Molecular Encoding at the Nanoscale: From Complex Cubes to Bimetallic Oxides**

Nanoparticles, 2] in particular those of semiconducting metal oxides like ZnO, have received increasing attention because of their high potential as components in nanotechnological devices. In general, materials containing more than one type of metal should display higher complexity and a wider range of properties. So far only little attention has been paid to bior multimetallic oxide nanostructures, although a diverse spectrum of properties can be envisioned for such oxides. Multimetallic oxides have been known in solid-state chemistry for a long time, but the high processing temperatures typically applied make them less suitable for the preparation of nanoscaled materials. In addition, optimum dispersity of the two metals inside the nanostructures would play a pivotal role for the rational synthesis and adjustment of properties of those systems. Optimum dispersity is ensured when the respective elements are distributed on the molecular level. In this respect, the use of molecular single-source precursors could potentially solve this problem by the creation of molecular building blocks suitable for bottom-up formation of oxides or other materials. Correlated to the molecular design of the precursor are low thermolysis temperatures resulting in reduced particle growth. However, the preparation of a specific precursor resulting in a specific oxide is difficult, especially if bior multimetallic oxide nanostructures are targeted. Some examples for such single-source precursors exist already. In most cases these heterometallic precursors are alkoxides and are thus very moisture sensitive. Our goal is to identify a new precursor system that enables access to a large variety of nanoscaled bimetallic oxides, and at the same time is readily available and stable. One particular type of bimetallic oxide of current interest is metal oxide semiconductors (for instance ZnO) doped with paramagnetic metal ions like Mn and Ni. Since such metal-doped oxides are expected to display considerable magnetoresistivity, they are promising materials for “spintronics”. 21] Heterometallic precursors for this type of materials are currently unknown. Herein, we report on the intriguing properties of molecular clusters having heterocubane architecture [M4 yM 2 y(LH)4] (OAc)4 x(ClO4)x (ffi [M4 yMyO4]; see Scheme 1) with a singly deprotonated dipyridyldiol (LH) as a chelating ligand, and their exploitation for the preparation of metal-doped transition-metal oxides. We show that 1) monometallic [M4O4] clusters are single-source precursors for nanoscaled oxides; 2) any combination and permutation of bimetallic clusters [M4 yM 2 yO4] containing the metals M = Mn, Co, Ni, Zn are easily accessible; and 3) these bimetallic clusters can be used to prepare nanoscaled bimetallic oxides. The oxo clusters introduced here are the smallest possible molecular building blocks for the desired bimetallic oxide nanostructures (see Scheme 1). 1) Preparation of nanoscaled oxides from [M4O4] precursors. In order to probe the capacity of the described clusters as reliable precursors, the thermolysis behavior of the different monometallic compounds [M4O4] was studied by thermogravimetric analysis (TGA) and powder X-ray diffraction (PXRD). The data for the [Zn4O4] cluster shown in Figure 1 are representative. Since phase-pure zinc oxide is obtained both under argon as well as under oxidative (20 vol% O2) conditions as proven by PXRD, [Zn4O4] is a suitable singlesource precursor for ZnO. The presence of oxygen facilitates the complete removal of the organic ligands as proven by TG analysis. Apart from the fact that one can obtain nanocrystalline ZnO, it is also possible to control the size of the ZnO particles by heating the [Zn4O4] precursor to a particular temperature Td and maintaining this temperature for 3 h in order to oxidize off the organic shell. Under these conditions, the size of the ZnO particles (determined by the Scherrer equation) depends nearly linearly on the thermolysis temperature Td (similar to the Ni–ZnO system shown below). Similar behavior was found for the alternative monometallic precursors with the difference that the oxidation state of the resulting materials may be sensitive to the presence of oxygen during thermolysis [Eq. (1a–d)]. Oxidizing conditions were selected for the preparation of bimetallic oxides.