, and 2,13-disubstituted helicenes, for example with bulky
alkyl silyl groups, display much higher angles (up to 59°).34
However, all of the seven fused rings show distortion from
planarity, and the dihedral angles between adjacent rings
increases passing from the “external” thiophene rings to the
Phosphinogold(I) chloride complexes are most commonly
prepared by substitution reaction of the weakly coordinated
tetrahydrothiophene (tht) in Au(tht)Cl with an appropriate
phosphine. Therefore, mono- and diphosphane 2 and 3 were
reacted with 1 and 2 equiv of Au(tht)Cl, respectively, in
CH2Cl2 at room temperature for 1 h (Scheme 2).
Evaporation of the solvent afforded air-stable yellow solids 6
and 7 in nearly quantitative yields. The complexes were fully
characterized by means of analytical and spectroscopic analyses
whose results were consistent with the proposed structures for
these compounds. In particular, the 31P{1H} NMR spectra of 6
and 7 display one sharp singlet at δ +22 ppm, that is a value
comparable with those found in other gold(I) complexes with
mono- and diphenyl phosphines.5d Coordination to Au(I)
produced the expected downfield shift (ca. 40 ppm) with
respect to the free phosphanes 2 and 3 (−15 ppm). FAB mass
spectra of 6 and 7 gave the corresponding parent peak at m/z
867 and 1283, respectively, attributable to the molecular ions
resulting from loss a chlorine atom [M − Cl]+. These results
are consistent with one gold center per ligand in complex 6 and
two gold centers per ligand in complex 7. As described below,
Scheme 1. Synthesis of Monoborane Adduct 4
D
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Scheme 2. Synthesis of Au(I) Complexes 6 and 7
property. The origin of Au(I) complexes’ optical (absorption−
emission) behaviors has been ascribed to different types of
electronic transitions. These include the limiting situations of
electronic transitions between metal centered states (MC) or
between ligand centered states (LC)36a according to the nature
of the phosphine,9,36b,c and/or the presence of metal−metal
cluster interaction.37
The gold(I) complex 7 shows luminescence emission both in
diluted room temperature solution as well as in the solid state.
For sake of comparison, the absorption and luminescence
features of 7 have been compared with those collected for the
phosphino-borane adduct 5. Adduct 5 has been chosen as
suitable reference chromophore since it is air-stable (contrarily
to the free phosphine 3),19 and the electronic environment
surrounding the P atoms is the same as in the metal complex 7
(i.e., the P lone pair is employed to coordinate a boron atom or
the gold one).
The preliminary results of this study are depicted in Figures 2
and 3. Figure 2 reports the absorption (inset), emission, and
Figure 1. ORTEP view of dinuclear complex 7. Hydrogen atoms are
omitted for clarity, and thermal ellipsoids are drawn at 50%
probability.
Table 2. Selected Bond Lengths and Angles for 7
Distances, Å
Au(1)−P(1)
Au(2)−P(2)
2.2438(14)
2.2317(14)
2.3078(14)
2.2868(16)
3.1825(3)
1.429(7)
C(5)−C(6) outer
C(8)−C(9) inner
C(11)−C(12) outer
C(15)−C(20) inner
C(17)−C(18) outer
C(20)−C(21) inner
C(9)−C(14) inner
1.360(8)
1.449(7)
1.396(8)
1.423(7)
1.364(8)
1.436(7)
1.428(7)
Au(1)−Cl(1)
Au(2)−Cl(2)
Au(1)−Au(2)
C(2)−C(3) inner
C(3)−C(8) inner
1.418(7)
Angles, deg
P(1)−Au(1)−Cl(1)
P(2)−Au(2)−Cl(2)
P(2)−Au(2)−Au(1)
175.44(5)
172.40(6)
93.82(4)
Cl(2)−Au(2)−Au(1)
C(22)−P(1)−Au(1)
C(1)−P(2)−Au(2)
93.78(5)
Figure 2. Normalized room temperature emission (straight lines) and
excitation (dashed lines) spectra of compounds 5 and 7 in DCM
solution (the solution of 7 was deaerated with argon bubbling).
