Synthesis of New Donor-Substituted Biphenyls: Pre-ligands for Highly Luminescent (C^C^D) Gold(III) Pincer Complexes

Article abstract of DOI:10.1002/chem.202003271

We herein report on new synthetic strategies for the preparation of pyridine and imidazole substituted 2,2’-dihalo biphenyls. These structures are pre-ligands suitable for the preparation of respective stannoles. The latter can successfully be transmetalated to K[AuCl₄] forming non-palindromic [(C^C^D)Au^III] pincer complexes featuring a lateral pyridine (D=N) or N-heterocyclic carbene (NHC, D=C’) donor. The latter is the first report on a pincer complex with two formally anionic sp² and one carbenic carbon donor. The [(C^C^D)Au^III] complexes show intense phosphorescence in solution at room temperature. We discuss the developed multistep strategy and touch upon synthetic challenges. The prepared complexes have been fully characterized including X-ray diffraction analysis. The gold(III) complexes’ photophysical properties have been investigated by absorption and emission spectroscopy as well as quantum chemical calculations on the quasi-relativistic two-component TD-DFT and GW/Bethe–Salpeter level including spin–orbit coupling. Thus, we shed light on the electronic influence of the non-palindromic pincer ligand and reveal non-radiative relaxation pathways of the different ligands employed.

Full text of DOI:10.1002/chem.202003271

Accepted Article
Accepted Article
Title: Synthesis of New Donor-Substituted Biphenyls: Pre-ligands for
Highly Luminescent (C^C^D) Gold(III) Pincer Complexes
Authors: Wolfram Feuerstein, Christof Holzer, Xin Gui, Lilli Neumeier,
Wim Klopper, and Frank Breher
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202003271
Link to VoR: https://doi.org/10.1002/chem.202003271
01/2020

10.1002/chem.202003271
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Synthesis of New Donor-Substituted Biphenyls: Pre-ligands for
Highly Luminescent (C^C^D) Gold(III) Pincer Complexes
Wolfram Feuerstein,ID[a] Christof Holzer,ID[b] Xin Gui,[c] Lilly Neumeier,[a] Wim Klopper*ID[c] and Frank
Breher*ID[a]
Abstract: We herein report on new synthetic strategies for the
preparation of pyridine and imidazole substituted 2,2’-dihalo biphenyls.
These structures are pre-ligands suitable for the preparation of
respective stannoles. The latter can successfully be transmetalated to
K[AuCl4] forming non-palindromic [(C^C^D)AuIII] pincer complexes
featuring a lateral pyridine (D = N) or N-heterocyclic carbene (NHC, D
= C’) donor. The latter is the first report on a pincer complex with two
formally anionic sp2 and one carbenic carbon donor. The
[(C^C^D)AuIII] complexes show intense phosphorescence in solution
at room temperature. We discuss the developed multi-step strategy
and touch upon synthetic challenges. The prepared complexes have
been fully characterized including X-ray diffraction analysis. The
gold(III) complexes’ photophysical properties have been investigated
by absorption and emission spectroscopy as well as quantum
chemical calculations on the quasi-relativistic two-component TD-
DFT and GW/Bethe-Salpeter level including spin-orbit coupling. Thus,
we shed light on the electronic influence of the non-palindromic pincer
ligand and reveal non-radiative relaxation pathways of the different
ligands employed.
Introduction
Phosphorescent emitters based on gold(III)[1] are far less studied
Scheme 1. Preparation of palindromic (C^N^C) and non-palindromic (C^C^N)
AuIII complexes reported in the literature and the synthetic approach presented
in this study. D: Donor.
in the context of phosphorescent organic light-emitting diodes
(PhOLEDs)[2] than systems incorporating other heavy metals, e.g.
The first example of [(C^N^C)AuIII] complexes luminescent in
frozen solution at 77 K was reported by Chi-Min Che and co-
workers in 1998.[9] In 2005, Vivian Wing-Wah Yam and co-
workers combined the (C^N^C) motif with alkynyl ligands to obtain
AuIII complexes which show phosphorescence in solution at room
temperature.[10] Later on, the field of luminescent [(C^N^C)AuIII]
complexes was further broadened by employing carbenes,[11]
alkyl donors[12] or thiolates[13] as ancillary ligands. The pincer’s
structure was modified as well, e.g. by substituting the central
pyridine by pyrazine, thus, affecting emission quantum yields and
wavelengths.[14]
We note that highly emissive tetradentate AuIII complexes
reported only recently may outperform many tridentate systems
in this regard,[15] however, complexes with tetradentate ligands
are less attractive for possible applications in chemical or catalytic
transformations because all coordination sites of the central AuIII
atom are occupied. That [(C^N^C)AuIII] complexes are valuable
candidates for the latter applications could impressively be shown
by Bochmann and co-workers who reported on the synthetic value
of [(C^N^C)AuIII] hydroxides[16] and hydrides.[17] In addition, they
prepared [(C^N^C)AuIII] olefin[18] and alkyne complexes[19] thereby
showing the suitability of these complexes for C−C bond forming
reactions. Finally, some [(C^N^C)AuIII] complexes are
investigated in the realm of anticancer drug research[20] with a
iridium(III),[3] ruthenium(II)[4] or platinum(II).[5] However, there is
increasing interest in gold(III)-based systems mainly employing a
2,6-diphenylpyridine (C^N^C)-based pincer ligand.[6] The
(C^N^C) pincer diminishes radiationless relaxation pathways of
excited complexes due to its rigid nature[7] and – in combination
with an additional strong donor ligand – results in high ligand field
splitting thereby shifting metal-centred d-states to higher energies.
