Effects of π-Electron Systems on Optical Activity of Au11 Clusters Protected by Chiral Diphosphines

Article abstract of DOI:10.1002/bkcs.12363

We systematically examined the effects of π-electron systems on the chiroptical activity of atomically precise gold clusters [Au₁₁(DP)₄L₂]⁺, where DP and L represent chiral diphosphines and achiral anionic ligands, respectively. Reducing the distance between the π-electron systems of the chiral DP and the Au₁₁ core enhanced the anisotropy factor of [Au₁₁(DP)₄L₂]⁺ in the range of 300–450 nm while extension of the π-electron system of the achiral L did not. This tendency supports our previous proposal that the proximity of the chiral π-electron system to the gold core amplifies the optical activity.

Full text of DOI:10.1002/bkcs.12363

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DOI: 10.1002/bkcs.12363
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N. Shinjo et al.
KOREAN CHEMICAL SOCIETY
Effects of π-Electron Systems on Optical Activity of Au11 Clusters
Protected by Chiral Diphosphines
†,‡,
Naoaki Shinjo,† Shinjiro Takano ,† and Tatsuya Tsukuda
*
†Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan
‡Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto
615-8520, Japan. *E-mail: tsukuda@chem.s.u-tokyo.ac.jp
Received June 8, 2021, Accepted July 9, 2021
Keywords: Au11 clusters, Chiroptical activity
The chiroptical activity of ligand-protected gold clusters
has attracted much interest since the first report of optically
active gold clusters protected by L-glutathione.1 Early stud-
ies have demonstrated the emergence of circular dichroism
(CD) in atomically precise Au clusters by protecting with
intrinsically chiral ligands having monodentate2–4 and mul-
tidentate5,6 binding sites or by noncovalent interaction with
chiral counterions.7,8 Whetten proposed that the chiroptical
activity originates either from a chiral core, a dissymmetric
field induced by the ligands, or a chiral footprint of the
ligands.1 However, determining factors of the optical rota-
tory strength and sign of optical rotation could not have
been identified due to the lack of structural information.
Later single crystal X-ray diffraction (SCXRD) analysis rev-
ealed the chiral geometric structures in the crystals of thiolate
(RS)-protected Au clusters such as Au38(SR)24,9 Au40(SR)24,10
Au44(SR)26,11 Au52(SR)32,10 Au102(SR)44,12 Au133(SR)52,13 and
Au246(SR)80,14 and diphosphine-protected Au13 clusters.15 The
isolation of enantiomeric Au clusters and their structural determi-
nation by SCXRD have significantly deepened our understanding
on the origin of the CD activity.16–23 Jin recently proposed a uni-
fied view of the chirality of Aux(SR)y based on the multitude of
examples in the literature24: the chirality mainly originates from
the surface structures rather than the core structure. Thermal-
induced racemization of enantiomeric Au38(SR)24 was observed
and ascribed to the rearrangement of the Au(I)SR bidentate oligo-
mers.18 The addition of an RC≡CAu unit to [Au9(BINAP)4]3+
suggest that the CD activity cannot be explained by a single
origin but by a combination of three origins.
Our group has previously determined the crystal struc-
tures of enantiomerically pure Au clusters protected by chi-
ral diphosphines: [Au11(R/S-DIOP)4Cl2]+ (DIOP = 1,4-bis
(diphenylphosphino)-2,3-O-isopropylidene-2,3-butanediol)
(1R/1S) and [Au8(R/S-BINAP)3(PPh3)2]2+ 19
CD activity of
.
[Au8(R/S-BINAP)3(PPh3)2]2+ much stronger than 1R/1S
cannot be explained solely by their HCM values. We proposed
that chiral arrangement of π-electron systems near the Au core
plays a role in the optical activity.19 To validate this proposal,
we herein studied the effects of π-electron systems on the optical
activity of Au clusters by employing [Au11(DP)4L2]+ as a plat-
form: DP and L represent chiral diphosphines and achiral
anionic ligands, respectively (Scheme 1). The chiral DP ligands
used in this study were DIOP, BINAP, and N,N0-[1,10-bin-
aphthalene]-2,20-diylbis[P,P-diphenylphosphinous
amide]
(BDPAB), whereas the achiral, monodentate ligands used were
Cl, phenylacetylene (PA), 9-ethynylphenanthrene (EPT), and
2-ethynylpyrene (EP) (Scheme 1). A comparison of the anisot-
ropy factors (Δε/ε, Δε: molar CD; ε: molar absorption coeffi-
cient) of [Au11(R-DIOP)4Cl2]+ (1R),19 [Au11(R-BINAP)4Cl2]+
(2R),5 and [Au11(R-BDPAB)4Cl2]+ (3R) revealed that the CD
was enhanced when the π-electron systems in a chiral configura-
tion were located in close proximity to the Au11 core. By com-
paring the CD activity of [Au11(R-DIOP)4L2]+ (4R, 5R, and 6R
for L = PA, EPT, and EP, respectively) with 1R, it was found
that achiral π-electron systems had little effect on the optical
activity. This study provides a synthesis guide for enhancing the
CD activity of small Au clusters.
