2
+
information). On the contrary, upon addition of anions to a solution of [ML] -type complexes (1b-PF
6 6 6
, 2b-PF and 3b-PF ), significant
downfield chemical shift of the protons in imidazole units were exhibited on the NMR timescale (Figs. S51–S65 in Supporting
2
+
information). This could be explained by the property of structures of the two different types of metallacalixarenes. In the [ML
2
] -type
complexes, the anion cannot affect the configuration of the monomer because the flexible ligand was locked by the central palladium
2
+
2
atom. Meanwhile, the anion only affected the stacking structure of these [ML ] -type complexes through changing the arrangement
2
+
mode between the monomer. In the [ML] -type complexes, the C-Himizole···anion hydrogen bond effect caused the particular NMR
signal shift. Importantly, the job plots (Figs. S66–S80 in Supporting information) demonstrated the predominant host-guest species in
2
+
these [ML] -type complexes were 1:1 stoichiometric complex. BindFit analysis of the titration data enabled the determination of the
a
anion association constants shown in Table 1 [33]. The K of these receptors became greater with the increase in number of the
hydrogen bond donor groups in the ligand units.
Fig. 4. Crystal structures of the complexes (top view) based on diimidazole ligands: di(1Hnaphtho[2,3-d]imidazol-1-yl)methane (A) [30], 2,6-
3
bis(1H-naphtho[2,3-d]imidazol-1-yl) (B) [31] pyridine and L (C).
In summary, a series of highly symmetrical palladium(II)-based [ML] -type and [ML
2
]2+-type metallacalixarenes using the flexible
2
+
pyridine-bridged diimidazole ligands were synthesized via coordination-driven self-assembly nearly in quantitative yields. Complexes
were characterized both in the solid state (X-ray diffraction, element analysis) and in the solution (NMR, CSI-MS, ESI-HRMS). The
flexibility of the ligands in these complexes plays a vital role in forming various configurations. At the same time, anions plays a vital
role in regulation of the crystal architectures through multiple C-H··· anion hydrogen bonds and other weak interactions. In addition,
2
+
these [ML] -type metallacalixarenes are capable of reorganizing and sensing anions and are being used as anion receptor and sensor
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21906002), Beijing Municipal Science and
Technology Project (No. KM202010005010), Beijing Municipal Natural Science Fund Project of Beijing Education Committee science
and technology project key project (No. KZ201710005001), Beijing municipal high level innovative team building program (No.
IDHT20180504) and Beijing Outstanding Young Scientist Program (No. BJJWZYJH01201910005017).
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