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 Ga-III-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',N'-tetrakis(2-pyridylmethyl)-1,2-ethanediamine, and the potentially hexadentate N,N,M,NI-tetrakis(2-pyridylmethyl)-1,2-ethanediamine. In solution, each of the three pyridylamine ligands appears to coordinate to the Ga-III 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.