The latter avoids population of these metal centred states
regularly seen to be responsible for radiationless relaxation
pathways.[8]
[a]
M. Sc. W. Feuerstein, Dr. L. Neumeier, Prof. Dr. F. Breher
Institute of Inorganic Chemistry
Karlsruhe Institute of Technology (KIT)
Engesserstr. 15, 76131 Karlsruhe (Germany)
E-mail: breher@kit.edu
[b]
[c]
Dr. C. Holzer,
Institute of Theoretical Solid State Physics
Karlsruhe Institute of Technology (KIT)
Wolfgang-Gaede-Straße 1, 76131 Karlsruhe (Germany)
X. Gui, Prof. Dr. W. Klopper
Institute of Physical Chemistry
Karlsruhe Institute of Technology (KIT)
Fritz-Haber-Weg 2, 76131 Karlsruhe (Germany)
E-mail: klopper@kit.edu
Supporting information for this article can be found at LINK.
1
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very recent study about photo-activatable cytotoxic [(C^N^C)AuIII]
hydrides.[21]
The (C^N^C) motif is introduced by reacting a mono-
cyclometalated mercury compound like 1 with gold(III) salts
(Scheme 1). In 2015, Nevado and co-workers reported on the
synthesis of the non-palindromic[22] [(C^C^N)AuIII] analogue which
exhibits exchanged positions of the central pyridine ring and one
lateral phenyl donor (4 in Scheme 1).[23] The latter is prepared
without need for toxic mercury compounds in an elegant way by
means of two successive, microwave-assisted C−H activations,
using pyridine substituted terphenyls designed to allow only one
kind of twofold cyclometalated products.
The non-palindromic (C^C^N) pincer was shown to exhibit
high emission quantum yields making its gold(III) complexes
particularly interesting for OLED fabrication.[24] This was assigned
to a higher ligand field splitting of these complexes compared to
the palindromic (C^N^C) congeners which was investigated by
TDDFT.[8b, 25]
The central phenyl donor of the (C^C^N) ligand exhibits a
stronger trans influence than the (C^N^C)’s central pyridine. This
made the preparation of stable AuIII fluorides[23] and formates[26]
possible and finds expression in notably different NMR shifts of
the respective hydrides.[17b] Moreover, the trans influence affects
the complexes’ reactivity: [(C^C^N)AuIII] carboxylates favour
thermal decarbonylation reactions[27] whereas the (C^N^C) based
carboxylates show elimination of CO2.[28]
There are no reports in the literature on gold(III) pincer
complexes with two aryl donors and a third donor other than
pyridine (D = N), e.g. carbene. However, bidentate, (C^C’)
cyclometalated (C’ = NHC carbon donor) AuIII complexes were
reported by von Arx et al.[29] and Crespo et al.[30]
In the present study, we report on the development of a
synthetic access to 5,5-dimethyl-5H-dibenzo[b,d]stannoles (5)
with a pyridine (D = N) or imidazolium (D = C’) substituent in 4-
position, which are suitable precursors for the new AuIII
complexes 6 and 7 (Scheme 1). On the one hand, this synthetic
approach constitutes a complementary variant of the synthesis
developed by Nevado and co-workers to prepare non-palindromic
[(C^C^N)AuIII] complexes. On the other hand, it is the first
example of a transition metal pincer complex with one NHC and
two formally anionic phenyl donors. Both pincer complexes are
combined with phenylethynyl or pentafluorophenyl ligands
resulting in compounds highly phosphorescent in solution at room
temperature. The photophysical properties are investigated
experimentally and by means of TDDFT and the Bethe-Salpeter
methodology including spin-orbit coupling described by us only
recently,[31] thereby showing the utility of these methods in the
field of photophysics and photochemistry.
