RELACIÓN DE REQUERIMIENTOS DE APTITUD PARA PUESTOS U OFICIOS

The kinetics of the acid-catalyzed substitution reactions of synthetic Fe-S-based clusters, particularly the dependence on the ratio ([NHEt3+]free /[NEt3]free), indicate that a single proton

rapidly binds to the cluster and labilises the terminal ligand of the cluster to substitution. This step involves an equilibrium step when the proton transfers from NHEt3+ acid to the cluster in

the forward reaction and the protonated cluster is deprotonated by NEt3 base in the back

reaction. The dependence on ([NHEt3+]free /[NEt3]free) is not consistent with NHEt3+ just

hydrogen bonding to the Fe-S-based cluster. Studies on the substitution reactions of [FeCl4]-

with PhS- demonstrate that NHEt3+ binds to the complex through hydrogen bonding and this

interaction modulates the rate of the substitution reaction39. In this situation the rate of substitution reaction will be accelerated not due to protonation but by formation of the ion-pair {[NHEt3+].[FeCl4-]} which is more labile to substitution than the parent [FeCl4]-. The proton in

this ion-pair {[NHEt3+].[FeCl4-]} may not transfer completely but it probably involve hydrogen

bonding between the acidic N-H and the chloro-ligands. The formation of this ion-pair increases the rate of substitution reaction but significantly less than the effect of proton transfer.

Returning to the mechanism shown in (Figure 1.9), the values of K1 (protonation equilibrium

constant of the cluster) and k2 (rate constant for substitution) can be calculated using Equation

(1.2). Moreover, the apparent value of the pKa of the cluster can be determined knowing the

synthetic Fe-S-based clusters have been performed, and the results are presented in Table (1.2)40. It can be seen that the values of the apparent pKa of all different clusters fall on the

limited range of 17.9-18.9 (in MeCN)30, 40, 41, in spite of change in either the composition of cluster or terminal ligands. Because of this insensitivity of the pKa to the composition and

structure of the cluster, it has been suggested that the protonation site is the bridging core sulfur of cluster, and not the terminal ligands. This suggestion has been supported through studies of the protonation for both [Fe4S4(SR)4]2- and [Fe4S4Cl4]2- clusters with the same acid (NHEt3+).

These studies show that the calculated pKas of these two clusters are very similar

notwithstanding that the pKas of the corresponding protonated ligands are very different (pKa

of PhSH > 2138 and pKa of HCl = 10.442 in MeCN), so this is a strong evidence that the detected

protonation is not on the terminal thiolate.

Table 1.2. The pKas of synthetic Fe-S-based clusters determined from the kinetics of the acid-

catalysed substitution reactions in MeCN30,40,41.

Fe-Cl clusters: Cluster pKa [Cl2FeS2VS2FeCl2]3- 17.9 [S2MoS2FeCl2]2- 17.9 [Fe4S4Cl4]2- 18.8 [{MoFe3S4Cl3}2(µ-SEt)3]3- 18.6 [{WFe3S4Cl3}2(µ-SEt)3]3- 18.2 [{WFe3S4Cl3)2(µ-OMe)3]3- 18.4 [Fe2S2Cl3(NCMe)]- 18.1 [Fe6S6Cl2(PEt3)4] 18.0

Fe-SR clusters: [Fe4S4(SPh)4]2- 18.6

[Fe4S4(SEt)4]2- 18.0

[{MoFe3S4(SEt)3}2(µ-SEt)3]3- 18.1

[{WFe3S4(SEt)3}2(µ-SEt)3]3- 18.3

[Fe6S9(SEt)2]4- 17.9

In most kinetic studies, the reactions of the synthetic Fe-S-based clusters are investigated in the aprotic solvent MeCN, not in a protic solvent. This is in contrast to the natural Fe-S-clusters

would be appropriate to observe the acid-catalyzed substitution reaction of the synthetic cluster in water. The substitution reaction of [Fe4S4(SCH2CH(OH)Me)4]2- with PhS- has been studied

in the presence of NHEt3+ as acid, in methanol (MeOH) as protic solvent43. This study shows

that the protonation labilises the terminal substituent by a dissociative mechanism, and the kinetics and mechanism are similar to that observed for the other synthetic Fe-S-based clusters reacting in the aprotic solvent (MeCN). The calculated pKa of the cluster [Fe4S4(SCH2CH(OH)Me)4]2-

is 8.5 (in methanol). Further studies on [Fe4S4(SCH2CH2CO2)4]6- show pKa = 7.4 for this cluster

in water44.

General investigation indicates that the structure of cuboidal {Fe4S4}2+ has four potential

protonation sites, which are core sulfur, Fe, terminal ligand and above a {Fe2S2} face, and the

structure of cuboidal {MFe3S4}n+ has eight potential protonation sites, which are core sulfur

bound to only Fe, core sulfur bound to M and Fe, terminal ligand bond to Fe, terminal ligand bound to M, above a {Fe2S2} face or above a {MFeS2} face. Nevertheless, the kinetic

observations indicated that the protonation occurs on bridging sulfur and this seems reasonable based on the expected relative basicities of the cluster components. The studies measuring the binding affinities of 4-YC6H4COCl (Y = MeO, H or Cl) to [Fe4S4(SR)4]2- (R = Ph, Et or But)

suggest that the acid chlorides probably bind to the cluster in a multi-site interaction, as shown in Figure (1.10). These studies focused on observation of maximum binding affinity of the acid chloride (which contained the most electron-withdrawing 4-Y-substituents). An analogous type of interaction, where the proton was proposed to bind above a Fe2S2 face, has been suggested44- 46_{. Figure 1.10 shows how the acid chlorides binding to the cluster with both the acyl oxygen}

binding to the Fe and the carbonyl carbon interacting with the sulfur of the terminal thiolate.

