Hangman dipyrrin complexes

Abstract

Metalloenzymes serve as inspiration for novel molecular catalysts and model systems. While chemists have designed complexes that mimic these biological catalysts, synthetic complexes often do not recapitulate the reactivity of the native enzymes. We hypothesise that the secondary coordination sphere surrounding the metal centre, which is indispensable for enzymatic activity, is the crucial component missing in biomimetic catalysts. To address this issue, we have designed and synthesised hangman dipyrrin complexes that emulate both the primary and secondary coordination sphere of the fungal enzyme polysaccharide monooxygenase by including a pendant phenol above the metal centre. Here, we report the synthesis of Zn(II), Pd(II), Co(III), Ga(III), Fe(III), and Mn(III) complexes, which are characterised by NMR spectroscopy, UV-vis absorption, X-ray crystallography, and cyclic voltammetry. Complementary density functional theory calculations were performed to support spin state assignments. The square planar and octahedral complexes exhibit atropisomerism, which arises from the 2-substituent on the meso aryl ring. This attribute ensures a precise, rigid position of the pendant phenol, akin to a protein active site, which can be controlled by leveraging differences in atropisomer solubility in the case of the Pd(II) phenol complex. We envision that this hangman ligand architecture will overcome the limitations of previous biomimetic catalysts.