Scheme 2. Preparation of biphenyldiyl gold(III) dimer 10 according to Usón et
al.[32]
Direct transmetalation of dilithiobiphenyls to AuIII is only possible
for very electron-poor biphenyls like perfluorinated ones[33] or
special AuIII precursors[34] due to the tendency of AuIII to become
reduced by organometallic reagents. Besides the findings of
Toste,[35] Bourissou[36] and Bertrand[37] on the oxidative addition of
biphenylene to several Au(I) species, the transmetalation of
stannoles is the method of choice for the preparation of
38]
cyclometalated biphenyldiyl AuIII complexes.[34,
Thus, we
anticipated the corresponding 4-substituted stannoles 5 would be
equally useful for the preparation of non-palindromic (C^C^D)
complexes of gold.
Dilithiobiphenyl 8 may be prepared by reaction of biphenyl
with two equivalents of nBuLi,[39] however, this variant suffers from
low yields and regioselectivity when employing substituted
biphenyls. Therefore, we focused on the synthesis of 2,2’-
dihalobiphenyls 13 and 16 with bromo or iodo substituents
(Scheme 3), which should serve as suitable pre-ligands for the
preparation of the pursued stannoles 5 (Scheme 1).
The anticipated dihalobiphenyls 13 and 16 should be
accessible by introducing a 2-halophenyl to phenylpyridine 12 and
phenyl imidazole 15 by means of palladium catalysed C−C cross-
couplings. The halogens may be obtained by transformation of
suitable N-based functional groups, i.e. anilines (X, X’ = NH2),
nitrobenzenes (X, X’ = NO2) or triazenes (X, X’ = NNNR2), by
means of SANDMEYER type reactions,[40] eventually after reduction
of NO2. The latter must be performed after building the core pincer
structure to ensure selectivity of the C−C cross couplings.
Phenylpyridine 12 might be obtained by C−C cross coupling as
well starting with 11 having (pseudo)halogen or metal
substituents in 2- and 6-position. Imidazole (15) should be
introducible by means of nucleophilic aromatic substitution at 2-
fluoro nitrobenzene 14.
Results and Discussion
Retrosynthetic Considerations
Our synthetic approach for the preparation of non-palindromic
[(C^C^D)AuIII] complexes (D = pyridine, NHC) is based on the
synthesis of biphenyldiyl gold(III) dimer 10 by transmetalation of
the stannole 9 with gold(III) salts, already described by Usón et al.
in 1980 (Scheme 2).[32]
Scheme 3. Retrosynthetic aspects of the preparation of dihalobiphenyls with
pyridine (C^C^N) (13) or NHC (C^C^C’) (16) donor. [Pd]: Pd catalysed C−C
cross coupling.
2
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Pre-Ligand Synthesis
With the above mentioned strategy at hand, we first coupled
commercially
available
nitroaniline
17
with
2-
(tributylstannyl)pyridine[41] followed by diazotization-iodination to
obtain iodine substituted phenylpyridine 19 in excellent yield.
Scheme 6: Synthesis of diiodo biphenyl 26. Conditions: (a) 1) nBuLi (pre-cooled
to –78 °C), –100 °C, 90 min 2) 1M ZnBr2 in THF (pre-cooled to –78 °C), –100 °C
→ –78 °C, 1 h, rt, 1 h 3) 1,2-diiodobenzene, [Pd(PPh3)4] (5 mol%), rx, 16 h; (b)
Amberlyst 15®, KI, MeCN, 70 °C, 10 min. The structure of 25 in the solid state
could be determined by X-ray diffraction analysis (Figure S 9, SI).
The latter was only successful under meticulous temperature
control: 24 was lithiated at –98 °C with precooled (–78 °C) nBuLi
in hexanes and subsequently transmetalated with ZnBr2 to obtain
a NEGISHI-reagent suitable for coupling with 1,2-diiodobenzene.
Prior to that, we tried magnesiation of 24 using turbo-Grignard
which did not occur even at elevated temperatures (THF, 80 °C)
leaving the bromide unaffected. The same holds for attempts to
directly zincate 24 using elementary zinc prepared by the method
of Rieke[52] or in the presence of LiCl.[53] This is probably rooted in
the electron-rich nature of 24 in contrast to the substrates reported
by the group of Knochel[53a] who used only electron-deficient
triazenes for direct zinc insertions. Reacting 24 with elementary
magnesium led only to decomposition products even in the
presence of ZnCl2. Consequently, we tried to lithiate 24 and
transmetalate with ZnBr2. Under standard conditions (nBuLi or two
Scheme 4. Attempt to obtain biphenyl 21. Conditions: (a) 2-
(tributylstannyl)pyridine, LiCl, [Pd(PPh3)4] (4 mol%), xylene, rx, 6 h; b) 1) HCl,
NaNO2, H2O, 0 °C, 30 min 2) KI, 0 °C → rt, 12 h; (c) 2-(aminophenyl)boronic
acid, K3PO4, XPhos Pd G3 (3 mol%),[42] 1,4-dioxane/H2O (4:1), 70 °C, 90 min;
(d) 1) HCl, NaNO2, H2O, 0 °C, 30 min 2) KI, 0 °C → rt, 12 h. The structure of 20
in the solid state could be determined by X-ray diffraction analysis (Figure S 9,
SI).
