Synthesis and characterization of μ-nitrido, μ-carbido and μ-oxo dimers of iron octapropylporphyrazine

Abstract

Three μ-X bridged diiron octapropylporphyrazine complexes having Fe^III–O–Fe^III, Fe^+3.5–NFe^+3.5 and Fe^IVCFe^IV structural units have been prepared and characterized by UV-vis, EPR, X-ray absorption spectroscopy and electrochemical methods. Single crystals of all the complexes were obtained from benzene-acetonitrile and their structures were determined by X-ray diffraction. In contrast to μ-oxo complex (6), μ-nitrido (7) and μ-carbido (8) dimers crystallized with one benzene molecule per two binuclear complex molecules arranged cofacially to the porphyrazine planes at Fe–C_benzene distances of 3.435–3.725 Å and 3.352–3.669 Å for 7 and 8, respectively. The short distances suggest an interaction between the iron sites and the benzene π-system which is stronger in the case of the Fe^IVCFe^IV unit with a higher Lewis acidity. The Fe–X–Fe angle increases in the sequence 6–7–8 from 158.52° to 168.5° and 175.10°, respectively, in agreement with the Fe–X bond order. However, the lengths of the Fe–X bonds do not follow this trend: Fe–O = 1.75/1.76 Å > Fe–C = 1.67/1.67 Å > Fe–N = 1.65/1.66 Å indicating unexpectedly long Fe–C bonds. This observation can be explained by back π-donation from the μ-carbido ligand to the Fe–C antibonding orbital thus decreasing the bond order which is confirmed by DFT calculations.