Monitoring of Photochemical Self-assembly of [Mo7O24]6¹to {Mo142}-blue Nanoring by Using Mo K-edge XAFSYu Mitani,1 Kazuki Oka,1 Yoshiyuki Shibata,1 Kazushi Konishi,1 Diaa M. Obaid,2Eri Ishikawa,2 Yasuo Izumi,*1 and Toshihiro Yamase*2,31
Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522
2
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
3(Inc.) MO device, 2-14-10 Kanaiwa-higashi, Kanazawa 920-0335(Received November 25, 2009; CL-091045; E-mail: yizumi@faculty.chiba-u.jp)

Synthetic pathway of {Mo142}-ring and the synmetry change of Mo sites.

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2010 p.132–133
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The Chemical Society of Japan
Published on the web January 16, 2010; doi:10.1246/cl.2010.132

doi:10.1246/cl.2010.132
Published on the web January 16, 2010

132

Monitoring of Photochemical Self-assembly of [Mo7O24]6¹
to {Mo142}-blue Nanoring by Using Mo K-edge XAFSYu Mitani,1 Kazuki Oka,1 Yoshiyuki Shibata,1 Kazushi Konishi,1 Diaa M. Obaid,2Eri Ishikawa,2 Yasuo Izumi,*1 and Toshihiro Yamase*2,3
1Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi, Inage-ku, Chiba 263-8522
2Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
3(Inc.) MO device, 2-14-10 Kanaiwa-higashi, Kanazawa 920-0335(Received November 25, 2009; CL-091045; E-mail: yizumi@faculty.chiba-u.jp)
The UV-induced self-assembly of [Mo7O24]6¹ to 28electron-reduced {Mo142} at pH 3.4 was monitored by Mo Kedge XAFS spectra of photolytes. The EXAFS curve ﬁt analysis
indicates that the accumulation of [MoO4]2¹ in an equilibriumwith {Mo8} was accompanied by the formation of two-electronreduced {Mo14}, and the subsequent construction of the {Mo20}
and {Mo21} building blocks provided {Mo142} through their
reductive condensation.

Twenty-eight-electron-reduced
Mo-bluenanorings,[Mo142O418(OH)14(H2O)58]26¹
and[Mo154O448(OH)14(H2O)70]14¹,1­3 have attracted attention in host­guest chemistry,1,3 shape-selective catalysis,4 and molecular magnetism.5 The
former shows a defect ring due to a removal of six linkers froman intact ring of the latter comprising fourteen [Mo2O5(H2O)2]2+linkers.1 The same type of carboxylate-coordinated Moblue ring, [Mo142O419(OH)10(H2O)49(CH3CO2)5(C2H5CO2)]30¹(≡{Mo142}), prepared photochemically in the presence of acetic
acid at pH 3.4, has provided a good model for clariﬁcation of themechanism of the self-assembly to the Mo-blue nanorings,4which is essential for the molecular design of Mo-blue-based
supramolecules.
[Mo7O24]6¹ (≡{Mo7}) and ¢-[Mo8O26]4¹ (≡{Mo8}) as
dominant species in aqueous solutions of molybdates at pH 3­
6 have been identiﬁed by 17O and 95Mo NMR spectra which
distinguish between terminal and bridging O atoms and among
three types of Mo sites, respectively.6,7 Two-electron-reducedMo-blue, [Mo14O46]10¹ (≡{Mo14}) as a condensed dimer of{Mo7}, is produced through one-electron reduction of {Mo7} inthe photolysis of {Mo7} at pH 5.4.8 Recently, an involvement ofmultielectron-reduced {Mo37}­{Mo42} intermediates through{Mo20} and {Mo21} building blocks in the photochemical selfassembly of {Mo7} to {Mo142} at pH 3.4 has been shown by
ESI-MS analysis.9 To obtain mechanistic details of this selfassembly, the electronic structure of the local Mo sites has beenmonitored by Mo K-edge XAFS. In this letter, it is strongly
supported that the {Mo20} and {Mo21} building blocks areproduced through the reaction of {Mo14} with [MoO4]2¹(≡{Mo1}) in an equilibrium with {Mo8}.
[NH4]6[Mo7O24]¢4H2O (1) (0.40 g, 0.31 mmol) was dissolved in water (20 mL), and acetic acid as electron donor wasadded until the pH level of the solution became 3.4. The
resultant solution was exposed to UV light from a 500-W xenon
lamp. Each photolyte sample from 6-, 12-, 24-, 48-, and 72-h
photolyses was kept in the dark and transported to the beamline.
Mo K-edge XAFS spectra were measured at 30­290 K intransmission mode on beamline NW10A of PF-AR, KEK. At

