Small molecule activation mediated by metal ligand cooperation via a dearomatization/rearomatization reaction sequence using redox active pyridine based ligand-systems in transition metal complexes

Abstract

This work demonstrates the bifunctional activation of different functional groups, predominantly C-O, C-N and S-O multiple bonds via metal-ligand-cooperation (MLC) under concomitant M—Y and C—Z (M = Re, Mn or Rh; Z = C or S; Y = N or O) bond formation triggered by a dearomatization/rearomatization reaction sequence in three different ligand systems based on simple 2-methyl-pyridine ligand frameworks. Eventually, the simple bidentate ligand frameworks showed similar reactivity with respect to the well-known tridentate pincer complexes, which previously showed such reaction sequences. The bond-activation reactions via MLC were investigated in rhenium(I), manganese(I) and rhodium(I) complexes. Due to the formation on anionic complexes the presented study also entails the investigation of effects imposed by the alkali counter cations with respect to the bound (activated) substrate in group 7 transition metal complexes (Re and Mn). Moreover, from a synthetic point of view, it is shown that the active dearomatized anionic complexes can be generated via two different synthetic routes: i.e. via double deprotonation or two electron reduction. The former contains the potential for the facile in situ preparation of such active cooperative species simply by the addition of base, without the need for strong reducing agents such as alkali metals. In addition to the cooperative binding of CO2, the anionic 2-amino/2-imino-methyl pyridine-based complexes show likewise the activation of further polarized multiple bonds such as aldehydes, ketones, nitriles and SO2. All activated carbonyl groups also show reversibility of the newly formed C-C and M-O (M = Re, Mn) bonds in exchange reactions of the activated substrate. The tridentate neutral Rh(I)-π complex with the newly developed ligand framework (dbap-py) shows, along with the activation of CO2, a cooperative binding of phenylisocyanate. The focus of the work is on the simplicity of the cooperative ligand frameworks, as well as the extended reactivity scope due to the direct interaction of the counterion with the activated substrate.