Electronic Modification of a Sterically Demanding Anionic Pyridine Ligand

Abstract

AbstractThe sterically demanding anionic pyridine ligand [4‐(Ph3B)‐2,6‐Mes2py]−([1]−) was used in salt metathesis reactions in order to obtain main group and transition metal complexes. Instead, reduction reactions were observed and identified as single electron transfer processes from [1]−to the respective Lewis acidic cations. The electron transfer was confirmed by reduction of the trityl cation. In order to reduce the reductive power of [1]−the borate function of the ligand was substituted with strongly electron withdrawing 3,5‐bis(trifluoromethyl)phenyl groups to give [1F]−. The redox potential was shifted from +0.56 V to +1.32 V and hence, [1F]−cannot reduce the trityl cation in contrast to [1]−. Consequently, the measured pKavalue of1F‐H (16.44) was lowered by two pKaunits compared with1‐H. Unfortunately, [1F]−still undergoes single electron transfer processes when used in salt metathesis reactions. We synthesized the anionic borane adduct [Li(THF)4][1F‐BH3] which demonstrates that [1F]−can be used as a ligand in main group chemistry. The determination of the crystal structures of [Li(THF)4][1F] and [Li(THF)4][1F‐BH3] revealed separated ion pairs while their analogs with the more electron rich [1]−are contact ion pairs.