112.90(17)
115.29(17)
“central” benzene ring of the helicene. Finally, the outer core
C−C bond lengths of 7 are shorter and those of the inner core
are longer than the normal value of 1.39 Å, in common with
similar compounds,19,34,35 implying a lower π character for the
inner-core C−C bonds and a higher π character for the outer-
core ones (i.e., higher π electron density along the periphery of
the molecule).
excitation profiles of 7 and 5 at room temperature in deaerated
dichloromethane solution, along with the observed emissions at
77 K in the rigid 2-MeTHF glass.
The UV−vis absorption spectra of 5 and 7 (inset Figure 2)
closely resemble each other, with 7 only 6 nm red-shifted
compared to 5. This is due to the presence of the gold atoms.
In both systems, the UV absorptions are characterized by a low
energy (350−420 nm) IL transition of HOMO to LUMO
Photoluminescence Studies on Complex 7. The
luminescence displayed by Au(I) complexes is an interesting
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Figure 4. Normalized emission spectra of the solid 7 at room
temperature and at 77 K.
Figure 3. Normalized emission (straight lines) and excitation (dashed
lines) spectra of compounds 5 and 7 at 77 K in 2-MeTHF rigid glass.
populated at room temperature and not accessible at low
temperature.
Quantum Chemical Calculations. The equilibrium
structures of the S0, S1, S2, and T1 electronic states of 7 have
been separately optimized, and selected geometrical parameters
are compiled in Table 3. As can be readily seen, the computed
(π−π*) character.38,17b This implies that, together with the
similar UV−vis shape, the 7-TH phosphine chromophore
dominates the optical feature of the gold derivative 7.
Complex 7 emits at 430 nm in diluted solution at room
temperature (red line, Figure 2), and the corresponding
excitation profile (dashed red line) matches the electronic
absorption spectra (inset Figure 2) already described. In the
same conditions, the borane adduct 5 (blue line) fluoresces at
420 nm. Therefore, it should be concluded that the 430 nm
emission of 7 could be attributed to the IL fluorescence of 7-
TH slightly perturbed by the presence of Au(I) atoms.
In rigid 2-MeTHF at 77 K, complex 7 displays two set of
emissions at 400−500 and 530−700 nm (Figure 3). The weak
fluorescence emission at 418 nm, due to rigidochromic effect, is
slightly blue-shifted compared to the room temperature
fluorescence (430 nm), and it is characterized by a well-
resolved vibronic progression with breathing mode of about
1500 cm−1. The low energy bands between 530 and 700 nm
and peaked at ca. 547 nm are characterized by vibronic
progression of about 1400 cm−1. These bands are attributed to
the phosphorescence emission of the aromatic helicene scaffold,
since they closely overlay to the bands observed in the same
region for borane 5 (blue line). Worth noting, compared to 5,
the overall 77 K emission of 7 differs in two significant
parameters, that are consequences of the presence of the Au
atoms: (a) the relative phosphorescence vs fluorescence
intensity ratio is higher in 7 than that in 5; (b) the
phosphorescence lifetime of 7 is at least 1 order of magnitude
shorter than that in 5 (τ4 ∼ 0.12 s).
Table 3. Selected Computed Bond Lengths and Angles for 7
Optimized in the Electronic Ground State S0, the First and
Second Excited Singlet States S1 and S2, and in the Lowest
Triplet State T1
S0
S1
S2
T1
Distances (Å)
Au(1)−P(1)
Au(2)−P(2)
Au(1)−Cl(1)
Au(2)−Cl(2)
2.305
2.307
2.389
2.391
3.320
1.440
1.422
1.376
1.463
1.391
1.425
1.376
1.440
1.423
2.308
2.310
2.393
2.394
3.309
1.422
1.438
1.397
1.408
1.449
1.439
1.394
1.424
1.454
2.307
2.309
2.392
2.394
3.312
1.431
1.423
1.398
1.434
1.409
1.424
1.397
1.431
1.417
2.306
2.308
2.391
2.391
3.322
1.428
1.443
1.395
1.390
1.489
1.434
1.382
1.435
1.478
Au(1)−Au(2)
C(2)−C(3) inner
C(3)−C(8) inner
C(5)−C(6) outer
C(8)−C(9) inner
C(11)−C(12) outer
C(15)−C(20) inner
C(17)−C(18) outer
C(20)−C(21) inner
C(9)−C(14) inner
Angles, deg
P(1)−Au(1)−Cl(1)
P(2)−Au(2)−Cl(2)
P(2)−Au(2)−Au(1)
Cl(2)−Au(2)−Au(1)
C(22)−P(1)−Au(1)
C(1)−P(2)−Au(2)
176.4
176.6
98.1
176.6
176.2
96.8
176.3
176.3
97.9
176.6
176.2
97.8
85.2
86.8
85.7
85.9
Both observations are the consequence of the so-called
“heavy atom effect”, due to the gold atoms through the indirect
spin orbit coupling (SOC).39
112.6
113.3
113.6
114.7
113.4
114.2
112.9
114.0
Au(I) modulates the overall photophysical kinetics in 7,
increasing both the rate of intersystem crossing (ISC) from the
first excited singlet state to the excited triplet one, and the
radiative rates of the phosphorescence emission.
