The design and synthesis of anion selective receptors and chemosensors continues to attract considerable interest within the supramolecular community. In recent years, increasing attention has focused on the use of neutral and cationic CH hydrogen bond donors as anion recognition elements. Over the last five years, motifs that support CHX (X = anion) hydrogen bonds have been actively used in various shape persistent macrocycles, foldamers and "molecular machines". This tutorial review highlights recent developments in host-guest chemistry based on the use of neutral and cationic CH hydrogen bond donors. Also discussed are various structural classifications, including alkyl CH, phenyl CH, triazole-based CH, imidazolium (CH)(+) and triazolium (CH)(+) hydrogen bond donor systems.

A pyrrolyl-based triazolophane, incorporating CH and NH donor groups, acts as a receptor for the pyrophosphate anion in chloroform solution. It shows selectivity for this trianion, followed by HSO 4 -> H 2 PO 4 -> Cl -> Br -(all as the corresponding tetrabutylammonium salts), with NH-anion interactions being more important than CH-anion interactions. In the solid state, the receptor binds the pyrophosphate anion in a clip-like slot via NH and CH hydrogen bonds.Pyrophosphate detection has aroused interest in the scientific community not only because pyrophosphate is the product of ATP hydrolysis under cellular conditions, 1 but also because it could afford a means of effecting real-time DNA sequencing. 2 Pyrophosphate monitoring may also have a role to play in cancer research since this anion is involved in DNA replication catalyzed by DNA polymerase. 3 Pyrophosphate is also of interest as a larger, more highly charged analogue of inorganic phosphate (H 2 PO 4 -/HPO 4 2-), which has physiological relevance in energy storage and signal transduction, in addition to being a structural component in teeth and bones. 4 Not surprisingly, therefore, considerable effort has been devoted recently to the development of synthetic receptors that allow for the recognition, detection, or extraction of pyrophosphate and related species, such as inorganic phosphate. 5 Systems incorporating neutral or cationic NH hydrogen bond donor groups (e.g., pyrrole, indole, ammonium, and guanidinium) or cationic CH hydrogen bond donor motifs (e.g. imidazolium and triazolium) have been particularly effective in this regard. However, to the best of our knowledge, receptors with neutral CH H-bond donors have yet to be exploited for the purpose of pyrophosphate (or phosphate) anion recognition. CH bonds are present in the overwhelming majority (97%) of chemical compounds. 6 Nevertheless, it is only recently that the importance of CH H-bond in biological and artificial anion recognition has come to be appreciated. 7 In recent pioneering work, Flood and co-workers reported the synthesis and anion binding properties of [3 4 ]triazolophanes. These new macrocycles have a diameter of about 3.8 Å, and display a high affinity for the chloride ion. 7(c) On the basis of this and previous theoretical and experimental studies, 8 it was suggested that the strength of neutral, triazole-derived C-H···X -(X -= halide) bonds can approach those of more traditional NH donors, such as pyrrole. We thus considered it of interest to combine both these recognition motifs within the same macrocyclic framework. Here, we report the first such system, namely the calix[2]1,3-bis(pyrro-2-yl)(1,4)-1,2,3-triazolo-phane (1), and show that it acts as a highly effective receptor for the pyrophosphate anion, both in the solid state and in organic media. 9 Our findings, supported by theory, provide support for the conclusion that NH-anion bonding interactions are more important than CH-anion interactions.The synthesis of 1 is shown in Scheme 1. Using "click" chemistry cond...

The pyrrole-based triazolium-phane 1 4+ $4BF 4 À has been prepared via the tetraalkylation of a macrocycle originally prepared via click chemistry. It displays a high selectivity for tetrahedral oxyanions relative to various test monoanions and trigonal planar anions in mixed polar organic-aqueous media. This selectivity is solvent dependent and is less pronounced in acetonitrile. Theoretical calculations were carried out in with the chloride anion in an effort to understand the influence of solvent on the intrinsic hydrogen bonding ability of the donor groups (pyrrole N-H, benzene C-H and triazolium C-H