Electronic Modification of a Sterically Demanding Anionic Pyridine Ligand

The sterically demanding anionic pyridine ligand [4-(Ph3B)-2,6-Mes(2)py](-) ([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 [1(F)](-). The redox potential was shifted from +0.56 V to +1.32 V and hence, [1(F)](-) cannot reduce the trityl cation in contrast to [1](-). Consequently, the measured pK(a) value of 1(F)-H (16.44) was lowered by two pK(a) units compared with 1-H. Unfortunately, [1(F)](-) still undergoes single electron transfer processes when used in salt metathesis reactions. We synthesized the anionic borane adduct [Li(THF)(4)][1(F)-BH3] which demonstrates that [1(F)](-) can be used as a ligand in main group chemistry. The determination of the crystal structures of [Li(THF)(4)][1(F)] and [Li(THF)(4)][1(F)-BH3] revealed separated ion pairs while their analogs with the more electron rich [1](-) are contact ion pairs.