The Gold(I)-Mediated Domino Reaction to Fused Diphenyl Phosphoniumfluorenes: Mechanistic Consequences for Gold-Catalyzed Hydroarylations and Application in Solar Cells

Article abstract of DOI:10.1002/chem.201800460

A domino sequence, involving a phosphinoauration and a gold-catalyzed 6-endo-dig cyclization step, was developed. Starting from modular and simple-to-prepare phosphadiynes, π-extended phosphoniumfluorenes were synthesized. The mechanistic proposal was sup

Full text of DOI:10.1002/chem.201800460

A Journal of
Accepted Article
Title: The Gold(I)-Mediated Domino Reaction to Fused Diphenyl
Phosphoniumfluorenes: Mechanistic Consequences for Gold-
Catalyzed Hydroarylations and the Application in Solar Cells
Authors: Sebastian Arndt, Jan Borstelmann, Rebeka Eshagh Saatlo,
Patrick W. Antoni, Frank Rominger, Matthias Rudolph,
Qingzhi An, Yana Vaynzof, and A. Stephen K. Hashmi
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of the final Version of Record (VoR). This work is currently citable by
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201800460
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The Gold(I)-Mediated Domino Reaction to Fused Diphenyl
Phosphoniumfluorenes: Mechanistic Consequences for Gold-
Catalyzed Hydroarylations and the Application in Solar Cells
Sebastian Arndt,[a] Jan Borstelmann,[a] Rebeka Eshagh Saatlo,[a] Patrick W. Antoni,[a] Frank Rominger,[a]
Matthias Rudolph,[a] Qingzhi An,[b,c] Yana Vaynzof,*[b,c] and A. Stephen K. Hashmi*[a,d]
Abstract: A domino sequence, involving a phosphinoauration and a
gold-catalyzed 6-endo-dig cyclization step, was developed. Starting
from modular- and simple-to-prepare phosphadiynes,-extended
phosphoniumfluorenes were synthesized. The mechanistic proposal
is supported by kinetic measurements and by the trapping of key-
intermediates. These led to important conclusions for the gold-
catalyzed hydroarylation mechanism. Cyclic voltammetry (CV) and
UV-Vis measurements indicated interesting properties for material
science. The phosphoniumfluorene structure was tested as a hole-
blocking layer in perovskite solar cells of inverted architecture.
Devices with the phosphoniumfluorene exhibit an efficiency of 14.2%
which is much higher than that of devices without (10.7%).
which might be attributed to the few preparative methods
provided. A generally accepted synthesis of phosphonium salts
is the addition of a forth substituent to a trivalent phosphane by
nucleophilic substitution. But while the installation of a methyl[9]
or benzyl[10] substituent is relatively simple, the coupling of an
aryl substituent is challenging. It requires the catalytic
employment of palladium[11] or nickel,[12] or stoichiometric
reactions with in situ-generated arynes[13] or aryl radicals.[14] Only
the aryne method has been used for the modification of a
phospha- to
a
phosphoniumfluorene by Chatani and
coworkers.[15,16] Using this method, a low isolated yield (27%)
was achieved for the simplest diphenyl phosphoniumfluorene,
which demonstrates the difficulty of phosphafluorene
modifications. A direct synthesis was described by Widhalm et
al.[17] They obtained a phosphoniumfluorene in a high yield
through a twofold halogen-metal exchange of a 2,2’-dibromo
Introduction
binaphthyl and
a subsequent addition of chlorodiphenyl-
Domino reactions allow the generation of structural complexity in
a simple procedure.[1] Without the need for isolation of the
intermediates, domino reactions enhance the efficiency and
sustainability with respect to costs, time and waste production.[2]
Among many examples of domino reactions in homogeneous
phosphane. Although this method could be a key-approach to
phosphoniumfluorenes, only one example was mentioned and
more complex biaryl-scaffolds might be expensive to synthesize.
