Palladium complexes bearing calixpyrrole ligands with pendant hydrogen bond donors: Synthesis, structural characterization, electrochemistry and dihydrogen evolution

Abstract

A series of three calixpyrrole ligands (1a-c) with pendant nitrogen-based hydrogen bond donors, R, were synthesized and coordinated to palladium to produce metal complexes (2a-c), where R = NH2 (a); NHC(O)CH3 (b); or NHC(O)OC(CH3)3 (c). The calixpyrrole compounds were generated using a Schiff-base reaction starting with 5,5′-diformyl-2,2′-diphenyldipyrromethane and an aniline precursor. The deprotonated calixpyrrole species were subsequently bound to palladium to generate distorted square planar complexes. The pendant groups were not coordinated to the metal with Pd-N distances of 3.36 to 4.79 Å. The electrochemical properties of the palladium complexes were also explored, and 2a-c displayed two irreversible oxidations above 0.0 V vs ferrocene/ferrocenium (Fc/Fc+), as well as two irreversible reductions below −0.70 V vs Fc/Fc+. Interestingly, the free ligands showed similar electrochemical features, suggesting redox non-innocence. Preliminary reactivity studies indicated the palladium complexes did not activate small molecules, but they did catalyze H2 evolution in the presence of acid. The onset of catalysis for 2a-c was approximately 0.4–0.5 V more positive than a glassy carbon electrode, and the active species could undergo 500 scans without significant changes in activity. The catalysts were found to be heterogeneous in nature and adsorbed onto the working electrode.