Iodide 19 was then coupled with 2-(aminophenyl)boronic acid by
means of
a
SUZUKI-MIYAURA coupling.[42-43] Unfortunately,
diazotization-iodination of 20 failed, although there are reports on
the successful diazotization-iodination of 2'-nitro-2-phenyl-
anilines.[44]
t
eq. BuLi –78 °C) we detected only decomposition products,
maybe due to decomposition by α-deprotonation at the pyrrolidine
which is known to readily happen at higher temperatures.[54]
The triazene 25 decomposes in the presence of HI[55] to the
diiodo biphenyl 26. We identified a variation employing proton
exchange resin and sodium iodide, i.e. in situ formed HI, in dry
acetonitrile to be the most effective.[56] This procedure avoids
shortcomings of other variations, e.g. reactions at the pyridine
ring.[57]
Beside the classical variant using HCl/NaNO2/KI, we also tried
DMSO-based reagents[45] or modifications employing CuBr2.[46]
Substitution of NH2 by Me3Sn according to a diazotization-
stannylation protocol led only to unidentified decomposition
products.[47] Attempts to isolate the corresponding diazonium salt
employing H[BF4]/ NaNO2 or [NO][PF6] were unsuccessful as well.
Reduction of the nitro-group of 20 and subsequent double
diazotization-iodination of the resulting amines is not effective,
because 2,2’-diaminobiphenyls cyclize to the corresponding
benzo[c]cinnolines under diazotization conditions.[48] We also
directly introduced 2-bromophenyl to 19, however, this approach
lacked of unacceptably low yields (Section S 1.2, SI).
For the preparation of a (C^C^C’) pre-ligand we started with
commercially available 1-bromo-3-fluoro-2-nitrobenzene (27)
(Scheme 7).
We then turned our attention to a different approach based on
the triazene 23 as key structure which was described by Knochel
and co-workers in the context of carbazole syntheses.[49]
Magnesiation
using
turbo-Grignard,[50]
subsequent
transmetalation with ZnBr2 and reaction with 2-bromopyridine by
means of a NEGISHI-coupling[40, 51] gave 24 in 87% isolated yield.
Scheme 7. Synthesis of (C^C^C’) pre-ligand 31. Conditions: (a) Imidazole,
NaOH, DMSO, rt, 2 h; (b) 2-bromphenyl boronic acid, [Pd(PPh3)4] (5 mol%),
K2CO3, THF/H2O (2:1), rx, 18 h; (c) Fe, EtOH/HOAc (1:1), rx, 3 h; (d) 1) 6 M HCl,
NaNO2, 0 °C, 30 min 2) KI, 0 °C → rt, 3 h. The structure of 29 in the solid state
could be determined by X-ray diffraction analysis (Figure S 9, SI).
Scheme 5: Synthesis of triazene 24. Conditions: (a) 1) HCl, NaNO2, H2O, 0 °C,
30 min 2) K2CO3, pyrrolidine, MeCN, 0 °C → rt, 2 h; (b) 1) iPrMgCl∙LiCl, THF, –
40 °C → –15 °C, 4 h 2) ZnBr2, –20 °C → 5 °C, 1 h 3) 2-bromopyridine,
[Pd(PPh3)4] (10 mol%), reflux, 16 h.
After introduction of imidazole by means of a nucleophilic
aromatic substitution[58] (28), we introduced 2-bromophenyl (29)
and subsequently reduced the nitro group to obtain the aniline 30.
The latter tends to cyclize with triazine formation (Section S 2.1,
SI), however, under strongly acidic conditions the dihalobiphenyl
31 can be obtained in 64 % yield, corresponding to a good overall
Then, we prepared the corresponding NEGISHI-reagent of 24 and
coupled with 1,2-diiodobenzene to yield 25 (Scheme 6).
3
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yield of 58 % over four steps. The presented route is especially
attractive, since all intermediates may be used without purification.
Although several synthetic steps are necessary, the
dihalobiphenyls 26 and 31 are easily preparable on a 10 to 20 g
scale.
Complex Synthesis and Characterization
t
Scheme 10. Synthesis of [(C^C^C’)AuIII] complex 38. Conditions: (a) 1) BuLi,
Et2O, –78 °C, 2 h 2) Me2SnCl2, –78 °C → rt, 1 h; (b) MeOTf, CH2Cl2, rt, 2 h. (c)
K[AuCl4], MeCN, K2CO3, rt, 2 h. The stannoles 36, 37 and [(C^C^C’)AuIII]
complex 38 could not be isolated in pure form, thus, yields are not given.
