Synthesis, structure, magnetism, and coordination chemistry of some self assembled polynuclear clusters and grids of first row transition metal ions derived from polytopic hydrazone ligands

Abedin, S. M. Tareque (2007) Synthesis, structure, magnetism, and coordination chemistry of some self assembled polynuclear clusters and grids of first row transition metal ions derived from polytopic hydrazone ligands. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

A collection of hydrazone based ligands ranging from simple ditopic to pentatopic have been synthesized and allowed to react with different transition metals ions (e.g. Mn(II), Cu(II) ). The ligands varied in terms of their coordination capability (ditopic vs. tritopic ), terminal functional group (e.g. -NH₂, vs. -CH₃), and overall ligand flexibility (e.g. rigid pyridine vs. -NH central piece). The ditopic ligand (e.g. pomp) gave tetranuclear M(II)₄(M = Mn(II), Cu(II)) complexes when treated with Mn(II) and Cu(II) metal salts. In all of the tetranuclear complexes, metal ions were hydrazone oxygen bridged except for the copper case where Cu(II) centers were bridged by a mixture of N-N diazine and hydrazone oxygen groups. Intramolecular antiferromagnetism was observed in all cases. Tritopic ligands with a flexible secondary -NH central piece yielded mostly trinuclear linear complexes with Cu(II) and trinuclear nonlinear complexes with Mn(II) ions. The inability of this class of ligands to form nonanuclear grids was attributed to the unusual flexible nature of the ligands which is unsuitable for grid formation. Intramolecular antiferromagnetism was observed in all of the trinuclear complexes. Tritopic ligands with pyridine as a relatively rigid center piece (e.g. 2pomp) yielded [3 x 3] nonanuclear Mn(II)₉ grids when reacted with Mn(II). However, with Cu(II) ions this ligand yielded octanuclear Cu(II)₈ pinwheel clusters. All of the Mn(II)₉ grids showed intramolecular antiferromagnetism whereas pinwheels were found to show ferromagnetism, which is relatively rare in this class of polynuclear complexes. Changing the choice to two different donor functional groupings gave two unprecedented outcomes. An oxalic acid dihydrazide based ditopic ligand yielded an inorganic [2] catenane, with mixed oxidation states when reacted with Co(II) ions. This is the first example of an inorganic cobalt catenane. However, due to large metal separations, no magnetic exchange coupling was observed in the complex. In another instance, an extended tritopic ligand formed an enormous Cu(II)₃₆ cluster using the extra coordination capability of the oxime substituents at two ends of the ligand. The cluster showed intramolecular antiferromagnetism. A logical and symmetric extension of the tritopic ligand led to a pentatopic ligand which gave a [5 x 5] square Mn(II)₂₅ cluster. STM imaging techniques were employed to explore the structure of the cluster. Topographic images clearly showed the [5 x 5] square grid-like array on a HOPG surface.

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