The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, mono-[Fe(TpFPP)(1MeIm)(NO)], has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe–N– O stretching and bending modes are strongly mixed with each other as well as with porphyrin ligand modes. The stretch is mixed with ν50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe–X–O (X = N, C, and O) complexes is correlated with the Fe–XO bond distances. The nature of highest frequency band at ~560 cm−1 has also been examined in two additional new derivatives. Previously assigned as the Fe–NO stretch (by resonance Raman), it is better described as the bend as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1-MeIm)(NO)] (TpFPP=tetra-para-fluorophenylporphyrin; 1-MeIm=1-methylimidazole), has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe-N-O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe-X-O (X=N, C, and O) complexes is correlated with the Fe-XO bond lengths. The nature of highest frequency band at ≈560 cm(-1) has also been examined in two additional new derivatives. Previously assigned as the Fe-NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

AbstractThe vibrational spectrum of a six‐coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1‐MeIm)(NO)] (TpFPP=tetra‐para‐fluorophenylporphyrin; 1‐MeIm=1‐methylimidazole), has been studied by oriented single‐crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in‐plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in‐plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in‐plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out‐of‐plane Fe‐N‐O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe‐X‐O (X=N, C, and O) complexes is correlated with the Fe−XO bond lengths. The nature of highest frequency band at ≈560 cm−1 has also been examined in two additional new derivatives. Previously assigned as the Fe−NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1-MeIm)(NO)] (TpFPP=tetra-para-fluorophenylporphyrin; 1-MeIm=1-methylimidazole), has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe-N-O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe-X-O (X=N, C, and O) complexes is correlated with the Fe-XO bond lengths. The nature of highest frequency band at ≈560 cm(-1) has also been examined in two additional new derivatives. Previously assigned as the Fe-NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1-MeIm)(NO)] (TpFPP = tetra-para-fluorophenylporphyrin; 1-MeIm=1-methylimidazole), has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe-N-O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v(50) as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe-X-O (X= N, C, and O) complexes is correlated with the Fe XO bond lengths. The nature of highest frequency band at 560 cm(-1) has also been examined in two additional new derivatives. Previously assigned as the Fe NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

The vibrational spectrum of a six‐coordinate nitrosyl iron porphyrinate, monoclinic [Fe(T p FPP)(1‐MeIm)(NO)] (T p FPP=tetra‐ para ‐fluorophenylporphyrin; 1‐MeIm=1‐methylimidazole), has been studied by oriented single‐crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in‐plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in‐plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in‐plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out‐of‐plane Fe‐N‐O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v 50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe‐X‐O (X=N, C, and O) complexes is correlated with the Fe−XO bond lengths. The nature of highest frequency band at ≈560 cm −1 has also been examined in two additional new derivatives. Previously assigned as the Fe−NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

The vibrational spectra of two five-coordinate nitrosyl iron porphyrinates, [Fe(OEP)(NO)] (OEP = dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin) and [Fe(DPIX)(NO)] (DPIX = deuteroporphyrin IX), have been studied by oriented single-crystal nuclear resonance vibrational spectroscopy. Single crystals (both are in the triclinic crystal system) were oriented to give vibrational spectra perpendicular to the porphyrin plane. Additionally, two orthogonal in-plane measurements that were also either perpendicular or parallel to the projection of the FeNO plane onto the porphyrin plane yield the complete set of vibrations with iron motion. In addition to cleanly enabling the assignment of the FeNO bending and stretching modes, the measurements reveal that the two in-plane spectra from the parallel and perpendicular in-plane directions for both compounds have substantial differences. The assignment of these in-plane vibrations were aided by density functional theory predictions. The differences in the two in-plane directions result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is thus found to be largely parallel and perpendicular to the projection of the FeNO plane on the porphyrin plane. These axial ligand effects on the in-plane iron motion are related to the strength of the axial ligand-to-iron bond.

The complete iron vibrational spectrum of the five-coordinate high-spin complex [Fe(OEP)(2-MeHIm)], where OEP = octaethylporphyrinato and 2-MeHIm = 2-methylimidazole, has been obtained by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS) data. Measurements have been made in three orthogonal directions, which provides quantitative information for all iron motion. These experimental data, buttressed by density functional theory (DFT) calculations, have been used to define the effects of the axial ligand orientation. Although the axial imidazole removes the degeneracy in the in-plane vibrations, the imidazole orientation does not appear to control the direction of the in-plane iron motion. This is in contrast to the effect of the imidazolate ligand, as defined by DFT calculations, which does have substantial effects on the direction of the in-plane iron motion. The axial NO ligand has been found to have the strongest orientational effect (Angew. Chem., Int. Ed., 2010, 49, 4400). Thus the strength of the directional properties are in the order NO > imidazolate > imidazole, consistent with the varying strength of the Fe-ligand bond.