Chem. Lett. 2010, 39, 132­133

beamline, the solutions were transferred without contact to air toan XAFS cell equipped with 25-¯m-thick Kapton windows onboth sides. XAFS analyses were done by use of XDAP 2.2.7(XAFS Services International). Multiple shell curve ﬁt analyses
used the Fourier-ﬁltered k3-weighted EXAFS data (k: wavenumber of photoelectrons).10 The ﬁt parameters for Mo=O andMo­O bonds were extracted from the EXAFS data measured forK2[MoO4] (2).11 The ﬁt parameters for Mo£Mo atomic distanceswere theoretically generated using FEFF 8.412 based on thecrystal structure of 1.13 Typical interatomic distances for Mo=O,Mo­O, and Mo£Mo are 1.66­1.76, 1.85­2.55, and 3.20­
4.04 ¡ for polyoxomolybdates. The best-ﬁt results of Mo
K-edge EXAFS14 for photolytes are summarized in Table 1.
Coordination numbers (N) of Mo=O, Mo­O, and Mo£Mo pairs
for the 0-h photolyte (before the photolysis) were 2.1, 4.1, and3.7, respectively, suggesting partial formation of {Mo1} in an
equilibrium with {Mo8} and {Mo7}.15 If the population ratio
between {Mo8} (or {Mo7}) and {Mo1} was 85:15, the calculated
N values were 2.0, 3.7, and 3.6, respectively. Interestingly, the6-h photolysis showed an increase of the N(Mo=O) value (2.6)
and a decrease of the N(Mo­O) and N(Mo£Mo) values (3.1 and2.7). If the population ratio of {Mo14} (or {Mo7}) and {Mo1}
Table 1. Best-ﬁt results to Mo K-edge EXAFS spectra of photolytes atvarious duration (h) of photolysis and site structural parameters ofcompoundsa
Photolytesand Compds
0612244872{Mo1}(2)d{Mo7}(1)d{Mo8}d{Mo14}d{Mo20}e{Mo142}d

Mo=ObMo­OcMo£MoNR(¡)

NR(¡)

NR(¡)

2.1(«0.7)
2.6(«0.9)
2.2(«0.6)
1.8(«0.2)
1.7(«0.4)
1.5(«0.5)
4
1.7
1.7
1.4
1.8
1.5

1.669
(«0.015)
1.648(«0.008)
1.665(«0.015)1.659(«0.008)
1.657(«0.012)1.657(«0.015)
1.76
1.73
1.70
1.71
®f
1.70

4.1
(«0.3)
3.1(«0.5)
4.1(«0.2)
4.3(«0.1)
4.9(«0.1)
4.6(«0.7)

1.932(«0.018)
1.918(«0.018)
1.925(«0.012)1.948(«0.013)1.941(«0.010)1.938(«0.013)3.7(«0.2)
2.7(«0.5)
4.1(«0.2)
4.5(«0.4)
4.6(«0.6)
4.9(«0.4)

3.314(«0.004)
3.308(«0.006)
3.329(«0.013)3.340(«0.004)
3.362(«0.004)
3.360(«0.002)

4.3
4.3
4.6
4.2
4.6

2.06
2.11
2.08
®f
1.97

3.4
4.2
3.7
4.3
4.9

3.31
3.35
®f
®f
®f

Goodnessof ﬁt
2.0 © 104
3.1 © 104
3.2 © 104
3.5 © 104
3.7 © 104
5.8 © 104
Ref. 11
Ref. 13
Ref. 4
Ref. 8
Ref. 4
Ref. 4a
Fit errors were indicated in parentheses. bFor terminal Mo=O (doubleor single and a half) bonds. cFor Mo­OH, Mo­OH2, and bridging Mo­
O­Mo bonds. dBased on X-ray crystallographic data for authenticsamples. eBased on hypothetical fragment structure from {Mo142}.
f
Exact values were diﬃcult to obtain due to bond strain.

