Asymmetry and steric hindrance in tripodal ligands: Reaching the limit for octahedral geometry with the newly synthesized [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod in FeCl2 complexes

We have synthesized the fully unsymmetrical [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod FBrTPA. The synthesis involves preparation of the already known [(6-bromo 2-pyridylmethyl) (2-pyridylmethyl)] amine BrDPA, which is obtained either by classical condensation of α-substituted pyridine carboxaldehyde with aminomethyl pyridine, or by a pathway involving the protection/deprotection sequence of this primary amine. This second way is useful for syntheses of monosubstituted DPAs in the cases where α-substituted pyridine carboxaldehydes are unavailable. The crystal structure of the FeCl2 complex shows that the ligand binds in a κ4-N fashion, with however relatively long ligand-to-metal distances. The spectroscopic and electrochemical studies support decoordination of the bromopyridyl substituent in solution, with κ3-N coordination mode of the tripod within a complex displaying a trigonal bipyramidal geometry at the metal centre. This complex reacts easily with dry oxygen to yield an unsymmetrical μ-oxo diferric complex, the structure of which is also reported.

Graphical abstractWe report in this paper the preparation of the fully unsymmetrical [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod, and the structure of the corresponding FeCl2 complex. In solid state, κ4-N coordination of the ligand is observed, whereas uncoordination of a pyridyl unit occurs in solution. The Fe(II) complex reacts with dry oxygen to yield an unsymmetrical μ-oxo diferric complex, the structure of which is also reported.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The fully unsymmetrical [(6-bromo 2-pyridylmethyl) (6-fluoro 2-pyridylmethyl) (2-pyridylmethyl)] amine tripod has been prepared. ► The structure of the corresponding FeCl2 complex is reported. ► In solid state, κ4-N coordination of the ligand is observed, whereas uncoordination of a pyridyl unit occurs solution. ► The Fe(II) complex reacts with dry oxygen to yield an unsymmetrical μ-oxo diferric complex, the structure of which is also reported.