Nitrogen donors cation exchangers

The nickel ) complex of 92 cannot be prepared directly via the template method, but can be prepared by a transmetallation procedure. Synthesis of the macrocycle in the presence of one of the metal ions known to be effective as a template is followed by a metal exchange process in solution to insert the nickel ) ion. This cation exhibits a strong preference for the square planar, square pyramidal, and octahedral geometries 79). Thus the failure of the nickel ) cation to behave as a template ion in the synthesis of 92 is probably due to the disinclination of the metal to accommodate the pentagonal array of donor nitrogen atoms necessary for reaction to occur. 

Methionine adds another degree of complexity because the radical cation, which is formed by the same pathway as in the cysteine case, namely, by exclusive electron transfer from sulphur, can stabilize by the formation of a cyclic structure with a two-centre three-electron bond between sulphur and nitrogen. CIDNP has provided unequivocal evidence for this species and allowed to probe its spin distribution, through the polarization pattern.Only an unprotonated amino group can function as a donor and effect the cyclic stabilization, so there is a pronounced dependence of the polarization pattern on the pPI. ° This pH-dependence was explained by a reversible pair substitution of the open-chain and cyclic radical cations, but later reinterpreted as arising from a fast (on the CIDNP timescale) equilibrium between the different forms in conjunction with degenerate electron exchange. ... 

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