Hydrogen bonding influence of 1,10-phenanthroline on five-coordinate high-spin imidazole-ligated iron(II) porphyrinates

The influence of a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron has been further studied in new complexes of five-coordinate high-spin imidazole-ligated iron(II) porphyrinates. With 1,10-phenanthroline (1,10-phen) as the hydrogen-bond acceptor, several new octaethylporphyrin dianion (OEP) and meso-tetraphenylporphyrin dianion (TPP) derivatives have been synthesized and characterized by X-ray crystallography and M?ssbauer spectroscopy. In all three new structures, the porphyrin molecules and 1,10-phenanthroline molecules have been found with a ratio of 1:1. All the porphyrin derivatives are five-coordinate 2-methylimidazole-ligated iron(II) species. 1,10-Phenanthroline is hydrogen bonded to the coordinated imidazole to form two unequal hydrogen bonds. The Fe-N p and Fe-N Im bond lengths and displacement of the iron atom out of the porphyrin plane are similar to those in imidazole-ligated species. M?ssbauer measurements showed remarkable temperature dependence; the analysis of the data obtained in applied magnetic field for [Fe(OEP)(2-MeHIm)].(1,10-phen) gave a negative quadrupole splitting value and large asymmetry parameters. All the structural and M?ssbauer properties suggest that these new hydrogen-bonded species have the same electronic configuration as imidazole-ligated species.     

Variable pi-bonding in iron(II) porphyrinates with nitrite, CO, and tert-butyl isocyanide: characterization of [Fe(TpivPP)(NO2)(CO)]-

The addition of the strongly pi-bonding ligands CO or tert-butyl isocyanide to the low-spin five-coordinate iron(II) nitrite species [Fe(TpivPP)(NO2)]- (TpivPP = picket fence porphyrin) gives two new six-coordinate species [Fe(TpivPP)(NO2)(CO)]- and [Fe(TpivPP)(NO2)(t-BuNC)]-. These species have been characterized by single-crystal structure determinations and by UV-vis, IR, and M?ssbauer spectroscopies. All evidence shows that in the mixed-ligand iron(II) porphyrin species, [Fe(TpivPP)(NO2)(CO)]-, the two trans, pi-accepting ligands CO and nitrite compete for pi density. The CO ligand however dominates the bonding. The Fe-N(NO2) bond lengths for the two independent anions in the unit cell at 2.006(4) and 2.009(4) A are lengthened compared to other nitrite species with either no trans ligands or non-pi-accepting trans ligands to nitrite. The Fe-C(CO) bond lengths are 1.782(4) A and 1.789(5) A for the two anions. The two Fe-C-O angles at 175.5(4) and 177.5(4) degrees are essentially linear in both anions. The quadrupole splitting for [Fe(TpivPP)(NO2)(CO)]- was determined to be 0.32 mm/s, and the isomer shift was 0.18 mm/s at room temperature in zero applied field. Both of the M?ssbauer parameters are much smaller than those found for six-coordinate low-spin iron(II) porphyrinates with neutral nitrogen-donating ligands as well as iron(II) nitro complexes. However, the M?ssbauer parameters are typical of other six-coordinate CO porphyrinates signifying that CO is the more dominant ligand. The CO stretching frequency of 1974 cm(-1) is shifted only slightly to higher energy compared to six-coordinate CO complexes with neutral nitrogen-donor ligands trans to CO. Crystal data for [K(222)][Fe(TpivPP)(NO2)(CO)].1/2C6H5Cl: monoclinic, space group P2(1)/c, Z = 8, a = 33.548(6) A, b = 18.8172(15) A, c = 27.187(2) A, beta = 95.240(7) degrees, V = 17091(4) A3.     

