The first nitrosyl porphyrin characterized was the {CoNO} 8 derivative, [Co(TPP)(NO)], 15 followed by the {FeNO} 7 complex, [Fe(TPP)(NO)]. 16 An undergraduate, Mark Frisse, developed simple anaerobic techniques necessary for

The preparation and molecular structures of several five-coordinate (nitrosyl)iron(II) porphyrinate derivatives are described. The derivatives reported include two crystalline forms of [Fe(OEP)(NO)] (OEP ) 2, 3,7,8,12,13,17,18-octaethylporphyrin dianion), three conformationally distinct forms of [Fe(TPPBr 4 )(NO)] (TPPBr 4 ) 2, 3,12,13-tetrabromo-5,10,15,20-tetraphenylporphyrin dianion), and [Fe(oxoOEC)(NO)] (oxoOEC ) 3,3,7,8,12,13,17,18-octaethyl-3H-porphin-2-onato(2-) dianion). These complexes differ in the nature and position of the β-pyrrolic and meso substituents, and in the conformation adopted by the porphyrinato cores in the crystalline state. For one form of [Fe(OEP)(NO)], the structure was determined at three temperatures (130(2), 213(2), and 293(2) K). For two of the structures the X-ray data were collected to exceedingly high resolution. For each structure, we observed a bent FeNO group (Fe-N-O angles ranging from 142.74(8)°to 147.9(8)°) and a significant off-axis tilt of the nitrosyl group irrespective of the nature and the conformation of the macrocycle. The tilt of the Fe-N(NO) vector from the heme normal ranges from 5.6 to 8.2°. In all cases, the off-axis tilt of the nitrosyl has an effect on the equatorial Fe-N p bond distances, leading to an asymmetric interaction of the iron atom with the porphyrinato nitrogen atoms. The structural distortion in the strongly bonding axial nitrosyl ligand appears to be intrinsic and supported only by bonding effects. The presence of a tilt/asymmetry in all ordered five-coordinate (nitrosyl)iron(II) porphyrinate derivatives strongly supports this as an intrinsic structural feature of the total bonding interaction in the five-coordinate complex.

The synthesis and characterization of six new high-spin deoxymyoglobin models (imidazole(tetraarylporphyrinato)iron(II)) are described. These have been intensively studied by temperature-dependent Mossbauer spectroscopy from 295 to 4.2 K. All complexes show a strong temperature dependence for the quadrupole splitting consistent with low-lying excited states of the same or lower multiplicity. An analysis of the data obtained in applied magnetic fields leads to the assignment of the sign of the quadrupole splitting. All model compounds as well as those of deoxymyoglobin and deoxyhemoglobin, previously studied, have a negative sign for the quadrupole splitting. Although not previously predicted, this experimental observation leads to the assignment of the ground-state electronic configuration for all high-spin imidazole-ligated iron(II) porphyrinates as (d(xz)())(2)(d(yz)())(1)(d(xy)())(1)(d(z)()()2)(1)(d(x)()()2(-)(y)()()2)(1). This is a distinctly different ground-state electronic configuration from other high-spin iron(II) porphyrinates; differences in structural details for the two classes of high-spin complexes are also discussed. The apparent anomaly of differing signs for the zero-field splitting constant between previously studied model complexes and the heme proteins is addressed; the difference appears to result from the fact that the assumptions used in the spin Hamiltonian approach that has been applied to these complexes are not adequately satisfied. Structures of four of the new five-coordinate species have been determined. Core conformations in these derivatives show variation, but these and previously studied compounds reveal a limited number of conformational patterns. The bond lengths and other geometrical parameters such as porphyrin core size and iron out-of-plane displacement support a high-spin state assignment for the iron(II).

We use nuclear resonance vibrational spectroscopy and computational predictions based on density functional theory (DFT) to explore the vibrational dynamics of (57)Fe in porphyrins that mimic the active sites of histidine-ligated heme proteins complexed with carbon monoxide. Nuclear resonance vibrational spectroscopy yields the complete vibrational spectrum of a Mössbauer isotope, and provides a valuable probe that is not only selective for protein active sites but quantifies the mean-squared amplitude and direction of the motion of the probe nucleus, in addition to vibrational frequencies. Quantitative comparison of the experimental results with DFT calculations provides a detailed, rigorous test of the vibrational predictions, which in turn provide a reliable description of the observed vibrational features. In addition to the well-studied stretching vibration of the Fe-CO bond, vibrations involving the Fe-imidazole bond, and the Fe-N(pyr) bonds to the pyrrole nitrogens of the porphyrin contribute prominently to the observed experimental signal. All of these frequencies show structural sensitivity to the corresponding bond lengths, but previous studies have failed to identify the latter vibrations, presumably because the coupling to the electronic excitation is too small in resonance Raman measurements. We also observe the FeCO bending vibrations, which are not Raman active for these unhindered model compounds. The observed Fe amplitude is strongly inconsistent with three-body oscillator descriptions of the FeCO fragment, but agrees quantitatively with DFT predictions. Over the past decade, quantum chemical calculations have suggested revised estimates of the importance of steric distortion of the bound CO in preventing poisoning of heme proteins by carbon monoxide. Quantitative agreement with the predicted frequency, amplitude, and direction of Fe motion for the FeCO bending vibrations provides direct experimental support for the quantum chemical description of the energetics of the FeCO unit.