Stabilising the Boat Conformation of Piperazines Coordinated to Iron(II): i Butyl Substituents Lead to Robust Oxidation Catalysts Via Hyperconjugation
Résumé
A known route to diastereomerically pure diketopiperazines starting from natural amino acids was utilised for the preparation of a chiral, 2,5-di-i butyl substituted 1,4-bis(2-pyridyl-methyl)piperazine, which can serve as a ligand in metal complexes. While it should be possible to extend the procedure described also to derivatives with other amino acid-derived substituents in the 2,5 positions, here two bulky i butyl residues were chosen with the aim of stabilising a tetradentate coordination mode at FeII centres thus avoiding coordination polymers. Reaction of the corresponding ligand 2i BuBPMP with FeCl2 and Fe(OTf)2 indeed led to the desired complexes (2i BuBPMP)FeX2 (X = Cl, OTf) with the piperazine ring in a boat conformation, as confirmed by single crystal X-ray diffraction analysis. However, unexpectedly the bulky i butyl residues were found in the axial positions. To explain this phenomenon DFT calculations were carried out. The results showed that in the most stable conformations the ligand is preorganised to adopt the observed conformation with the axial i butyl residues after complexation with iron(II). Moreover, it turned out that the resulting complex structures are also favoured thermodynamically over the corresponding ones with the residues in equatorial positions. Having thus stabilised the tetrapodal binding of a BPMP ligand to single iron centres, the potential of the complexes as catalysts for the oxidation of hydrocarbons with H2O2 was tested employing cyclooctene as a representative example. It turned out that the two complexes mentioned above show similar (moderate) activities which are comparable to those displayed by corresponding complexes of other polydentate pyridylmethyl-amino ligands. Interestingly, the two complexes described here are superior to an Fe(OTf)2 complex of the unsubstituted ligand.
Origine : Fichiers produits par l'(les) auteur(s)