The low-frequency vibrational characterization of the spin-crossover complex, five-coordinate cyano(tetraphenylporphyrinato)iron(II), [Fe(TPP)(CN)](-), is reported. Nuclear resonance vibrational spectroscopy has been used to measure all low-frequency vibrations involving iron at several temperatures; this yields vibrational spectra of both the low- (S = 0) and high-spin (S = 2) states. Multitemperature oriented single-crystal measurements facilitate assignments of the vibrational character of all modes and are consistent with the DFT-predicted spectra. The availability of the entire iron vibrational spectrum allows for the complete correlation of the modes between the two spin states. These data demonstrate that not only do the frequencies of the vibrations shift to lower values for the high-spin species as would be expected owing to the weaker bonds in the high-spin state, but also the mixing of iron modes with ligand modes changes substantially. Diagrams illustrating the changing character of the modes and their correlation are given. The reduced iron-ligand frequencies are the primary factor in the entropic stabilization of the high-spin state responsible for the spin crossover.

Nuclear Resonance Vibrational Spectroscopy (NRVS) is a sensitive vibrational probe for biologically important heme complexes. The exquisite sensitivity of the NRVS data to the electronic structure provides detailed insights into the nature of these interesting compounds, but requires highly accurate computational methods for the mode assignments. To determine the best combinations of density functionals and basis sets, a series of benchmark DFT calculations on the previously characterized complex [Fe(OEP)NO] (OEP(2-)=octaethylporphyrinatio dianion) were performed. A test set of 21 methodology combinations including 8 functionals (BP86, mPWPW91, B3LYP, PBE1PBE, M062X, M06L, LC-BP86 and ωB97X-D) and 5 basis set (VTZ, TZVP, Lanl2DZ for iron and 6-31G*, 6-31+G* for other atoms) was carried out to calculate electronic structures and vibrational frequencies. We also implemented the conversion of frequency calculations into orientation-selective mode composition factors (e(2)), which can used to simulate the Vibrational Density Of States (VDOS) using Gaussian normal distribution functions. These use a series of user-friendly scripts for their application to NRVS. The structures as well as the isotropic and anisotropic NRVS of [Fe(OEP)NO] obtained with the M06L functional with a variety of basis sets are found to best reproduce the available experimental data, followed by B3LYP/LanL2DZ calculations. Other density functionals and basis sets do not produce the same level of accuracy. The noticeably worse agreement between theory and experiment for the out-plane NRVS compared with the excellent performance of the M06L functional for the in-plane prediction is attributed to deficiencies of the physical model rather than the computational methodology.

We report our studies of the vibrational dynamics of iron for three imidazole-ligated oxyheme derivatives that mimic the active sites of histidine-ligated heme proteins complexed with dioxygen. The experimental vibrational data are obtained from nuclear resonance vibrational spectroscopy (NRVS) measurements conducted on both powder samples and oriented single crystals, and which includes several in-plane (ip) and out-of-plane (oop) measurements. Vibrational spectral assignments have been made through a combination of the oriented sample spectra and predictions based on density functional theory (DFT) calculations. The two Fe-O(2) modes that have been previously observed by resonance Raman spectroscopy in heme proteins are clearly shown to be very strongly mixed and are not simply either a bending or stretching mode. In addition, a third Fe-O(2) mode, not previously reported, has been identified. The long-sought Fe-Im stretch, not observed in resonance Raman spectra, has been identified and compared with the frequencies observed for the analogous CO and NO species. The studies also suggest that the in-plane iron motion is anisotropic and is controlled by the orientation of the Fe-O(2) group and not sensitive to the in-plane Fe-N(p) bonds and/or imidazole orientations.

