Chapter 4 - Jacobs University

Chapter 4
16-Iron Metal Center Nanocluster
4.5 Conclusion
We have prepared the 16-Fe 3+ containing 48-tungsto-8-phosphate
[P 8 W 48 O 184 Fe 16 (OH) 28 (H 2 O) 4 ] 20- (2) as the mixed cation salts
Li 4 K 16 [P 8 W 48 O 184 Fe 16 (OH) 28 (H 2 O) 4 ]×66H 2 O×2KCl (2a) and
Na 9 K 11 [P 8 W 48 O 184 Fe 16 (OH) 28 (H 2 O) 4 ]∙100H 2 O (2b). Polyanion 2 contains 16 edge- and cornersharing
FeO 6 octahedra in the form of a cyclic, unprecedented {Fe 16 (OH) 28 (H 2 O) 4 } 20+ ironhydroxo-aqua
nanocluster, grafted on the inner surface of the crown-shaped [H 7 P 8 W 48 O 184 ] 33-
(P 8 W 48 ) precursor. The synthesis of 2 was accomplished by reaction of hydrate complexes of
Fe 2+ (in presence of O 2 ), Fe 3+ and [Fe 3 O(CH 3 COO) 6 (H 2 O) 3 ] + with P 8 W 48 in aqueous, acidic
medium (pH ~4).
Besides the unprecedented {Fe 16 (OH) 28 (H 2 O) 4 } 20+
iron-hydroxo-aqua nanocluster, the
central, (empty) cavity of the title polyanion has another highly interesting feature. Access of an
oxidant/substrate to the “iron active site” is easily possible and therefore 2 is very attractive for
catalytic applications. In fact, initial oxidation catalysis studies using air-oxygen as oxidant are
highly promising. 11
Furthermore, it is very likely that the cavity in 2 can be filled with additional metal
centers, e.g. those different from Fe 3+ . We are currently engaged in the process of preparing
mixed-metal derivatives of 2 (e.g. “Fe 16-x M x P 8 W 48 ”) with one or more of the iron centers
substituted by other transition metal ions (e.g. Mn 2+ , Co 2+ , Zn 2+ ). Such derivatives could lead to
interesting magnetic as well as catalytic properties. 13
The study of a reaction of a solution of P 8 W 48 with metal cations offers the possibility to obtain
basic information about principles of directed assembly processes under geometrically confined
conditions. This can also lead, as in the present case, to cationic nanoclusters not obtainable
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