Catalysis of Alkene Epoxidation by a Series of Gallium(III) Complexes with Neutral N‑Donor Ligands

Six
gallium­(III) complexes with N-donor ligands were synthesized to study
the mechanism of GaIII-catalyzed olefin epoxidation. These
include 2:1 ligand/metal complexes with the bidentate ligands ethylenediamine,
5-nitro-1,10-phenanthroline, and 5-amino-1,10-phenanthroline, as well
as 1:1 ligand/metal complexes with the tetradentate N,N′-bis­(2-pyridylmethyl)-1,2-ethanediamine,
the potentially pentadentate N,N,N′-tris­(2-pyridylmethyl)-1,2-ethanediamine,
and the potentially hexadentate N,N,N′,N′-tetrakis­(2-pyridylmethyl)-1,2-ethanediamine.
In solution, each of the three pyridylamine ligands appears to coordinate
to the GaIII through four donor atoms. The six complexes
were tested for their ability to catalyze the epoxidation of alkenes
by peracetic acid. Although the complexes with relatively electron-poor
phenanthroline derivatives display faster initial reactivity, the
gallium­(III) complexes with the polydentate pyridylamine ligands appear
to be more robust, with less noticeable decreases in their catalytic
activity over time. The more highly chelating trispicen and tpen are
associated with markedly decreased activity.