Electronic configuration of high-spin imidazole-ligated iron(II) octaethylporphyrinates

The preparation and characterization of two new five-coordinate, imidazole-ligated, high-spin iron(II) octaethylporphyrinates is described. [Fe(OEP)(1,2-Me2Im)] and [Fe(OEP)(2-MeHIm)] have been characterized by X-ray structure determinations and temperature-dependent M?ssbauer spectroscopy in zero and applied magnetic fields. The distinction between imidazole-ligated and other ligands in high-spin iron(II) porphyrinates, noted for a series of tetraarylporphyrinate derivatives (Hu, C.; Roth, A.; Ellison, M. K.; An, J.; Ellis, C. M.; Schiltz, C. E.; Scheidt, W. R. J. Am. Chem. Soc. 2005, 127, 5675), is seen here as well. The sign of the quadrupole-splitting constant is again negative, which is unique to the imidazole-ligated derivatives and suggests a distinct electronic structure. The derivatives again display a remarkable temperature dependence in the quadrupole splitting, which is also seen for deoxymyoglobin and -hemoglobin. Structural features for the two new derivatives are similar to those seen earlier, although the core conformations show somewhat more doming character.     

Sulfoxide as a ligand in iron(II) porphyrinates: S- or O-bound?

In order to study the bonding of sulfoxides to iron(II) porphyrinates, an equilibrium study of Fe(TPP) with tetramethylenesulfoxide (TMSO) has been performed. UV-vis spectra at different concentrations of TMSO have shown distinct character belonging to three species: four-coordinate Fe(II)(TPP), five-coordinate [Fe(II)(TPP)(TMSO)], and six-coordinate [Fe(II)(TPP)(TMSO)2]. The isosbestic points for the low TMSO concentrations suggest that the equilibrium constant K1 is much larger than K2. Analysis of spectral data by the nonlinear least-squares program SQUAD gives K1 = 267 and K2 approximately 1. Even though the five-coordinate species is the dominant species under the synthetic conditions, only the six-coordinate species was crystallized and characterized by an X-ray diffraction study. [Fe(TPP)(TMSO)2] (C52H44Fe-N4O2S2): monoclinic, P2(1)/c, a = 11.2580(3) A, b = 15.9262(5) A, c = 12.3930(4) A, beta = 116.246(1) degrees , V = 1992.95(10) A3, Z = 2. X-ray crystallography demonstrates the complex is a low-spin bis-TMSO ligated species. The average Fe-Np distance is 1.999(4) A. The most important feature is that TMSO is coordinated to iron(II) by the sulfur donors, not oxygen. The Fe-S distance is 2.2220(3) A.     

Five- to six-coordination in (nitrosyl)iron(II) porphyrinates: effects of binding the sixth ligand

We report structural and spectroscopic data for a series of six-coordinate (nitrosyl)iron(II) porphyrinates. The structures of three tetraphenylporphyrin complexes [Fe(TPP)(NO)(L)], where L = 4-(dimethylamino)pyridine, 1-methylimidazole, 4-methylpiperidine, are reported here to a high degree of precision and allow observation of several previously unobserved structural features. The tight range of bonding parameters for the [FeNO] moiety for these three complexes suggests a canonical representation for six-coordinate systems (Fe-N(p) = 2.007 A, Fe-N(NO) = 1.753 A, angle FeNO = 138.5 degrees ). Comparison of these data with those obtained previously for five-coordinate systems allows the precise determination of the structural effects of binding a sixth ligand. These include lengthening of the Fe-N(NO) bond and a decrease in the Fe-N-O angle. Several other aspects of the geometry of these systems are also discussed, including the first examples of off-axis tilting of a nitrosyl ligand in a six-coordinate [FeNO](7) heme system. We also report the first examples of M?ssbauer studies for these complexes. Measurements have been made in several applied magnetic fields as well as in zero field. The spectra differ from those of their five-coordinate analogues. To obtain reasonable fits to applied magnetic field data, rotation of the electrical field gradient is required, consistent with differing g-tensor orientations in the five- vs six-coordinate species.     

Coordination of diatomic ligands to heme: simply CO

The synthesis and molecular structures of three iron(II) porphyrinates with only CO as the axial ligand(s) are reported. Two five-coordinate [Fe(OEP)(CO)] derivatives have Fe-C = 1.7077(13) and 1.7140(10) A, much shorter than those of six-coordinate [Fe(OEP)(Im)(CO)], although nu(C-O) is 1944-1948 cm(-1). The six-coordinate species [Fe(OEP)(CO)2] has also been studied. The competition for pi-back-bonding of two CO ligands leads to Fe-C distance of 1.8558(10) A and nu(C-O) being increased to 2021 cm(-1). The M?ssbauer spectrum has a quadrupole splitting constant of 0 mm/s at 4.2 K, indicating high electronic symmetry.     