The most advanced strategy was reported by Wang et al. just
recently. They developed a powerful copper-mediated method
for the synthesis of versatile phospholium structures. However,
the major drawback is the consumption of two equivalents of the
copper salt which finally ends up as a byproduct.[18] Herein, we
gold-catalysis,[3,4]
a
phosphonylation-6-endo-dig-cyclization
cascade has not yet been reported. This is due to the strong
coordination ability of phosphanes to gold(I), inhibiting the
catalyst effectively. Recently, we reported phosphinoauration[5]
and protophosphonylation[6] reactions as tool for the synthesis of
phosphindolium structures. By expanding the phosphane-tolane
scaffolds with an additional alkyne moiety, we envisioned a
report
a mild and simple access to -extended diphenyl
phosphoniumfluorenes.
Ph
Ph
possible
cycloisomerization
towards
fused
diphenyl
Ph
Phosphinoauration
or Protophosphonylation
P
Ph
phosphoniumfluorenes (Scheme 1). Phosphorus-containing
fluorenes have recently become interesting molecules for
organic materials as they act as stable electron acceptors at
relatively low reduction potentials.[7] However, compared to oxo
or thioxo phosphafluorenes,[8] the phosphoniumfluorene
structure is largely unexplored to date. Only 2% of the literature
covering phosphafluorenes comprises phosphoniumfluorenes,
X
P
[Au]+ and H+
X = [Au] or H
X
Ph Ph
Ar
Ar
P
6-endo-dig cyclization
[a]
[b]
[c]
Organisch-Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg
(Germany).
Kirchhoff-Institute for Physics, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg
(Germany).
Centre for Advanced Materials, Ruprecht-Karls-Universität
Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg
(Germany).
Ar
X
Scheme 1. Domino sequence. Counter anions are omitted.
Results and Discussion
Our initial experiments addressed the assignment of the
intermittent phosphindolium species. Therefore, stoichiometric
amounts of gold(I) were used and the reactions were conducted
without the addition of acids. [Phosphadiyne-Au(NHC)][X] 1a-Au
was generated in situ by dissolving the enediyne 1a and 1 eq. of
[d]
*
Chemistry Department, Faculty of Science, King Abdulaziz
University (KAU), Jeddah 21589, Saudi Arabia.
E-mail: hashmi@hashmi.de, vaynzof@uni-heidelberg.de
Supporting information for this article is given via a link at the end of
the document.
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Ph
Ph
P
X
Ph
Ph
Ph Ph
P
Ph
P
Ph
1 eq. [IPrAu][NTf2]
Ph
+
+
P
bromobenzene
90 °C, 1 d
Ph
Ph
X
X
X
2a-H (31P = 30 ppm, 3%)
2a-Au (31P = 28 ppm, 62%)
3a-H
4a-H
(
(
31P = 22 ppm)
3a-Au (31P = 21 ppm, 12%)
31P = 13 ppm)
(
31P = 12 ppm, 23%)
4a-Au
N
N
1a 31P
(
= -9 ppm)
1a-H X = H (31P = 2 ppm)
= IPr
1a-Au X = IPrAu (31P = 39 ppm)
-
Scheme 2. Initial reaction and structural assignment. Counter anions (NTf2 ) are omitted for clarity.
[IPrAu][NTf2] in bromobenzene. The P-Au bond was clearly
established as confirmed by 31P NMR spectroscopy
( = 38.5 ppm). Upon heating to 90 °C, the 31P NMR monitoring
revealed the generation of mainly four species with chemical
shifts at  = 30 ppm, 28 ppm, 21 ppm, and 12 ppm
(ratio = 3:62:12:23; Scheme 2). The signals at  = 30 ppm and
 = 28 ppm were assigned to the phosphindolium salts 2a-H and
2a-Au. The signal at  = 21 ppm was assigned to the desired
3a-Au. Later, a crystal structure of another derivative revealed
that the species resonating at  = 12 ppm (4a-Au) can be
assigned to the 5-exo-dig cyclization product. Further
experiments addressing the protodeauration of the intermediate
species gave signals at  = 22 ppm and  = 13 ppm, which were
assigned to the protonated structures 3a-H and 4a-H. All the
assignments are in good agreement with the literature.[5,6,15,16]
After screening experiments, we found optimal reaction
conditions to be at a temperature of 80 °C and acetonitrile as
solvent. We observed full conversion to the two-fold cyclized
product 3a-Au within 2 d. Lower temperatures led to a very slow
conversion rate of 35% within 3 d at 50 °C, whereas higher
temperatures accelerated the rearrangement (full conversion
within 1 h at 150 °C). No reaction was observed within a period
of 10 d when performed at room temperature.
