Chemistry Letters 2002
71
Figure 2. Raman spectra of [(Nb6Cl12)Cl2(H2O)4]Á4H2O (1)
Figure 3. XRD patterns of (H3O)2[Mo6Cl14]Á6H2O (2) treated
at various temperatures in a helium stream for 1 h. All the
identified compounds have two strong diffraction peaks at 12.4–
treated at various temperatures in a helium stream for 1 h.
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acetate at 150 C. Clusters 1 and 2 revealed the maximum
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1
2.6 and 15.3–15.6 degree.Unique diffraction peaks of each
compound are indicated: (a) for 2, (b) for [Mo6Cl12]Á8H2O, (c) for
Mo6Cl12]Á2H2O, and (d) for Mo6Cl12.
activities by treatment at around 250 C and 400 C respectively.
The Raman spectra of 1 treated at various temperatures for1 h
in helium streams are shown in Figure 2. The spectra markedly
[
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changed beyond 250 C with increasing temperature. The Raman
a
shift caused by the vibration of Nb–Cl or Nb–O has not been
À1
change shown in Figure 2, indicating partial loss of some internal
Cl ligands. The X-ray diffraction patterns of 2 are shown in Figure
3. The cluster changed from (H3O)2[Mo6Cl14]Á6H2O via
[Mo6Cl12]Á8H2O and [Mo6Cl12]Á2H2O to an extended M–Cl–M
attributed for this complex. However, the band at 239 cm is
assigned to the breathing motion of the Nb6 octahedron (A1g),
À1
whereas those at 161 and 152 cm areattributed toedge bridging
i
8
i
a
a
7
Nb–Cl breathing vibrations (Eg and T2g, respectively). Crystal
structural data revealed that the differences in M–M distances
bonded solid compound Mo6Cl12 ([Mo6Cl 8]Cl 2Cl 4=2) when
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heat was applied up to 450 C. Thus in contrast to 1, formation of
9
parallel the observed variations in the Raman shift. The
variations of the observed shift of the A1g M6 band are almost
entirely accounted for by differences in the M–M force constant
the solid state network is likely to keep the activated non-
coordinated metal atoms intact, and hence retain the catalytic
activity.
9
that accompany dissimilarities in the M–M bond distances. The
band of the Nb6 A1g vibration mode begins to shift to higher
We thank Mr. T. Mori and Mr. M. Watanabe of Shibaura
Institute of Technology for their help in experiments.
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energy by treatment and has the maximum shift at 250 C. The
loss of some internal Cl ligands can increase the Nb–Nb bond
order of the Nb6 octahedron, which may be ascribed to the higher
energy shift. However, the Nb6 metal framework remains the
References and Notes
1
S. C. Lee and R. H. Holm, Angew. Chem., Int. Ed. Engl., 29,
840 (1990).
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same. Above 250 C, the two Raman bands originally at 161 and
i
À1
1
52 cm due to edge bridging Nb–Cl breathing vibrations were
2
3
H. Imoto and J. D. Corbett, Inorg. Chem., 19, 1241 (1980).
N. Prokopuk and D. F. Shriver, Adv. Inorg. Chem., 46, 1
(1999); G. J. Miller, J. Alloys Compd., 229, 93 (1995).
F. W. Koknat, J. A. Parsons, and A. Vongvusharintra, Inorg.
Chem., 13, 1699 (1974).
replaced with a new band, which suggests that the structure of the
coordinating Cl ligands changed above this temperature. It is at
this temperature that the catalytic activity is the highest. Hence,
the catalytic activity of 1 may be attributed to the appearance of
the non-coordinated metal atoms of the cluster metal core. The X-
ray diffraction patterns of the same samples were recorded. The
change of the X-ray diffraction pattern shows that treatment
4
5
6
E. Ghibaudi and A. J. Colussi, Int. J. Chem. Kinet., 16, 1575
(1984).
Y. Pouilloux, J. P. Bodibo, I. Neves, M. Gubelmann, G. Perot,
and M. Guisnet, Stud. Surf. Sci. Catal., 59, 513 (1991).
P. Nannelli and B. P. Block, Inorg. Synth., 12, 170 (1970).
K. Harder and W. Preetz, Z. Anorg. Allg. Chem., 591, 32
(1990).
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above 100 C changes 1 to an amorphous compound that is not
the extended M–Cl–M bonded solid compound Nb6Cl14
7
8
i
iðaÞ
a
aðiÞ
(
[Nb6Cl 10Cl 2=2]Cl 4=2Cl 2=2) prepared by
a different
1
0
route. Furthermore, the change of the crystal structure, which
could be caused by removal of the outer halogen ligand or
coordinated water, did not give rise to the catalytic activity.
The Raman spectra of 2 treated at various temperatures for
9
J. R. Schoonover, T. C. Zietlow, D. L. Clark, J. A. Heppert,
M. H. Chisholm, H. B. Gray, A. P. Sattelberger, and W. H.
Woodruff, Inorg. Chem., 35, 6606 (1996).
1
Cl vibrations beyond 200 C is quite similar to the corresponding
h in helium streams were measured. A marked change of Mo–
i
10 A. Simon, H. G. Schnering, H. W o¨ hrle, and H. Sch a¨ fer, Z.
Anorg. Allg. Chem., 339, 155 (1965).
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