Structural and electronic characterization of nitrosyl(octaethylporphinato)iron(III) perchlorate derivatives

The synthesis and crystallographic characterization of the five-coordinate iron(III) porphyrinate complex [Fe(OEP)(NO)]ClO4 are reported. This [FeNO]6 complex has a nearly linear Fe-N-O group (angle = 173.19(13) degrees) with a small off-axis tilt of the Fe-N(NO) vector from the heme normal (angle = 4.6 degrees); the Fe-N(NO) distance is 1.6528(13) A and the iron is displaced 0.32 A out-of-plane. The complex forms a tight cofacial pi-pi dimer in the solid state. M?ssbauer spectra for this derivative as well as for a related crystalline form are measured both in zero applied magnetic field and in a 7 T applied field. Fits to the measurements made in applied magnetic field demonstrate that both crystalline forms of [Fe(OEP)(NO)]ClO4 have a diamagnetic ground state at 4.2 K. The observed isomer shifts (delta = 0.22-0.24 mm/s) are smaller than those typically observed for low-spin iron(III) porphyrinates. Analogous M?ssbauer measurements are also obtained for a six-coordinate derivative, [Fe(OEP)(Iz)(NO)]ClO4 (Iz = indazole). The observed isomer shift for this species is smaller still (delta = 0.02 mm/s). All derivatives show a strong temperature dependence of the isomer shift. The data emphasize the strongly covalent nature of the FeNO group. The M?ssbauer isomer shifts suggest formal oxidation states greater than +3 for iron, but the NO stretching frequencies are not consistent with such a large charge transfer to NO. Differences in the observed nitrosyl stretching frequencies of the two crystalline forms of [Fe(OEP)(NO)]ClO4 are discussed.     

Nuclear resonance vibrational spectra of five-coordinate imidazole-ligated iron(II) porphyrinates

Nuclear resonance vibrational spectra have been obtained for six five-coordinate imidazole-ligated iron(II) porphyrinates, [Fe(Por)(L)] (Por = tetraphenylporphyrinate, octaethylporphyrinate, tetratolylporphyrinate, or protoporphyrinate IX and L = 2-methylimidazole or 1,2-dimethylimidazole). Measurements have been made on both powder and oriented crystal samples. The spectra are dominated by strong signals around 200-300 cm(-1). Although the in-plane and out-of-plane vibrations are seriously overlapped, oriented crystal spectra allow their deconvolution. Thus, oriented crystal experimental data, along with density functional theory (DFT) calculations, enable the assignment of key vibrations in the spectra. Molecular dynamics are also discussed. The nature of the Fe-N(Im) vibrations has been elaborated further than was possible from resonance Raman studies. Our study suggests that the Fe motions are coupled with the porphyrin core and peripheral groups motions. Both peripheral groups and their conformations have significant influence on the vibrational spectra (position and shape).     

Electronic, magnetic, and structural characterization of the five-coordinate, high-spin iron(II) nitrato complex [Fe(TpivPP)(NO3)]-

The preparation and characterization of the five-coordinate iron(II) porphyrinate derivative [Fe(TpivPP)(NO3)]- (TpivPP = picket-fence porphyrin) is described. Structural and magnetic susceptibility data support a high-spin state (S = 2) assignment for this species. The anionic axial nitrate ligand is O-bound, through a single O atom, with an Fe-O bond length of 2.069(4) A. The planar nitrate ligand bisects a N(p)-Fe-N(p) angle. The average Fe-N(p) bond length is 2.070(16) A. The Fe atom is located 0.49 A out of the 24-atom mean porphyrin plane toward the nitrate ligand. From solid-state M?ssbauer data, the isomer shift of 0.98 mm/s at 77 K is entirely consistent with high-spin iron(II). However the quadrupole splitting of 3.59 mm/s at 77 K is unusually high for iron(II), S = 2 systems but within the range of other five-coordinate high-spin ferrous complexes with a single anionic axial ligand. Crystal data for [K(222)][Fe(TpivPP)(NO3)] x C6H5Cl: a = 17.888 (5) A, b = 21.500 (10) A, c = 22.514 (11) A, beta = 100.32 (3) degrees, monoclinic, space group P2(1)/n, V = 8519 A3, Z = 4.     