yielded a mixture of inseparable constitution isomers in a 1:2
ratio, as confirmed by X-ray single crystal structure analysis. A
superimposed molecular structure of the 6-endo-dig and the 5-
exo-dig isomers was determined (Figure 1, right, the 6-endo-
isomer was omitted for clarity). The 5-exo/6-endo selectivity can
be explained by the hyperconjugation of the alkyne *-orbital
with the vinyl gold carbenoidic carbanion in 2-Au during the
transition state. In case of terminal electron acceptors, the orbital
coefficient favors the exo-attack, whereas with donor groups it
favors the endo-attack.[19–21]
We next sought to isolate 3a-Au but it turned out to be sensitive
towards protodeauration. For the purpose of clean analytical
data, we added 1 eq. of HNTf2 to 3a-Au for complete
protodeauration. Full conversion towards bench-stable 3a-H was
obtained. The crude product was purified by recrystallization in
CH2Cl2/Et2O and the clean product was isolated in 83% yield.
The [IPrAu][NTf2] was cleanly recovered in 57% isolated yield,
Figure 1. Solid state molecular structures of 3a-H (left, twinned), 4f-H (right,
superimposed with 3f-H).[22] Twin, superposition, counter anions, and solvent
molecules are omitted for clarity.
i) 1.00 eq.
[IPrAu][NTf2]
ii) 1.00 eq.
H+ or I+
PPh2
Ph
Ph
P
1
which might improve at large-scale reactions. H and 13C NMR
MeCN,
80 °C, 1-7 d
spectroscopy and crystal structure analysis confirmed the
anticipated connectivity and constitution (Figure 1, left). In
conclusion, we were able to afford the desired structure in good
yields and in a simple procedure.
R
X
3a-f
R
1a-f
With the conditions in hand, we tested the scope of the reaction.
Electron-rich diynes (1a-c) were converted to the
phosphoniumfluorenes (3a-c) in good to excellent yields under
the standard conditions (Table 1). Electron-poor and a bulky tert-
butyl-substituted diyne (1d, 1e and 1f) required higher
temperatures or prolonged reaction times. In case of 1f, a
temperature above 100 °C was necessary and the reaction
X = H
yield X = I
yield
3a-H (R = Ph)
83%; 3a-I (R = Ph)
80%
99%
3b-H
3b-I
(R = p-Tol)
69%;
(R = p-Tol)
3c-H
3c-I
(R = p-OMe-C6H4) 96%;
(R = p-OMe-C 6H4) 99%
3d-H (R = t-Bu)
3e-H
n. d.; 3d-I (R = t-Bu)
3e-I
(R = p-F-C6H4)
39%
81%
(R = p-F-C6H4)
75%;
3f-Au/4f-Au
(R = p-CF3-C6H4, X = IPrAu) ratio = 1:2, not isolated
-
Table 1. Scope. Counter anion (NTf2 ) is omitted for clarity.
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releasing the active gold complex. The gold then
coordinates to either one of the alkyne bonds. In pathway 1,
H
N
H
N
the
phosphinoauration and protodeauration (
through a protophosphonylation by traces of water (
yielding 2-H as intermediate. This is followed by a gold-
catalyzed hydroarylation ( ), which is consistent with the
rearrangement
either
proceeds
through
) or directly
),
a
Ohno et al.
a, b
Hydroarylation
mechanism
H
H
c
5
Ar
[Au]
[Au]
Ar
d
Scheme 3. Key elementary step for the gold(I)-catalyzed hydroarylation
according to the mechanistic proposal by Ohno et al. Counter anions are
omitted for clarity.
