Polymeric oxygen donor ligands

Metal Ion Binding at Polymeric Oxygen Donor Ligands... 

Complexes of uranyl chloride with a variety of oxygen donor ligands continue to be reported, notably with phosphine oxides (55), pyridine N-oxides (27), N,IV-dimethylformamide (7J), acetamide (15), and with the N,IV,IV, IV -tetramethyldicarboxylic acid amides (14), the last being mainly polymeric compounds. A few similar complexes of uranyl bromide (54) and iodide (71) are also known. 

The sterically encumbered R2 substituents give steric protection to the oxygen-donors that are attached to the metal centers from coordination with Lewis acids such as MAO, or from another molecule of the catalytically active cationic species, which are supposed to be highly electrophilic. The coordination increases steric congestion near the polymerization center, which at least hampers ethylene coordination to the metal. Even worse, it may cause catalyst decay by, for instance, loss of the ligand. 

An example of a macrocycle with oxygen donors to support a Ti catalyst for lactide polymerization has been reported by Frediani et a/. " The authors described several titanium chloride complexes bearing calix[4]arene ligands, which act as catalysts for solvent-free lactide polymerization (Figure 18). These complexes acted as dual-site catalysts with two polymer chains growing from one metal center. 

Two articles follow which have very little in the way of polymerization data but point to exciting new possibilities for catalyst development. The first, by Siemeling, reviews chelate complexes of cyclopenta-dienyl ligands that bear pendant oxygen donors. The second, by Butenschon, treats analogous complexes with pendant phosphorus, arsenic, and sulfur donors. The constrained geometry catalysts that have been commercialized over the last 8 years feature similar chelates. 

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