the Coulombic repulsion between the cationic building blocks. This feature also
brings about a unique property of this catenane. That is, in order to decrease the
Coulombic repulsion, one or two of the BIPY
2+ units in the catenane prefer to
stay in the radical state, namely, BIPY
•+ . As a consequence, the radical is highly
stabilized, opening up opportunities to develop purely organic paramagnetic
materials.
3.5.2 Rotaxanes Containing CBPQT
4+ Ring
The donor-acceptor rotaxanes are obtained in a similar approach as their catenanes
counterparts, relying on the ability of a CBPQT
4+ ring or its precursors to recognize
the π-electron-rich guests. Clipping reaction of α,α
0 -dibromo-p-xylene and 1
2+
occurs in the presence of a dumbbell, yielding the corresponding rotaxanes [43].
Because the C–H•••O hydrogen bonds play an even more important role than that of
donor-acceptor interactions, it was observed that the dumbbells containing more
ethylene glycol units often produced rotaxanes in higher yields. The threadingfollowed-by-stoppering strategy is also often used in the synthesis of rotaxanes
containing a CBPQT
4+ ring. Here the CBPQT
4+ ring encircles a thread on its πelectron-rich binding station. The two terminal OH groups of the thread react with
two larger bulky silicon derivatives whose volume should be larger than the cavity of
CBPQT
4+ ring to trap the corresponding rotaxane architectures [26] (Fig. 15).
CuAAC [44], which has been referred to as click reaction, is an ideal reaction for
rotaxanes synthesis [45], because of (i) high yield and (ii) room temperature condition that favors complexation.
The capability of CBPQT
2(•+) to recognize a guest containing BIPY
•+ was also
taken advantage (Fig. 16) of by the Stoddart group in the synthesis of rotaxanes
[46]. A complex 11
•+
&CBPQT
2(•+) was self-assembled. Tris(2,2
0 -bipyridine)
dichlororuthenium(II) was used as a sensitizer, which reduces the BIPY
2+ units
in both CBPQT
4+ and 11
2+ to their radical states, in the presence of amino
sacrificial reductant under visible light. The two azide terminal groups in 11
•+
were introduced to undergo a type of click reaction, namely, azide-alkyne cycloaddition. An electron-deficient alkyne 12 was chosen, because it has a low-lying
LUMO and therefore can undergo azide-alkyne cycloaddition without Cu(I) catalyst. Avoiding Cu(I) catalyst is of importance, due to its oxidative nature that might
quench the BIPY
•+ radicals. After the click reaction was accomplished, a rotaxane
13
6+ was obtained, whose dumbbell and ring components are repulsive to each
other. A few years later, the Stoddart group also discovered [47] that the length of
rotaxane could determine the stability of BIPY
•+ radicals in the rotaxane against
oxidation. In a shorter rotaxane, the distance between the CBPQT
4+ ring and the
central BIPY
2+ unit in the dumbbell is smaller, which introduces larger Coulombic
repulsion. The unfavorable repulsive interaction increases the tendency of the
BIPY
2+ unit to be reduced, as a consequence of which, the radical state of the
short rotaxane is remarkably stabilized.
66
H. Li et al.
brings about a unique property of this catenane. That is, in order to decrease the
Coulombic repulsion, one or two of the BIPY
2+ units in the catenane prefer to
stay in the radical state, namely, BIPY
•+ . As a consequence, the radical is highly
stabilized, opening up opportunities to develop purely organic paramagnetic
materials.
3.5.2 Rotaxanes Containing CBPQT
4+ Ring
The donor-acceptor rotaxanes are obtained in a similar approach as their catenanes
counterparts, relying on the ability of a CBPQT
4+ ring or its precursors to recognize
the π-electron-rich guests. Clipping reaction of α,α
0 -dibromo-p-xylene and 1
2+
occurs in the presence of a dumbbell, yielding the corresponding rotaxanes [43].
Because the C–H•••O hydrogen bonds play an even more important role than that of
donor-acceptor interactions, it was observed that the dumbbells containing more
ethylene glycol units often produced rotaxanes in higher yields. The threadingfollowed-by-stoppering strategy is also often used in the synthesis of rotaxanes
containing a CBPQT
4+ ring. Here the CBPQT
4+ ring encircles a thread on its πelectron-rich binding station. The two terminal OH groups of the thread react with
two larger bulky silicon derivatives whose volume should be larger than the cavity of
CBPQT
4+ ring to trap the corresponding rotaxane architectures [26] (Fig. 15).
CuAAC [44], which has been referred to as click reaction, is an ideal reaction for
rotaxanes synthesis [45], because of (i) high yield and (ii) room temperature condition that favors complexation.
The capability of CBPQT
2(•+) to recognize a guest containing BIPY
•+ was also
taken advantage (Fig. 16) of by the Stoddart group in the synthesis of rotaxanes
[46]. A complex 11
•+
&CBPQT
2(•+) was self-assembled. Tris(2,2
0 -bipyridine)
dichlororuthenium(II) was used as a sensitizer, which reduces the BIPY
2+ units
in both CBPQT
4+ and 11
2+ to their radical states, in the presence of amino
sacrificial reductant under visible light. The two azide terminal groups in 11
•+
were introduced to undergo a type of click reaction, namely, azide-alkyne cycloaddition. An electron-deficient alkyne 12 was chosen, because it has a low-lying
LUMO and therefore can undergo azide-alkyne cycloaddition without Cu(I) catalyst. Avoiding Cu(I) catalyst is of importance, due to its oxidative nature that might
quench the BIPY
•+ radicals. After the click reaction was accomplished, a rotaxane
13
6+ was obtained, whose dumbbell and ring components are repulsive to each
other. A few years later, the Stoddart group also discovered [47] that the length of
rotaxane could determine the stability of BIPY
•+ radicals in the rotaxane against
oxidation. In a shorter rotaxane, the distance between the CBPQT
4+ ring and the
central BIPY
2+ unit in the dumbbell is smaller, which introduces larger Coulombic
repulsion. The unfavorable repulsive interaction increases the tendency of the
BIPY
2+ unit to be reduced, as a consequence of which, the radical state of the
short rotaxane is remarkably stabilized.
66
H. Li et al.