Effect of the sixth axial ligand in CS-ligated iron(II)octaethylporphyrinates: structural and Mössbauer studies

The effect of a sixth ligand in a series of low-spin thiocarbonyl-ligated iron(II)octaethylporphyrinates has been investigated. Six-coordinate complexes have been synthesized and characterized by M?ssbauer and infrared spectroscopy and single-crystal X-ray structure determinations. The results are compared with the five-coordinate parent complex. The crystal structures of [Fe(OEP)(CS)(1-MeIm)] and [Fe(OEP)(CS)(Py)] are reported and discussed. The 1-methylimidazole and pyridine derivatives exhibit Fe-C(CS) bond distances of 1.703(4) and 1.706(2) A that are significantly longer than the 1.662(3) A reported for five-coordinate [Fe(OEP)(CS)] (Scheidt, W. R.; Geiger, D. K. Inorg. Chem. 1982, 21, 1208). The trans Fe-N(ligand) distances of 2.112(3) and 2.1550(15) A observed for the 1-methylimidazole and pyridine complex are approximately 0.13 A longer than those observed for analogous bis-ligated complexes and are consistent with a significant structural trans effect for the CS ligand. M?ssbauer investigations carried out for five- and six-coordinate thiocarbonyl derivatives with several different sixth axial ligands reveal interesting features. All derivatives exhibit very small isomer shift values, consistent with a very strong interaction between iron and CS. The five-coordinate derivative has delta(Fe) = 0.08 mm/s, and the six-coordinate complexes exhibit delta(Fe) = 0.14 to 0.19 mm/s at 4.2 K. The five-coordinate complex shows a large quadrupole splitting (DeltaE(q) = 1.93 mm/s at 4.2 K) which is reduced on coordination of the sixth ligand (DeltaE(q) = 0.42-0.80 mm/s at 4.2 K). Addition of a sixth ligand also leads to a small decrease in the value of nu(CS). Correlations in structural, IR, and M?ssbauer results suggest that the sixth ligand effect is primarily induced by changes in sigma-bonding. The structure of [Fe(OEP)(CS)(CH(3)OH)] is briefly reported. Crystal data: [Fe(OEP)(CS)(1-MeIm)] crystallizes in the monoclinic system, space group P2(1)/n, Z = 4, a = 9.5906(5) A, b = 16.704(4) A, c = 23.1417(6) A, beta = 100.453(7) degrees. [Fe(OEP)(CS)(Py)] crystallizes in the triclinic system, space group P1, Z = 5, a = 13.9073(6) A, b = 16.2624(7) A, c = 22.0709(9) A, alpha = 70.586(1) degrees, beta = 77.242(1) degrees, gamma = 77.959(1) degrees. [Fe(OEP)(CS)(CH(3)OH)] crystallizes in the triclinic system, space group P1, Z = 1, a = 9.0599(5) A, b = 9.4389(5) A, c = 11.0676(6) A, alpha = 90.261(1) degrees, beta = 100.362(1) degrees, gamma = 114.664(1) degrees.     

Spin density distribution in five- and six-coordinate iron(II)-porphyrin NO complexes evidenced by magnetic circular dichroism spectroscopy

Using magnetic circular dichroism (MCD) spectroscopy together with DFT calculations, the spin density distributions in five-coordinate [Fe(TPP)(NO)] (I) and six-coordinate [Fe(TPP)(MI)(NO)] (II, MI = 1-methylimidazole) are defined. In the five-coordinate complex, a strong Fe-NO sigma bond between pi(*)(h) and d(z)(2) is present that leads to a large transfer of spin density from the NO ligand to Fe(II) corresponding to an electronic structure with noticeable Fe(I)-NO(+) character. Consequently, the MCD spectrum is dominated by paramagnetic C-term contributions. On coordination of the sixth ligand, the spin density is pushed back from the iron toward the NO ligand, resulting in an Fe(II)-NO(radical) type of electronic structure. This is reflected by the fact that the MCD spectrum is dominated by diamagnetic contributions.     

