II -Diimine Complexes

The inertness of iron(II) when it is in the low-spin state (/jg) is what qualifies this center for inclusion in this chapter. So high-spin low-spin interconversions can be considered of fundamental importance here, even if they do take place extremely rapidly. The demonstration of a thermal and photoinduced high-spin to low-spin transition of hexakis(l-methyl-l/f-tetrazole)iron(II), [Fe (38) 6], is the first example of such generation of a 

AF p The very similar complex (39) with X= —CHjCHj— undergoes 

8 Inert-Metal Complexes Other Inert Centers 

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. 

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. 

The first step in reactions of iron(II)-diimine complexes with sluggish oxidants is sometimes rate-determining dissociation. This is the case for the reaction with peroxodiphosphate, and for peroxodisulfate also when 

8 Coordination Numbers 6 and Above Other Inert Centers 

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, 

Reaction of [Fe(phen)3] or [Fe(5N02phen)3] with hydroxide is more than 1000 times faster in microemulsions than in water, and reaction with cyanide shows an even greater acceleration but aquation of these complexes shows an acceleration of only a few times in the microemulsions. The larger accelerations of the hydroxide and cyanide reactions are more likely to be due to activation via desolvation of these anions than simply to electrostatic effects, since hydroxide attack at 2,4-dinitrochlorobenzehe is also accelerated by about 1000 times under similar conditions. A much more dramatic change of medium is to the solid state, where it is of interest to note that the mechanisms of racemization of the very similar complexes [Fe(bipy)3] and [Fe(phen)3 appear to differ significantly.  

The increasing values of the rate constant (298.2 K) for racemization of the [Fe(phen)3] cation  

Rate constants for racemization of the [Fe(gmi)3] cation, where gmi is the simple aliphatic diimine ligand 5, in a range of solvents increase in the order 

Before leaving the question of magnetic moments in reactions of iron(II)-diimine complexes it is necessary to mention the strange case of complexes of the Schiff base ligand 6, sb. Many years ago the tris(ligand) 

Kinetics of aquation, base hydrolysis, cyanide attack, and peroxodisul-fate oxidation have been investigated for the 4-methyl-1,10-phenanthroline complex [Fe(4Me phen)3]. The main interest here is that another demonstration is provided of the dominant role of solvation changes in determining activation volumes when heavily solvated ions, such as cyanide here, are involved. The minimal change in AF on going from water as solvent to 33% methanol is also ascribable to heavy and strongly preferential hydration of the cyanide. In a similar vein, the zero activation volume for peroxodisulfate oxidation of the [Fe(bipy)(CN)4] anion, surely simple bimolecular outer-sphere electron transfer, can also be explained by balanc- 

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 

Kinetic studies of aquation of [Fe(phen)3] and derivatives in binary aqueous media remain popular. A group additivity approach has been applied to aquation of [Fe(5N02phen)3] in aqueous alcohols (faster reaction) and formic and acetic acids (slower), to investigate its potential for mechanism diagnosis. Rate constants for dissociation of the parent complex increase tenfold on going from water to 100% dimethylformamide. Aquation rate constants and activation parameters have also been reported for the 5-nitro, 5-phenyl, and 4,7-diphenyl derivatives in water-dioxan mixtures. Both papers contain obscure discussions of solvolysis mechanisms in DMF-rich and dioxan-rich media. In the latter media it seems that ion pairs play a key role, as evidenced by activation entropies. The discussion of reactivities in terms of hydrophobicities of the complexes and their respective transition states represents a qualitative initial state-transition state analysis. An explicit analysis of this type has been published for the iron(II) complexes of the 

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.  

The debate on hydroxide attack at coordinated diimine and pyridine ligands coordinated to transition metal ions such as Fe continues. Catalysis of hydrolysis of the diimine ligand tptz, (34), by Cu has been suggested to proceed by attack by hydroxide or by water at one of the pyridine rings of tptz when it is coordinated to the copper.  

---

J. Burgess, A. J. Duffield and R. Sherry, J. Chem. Soc. Chem. Commons. 350 (1980). It is difficult to rationalize AEj values of 20cm mol with an associative mechanism for nucleophiles reacting with low spin Fe(II) diimine complexes. This value however represents a balance between contributions from AFj (negative) and anion desolvation in forming activated complex (positive AK-). 