The diiodo biphenyl 26 was doubly lithiated and reacted with
Me2SnCl2 to obtain the stannole 32, which slowly decomposes
upon chromatography; thus, analytically pure samples were
obtained with moderate yields only (43 %). However, the crude
stannole may be reacted with K[AuCl4] to obtain the non-
palindromic [(C^C^N)AuIII] complex 33 in 81 % yield (Scheme 8).
Unfortunately, we were not able to obtain the stannoles 36 and 37
in pure form: Both are unstable against chromatography and did
not crystallize. Nevertheless, we could prove their formation by
NMR, APCI MS (36) and ESI MS (37) analyses of the crude
reaction mixtures. The [(C^C^C’)AuIII] complex 38 which we
obtained by reacting 37 with K[AuCl4] in the presence of base
rapidly decomposes in solution and in the presence of moisture
too, which made the isolation of pure samples impossible as well.
However, crude 38 could be reacted with lithium
phenylacetylide
or
[(MeCN)AgC6F5][59]
to
obtain
the
Scheme 8. Synthesis of [(C^C^N)AuIIICl] (33). Conditions: (a) 1) tBuLi, Et2O, –
78 °C, 2 h 2) Me2SnCl2, –78 °C → rt, 16 h; (b) K[AuCl4], MeCN, 0 °C → rt, 1 h.
[(C^C^C’)AuIII] complexes 39 and 40 in 43 % and 49 % yield
(Scheme 11), respectively, over four steps starting from
dihalobiphenyl 31. The latter two complexes are stable towards
moisture, air and chromatography.
Complex 33 was obtained as colourless solid which dissolves
readily in dichloromethane and toluene. However, after a couple
of hours, it precipitates in form of yellow blocks which are suitable
for X-ray diffraction analysis. Furthermore, 33 crystallizes from
CH2Cl2/n-hexane with one molecule of n-hexane in the unit cell
(Figure S 9, SI). The yellow blocks only modestly dissolve in hot
organic solvents, thus, NMR analysis was performed in DMSO-
d6 at 100 °C. Due to the poor solubility, chromatographic
purification of 33 (instead of crystallization) results in notably
reduced yields (62 %), however, the yellow blocks are fully
suitable for further transformations. We note that 33 – if not
already precipitated – decomposes in solution after three days
forming elemental gold when exposed to light.
Scheme 11. Synthesis of [(C^C^C’)AuIII] complexes 39 and 40. Conditions: (a)
LiCCPh, Et2O/toluene (1:1), rt, 3 h; (b) [(MeCN)AgC6F5], CH2Cl2, rt, 16 h.
Thus, we reacted 33 with lithium phenylacetylide or
[(MeCN)AgC6F5][59] to obtain the [(C^C^N)AuIII] complexes 34 and
35 in 86 % and 87 % yield, respectively (Scheme 9).
Crystals suitable for X-ray diffraction analyses could be obtained
from the pentafluorophenyl complexes 35 and 40 as well as the
PhCC derivative 39 (Figure 1). Overall, the structural parameters
of the investigated complexes are very similar. The Au−C17 bond
of 39 has the same length as its [(C^C^N)AuIII] pendant,[23] but the
Au−C17 bond of 40 (206.2(14) pm) is about 5 pm shorter
compared to the respective bond of its (C^C^N) congener 35
(Au−C18: 211.2(10) pm). The latter may be a result of the better
p acceptor properties of the lateral carbene as compared to the
pyridine donor-ligand. The molecules are rather closely packed in
the solid state with distances between the planes spanned by the
pincers ranging between 345 pm and 360 pm. The
pentafluorophenyl complexes 35 and 40 are aligned in two
different planes notably tilted against each other in the solid state,
whereas the molecules of 39 are packed in a nearly parallel
manner (Figure S 10, SI).
Scheme 9. Synthesis of [(C^C^N)AuIII] complexes. (a) LiCCPh, Et2O/toluene
(1:1), rt, 3 h; (b) [(MeCN)AgC6F5], CH2Cl2, rt, 16 h.
In a similar manner, we prepared stannole 36 and its methyl
imidazolium salt 37 (Scheme 10).
All complexes adopt structures with the aryl entities of the
ancillary ligands tilted against the plane spanned by the donor
atoms of the pincer ligand. They are not perpendicular aligned to
the pincer’s plane, however, the preference for the tilted
arrangement is important with respect to photophysical
considerations. Thus, for the following discussion, we want to rely
on an idealized situation of perfectly perpendicular aryl entities.
The structural feature of tilted/perpendicular aryl entities are also
found for most palindromic (C^N^C) analogues, either
4
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Figure 1. Solid state molecular structure of [(C^C^D)AuIII] complexes 35 (left), 39 (middle) and 40 (right). Thermal ellipsoids are set at 30 % probability. Hydrogen
atoms are omitted for clarity. Only one molecule of the asymmetric unit of 35 is shown. The asymmetric unit of 35 contains half a molecule hexane (not shown).