mechanistic proposal of Ohno et al. made during their
investigation on polyyne cascades.[4] In order to check
pathway 1,
2a-H
was
synthesized
by
the
protophosphonylation of 1a with 1 eq. of HNTf2. The
consecutive transformation to 3a-H was attempted,
however neither the addition of catalytic nor stoichiometric
amounts of [IPrAu][NTf2] gave a further conversion at
temperatures between 80 °C and 150 °C within 5 d (even
under basic conditions using DBU, Scheme 5a). These
results account for an irreversible formation of 2-H which
was again confirmed by another control experiment: In the
2800
2300
1800
1300
800
1c-Au
2c-Au
3c-Au
300
presence of the phosphindolium complex 2e-H
complex 1c-Au was transformed into 3c-Au. While 1c was
fully converted to 3c 2e-H remained completely unaffected
, the
-200
,
0
5
10
15
20
25
30
35
demonstrating that 2-H is not involved in the reaction
(Scheme 5b). In conclusion, Ohno’s mechanism seems not
to be operative. Please note that the phospholium double
bond in 2-H is less nucleophilic than in indoles and
therefore its attack might be unfavored. In pathway 2, the
t [h]
Figure 2. Time/conversion curve for the reaction of 1c-Au to 3c-Au
Conditions: 1c-Au (20.3 µmol), [d5]-bromobenzene (0.5 mL), 80 °C.
.
mechanism proceeds through
a
concerted diyne
), which
In addition to protodeauration, I2 and N-iodosuccinimide (NIS)
were tested as iododeauration agents. To our delight, both
reagents cleanly provided the iodinated species 3(a-e)-I in good
yields. The installation of an aryl iodine additionally allows for
follow-up chemistry using simple cross-coupling methods.[23]
Control experiments without gold complexes were carried out
using HNTf2, HCl, B(C6F5)3,[6] Iodine[24] or NIS. All reactions were
unspecific or extremely slow. With HCl, a clean reaction was
observed at 80 °C, however the reaction was aborted after 19 d
at 95% conversion to 3a-H. Hence, gold(I) was found to be most
efficient in this cascade.
cyclization[26,27] generating a phenyl cation (
e
represents the zwitterionic version of the Bergmann
cyclization[20,21]. For the diyne mechanism we anticipated a
dramatic increase of the reaction velocity in the presence of
the free gold(I). However, the velocity did not substantially
increase when 2 eq. of [Au]+ were used.
Ph
Ph
PPh2
P
X
e
2
According to mechanistic proposal for the hydroarylation
Ar
of alkyne-extended indole intermediates (5, Scheme 3)
Ar
1 [Au]
[Au]
Ph
reported by Ohno et al., the mechanism proceeds through
electrophilic activation of the alkyne which is subsequently
attacked by the indole double bond.[4] Obtaining the twofold-
Ph
P
cyclized product
surprising, as
3
in the first instance was therefore
consecutive phosphinoauration and
PPh2
a
f
a
hydroarylation would have required the protodeaurated
phosphindolium intermediate. The employment of an acid,
which was not added, would have been required. This
prompted us to start mechanistic investigations. In order to
investigate the mechanism, kinetic measurements were
conducted (Figure 2 and S1). Species 2-Au was recognized
as an intermediate, which suggests that the cascade
proceeds through a phosphinoauration in the initial step.
The potential intermediate 2-H was not detected by 31P
NMR which can be explained by the absence of any proton
source or by a fast further transformation to the aurated
3-Au
[Au]
Ar
3
[Au]
2-Au
[Au]
Ar
1
H , -[Au]
slow
X
d
b
PPh2
Ph
Ph
1
P
X
c
H
H
product in the case that traces of acid led to
a
Ar
2-H
1
[Au]
protodemetallation. Based on the kinetic results, we
hypothesized[25] three mechanistic pathways (1-3,
Scheme 4): In each case, the reaction is induced by the
thermal cleavage of the phosphane-gold bond in 1-Au
Ar
Scheme 4. Overview of stoichiometric reaction pathways under standard
conditions. Counter anions are omitted for clarity.
