Synthesis, structural and magnetic properties of high nuclearity self-assembled cluster complexes

Zhao, Liang (2001) Synthesis, structural and magnetic properties of high nuclearity self-assembled cluster complexes. Masters thesis, Memorial University of Newfoundland.

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Abstract

This thesis describes synthesis, structural and magnetic properties of high nuclearity cluster complexes self assembled from polydentate alkoxo-diazine ligands. In Chapter 1. clusters and self-assembly have been briefly reviewed. In Chapter 2. synthesis, characterization. X-ray structures and the magnetic properties of three trinuclear copper(II) complexes are described. These compounds exhibit moderate to strong antiferromagnetic coupling as a result of superexchange via the N-N linkages. The magnetic properties are explained on the basis of large rotational angles of the copper magnetic planes about the diazine bridges. In Chapter 3. four predominantly square grid M₄(µ-O)₄ (M = Cu(II), Ni(II) and Co(II)) complexes and a rectangular Mn₄(N-N)₂O₂ grid complex are synthesized and structurally characterized by X-ray techniques. The square [2x2] copper(II) cluster exhibits intramolecular ferromagnetic spin exchange, associated with the orthogonal alkoxide bridging arrangement and the close proximity of the copper centers. Both the Ni₄(µ-O)₄ and Co₄(µ-O)₄ clusters are antiferromagnetically coupled due to the large M-O-M angles and for the rectangular Mn₄ complex the exchange is dominated by antiferromagnetic coupling via the N-N bridge. Chapter 4 describes the ligand paoh. which has two widely separated tridentate (N₂O) terminal coordination pockets, and uncoupled square [2x2] Fe(II)₄ grid complex. Replacement of the NH₂ groups with 2-pyridyl residues gives the ligand dpkoh with the additional coordination capacity of an exo-N₄ coordination pocket, leading to an octanuclear metallacyclic cluster complex [Ni₈(dpkoh-2H)₄(H₂O)₈](ClO₄)₈.4H₂O, in which eight nickel(II) centers are bridged just by the diazine N-N single bond. Substantial antiferromagnetic coupling exists between the nickel(II) ions throughout the metallacycle. In Chapter 5. the synthesis, structural characterization and magnetism studies of [3x3] Mn(ll) and Cu(II) nanonuclear square grid complexes are described. The Mn(II)o systems exhibit intramolecular antiferromagnetic coupling, while for the Cu(II)q complexes ferromagnetic exchange is observed with an S = 7/2 ground state. The aggregation of nine octahedral Mn(II) and Cu(II) centers (fifty four coordination positions) into an alkoxo-bridged, portcullis-like [3x3] grid by six heptadentate 2poap ligands (fifty four donor positions) is a unique example of a self assembly process.

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