The known clusters 1R and 2R were synthesized
according to the previous reports.5,19 New Au11 clusters 3R
and 3S were synthesized via a procedure similar to that for
[Au11(PPh3)8Cl2]+.34 Briefly, (AuCl)2(R-/S-BDPAB) precur-
sors were synthesized by the ligand exchange of (AuCl)S
(CH3)2 with R-/S-BDPAB. Then, a methanol solution of
NaBH4 (0.5 eq.) was added to the dichloromethane (DCM)
solution of (AuCl)2(R-/S-BDPAB) at room temperature. The
crude product obtained by stirring for 6 h was purified by
alumina column chromatography. Details of synthesizing
(BINAP
=
2,20-bis(diphenylphosphino)-1,10-binaphthyl)
resulted in [Au10(BINAP)4(C≡CR)]3+ having the hitherto largest
anisotropy factor (ꢀ6.6 ꢁ 10ꢂ3).23 The origin of the optical
activity in the ligand-protected metal clusters has been theoreti-
cally studied.25–33 Garzon maintained that the known trends in
ꢀ
experimental CD activity are explained in terms of the Hausdorff
chirality measure (HCM),25,30 which is a geometrical quantifica-
tion of chirality. Aikens reported that the deformation of the Au11
and Au8 clusters due to the ligation played the most important
role in the CD activity.26,31 Häkkinen proposed that the chirality
of the core as well as the footprint of the outermost Au–SR
layer contributes to the CD activity of Au144(SR)60 and
Au144(CCR)60.33 These experimental and theoretical studies
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these clusters are given in the Supporting Information.
Electrospray ionization mass spectrometry (ESI-MS, Figure 1
(a) and (b)) indicated that 3R and 3S were synthesized in a
pure form. Weakly structured profiles of the UV–visible
absorption spectra of 3R and 3S (Figure 1(c)) overlapped
almost completely. The anisotropy factors (Δε/ε) of 3R and
3S (Figure 1(d)) were calculated based on the CD spectra of
the acetonitrile solutions of 3R and 3S. The plots of the
anisotropy factor are in a mirror image relationship. These
results suggest that enantiomerically pure 3R and 3S were
successfully synthesized.
Figure 2(a) and (b) compare the UV–visible absorption
spectra and the anisotropy factors of 1R, 2R, and 3R in ace-
tonitrile, respectively. Although SCXRD data were not
available for 2R and 3R, the similarity of their optical spec-
tra with the well-known [Au11(PPh3)8Cl2]+ (Ref. 34) sug-
gests that their Au11 cores have a similar quasi-icosahedral
motif. Figure 2(b) indicates that 1R, 2R, and 3R exhibited
chiroptical activity in the wavelength range of <450 nm
where photoabsorption is associated with optical transition
between the electronic states within the Au11 core. The
anisotropy factors of 1R, 2R, and 3R behave differently
with respect to the wavelength and become smaller in the
order of 2R > 3R > 1R. The different profiles of Figure 2
(b) may be ascribed to the different coordination geometries
of the DP and Cl ligands in 1R, 2R, and 3R according to
the theoretically predicted CD spectra of coordination
Scheme 1. Single-crystal structure of [Au11(R-DIOP)4Cl2]+
(1R)19 and the list of the Au11 clusters used in this study.
Figure 1. Positive-mode ESI mass spectra of (a) 3R and (b) 3S in acetonitrile. Insets show enlarged views of the dominant peaks and the
calculated isotope patterns of [Au11(BDPAB)4Cl2]+. (c) UV–visible absorption spectra and (d) anisotropy factors of 3R and 3S in
acetonitrile.
