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
Inorganica Chimica Acta(2000)
摘要
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 NtripodalFe 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 NtripodalFe 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.
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关键词
Conformational change,Seven-coordinate,Spin conversion,Intermediate spin,Iron(II) complexes,Density functional calculation
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