The chemistry of half-sandwich vanadium imido-amido complexes

Many important industrial processes make use of catalyzed reactions. One of the best known catalytic processes is the production of polyethylene and polypropylene plastics used in everyday life by means of olefin polymerisation (carbon-carbon bond formation). Another catalyzed chemical transformation that received widespread attention is the formation of carbon-heteroatom bonds. The hydroamination reaction, for instance, creates a carbon-nitrogen bond from amines and alkenes or alkynes. It is of great importance for the pharmaceutical industry and organic synthesis in general. Complexes of a wide range of metals (transition metals, rare earth metals, alkaline earth metals) are able to catalyze this reaction, and various reactive pathways are available. Nevertheless, it is not straightforward to find the best catalyst/substrate combinations, and more insight is needed into the factors that determine catalyst efficiency. For the early transition-metal catalyzed hydroamination, the group 4 metals titanium and zirconium have been the most intensively investigated. This research suggested that species with either metal-nitrogen single bonds (amido) or double bonds (imido) can be viable reaction intermediates. In this thesis, cyclopentadienyl complexes of the group 5 metal vanadium that carry both these functionalities were studied, in order to obtain more insight in the relative effectiveness of the various reactive pathways for hydroamination. These complexes can indeed catalyze intramolecular hydroamination reactions, although the studies did not allow for a positive identification of the preferred reaction route. For a number of substrates, the catalysts were found to yield products with a 2:1 alkyne:amine ratio. In addition, unique structural evidence was obtained for a weak intramolecular V-η1-arene interaction in complexes with aryl-bearing cyclopentadienyl ligands.

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