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a)
Ph
Ph
Ph Ph
P
P
1 eq. [IPrAu][NTf2]
BrPh, 80-150 °C, 5 d
H
2a-H
H
3a-H
Ph
Ph
Ph2P
b)
Ph Ph
P
1 eq. 2e-H
1 eq. [IPrAu][NTf2]
[d3]-MeCN,
80 °C, 2 d
[Au]
(MeO)C6H4
1c
p-C6H4OMe
3c-Au
(2e-H not consumed)
c)
Ph
Ph
Ph
Ph
P
P
1d
1 eq.
X
[d3]-MeCN, rt
[Au]
[Au]
Figure 3. Solid state molecular structure of 3c-Au [22]
Selected bond
.
(MeO)C6H4
F
p- -C6H4
2d-Au
0.8 eq.
0.8 eq. 2c-Au
(+0.2 eq.
lengths [Å] and angles [°]:Au1-C61 2.029(3), Au1-C4 2.050(3), P1-C1
1.781(3), P1-C21 1.780(3), C21-C26 1.407(4), C2-C26 1.497(4), C1-C2
1.393(4), C2-C3 1.420(4), C3-C4 1.398(4); C61-Au1-C4 174.37(11),
C21-P1-C1 94.01(13), C2-C1-P1 109.48(19), C26-C21-P1 109.3(2), C2-
C3-C51 119.7(2), C31-P1-C41 110.11(14). Counter anions, and solvent
molecules are omitted for clarity.
1c
)
Ph Ph
d)
P
0.3 eq. [IPrAu][NTf2]
[d3]-MeCN, rt, 1 h
0.8 eq. 2c-Au
(+0.2 eq. 1c)
[Au]
(MeO)C6H4
3c-Au
1c-Au
(+0.2 eq.
)
complex 2c-Au was synthesized from 1c using 0.8 eq. of
[IPrAu][NTf2] at 80 °C. After the full conversion of the
[IPrAu][NTf2], an excess of 10 mol% [IPrAu][NTf2] was
generated by the addition of further 0.3 eq. (the remaining
0.2 eq. were consumed by the complexation with the
residual 1c). Herein, a complete conversion of 2c-Au to 3c-
Au was observed after 1 h at room temperature
(Scheme 5d). The corresponding molecular structure of 3c-
Au in the solid state was confirmed by single crystal X-ray
structure analysis (Figure 3). Similar results were obtained
by adding 0.5 or 1 eq. of HNTf2 to 2c-Au. The addition led
to full conversion to 3c in less than 1 h at room
temperature. On basis of these results, we propose a
mechanism proceeding through gold exchange. We
conclude that in our experiments 2c-Au underwent
protodeauration in catalytic quantities. The released [Au]
then activated the remaining alkyne bond and the vinyl-gold
electron pair attacked the activated carbon atom under
exchange of the [Au]-center. Thus, the second step of the
sequence is gold-catalyzed. In context of the work by
Ohno et al., the experiments clearly demonstrate a higher
nucleophilicity of the vinyl-gold versus the vinyl-H electron
pair which to the best of our knowledge was not
investigated before. This changes the mechanistic view of
gold-catalyzed hydroarylations: Beside the mechanistic
proposal by Ohno et al., the contribution of the gold-
exchange mechanism has to be considered for electron-
poor aromatics.
Scheme 5. a) Gold-mediated conversion of 2a-H in 3a-H (pathway 1). b)
Conversion of 1c in presence of 2e-H (pathway 1). c) Equilibration experiment
(pathway 2) d) Gold-catalyzed conversion of 2c-Au in 3c-Au (pathway 3).
Furthermore, a reversible reaction from the phosphindolium
species 2-Au to 1-Au would be necessary for pathway 2 in
order to match the kinetic results. However, the reversibility
of the formation of 2-Au was disproven in a stepwise
experiment using the phosphindolium gold complex as
starting material (Scheme 5c). First, 2c-Au was generated
reacting 0.8 eq. of [IPrAu][[NTf2] with 1 eq. of 1c at 80 °C.
