Receptor-anion complex

Lewis acidic hosts (Section V.A) illustrated important theoretical concepts such as the chelate effect and binding cooperativity, which have now been shown to exist for anion as well as cation binding. This work has also resulted in the crystallographic determination of eye-catching solid state receptor-anion complexes, while heteroelement NMR has allowed an accurate means of probing the solution phase structure of these complexes. Already, multinuclear tin systems are being built into functioning anion selective electrodes. 

When a redox-active transition metal is used as the signalling imit of a receptor, anion binding is coupled to electron transfer, i.e. anion binding changes the redox potential (couple) of the transition metal. This electrochemical shift can be represented as A, the difference in redox potentials between the receptor anion complex and the receptor alone. Concomitantly, electron transfer at the redox centre also changes the affinity of the receptor for the guest species. These coupled processes are linked thermodynamically by Eq. 1, where Kred and Kox are the stability constants of the reduced and oxidised forms of the receptor anion complex respectively [7]. 

From a thermodynamic standpoint, the value of the shift in redox potential is determined by the ratio of Kox/-f red> instead of the absolute value of either Kox or Kted- As a consequence a receptor need not necessarily have a very high binding strength for the anion to be sensed. If electron transfer leads to a sufficiently large change in the stability of the receptor anion complex, a measurable change in redox potential can be observed. 

Loeb, Gale and co-workers have used Pt(II) as a structural template for the self-assembly of a series of anion receptors. Receptors 129-132 were found to bind a variety of anions in DMSO-d solution [92,93]. Even the simple tetrapyridine receptor 129 has an affinity for anions due to its electrostatic charge. The addition of pyrrole hydrogen-bond donors increases the stability of the receptor anion complexes by more than five-fold in 130, but in 131... 

More recently, some simple and yet highly effective amide-cleft-based anion receptors were synthesized by Crabtree and coworkers. Receptors 10-14 are based upon benzene or pyridine rings that are substituted in the 1- and 3-positions, or in the case of pyridine 2- and 6-positions, with either amide or sulfonamide groups. Proton NMR titrations in dichIoromethane-fi 2 were used to determine the association constants of the receptors with halide anions and acetate. All the receptors displayed solely 1 1 receptor anion complex stoichiometry in solution. except receptor 13 that showed behavior consistent... 

Kubo and coworkers synthesized a 7M-thiourea functionalized dibenzo-diaza-30-crown-10 51 that wraps around potassium cations, so bringing the two thiourea groups into close proximity, forming an anion binding site. NMR titration experiments with (Ph0)2P(0)0 anions (which form both 1 1 and 1 2 receptor anion complexes with 51) in acetonitrile-r/j solution in the absence and presence of potassium cations show an increase in Ki from 490 to 9200 M upon addition of three equivalents of potassium cations. 

In the solid state, a series of anion receptors may assemble in various assembling modes in the forms of receptor-anion complexes by combination with counter cations. jS-Unsubstituted lb afforded a planar [1 -1- l]-type pentacoordi-nated CF complex Ib CF that involved the pyrrole NH, the bridging CH, and phenyl ortho-CH units (Fig. 9.3a). Planar anion Ib CF stacked with tetrapropy-lammonium cations (TPA" ) to form a charge-by-charge columnar structure with a Cl -Cl distance of 8.54 A, and a separation of 7.29 A between the Ib CF units... 

Y. Haketa, S. Sasaki, N. Ohta, H. Masunaga, H. Ogawa, N. Mizuno, F. Araoka, H. Takezoe, H. Maeda, Oriented salts dimension-controlled charge-by-chaige assemblies from planar receptor-anion complexes. Angew. Chem. Int Ed. 49, 10079-10083 (2010)... 

B. Dong, Y. Terashima, Y. Haketa, H. Maeda, Charge-based assemblies comprising planar receptor-anion complexes with bulky alkylammonium cations. Chem. Eur. J. 18, 3460-3463 (2012)... 

B. Dong, T. Sakurai, Y. Honsho, S. Seki, H. Maeda, Cation modules as building blocks forming supramolecular assemblies with planar receptor-anion complexes. J. Am. Chem. Soc. 135, 1284-1287 (2013)... 

The use of labile metal atoms, that is, those in which ligand association and disassociation are fast on an experimental timescale, has also been investigated, where the receptor-anion complex is essentially self-assembled in solution as the thermodynamic product. 

The Cl -Cl distances of 12.43 and 10.57 A for 2b CF-TATA+ and 3b CF-TATA+, respectively, along the column are much longer than the 6.85 A observed for lb Cl-TATA+. Smaller overlaps between receptor-anion complexes and TATA in 2b CF-TATA and 3b CF-TATA implied that the p-substituents of the receptor molecules interfered with the formation of stable charge-by-charge columnar structures. ... 

Sessler and co-workers further synthesized strapped calixpyrroles, including isophthalamide strapped system 74 [136, 137]. The aliphatic bridges were found to increase the binding affinity for halide anions in acetonitrile, but failed to provide size-based selectivity amongst anions (Br, CF and F) due to the tilt of the strap to one side of the receptor, which allowed the formation of 2 1 (receptor anion) complexes. 

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