3.2
Structural Features
CBPQT
4+ has been referred to as “Blue Box” in the community of supramolecular
chemistry including the group of Sir Fraser Stoddart. Two major reasons might
account for this “nickname.” First, the two 4,4
0 -bipyridinium (BIPY
2+ ) units in
CBPQT
4+ can easily undergo reduction, producing blue-colored solution of
CBPQT
2(•+) containing two BIPY
•+ moieties that absorb red wavelengths of light
[21]. Second, the color of blue is often used to represent electron-deficient part in a
molecule in the community of chemistry. The tetracationic CBPQT
4+ ring has a πelectron-deficient nature. We therefore often employ blue color to draw the rectangular molecular structural formula of CBPQT
4+ , in order to indicate that this ring is
electron-poor. This might be another origin of the name of “Blue Box.”
In the solid-state framework of CBPQT
4+ [20] (Fig. 1), two BIPY
2+ units are
bridged in a face-to-face manner by two p-xylyl linkers. The rigidity of the building
blocks, including both the BIPY
2+ and the p-xylyl spacers, affords the cyclophane a
preorganized and rigid cavity. The distance between the two BIPY
2+ in their middle
parts is around 6.8 Å, which is twice of π-π interaction distance. The implication is
that, when a π-electron guest inserts into the macrocycle cavity, the interplane
distances between the guest and each of the two BIPY
2+ moieties would be around
3.4 Å, an optimized distance for the occurrence of π-π interactions. As a consequence, both of the two BIPY
2+ units are able to undergo π-π interactions with the
guest in the host cavity in a cooperative manner. This feature explains the phenomena that CBPQT
4+ is highly promiscuous in binding a variety of π-electron-rich
guests with complementary geometries.
It is also noteworthy that the BIPY
2+ contains a few acidic protons, including
the pyridinium protons in four α-positions with respect to the two pyridinium
nitrogen atoms, as well as the methylene protons in the p-xylyl linkers. Their
acidity results from the electron-withdrawing nature of the nitrogen atoms, on
account of either conjugation or inductive effects. The consequence is that these
protons represent promising hydrogen bond donors in host-guest recognition.
More specifically, CBPQT
4+ can provide larger binding affinities for those πelectron-rich guests that bear ethylene glycol side chains, whose oxygen atoms
are considered hydrogen bond acceptors. This issue is discussed in more detail in
the coming section.
The rectangle architecture of CBPQT
4+ introduces ring strain. It is well-known
that a sp
3 -hybridized carbon atom should have an optimized bond angle of around
109.5
in order to minimize the repulsion between the four bonding electron pairs.
Considering each of the four methylene linkers in CBPQT
4+ framework contains
two less steric bulky protons, the optimized C–C–C bond angle (i.e., the central
carbon is the methylene one) is supposed to be even larger than 109.5
. This value
significantly deviates from 90
in a regular rectangle framework. This deviation
indicates that in a CBPQT
4+ framework, either the C–C–C bond angle is smaller
than the optimized value, namely, 109.5
, or the BIPY
2+ moiety or p-xylyl linker
undergo bend. Both of these two behaviors introduce ring strain. In fact, in a solidstate structure of CBPQT
4+ , the C–C–C bond angle is observed to be around 108
.
In addition, the two pyridinium moieties in a BIPY
2+ are not in the same plane,
3 Host-Guest Chemistry of a Tetracationic Cyclophane, Namely, Cyclobis. . .
51
Structural Features
CBPQT
4+ has been referred to as “Blue Box” in the community of supramolecular
chemistry including the group of Sir Fraser Stoddart. Two major reasons might
account for this “nickname.” First, the two 4,4
0 -bipyridinium (BIPY
2+ ) units in
CBPQT
4+ can easily undergo reduction, producing blue-colored solution of
CBPQT
2(•+) containing two BIPY
•+ moieties that absorb red wavelengths of light
[21]. Second, the color of blue is often used to represent electron-deficient part in a
molecule in the community of chemistry. The tetracationic CBPQT
4+ ring has a πelectron-deficient nature. We therefore often employ blue color to draw the rectangular molecular structural formula of CBPQT
4+ , in order to indicate that this ring is
electron-poor. This might be another origin of the name of “Blue Box.”
In the solid-state framework of CBPQT
4+ [20] (Fig. 1), two BIPY
2+ units are
bridged in a face-to-face manner by two p-xylyl linkers. The rigidity of the building
blocks, including both the BIPY
2+ and the p-xylyl spacers, affords the cyclophane a
preorganized and rigid cavity. The distance between the two BIPY
2+ in their middle
parts is around 6.8 Å, which is twice of π-π interaction distance. The implication is
that, when a π-electron guest inserts into the macrocycle cavity, the interplane
distances between the guest and each of the two BIPY
2+ moieties would be around
3.4 Å, an optimized distance for the occurrence of π-π interactions. As a consequence, both of the two BIPY
2+ units are able to undergo π-π interactions with the
guest in the host cavity in a cooperative manner. This feature explains the phenomena that CBPQT
4+ is highly promiscuous in binding a variety of π-electron-rich
guests with complementary geometries.
It is also noteworthy that the BIPY
2+ contains a few acidic protons, including
the pyridinium protons in four α-positions with respect to the two pyridinium
nitrogen atoms, as well as the methylene protons in the p-xylyl linkers. Their
acidity results from the electron-withdrawing nature of the nitrogen atoms, on
account of either conjugation or inductive effects. The consequence is that these
protons represent promising hydrogen bond donors in host-guest recognition.
More specifically, CBPQT
4+ can provide larger binding affinities for those πelectron-rich guests that bear ethylene glycol side chains, whose oxygen atoms
are considered hydrogen bond acceptors. This issue is discussed in more detail in
the coming section.
The rectangle architecture of CBPQT
4+ introduces ring strain. It is well-known
that a sp
3 -hybridized carbon atom should have an optimized bond angle of around
109.5
in order to minimize the repulsion between the four bonding electron pairs.
Considering each of the four methylene linkers in CBPQT
4+ framework contains
two less steric bulky protons, the optimized C–C–C bond angle (i.e., the central
carbon is the methylene one) is supposed to be even larger than 109.5
. This value
significantly deviates from 90
in a regular rectangle framework. This deviation
indicates that in a CBPQT
4+ framework, either the C–C–C bond angle is smaller
than the optimized value, namely, 109.5
, or the BIPY
2+ moiety or p-xylyl linker
undergo bend. Both of these two behaviors introduce ring strain. In fact, in a solidstate structure of CBPQT
4+ , the C–C–C bond angle is observed to be around 108
.
In addition, the two pyridinium moieties in a BIPY
2+ are not in the same plane,
3 Host-Guest Chemistry of a Tetracationic Cyclophane, Namely, Cyclobis. . .
51