Subsequently, 1 eq. of 1e was added and the sample was
further heated at 80 °C. The formation of 2e-Au that would
have been expected in case of a reverse reaction was not
observed. This result indicates that 1-Au and 2-Au are not
in equilibrium and hence that the diyne mechanism does
not apply. These results can be rationalized by the strong
coordination of the gold center to the phosphane moiety
restricting the full spectrum of the gold’s reactivity. Over the
diyne mechanism, the phosphinoauration presumably is
preferred due to the proximity of the corresponding alkyne
bond. In case of the reaction with 2 eq. of [IPrAu][NTf2], the
phosphane is occupied as a nucleophile which prevents a
successful Bergmann cyclization. Interestingly, when using
2 eq. of [IPrAu][NTf2], the germinal diaurated species was
not observed. Presumably, its formation is unfavored due to
the electron-deficiency of the phosphoniumfluorene.[27]
In pathway 3, a phosphinoauration leads to 2-Au (a) first,
but instead of protodeauration, the alkyne-activation by a
second [Au]-center triggers the nucleophilic attack in a
direct fashion (f). Please note that a concerted trans-
Next, the material properties were investigated and CV
measurements were conducted. A representative cyclic
addition with a single [Au]-center in 2-Au would be
geometrically impossible. The phosphindolium-gold
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voltammogram of 3a-H is depicted in Figure 4. For the
By UV-Vis spectroscopy
a
strong absorbance was
protonated fluorenes 3a-H
Ep = -1.7 V were recorded which are about 0.2 V lower than
the literature values for the methyl phenyl
,
3b-H, and 3e-H, potentials of
observed in the UV region with absorption maxima at
251 nm, 254 nm, and 252 nm for 3a-H 3b-H, and 3e-H
,
(Figure 6). For these structures, a small shoulder was
noticed at around 289 nm. 3b-I and 3e-I showed maxima at
264 nm and 265 nm with a larger shoulder at 287 nm. For
all compounds, a second absorption band is observed in
the visible region with maxima between 367 nm and
370 nm. The iodinated structures additionally show two
distinct shoulders at 353 nm and 390 nm. The LUMO levels
phosphoniumfluorene.[28] For the iodinated fluorenes 3b-I
and 3e-I, a two-electron acceptance was observed at
potentials of Ep1 = -1.5 V and Ep2 = -1.7 V. A representative
cyclic voltammogram of 3b-I is depicted in Figure 5. All
compounds were found to be irreversibly reduced as
indicated by the ∆Ep values and the Ip,Ox/Ip,Red-ratios (see
SI).
and HOMO levels for
and UV-Vis measurements. The LUMO and HOMO levels
for the protonated compounds 3a-H 3b-H, and 3e-H were
3 were estimated based on the CV
Potential [V] vs. Fc+/Fc
,
-2
-1,5
-1
-0,5
0
0,5
1
approximated to ELUMO = -3.11 eV and EHOMO = -6.49 eV.
For 3b-I and 3e-I, they were approximated to ELUMO = -
3.3 eV and EHOMO = -6.7 eV. All compounds showed a
similar HOMO-LUMO gap of Egap = 3.4 eV. TDDFT
calculations showed good agreement with the measured
values (Egap = 3.7 eV for 3a-H, CAM-B3LYP/Def2TZVPP,
see SI). The values for the HOMO-LUMO gaps suggest a
potential application for these materials as a hole-blocking
layer (HBL) in solar cells.
1,50E-05
1,00E-05
5,00E-06
0,00E+00
-5,00E-06
-1,00E-05
-1,50E-05
-2,00E-05
1.0 V/s
0.5 V/s
0.3 V/s
0.1 V/s
The phosphoniumfluorene structure 3c-H was tested as
a HBL in inverted architecture perovskite solar cells
(Figure 7a). As perovskite material, CH3NH3PbI3 (MAPbI3)
was used, while [6,6]-phenyl-C61-butyric acid methyl ester
(PCBM) was used as electron transport layer. Inverted
architecture perovskite solar cell means that in this
architecture the indium tin oxide (ITO) is the anode,
whereas the metal contact constitutes the cathode. 3c-H
devices were compared to devices without a HBL and with
Figure 4. Cyclic voltammogram of 3a-H at different scan rates.
