Amine conformational change and spin conversion induced by metal-assisted ligand oxidation: From the seven-coordinate iron(II)-TPAA complex to the two oxidized iron(II)-(py)₃tren isomers. Characterization, crystal structures, and density functional study

The tripodal heptadentate ligand TPAA (TPAA = tris[N-(2-pyridylmethyl)-2-aminoethyl]amine) is found to undergo an iron(II)-assisted oxidative dehydrogenation with its three amine functions oxidized into three imine groups, giving the tripodal potentially heptadentate ligand (py)3tren ((py)3tren = tris[N-(2-pyridylmethyl)-2-iminoethyl]amine). This oxidative process induces structural changes and spin conversion in the three identified iron(II) complexes, Fe-TPAA complex (1), Fe-py3tren complexes (2) and (3). X-ray crystallographic studies revealed differences in the coordination geometry of the bridging nitrogen atom and shifts in the coordination number from seven to six. Complex 1 is seven-coordinate and is characterized by a pyramidal environment of the tripodal centered nitrogen and a short N(tripodal)-Fe distance equal to 2.504 Å. Complex 3 is hexa-coordinate with a planar environment of the centered nitrogen located at 3.435 Å from iron(II) and therefore not bound to the iron. Complex 2 which is an isomeric form of complex 3 is found to be a structural intermediate between complexes 2 and 3 with a pseudo-coordination of six as shown by the N(tripodal)-Fe distance equal to 2.754 Å. Electronic changes were recognized from NMR studies in solution and EPR and SQUID measurements of susceptibilities in solid state. Complex 1 is high-spin with the S = 2 state characterized by a g factor equal to 2.25. Complex 3 is low-spin but its isomer, the intermediate complex 2 exhibits a temperature-dependent spin conversion from the S = 2 high-spin form at room temperature to a lower spin that we provisionally identify as a S = 1 intermediate spin form. DFT Becke3LYP calculations were carried out on the two isomeric complexes 2 and 3. The planar complex was found to be 3.2 kcal mol-1 more stable than the pyramidal isomer, in agreement with the experiment. (C) 2000 Elsevier Science S.A.