Base hydrolysis iron complexes

Structures have been determined for [Fe(gmi)3](BF4)2 (gmi = MeN=CHCF[=NMe), the iron(II) tris-diazabutadiene-cage complex of (79) generated from cyclohexanedione rather than from biacetyl, and [Fe(apmi)3][Fe(CN)5(N0)] 4F[20, where apmi is the Schiff base from 2-acetylpyridine and methylamine. Rate constants for mer fac isomerization of [Fe(apmi)3] " were estimated indirectly from base hydrolysis kinetics, studied for this and other Schiff base complexes in methanol-water mixtures. The attenuation by the —CH2— spacer of substituent effects on rate constants for base hydrolysis of complexes [Fe(sb)3] has been assessed for pairs of Schiff base complexes derived from substituted benzylamines and their aniline analogues. It is generally believed that iron(II) Schiff base complexes are formed by a template mechanism on the Fe " ", but isolation of a precursor in which two molecules of Schiff base and one molecule of 2-acetylpyridine are coordinated to Fe + suggests that Schiff base formation in the presence of this ion probably occurs by attack of the amine at coordinated, and thereby activated, ketone rather than by a true template reaction. ... 

Transfer chemical potentials for the low-spin amine-diimine complexes [Fe(tsba)2] " with tsba = (8 were estimated from the solubilities of their perchlorate salts, in methanol-water mixtures.Solubility and transfer chemical potential data are also available for [Fe(Me2bsb)3] " " in several nonaqueous solvents. One of the main purposes in determining transfer chemical potentials for these iron(II)-diimine complexes is to enable dissection of reactivity trends into initial state and transition state components for base hydrolysis (see next section) in binary aqueous solvent mixtures. Systems for which this has been achieved are indicated in Table 8. 

Activation volumes for dissociation, base hydrolysis, cyanide attack, and peroxodisulfate oxidation (see following pages) of iron(II)-diimine complexes are collected together in Table 9. 

Figure 2 Diagrammatic summary of selected structural, substituent, and solvent effects on rate constants (kj, at 298 K) for base hydrolysis of low spin iron(II)-diimine complexes. Ligand abbreviations not appearing in the list at the end of this chapter are apmi = (73) with = Me BOH cage = (78) with X = OH ...

Fe(gmi)3] in glycol-water and a range of other binary aqueous solvent mixtures. These results, along with further results for AV for base hydrolysis of [Fe(phen)3] " and of [Fe(bipy)3] " in alcohol-water mixtures, have permitted the construction of a scheme combiniim solvent and ligand effects on AF for base hydrolysis of a range of diimine-iron(II) complexes. ... 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

Reaction kinetics and mechanisms for oxidation of [Fe(diimine)2(CN)2], [Fe(diimine)(CN)4] (diimine = bipy or phen) (and indeed [Fe(CN)6] ) by peroxoanions such as (S20g, HSOs", P20g ) have been reviewed. Reactivity trends have been established, and initial state— transition state analyses carried out, for peroxodisulfate oxidation of [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4] , and [Fe(Me2bsb)(CN)4] in DMSO—water mixtures. Whereas in base hydrolysis of iron(II)-diimine complexes reactivity trends in binary aqueous solvent mixtures are generally determined by hydroxide solvation, in these peroxodisulfate oxidations solvation changes for both partners affect the observed pattern. ... 

Kinetic parameters, including activation volumes, for base hydrolysis of a variety of iron(II)-diimine complexes provide useful indicators of ligand, substituent, and medium effects on reactivity. ... 

Basolo noted that reactions of Rh" amine complexes were not dramatically accelerated by hydroxide ion, but did show that substitutions in base do follow the standard = A , + At2[OH ] format, with A, representing the first-order aquation observed in acidic solution, and 2 representing the second-order base-catalyzed path. Poe has studied the kinetics of the base hydrolysis of a variety of frans-[Rh(en)2XY] complexes (Table 41). Studies on the base hydrolyses of irans-[Rh(en)2(OH)X] (X = Cl, Br, I) showed that the coordinated hydroxide has an intrinsic kinetic irons effect comparable to that of Cl but that its position in a thermodynamic irons effect series is much higher. For frauy-[Rh(en)2X2] (X = Cl, Br), virtually complete irons cis isomerization occurs upon hydrolysis in base, and ca. 50% isomerization is observed when X = I. No such... 

Inert metal ion aqua complexes such as [Cr(NH3)5(OH2)] can accelerate the hydrolysis of phosphate esters. These species are believed to act as a general base catalyst. On the other hand, CrOj inhibits the enzyme action of phosphatase and sulfatase and the inhibitary action is enhanced by the addition of phenol. Certain Cr(III) complexes can also act as catalysts in the electrocatalytic reduction of CO2 to MeOH, but they are not as efficient as iron complexes. 

Micellar and microemulsion effects on reactivity in aquation and base hydrolysis reactions of iron(II)-diimine complexes have been much studied/ The latest contribution deals with the effects of added potassium chloride or bromide to micelles of the respective cetyltrimethylammonium halides. Effects on base hydrolysis of [Fe(phen)3] and its 4,7-diphenyl and 3,4,7,8-tetramethyl derivatives can be interpreted in terms of competitive binding to the micelles in a pseudophase-ion exchange model. In connection with these secondary effects of added halides it should be mentioned that further studies of kinetics of aquation of [Fe(bipy)3] and of [Fe(phen)3] in strong aqueous solutions of chlorides have been interpreted in terms of water and of chloride attack, with the postulation of transient diimine-chloride-iron(II) intermediates. ... 

As for the pentacyano-complexes just dealt with, nucleophilic substitution at low-spin iron(ii) di-imine complexes, [Fe(LL)3], appears in this section as well as in the chapters on base hydrolysis and ligand replacement. Schiff-base di-imine ligands such as (1) are unsymmetrical, with non-equivalent iron-nitrogen bonds in their complexes. There is thus the possibility of two simultaneous paths for the aquation of such complexes. Kinetic parameters for the various paths and steps in the reaction sequence for aquation of the complex of (I ... 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly... 

Tris-diimine-iron(II) complexes of unsymmetrical ligands can exist in mer and fac isomers. Interconversion between such geometrical isomers is often slow, which complicates the establishment of rate constant and solubility data. Rate constants for hydroxide attack at the particularly stable and inert complex of the ligand (40) were found to decrease over a matter of hours or days as stock solutions aged. Indeed it proved possible to obtain fairly good estimates for isomerization rate constants for this complex from the time dependence of base hydrolysis rate constants. Iron(II) complexes of the triazine-diimine ligand (41), whose disulfonate ( ferene ) is an important analytical reagent for iron determination, have been rediscovered both mer and fac isomers have been observed. 

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