Racemization iron diimines

Although rate constants for high-spin low-spin interconversion for [Fe(pyim)3], pyim = 2-(2 -pyridyl)imidazole (29), are extremely fast, they are relevant to discussions of racemization and dissociation of iron(II) diimine complexes, as in both cases significant participation by the high-spin form is often implicated in reactions involving the substitution-inert low-spin form. Thus... 

Many investigators have studied substitution at iron(II)-diimine complexes in binary aqueous mixed solvents and other investigators in aqueous salt solutions. Some years ago the results of addition of salts and a cosolvent were assessed, for [Fe(5N02phen)3] in water, t-butyl alcohol, acetone, dimethyl sulfoxide, and acetonitrile mixtures containing added potassium bromide or tetra-n-butylammonium bromide. " Now the effects of added chloride, thiocyanate, and perchlorate on dissociation and racemization rates of [Fe(phen)3] in water-methanol mixtures have been established. The main explanation is in terms of increasing formation of ion pairs as the methanol content of the medium increases, but it is somewhat spoiled by the (unnecessary) assumption of a mechanism involving interchange within the ion pairs. Kip values (molar scale) of 11,18, and 25 were estimated for perchlorate, chloride, and thiocyanate in 80% (volume) methanol at 298.2 K. These values may be compared with values of 20, 7, and 4 for association between [Fe(phen)3] and iodide, " [Fe(bipy)3] and iodide, " and [Fe(phen)3] and cyanide " " in aqueous solution (at 298.2,... 

A >600s at -31 °C, and dissociates very rapidly (for a complex of this kind), with a half-life of only a few minutes in dilute perchloric acid. The relation between these two kinetic time scales indicates that enantiomerization must be an intramolecular process the rapidity both of this and of the dissociation can be attributed to the distorted structure forced by ligand geometry. A short review of inter- and intramolecular racemizations of low-spin iron(II)-diimine complexes has appeared. 

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