The room temperature solid state emission of 7 is reported
in Figure 4, together with the 77 K data collected on the same
sample. In the solid state, 7 displays a broad featureless
fluorescence between 420 and 600 nm (blue line). Upon
cooling the same sample at 77 K (red line), the fluorescence
emission gains a slight structured vibronic progression, and
phosphorescence emission appears (560−700 nm). This
“thermochromic effect”36a might point to the presence of
energetically close and accessible deactivating states, thermally
values for the electronic ground state S0 agree very nicely with
those determined by X-ray analysis (Table 2). The largest
deviations of approximately 0.1 Å occur for the bond where the
gold atoms are involved. In particular, the computed Au−Au
distance is found to be slightly larger with 3.32 Å as compared
to the experimental value of 3.18 Å. Most likely this deviation is
due to the use of a rather small basis set and an effective core
potential for the gold atoms. All other bond lengths deviate by
less than 0.1 Å. The largest deviations in the bond angles is
found for the P(2)−Au(2)−Au(1) and Cl(2)−Au(2)−Au(1)
angles. Here, in addition to the limited basis set size also crystal
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packing effects may play a role, which are not included in the
calculations.
At the optimized S0 equilibrium geometry, the lowest excited
singlet states S1 to S4 have been computed including the
continuum effects of DCM solvation (Table 4). All these states
compared to the experimental phosphorescence wavelength of
545 nm (2.27 eV). In general, one can not expect a better
agreement, since the computed vertical absorption, fluores-
cence, and phosphorescence energies are given as energy
differences of the electronic surfaces only, while the peaks in
the spectra correspond to vibronic 0−0 transitions.
For comparison, also the energetically lowest excited states of
the free ligands 3 and 5 have been computed. They exhibit
practically identical excitation energies and oscillator strengths
as gold complex 7. In analogy, they correspond to ππ* excited
state with the same frontier orbital structure. Hence, it is safe to
conclude that the photoluminescence properties can be
attributed to the tetrathiahelicene ligand only, with the gold
atoms inducing only a “heavy atom” effect on the
phophorescence quantum yield without largely changing the
electronic structure of the involved states.
Table 4. Excitation Energies and Oscillator Strengths of the
Four Lowest Excited Singlet States of 7 at the Calculated
Equilibrium Structures of S0, S1, and S2 at the Level of
TDDFT/ωB97X/6-31G*
state
S0
S1
S2
S1
S2
S3
S4
3.81 (0.22)
3.94 (0.38)
4.45 (0.22)
4.50 (0.68)
3.03 (0.38)
3.55 (0.66)
4.12 (1.01)
4.18 (0.46)
3.46 (0.32)
3.52 (0.62)
4.26 (0.44)
4.28 (0.93)
Catalysis Study. Having secured good access to mono- and
dinuclear gold(I) complexes 6 and 7, respectively, we then
explored the possibility of using them as new catalysts in the
field of the homogeneous gold catalysis. In particular, we
selected four different gold(I)-catalyzed cycloisomerizations as
screening reactions for new catalysts, and the results of this
study are listed in Table 5.
correspond to typical ππ* excited states located on the
tetrathiahelicene ligand. In particular, S1 and S2 correspond to
typical electronic transitions from the highest occupied
molecular orbital (HOMO) and HOMO − 1 into the lowest
unoccupied molecular orbital (LUMO) (Figure 5). S3 and S4
are not easily described within the molecular orbital picture,
since they correspond to mixtures of several electronic
transitions. However, analysis of the transition densities reveals
their ππ* character. According to our calculations, S1 and S2 are
only slightly energetically separated (Table 3) and could both
be contained in the first peak of the experimental spectrum.
The energy spacing between the vertical excitation energies is
only 0.12 eV.
