Unusual Attractive Au–π Interactions in Small Diacetylene-Modified Gold Clusters

Article abstract of DOI:10.1002/anie.201814359

It is well known that alkynes act as π-acids in the formation of complexes with metals. We found unprecedented attractive Au–π interactions in diacetylene-modified [core+exo]-type [Au₈]⁴⁺ clusters. The 4-phenyl-1,3-butadiynyl-modifie

Full text of DOI:10.1002/anie.201814359

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Accepted Article
Title: Unusual Attractive Au-π Interactions in Diacetylene-modified
Small Gold Clusters
Authors: Mitsuhiro Iwasaki, Yukatsu Shichibu, and Katsuaki Konishi
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Unusual Attractive Au-π Interactions in Diacetylene-modified Small
Gold Clusters
Mitsuhiro Iwasaki, Yukatsu Shichibu, and Katsuaki Konishi *
[
16-20]
Abstract: It is well known that alkynes serve as π-acid to form
metal complexes through strong π-coordination bonds. In this
work, we highlight unprecedented “attractive” Au-π
interactions found in diacetylene-modified [core+exo]-type
Au-C≡C-Au-C≡C-.
These motifs are analogous to the staple
units found in the thiolate-protected clusters but to note is the
capability to accommodate additional gold atoms through π-
coordination. Such diverse coordination modes of alkynyl
ligands are unique features that are not found for conventional
σ-only type ligands, which expand the scope and aspects of
molecular gold clusters.
4
+
[
Au
8
]
clusters (2). The 4-phenyl-1,3-butadiynyl-modified
distances in the
cluster (2a) showed unusually short Au-C
α
crystal structure, revealing the presence of attractive
interactions of the coordinating C≡C moieties with the
In the examples mentioned above, mono-acetylenic ligands,
such as phenylethynyl, have been used as the protecting ligands.
Herein we show that diacetylenic (1,3-diynyl) ligands (C≡C-
C≡CR) have unusual interaction activities with gold frameworks,
which were firmly identified by X-ray crystal structures, IR
spectra, and solution NMR spectra. The attractive interaction
affected HOMO-LUMO gap energies, causing substantial red
shifts of the visible absorptions. The present finding
demonstrates that diacetylenic ligands offer opportunities to
create novel structures / properties that cannot be achieved by
the use of simple mono-acetylenic ligands.
neighboring bitetrahedral Au
6
core, which was further
supported by IR and NMR spectra. To note is the fact that such
weak interactions were not found in mono-acetylene-modified
clusters (1), indicating they are specific for diacetylenic ligands.
The Au-π attractive interactions are likely associated with the
low π* energy level of the diacetylenic moieties, to which the
valence electrons in the gold core may be donated in a back-
donation-like manner. 2 showed clear red shifts of > 10 nm
with respect to the corresponding mono-acetylenic clusters (1)
for the visible absorption bands, indicating the substantial
electronic perturbation effects of the Au-π interactions. This
work implies potential utility of diacetylenic ligand for the
emergence of unique binding motif and cluster structures.
4
-Aryl-1,3-butadiynes (aryl
synthesized by organic syntheses and introduced at the two exo
positions of dppp (Ph P(CH PPh )-coordinated [core+exo]-type
Au cluster ([Au (dppp) similar manner to that
reported for the synthesis of mono-acetylenic-ligated clusters (1,
= phenyl or 4-pyridyl) were
2
2
)
3
2
2
+
8
8
4 2
R ] ) in a
Ligand-protected gold clusters with defined nuclearity and
geometrical structure have attracted considerable interests in
relation to the applications in optoelectronic materials and
[
12]
2+
Fig. 1).
Thus, the reaction of [Au
8
(dppp)
4
]
with the alkynyl
anions proceeded smoothly at ambient conditions. The product
clusters with two diacetylenic fragments were isolated as nitrate
salts (2), which were characterized by electrospray ionization
(ESI) mass (Fig. S3) and NMR spectra.
[
1-6]
[1-3]
[6-10]
catalysis.
Thiolates
and phosphines
have been
frequently used as protecting ligands, while alkynyl (acetylide)
ligands have recently emerged as unique and suitable ligands
[
4]
stabilizing gold clusters.