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. 

Table 8 Solvation of iron(II)-diimine complexes in binary aqueous solvent mixtures. ... 

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. 

Table 9 Activation volumes (cm mol ) for reactions of iron(II)-diimine complexes, in aqueous solution at...

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 ...

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. ... 

Rounding off this section, the Pt(ii)(diimine) complex 79 [96], a possible intermediate in C,H-activation chemistry, shows a 23.5 Hz J( Pt, H) coupling to the (averaged) protons, thus helping to support the t olefin stmcture. 

Synthesis, photophysical, and electrochemical properties of dinuclear Cd(II) diimine complexes with bridging chalcogenolate ligands were described [91]. 

Also, Yam and Cheung [62] have carried out the synthesis of a series of novel polynuclear mercury(II) diimine complexes with bridging chalcogenate ligands and studied their luminescence and electrochemical properties. 

The iron(II) diimine complex (30) displays unusual stability towards acids, alkali, reducing and oxidizing agents and has been termed quasi-aromatic .44 Complex (30) undergoes bromination in acetic acid to give the dibromo analogue (31) (equation 20).45... 

Leland and Powell also studied ECL obtained from reaction of [(bpy)3Ru]3+ with trialkylamines [47], Since the mechanism involves an electron transfer from the amine to Ru3+, there exists an inverse relationship between the first ionization potential of the amine and ECL intensity. The relative intensity of [(bpy)3Ru]2+ ECL was found to be ordered tertiary > secondary > primary. Quaternary ammonium ions and aromatic amines do not produce ECL with Ru(II) diimine complexes. Brune and Bobbitt subsequently reported the detection of amino acids by [(bpy)3Ru]2+ ECL [28,29], Employing capillary electrophoresis for separation, the presence of various amino acids can be detected directly by reaction with [(bpy)3Ru]3+ generated in situ with up to femtomo-lar sensitivity and with a selectivity for proline and leucine over other amino acids. The formation of an amine radical cation intermediate is characteristic of proposed mechanisms of both aliphatic amines and amino acids. 

Other recent work on the temperature dependence of the luminescence behavior of Os(II) diimine complexes was reported by Ogawa and coworkers in the investigation of [Os(bpy)2(4,4/-dcbpy)] and [Os(bpy)2(3,5-dcbpy)], where 4,4 -dcbpy = 4,4/-dicarboxy-2,2/-bipyridine and 3,5-dcbpy = 3,5-dicarboxy-2,2/-bipyridine. The 4,4 -dcbpy complex exhibited a decrease in the luminescence lifetime with increasing temperature and had an activation barrier of approximately 350 cm x. The luminescence lifetime of the 3,5-dcbpy complex, however, increased with increasing temperature. This surprising observation remains unexplained [14]. 

Fig. 2 DFT model (left) and full DFT/MM (right) descriptions commonly used in DFT/MM calculations of olefin polymerization catalyzed by Ni(II) diimine complexes...

A series of ruthenium (II) diimine complexes containing oxa-thiacrown derived from 1,10-phenanthroline have been synthesized and characterized <2007IC720>. The crystal stmctures of [Ru(bpy)2200](PF6)2, [Ru(bpy)2201](ClC>4)2, [Ru(bpy)2202](C104)2 have been determined. The luminescence properties of [Ru(bpy)2200](C104)2 were found to be sensitive and selective toward the presence of Hgz+ ions in an acetonitrile solution. 

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. ... 

Del Guerzo, A. Leroy, S. Fages, F. Schmehl, R. H. Photophysics of Re(I) and Ru(II) diimine complexes covalently linked to pyrene contributions from intra-ligand charge transfer states. Inorg. Chem. 2002, 41, 359-366. 

The second-order term in the rate laws for reactions of low-spin iron(II) diimine complexes with such nucleophiles as hydroxide and cyanide ions has been established as arising from a bimolecular reaction between complex and nucleophile.182 Activation volumes that were obtained for reactions of CN and OH with Fc(phcn)2 1 and Fe(bpy)3 + were in the range of +19.7 to +21.5cm3mol-1.183 Because these observations were not readily accounted for by an associative mechanism, a mechanism analogous to the Eigen-Wilkins mechanism of complex formation was introduced in which dissociative activation dominates in determining the observed activation volumes. However, subsequently it was shown that solvation... 

---