Unexpected nitrosyl-group bending in six-coordinate {M(NO)}6 σ-bonded aryl(iron) and -(ruthenium) porphyrins

The six-coordinate nitrosyl sigma -bonded aryl(iron) and (ruthenium) porphyrin complexes (OEP)Fe-(NO)(p-C6H4F) and (OEP)Ru(NO)(p-C6H4F) (OEP octaethylporphyrinato dianion) have been synthesized and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting of the MNO group for both (MNO)(6) species as well as significant lengthening of trans axial bond distances. In (OEP)Fe(NO)(p-C6H4F) the Fe-N-O angle is 157.4(2)degrees, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2 degrees, Fe-N(NO) 1.728(2) Angstrom, and Fe-C(aryl) 2.040(3) Angstrom. In (OEP)Ru (NO)(p-C6H4F) the Ru-N-O angle is 154.9(3)degrees, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8 degrees, Ru-N(NO) 1.807(3) Angstrom, and Ru-C(aryl) = 2.111(3) Angstrom. We show that these structural features are intrinsic to the molecules and are imposed by the strongly a-donating aryl ligand trans to the nitrosyl. Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe(NO)(p-C6H4F) and, combined with the results of extended Huckel calculations, show that the observed bending and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO pi -bonding as measured by infrared and Fe-57 Mossbauer spectroscopies. The solid-state nitrosyl stretching frequencies for the iron complex (1791 cm(-1)) and the ruthenium complex (1773 cm(-1)) are significantly reduced compared to their respective (MNO)(6) counterparts. The Mossbauer data for (OEP)Fe(NO)(p-C6H4F) yield the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins,