太陽光エネルギーの有効利用を目指した金属クラスター型人工光合成モデルの構築
Artificial photosynthetic model composed of noble metal clusters towards efficientutilization of sunlight
研究代表者 北海道大学 大学院工学研究院 助教 石田 洋平
Yohei Ishida, Assistant Professor of Faculty of Engineering, Hokkaido University
1．Abstract
I have been studying the control of molecular assembly structure based onelectrostatic interaction and its application to artificial photosynthetic systems.
Recently, I have succeeded in synthesizing a group of cationic metal clusters with acompletely single composition for the first time. By applying the ordered assembly
control technique to molecular metal clusters, I aimed to propose an artificial
photosynthetic model. I synthesized new metal cluster compounds with surface charges,
but it was difficult to isolate clusters with visible light emission due to the low thermal
stability of the cluster compounds with very dense surface charges. I therefore tried toestablish a new method to obtain cluster compounds with the desired composition while
controlling the number of surface charges. Surface chemical reactions on atomically
precise metal clusters have considerable attention for opening a new platform for cluster
functionalization. In this study, basic Au25(4-PyET)18 (4-PyET = –SCH2CH2Py; Py =pyridyl) clusters were successfully transformed into cationized Au25(4-PyET-CH3+)x(4PyET)18-x clusters, without altering their Au25 cores, through the Menshutkin SN2reaction of their surface Py moieties. This study offers not only a simple cationization
method, but also a protocol for modifying the surface functionalities of molecular metalclusters via a synthetic reaction. The use of metal clusters as supramolecular devices
with functional sites on the surface is now in progress.
2．Introduction
Photosynthesis in natural plants is one of the ideal photochemical reactions. It is anextremely efficient light-harvesting system that collects visible light and realizes lightenergy conversion reactions using water as an electron source. In the light-harvesting
system, the chlorophyll pigments are appropriately aligned and oriented by thesurrounding proteins, resulting in highly efficient light energy collection and transfer.
Based on the guideline that regular molecular arrangements are important for highly
efficient photochemical reaction systems, research is being conducted worldwide.
Although covalent linkage is a typical method for constructing a regular assembly－ 201 －

structure of organic molecules, I have succeeded in precisely controlling the assembly
structure of molecules based on electrostatic interactions for anionic inorganic
nanosheet materials, and have constructed an artificial light-harvesting model withalmost 100% quantum efficiency and a new artificial photosynthesis model.
In this study, we focused on the possibility of using atomically precise molecular
metal cluster compounds as a chemical reaction field in order to mimic the precise
molecular arrangement structure of the photosynthetic system in a three-dimensional
system. Atomically precise, thiolate-protected gold clusters (Aum(SR)n) have potential
applications in diverse fields, such as catalysis,1,2 sensing,3,4 and bio-applications,5,6
owing to their interesting physicochemical properties. Surface SR ligands are ofparticular interest as they affect the magic size (stable number of metal atoms) andstructures of the clusters.6,7 Furthermore, the R-groups on the cluster surface areexposed to solutions and other phases; hence, they determine the molecular-likecharacteristics

ofclusters,8

such

as

their

hydrophobicities/hydrophilicities,9chiralities,10 catalytic activities,2,11 optical properties,12 and interactions with hostmaterials.5,6 These R-groups are therefore crucial to the rational design of thefunctional properties and applications of clusters. In peculiar, clusters capped bycationic R-groups are desirable as they widen the applications of bioimaging andsensing;13 however, the synthesis of cationized clusters is rarely reported owing to thedifficulty in handling the positive charges of R-groups.14-16 By preventing theformation of [Au(I)]-–SR+ ionic polymer complexes that hinder the homogeneous
reduction of Au precursors and the formation of atomically pure clusters, we recently
succeeded in the synthesis of cationized Au clusters (Au25(S(CH2)11N(CH3)3+)18,14
Au144(S(CH2)11N(CH3)3+)60,15 and Au25(S(CH2)11N(CH3)3+)x(S(CH2)2Ph)18-x (x=1-6)).16
To date, major protocols for the preparation of atomically precise clusters with various
thiolate ligands have been categorized into size-focusing14,15 and ligand-exchange
method.16,17 In the former case, the Au(I)–SR complex is prepared by mixing Au ion andthiols (HSR), followed by reduction into objected Au clusters. The latter involves thesynthesis of Au clusters, followed by the replacement of their initial ligands with thedesired ligands. Besides these methods, surface reactions have also received significant
attention in recent years; however, there are few reports on such reactions, two of which
are azido- and amide-bond formation reactions.18,19
The Menshutkin SN2 reaction is a common method to obtain quaternary ammonium
salt ([–N(CH3)3]+) by reacting a tertiary amine with an alkylating agent.20 In this study,
we demonstrate the Menshutkin (methylation) reaction of the surface pyridyl (Py)moieties on basic Au25(4-PyET)18 clusters (4-PyET = –SCH2CH2Py) for their successful－ 202 －