Proton-mediated electron configuration change in high-spin iron(II) porphyrinates

The synthesis, molecular structure, and electronic structure characterization of two five-coordinate high-spin imidazolate-ligated iron(II) porphyrinates are reported. Their electronic structure, as deduced from M?ssbauer spectra obtained in strong magnetic fields, is distinctly different from that of the analogous imidazole-ligated species. The resulting electronic structure models are consistent with all observed differing features in the two classes.     

Ligand orientation control in low-spin six-coordinate (porphinato)iron(II) species

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)(2)] which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened equatorial bonds of 1.963(6) A and slightly elongated axial Fe-N bond lengths of 2.034(9) A that are about 0.04 A shorter and 0.03 A longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with parallel oriented axial ligands. The Mossbauer spectrum shows a pair of quadrupole doublets that can be assigned to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has DeltaE(Q) = 2.25 mm/s and delta = 0.90 mm/s at 15 K. The quadrupole splitting, DeltaE(Q), for [Fe(TMP)(2-MeHIm)(2)] is 1.71 mm/s, and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mossbauer spectra thus provide a probe for ligand orientation when structural data are otherwise not available.     

Structure of the deoxymyoglobin model [Fe(TPP)(2-MeHIm)] reveals unusual porphyrin core distortions

The preparation and characterization of the deoxymyoglobin model (2-methylimidazole)(tetraphenylporphinato)iron(II) is described. The preparation and crystallization from chlorobenzene leads to a new crystalline phase that has been structurally characterized. The complex is the most ordered example of a deoxymyoglobin model yet characterized. The X-ray structure determination reveals a number of distortions both in the iron coordination group and in the porphyrin core that result from the steric bulk of the axial ligand. Some of these distortions have been noted previously in related species; however, the demonstration of porphyrin core distortions and an asymmetry in the Fe-N(p) bond distances are new observations. These may have functional significance for this important type of heme protein coordination group. The new structure emphasizes that high-spin iron(II) porphyrinate derivatives display substantial structural pliability with significant variations in iron atom displacements, porphyrin core hole size, and axial and equatorial Fe-N bond lengths. The new complex has also been characterized by zero-field and applied field magnetic M?ssbauer spectroscopy. M?ssbauer parameters are characteristic for high-spin iron, although they also reveal an extremely rhombic site for iron(II). Crystal data at 130 K for [Fe(TPP)(2-MeHIm)].1.5C(6)H(5)Cl: a = 12.334(3) A, b = 13.515(6) A, c = 14.241(7) A, alpha = 70.62(3) degrees, beta = 88.29(2) degrees, gamma = 88.24(3) degrees, triclinic, space group, P, V = 2238(2) A(3), Z = 2.     

Molecular structure of an Fe(IV) species:

The molecular structure of the formal iron(IV) porphyrinate derivative, [[Fe(TTP)]2N]SbCl6 (TTP = tetratolylporphyrinate), is reported. The structural parameters are compared to the previously reported species [Fe(TPP)]2N, in which the iron oxidation state is +3.5. Both the equatorial and axial bond distances in [[Fe(TTP)]2N]SbCl6 are slightly shortened and consistent with an increased formal charge on iron. The value for the axial Fe-N distance is 1.6280(7) A, and the average value of the equatorial Fe-Np distances is 1.979(5) A. The M?ssbauer isomer shift decreases upon oxidation, again consistent with an increase in formal charge. Values for the isomer shift at room temperature are -0.13 mm/s for [[Fe(TTP)]2N]SbCl6 and 0.04 mm/s for [Fe(TTP)]2N. Crystal data for [[Fe(TTP)]2N]SbCl6 are as follows: orthorhombic, space group Fddd, Z = 8, a = 23.689(2) A, b = 31.056(3) A, c = 22.7788(18) A.     

Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.     

