Diimines formation constant

Over recent years there have been a number of publications concerned with formation constants of [Fe(a,a -diimine)3]2+ complexes at various temperatures. Consequently additional AH° and AS° values are now available (Table 13),425428 and some data concerning mixed solvents have also been reported.424 459 4,1 Most of the substitution reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,a -diimine ligands are 2,2 -bipyridine and 1,10-phenanthroline, and their iron(II) complexes are dealt with first before considering complexes of other a,a -diimines. 

In solution yellow iron(II) species containing one a,a -diimine ligand can be obtained from an excess of Fe + in an acidified solution of the ligand (which controls the amount of available free ligand via protonation). Formation constants for many such complexes of substituted 1,10-phenanthrolines have been reported and tabulated. The rate of formation of [Fe(phen)(H20)4] is rapid and has a very unfavourable entropy of activation ( = 53.6 kJ mol , A5 = — 66.9 J K mor Kinetics of the corresponding reactions with 2,2 -bipyridine have also... 

An important aspect of the photophysics of the Pt(diimine)(dithiolate) photochemistry that has received increasing attention is the ability of the excited-state complexes to undergo self-quenching. Initial work by Connick and Gray (111) showed that the lifetime of the complex Pt(bpy)(bdt) (bdt = benzene-1,2-dithiolate, 31) decreased with increasing solution concentration. The bimolecular self-quenching rate constant, calculated from a Stem-Volmer quenching analysis, was found to be 9.5 x 109 A/-1 s-1 in acetonitrile and 4 x 109 M 1 s 1 in chloroform. However, no evidence of excimer formation... 

The mechanism may involve formation and disproportionation of diimine (N2H2). The rate of the hydrogen ion-catalysed reaction has been studied with solutions containing 0-60 % dioxan, and the results analysed in terms of the dielectric constants. This presence of dioxan causes the rate to rise by a factor of as much as 140. Changes occur in the entropy of activation, whilst the energy of activation remains virtually constant (10.2 kcal.mole" in water). 

The original workers2 had shown that under different conditions an alternative product, the bisoxoenamine 4, was formed. Indeed some earlier workers7 had described the formation of only the bisoxoenamine from these reactants, while other workers3 had reported the preparation of the diazepine but quoted physical constants which are those of the bisoxoenamine. The structures of these alternative open-chain products, e.g., 4, as bisoxoenamines rather than as tautomeric diimines, was later confirmed by NMR spectroscopy.8 9 A detailed examination of the reactions... 

---