Journal of the American Chemical Society
COMMUNICATION
the bulky C60Ar5 ligands. It is noteworthy that a small change at the
n
t
substituents on the phenyl groups, from Bu (2b) to Bu (2a),
strongly affected the reactivity: the more bulky trisulfide complex 2a
did not react with PnBu3 at all, showing the tunability of the bowl-
shaped confined spaces. These results also demonstrate that
trisulfide complexes can be used as starting materials for precise
syntheses of metalꢀsulfur clusters.
In conclusion, by utilizing the bowl-shaped confined space of
pentaaryl[60]fullerene, we successfully prepared a series of 6π-
electron cobalt trichalcogenide complexes, which are neutral
four-membered aromatic species. The following evidence sup-
ports the conclusion that these four-membered rings are aro-
matic: (1) high planarity of the four-membered ring, (2) short
CoꢀS bond distances, (3) high stability and excellent selectivity
in formation of complexes, (4) strongly delocalized 6π-electron
molecular orbitals with high resonance energies, (5) a large
negative NICS(1) value, and (6) severe destabilization upon
disruption of the 6π-electron structure. The findings reported
here will advance the understanding of exotic aromatic com-
pounds and open new avenues of materials science research on
interesting π-electron systems
Figure 4. Crystal structure of 4b with thermal ellipsoid plotted at 30%
probability (hydrogen atoms and solvent molecules are omitted for
clarity).
of π-aromaticity was obtained in NICS-scan calculations19 for an
out-of-plane component (NICSzz), which showed respectable
negative values (up to ꢀ8) at 1ꢀ2 Å above the CoS3 plane,
indicating the magnetic shielding from aromatic π-electrons of the
CoS3 ring (Figure S4, SI).
Resonance in the CoS3 ring was studied by using second-order
perturbation theory to estimate the structure of a model compound,
CpCoS3 (Figures S5 and S6, SI). The donorꢀacceptor stabilization
from the pz-orbital on the sulfur atoms (lone pair) to the dyz-orbital
on Co (vacant d-orbital) was estimated to be 22.5 kcal/mol (form B
in Figure 1b). In addition, resonance stabilization from one SdS
bond (form C in Figure 1b) was estimated to be 4.1 kcal/mol. In our
analysis, the degree of π-electron delocalization in the CoS3 ring was
comparable to that in a thiophene as a representative sulfur-con-
taining aromatic molecule (SI).
The uniqueness of this CoE3 system can also be seen through a
comparison with previously reported MS3 complexes (M = Ti
and Re).20 Because these complexes have coordinatively satu-
rated metal centers or nonplanar MS3 rings, the systems are not
aromatic. In contrast, the present cobalt C60Ar5 compounds have
a sterically confined space, which rigorously excludes highly
coordinating compounds; this allows isolation of coordinatively
unsaturated CoS3 species. When an ordinary cyclopentadienyl
derivative, CpCo(CO)2, is used for a reaction with elemental
sulfur under the same conditions, the reaction gives a mixture of
oligomerized CoꢀS clusters such as Cp4Co4S4.21 A less bulky
C60Me5 derivative, Co(η5-C60Me5)(CO)2, did not afford the
corresponding CoS3 complex, suggesting the importance of the
bowl-shaped confined space.
Unique reactivity of the CoS3 unit was also demonstrated by
intentionally disrupting the 6π conjugation. From trisulfide
complex 2b, one sulfur atom was abstracted by reaction with
one equivalent of PnBu3 in toluene at room temperature. The
product was a cluster complex 4b, which we postulate to be the
dimerization product of the most likely intermediate CoS2-
(C60Ar5) (Scheme 1, Figures 4 and S8 [SI]). A nonaromatic
coordinatively unsaturated CoS2 complex, generated in situ, is
highly unstable and readily dimerizes to achieve an 18-electron
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures, spectral
b
data, details of DFT studies on model compound, and crystal-
lographic data for 2a and 4b (CIF). This material is available free
’ AUTHOR INFORMATION
Corresponding Author
’ ACKNOWLEDGMENT
This work was financially supported by the Japan Society for
the Promotion of Science (JSPS) through its “Funding Program
for Next Generation World-Leading Researchers” and MEXT,
Japan (KAKENHI, No. 22000008 and the Global COE
program).
’ REFERENCES
(1) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and
Antiaromaticity: Electronic and Structural Aspects; John Wiley & Sons:
New York, 1994.
(2) Saito, M.; Sakaguchi, M.; Tajima, T.; Ishimura, K.; Nagase, S.;
Hada, M. Science 2010, 328, 339–342.
(3) Abersfelder, K.; White, A. J. P.; Rzepa, H. S.; Scheschkewitz, D.
Science 2010, 327, 564–566.
(4) Li, X.; Kuznetsov, A. E.; Zhang, H.-F.; Boldyrev, A. I.; Wang, L.-S.
Science 2001, 291, 859–861.
(5) (a) Estarellas, C.; Rotger, M. C.; Capꢀo, M.; Qui~nonero, D.;
Frontera, A.; Costa, A.; Deyꢁa, P. M. Org. Lett. 2009, 11, 1987–1990. (b)
Passmore, J.; Sutherland, G. W.; White, P. S. J. Chem. Soc., Chem. Comm.
1980, 330–331.
(6) (a) Sekiguchi, A.; Matsuo, T.; Watanabe, H. J. Am. Chem. Soc.
2000, 122, 5652–5653. (b) Lee, V. Y.; Takanashi, K.; Matsuno, T.;
Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2004, 126, 4758–4759.
(7) (a) Mikulski, C. M.; Russo, P. J.; Saran, M. S.; MacDiarmid,
A. G.; Garito, A. F.; Heeger, A. J. J. Am. Chem. Soc. 1975, 97, 6358–6363.
(b) Niecke, E.; Fuchs, A.; Baumeister, F.; Nieger, M.; Schoeller, W. W.
Angew. Chem., Int. Ed. Engl. 1995, 34, 555–557.
configuration. From X-ray crystallographic analysis of 4b (C7H8)2,
3
the geometry of the Co2S4 moiety was found to be essentially the
same as that of Cp*2Co2S4,14b but the CoꢀCo distance was
elongated (3.47 Å vs 3.38 Å), likely owing to steric repulsion of
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