Direct determination of the complete set of iron normal modes in a porphyrin-imidazole model for carbonmonoxy-heme proteins: [Fe(TPP)(CO)(1-MeIm)

Detailed Fe vibrational spectra have been obtained for the heme model complex [Fe(TPP)(CO)(1-MeIm)] using a new, highly selective and quantitative technique, Nuclear Resonance Vibrational Spectroscopy (NRVS). This spectroscopy measures the complete vibrational density of states for iron atoms, from which normal modes can be calculated via refinement of the force constants. These data and mode assignments can reveal previously undetected vibrations and are useful for validating predictions based on optical spectroscopies and density functional theory, for example. Vibrational modes of the iron porphyrin-imidazole compound [Fe(TPP)(CO)(1-MeIm)] have been determined by refining normal mode calculations to NRVS data obtained at an X-ray synchrotron source. Iron dynamics of this compound, which serves as a useful model for the active site in the six-coordinate heme protein, carbonmonoxy-myoglobin, are discussed in relation to recently determined dynamics of a five-coordinate deoxy-myoglobin model, [Fe(TPP)(2-MeHIm)]. For the first time in a six-coordinate heme system, the iron-imidazole stretch mode has been observed, at 226 cm(-)(1). The heme in-plane modes with large contributions from the nu(42), nu(49), nu(50), and nu(53) modes of the core porphyrin are identified. In general, the iron modes can be attributed to coupling with the porphyrin core, the CO ligand, the imidazole ring, and/or the phenyl rings. Other significant findings are the observation that the porphyrin ring peripheral substituents are strongly coupled to the iron doming mode and that the Fe-C-O tilting and bending modes are related by a negative interaction force constant.     

New insights on the electronic and molecular structure of cyanide-ligated iron(III) porphyrinates

The preparation and characterization of several new cyano-ligated six-coordinate low-spin iron(III) porphyrinates are reported. The synthesis and structure of the new bis(cyanide) derivative K(222)][Fe(TMP)(CN)2] (TMP = tetramesitylporphyrinate) is described. Three mixed-ligand species of the general form [Fe(Porph)(CN)(L)], where L = 1-methylimidazole or pyridine, have also been prepared and structurally characterized. All complexes have been studied with EPR spectroscopy in frozen solution and in microcrystalline form. In some cases, especially those of the bis(cyanide) derivative above and the previously reported [Fe(TPP)(CN)2](-), there are significant differences in the EPR spectra as a result of the state change. These spectral differences can be correlated with changes in the electron configuration that are the likely result of a differing environment of the coordinated cyanide ligands; the core conformation and electronic structure of the porphyrin ligand are unlikely to play a role. All four new complexes and [Fe(TPP)(CN)2](-) have been studied by M?ssbauer spectroscopy with variable-temperature and applied magnetic-field measurements. The sign of the quadrupole splitting value has been established as negative. These measurements have allowed us to give estimates of the energy difference between the two close-lying dpi (dxz and dyz) orbitals. These splitting values range from approximately 267 cm-1 for [Fe(TPP)(CN)2](-) to approximately 614 cm(-1) for [Fe(TPP)(CN)(Py)].     

Electronic configuration assignment and the importance of low-lying excited states in high-spin imidazole-ligated iron(II) porphyrinates

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).     

Hydrogen bonding effects on the electronic configuration of five-coordinate high-spin iron(II) porphyrinates

The characterization of a new five-coordinate derivative of (2-methylimidazole)(tetraphenylporphinato)iron(II) provides new and unique information about the effects of forming a hydrogen bond to the coordinated imidazole on the geometric and electronic structure of iron in these species. The complex studied has two crystallographically distinct iron sites; one site has an axial imidazole ligand modified by an external hydrogen bond, and the other site has an axial imidazole ligand with no external interactions. The iron atoms at the two sites have distinct geometric features, as revealed in their molecular structures, and distinct electronic structures, as shown by M?ssbauer spectroscopy, although both are high spin (S = 2). The molecule with the external hydrogen bond has longer equatorial Fe-N(p) bonds, a larger displacement of the iron atom out of the porphyrin plane, and a shorter axial bond compared to its counterpart with no hydrogen bonding. The M?ssbauer features are distinct for the two sites, with differing quadrupole splitting and isomer shift values and probably differing signs for the quadrupole splitting as shown by variable-temperature measurements in applied magnetic field. These features are consistent with a significant change in the nature of the doubly populated d orbital and are all in the direction of the dichotomy displayed by related imidazole and imidazolate species where deprotonation leads to major differences. The results points out the possible effects of strong hydrogen bonding in heme proteins.