Potential [V] vs. Fc+/Fc
-2
-1,5
-1
-0,5
0
0,5
1
2,00E-05
1,50E-05
1,00E-05
5,00E-06
0,00E+00
-5,00E-06
-1,00E-05
-1,50E-05
-2,00E-05
the
Summarizing the results of the CV, UV-Vis, and the
ultraviolet photoemission spectroscopy (UPS)
commonly
used
bathocuproine
(BCP).[29,30]
1.0 V/s
0.5 V/s
0.3 V/s
0.2 V/s
0.1 V/s
measurements, the corresponding energy diagram of the
solar cell is shown in Figure 7b. Compared to the PCBM
HOMO level, the low-lying HOMO level of 3-H can
effectively block the holes from the PCBM to the metal
contacts. Figure 7c shows the J-V curves obtained from the
three types of devices. Devices without HBL show a
reduced performance with a characteristic S-shape of the J-
V curve that originates from the increased series resistance
of the device.[31] Additionally, devices without a HBL show a
minor hysteresis. In comparison, devices with 3c-H as a
Figure 5. Cyclic voltammogram of 3b-I at different scan rates.
1
0,9
0,8
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0
3e-I
3b-H
3e-H
3b-I
HBL show
a
significantly improved photovoltaic
performance, reaching a power conversion efficiency (PCE)
of 14.17%. The devices exhibit a higher fill factor (FF), a
much lower series resistance (Rs), and show nearly no
hysteresis. Reference devices with BCP as HBL show a
very similar performance to those with 3c-H, demonstrating
that the phosphoniumfluorene structure can be effectively
applied as hole blocking layer in perovskite solar cells. The
photovoltaic parameters and the extracted series
resistances are summarized in Table 2.
3a-H
230
280
330
380
430
480
530
λ [nm]
Figure 6. UV-Vis spectra of compounds 3a-H
,
3b-H, 3b-I, 3e-H, and 3e-
I
.
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10.1002/chem.201800460
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a)
Conclusions
In summary, we developed the domino synthesis of -extended
phosphoniumfluorenes in a simple procedure and with up to
quantitative yields. The gold-complex could be recovered which
increases the efficiency. Furthermore, the electron-deficient
nature of the phosphoniumfluorene allowed the trapping of the
key-intermediates in the mechanism, which resulted in the
determination of the “gold-exchange mechanism”. The reaction
accordingly follows a phosphinoauration in the first step leading
to the phosphindolium-gold complex. The remaining triple bond
is then activated by a second gold-center which triggers the 6-
endo-dig cyclization to the phosphoniumfluorene under
exchange of the gold centers. Hence, the 6-endo-dig cyclization
in the second step is gold-catalyzed, whereas the
phosphinoauration requires stoichiometric amounts of gold. Our
finding regarding the gold-exchange mechanism provides a new
perspective by means of gold-catalyzed hydroarylation reactions
and thus is significant for future developments in this field.
TDDFT calculations, CV, UV-Vis measurements, and the
b)
application
in
solar
cell
devices
show
that
phosphoniumfluorenes are promising candidates to be applied
as efficient hole blocking layer materials in perovskite
photovoltaics. As part of our interest in gold-catalysis, the
development of
a catalyzed phosphinoauration is under
investigation. This requires an efficient deauration mechanism
which neither interferes with the gold-catalytic cycle nor leaves a
strongly coordinating anion that effectively inhibits the gold-
catalyst.
c)
Experimental Section
General Procedure for the Cyclization Cascade: Under argon
atmosphere, the phosphane-substituted enediyne and the given amount
of [IPrAu][NTf2] were dissolved in 0.5 mL solvent. The mixture was
transferred to
a Young-NMR tube and heated at the described
temperature. The reactions were monitored using 31P NMR spectroscopy
and upon complete conversion, the reaction mixture was treated with a
deauration agent (i. e. HNTf2 or NIS). The solvent was removed under
reduced pressure and the crude product was recrystallized in a mixture
of CH2Cl2/Et2O/n-pentane (1:5:5 v/v/v, 15 mL, -20 °C). For large-scale
reactions, precipitation in CH2Cl2/Et2O is sufficient. The crystals were
filtered off and washed with pentane to obtain the pure product.
