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

The preparation and molecular structures of several five-coordinate (nitros yl)iron(II) porphyrinate derivatives are described. The derivatives reporte d 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-po rphin-2-onato(2-) dianion). These complexes differ in the nature and positi on of the beta-pyrrolic and meso substituents, and in the conformation adop ted 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 FeN O group (Fe-N-O angles ranging from 142.74(8)degrees to 147.9(8)degrees) an d a significant off-axis tilt of the nitrosyl group irrespective of the nat ure 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 dist ances, leading to an asymmetric interaction of the iron atom with the porph yrinato nitrogen atoms. The structural distortion in the strongly bonding a xial nitrosyl ligand appears to be intrinsic and supported only by bonding effects. The presence of a tilt/asymmetry in all ordered five-coordinate (n itrosyl)iron(II) porphyrinate derivatives strongly supports this as an intr insic structural feature of the total bonding interaction in the five-coord inate complex.