Electronic Modification of a Sterically Demanding Anionic Pyridine Ligand

Abstract

Dedicated to Professor Cameron Jones on the occasion of his 60 th birthdayThe sterically demanding anionic pyridine ligand [4-(Ph 3 B)-2,6-Mes 2 py] À ([1] À ) 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] À 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] À the borate function of the ligand was substituted with strongly electron withdrawing 3,5-bis(trifluoromethyl)phenyl groups to give [1 F ] À . The redox potential was shifted from + 0.56 V to + 1.32 V and hence, [1 F ] À cannot reduce the trityl cation in contrast to [1] À . 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 ] À still undergoes single electron transfer processes when used in salt metathesis reactions. We synthesized the anionic borane adduct [Li(THF) 4 ][1 F -BH 3 ] which demonstrates that [1 F ] À 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 -BH 3 ] revealed separated ion pairs while their analogs with the more electron rich [1] À are contact ion pairs.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor–Acceptor Systems in Main Group Chemistry

“…The reduction of metals by pyridine scaffolds was observed by Boẗtcher et al during their attempts of coordinating paraborane-substituted pyridines 31 and 31 F (Scheme 10C) to metal centers. 54 To study the SET process in more detail, they mixed 31 with the trityl cation (CPh 3 + ) in DCM. The obtained products were the protonated pyridine 33 and Gomberg's dimer, 34.…”

Electron-rich pyridines with para-N-heterocyclic imine substituents: ligand properties and coordination to CO2, SO2, BCl3 and PdII complexes

“…The attempt to prepare transition metal complexes with VII was unsuccessful due to single electron transfer processes from the electron-rich pyridine. 31,32 2,3,5,6-Tetrakis(tetramethylguanidino)pyridine also readily undergoes electron transfer processes, yet Cu I complexes have been prepared. However, the complexation of the Cu I ions occurs via the guanidino rather than the pyridine donor atoms.…”