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

摘要

Three mu-X bridged diiron octapropylporphyrazine complexes having Fe-III-O-Fe-III, Fe+3.5-NvFe(+3.5) and Fe-IV=C=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 mu-oxo complex (6), mu-nitrido (7) and mu-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 angstrom and 3.352-3.669 angstrom for 7 and 8, respectively. The short distances suggest an interaction between the iron sites and the benzene pi-system which is stronger in the case of the Fe-IV=C=Fe-IV unit with a higher Lewis acidity. The Fe-X-Fe angle increases in the sequence 6-7-8 from 158.52 degrees to 168.5 degrees and 175.10 degrees, 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 angstrom > Fe-C = 1.67/1.67 angstrom > Fe-N = 1.65/1.66 angstrom indicating unexpectedly long Fe-C bonds. This observation can be explained by back pi-donation from the mu-carbido ligand to the Fe-C antibonding orbital thus decreasing the bond order which is confirmed by DFT calculations.