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

The six-coordinate nitrosyl sigma -bonded aryl(iron) and (ruthenium) porphy rin complexes (OEP)Fe-(NO)(p-C6H4F) and (OEP)Ru(NO)(p-C6H4F) (OEP octaethyl porphyrinato dianion) have been synthesized and characterized. Single-cryst al X-ray structure determinations reveal an unprecedented bending and tilti ng of the MNO group for both (MNO)(6) species as well as significant length ening of trans axial bond distances. In (OEP)Fe(NO)(p-C6H4F) the Fe-N-O ang le is 157.4(2)degrees, the nitrosyl nitrogen atom is tilted off of the norm al to the heme plane by 9.2 degrees, Fe-N(NO) 1.728(2) Angstrom, and Fe-C(a ryl) 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 1 0.8 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 iden tified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO pi -bondi ng as measured by infrared and Fe-57 Mossbauer spectroscopies. The solid-st ate nitrosyl stretching frequencies for the iron complex (1791 cm(-1)) and the ruthenium complex (1773 cm(-1)) are significantly reduced compared to t heir 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 porphy rins,