Diketonate ligands, polymeric metal

A third type of polymerization may occur through / -diketonate ligands which span two metal centers via oxygen coordination. However, only one compound has been su ested as an example of this type of bonding. 

One of the promising superconducting materials, YBa2Cu307-3 (YBCO or Y-123), requires barium in the correct stoichiometric ratio. However, polymerization results in the /8-diketonate precursors when barium is the metal coordination site. This reaction can be avoided by coordinating a third neutral ligand such as tetrahydrofuran, trimethylamine, or suitable polyethers to the barium atom. Tetraglyme, a methoxy-terminated polyether, and alkoxide ligands have also been used to increase the stability of /3-diketonates. ... 

Lanthanide(III) ions form three distinct types of complexes with 1,3-diketones (p-diketones) (a) tris complexes with a metal-to-ligand ratio of 1 3, (b) Lewis-base adducts of tris complexes, and (c) tetrakis complexes with a metal-to-ligand ratio of 1 4 (Binnemans, 2005a). It is very difficult to obtain pure monomeric tris complexes, because the coordination number of the lanthanide ion in this type of compounds is six and this is too low for saturation of the first coordination sphere of a trivalent lanthanide ion (except for tris complexes of very bulky 1,3-diketones). Therefore, most of the tris complexes form either dimeric (or even oligomeric or polymeric) complexes, or they form hydrates. The water molecules act as Lewis bases and by addition of... 

The decomposition of the transition complex (Scheme 12) at elevated temperatures causes the reduction of the metal cation, whereas the carbocation (Scheme 13) initiates the polymerization of the epoxy oligomers. In that way, the metal type and the stereochemistry of the mixed-ligand complexes as well as of the metal )S-diketonates bring about the reactivity... 

The polymerization of bis(pyrazinylmethyl) diketones with metal acetates proceeds via the enol form (<5). Treating the bis(ligand) in pyridine solution at 50°-90°C with an aqueous metal salt gave intractable colored precipitates. Only elemental analyses were presented to substantiate the proposed structures for the products. They are shown in Eq. (VII-24). 

If one simply considers the number of bis(ligands) that have been synthesized and polymerized with metals, poly[metal bis(j3-diketonates)] are the most widely investigated class of chelate polymers. Perhaps, as we shall see below, this contributed to the fact that the greatest success in synthesizing truly high molecular weight chelate polymers has been achieved with this ligand class. 

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