Chapter 4 - Jacobs University

Chapter 4
16-Iron Metal Center Nanocluster
mentioned below). For example, 2 can be prepared by reaction of a solution of P 8 W 48 with (i)
FeCl 3 in 0.5 M LiCH 3 COO/CH 3 COOH buffer, pH 4.0 and a few drops of 30% H 2 O 2 , (ii)
Fe(ClO 4 ) 3 in 0.5 M LiCH 3 COO/CH 3 COOH buffer, pH 4.0, (iii) FeSO 4 in 0.5 M
LiCH 3 COO/CH 3 COOH buffer, pH 4.0 and a few drops of 30% H 2 O 2 , (iv)
[Fe 3 O(CH 3 COO) 6 (H 2 O) 3 ]Cl∙H 2 O (Fe 3 Ac 6 ) in 1M NaCH 3 COO/CH 3 COOH buffer, pH 4.2 and (v)
FeCl 2 in 1M NaCH 3 COO/CH 3 COOH buffer, pH 4.2 in presence of O 2 . Interestingly, the latter
reaction can be considered as a model for the formation of the Fe 3+ oxide nucleus of ferritin.
It became apparent that, as expected, the pH is a crucial parameter besides the acetate
medium. On the other hand, it is possible to use a large variety of iron salts as educts, ranging
from mononuclear Fe 2+ and Fe 3+ complexes (the former requires addition of an oxidant) to
trinuclear Fe 3+ carboxylates. These observations support earlier knowledge that POM syntheses
in general depend very much on the boundary conditions in the reaction vessel (e.g. pH and
solvent).
Polyanion 2 exhibits a highly attractive symmetrical D 4h structure (Figures 4.3 and 4.4).
The large cavity (roughly 9 Å x 9 Å x 7 Å = 567 Å 3 ) of the “cyclic template/host” P 8 W 48 has
been “decorated” with a cationic nanocluster built up by 16 FeO 6 octahedra, resulting in a
smaller, central cavity (roughly 6 Å x 6 Å x 5 Å = 180 Å 3 ). The related “Fe 16 ring” is composed
of eight pairs of structurally equivalent, edge-shared FeO 6 octahedra which are connected to each
other via corners. While most of the Fe-O-Fe bridges are monoprotonated, four are diprotonated
(presence of H 2 O ligands). This can be confirmed by looking at the related bond valence sums
(BVS) of these oxygen atoms. 12 For example, the protonated oxygens (with the corresponding
BVS values) of the polyanion in the mixed lithium-potassium salt 2a are O14F (BVS 0.69),
O23F (0.71), O13F (1.08), O24F (1.11), O14G (1.17), O23G (1.27), O1FE (1.31), O2FE (1.32),
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