The similarity of the electronic structures of the excited S1
and S2 states is further confirmed by their closely related
equilibrium structures. By analyzing the geometrical parameters
(Table 3) only marginal differences can be identified. Most
strikingly, for all considered states, the geometrical parameters
in the vicinity of the gold atoms do hardly change upon
excitation of 7, which also demonstrates that orbitals at the gold
atoms are not involved in the low-lying excited states of 7. Of
course, the vertical excitation energies decrease upon geometry
relaxation, and the S1 state exhibits an excitation energy of only
3.03 eV at its equilibrium geometry, which can be compared to
the fluorescence wavelength of 430 nm (2.9 eV). The
phosphorescence wavelength of 7 can be computed as energy
difference between the total energies of the optimized S0
structure and optimized T1 structure both obtained at DFT/
ωB97X level. Here we find an energy difference of 2.58 eV
Thanks to the diamagnetic character of gold(I), all
experiments were performed in an NMR tube, and then
1
monitored by H NMR analysis. The electrophilic gold(I)
active species [AuL]+ in the catalytic activation of alkynes or
allenes are often formed by chloride abstraction from
[AuCl(L)] complexes using silver(I) salts.40 In our test
reactions, complexes 6 and 7 were converted into the active
catalysts in situ by removal of the chloride with a silver salts
(AgNTf2 or AgOTf) of a noncoordinating counterion. Catalysts
6 and 7 were then tested in the cyclization of ω-alkynylfuran
8
41 (entries 1−3), the cyclization of 1,6-enyne 1042 (entries 4−
5), and the intramolecular hydroarylation of allene 1343 (entries
6−8), and catalyst 7 was also tested in the hydroxycarbox-
ylation of allene-carboxylate 1544 (entry 9). As reported in
Table 5, gold(I) complexes 6 and 7 have been shown to be
ineffective catalysts for the cyclization of ω-alkynylfuran 8 to
give phenol 9, which was obtained in 18% and 4% yield, using
0.25 mol % of 6 and 7, respectively (entries 1 and 2). Increasing
the catalyst loading of 7 to 2 mol %, the yield of 9 did not
significantly improve (entry 3). Slightly better results in terms
of yield were obtained in the isomerization reaction of 1,6-
enyne 10 (entries 4 and 5). However, the use of 2 mol % of
both catalysts 6 and 7 yielded a mixture of cyclohexene
Figure 5. Frontier molecular orbitals of 7 involved in the two lowest excited electronic states S1 and S2.
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Table 5. Gold(I)-Catalyzed Cyclizations with 6 and 7
a
b
The reactions were performed in NMR tubes at room temperature. Before adding the catalyst, an NMR spectrum was measured. Unless otherwise
c
indicated, equimolar amounts of AgNTf2 were added together with the catalysts 6 and 7. Determined by integration and comparison with an
internal standard. 5 mol % of AgOTf was added together with catalyst 7.
d
derivative 11 and vinyl cyclopentene 12 in scarce regioselec-
tivity, with the diene 11 as the major isomer. In the case of
catalyst 6 the selectivity is slightly better (11:12 = 3:1), but the
conversion and yield are lower (entry 4), while in the case of
the more reactive 7 (conversion and combined yield is higher)
the selectivity is lower (11:12 = 3:2, entry 5). The selectivity
toward the formation of 11 was also observed in the
cycloisomerization of enyne 10 using Au(PPh3)Cl/AgSbF6 as
catalyst system, although in this case the selectivity in favor of
diene 11 was much higher.42a
On the contrary, 6 and 7 both proved to be highly efficient
catalysts in the intramolecular allene hydroarylation reactions
(entries 6−8), and in the hydroxycarboxylation of allene-
carboxylate 15 (entry 9). In particular, both 6 and 7 efficiently
react with 4-allenyl arene 13 in an exo fashion to provide the
corresponding vinylbenzocycle 14 in excellent yield (99%,
entries 6 and 8). On the other hand, a good yield of product 14
was also obtained when the catalyst loading of 6 was reduced
from 5 to 1 mol % (65%, entry 7). Finally, the cyclization of the
carboxylate 15 in the presence of 2.5 mol % of gold(I)
phosphine 7 in combination with 5 mol % of AgOTf gave the
corresponding lactone 16 in almost quantitative yield (entry 9).
