Pyridine, complex cation, with iron

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

Amination of AT-alkylpyridinium salts with amide ions, which in principle should be easier than the reaction with the parent pyridine, has been little studied. The main reason for this is that solvent selection is difficult. Metal amides are only soluble in liquid ammonia (with which pyridinium salts react easily, vide infra), and pyridinium salts are soluble in solvents that are not suitable for use with metal amides. The A/ -methylacridinium cation undergoes direct imination to give (153) in 35% yield by treatment with potassium amide and iron (III) nitrate in liquid ammonia. Two other products (154) and (155) are also formed, probably by hydrolysis and subsequent disproportionation (Scheme 90). One might question whether sodamide is necessary to the above transformation in light of the fact that quin-olinium, isoquinolinium and certain pyridinium ions give cr-complexes (156), (157) and (158) in liquid ammonia alone at 0 °C (73JOC1949). 

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