Unlike thiolates and phosphines
having only σ-bonding coordination characters, π-electron
functionalities of acetylide ligands serve as both σ- and π-donors
to interact gold atoms, enabling the formation various
coordination motifs on the cluster surface. The first example of
such alkynyl-protected gold clusters is Au54(C≡CPh)26 reported
[
11]
by Tsukuda et al in 2012,
determination has not been achieved. We have also introduced
alkynyl ligands in Au and Au13 clusters as coligands and
but the crystal structure
8
showed the substantial perturbation effects of particular π-
electron systems on the electronic properties of the gold
Figure 1. Structures oft he cationic moieties of [core+exo]-type
Au cluster nitrate salts (1 and 2).
8
[
12-15]
clusters.
In addition, Q.-M. Wang et al have recently
reported numerous homoleptic and heteroleptic alkynyl-
protected gold clusters, demonstrating the formation of various
surface Au(I)-acetylide motifs such as -C≡C-Au-C≡C- and -C≡C-
X-ray crystallography was successfully performed for the 4-
phenyl-1,3-butadiynyl-modified cluster (2a), whose structure was
[
12]
compared with a mono-acetylenic analogue (1a). As shown in
Fig. 2, 1a and 2a both had similar [core+exo]-type Au
frameworks composed of a bitetrahedral Au core and two
attached gold atoms at exo positions. The Au cores gave
8
[
[
*]
M, Iwasaki, Dr. Y. Shichibu, Prof. K. Konishi
Graduate School of Environmental Science, Hokkaido
University, North 10 West 5, Sapporo 060-0810 Japan
E-mail: konishi@ees.hokudai.ac.jp
6
6
almost same structural parameters (Table S3), but marked
differences were found around the coordinating C≡C moieties.
Thus, as shown in Fig. 2b, the diacetylenic units of 2a are
b]
Dr. Y. Shichibu, Prof. K. Konishi
Faculty of Environmental Earth Science, Hokkaido University,
North 10 West 5, Sapporo 060-0810 Japan
substantially tilted from the terminal Au
angle: 65.7 and 77.5º, Table 1), which is contrastive to the
almost perpendicular orientation of the ethynyl units of 1a (Fig.
angles of 84.8 and 86.7º (Table 1). It
should be noted that the coordinating carbon atoms (C ) of 2a
3
triangles (Auedge-Auexo-
Supporting information and the ORCID identification number(s)
for the author(s) of this article can be found under:
C
α
http://dx.doi.org/10.1002/xxx
2
a) with Auedge-Auexo-C
α
α
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Angewandte Chemie International Edition
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are located in proximity to the gold atoms on the exo-bridged
a
Table 1. Selected Structural Parameters of 1a, 1c and 2a.
1a 1c 2a
edges of the bitetrahedron (Auedge). The Auedge-C
α
distances are
2
.98 and 3.26 Å (Table 1) which are longer than the distance of
normal Au-C bonds but shorter than the sum of the van der
Waals radii (3.36 Å). Thus, in addition to the conventional atop
Auedge–Auexo
Auedge–C
Auexo–C
2.93, 3.02 Å 2.94, 3.03 Å 3.05, 3.18 Å
3.41, 3.53 Å 3.24, 3.44 Å 2.98, 3.26 Å
α
α
σ-coordination of terminal alkynyl units, the α-carbon atoms (C )
of 2a have attractive interaction with neighboring gold atoms. On
the other hand, the corresponding distances of 1a (3.41 and
α
2.02 Å
2.02 Å
2.00 Å
Auedge–Auexo–C
α
84.8, 86.7º
2.0 º
79.2, 83.4º
65.7, 77.5º
14.2 º
3
.53 Å, Table 1) were sufficiently above the distance of van der
Waals contacts, indicating the absence of the attractive
interactions. Since the distances of the Auexo-C coordination
bonds of the two clusters were similar to each other (2.02 and
.00 Å, respectively) (Table 1), the unusually short Auedge-C
distances in 2a are likely solely due to the attractive force with
b
Tilt angle
5.1 º
a
b
For atom labeling, see Figure 2. Angles between the terminal
α
Au
3
triangles and the mean plane composed of four Auedge atoms.
2
α
[12]
cluster (1c).
