Intrinsic structural distortions in five-coordinate (nitrosyl)iron(II) porphyrinate derivatives

The preparation and molecular structures of several five-coordinate (nitrosyl)iron(II) porphyrinate derivatives are described. The derivatives reported include two crystalline forms of [Fe(OEP)(NO)] (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrin dianion), three conformationally distinct forms of [Fe(TPPBr4)(NO)] (TPPBr4 = 2,3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrin dianion), and [Fe(oxoOEC)(NO)] (oxoOEC 3,3,7,8,12,13,17,18-octaethyl-3H-porphin-2-onato(2-) dianion). These complexes differ in the nature and position of the beta-pyrrolic and meso substituents, and in the conformation adopted by the porphyrinato cores in the crystalline state. For one form of [Fe(OEP)(NO)], the structure was determined at three temperatures (130(2), 213(2), and 293(2) K). For two of the structures the X-ray data were collected to exceedingly high resolution. For each structure, we observed a bent FeNO group (Fe-N-O angles ranging from 142.74(8)degrees to 147.9(8)degrees) and a significant off-axis tilt of the nitrosyl group irrespective of the nature and the conformation of the macrocycle. The tilt of the Fe-N(NO) vector from the heme normal ranges from 5.6 to 8.2 degrees. In all cases, the off-axis tilt of the nitrosyl has an effect on the equatorial Fe-N-p bond distances, leading to an asymmetric interaction of the iron atom with the porphyrinato nitrogen atoms. The structural distortion in the strongly bonding axial nitrosyl ligand appears to be intrinsic and supported only by bonding effects. The presence of a tilt/asymmetry in all ordered five-coordinate (nitrosyl)iron(II) porphyrinate derivatives strongly supports this as an intrinsic structural feature of the total bonding interaction in the five-coordinate complex.