Ligands tris(2-pyridylmethyl)amine with nitrile group in - substituted and the corresponding FeCl2 complexes: a diversity preceding a possible using of dioxygen in soft conditions

Abstract

We have synthesized the mono, bis and tris nitrile at the  position ligands in the tris(2-pyridylmethyl)amine (TPA) series, respectively CNTPA, (CN)2TPA and (CN)3TPA. Nitrile at the  position of these nitrogen- containing tripods shifts the oxidation potential of the ligand by comparison with the TPA ligand, it is observed that the oxidation potentials of the ligands increase regularly with the number of nitrile groups. For each group added, the oxidation potential is increased by an average of 110mV / ECS. This reflects the strong electron- withdrawing effect of the nitrile groups.. The crystal structure analysis of the dichloroferrous complexes with CNTPA and (CN)2TPA reveals that the iron lies in a distorted octahedral geometry comparable to that already ^{found in TPAFeCl}2^{. All spectroscopic data indicate that the geometry is retained in solution. These three} isostructural complexes all react with molecular dioxygen to yield µ-oxo diferric complexes as stable compounds. Upon reduction, all µ-oxo diferric complexes convert back into the starting ferrous species. The oxygenation reaction parallels the well known formation of µ-oxo derivatives from dioxygen in chemistry of porphyrins reported almost three decades ago. The crystal structure analysis is reported for each of these three µ-oxo compounds. With TPA, a symmetrical structure is obtained for a dicationic compound, the tripod coordinating in the (⁴-N) coordination mode. With CNTPA, the compound is a neutral µ-oxo di-ferric complex. When oxygenation is carried out on the (CN)2TPA complex, a neutral asymmetrical compound is obtained. The striking point within the series of ferrous precursors is the effect of the ligand on the kinetics of oxygenation of the complexes. Whereas the parent complex undergo full oxygenation over 14 hours, the mono nitrile ligand provides a complex that has fully reacted over 14 hours, the reaction time being only 2 hours for the complex with the dinitriles ligand,and for trinitriles ligand it will tack 30min. Thus electron-deficiency ( Lewis acidity at the metal centre) seems to govern the kinetics of oxygenation. This strongly suggests coordination of the dioxygen as an initial step in the process leading to formation of µ-oxo diferric compounds, by contrast with un unlikely outer sphere reduction of dioxygen, that generally occurs at very negative potentials. Finally, we report that cyclohexane is catalytically converted into cyclohexanone when reacted with dioxygen in the presence of zinc amalgam, and that the conversion rate also parallels the Lewis acidity at the metal center, with the best rate obtained for with ^(CN)2TPAFeCl2^.