When compared with 1a, the C
core (Fig. S4), suggesting
the presence of some degree of steric hindrance in 1a.
Accordingly, the Auedge-C distances of 1c were shorter than 1a,
but they are around the distance of the van der Waals contact
Table 1). Thus, the C ≡C-mediated attractive interactions in 1c
α
≡C units of 1c
were slightly oriented towards the Au
6
the bitetrahedral Au
6
core, rather than the enhancement of the
σ-coordination of C
α
to Auexo. The intramolecular interactions
α
appeared to be correlated with the substantial distortion of the
Au framework. The terminal Au triangles of 2a were tilted from
the mean plane of the four Auedge atoms by ~14.2°, whereas in
8
3
(
α
seem negligible, similarly to 1a. The similarity of the structural
features of 1a and 1c was further supported by the IR and NMR
spectra (vide infra). Therefore, it is likely that electronic factors,
rather than the steric effects, primarily responsible for the
diacetylenic-specific attractive interaction.
1
2
1
a they are almost in plane (angle: ~2.0°) (Table 1). Accordingly,
a showed longer Auedge-Auexo distances (3.05 and 3.18 Å) than
a (2.93 and 3.02 Å) (Table 1).
As mentioned, the diacetylenic cluster (2a) had
α
intramolecular interactions involving the C atoms, which were
6 α
The unusual interaction between the Au core and the C
not found in the mono-acetylenic cluster (1a). One of the
possible factors to contribute to this difference is the steric
hindrance of the Ph groups attached to the ethynyl ligands of 1a,
atoms of 2a is considered to be associated with the π-systems
of the diacetylenic moieties. To obtain further insights into this
point, we measured IR spectra of 2a and also the 4-pyridyl-
substituted analogue (2b) in order to check the generality. As
shown in Table 2 (entry 4) and Fig. S5, 2a (KBr pellet) gave two
which may hamper the access of the C
atoms owing to the repulsion with the neighboring P-Ph groups
Fig. 1(i)). However, such steric effects seem not critical since no
substantial interactions were observed in the 1-hexynyl-modified
α
≡C moiety to the Auedge
(
-1
vibration bands at 2054 and 2179 cm . A related σ-gold(I)
complex of 4-phenyl-1,3-butadiyne (Ph
3 4
PAuC Ph) was reported
-
1
to show vibrations at 2134 and 2181 cm , but the assignments
Figure 2. X-ray crystal structures of (a) 1a and (b) 2a. (i) Entire and (ii) (iii) partial structures of the cationic moieties with hydrogen
atoms omitted for clarity.
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[
21]
were not given.
assigned due to the terminal coordinating C
shifts of the vibrations of 2a from the above reference complex
If the lower-energy bands are tentatively
diacetylene π* orbital level, is likely responsible for the unusual
attractive Au-π interaction in 2.
α
≡C bonds, the red
-
1
are evaluated 80 and 2 cm for the coordinating and distal C≡C
moieties, respectively. This assignment seems reasonable
considering that the C
α
atoms of 2a are involved in additional
interactions in the crystal structure. 2b also showed two bands
-
1
at similar wavenumbers (2060 and 2182 cm , entry 5). On the
other hand, their mono-acetylenic analogues (1a – 1c) gave
-
1
C≡C vibrations at ~2100 cm (entries 1 - 3) (Fig. S5a-c).
Accordingly, vibration bands of the coordinating C ≡C bonds of
α
Figure 3. Schematic illustration of a possible mechanism of the
Au-π interaction. Back-bonding-like electron donation from the
Au core to the σ-bonded C≡C unit in 2.
6
-
1
the diacetylenic clusters (2) were red-shifted by ~50 cm from
those of corresponding mono-acetylenic counterparts (1), which
appears to be concerned with the attractive interaction found in
the crystal structure of 2a.
To obtain further insights into the attractive Au-π interactions
specific in the diacetylenic clusters (2), we conducted DFT
calculation studies of 1a and 2a. As shown in Fig. S6, orbital
distribution features of 1a and 2a were similar to each other for
HOMO and LUMO, but notable differences were found in the
lower-energy occupied orbitals. Specifically, in 2a, two bonding
interactions between the superatomic cluster orbitals located
Table 2. IR and UV-vis Absorption Data of Alkynyl-ligated Au
8
Clusters.
