Reversible NO Motion in Crystalline [Fe(Porph)(1-MeIm)(NO)] Derivatives

Abstract

The synthesis, characterization, and X-ray structures of three low-spin (nitrosyl)iron(II) tetraarylporphyrinates, [Fe(TpXPP)(NO)(1-MeIm)], where X = F (in a triclinic and a monoclinic form) and OCH 3 are reported. All three molecules, at 100 K, have a single orientation of NO. These structures are the first examples of ordered NO's in [Fe(Porph)(NO)(1-MeIm)] complexes. The three new derivatives have similar structural features including a previously unnoted "bowing" of the N NO -Fe-N Im angle caused by a concerted tilting of the axial Fe-N NO and Fe-N Im bonds. Structural features such as the displacement of Fe out of the mean porphyrin plane toward NO, tilting of the Fe-N NO bond off the heme normal, and the asymmetry of the Fe-N por bonds further strengthen and confirm observations from earlier studies. The [Fe(TpXPP)(NO)(1-MeIm)] complexes were also studied at temperatures between 125-350 K to investigate temperature-dependent variations and trends in the coordination group geometry. At varying temperatures (above 150 K), all three derivatives display a second orientation of the NO ligand. The population and depopulation of this second orientation is thermally driven, with no apparent hysteresis. Crystal packing appears to be the significant feature in defining the order/disorder of the NO ligand. The length of the bond trans to NO, Fe-N Im , was also found to be sensitive to temperature variation. The Fe-N Im bond length increases with increased temperature, whereas no other bonds change appreciably. The T-dependent Fe-N Im bond length change and cell volume changes are consistent with a "soft" Fe-N Im bond. Variable temperature measurements show that the N-O stretching frequency changes with the Fe-N Im bond length. Temperature-dependent changes in the Fe-N Im bond length and N-O stretching frequency were also found to be completely reversible with no apparent hysteresis.

Nuclear Resonance Vibrational Spectroscopy Applied to [Fe(OEP)(NO)]: The Vibrational Assignments of Five-Coordinate Ferrous Heme−Nitrosyls and Implications for Electronic Structure

“…In form I (Scheme 2, left), 5 neighboring ethyl groups of the macrocycle point to one face of the porphyrin, whereas the remaining 3 point in the opposite direction. In addition to this complication, the bound NO ligand can rotate around the Fe-NO bond, and occupy four basically isoenergetic positions on each side of the porphyrin ring where the oxygen atom of NO is placed between two adjacent nitrogen atoms of the macrocycle 19. Since the two faces of the porphyrin ring are not equivalent, form I can potentially give rise to eight different conformers (A – H) as shown in Figure 1.…”

“…This is evident from the solid state structures on these compounds which frequently show disorder in the NO orientation (see, for example, ref. 19). Our calculations show that the conformation adopted by the NO ligand relative to the ethyl substituents has a distinct effect on the NRVS data in the region of Fe-porphyrin core vibrations, whereas the electronic structure of the FeNO subunit is hardly affected by changes in the conformation.…”

Correlated Ligand Dynamics in Oxyiron Picket Fence Porphyrins: Structural and Mössbauer Investigations

“…Given the interesting ligand dynamics and other issues, we thought that a series of multiple-temperature X-ray studies could provide information on the O 2 ligand rotations, much like those we were able to map out for several six-coordinate nitrosyl iron porphyrinates. 23, 24 In these cases, the lower temperature structures resolved disorder, even though the rotation about the axial Fe–NO bond is a very low barrier process.…”

Explorations in metalloporphyrin stereochemistry, physical properties and beyond

“…This has been further demonstrated in a series of temperature-dependent crystal structures and IR investigation [87]. There are other effects on the structure around iron on the addition of the sixth ligand that have been summarized in references [87][88][89]. We have also characterized additional iron(III) species, espcially six-coordinate species and the original references include the following [90][91][92][93][94].…”

“…These complexes showed that the NO stretching frequency is a remote trans bond detecting system [86]. This has been further demonstrated in a series of temperature-dependent crystal structures and IR investigation [87]. There are other effects on the structure around iron on the addition of the sixth ligand that have been summarized in references [87][88][89].…”