Guest complexes amines

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

Enantioselective reagents for ammonium ions include, for example, a mixture containing a host chiral crown ether such as 196, possessing four (R) centers and symbolized as M, a host achiral crown ether of similar functionality, symbolized as R, and a salt of a guest chiral amine, symbolized as A, which is analyzed by fast atom bombardment MS (FAB-MS), and the relative peak intensity of the equilibrium complexes 7(MA)//(RA) is measured and correlated with the chirality of the guest molecule. Many host and guest molecules have been investigated405. 

Figure 13. Schematic representations of the geometries of host-guest complexes, (a) Two possible geometries of the host-guest complex between calix[6jarene hexaester (29) and a protonated primary amine guest, (b) Plausible geometry of the host-guest complex between dibenzo-18-crown-6 (32) and a protonated primary amine guest...

Such a chiral host is able to bind ammonium ions by inclusion and triple hydrogen bonds between the ammonium ion and three oxygens of the crown enantiodiscrimina-tion is due to steric reasons so that the host-guest complexation of one enantiomer is favoured over the other. Accordingly, the spectrum of application is quite narrow as it is restricted only to chiral primary amines including primary a- and f)-amino acids. 

Fig. 10a-f Examples of the structural motifs of p-ferf-butylcalix[4] arene derivatives a guest-free structure b 1 1 complex c 2 1 host-guest complex d hydrogen-bonded structure with an amine guest e self-inclusion structure f 1 1 host-guest clay mimic structure. (Adopted from [44] with permission)... 

Only few investigations have been published on the gas-phase ion chemistry of host-guest complexes of calixarenes. With the advent of ESI mass spectrometry, especially when combined with ion-trap and FT-ICR mass spectrometry, this field has started to be developed. Binding selectivities of alkali metal ions to cahxarene-based crown ethers and open-chain ethers have been studied , the inclusion of neutral guests into the protonated resorcarene-based cavitand hosts by gas-phase ion-molecule reactions with amines have been studied and the formation of capsules from various calixarene tetraether derivatives and alkylammonium ions as ionic guests (notably enabling their detection by mass spectrometry) have been described recently . ... 

Microcrystalline cellulose triacetate, cyclodextrin- and crown ether-derived CSPs, as well as some chiral synthetic polymers, achieve enantiomer separation primarily by forming host-guest complexes with the analyte in these cases, donor-acceptor interactions are secondary. Solutes resolved on cyclodextrins and other hydrophobic cavity CSPs often have aromatic or polar substituents at a stereocenter, but these CSPs may also separate compounds that have chiral axes. Chiral crown ether CSPs resolve protonated primary amines. 

Although a macrocyclic nature of the host is not a prerequisite for binding organic guests, the discovery of crown ethers by Pedersen [45], followed by extensive smdies of Cram [1] and Lehn [2], gave impetus to the birth of host-guest complex chemistry. Amines and crown ethers, particularly those of the 18-crown-6 family, constitute the first and most thoroughly studied to date subject in the field. 

The reduced capability of other than primary amines for host-guest complexation decreases their extraction greatly (Table I). 

A. Mehdizadeh, M. C. Letzel, M. Klaes, C. Agena, J. Mattay, Chiral discrimination on the host-guest complexation of resorcin[4]arenes with quaternary amines, Eur. J. Mass Spectrom., 2004, 10, 649-655. 

The most thoroughly studied host-guest system with a porphyrin-based host is the complex formed between amines and metalloporphyrins. Tire stoichiometry of the host-guest complex and the binding affinity will depend on the nature of the metal, the basicity of the amine, the reaction media and the pH in the case of water. Porphyiin-amine equilibria in water were studied by Pasternack et u/. [5.10,15,20-tetrakis(/V-methylpyridinium-4-yl)porphyrina-tolCo(IIl) (27) binds pyridine in water, with the binding constants and the rale constants of complex formation depending upon pH. At acidic pH, pH < 4, the first replacement of water by pyridine. 

Hereafter the values of AG A// and A5 refer to the standard state of 1 M of each species. We also use notation, A-B, to represent a host-guest complex between compound A and compound B. Table I summarizes the binding free energy for complex formation for various amine-metallo-porphyrin complexes. Monomeric porphyrins bind amines with — AG values in the range of 10-25 kJ/mol, while dimeric porphyrins bind ditopic guests with a larger — AG of 30-43 kJ/mol. 

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