-
1
a
b
entry
cluster
ν
C≡C (cm )
λ (nm)
1
2
3
4
5
1a
1b
1c
2a
2b
2101
2107
2113
509
512
509
520
524
around the Auedge atoms and C
the HOMO-2 and HOMO-4, which respectively involve each of
the two orthogonally oriented C ≡C π orbitals (e.g., π2py, π2pz
Fig. 4). Therefore, the attractive interaction found in the
α
≡C units were explicitly found in
α
)
2054, 2179
(
2060, 2182
diacetylenic cluster appears to be augmented by dual
interactions of the cluster valence electrons with the two π
orbitals. In contrast, for 1a, there were no signs of bonding
interactions between the molecular orbitals of the gold cluster
a
b
In KBr pellet at room temperature. In methanol at 25°C.
The above crystallographic and IR profiles implied that the σ-
coordinated C ≡C bonds of the diacetylenic Au clusters (2) are
weakened as a result of the additional interaction with the
neighboring gold atoms in the Au framework. Such a bond
weakening has been well established in many alkyne π-
α
8
α
units and the C ≡C moieties in the lower-energy orbitals as far
as inspected to HOMO-8 (Fig. S6a). In this relation, natural bond
6
orbital (NBO) analyses revealed that 2a has more positive
α
charges on the Auedge atoms and negative charges on the C ≡C
[
22-23]
complexes
and also found in phenylethynyl-capped gold
In the present case, one can also postulate the
units than 1a (Fig. S7). These results may reflect the preference
[
11, 17, 24]
clusters.
of the back-donation character of the electronic interaction of the
possibility of σ- and π-complexation for the additional interaction.
Actually, recent crystallographic studies of phenylethynyl-capped
gold clusters have shown diverse binding motifs, some of which
contain σ-bonded Au-C≡C- moieties that simultaneously bind
Au
6
core with the diacetylenic moieties.
[
4]
neighboring gold atoms via σ- or π-coordination. However,
such σ- or π-bonding is unlikely in 2, considering the long
α 6
distances and relative orientation between the C ≡C and Au
framework in the crystal structure (Fig. 2b). Furthermore, as
mentioned, the additional interaction is specifically observed for
diacetylenic clusters (2), not for the mono-acetylenic clusters (1).
In this relation, the energy level of π* orbital of 1,3-butadiynyl
anion was shown to be substantially lower than that of the
Figure 4. DFT molecular orbital plots for (a) HOMO-2 and
[
25-26]
(
b) HOMO-4 of 2a.
corresponding ethynyl anion.
On the basis of this
information, we would propose the involvement of “back-
donation-type” mechanism for the attractive interaction, as
illustrated in Fig. 3. Thus, the low energy level of π* orbital of the
conjugated diacetylenic system may facilitate the through-space
As described, crystallographic and IR studies have clearly
revealed the [core+exo]-type Au clusters bearing mono- (1) and
8
di-acetylenic units (2) attached to the exo gold atoms were
structurally different, which are associated with the
intramolecular attractive Au-π interaction. Based on these
structural features in solid states, we next measured NMR
electron donation from the bitetrahedral Au
6
core. For the
electron donation from the gold core, our previous theoretical
and experimental studies on a series of [core+exo]-type gold
clusters demonstrated that the HOMO is localized around the
3
1
spectra to investigate the solution structures. The P NMR
spectra of the four clusters all displayed three signals with an
integrated intensity ratio of 1 : 2 : 1, in agreement with the
[
27]
exo-bridged edges of the polyhedral core, and the core serves
[
14, 28]
as an electron pool to induce unique behaviors.
Thus, the
electron-donating capability of the Au core, as well as the
6
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Angewandte Chemie International Edition
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COMMUNICATION
[
core+exo]-type Au
8
framework decorated by four dppp ligands.
