Issue 4, 2016

Binding small molecules and ions to [Fe4S4Cl4]2− modulates rate of protonation of the cluster

Abstract

The mechanism of the acid-catalyzed substitution reaction of the terminal chloro-ligands in [Fe4S4Cl4]2− by PhS in the presence of NHBun3+ involves rate-limiting proton transfer from NHBun3+ to the cluster (k0 = 490 ± 20 dm3 mol−1 s−1). A variety of small molecules and ions (L = substrate = Cl, Br, I, RNHNH2 (R = Me or Ph), Me2NNH2, HCN, NCS, N3, ButNC or pyridine) bind to [Fe4S4Cl4]2− and this affects the rate of subsequent protonation of [Fe4S4Cl4(L)]n. Where the kinetics allow, the equilibrium constants for the substrates binding to [Fe4S4Cl4]2− (KL) and the rates of proton transfer from NHBun3+ to [Fe4S4Cl4(L)]n (kL0) have been determined. The results indicate the following general features. (i) Bound substrates increase the rate of protonation of the cluster, but the rate increase is modest (kL0/k0 = 1.6 to ≥72). (ii) When KL is small, so is kL0/k0. (iii) Binding substrates which are good σ-donors or good π-acceptors lead to the largest kL0/k0. This behaviour is discussed in terms of the recent proposal that protonation of [Fe4S4Cl4]2− at a μ3-S, is coupled to concomitant Fe–(μ3-SH) bond elongation/cleavage.

Graphical abstract: Binding small molecules and ions to [Fe4S4Cl4]2− modulates rate of protonation of the cluster

Supplementary files

Article information

Article type
Paper
Submitted
17 Nov 2015
Accepted
08 Dec 2015
First published
08 Dec 2015

Dalton Trans., 2016,45, 1373-1381

Author version available

Binding small molecules and ions to [Fe4S4Cl4]2− modulates rate of protonation of the cluster

T. M. M. Al-Rammahi and R. A. Henderson, Dalton Trans., 2016, 45, 1373 DOI: 10.1039/C5DT04523A

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