Copper complexes containing o-phenylenediamine-based pentadentate ligands catalyze TEMPO-mediated alcohol oxidation

Abstract

A set of o-phenylenediamine-based pentadentate ligands having pyridine and quinoline binding sites was prepared in this work. The weak metal-binding abilities of anilinic nitrogen atoms and rigid chelate structure of the o-phenylenediamine skeleton serve as unique metal coordination properties of the ligand library presented herein. In addition to variations in the pyridine and quinoline binding sites, the non-coordinating alkyl groups (CH<inf>3</inf> or CH<inf>2</inf>Ph) in the ligand structure cause significant differences in the coordination structures of copper(II) complexes. In the aerobic alcohol oxidation reaction in the presence of CuBr and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), the ligand donating fewer electrons, namely, N-benzyl-N,N’,N’-tris(2-quinolylmethyl)-1,2-phenylenediamine (Bn-TQPHEN, L4), exhibited greater activity than N-methyl-N,N’,N’-tris(2-pyridylmethyl)-1,2-phenylenediamine (Me-TPPHEN, L1). Thus, the present study proposes future directions for the utilization of pentadentate ligands and their relevance to redox-active copper metalloenzymes.