In conclusion, we have demonstrated that C≡C ligands and
However, the careful comparison revealed that there is a
marked trend to discriminate 1 and 2 in terms of the chemical
shifts of the signals assignable to the phosphorus atoms bonded
to the Auedge and Aucenter atoms (Pedge and Pcenter) (Fig. S8). The
8
gold frameworks of diacetylene-modified Au clusters undergo
not only atop-type σ-bonding but also additional Au-π attractive
interaction. Such interactions were not found in simple mono-
acetylene-modified analogues and hence are specific for the
diacetylenic clusters. The involvement of back-bonding type
electron donation from the cluster core to π* orbital of
diacetylenic units was proposed. With respect to the bonding
patterns of C≡C units in protected gold clusters, there are
numerous configurations involving well-known σ- / π-bonding,
such as Au-C-Au bridging. The present interaction does not fall
into these categories, and, rather may be related to the hollow-
P
center signals of the 2a and 2b were observed at δ = ~54.9,
which were upfield from those of the mono-acetylenic clusters
1a – 1c) (δ = ~55.6). On the contrary, Pedge signals appeared at
(
a lower magnetic field for 2 (δ = ~51.6) than for 1 (δ = ~51.0).
1
Such a difference between 1 and 2 was also observed in the H
NMR spectra for some protons of phenyl groups of dppp ligands
(
Fig. 5 and Table S4). The signals of o-protons of Ph2 of 2a and
b are observed at δ = ~8.0 ((c) and (d)), which are upfield from
[
29-
2
site adsorption found in the interactions on substrate surface.
3
0]
corresponding signals of 1a, 1b and 1c (δ = ~8.1, (a - c)). Since
the Ph2 groups are located closed to the coordinating C≡C
moieties, the observed NMR profiles may reflect the
presence/absence of the Au-π attractive interaction. On the
other hand, the m- and p-protons of Ph1 of 1a and 1b were
observed upfield from those of 1c, 2a, and 2b (Fig. 5),
suggesting the spatial proximity of the Ph1 groups to the
terminal aryl groups of the acetylide ligands of 1a and 1b.
The unique capability of diacetylenic ligands would endow
intriguing structures and properties in metal cluster chemistry.
Experimental Section
Diynes were prepared by organic synthesis (see Supporting Information)
and were reacted with [Au
reported for the synthesis of mono-acetylenic clusters (1).
methanolic solution (20 mL) of [Au (dppp) ](NO (20.0 mg, 5.47 µmol)
8 4 3 2
(dppp) ](NO ) according to the method
[
8]
2a: To a
8
4
3 2
)
was added buta-1,3-diyn-1ylbenzene (3.0 mg, 23.9 µmol) and sodium
methoxide (6.4 mg, 119 µmol), and the mixture was stirred for 13 h at
room temperature under nitrogen and room light. The product was
further purified by vapor diffusion of ether into a cluster solution in
methanol to give 2a as pink crystals (6.5 mg, 59%). CCDC 1834911 (2a)
and 1885501 (1c) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge
Crystallographic
Data
Centre
via www.ccdc.cam.ac.uk/data_request/cif. 2b was prepared in a similar
manner to the preparation of 2a. Characterization data (ESI-MS and
NMR) are given in Supporting Information.
Acknowledgements
This work was supported by MEXT/JSPS Grants-in-Aid
1
Figure 5. Aromatic region of H NMR spectra of (a) 1a, (b) 1b, (c)
1
(
KAKENHI 18H01987 to K.K., 16H05961 to Y.S., and 18J11586
2 2 3
c, (d) 2a, and (e) 2b in CD Cl / CD OD (1/1) at 25 °C. The
asterisks indicate the signals assignable to p-Ph1 (δ = ~7.0) and m-
Ph1 (δ = ~6.8). Structure with Ph-group labeling is also shown.
to M.I) and Mitsubishi Foundation. We thank the help of Dr. K.
Yoza of Bruker AXS in the crystal data refinement.
Finally, we accessed the electronic properties to investigate
the perturbation effects of the intramolecular Au-π interactions.
Keywords: cluster compounds · gold · diyne · π-complex ·
We have previously reported that the [core+exo]-type Au
8
ligand effect
clusters exhibit single absorption bands at 500-550 nm due to
the HOMO-LUMO transitions within the gold framework, and the
band
positions
(wavelengths)
sensitively
reflect
the
electronic/attractive interactions with the proximal ligand
moieties. As summarized in Table 2, the diacetylenic clusters
[
[
1]
2]
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[
5]
6]
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