Stereoisomers and functional groups in oxidorhenium(v) complexes: effects on catalytic activity.

Abstract

The syntheses of oxidorhenium(v) complexes [ReOCl(L1a-c)2] (3a-c), equipped with the bidentate, mono-anionic phenol-dimethyloxazoline ligands HL1a-c are described. Ligands HL1b-c contain functional groups on the phenol ring, compared to parent ligand 2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol H1a; namely a methoxy group ortho to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-6-methoxyphenol, H1b), or a nitro group para to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol, H1c). Furthermore, oxidorhenate(v) complexes (NBu4)[ReOCl3(L1a-b)] (2a-b) were synthesized for solid state structural comparisons to 3a-b. All novel complexes are fully characterized including NMR, IR and UV-Vis spectroscopy, MS spectrometry, X-ray crystallography, elemental analysis as well as cyclic voltammetry. The influence of functional groups (R = -H, -OMe and -NO2) on the catalytic activity of 3a-c was investigated in two benchmark catalytic reactions, namely cyclooctene epoxidation and perchlorate reduction. In addition, the previously described oxidorhenium(v) complex [ReOCl(oz)2] (4), employing the phenol-oxazoline ligand 2-(4,5-dihydro-2-oxazolyl)phenol Hoz, was included in these catalysis studies. Complex 4 is a rare case in oxidorhenium(v) chemistry where two stereoisomers could be separated and fully characterized. With respect to the position of the oxazoline nitrogen atoms on the rhenium atom, these two stereoisomers are referred to as N,N-cis and N,N-trans isomer. A potential correlation between spectroscopic and structural data to catalytic activity was evaluated.