transformation into cationized Au25(4-PyET-CH3+)x(4-PyET)18-x (x ≈ 18) clusters (Scheme1). This work proposes a novel strategy for not only the cationization of Au clusters by
surface chemical reactions, but their functionalization as well.

Scheme 1 Menshutkin (methylation) reaction on Au25(4-PyET)18 clusters(4-PyET = –SCH2CH2Py, where Py = pyridyl group)
3．Results and Discussion
The following 4 chemicals were evaluated as methylating reagents: iodomethane(MeI), dimethyl sulfate (Me2SO4), methyl trifluoromethanesulfonate (TfOMe), andtrimethyl oxonium tetrafluoroborate (Me3OBF4) (reactivity with Py group: MeI <Me2SO4 < TfOMe < Me3OBF4). The surface-reactive basic Au cluster Au25(4-PyET)18 wassynthesized according to our method as reported previously and characterized byelectrospray ionization mass spectrometry (ESI-MS) and UV-vis absorption.21 Figures
1(a)-(d) show the UV-vis absorption spectra of the samples before (0 min) and after the
methylation reaction (30 min or 60 min) using each of the 4 methylating reagents. When
MeI and Me3OBF4 were used (Figures 1(a) and (d), respectively), the characteristic
absorption peaks of Au25 clusters22,23 at 400, 450, 560, 670 and 780 nm disappeared after
the reaction, indicating that these reagents decomposed the Au25 clusters. On the other
hand, no obvious decreases in the absorption spectra were observed when using Me2SO4
and TfOMe (Figures 1(b) and (c)). The decomposition of the clusters by MeI is due to the
nucleophilicity of the I- ion generated by the reaction, as reported elsewhere.24 Besides,
methylating agents not only react with the Py group of the ligands, but with the –S
moiety of the Au–S bonds as well. Their reaction with the –S group causes the Au–S
bond to break, leading to the decomposition of the clusters. Me3OBF4 likely decomposed
the clusters owing to its strong reactivity. On the other hand, Me2SO4 and TfOMe－ 203 －

Figure 1 Selection of methylating reagents(UV-vis absorption spectra of the samples before (0 min) and after reaction (30 min or
60 min) with (a) MeI, (b) Me2SO4, (c) TfOMe, and (d) Me3OBF4)
selectively react with the outer Py groups over the inner –S groups because of theirmoderate reactivity, allowing the methylation reaction to proceed without any
decomposition of the clusters.
TfOMe, which proceeded the most efficiently, was selected as the methylating reagent.
The Menshutkin reaction on the Au25(4-PyET)18 cluster was conducted to obtainmethylated, cationized Au25 clusters (see the detailed procedure in the Supporting
Information). Trifluoromethanesulfonic acid (CF3SO2OH) was noted as a decomposition
product of TfOMe, which could hinder the Menshutkin reaction by protonating theunreacted Py groups. By performing the 1h reaction 3 times (i.e., 3 cycles), CF3SO2OH
was effectively removed and the reaction was proceeded efficiently. Notably, running 1
cycle for up to 2 days did not produce fully methylated Au clusters as the main productand was less efficient than the 3-cycle reaction. UV-vis absorption spectra of the sample
before and after 1 cycle showed slight increases in the characteristic Au25 cluster
bands22,23 at 400 and 560 nm, whereas those at 450, 670, and 780 nm slightly decreased(Figure 2). Such absorption changes were also observed when Au25(4-PyET)18 clusters－ 204 －

