Bt

Scheme 3.

In addition, the nature of bonding is generally different in the two cases; the class of MBNs is dominated by molecular architectures and assemblies based largely on noncovalent interactions, where the main driving forces for the formation of a network are metal-ligand interaction, hydrogen bonding, ft-ft stacking, ion templation, chiral interactions, etc. On the other hand, the MDNs are based on covalent and/or ionic interaction. Furthermore, in the case of an MBN, the common principles of synthetic organic, organometal-lic, inorganic, and polymer chemistry are used to obtain the desired material. This methodology is also used to obtain the precursors to MDNs; however, the target nanomate-rial is obtained only after a few processing steps, which involve the loss of organic ligands, structural fragments, or elements to reach the final solid. Moreover, a molecular building block of an MDN may contain many more elements than those required to form the end product. For instance, iron pentacarbonyl (Fe(CO)5) is used as a source of iron nanoparticles, and nanostructured films contain C and O as carbonyl ligands, which are knocked off during the processing steps to nanocrystalline iron. Similarly, titanium isopropoxide (Ti(OPr')4) loses the organic periphery in the formation of titanium dioxide (TiO2).

The molecular self-assembly, which can be seen as a case of MBN, seems to open new pathways to nanostructures formed by the organization of molecular building blocks through controlled molecular interactions. A representative example is shown in Figure 1, which displays a supramolec-ular ribbon structure based on hydrogen bonding between barbituric acid and 2,4,6-triaminopyrimidine units [32].

It must be emphasized that despite significant differences, the distinction between the MBN and MDN approaches is not absolute because examples are now known in which the structural features of an MDN source are carried forward to the extended solid network. For instance, the use of cubane precursor molecules, [(Bu')GaS]4, in the MOCVD process results in the growth of a new cubic GaS phase, whereas using the dimeric species [(Bu')2GaS]2 as molecular precursor forms the thermodynamically stable hexagonal GaS. It has been shown that the formation of cubic GaS is a consequence of the retention of the cubane Ga4S4 core during the deposition and growth process (Scheme 4). Interestingly, the cubic phase is not obtained by other routes and precursors. In view of the general definitions applied to MBN and

Figure 1. Self-assembly of a supramolecular ribbon from barbituric acid and 2,4,6-triaminopyrimidine units. Reprinted with permission from [32], J. M. Lehn et al., J. Chem. Soc., Perkin Trans. 2, 461 (1992). © 1992, Royal Society of Chemistry.

Scheme 2.

Figure 1. Self-assembly of a supramolecular ribbon from barbituric acid and 2,4,6-triaminopyrimidine units. Reprinted with permission from [32], J. M. Lehn et al., J. Chem. Soc., Perkin Trans. 2, 461 (1992). © 1992, Royal Society of Chemistry.

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