Anion chelation

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

As seen in the preceding chapters, macrocyclic polyamines such as [18]aneN6 can be cation chelators, anion chelators, or both depending on the conditions. It was expected that with these dual properties, the macrocyclic polyamines might serve as litholytic agents by removing Ca2+ and phosphate or oxalate anions from insoluble calculi. 

Many cationic and anionic chelates which are not extractable by the usual organic solvents due to residual charge can be extracted in the presence of a... 

Proton nmr halide anion titrations reveal that the ethyl- [79], propyl-[80] and butyl- [81] linked derivatives (Fig. 43) form complexes of 1 1 stoichiometry in acetonitrile solution. Stability constant determinations suggest that the ethyl derivative [79] exhibits selectivity for the chloride anion in preference to bromide or iodide. As the chain length increases, so the selectivity for chloride decreases and also the magnitude of the stability constant which is evidence for an anionic chelate effect with the chloride anion. Receptors containing larger aryl [81], [83], [84] and alkylamino spacers [85] (Fig. 43) form complexes of 2 1 halide anion receptor stoichiometry. 

Subsequent decreases in solubility may be due to re-sorption or precipitation on clays and oxides as soil conditions continue to change, and decomposition of organic anions chelating P or chelating A1 and Fe with which it would otherwise react. 

Until recently only complexes of low stability were known for AC s even in the case of anionic chelating ligands (especially in water Table 7). Numerous and much more stable chelate complexes of AEC s with multidentate anionic ligands are known however (see Table 7). The complexation constants (3) always follow the stability sequence Ca2+ > Sr2+, Ba2+ and are in general difficult to modify in a progressive, stepwise fashion. It is obviously of interest to be able to control both stability and selectivity of AC and AEC complexes, especially the former. 

As a part of our program to develop new adjuvants for the into-cell delivery of phosphorylated nucleotide-type antiviral agents (see Section 3 of this chapter), we became interested in developing a sapphyrin-based approach to phosphate anion chelation. As proved true for halide anion recognition, important initial support for the idea that sapphyrins could function as phosphate anion receptors came from single crystal X-ray diffraction studies. In fact, to date, five X-ray structures of sapphyrin-phosphate complexes have been obtained. ... 

More detailed analyses of sapphyrin anion chelation in solution were made using a full range of tricks borrowed from the supramolecular field. Thus both spectroscopic techniques (e.g., NMR, UV-vis, fluorescence spectroscopy) and transport studies (carried out in a model Pressman type U-tube membrane system ) were employed. From these analyses, it became clear that sapphyrin does in fact bind various negatively charged substrates in solution, but does so both with variable affinity and oft-times remarkable selectivity. These findings/conclusions are detailed further in the paragraphs below. 

As expected, system 13 did in fact bind and transport zwitterionic a-amino acids through a model membrane barrier with good selectivity under conditions where the porphyrin-derived control system (14), lacking the carboxylate anion chelation ability inherent in 13, would not. Specifically, it was found that at neutral pH compound 13 acts as a very efficient carrier for the through model membrane (H2O-CH2CI2-H2O) transport of phenylalanine and tryptophan. Further, in direct competition experiments, L-phenylalanine was found to be transported four times faster than L-tryptophan and 1000 times faster than L-tyrosine. As implied above, little or no transport was observed when a porphyrin control (14) was used. Nor was significant transport observed when a mixture of sapphyrin and lasalocid was used. 

Analogous to the work on dipicolinic acid reported in Section 31.5.2, a range of titanium(IV) peroxo complexes (35) have been prepared which encompass an oxygen-nitrogen anionic chelating ligand and hexamethylphosphortriamide.171 The crystal structure of... 

In the case of anionic chelating extractants such as HDEHP, HDBP, HTTA or generally HA, the reaction stoichiometry with respect to the metal, Mz+, and extractant, HA, is given by equation (134). In this case, DM is given by equation (135),90 where [MAJ is the neutral extracted species and [MA ] represents all the aqueous phase species for n = 0-N of which MAX is one. A plot... 

Iminophosphoranes typically form N,P-chelates 390. For the compounds with mixed phosphino- and amino-functions, apart from expected N,N,P-chelation 391, there are frequent cases when an amino group is excluded from coordination, 392 and 393. Diphenylphosphino-2-amino-pyridine may exhibit various co-ordination modes, monodentate 394, neutral and anionic chelating, 395 and 396, and bridging 397. Other chelating environments may be classical pincer 398, P,P- 399, or double 400. Also, transformation from the monodentately coordinated 401 to the bridging heterodinuclear 402 occurs quite frequently. 

Scheme 4.4 Use of an inert Ru(II) centre to organise an anion chelate ligand.

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