© 2010 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/133
was 7:3, N(Mo=O), N(Mo­O), and N(Mo£Mo) were 2.2, 3.2,and 2.6, respectively, which are close to the values ﬁtted for the6-h photolyte comprising high amounts of {Mo14} derived from{Mo7}.1 A similarity in EXAFS values between {Mo14} and{Mo7} with similar coordination of Mo sites implies thatEXAFS is unexpectedly insensitive to the MoVI ¼ MoV photoreduction.16
For more than 12-h photolytes, the N(Mo=O) value
decreased to 1.5 at 72-h photolysis, while N(Mo­O) andN(Mo£Mo) values increased to 4.6 and 4.9, respectively(Table 1). The bond lengths (R) for Mo=O and Mo­O remained
nearly constant within 0.021 and 0.030 ¡, respectively, throughout the photolysis. On the other hand, the Mo£Mo distanceselongated gradually from 3.308 ¡ at 6 h to 3.362 ¡ at 48 h(¦ = 0.054 ¡), due to an increase of the bond strain withincreasing ring geometry to {Mo142}.1
Assuming that the 12-h photolyte consisted 90% of {Mo20}
or {Mo21} (higher nuclearity species than {Mo14}) and 10%{Mo1}, the values estimated for N(Mo=O), N(Mo­O), andN(Mo£Mo) were 2.0, 3.8, and 3.9, respectively, which were ingood agreement with the result ﬁtted for the 12-h photolyte(Table 1) but only based on estimated N values, the possibilityof inclusion of {Mo8} cannot be excluded. Together with thefact that N values for the three interatomic lengths at the 24-hphotolysis are close to the values for {Mo20} (Table 1), theEXAFS best-ﬁt results support the formation of {Mo20} and{Mo21} building blocks during 12­24 h photolysis. Also, Nvalues for the three lengths for the 72-h photolyte are consistent
with those for {Mo142}, suggesting the formation of {Mo142}
through the reductive cyclic heptamerization of the {Mo20} and{Mo21} building blocks during 24­72 h photolysis. Although
the precipitation of {Mo8} as [Me3NH]+ salt from the 72-h
photolyte strongly indicates the equilibrium between {Mo8}and {Mo1} at pH 3.4,4,15 it should be noted that the dominant
molybdates in the 72-h photolyte are likely to be {Mo142} and{Mo8}. The values of R(Mo=O) and R(Mo­O) for photolytes
were always smaller than for crystals (Table 1), implying morecovalent bonding in solution than in crystal. The reason is notclear at present. Peaks associated with further distant Mo sites
than Mo£Mo were not clearly observed.
A shoulder peak at 19994.5 eV due to the dipole-forbidden
electronic Mo 1s ¼ 4d transition near the Mo K absorption edge
reﬂects inversion symmetry of Mo center,17 and its intensities for
2 and MoO3 corresponding to MoO4-tetrahedral and MoO6octahedral compounds were 0.43 and 0.31, respectively. Thepeak intensity of the photolyte was almost unchanged during 6-h
photolysis (0.31) and decreased during 6­72 h photolysis (0.27),
supporting an increase of the MoO4-tetrahedra during 6-hphotolysis and their decrease during further photolysis with acompensating increase of the MoO6-octahedra through {Mo142}
formation. The 1s ¼ 4d peak for {Mo142} crystal is also shownin Figure 1.
Stepwise photoreaction pathway of {Mo7} to {Mo142} at
pH 3.4 is consistent to the theory of the condensation as
depicted in Scheme 1: {Mo7} is in equilibrium with {Mo8}through the dissociation to {Mo1}.15 The accumulation of{Mo1} through the dissociation of {Mo8} resulting from thephotochemical formation of {Mo14} was indicated by EXAFS
data for the 6-h photolyte (ca. 30% of {Mo1}). Subsequent
decrease of the {Mo1} population for the 12-h photolyte
Chem. Lett. 2010, 39, 132­133

Figure 1. Normalized Mo K-edge XANES spectra. Change of the Mo
1s ¼ 4d pre-edge region during the photolysis (a). Fit evaluation of
1s ¼ 4d peak intensity (0.32 and 0.29) for 0-h photolyte (b) and {Mo142}
crystal (c).

Scheme 1. Pathway to {Mo142} through {Mo20} and {Mo21}.

supports the formation of the {Mo20} and {Mo21} building
blocks through the reaction of {Mo1} with {Mo14}, followed by
the heptameric cyclic condensation of the building blocks to{Mo142} during 24- to 72-h photolysis.4,8References and Notes
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14 EXAFS »-function, associated Fourier transform, and ﬁts are in
Supporting Information. Supporting Information is available electronically on the CSJ-Journal Web site, http://www.csj.jp/journals/chemlett/index.html.
15 F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochman, Advanced
Inorganic Chemistry, 6th ed., Wiley-Interscience, 1999, pp. 920­951.
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