353523-93-0Relevant academic research and scientific papers
Unexpected nitrosyl-group bending in six-coordinate {M(NO)}6 σ-bonded aryl(iron) and -(ruthenium) porphyrins
Richter-Addo,Wheeler,Hixson,Chen,Khan,Ellison,Schulz,Scheidt
, p. 6314 - 6326 (2001)
The six-coordinate nitrosyl σ-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)°, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2°, Fe-N(NO) = 1.728(2) A, and Fe-C(aryl) = 2.040(3) A. In (OEP)Ru-(NO)(p-C6H4F) the Ru-N-O angle is 154.9(3)°, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8°, Ru-N(NO) = 1.807(3) A, and Ru-C(aryl) = 2.111(3) A. We show that these structural features are intrinsic to the molecules and are imposed by the strongly σ-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 Hueckel 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 π-bonding as measured by infrared and 57Fe Moessbauer 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 Moessbauer 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.
