Premium
Stabilizing the Boat Conformation of Piperazines Coordinated to Iron(II): iso ‐Butyl Substituents Lead to Robust Oxidation Catalysts via Hyperconjugation
Author(s) -
Ostermeier Marc,
Limberg Christian,
Herwig Christian,
Ziemer Burkhard
Publication year - 2009
Publication title -
zeitschrift für anorganische und allgemeine chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.354
H-Index - 66
eISSN - 1521-3749
pISSN - 0044-2313
DOI - 10.1002/zaac.200900275
Subject(s) - piperazine , chemistry , ligand (biochemistry) , ring (chemistry) , cyclohexane conformation , crystallography , stereochemistry , lone pair , crystal structure , molecule , organic chemistry , receptor , hydrogen bond , biochemistry
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‐pyridyl‐methyl)piperazine, 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 2 i Bu BPMP with FeCl 2 and Fe(OTf) 2 indeed led to the desired complexes( 2 i Bu BPMP)Fe X 2 ( 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 conformation with carbon atoms at the tips of the boat since repulsive interactions between the lone‐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 H 2 O 2 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.