Selected bond lengths (pm) and angles (°), 35: Au–C18 211.2 (10), Au–C1 209.4(10), Au–C12 200.7(10), Au–N 212.4(9), C1–Au–C12 78.9(4), C12–Au–N 80.1(4),
C1–Au–C18–F 62.6(9); 39: Au–C17 204.5(8), Au–C1 206.4(7), Au–C12 197.9(8), Au–C13 207.4(8), C1–Au–C12 79.6(4), C12–Au–C13 78.2(3); 40: Au–C17
206.2(14), Au–C1 206.6(12), Au–C12 200.3(12), Au–C13 209.1(12), C1–Au–C12 78.9(6), C12–Au–C13 78.7(4), C1–Au–C17–C 97.9(1).
crystalographically[10a] or by means of DFT calculations.[10b]
Nonetheless, the role of the tilted PhCC ligand is somewhat
overlooked in the literature: Some (TD)DFT investigations are
based on structures with a phenylethinyl ligand being in coplanar
arrangement with the pincer moiety.[8b, 25] This may be misleading
with respect to the interpretation of photophysical properties. The
[(C^N^C)AuIII] motif belongs to the C2v point group. Thus,
The investigated complexes show intense luminescence in
solution at room tempreature with emission lifetimes in the micro-
and sub-microsecond region indicating phosphorescence. The
complexes are not emissive in the solid state maybe due to triplet-
triplet annihilation[62] facilitated by the close proximity of the
complex molecules in the solid state: for instance, the distance of
the planes spanned by the pincer ligand between two molecules
of 40 in the solid state is found to be only 347 pm (Figure S 10,
SI).
The emission pattern of all complexes is very similar. The vibronic
structure with band distances of about 1300 cm−1 indicates pincer
p*→p centred transitions. Interestingly, the carbene donor does
not alter the emission profile. The emission wavelengths of the
(C^C^C’) complexes 39 and 40 are negligibly shifted to higher
energies. In addition, the PhCC or C6F5 ligands do not have any
impact on the emission profile, thus, the transition is detached
from these ligands. The emission spectra resemble the ones of
other [(C^C^N)AuIII][23] and [(C^N^C)AuIII] complexes[10, 16] as well
as biphenyl-based [(C^C)AuIII][63] or cyclometalated pyridine[64]
and carbene [(C^C‘)AuIII] complexes.[29] Thus, the p orbital of the
emissive p→p* 3IL state is almost exclusively centred at the
(bi)phenyl unit of the respective pincer ligand.
Emission quantum yields F in dichloromethane solution at
room temperature range between 3 and 10 % being higher than
the yields of many (C^N^C) analogues.[10] Obviously, the quantum
yields are not affected by the lateral donor of the pincer, but the
ligand besides the pincer: The C6F5 substituted systems 39 and
40 phosphoresce up to three times more efficient than the PhCC
analogues 34 and 35. This is especially noteworthy, since the
opposite finding was reported by Venkatesan and co-workers
about the luminescence of cyclometalated [(C^N)Au(III)][64] and
[(C^C’)Au(III)][29] complexes. The authors found higher emission
quantum yields for phenylethinyl substituted complexes than for
pentafluorophenyl substituted ones. Furthermore, Bochmann and
co-workers have shown for palindromic [(C^N^C)AuIII] phenyl
complexes that the phosphorescence quantum yield increases
when going from pentafluorophenyl [(C^N^C)AuIIIC6F5][65] to less
fluorinated variants like [(C^N^C)AuIIIC6F4H] (F = 0.6 %) and
[(C^N^C)AuIIIC6H4F] (F = 1.3 %).[16] This may be understood from
the weaker donor strength of (per)fluorinated aryl ligands resulting
in smaller ligand field splitting of the gold(III) atom. In contrast, the
pCNC→p*CNC intra ligand transitions (IL) are of B1 symmetry, i.e.
dipole allowed. The same holds for the non-palindromic
[(C^C^D)AuIII] structure, whose symmetry is Cs (A’). The character
of pethinyl→p*CNC inter ligand transitions (LL’CT) depends on the
orientation of the PhCC: In a coplanar arrangement, LL’CT is of
A1 symmetry (A’ in Cs), i.e. an allowed transition; a tilted
arrangement changes the p→p* LL’CT to A2, which is dipole
forbidden in C2v. In Cs, the latter is dipole allowed in one direction
(A’’). The transition intensities are strongly affected by these
symmetry properties, thus, being important for a sound discussion
of photophysics.
Absorption and Emission Properties
Absorption and emission spectra of complexes 34, 35, 39 and
40 are shown in Figure 3. The photophysical properties are
summarized in Table 1. We note that we did not observe any
degradation of these complexes in the presence of light over the
course of a couple of days neither in toluene nor in CH2Cl2.