Figure 1.10. Suggested binding of acid chloride to [Fe4S4(SR)4]2-, on the left showing

involvement of terminal thiolate ligand and on the right, is a possible structure for proton binding to the cluster involving a similar interaction with the terminal thiolate and core sulphides.

In acid-catalysed substitution reactions of synthetic Fe-S-based clusters, the stereochemical relationship between the site of protonation and the site of substitution has been investigated 47. In Fe-S-based clusters, there are several sites of protonation as well as several sites of substitution, so the problem of matching up a particular protonation site with a particular substitution site is complicated. However, the study on [Cl2FeS2VS2FeCl2]3-, indicates that the site of protonation

should be adjacent to the site of substitution47_.

The metal sites in most Fe-S-based clusters are magnetically coupled and hence communicate with one another. However, in the linear trinuclear cluster [Cl2FeS2VS2FeCl2]3-, it is notable

that the two Fe sites are chemically equivalent but the two Fe atoms are magnetically isolated by the central V atom48_{. Studies on the substitution reaction of the terminal chloro-ligands in}

[Cl2FeS2VS2FeCl2]3- with PhSH in the presence of NHEt3+ and NEt3 indicate that the Fe sites

undergo both uncatalysed and acid-catalysed substitution reactions. The uncatalysed substitution reaction is slow and dissociative. However, the acid-catalysed substitution reaction, which displays a first-order dependence on the concentration of PhSH, is fast and associative. The species [Cl2FeS2VS(SH)FeCl2]2- is produced after the initial protonation step and then this

species undergoes substitution. The associative substitution step can happen either on the Fe atom next to SH and this substitution is fast. Alternatively, the substitution can occur on the Fe centre the other side of the V atom. In this case, the V shields the labilising effect of the SH group and hence, the substitution at the remote Fe is little perturbed by the protonation, as shown in Figure (1.11).

Figure 1.11. Alternative pathways for the acid-catalysed substitution reaction of [Cl2FeS2VS2FeCl2]3- with PhSH in presence of NHEt3+ and NEt3. The top pathway involves

protonation and substitution at sites remote from one another. The bottom pathway involves protonation and substitution at adjacent sites.

In the acid catalysed substitution reactions of the Fe-S-based clusters, if the nucleophile-binding site must be adjacent to the protonation site, it is worth considering the corresponding reactions of cuboidal clusters as shown in Figure (1.12). It can be noted that all Fe atoms are equivalent in cuboidal [Fe4S4X4]2-, but after the protonation step three of these Fe atoms are adjacent to

SH whilst one Fe is remote from the protonated site. Hence, it could be expected that the substitution reaction on the three equivalent Fe atoms is faster than protonation at the single remote Fe. In the cuboidal [MFe3S4X3]n- (M=Mo, V, W, Nb or Re), despite all three Fe sites

are equivalent, the S sites are differentiated: three sulfurs are bound to M, but the fourth sulfur is bound only to Fe. In addition, in cuboidal [MFe3S4X3]n-, if the protonation occurs at any of

the µ3-S sites bound to M that means just two Fe sites will be adjacent to protonated site and

consequently the substitution is facilitated at these sites. In contrast, all three Fe sites are labilised when protonation occurs at the unique µ3-S as shown in Figure (1.12).

Figure 1.12. Fe site discrimination in cuboidal Fe-S-based clusters after protonation of core S. The kinetic studies of the substitution reaction of Fe-S-based clusters, for both associative and dissociative mechanisms, indicate that the substitution step is facilitated by prior protonation. Here will be considered the electronic factors that can occur upon protonation. It would be anticipated that protonation would distort the electron density of the parent cluster, and the electron density will be pulled towards the protonation site and that will lead to decrease the electron density at the site of substitution (particularly Fe sites adjacent to the SH). Thus, the protonation will facilitate attack of the nucleophile in an associative substitution step3.

A broader understanding of the electronic effects on the lability of the clusters, and particularly the rate of dissociation of the terminal chloro-ligands in the reactions of [Fe4S4Cl4]2-, was

revealed in reactions using the series of 4-RC6H4S- nucleophile (R = H, Me, MeO, Cl or CF3)49.

The substitution reactions of [Fe4S4Cl4]2- with 4-RC6H4S- to produce [Fe4S4(SC6H4R-4)Cl3]2-

occurs by an associative mechanism where the thiolate ion binds to the cluster and then the chloro-group dissociates. The effect of the 4-R-thiolate substituent on the lability of the Fe-Cl bond can be observed by analysis of the kinetics data which allows calculation of the rate constant for dissociation of the chloro-ligand from the intermediate [Fe4S4(SC6H4R-4)Cl4]3-.

The surprising observation is that the lability of the chloro-group increases when the 4-R- substituent becomes more electron-withdrawing. From the mechanism, it would have been expected that electron-withdrawing 4-R-substituents would increase Fe-Cl bond strength and hence decrease the rate of dissociation. This result suggests that the electron-withdrawing substituents reduce the anion-anion repulsion felt in the transition state as the thiolate approaches the cluster and this is an important factor in facilitating the reaction.