Unexpected explanation for the enigmatic acid-catalysed reactivity of [Fe₄S₄X₄]²⁻ clusters

Abstract

Density functional calculations show that Fe-S clusters undergo unexpected large structural changes when protonated at S. Protonation of prototypical cubanoid [Fe4S4X4](2-) to [Fe4S3(SH)X4](-) (X = Cl, SR, OR) results in formation of doubly-bridging SH, severance of one Fe-S bond, and creation of a three-coordinate Fe. These findings explain previously enigmatic results concerning the reactivity of these clusters, including the rates of protonation, pKa data, and the kinetics of acid-catalysed ligand substitution.

Protonation and Proton‐Coupled Electron Transfer at S‐Ligated [4Fe‐4S] Clusters

“…[17] This result that p K a ( 1-H ) < 14.4 contrasts with very recent conclusions by Dance and Henderson, from computational and experimental (kinetic) studies, that there is complete initial protonation of a thiolate ligand [Fe 4 S 4 (SR) 4 ] 2- (R = Et, Ph) by Et 3 NH + , also in MeCN. [18] Since the p K a of Et 3 NH + in MeCN is 18.9, their study implies p K a values for the protonated clusters of >18.9. This discrepancy could be due to the different thiolate ligands used (SPh vs. SC 6 H 2 i Pr 3 ), although the >5.5 unit p K a difference seems large for a change of three alkyl substituents.…”

X-ray crystal structures of [NHR3]2[Fe4S4X4] (X = PhS, R = Et or n Bu; X = Cl, R = n Bu): implications for sites of protonation in Fe–S clusters

“…We will now consider how the structures of [NHR 3 ] 2 [Fe 4 S 4 X 4 ] relate to the earlier proposal (based on the kinetics and DFT calculations) that protonation of the cluster is associated with major structural changes [9][10][11][12][13][14][15][16]. It is difficult to study the protonation of Fe-S-based clusters directly because of the poor spectroscopic changes associated with this process.…”

“…This hydrogen bonding suggests potential sites of protonation. The kinetic studies suggest that the 'labilising' protonation (which facilitates substitution of terminal ligands) causes significant structural changes to the cluster, and DFT calculations [12][13][14] indicate this is a Fe-(l 3 -SH) bond cleavage (Figs. 2 and 4).…”

“…It has been suggested that these observations indicate protonation is associated with significant structural changes to the cluster [12][13][14]. Recent DFT calculations indicate that these structural changes are localised within the cluster to a significant elongation/cleavage of the Fe-(l 3 -SH) bond (Fig.…”

“…2). This cluster disruption produces a 3-coordinate Fe site which appears to be primed for nucleophilic attack in the substitution stage of the reaction [12][13][14].…”