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

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 OCH3 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 NNO−Fe−NIm angle caused by a concerted tilting of the axial Fe−NNO and Fe−NIm bonds.
Structural features such as the displacement of Fe out of the mean porphyrin plane toward NO, tilting of the
Fe−NNO bond off the heme normal, and the asymmetry of the Fe−Npor bonds further strengthen and confirm
observations from earlier studies. The [Fe(TpXPP)(NO)(1-MeIm)] complexes were also studied at temperatures
between 125 and 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 are 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−NIm, was also found to be sensitive to temperature variation. The Fe−NIm bond length
increases with increased temperature, whereas no other bonds change appreciably. The temperature-dependent
Fe−NIm bond length change and cell volume changes are consistent with a “soft” Fe−NIm bond. Variable-temperature
measurements show that the N−O stretching frequency changes with the Fe−NIm bond length. Temperature-dependent changes in the Fe−NIm bond length and N−O stretching frequency were also found to be completely
reversible with no apparent hysteresis.