Cleft hosts

Molecular cleft host Octopus host Pincer host... 

Compound 2.61 is also an example of a rigid urea-based molecular-cleft host. This species was designed to bind wefa-disubstituted aromatic guests and is also able to chelate carboxylates, phosphates and sulfonates. 

Most network structures are effective host structures for small guest molecules, often the solvent. Exceptions arise when there is a high degree of interpenetration, i.e. where two or more networks are entangled [7]. This type of host-guest behaviour is not intrinsic to the molecular components themselves, but occurs in cavities or clefts created by the assembly of the network structure. Mol-... 

TT-Stacking interactions and solvation effects within the highly preorganized cleft of a bifunctional C-shaped host are believed to benefit the base-promoted conversion of 5-nitrobenzisoxazole to 2-cyano-5-nitrophenolate relative to the acetate-promoted reaction structural variation of the host has been explored. ... 

Protonated forms of the large-ring macrocycle [24]Ng02 (5) and related compounds have been shown to be active as synthetic phosphorylation catalysts in ATP synthesis. It is likely that in this case the substrate enters the macrocyclic cavity to some extent, or is enveloped by it. Evidence for this possibility comes from the crystal structure of the chloride salt of 5-6H (Figure 1) in which a chloride ion is enveloped within a cleft formed by the boat-shaped conformation of the macrocy-cle. The crystal structure of the nitrate salt of 5-4H has also recently been determined and the host again adopts a boat-like conformation as it interacts with the anion. The hydrochloride salt of the smaller [22]Ng binds two chloride anions above and below the host plane in a similar way to 1. Molecular dynamics simulations indicate that the pocket-like conformation for 5-6H is maintained in solution, although Cl NMR experiments demonstrate that halide ions are in rapid exchange between the complexed and solvated state. 

Receptor models 71a and 71b for dinucleotides (dinucleoside monophosphates) have been developed out of 69 by replacing the naphthoyl residue, which is not involved in any host-guest interactions in the complexes, by a second carbazolyl cleft. The adenine selectivity of the two pincers was reflected in single extraction experiments dissolved in CH2CI2,71a removes one full equivalent of both adenylyl(3 -+ 5 )adenosine (ApA) and 2 -deoxyadenylyl(3 ->5 )-2 -deoxyadenosine [d(ApA)] 72 out of aqueous solutions of guest (8-fold excess) but only half the amount of d(ApG) [103]. 

TTie creation of molecules which have this property is not a trivial concern. It is obvious from die five indiviual chapters covered in the book, there are outstanding experts with a master s touch in the field who have constructed molecular holes, niches, cavities and clefts that are capable of forming selective host-guest complexes and inclusion compounds. 

The X-ray crystal structure of the hexachloride salt of the protonated macrocycle shows that it adopts a cleft-like conformation as shown in the diagram, and modelling studies support the existence of this conformation in solution. The electrostatic nature of the anion binding by 4.12-nH+ means that it binds particularly strongly to multiply changed anions, and ATP, with its 4- charge, is bound in water at pH 4, with logXn = 11 for the hexaprotonated host. 

NMR titrations (of anion into ligand at fixed pH) and pH-potentiometric titrations (of pH at fixed anion ligand ratios) provide comparable values of the stability constants for binding of mononegative oxoanions by protonated R3Bm, R3F, and R3P hosts [15,20,21] Table 2. The weak complexation at hexaprotonated levels for tetrahedral monoanionic oxoanions makes it difficult to obtain reliable data for protonation levels below 5. This has however been achieved for nitrate with the cleft binding host R3P as well as for Re O4 with the most basic cryptand R3Bm. 

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