Insight into the conformational selectivity of cobalt(iii) complexes with a tetradentate salen-type Schiff base ligand and their biorelevant catalysis: a combined experimental and theoretical study

Abstract

Salen-based octahedral trans metallo-salen complexes with monodentate co-ligands may exist as several conformational isomers, including stepped and umbrella shaped conformations. However, the factors governing the selective isolation of a particular conformer have not been investigated properly. In this report, six mononuclear cobalt(III) complexes (1–6) with a tetradentate salen-type Schiff base ligand, N,N′-bis(3-methoxy-5-methylsalicylidene)-ethane-1,2-diamine (H₂L), and different heteroaromatic co-ligands (pyridines, imidazole and pyrazole) have been synthesized and characterized to unveil the influence of heteroaromatic co-ligands on the conformational (stepped versus umbrella) selectivity of the resulting complexes. X-ray crystallography reveals that the structures of 1–6 are eventually similar with only differences in the coordinated heteroaromatic moieties at axial positions and the accommodation of a different number of lattice solvent/guest molecules in the crystal voids. Interestingly, although a landscape of intermolecular non-covalent interactions are present in the solid state in these systems, the pyridyl co-ligands exclusively stabilize the umbrella-shaped conformation, while five-membered heteroaromatic bases prefer the stepped-shaped conformation, disclosing that the different degrees of intramolecular interactions (repulsion/attraction) of the six and five membered heteroaromatic ligands at the axial positions play the most important role in the selective stability of the different conformations in the solid state. All these complexes exhibit efficient catalytic activity mimicking the functions of phenoxazinone synthase and catechol oxidase. The catalyst recyclability test was also performed and it showed a very negligible change in catalytic efficiency even after five catalytic cycles. Theoretical calculations were performed to gain in-depth understanding of the preference of pyridine ligands for the umbrella-shaped conformation and the five-membered ring co-ligands for the stepped-shaped one, as well as for the mechanistic investigation of the aerobic oxidation of o-aminophenol in the presence of 1–6 as catalysts.