Like the (C^C^N) complexes reported by Nevado et al.,[23] the
absorption bands of 34 and 35 are blue shifted compared to the
palindromic (C^N^C) analogues.[10a, 16] The vibronically structured
bands between 330 and 280 nm (ε ~ 5000 – 15000 dm3 mol−1
1
1
cm−1) with peak distances between 1200 cm− and 1400 cm−
being typical for the pincer’s ligand breathing modes probably
23, 60]
arise from p→p* IL transitions.[10a,
LL’CT transitions
responsible for the first absorption bands can be ruled out
because in this case the absorption profile of phenylethynyl (34)
and pentafluorophenyl (35) derivate should considerably differ.
The (C^C^C’) complexes 39 and 40 exhibit further blue-
shifted absorptions. Obviously, the lateral carbene donor leads to
electronic states shifted to higher energies compared to the
(C^C^N) congeners. This is rooted in the higher energy levels of
imidazole p* orbitals[61] as well as the stronger ligand field splitting
causing higher lying gold centred d-orbitals. Thus, the absorption
spectra of 39 and 40 clearly render the electron rich nature of the
goldIII atom entailed by the (C^C^C’) pincer.
PhCC ligand is
a
sufficiently strong donor to enable
Obviously, the non-palindromic pincer
66]
phosphorescence.[10,
motifs reported in the present study overcompensate the weaker
5
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10.1002/chem.202003271
Chemistry - A European Journal
FULL PAPER
E / eV
E / eV
2.8
2.6
2.4
2.2
2
1.8
4.5
4
3.5
3
2.5
2.0
1.5
1.0
0.5
0.0
34
35
39
40
1.0
0.8
0.6
0.4
0.2
0.0
34
35
39
40
250
300
350
400
450
500
450
500
550
600
650
700
l / nm
l / nm
Figure 3. Left: UV/Vis spectra of [(C^C^N)AuIIICCPh)] (34), [(C^C^N)AuIIIC6F5)] (35), [(C^C^C‘)AuIIICCPh)] (39) and [(C^C^C‘)AuC6F5)] (40). Right: Emission spectra
in solution. All spectra were recorded at 293 K in dry and degassed CH2Cl2. The excitation wavelength was chosen to match the respective first absorption maximum.
Table 1. Photophysical data of gold complexes 34, 35, 39 and 40 in solution[a] at 293 K.
[ꢒ]
[ꢏ]
[ꢉ]
ꢀꢁꢂꢃ
nm
ꢄ
ꢀꢉꢊ
nm
ꢎꢌ
µs
ꢐꢌ
10ꢑ sꢇꢈ
ꢐꢓꢔ
10ꢑ sꢇꢈ
[ꢍ]
complex
ꢋꢌ
10ꢅ dmꢆ molꢇꢈ cmꢇꢈ
34
35
39
40
298, 312, 327, 376
1.52, 1.34, 1.25, 0.15
473, 507, 543, 588
475, 410, 546, 589
471, 505, 540, 581
470, 504, 539, 582
0.03
0.09
0.03
0.10
0.8
4.1
1.0
0.5
0.38
0.22
0.30
2.0
12.13
2.22
0.97
1.80
278, 285, 295, 314,
326, 370
1.27, 1.16, 0.87, 1.00,
1.29, 0.12
284, 303, 314
302, 314, 336
1.51, 1.13, 1.02
0.58, 0.54, 0.06
[a] Dried and deoxygenated dichloromethane. [b] Absolute quantum yields, determined by use of an integrating sphere (c = 10−4 mol dm−3). [c] Phosphorescence
lifetime. [d] Rate constant phosphorescence, ꢐꢌ = ꢋꢌ/ꢎꢌ. [e] Rate constant radiationless relaxation, ꢐꢓꢔ = (1 − ꢋꢌ)/ꢎꢌ.
ligand-field splitting, i.e. the ligand field splitting of 35 and 40 is
strong enough despite the weak pentafluorophenyl donor.
However, this does not explain why the latter complexes
outperform the phenylethynyl 34 and 39 regarding
phosphorescence quantum yields. In order to shed some more
light on these aspects, we performed quantum chemical
calculations.
calculations are in good mutual agreement. GW/BSE excitation
and emission energies are slightly red shifted by approx. 0.15 eV.
Even though there is good agreement between the predicted
and observed spectra, the quantum yield needs further
investigation, as the difference between the PhCC and C6F5-
ligated systems cannot be explained by the emission spectra
Quantum Chemical Calculations
To investigate the phosphorescence properties of the four
complexes, and especially the different quantum yields observed
for the differently substituted complexes, GW/Bethe-Salpeter and
time-dependent density-functional theory (TD-DFT) calculations
using the TPSSh functional were performed at the quasi-
relativistic two-component level including spin-orbit coupling.
