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

Three μ-X bridged diiron octapropylporphyrazine complexes having Fe(III)-O-Fe(III), Fe(+3.5)-N[double bond, length as m-dash]Fe(+3.5) and Fe(IV)[double bond, length as m-dash]C[double bond, length as m-dash]Fe(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 (), μ-nitrido () and μ-carbido () dimers crystallized with one benzene molecule per two binuclear complex molecules arranged cofacially to the porphyrazine planes at Fe-Cbenzene distances of 3.435-3.725 Å and 3.352-3.669 Å for and , 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(IV)[double bond, length as m-dash]C[double bond, length as m-dash]Fe(IV) unit with a higher Lewis acidity. The Fe-X-Fe angle increases in the sequence -- 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.