Outside the confines and well-established domain of organic chemistry, the systematic building of large macromolecular arrangements based on non-carbon elements represents a significant and exciting challenge. Our aim in the past two decades has been to develop robust synthetic methods to construct new types of main group architectures in a methodical way, principles of design that parallel those used in the organic arena. This Concept article addresses the fundamental thermodynamic and kinetic problems involved in the design and synthesis of main group macrocycles and looks to future developments of macromolecules in this area, as well as new applications in coordination chemistry.
Main group inorganic macrocycles, based on p-block element backbones other than carbon, are a challenging synthetic target that has been largely overlooked. In this study, we show that a simple strategy based on the combination of electrophilic and nucleophilic phosphazane building blocks can be extended to readily accessible [E(tBuN)P(μ-NtBu)]22- nucleophilic components, as exemplified by the Se-bridge PIII/PV phosphazane macrocycle [{(tBuN[double bond, length as m-dash])PV(μ-NtBu)}2(μ-Se)2{PIII(μ-NtBu)}2]3.
We report here the synthesis of a 1,3-alternate calix[4]arene
8
, with bis-pyrazolylmethylpyrenes on the one end and bis-triazolylmethylphenyls on the other end, as a homoditropic fluorescent sensor for both Hg
2+
and Ag
+
ions. Calix[4]arene
3
, with lower-rim bis-pyrazolylmethylpyrenes in cone conformation, was also synthesized as a control compound. UV-Vis and fluorescence spectra were used for metal ions screening, and we found that both ligands
8
and
3
showed strong excimer emission of pyrenes when they are as a free ligand in CHCl
3
/MeOH (v/v, 3:1) solution; however, they both showed a high selectivity toward Hg
2+
and Ag
+
ions with strong fluorescence quenching and yet with different binding ratios. The fluorescence of ligand
8
was strongly quenched by Hg
2+
but was only partially quenched by Ag
+
ions; however, the fluorescence of ligand
3
was strongly quenched by Hg
2+
, Ag
+
, and Cu
2+
ions. Job plot experiments showed that ligand
8
formed a 1:2 complex with both Hg
2+
and Ag
+
ions; ligand
3
formed a 1:1 complex with Hg
2+
, but it formed a 2:3 complex with Ag
+
. The binding constant of ligand
3
with Hg
2+
and Ag
+
ions was determined by the Benesi-Hildebrand plot of UV-vis titration experiments to be 2.99 × 10
3
and 3.83 × 10
3
M
−1
, respectively, while the association constant of ligand
8
with Hg
2+
and Ag
+
was determined by Hill plot to be 1.46 × 10
12
and 9.24 × 10
11
M
−2
, respectively. Ligand
8
forms a strong complex with either two Hg
2+
or two Ag
+
ions using both the upper and lower rims of the 1,3-alternate calix[4]arene as the binding pockets; hence, it represents one of the highly selective fluorescent sensors for the homoditropic sensing of Hg
2+
and Ag
+
ions.
The simple inorganic cyclodiphosph(iii)azane chiral derivatisation agent ClP(μ-tBuN)2POBorn (Born = endo-(1S)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-yl) is shown to be effective in the measurement of ee’s of chiral amines using 31P NMR spectroscopy.
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