Molecular structure of Fe(III) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin perchlorate:: a saddle-shaped porphyrin with an intermediate S=3/2 spin state

Recent crystallographic evidence of nonplanarity of several prosthetic groups in heme proteins has prompted the design of conformationally distorted synthetic porphyrins in order to assess the consequences of the nonplanarity observed in vivo. The crystal structure of the sterically crowded Fe(III) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin perchlorate has been determined and shows the macrocycle to be saddle-shaped with a small degree of ruffling. The combination of severe nonplanarity and of the weak ligand-field counterion results in a nearly pure S = 3/2 intermediate spin state of the Fe(III) as evidenced by its electron paramagnetic resonance spectrum in frozen solution at low temperatures. Published in 2001 by John Wiley & Sons, Ltd.