Supramolecular Self-Assembled Polynuclear Complexes from Tritopic, Tetratopic, and Pentatopic Ligands: Structural, Magnetic and Surface Studies

Polymetallic, highly organized molecular architectures can be created by “bottom-up” self-assembly methods using
ligands with appropriately programmed coordination information. Ligands based on 2,6-picolyldihydrazone (tritopic
and pentatopic) and 3,6-pyridazinedihydrazone (tetratopic) cores, with tridentate coordination pockets, are highly
specific and lead to the efficient self-assembly of square [3 × 3] Mn9, [4 × 4] Mn16, and [5 × 5] Mn25 nanoscale
grids. Subtle changes in the tritopic ligand composition to include bulky end groups can lead to a rectangular 3 ×
[1 × 3] Mn9 grid, while changing the central pyridazine to a more sterically demanding pyrazole leads to simple
dinuclear copper complexes, despite the potential for binding four metal ions. The creation of all bidentate sites in
a tetratopic pyridazine ligand leads to a dramatically different spiral Mn4 strand. Single-crystal X-ray structural data
show metallic connectivity through both μ-O and μ-NN bridges, which leads to dominant intramolecular
antiferromagnetic spin exchange in all cases. Surface depositions of the Mn9, Mn16, and Mn25 square grid molecules
on graphite (HOPG) have been examined using STM/CITS imagery (scanning tunneling microscopy/current imaging
tunneling spectroscopy), where tunneling through the metal d-orbital-based HOMO levels reveals the metal ion
positions. CITS imagery of the grids clearly shows the presence of 9, 16, and 25 manganese ions in the expected
square grid arrangements, highlighting the importance and power of this technique in establishing the molecular
nature of the surface adsorbed species. Nanoscale, electronically functional, polymetallic assemblies of this sort,
created by such a bottom-up synthetic approach, constitute important components for advanced molecule-based
materials.