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Figure 2. (a) UV–visible absorption spectra and (b) anisotropy
factors of 1R, 2R, and 3R in acetonitrile.
isomers of model clusters [Au11(diphosphine)8Cl2]+.31 In
addition, less structured optical absorption spectra of 2R and
3R (Figure 2(a)) than that of 1R may be due to the coexis-
tence of configurational isomers. Although we cannot exclude
a possible contribution of configurational isomers in 2R and
3R, it is safe to conclude based on the theoretical studies31
that the amplitude of the anisotropy factor is not significantly
affected by the coordination geometries of the ligands, but is
mainly associated with the ligand structure. Since BDPAB
has a secondary amine group between the binaphthyl moiety
and the phosphorous atoms, the distance between the
π-electron systems and the Au11 core in 3R is thought to be
larger than that in 2R. These results support our previous pro-
posal that the close proximity of the chiral π-electron system
to the Au core enhances the CD activity.19
Figure 3. Positive-mode ESI mass spectra of the ligand exchange
between 1R and (a) PA (4 R), (b) EPT (5 R), and (c) EP (6 R) in
acetonitrile. Insets show enlarged views of the most dominant
peak in each spectrum and the calculated isotope patterns of
[Au11(R-DIOP)4(L)2]+ (L = PA, EPT, EP). Asterisks are
assigned to the impurities of [Au11(R-DIOP)4(EPT)Br]+ (*),
[Au12(DIOP)4(EPT)3]+
(**),
[Au13(R-DIOP)3(EPT)Cl]2+
(***), and [Au11(R-DIOP)4(EP)Br]+ (****), respectively.
Next, [Au11(R-DIOP)4(PA)2]+ (4R), [Au11(R-DIOP)4(EPT)2]+
(5R), and [Au11(R-DIOP)4(EP)2]+ (6R) were synthesized by the
ligand exchange of 1R.35 Briefly, PA, EPT, or EP (50 eq.)
and subsequently N-ethyldiisopropylamine (50 eq.) were
added dropwise to the methanol–acetonitrile (1:1, v/v)
solution of 1R at room temperature. After stirring the mix-
ture for 3 days at room temperature, the crude products
were purified by silica gel column chromatography. The
products were characterized by ESI-MS (Figure 3). In all
the mass spectra, the most dominant peaks were assigned
to [Au11(R-DIOP)4L2]+ (L: PA, EPT, EP). This indicates
that two Cl ligands of 1R were successfully replaced with
L while retaining their compositions. The common compo-
sitions of 4R–6R suggest that the L ligands coordinate to
the same sites in these clusters although their crystal struc-
tures are not available.
Figure 4. (a) UV–visible spectra and (b) anisotropy factors of
[Au11(R-DIOP)4L2]+ (L: Cl (1R), PA (4R), EPT (5R), EP (6R)) in
acetonitrile. The dotted line in panel (a) shows the absorption
spectrum of free EP in DCM.
The UV–visible absorption spectra of 4R–6R (Figure 4
(a)) showed similar spectral profiles to that of 1R, especially
in the onset region: the spectrum of 6R exhibits sharp addi-
tional peaks due to the localized photoexcitation of the EP
ligands. These results suggest that the electronic structure of
the Au11 core did not change appreciably by the replacement
of the achiral ligand L. The anisotropy factors are plotted in
Figure 4(b). Although the peaks in the 300–400 nm range
are slightly shifted, the anisotropy factors do not show any
clear correlation with the size of the π-electron systems of
L. This suggests that the existence of achiral π-electron
systems does not have a large influence on the optical activ-
ity. These results further support the importance of the chiral
π-electron system near the gold core for enhancing the opti-
cal activity. One straightforward approach to enhance the
optical activity of gold clusters is to modify the nearest
region of their surfaces with chiral ligands bearing highly
extended π-electron systems. Further experimental and theo-
retical efforts will open new avenues for imparting novel
optical properties based on the ligand design.
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In summary, two types of effects of ligands on the
optical activity were demonstrated. First, the anisotropy
factor of [Au11(R-BDPAB)4Cl2]+ was smaller than that
of [Au11(R-BINAP)4Cl2]+, but larger than that of
[Au11(R-DIOP)4Cl2]+. This tendency supports our pre-
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alkynyl ligands with achiral π-electron systems. This sug-
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large optical activity. This research provides a guide for
controlling the optical activity of ligand-protected gold
clusters by designing ligands with appropriate distances
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Acknowledgments.
This research was financially
supported by JST, CREST (grant no. JPMJCR20B2), the
Elements Strategy Initiative for Catalysts & Batteries
(ESICB) (grant no. JPMXP0112101003) of MEXT, and by
JSPS KAKENHI (grant no. JP20H00370).
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
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