Experimental and density functional theoretical investigations of linkage isomerism in six-coordinate {FeNO}6 iron porphyrins with axial nitrosyl and nitro ligands

A critical component of the biological activity of NO and nitrite involves their coordination to the iron center in heme proteins. Irradiation (330 < lambda < 500 nm) of the nitrosyl-nitro compound (TPP)Fe(NO)(NO2) (TPP = tetraphenylporphyrinato dianion) at 11 K results in changes in the IR spectrum associated with both nitro-to-nitrito and nitrosyl-to-isonitrosyl linkage isomerism. Only the nitro-to-nitrito linkage isomer is obtained at 200 K, indicating that the isonitrosyl linkage isomer is less stable than the nitrito linkage isomer. DFT calculations reveal two ground-state conformations of (porphine)Fe(NO)(NO2) that differ in the relative axial ligand orientations (i.e., GS parallel to and GS-perpendicular to). In both conformations, the FeNO group is bent (156.4 degrees for GS parallel to, 159.8 degrees for GS perpendicular to) for this formally {FeNO}(6) compound. Three conformations of the nitrosyl-nitrito isomer (porphine)Fe(NO)(ONO) (MSall, MSa perpendicular to, and MSa(L)) and two conformations of the isonitrosyl-nitro isomer (porphine)Fe(ON)(NO2) (MSbll and MSb perpendicular to) are identified, as are three conformations of the double-linkage isomer (porphine)Fe(ON)(ONO) (MSc parallel to, MSc perpendicular to, MSCL). Only 2 of the 10 optimized geometries contain near-linear FeNO (MSa(L)) and FeON (MSCL) bonds. The energies of the ground-state and isomeric structures increase in the order GS < MSa < MSb < Me. Vibrational frequencies for all of the linkage isomers have been calculated, and the theoretical gas-phase absorption spectrum of (porphine)Fe(NO)(NO2) has been analyzed to obtain information on the electronic transitions responsible for the linkage isomerization. Comparison of the experimental and theoretical IR spectra does not provide evidence for the existence of a double linkage isomer of (TPP)Fe(NO)(NO2).