Figure 2 UV-vis absorption spectra of the sample before (0h) and after 1-cycle reaction(1h) with TfOMe
were treated with HCl, in which the reversible protonation of the Py group gave its
positive form, –C5H4NH+.21 These results strongly suggest that the geometric structureof Au25 core was distorted due to the surface charge anisotropy12,21 during the additionof positive charges. The absorption spectra showed negligible changes after the 2nd and3rd reaction cycles, demonstrating the good stability of the methylated, cationized Au25
clusters and lack of geometrical changes in the Au25 core (discussed in detail in Figure
4).
The positive-mode ESI-MS of the methylated Au25 clusters (Figure 3, left)showed 3 different charged states +2, +3, and +4 (depending on the number ofattached PF6- anions). Specifically, [Au25-1(4-PyET-CH3+)x(4-PyET)18-x·(PF6-)x-3]2+ (11≤ x ≤ 18), [Au 25 -1 (4-PyET-CH 3 + ) x (4-PyET) 18- x ·(PF 6 - ) x -4 ] 3+ (12 ≤ x ≤ 18), and[Au 25 -1 (4-PyET-CH 3 + ) x (4-PyET) 18- x ·(PF 6 - ) x -5 ] 4+ (14 ≤ x ≤ 18) (groups Au25(+2),
Au25(+3), and Au25(+4), respectively) were observed. Figure 3 (right) shows the expanded
ESI-MS of the methylated Au25 clusters with +3 charged state (group Au25(+3)) observedwith relatively high intensity. After 1st reaction cycle (1h in total), the x values (thenumber of methylated ligands) ranged between 12 and 18. Note that after 2 min ofreaction, x had already reached 10-18, indicating the reaction rate became very
inefficient due to the decrease in the number of reaction sites and bulkiness of thecounter anions CF3SO3- and PF6-, which geometrically impede further reaction of theunreacted ligands. After the 2nd and 3rd reaction cycles (2h and 3h in total,
respectively), the x values increased to x=14-18 and 15-18 respectively. The average
number of methylated ligands was calculated from the peak intensity ratio, assuming
the mass spectral signal intensities are equal to the solution concentrations of the－ 205 －

Figure 3 Positive-mode ESI-MS of the samples after different numbers of reaction
cycles with TfOMe (left), and the expanded ESI-MS of group Au25(+3) (right)(The red and blue numbers indicate the x values of [Au25-1(4-PyET-CH3+)x(4-PyET)18-x·(PF6-)x-4]3+ and fragmentation product [Au240(4-PyET-CH3+)x(4-PyET)16-x·(PF6-)x-3]3+,
respectively)
corresponding cluster species; the numbers for cycles 1-3 were 15.0 (83.1%), 16.7 (92.7%),
and 17.3 (95.9%), respectively. The 1H NMR spectrum of the sample after the 3rdreaction cycle also showed a similar x value of 17.3 (96.0%), confirming that the x values
calculated from the mass spectral signal intensities are reliable in the selected mass
region (m/z = 2220-4950). The average x value gradually increased by the number ofreaction cycles, and nearly per-methylated Au25 clusters were successfully obtained
after 3 cycles. By further increasing the number of reaction cycles to 4 (4h in total), moreclusters, with x=18, could be produced; however they exhibited more impurity peaks
due to their decomposition. The compositions and charge assignments of the observed
Au25 cluster peaks are summarized in Table 1.

－ 206 －

Table 1 The compositions and charge assignments of the observed Au25 cluster peaks
in Figure 3x1hComposition

