In 1989, it was predicted that a change in dendritic shape to a nearly spherical one should occur upon increasing the generation number (Naylor, A. M.; Goddard, W. A., III; Kiefer, G. E.; Tomalia, D. A. J. Am. Chem. Soc. 1989, 111, 2339). The absence of long-range order required for X-ray analysis allowed only indirect evidence to be provided for this concept. This publication reports the synthesis of three generations of self-assembling monodendrons based on the AB3 building block methyl 3,4,5-trihydroxybenzoate. The first 3,4,5-tris[p-(n-dodecan-1-yloxy)benzyloxy]benzoic acid and the second-generation methyl 3,4,5-tris{3‘,4‘,5‘-tris[p-(n-dodecan-1-yloxy)benzyloxy]benzyloxy}benzoate monodendrons self-assemble into cylindrical supramolecular dendrimers that self-organize in a two-dimensional p6mm lattice. The third-generation monodendron 3,4,5-tris(3‘,4‘,5‘-tris{3‘‘,4‘‘,5‘‘-tris[p-(n-dodecan-1-yloxy)benzyloxy]benzyloxy}benzyloxy)benzoate self-assembles in a spherical dendrimer that self-organizes in a three-dimensional cubic Pm3̄n lattice. Structural analysis of these lattices by X-ray diffraction provided the first direct demonstration of the supramolecular dendrimer shape change from cylindrical to spherical and indirect determination of the average shape change of the monodendron from a quarter of a disk to a half of a disk and to a sixth of a sphere as a function of generation number. These results have demonstrated the concept of monodendron and supramolecular dendrimer shape control by generation number.

The way in which alignment of hexagonal columnar mesophases depends on molecular structure, film thickness, and surface interactions is investigated. Homeotropic orientation of tapered nonpolymeric and polymeric amphiphilic columnar molecules occurs most readily on carbon substrates. The planar alignment of asymmetric dendrimers is favored on a water surface but is not possible for a symmetric dendrimer. For the polymeric material, film thickness has a significant influence on the morphology. Thick films align homeotropically, but films that are only a few column diameters in thickness are planar, and their thickness is quantized. Because they must contain an integral number of layers of cylinders, the resulting morphology is terraced.

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The catalytic reduction of cis-dimethyldiazene by the (Et4N)2[(Cl4-cat)(CH3CN)MoFe3S4Cl3] cluster (Cl4-cat = tetrachlorocatecholate) is reported. Unlike the reduction of cis-dimethyldiazene by the Fe/Mo/S center of nitrogenase, which yields methylamine, ammonia, and methane (the latter from the reduction of the C−N bond), the reduction of cis-dimethyldiazene by the synthetic cluster yields exclusively methylamine. In separate experiments, it was shown that the C−N bond of methylamine is not reduced by the [MoFe3S4]3+ core, perhaps accounting for the differences observed between the biological and abiological systems. 1,2-Dimethylhydrazine, a possible partially reduced intermediate in the reduction of cis-dimethyldiazene, was also shown to be reduced to methylamine. Interaction of methylamine with the Mo atom of the cubane was confirmed through the synthesis and structural characterization of (Et4N)2[(Cl4-cat)(CH3NH2)MoFe3S4Cl3]. Phosphine inhibition studies strongly suggest that the Mo atom of the [MoFe3S4]3+ core, which has a Mo coordination environment very similar to that in nitrogenase, is responsible for the binding and activation of cis-dimethyldiazene. The reduction of a NN bond exclusively at the heterometal site of a nitrogenase-relevant synthetic compound may have implications regarding the function of the nitrogenase Fe/Mo/S center, particularly in the latter stages of dinitrogen reduction.