CYCLOTRIVERATRYLENE MODELS FOR 4FE-4S PROTEINS : 3:1 SUBSITE DIFFERENTIATION AND MODULATION OF THE REDOX POTENTIAL
1997
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.
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