One-and Two-Electron Coordinatively-Induced Reduction of N-Heterocycles by Divalent Rare Earth Terphenyl Anilide Complexes

Abstract

Molecular rare-earth (RE) complexes in the divalent oxidation state have been used as potent one-electron reductants, yielding bonding motifs not accessible elsewhere in the periodic table. A deeper understanding of the chemical properties which dictate their reducing power is required to tame their reduction potentials and unlock new controlled reactivity. Here, we report the electrochemical characterisation of a series of divalent bis-(tethered)arene complexes, [M II (NHAr iPr6 )2] (1M, M = Y, La, Sm, Eu, Yb, Tm, Lu; NHAr iPr6 = {N(H)C6H3-2,6-(C6H2-2,4,6-iPr3)2}), and their reactivity towards the nitrogen heterocycles pyridine and 4,4′-bipyridine. Complexes 1Y, 1La, 1Tm, and 1Lu reduce 4,4′-bipyridine and reductively couple pyridine to afford the dimeric [{M III (NHAr iPr6 )2}2(µ-N2C10H8)] (2M, M = Y, La, Tm, Lu) and [{M III (NHAr iPr6 )2}2(µ-N2C10H10)] (3M, M = Y, La, Tm, Lu) respectively. These results contrast with electrochemical measurements, which place the RE(III/II) reduction potentials for all 1M complexes as insufficiently reducing for either heterocycle. These results imply the reactions proceed through coordination-induced reduction. Synthesis of 2M complexes also results in the formation of 4,4′-bipyridine radical species, which are probed by NMR, EPR, and UV-Vis-NIR spectroscopies.