Absorption spectra were found to be in good agreement with
experimental data. When optimizing the first excited triplet state,
we find a shift of the emission lines for all four complexes to a
nearly constant value. Calculated 0←0 triplet emission energies
are found at 619 nm/2.00 eV (34), 620 nm/2.00 eV (35), 623
nm/1.99 eV (39), and 626 nm/1.98 eV (40) while a natural-
transition-orbital (NTO) analysis confirms the p←p* triplet intra-
ligand (3IL) character of this excitation as described in the
previous section. The TD-DFT (TPSSh) and GW/BSE
Figure 2. Natural transition orbitals of the hole (red/blue)
(green/orange) pairs of the first triplet excitation in the planar (a,b) and tilted
(c,d) configuration of complex 39.
- particle
6
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alone. A main difference between these complexes is given by
their ability to rotate about the Au-C axis. For the phenylethynyl
complexes 39 and 34, we have obtained rotational barriers of only
2.0 kJ/mol and 1.1 kJ/mol, respectively, and therefore assume a
rapid rotation of the ligand at room temperature, as it was done
by Yam et al.[8b] Due to steric hindrance, however, no rotation is
observed for the bulkier pentafluorophenyl ligand, and a stable
transition-state geometry could not be located. In the vicinity of
the transition-state geometry, where the phenylethinyl and pincer
ligands are nearly coplanar, the first triplet excited state changes
its character from intra- (IL) to inter-ligand charge-transfer (LL’CT,
Figure 2). This allows for an efficient pathway to release the
excess energy of the excited state, suppressing the
phosphorescence of complexes 34 and 39. In the tilted geometry,
this LL’CT state is also present, but only as a higher-lying excited
state. Therefore, for the bulkier -C6F5 ligand, this non-radiative
relaxation pathway is closed as the overlap between the
p systems is never sufficiently high at any reasonable geometry
to yield a significant transition moment dipole.
Upcoming studies currently performed in our group are
focused on modified procedures to introduce functional groups
into the pincer moieties to systematically investigate and tune the
chemical and photophysical properties of derived complexes.
Furthermore, the dihalobiphenyls are examined with regard to
their applicability to other transition metal complexes and main
group elements probably opening up a rich chemistry with
possible applications in catalytic, photophysical/-chemical,
materials or pharmaceutical applications.
Acknowledgements
We thank Dr. Waldemar Konrad for valuable support with
chromatographic separations. We thank Prof. Joachim Podlech
for proofreading the manuscript. W. F. gratefully acknowledges
the Carl-Zeiss Stiftung for a PhD scholarship as well as the
Studienstiftung des deutschen Volkes for general support.
Financial support by the Collaborative Research Centre
CRC/Transregio 88, “Cooperative effects in homo- and
heterometallic complexes (3MET)” is gratefully acknowledged
(Projects B4 and C1).
But even for the PhCC complexes, the overall transition
probability is still not high in the coplanar configuration, explaining
why only diminished emission quantum yields are observed
instead of full quenching. The different transition dipole moment’s
magnitude for tilted (LL’CT: A’’) and coplanar geometry (LL’CT:
A’) nicely correspond to a qualitative picture based on group
theory considerations.
Keywords: pincer ligand • luminescence • gold complex •
photophysics • quantum chemistry
We note that rotation is also possible for alkyl ligands, for
which high phosphorescence quantum yields have been
observed.[12] However, the non-radiative LL’CT relaxation
pathway does not exist for alkyl ligands due to the lack of an
adjacent p system.
For a detailed overview of the methods used, excitation and
emission spectra including spin orbit coupling, and a detailed
analysis of the character of the corresponding relevant excitations,
we refer to the SI.
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absorptions and high emission quantum yields in solution at room
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The pentafluorophenyl derivative shows luminescence
with a quantum yield of about 1 %, however, this is rooted
in a fluorescence as clearly seen from a small Stokes shift,
emission lifetimes in the nanosecond region and different
vibronic bands.
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The stronger donor properties of phenylethinyl compared
to pentafluorophenyl is also reflected in a blue shifted
absorption of the [(C^N^C)AuIII] phenylethinyl complex
compared to its C6F5 pendant.
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Entry for the Table of Contents
Capture the Gold(fish)! Donor-substituted dihalobiphenyls are suitable pre-ligands for the preparation of luminescent, non-
palindromic [(C^C^D)AuIII] complexes with a lateral pyridine or carbene donor. Like an anglerfish’s rod enmeshes its prey, the lateral
donor tunes the gold`s electronic properties which is hold by the rigid biphenyl mouth. Synthesis, spectroscopy and quantum
chemistry draw a comprehensive picture about the new complexes prepared.
Institute and/or researcher Twitter usernames: @BreherGroup @74_feuerstein @KITKarlsruhe
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