meso-Aryl substituents modify the electrochemical profile and palladium(ii) coordination of redox-active tripyrrin ligands

Abstract

The tripyrrin coordination motif, namely a conjugated tripyrrolic fragment of the porphyrin scaffold, is found in numerous metal complexes of oligopyrrolic macrocycles. Because of their typically limited stability, linear tripyrrins are underutilized in coordination chemistry; however, hexaalkyl-tripyrrindiones featuring the pyrrolinone termini characteristic of biopyrrin pigments have recently emerged as versatile redox-active ligands. Herein, we report the synthesis of the first example of meso-di(pentafluorophenyl) tripyrrin-1,14-dione through the demethylation of a stable 1,14-dimethoxytripyrrin precursor. The two tripyrrin ligands coordinate palladium(II) in square planar geometries in completely distinct ways: the dimethoxytripyrrin forms a cyclopalladate following C(sp³)–H bond activation at one of the methoxy groups, whereas the tripyrrindione binds as a trianionic ligand engaging an adventitious aqua ligand in hydrogen-bonding interactions. When compared to the hexaalkyl substituents of previous tripyrrindiones, the meso-aryl groups significantly alter the electrochemical profile of the Pd(II) tripyrrindione complex, shifting anodically by ∼500 mV the one-electron processes to attain the other redox states of the ligand. The formation of a ligand-based radical on the Pd(II)-bound meso-aryl tripyrrindione is confirmed by spectroelectrochemical and electron paramagnetic resonance (EPR) methods.