This result is very similar to that previously reported in the
cyclization of 15 using 2.5 mol % of [dppm(AuCl)2] activated
with 5 mol % of silver salt, in which 16 was obtained in 91%
yield.44
CONCLUSION
■
In this Article, we have reported the synthesis and the
characterization of new gold(I) complexes 6 and 7 with
phosphanes 2 and 3, respectively, which represent the first
example of gold complexes of phosphine based on the helicene
scaffold. The molecular structure of dinuclear complex 7 was
determined by X-ray diffraction analysis, showing an aurophilic
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37, 1806−1813. (d) Pintado-Alba, A.; de la Riva, H.; Nieuwhuyzen,
interaction, with the Au−Au bond distance of 3.18 Å, which
could explain the unusual short value of 45° of the dihedral
angle between the two terminal thiophene rings of the helical
skeleton. First studies on the photophysical properties of 7
along with experiments on the use of 6 and 7 as catalysts in
typical Au(I) cycloisomerization reactions have been also
investigated. In particular, organophosphane−Au(I) complex 7
shows interesting luminescent properties, in solution as well as
in the solid state, which are ascribed to both fluorescence and
(only at 77 K) phosphorescence emission. Quantum chemical
calculations demonstrate that the photophysical properties of 7
are determined by transitions between ligand centered states
only (both singlet and triplet), slightly perturbed by the
presence of gold atoms through the indirect spin orbit coupling
(SOC). Finally, complexes 6 and 7 have been shown to be
efficient catalysts in the intramolecular hydroarylation of allene
13, and the hydroxycarboxylation of allene-carboxylate 15. The
enantiopure form of this new class of gold(I) complexes based
on 7-TH phosphanes may find important applications in the
field of the chiral nonlinear optical materials, as well as in the
asymmetric catalysis.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: emanuela.licandro@unimi.it (E.L.); hashmi@hashmi.
de (A.S.K.H.).
■
Lescop, C.; Roussel, C.; Auchtsbach, J.; Crassous, J.; Rea
́
u, R. Angew.
Author Contributions
Chem., Int. Ed. 2010, 49, 99−102.
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(12) (a) Nuckolls, C.; Katz, T. J. J. Am. Chem. Soc. 1998, 120, 9541−
9544. (b) Katz, T. J. Angew. Chem., Int. Ed. 2000, 39, 1921−1923.
(c) Nuckolls, C.; Katz, T. J.; Katz, G.; Collings, P. J.; Castellanos, L. J.
Am. Chem. Soc. 1999, 121, 79−88. (d) Rahe, P.; Nimmrich, M.;
Notes
Greuling, A.; Schutte, J.; Stara,
́
I. G.; Rybac
́ ̌
ek, J.; Huerta-Angeles, G.;
̈
The authors declare no competing financial interest.
Stary, I.; Rohlfing, M.; Kuhnle, A. J. Phys. Chem. C 2010, 114, 1547−
́
̈
1552. (e) Katz, T. J.; Sudhakar, A.; Teasley, M. F.; Gilbert, A. M.;
Geiger, W. E.; Robben, M. P.; Wuensch, M.; Ward, M. D. J. Am. Chem.
Soc. 1993, 115, 3182−3198.
ACKNOWLEDGMENTS
■
S.C. acknowledges the University of Milan for the postdoctoral
fellowship. M.C.B.J. is grateful to the DAAD for a fellowship.
Gold salts were generously donated by Umicore AG & Co. KG.
(13) (a) Reetz, M. T.; Beuttenmuller, E. W.; Goddard, R. Tetrahedron
̈
Lett. 1997, 38, 3211−3214. (b) Reetz, M. T.; Sostmann, S. J.
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Organomet. Chem. 2000, 603, 105−109. (c) Krausova, Z.; Sehnal, P.;
̀
This work was supported by the Ministero dell’Universita e
della Ricerca (MIUR) and the University of Milan, PRIN 2007
(prot. 2007XFA27F_004), PUR 2008.
ABBREVIATIONS; 7-TH, tetrathiahelicene; tht, tetrahydro-
thiophene; DCM, dichloromethane; FAB, fast atom bombard-
ment; SOC, spin orbit coupling; 2-MeTHF, 2-methyl
tetrahydrofuran; TLC, thin layer chromatography
̌
́
I. G.; Saman, D.;
Bondzic, B. P.; Chercheja, S.; Eilbracht, P.; Stara,
Stary, I. Eur. J. Org. Chem. 2011, 3849−3857.
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