value

2h

3h

m/z (obs.) m/z (obs.) m/z (obs.)-1+

m/z (calc.)-

Total charge +2 : Au25 (4-PyET-CH3 )x(4-PyET)18-x·(PF6 )x-3
-1

+

-

11

Au25 (4-PyET-CH3 )11(4-PyET)7(PF6 )84368.38--4368.40

12-1+
Au25 (4-PyET-CH3 )12(4-PyET)6(PF6 )94448.39--4448.40

13-1+
Au25 (4-PyET-CH3 )13(4-PyET)5(PF6 )104528.38--4528.38

14

Au25 (4-PyET-CH3 )14(4-PyET)4(PF6 )11

4608.364608.36-4608.38

15-1+
Au25 (4-PyET-CH3 )15(4-PyET)3(PF6 )124688.374688.384688.314688.37

16-1+
Au25 (4-PyET-CH3 )16(4-PyET)2(PF6 )13

4768.35

4768.364768.374768.36

17

Au25 (4-PyET-CH3 )17(4-PyET)1(PF6 )144848.334848.364848.364848.36

18-1+
Au25 (4-PyET-CH3 )18(PF6 )15-

4928.34

4928.34

4928.35

-1

+

-1

-

+

-

-1
+
Total charge +3 : Au25 (4-PyET-CH3 )x(4-PyET)18-x·(PF6 )x-4
-1

+

-

12

Au25 (4-PyET-CH3 )12(4-PyET)6(PF6 )82917.30--

2917.28

13-1+
Au25 (4-PyET-CH3 )13(4-PyET)5(PF6 )92970.62--2970.61

14

Au25 (4-PyET-CH3 )14(4-PyET)4(PF6 )103023.943023.95-

3023.94

15-1+
Au25 (4-PyET-CH3 )15(4-PyET)3(PF6 )11

3077.273077.283077.273077.27

16-1+
Au25 (4-PyET-CH3 )16(4-PyET)2(PF6 )123130.59

3130.60

3130.613130.60

17

Au25 (4-PyET-CH3 )17(4-PyET)1(PF6 )13

3183.92

3183.92

3183.933183.92

18-1+
Au25 (4-PyET-CH3 )18(PF6 )14

3237.24

3237.253237.253237.25

-1

+

-1

-

+

-

-1
+
Total charge +4 : Au25 (4-PyET-CH3 )x(4-PyET)18-x·(PF6 )x-5
-1

+

-

14

Au25 (4-PyET-CH3 )14(4-PyET)4(PF6 )92231.72--

2231.71

15-1+
Au25 (4-PyET-CH3 )15(4-PyET)3(PF6 )10

2271.71

2271.71

2271.71

2271.71

16

Au25 (4-PyET-CH3 )16(4-PyET)2(PF6 )11

2311.702311.712311.702311.70

17-1+
Au25 (4-PyET-CH3 )17(4-PyET)1(PF6 )12

2351.70

2351.70

2351.69

2351.70

18-1+
Au25 (4-PyET-CH3 )18(PF6 )13

2391.692391.692391.692391.69-1+-

The isotope distributions matched well with the simulated isotope patterns. The wellknown

fragmentation

product[Au 4 0 (4-PyET-CH 3 + ) x (4-PyET) 4- x ·(PF 6 - ) x -1 ] 1+

(1 ≤ x ≤ 4) was also observed. Moreover, the fragmentation products during the ESI
process[Au 24 0 (4-PyET-CH 3 + ) x (4-PyET) 16- x ·(PF 6 - ) x -3 ] 3+(8≤x ≤ 16) and[Au 23 0 (4-PyET-CH 3 + ) x (4-PyET) 15- x ·(PF 6 - ) x -3 ] 3+ (8 ≤ x ≤ 15) were observed forAu25(SR)18; these corresponded to losses of Au1–SR2 and Au2–SR3. Interestingly, like－ 207 －

those of the whole Au25 clusters, the x values of these fragmentation products also
increased with the number of reaction cycles.

Figure 4 The abundance ratios of each cluster species with different x values under
representative conditions

We also evaluated the weaker methylating reagent, Me2SO4, by varying the reaction
time and number of reaction cycles; this gave clusters with various x values. Specifically,
those with x=8.5, 9.7, 12.6, and 14.1 were formed by reactions with Me2SO4 for times of
2, 5, 15, and 60 min, respectively. The abundance ratios of each cluster species withdifferent x values, determined by the intensities of their ESI-MS peaks, are shown in
Figure 4. Monitoring the reaction with Me2SO4 revealed that the absorption spectralchanges (i.e., the structural change of Au25 core) were completed at 15 min, where theminimum x value of the contained Au clusters was 9. These observations indicate thatthe structural changes are complete when approximately half number of the surface Pygroups are cationized. To further understand the mechanisms of the spectral andstructural changes caused by the dense surface charges, we are currently isolating eachcluster species and analyzing them by single-crystal X-ray structure analysis.

－ 208 －

4．Conclusion
We have demonstrated a novel method for obtaining cationized Au clusters via thesurface Menshutkin SN2 reaction. By changing the methylating reagent, reaction time,and number of reaction cycles, we succeeded in controlling the number of methylated
ligands (x); an x of approximately 18 (i.e., fully methylated Au25(4-PyET-CH3+)18) was
obtained under the optimal condition. This work proposed not only a new synthetic
pathway for cationized Au clusters, but also provides a novel idea for diversifying theR-groups of molecular metal clusters through the surface chemical reaction.
Acknowledgement
Y.I. thanks nancial supports from JFE 21st Century Foundation.
謝辞
本研究は公益財団法人 JFE21 世紀財団研究助成により行われました。ここに記して感謝申し上げます。
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