Rates of Proton Transfer to Fe−S-Based Clusters:  Comparison of Clusters Containing {MFe(μ₂-S)₂}ⁿ⁺ and {MFe₃(μ₃-S)₄}ⁿ⁺ (M = Fe, Mo, or W) Cores

Abstract

The rates of proton transfer from [pyrH]+ (pyr = pyrrolidine) to the binuclear complexes [Fe2S2Cl4]2- and [S2MS2FeCl2]2- (M = Mo or W) are reported. The reactions were studied using stopped-flow spectrophotometry, and the rate constants for proton transfer were determined from analysis of the kinetics of the substitution reactions of these clusters with the nucleophiles Br- or PhS- in the presence of [pyrH]+. In general, Br- is a poor nucleophile for these clusters, and proton transfer occurs before Br- binds, allowing direct measure of the rate of proton transfer from [pyrH]+ to the cluster. In contrast, PhS- is a better nucleophile, and a pathway in which PhS- binds preferentially to the cluster prior to proton transfer from [pyrH]+ usually operates. For the reaction of [Fe2S2Cl4]2- with PhS- in the presence of [pyrH]+ both pathways are observed. Comparison of the results presented in this paper with analogous studies reported earlier on cuboidal Fe-S-based clusters allows discussion of the factors which affect the rates of proton transfer in synthetic clusters including the nuclearity of the cluster core, the metal composition, and the nature of the terminal ligands. The possible relevance of these findings to the protonation sites of natural Fe-S-based clusters, including FeMo-cofactor from nitrogenase, are presented.

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

“…Kinetic studies show that substitution is faster upon protonation of these 'm-S only' clusters, and the rate of protonation of m-S is similar to that observed for m 3 -S in cubanoid clusters. 24,25 Protonation of clusters containing only m-S would be expected to form m-SH, without severance of an Fe-S bond, and this has been confirmed with DFT calculations. Therefore an acid-catalysed substitution mechanism involving three-coordinate Fe is not available in these systems.…”

“…The significant result here is that protonation of m-S to form m-SH, while not involving Fe-S cleavage, does involve substantial structural changes, resulting in slow proton transfer. 25 23 The rate-limiting step is dependent on X, as marked. This mechanism is consistent with all kinetic data.…”

Large structural changes upon protonation of Fe4S4 clusters: the consequences for reactivity

“…38 Direct measurement of the rates of proton transfer to the same cluster have been made using pyrrolidinium ion (thermodynamically-unfavourable proton transfer) and k = 2.4 × 10 4 dm 3 mol −1 s −1 . [39][40][41][42] The reverse reaction is thermodynamically-favourable and can be calculated to be k ∼ 1 × 10 7 dm 3 mol −1 s −1 , confirming the slowness of proton transfer reactions involving [Fe 4 S 4 X 4 ] 2− . It seemed likely that the reason for this slow protonation was because the proton transfer step was associated with significant structural reorganisation of the cluster core dimensions.…”

Ligandenumlagerungen an Fe/S‐Cofaktoren: langsame Isomerisierung eines biomimetischen [2Fe‐2S]‐Clusters

“…[8][9][10] Diese Ergebnisse lassen die Frage offen, wie Ligandenaustausch-und Ligandenumlagerungsprozesse in Eisen-Schwefel-Clustern auf molekularer Ebene mechanistisch ablaufen. [21][22][23][24][25] Diese Studien betonten vor allem den Einfluss des gewählten Lçsungsmittels und auch der Gegenwart von Protonen auf den Ablauf der Substitutionsreaktion. Die kinetischen und mechanistischen Details derartiger Ligandenaustauschund Isomerisierungsprozesse sind bisher allerdings kaum untersucht.…”

“…Die Kinetik der Ligandensubstitution an [Cl 2 Fe-(m-S) 2 FeCl 2 ] 2À und verschiedenen [4Fe-4S]-Clustern wie 3 wurde sowohl in MeCN als auch in Wasser beschrieben. [21][22][23][24][25] Diese Studien betonten vor allem den Einfluss des gewählten Lçsungsmittels und auch der Gegenwart von Protonen auf den Ablauf der Substitutionsreaktion. Während der Fokus dieser früheren Studien vor allem auf [4Fe-4S]-Clustern lag, spielen in der Biosynthese von Eisen-Schwefel-Clustern besonders der Aufbau und Tr ansfer und somit Umlagerungen in der Koordinationsumgebung von [2Fe-2S]-Clustern eine zentrale Rolle.U nseres Wissens ist bisher keine detaillierte Beschreibung der Kinetik und des Mechanismus der Ligandenumlagerung an biomimetischen, S-koordinierten [2Fe-2S]-Clustern verfügbar.…”