12.4 Structures of Biphen[n]arenes
The configuration and conformation of macrocyclic molecules are very important for
novel macrocyclic hosts because the structural features influence their host–guest
properties and self-assembly behavior. For example, calixarenes exhibit a basketshaped structure and pillararenes display a symmetrical pillar structure. Biphen[n]
arenes, as a new generation of macrocycles, also have a unique topological structure.
In this part, we will detailedly present the topological structures of biphen[n]arenes.
To date, four high-quality crystals of these macrocycles have been obtained. In
2015, we report the X-ray crystal structures of the 4,4
0 -biphen[n]arenes 44BP3 and
44BP4 [25]. The structures of these two macrocylces are completely different.
44BP3 exhibits a distorted triangular-prism structure and does not have an effective
cavity in the solid state. It is noted that three biphenyl units in 44BP3 are in the cisconformation according to the position of the methylene bridge linkage (Fig. 1a).
Interestingly, there are four conformations in the unit cell. As shown in Fig. 1, the
angles between two bridging carbon-carbon bonds in conformation I and IV are
115.81
, 112.37
, and 115.35
, respectively, which are completely different from the
configurations of II and III (113.64
, 112.88
, and 109.94
). What’s more, we found
conformations I with IV and II with III were two pairs of enantiomers in the solid
state (Fig. 1c, d).
In contrast, 44BP4 is a centrosymmetric molecule rather than a chiral molecule,
and the O point is a symmetric center (Fig. 2b). It has a cuboid-like cyclic structure
from the top view. From the side view, 44BP4 exists in the form of a “partial chair”
topology. The distances between the methylene bridges are 8.77 Å and 7.19 Å,
respectively. It is also interesting to note that the two biphenyl units located on
opposite sides are in the cis and trans conformations according to the relative
position of the two methylene linkers, respectively (Fig. 2a). Topologically, 44BP4
can be recognized as a “molecular box”-type macrocycle. Distinctly different with
electron-deficient cationic “blue box” and “Texas-sized” box, 44BP4 is a new type
of neutral molecular box with a π-electron-rich cavity.
For 2,2
0 -biphen[n]arenes, only one single crystal of 22BP4 was obtained after
many efforts as the crystals of cyclic molecules are easily weathered [26]. The
Table 1 (continued)
Entry Guest
Host
Solvent
K a (M
À1
)
Method References
75
G37ÁCl
44CBP4
H 2 O
(3.51 Æ 0.42) Â 10
3 ITC
[41]
76
G38ÁCl
44CBP4
H 2 O
b
ITC
[41]
77
G39ÁCl
22CBP4
H 2 O (pH 7.4)
(5.87 Æ 0.24) Â 10
5 F
d
[43]
78
G40ÁCl
22CBP4
H 2 O (pH 7.4)
(2.29 Æ 0.27) Â 10
6 F
[ 43]
a Determined by the NMR titration method
b
No interactions were found or the association constants were too small (<10 M
À1
) to be calculated
accurately
c Determined by isothermal titration microcalorimetry (ITC) experiments
d
Determined by fluorescence titration
322
B. Li et al.
The configuration and conformation of macrocyclic molecules are very important for
novel macrocyclic hosts because the structural features influence their host–guest
properties and self-assembly behavior. For example, calixarenes exhibit a basketshaped structure and pillararenes display a symmetrical pillar structure. Biphen[n]
arenes, as a new generation of macrocycles, also have a unique topological structure.
In this part, we will detailedly present the topological structures of biphen[n]arenes.
To date, four high-quality crystals of these macrocycles have been obtained. In
2015, we report the X-ray crystal structures of the 4,4
0 -biphen[n]arenes 44BP3 and
44BP4 [25]. The structures of these two macrocylces are completely different.
44BP3 exhibits a distorted triangular-prism structure and does not have an effective
cavity in the solid state. It is noted that three biphenyl units in 44BP3 are in the cisconformation according to the position of the methylene bridge linkage (Fig. 1a).
Interestingly, there are four conformations in the unit cell. As shown in Fig. 1, the
angles between two bridging carbon-carbon bonds in conformation I and IV are
115.81
, 112.37
, and 115.35
, respectively, which are completely different from the
configurations of II and III (113.64
, 112.88
, and 109.94
). What’s more, we found
conformations I with IV and II with III were two pairs of enantiomers in the solid
state (Fig. 1c, d).
In contrast, 44BP4 is a centrosymmetric molecule rather than a chiral molecule,
and the O point is a symmetric center (Fig. 2b). It has a cuboid-like cyclic structure
from the top view. From the side view, 44BP4 exists in the form of a “partial chair”
topology. The distances between the methylene bridges are 8.77 Å and 7.19 Å,
respectively. It is also interesting to note that the two biphenyl units located on
opposite sides are in the cis and trans conformations according to the relative
position of the two methylene linkers, respectively (Fig. 2a). Topologically, 44BP4
can be recognized as a “molecular box”-type macrocycle. Distinctly different with
electron-deficient cationic “blue box” and “Texas-sized” box, 44BP4 is a new type
of neutral molecular box with a π-electron-rich cavity.
For 2,2
0 -biphen[n]arenes, only one single crystal of 22BP4 was obtained after
many efforts as the crystals of cyclic molecules are easily weathered [26]. The
Table 1 (continued)
Entry Guest
Host
Solvent
K a (M
À1
)
Method References
75
G37ÁCl
44CBP4
H 2 O
(3.51 Æ 0.42) Â 10
3 ITC
[41]
76
G38ÁCl
44CBP4
H 2 O
b
ITC
[41]
77
G39ÁCl
22CBP4
H 2 O (pH 7.4)
(5.87 Æ 0.24) Â 10
5 F
d
[43]
78
G40ÁCl
22CBP4
H 2 O (pH 7.4)
(2.29 Æ 0.27) Â 10
6 F
[ 43]
a Determined by the NMR titration method
b
No interactions were found or the association constants were too small (<10 M
À1
) to be calculated
accurately
c Determined by isothermal titration microcalorimetry (ITC) experiments
d
Determined by fluorescence titration
322
B. Li et al.