Synthesis and Reactivity of Iron(II) Complexes with a New Tripodal Imine Ligand

Abstract

A new tripodal imine ligand tris(2-(propan-2ylideneamino)ethyl)amine (imine 3 tren) was prepared in order to stabilize high valent iron-oxido complexes. Iron complexes were synthesized in template reactions from iron(II) salts, tris(2-aminoethyl)amine (tren) and acetone. Due to the reversibility of the imine formation, complexes with different ligands were obtained depending on the reaction conditions. Three complexes, [Fe(imine 3 tren)(OAc) 2 ] (1), [Fe(imine 3 tren) (OAc)]OTf (2) and [(imine 3 tren) 2 Fe 2 (F) 2 ](SbF 6) 2 (3), could be synthesized and structurally characterized. However, reactions with hydrogen peroxide, iodosobenzene or ozone did not lead to any kind of "oxygen adduct" complex that could be spectroscopically observed.

Investigations of metal complexes with the tripodal tetradentate ligand tris(2‐(propan‐2‐ylideneamino)ethyl)amine (imine3tren)

“…The molecular structure of 4 is similar to the corresponding iron complex [Fe (imine 3 tren)(OAc)]OTf reported previously. [18] There is nothing unusual about the UV-vis spectrum of 4 and the cyclic voltammogram is complex as expected (Supporting Information).…”

“…Prior to the successful preparation described herein by applying the free ligand all our attempts to isolate a nickel complex with this ligand had ended with the crystallization of nickel tren complexes instead. [18,19] Furthermore, we did not succeed to crystallize nickel(II) imine 3 tren complexes with other anions. The molecular structure of 4 is similar to the corresponding iron complex [Fe (imine 3 tren)(OAc)]OTf reported previously.…”

“…Previously, imine 3 tren was synthesized in a template reaction with different iron salts, however, we did not succeed to obtain the free ligand that way. [18] While it was possible to prepare and structurally characterize some iron(II) complexes such as [Fe(imine 3 tren)(OAc) 2 ], different attempts to optimize the reaction with regard to higher yields failed and side reactions turned out to be a big problem. However, these iron complexes did not show any reactivity towards dioxygen.…”

“…An additional problem were the limited yields and side reactions that led to the formation of iron complexes with a macrocyclic ligand (formed during the template reaction). [18,19] During our efforts to obtain pure iron complexes in better yields with this ligand and different iron salts, we synthesized a sodium complex instead. Later on we obtained suitable crystals of [Na(imine 3 tren)]BPh 4 (1) for XRD characterization and the molecular structure of the cation of this complex is presented in Figure 1 (crystallographic data are reported in the Supporting Information) which is coordinated in a distorted trigonal bipyramidal geometry.…”

“…Finally, after many attempts (over years) it was possible to obtain the ligand imine 3 tren in nearly pure form that allowed us to synthesize and structurally characterize zinc, nickel and two copper(I) complexes with this ligand. While the zinc and the nickel complex as redox inactive complexes have been prepared to investigate the coordination behavior with this ligand additionally to iron complexes reported previously, [18] the copper(I) complexes were synthesized to investigate their reactivity towards dioxygen. With a ligand that can be classified in between the pure aliphatic tren alkyl ligands and the guanidine derivatives we had hoped to obtain a copper complex that would form a quite stable but still reactive end-on superoxido complex.…”