Cyclotriveratrylene models for [4Fe–4S] proteins: 3∶1 subsite differentiation and modulation of the redox potential

Abstract

The potential of cyclotriveratrylene (ctv) (2,3,7,8,12,13-hexamethoxy-10,15-dihydro-5H-tribenzo[a, d,g]cyclononene) trithiols as ligands that can easily be functionalised and show subsite differentiation in their complexes with [4Fe–4S] clusters has been explored. The cluster complexes of tris(2-sulfanylethoxy)- and tris(3-sulfanyl-methylbenzyloxy)-functionalised ctvs have been studied by core-extrusion experiments, spectroscopy and electrochemical techniques. With [Fe ₄ S ₄ Cl ₄ ] ^2- as starting material a cluster complex was obtained in which the unique Fe and its co-ordinating Cl was turned into the cavity and show no reactivity. Starting with the more bulky [Fe ₄ S ₄ (SBu ^t ) ₄ ] ^2- the unique iron points outwards and is susceptible to substitution reactions. The effects of hydrogen bonding and electron density on the redox potential of the cluster complex have been investigated. The redox potential becomes more negative when the length of the spacer between the ctv and cluster core is increased, which is explained by the longer distance between the cluster and the electron-withdrawing phenoxy moiety of the ctv. The synthesis of ctv derivatives with one thiol and one alcohol functionality per phenyl unit, and comparison with corresponding derivatives where hydrogen bonding is not possible, showed that no significant differences were found. The effects of a substituent in an aromatic amide group that could hydrogen bond to the co-ordinated thiol were investigated. A weak effect, in the direction expected, was found upon substitution of methyl for H.