Structures of FI Catalysts

Because an FI catalyst has a pair of non-symmetric phenoxy-imine ligands, it potentially possesses five isomers stemming from the coordination modes of ligands. Zr-, Ti-, and Hf-FI catalysts 1-3 display three sets of signals in XH NMR, attributed to the imine proton, suggesting that these FI catalysts exist as isomeric mixtures in solution, which is probably an intrinsic feature of FI catalysts. On the basis of the symmetry of the possible isomers A-E (Fig. 10), as well as the relative formation 

X-ray crystallographic analyses demonstrate that, in the solid state, Zr- and Ti-FI catalysts 1 and 2 possess approximately octahedrally coordinated metal centers and C2 symmetry, with a trans-O, cis-N, and cis-C 1 disposition (isomer A) (Fig. 11). 

These results indicate that FI catalysts favor the isomer A structure, and that they can be present as a mixture of isomers A and B in solution. It is important to note that an FI catalyst generally exhibits fluxional character in solution, which can result in unique polymerization catalysis. 

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