In-depth probing: A NRVS single-crystal study (NRVS=nuclear resonance vibrational spectroscopy) has provided detailed information on the in-plane modes of nitrosyl iron porphyrinate [Fe(oep)(NO)] (see picture; oep=octaethylporphyrin). The axial nitrosyl ligand controls the direction of the in-plane iron motion. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Recent reports of potential physiological roles of hydrogen sulfide have prompted interest in heme-sulfide interactions. Heme-H(2)S and/or heme-HS(-) interactions could potentially occur during endogenous production, transport, signaling events, and catabolism of H(2)S. We have investigated the interaction of the hydrosulfide ion (HS(-)) with iron porphyrinates. UV-vis spectral studies show the formation of [Fe(Por)(SH)](-), [Fe(Por)(SH)(2)](2-), and the mixed-ligand species [Fe(Por)(Im)(SH)](-). UV-vis binding studies of [Fe(OEP)] and [Fe(T-p-OMePP)] (OEP = octaethylporphyrinate; T-p-OMePP = tetra-p-methoxyphenylporphyrinate) with HS(-) allowed for calculation of the formation constants and extinction coefficients of mono- and bis-HS(-) complexes. We report the synthesis of the first HS(-)-bound iron(II) porphyrin compounds, [Na(222)][Fe(OEP)(SH)].0.5C(6)H(6) and [Na(222)][Fe(T-p-OMePP)(SH)].C(6)H(5)Cl (222 = Kryptofix-222). Characterization by single-crystal X-ray analysis, mass spectrometry, and Mossbauer and IR spectroscopy is all consistent with that of known sulfur-bound high-spin iron(II) compounds. The Fe-S distances of 2.3929(5) and 2.3887(13) A are longer than all reported values of [Fe(II)(Por)(SR)](-) species. An analysis of the porphyrin nonplanarity for these derivatives and for all five-coordinate high-spin iron(II) porphyrinate derivatives with an axial anion ligand is presented. In our hands, attempts to synthesize iron(III) HS(-) derivatives led to iron(II) species.

Nitric oxide (NO) is a signaling molecule employed to regulate essential physiological processes. Thus, there is great interest in understanding the interaction of NO with heme, which is found at the active site of many proteins that recognize NO, as well as those involved in its creation and elimination. We summarize what we have learned from investigations of the structure, vibrational properties, and conformational dynamics of NO complexes with ferrous porphyrins, as well as computational investigations in support of these experimental studies. Multitemperature crystallographic data reveal variations in the orientational disorder of the nitrosyl ligand. In some cases, equilibria among NO orientations can be analyzed using the van't Hoff relationship and the free energy and enthalpy of the solid-state transitions evaluated experimentally. Density functional theory (DFT) calculations predict that intrinsic barriers to torsional rotation are smaller than thermal energies at physiological temperatures, and the coincidence of observed NO orientations with minima in molecular mechanics potentials indicates that nonbonded interactions with other chemical groups control the conformational freedom of the bound NO. In favorable cases, reduced disorder at low temperatures exposes subtle structural features including off-axis tilting of the Fe-NO bond and anisotropy of the equatorial Fe-N bonds. We also present the results of nuclear resonance vibrational spectroscopy measurements on oriented single crystals of [Fe(TPP)(NO)] and [Fe(TPP)(1-MeIm)(NO)]. These describe the anisotropic vibrational motion of iron in five- and six-coordinate heme-NO complexes and reveal vibrations of all Fe-ligand bonds as well as low-frequency molecular distortions associated with the doming of the heme upon ligand binding. A quantitative comparison with predicted frequencies, amplitudes, and directions facilitates identification of the vibrational modes but also suggests that commonly used DFT functionals are not fully successful at capturing the trans interaction between the axial NO and imidazole ligands. This supports previous conclusions that heme-NO complexes exhibit an unusual degree of variability with respect to the computational method, and we speculate that this variability hints at a genuine electronic instability that a protein can exploit to tune its reactivity. We anticipate that ongoing characterization of heme-NO complexes will deepen our understanding of their structure, dynamics, and reactivity.

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 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-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 temperature-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-NIm bond length and N-O stretching frequency were also found to be completely reversible with no apparent hysteresis.

The isolation of two polymorphic forms of nitrosyl(1-methylimidazole)(tetra-p-fluorophenylporphinato)iron(II) provides a unique opportunity to explore the interplay of structure and vibrational dynamics in six-coordinate {FeNO}(7) nitrosyliron porphyrinates. The two species display differing vibrational spectroscopic properties both in nu(NO) (IR) and the iron vibrational modes obtained through the use of nuclear resonance vibrational spectroscopy. Structural characterization of the two complexes has yielded extremely high-quality structures that further confirm that coordination of NO leads to ligand tilting and asymmetry in the equatorial Fe-N-p bond distances. The two polymorphic structures (monoclinic and triclinic crystal systems) differ in the relative orientations of the two axial ligands and small but significant differences in coordination group bond distances. Although DFT calculations suggest that the NO/imidazole orientations should be indistinguishable, real experimental (structural and vibrational) differences between the two are found. The observed variation in the axial and equatorial Fe-N bonds is strongly correlated to the dynamics of the Fe-NO unit and other motions of iron. Other structural differences appear to have little effect on the vibrational properties of the two forms. The in-plane motions of iron in CO versus NO heme complexes illustrate distinct dynamic differences.