Iron(ii) tetrafluoroborate complexes of new tetradentate C-scorpionates as catalysts for the oxidative cleavage of trans-stilbene with H2O2

Abstract

Attachment of a 2-methylpyridyl group onto the unique 1-nitrogen atom on nitrogen-confused C-scorpionates with either pyrazol-1-yl or 3,5 dimethylpyrazol-1-yl donors gives two new cis-directing tetradentate-N₄ ligands (L and L*). The complexes [(L or L*)Fe(CH₃CN)₂](BF₄)₂ (1 or 2) were prepared, fully characterized, and investigated for their ability to catalyse the oxidative cleaveage of trans-stilbene in CH₃CN. Complexes 1 and 2 are capable of catalysing stilbene cleavage when H₂O₂ is used as an oxidant but up to six different products are formed, with CC cleavage products (benzaldehyde and benzoic acid) dominating over four products of oxygen transfer. Catalytic amounts of 1 or 2 enhance the ability for the organic photocatalyst riboflavin tetraacetate to use atmospheric oxygen and blue light irradation (450–460 nm) to selectively cleave stilbene to benzaldehyde. However, when benzaldehyde oxidizes further to benzoic acid, the iron species begin giving increasing amounts of stilbene oxygenation products.