For endohedral metallofullerenes
(EMFs), it has been well established
that the cage shape and size should match those of the endohedral
cluster. As a result, sufficient cluster–cage interaction can
be achieved, which is essential for mutual stabilization. Nevertheless,
how a small endohedral cluster nests in a giant fullerene has been
less explored. Herein, we report a pair of large oxide-cluster fullerene
(OCF) isomers, denoted as Ho2O@C92-I and -II.
Crystallographic studies reveal that major isomer-I possesses a D
3
(85)-C92 cage with
a highly stretched Ho2O cluster inside, which contributes
to achieving regular metal–cage contacts. Density functional
theory (DFT) computations also reveal the predominant abundance of
the D
3
(85) isomer relative
to the other two possible minor species including C
1
(67) and C
2
(64) isomers. Moreover, electrochemical (EC) studies
verify that the isomers exhibit almost identical redox behaviors,
indicating their similar cage structures. On the basis of the remarkable
topological similarity of D
3
(85) and C
1
(67) isomers,
isomer-II is likely to be Ho2O@C
1
(67)-C92, though it remains to be confirmed.
Our studies thus provide new insights into the cage–cluster
interplay and cage isomerization, both contributing to a better understanding
of large EMFs.
Carbide clusterfullerenes (CCFs) have been of great concern due to their potential applications in materials science, in which the internal carbide cluster plays vital roles in the stability and properties of CCF. However, there still remains a debate about what configuration is ideal for the internal carbide cluster. In this work, we isolated two isomers (I and II) of Ho 2 C 94 and studied them by means of mass spectrometry, UV−vis-NIR spectroscopy, and cyclic/differential pulse voltammetry. A combined study of single-crystal X-ray diffraction (SC-XRD) and density functional theory (DFT) computation ascertains isomer-I as Ho 2 C 2 @C 2 (61)-C 92 , in which the Ho 2 C 2 cluster displays variable configurations from planar zigzag to folded butterfly with very small distortion energy (∼10 kJ/mol). This study hence confirms that the internal carbide cluster is intrinsically flexible over a broad geometrical range in a relatively large fullerene cage, where the nanoscale compression effect is almost negligible.
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