Structurally Modelling the 2-His-1-Carboxylate Facial Triad with a Bulky N,N,O Phenolate Ligand

Abstract

We present the synthesis and coordination chemistry of a bulky, tripodal N,N,O ligand, ImPh2NNOtBu (L), designed to model the 2-His-1-carboxylate facial triad (2H1C) by means of two imidazole groups and an anionic 2,4-di-tert-butyl-subtituted phenolate. Reacting K-L with MCl2 (M = Fe, Zn) affords the isostructural, tetrahedral non-heme complexes [Fe(L)(Cl)] (1) and [Zn(L)(Cl)] (2) in high yield. The tridentate N,N,O ligand coordination observed in their X-ray crystal structures remains intact and well-defined in MeCN and CH2Cl2 solution. Reacting 2 with NaSPh affords a tetrahedral zinc thiolate complex, [Zn(L)(SPh)] (4), that is relevant to isopenicillin N synthase (IPNS) biomimicry. Cyclic voltammetry studies demonstrate the ligand's redox non-innocence, where phenolate oxidation is the first electrochemical response observed in K-L, 2 and 4. However, the first electrochemical oxidation in 1 is iron-centred, the assignment of which is supported by DFT calculations. Overall, ImPh2NNOtBu provides access to well-defined mononuclear, monoligated, N,N,O-bound metal complexes, enabling more accurate structural modelling of the 2H1C to be achieved.