Alternatively, the crude product can be purified by flash column
chromatography on silica gel (pure PE to pure Et2O) and by subsequent
extraction with CH2Cl2/H2O and with MeCN/pentane.
Figure 7. (a) Perovskite solar cell device structure. (b) Energy diagram
of Perovskite/PCBM/hole blocking layer/Ag. UPS was used to determine
the HOMO and LUMO levels of MAPbI3 and PCBM in a thin film. (c) J-V
curves of devices with and without different hole blocking materials. Both
reverse and forward scan directions are shown.
General Procedure for the Fabrication and Characterization
Perovskite Solar Cell: ITO coated glass substrates were ultrasonic
cleaned by acetone and isopropanol for 10 min, followed by 10 min
plasma treatment. PEDOT:PSS was spin coated on cleaned ITO
substrate with 150 °C 15 min annealing. Lead acetated trihydrate
perovskite solution was spin coated onto PEDOT:PSS in a drybox
(RH < 5%) with drying 5 min and 100 °C 5 min annealing.[29] The
Table 2. Photovoltaic performance parameters with and without BCP or
3c-H as HBL for both reverse and forward scan directions. Rs was
extracted from the J-V curve at V = Voc.
scan
Voc
Jsc
FF
PCE
Rs
direction
Backward
Forward
Backward
Forward
Backward
Forward
(V)
(mA/cm3)
-20.74
-20.74
-20.81
-20.81
-22.22
-22.22
(%)
(%)
(Ωcm2)
prepared samples were transferred into
Subsequently, PCBM in chlorobenzene
a
N2 filled glovebox.
(20 mg/ml) and
0.88
0.87
0.92
0.92
0.89
0.89
58.23
55.3
10.68
9.95
17.25
PCBM
BCP(0.5 mg/ml)/3c-H(1 mg/ml) in isopropanol was dynamically spin-
coated on the samples at 2000 rpm and 4000 rpm, respectively. To
complete the device, 80 nm silver electrode was deposited via thermal
evaporation under high-vacuum. The J–V measurements were
performed under simulated AM 1.5 sunlight at 100 mW cm−2 irradiance
(Abet Sun 3000 Class AAA solar simulator) with a Keithley 2450 Source
Measure Unit. The light intensity was calibrated with a Si reference cell
(NIST traceable, VLSI).
76.64
75.27
71.93
70.89
14.60
14.39
14.17
14.09
3.99
3.98
PCBM/BCP
PCBM/3c-
H
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Acknowledgements
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SA and PWA thank Florian F. Mulks for computational advices.
QA and YV thank the DFG for financial support (SFB 1249,
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Keywords: Protophosphonylation • Phosphinoauration •
enediyne cyclization • domino reaction • cascade • hole blocking
layer • perovskite solar cells
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Sebastian Arndt,[a] Jan Borstelmann,[a]
Rebeka Eshagh Saatlo,[a] Patrick W.
Antoni,[a] Frank Rominger,[a] Matthias
Rudolph,[a] Qingzhi An,[b,c] Yana
Vaynzof,*[b,c] and A. Stephen K.
Hashmi*[a,d]
Page No. – Page No.
A domino reaction to phosphoniumfluorenes with up to quantitative yields was
developed. Mechanistic studies support the proposal of a gold-exchange
mechanism. The properties were investigated by CV, UV-Vis, and TDDFT studies.
The material was tested as a hole blocking layer in perovskite solar cells.
The Gold(I)-Mediated Domino
Reaction to Fused Diphenyl
Phosphoniumfluorenes: Mechanistic
Consequences for Gold-Catalyzed
Hydroarylations and the Application
in Solar Cells
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