Stabilizing the Boat Conformation of Piperazines Coordinated to Iron(Ii): Iso-Butyl Substituents Lead to Robust Oxidation Catalysts Via Hyperconjugation

A known route to diastereomerically pure diketopiperazines starting from natural amino acids was utilized for the preparation of a chiral, 2,5-di-iso-butyl-substituted 1,4-bis(2-pyridyi-metllyl)pipera-zine, which can serve as a ligand in metal complexes. Whereas 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 iso-butyl residues were chosen with the aim of stabilizing a tetradentate coordination mode at Fe-II atoms thus avoiding coordination polymers. Reaction of the corresponding ligand (BPMP)-B-2iBu with FeCl2 and Fe(OTf)(2) indeed led to the desired complexes ((BPMP)-B-2iBu)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 iso-butyl residues were found in the axial positions. To explain this phenomenon DFT calculations were carried out. The results showed that the most stable conformations of the free ligand contain the piperazine ring in a boat confirmation with carbon atoms at the tips of the boat since repulsive interactions between the tone-pairs at the nitrogen atoms are avoided thereby. This way the ligand is preorganized to adopt the observed conformation with the axial iso-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 stabilized the tetrapodal binding of a BPMP ligand to single iron atoms, 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 herein are superior to an Fe(OTf)(2) complex of the unsubstituted ligand.