Alkali-Metal Aminotroponiminates: Selectivities and Equilibria in Reversible Radical Coupling of Delocalized π-Electron Systems

Aminotroponiminates (ATIs) have recently been shown to belong to the growing class of redox-active ligands. The choice of the metal center allowed to switch between reversible electron transfer (M=Rh) and reductively induced dimerization (M=Na). Here, we investigate if the reductively induced dimerization of ATIs is a more general phenomenon for their alkali-metal complexes. Lithium ATI complexes are shown to undergo reductively induced dimerizations, which are equilibrium reactions and chemically reversible. The choice of the metal center (Li vs. Na), the sub-stitution pattern at the nitrogen atoms of the ATI ligands, and the solvent critically influence the regioselectivity and diastereoselectivity of the radical-dimerization reactions. Potassium ATIs are shown to be susceptible to side reactions, more specifically a reduction accompanied by hydrogenatom transfer. Products and intermediates of the reductively induced dimerizations were characterized by techniques including NMR and EPR spectroscopy, cyclic voltammetry, DFT calculations, single-crystal X-ray diffraction, and mass spectrometry.