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.     

Low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm)(2)]Cl): a consequence of hydrogen bonding?

The synthesis and characterization of low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp[Fe(OEP)(2-MeHIm)2]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened equatorial bonds of 1.974(4) A and slightly elongated axial Fe-N bond lengths of 2.005(10) A that are about 0.02 A shorter and 0.03 A longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)2]ClO4, hydrogen bonding may play an important role in the differences in the two structures. M?ssbauer spectra show broadened quadrupole doublets with quadrupole splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These confirm that the title complex is a low-spin iron(III) species with the ground state (dxy)2(dxz,dyz)3. Crystal data: monoclinic, space group P2(1)/c, a = 14.066(3) A, b, 20.883(4) A, c = 19.245(4) A, beta = 109.67 degrees , and Z = 4.     

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.     

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.     

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.     

Models of the membrane-bound cytochromes: mössbauer spectra of crystalline low-spin ferriheme complexes having axial ligand plane dihedral angles ranging from 0 degree to 90 degrees

Crystalline samples of four low-spin Fe(III) octaalkyltetraphenylporphyrinate and two low-spin Fe(III) tetramesitylporphyrinate complexes, all of which are models of the bis-histidine-coordinated cytochromes of mitochondrial complexes II, III, and IV and chloroplast complex b(6)f, and whose molecular structures and EPR spectra have been reported previously, have been investigated in detail by M?ssbauer spectroscopy. The six complexes and the dihedral angles between axial ligand planes of each are [(TMP)Fe(1-MeIm)(2)]ClO(4) (0 degree), paral-[(OMTPP)Fe(1-MeIm)(2)]Cl (19.5 degrees), paral-[(TMP)Fe(5-MeHIm)(2)]ClO(4) (26 degrees, 30 degrees for two molecules in the unit cell whose EPR spectra overlap), [(OETPP)Fe(4-Me(2)NPy)(2)]Cl (70 degrees), perp-[(OETPP)Fe(1-MeIm)(2)]Cl (73 degrees), and perp-[(OMTPP)Fe(1-MeIm)(2)]Cl (90 degrees). Of these, the first three have been shown to exhibit normal rhombic EPR spectra, each with three clearly resolved g-values, while the last three have been shown to exhibit "large g(max)" EPR spectra at 4.2 K. It is found that the hyperfine coupling constants of the complexes are consistent with those reported previously for low-spin ferriheme systems, with the largest-magnitude hyperfine coupling constant, A(zz), being considerably smaller for the "parallel" complexes (400-540 kG) than for the strictly perpendicular complex (902 kG), A(xx) being negative for all six complexes, and A(zz) and A(xx) being of similar magnitude for the "parallel" complexes (for example, for [(TMP)Fe(1-MeIm)(2)]Cl, A(zz) = 400 kG, A(xx) = -400 kG). In all cases, A(yy) is small but difficult to estimate with accuracy. With results for six structurally characterized model systems, we find for the first time qualitative correlations of g(zz), A(zz), and DeltaE(Q) with axial ligand plane dihedral angle Deltavarphi.     

Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH(3)PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species, such as an expanded porphyrinato core, large equatorial Fe-N(p) bond distances, and a significant out-of-plane displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N(p) bond distances are similar to those in imidazole-ligated species. These suggest that the coordination abilities to iron(II) for imidazole and pyrazole are very similar even though pyrazole is less basic than imidazole. Mo?ssbauer studies reveal that [Fe(TPP)(Hdmpz)] has the same behavior as those of imidazole-ligated species, such as negative quadrupole splitting values and relative large asymmetry parameters. Both the structures and the Mo?ssbauer spectra suggest pyrazole-ligated five-coordinate iron(II) porphyrinates have the same electronic configuration as imidazole-ligated species.     

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.     

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.     

Low-spin ferriheme models of the cytochromes: correlation of molecular structure with EPR spectral type

The preparation and characterization of the following bis-imidazole and bis-pyridine complexes of octamethyltetraphenylporphyrinatoiron(III), Fe(III)OMTPP, octaethyltetraphenylporphyrinatoiron(III), Fe(III)OETPP, and tetra-beta,beta'-tetramethylenetetraphenylporphyrinatoiron(III), Fe(III)TC(6)TPP, are reported: paral-[FeOMTPP(1-MeIm)(2)]Cl, perp-[FeOMTPP(1-MeIm)(2)]Cl, [FeOETPP(1-MeIm)(2)]Cl, [FeTC(6)TPP(1-MeIm)(2)]Cl, [FeOMTPP(4-Me(2)NPy)(2)]Cl, and [FeOMTPP(2-MeHIm)(2)]Cl. Crystal structure analysis shows that paral-[FeOMTPP(1-MeIm)(2)]Cl has its axial ligands in close to parallel orientation (the actual dihedral angle between the planes of the imidazole ligands is 19.5 degrees ), while perp-[FeOMTPP(1-MeIm)(2)]Cl has the axial imidazole ligand planes oriented at 90 degrees to each other and 29 degrees away from the closest N(P)-Fe-N(P) axis. [FeOETPP(1-MeIm)(2)]Cl has its axial ligands close to perpendicular orientation (the actual dihedral angle between the planes of the imidazole ligands is 73.1 degrees ). In all three cases the porphyrin core adopts relatively purely saddled geometry. The [FeTC(6)TPP(1-MeIm)(2)]Cl complex is the most planar and has the highest contribution of a ruffled component in the overall saddled structure compared to all other complexes in this study. The estimated numerical contribution of saddled and ruffled components is 0.68:0.32, respectively. Axial ligand planes are perpendicular to each other and 15.3 degrees away from the closest N(P)-Fe-N(P) axis. The Fe-N(P) bond is the longest in the series of octaalkyltetraphenylporphyrinatoiron(III) complexes due to [FeTC(6)TPP(1-MeIm)(2)]Cl having the least distorted porphyrin core. In addition to these three complexes, two crystalline forms each of [FeOMTPP(4-Me(2)NPy)(2)]Cl and [FeOMTPP(2-MeHIm)(2)]Cl were obtained. In all four of these cases the axial planes are in nearly perpendicular planes in spite of quite different geometries of the porphyrin cores (from purely saddled to saddled with 30% ruffling). The EPR spectral type correlates with the geometry of the OMTPP, OETPP and TC(6)TPP complexes. For the paral-[FeOMTPP(1-MeIm)(2)]Cl, a rhombic signal with g(1) = 1.54, g(2) = 2.51, and g(3) = 2.71 is consistent with nearly parallel axial ligand orientation. For all other complexes of this study, "large g(max)" signals are observed (g(max) = 3.61 - 3.27), as are observed for nearly perpendicular ligand plane arrangement. On the basis of this and previous work, the change from "large g(max)" to normal rhombic EPR signal occurs between axial ligand plane dihedral angles of 70 degrees and 30 degrees.     

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)].     

Iron complexes of 5,10,15,20-tetraphenyl-21-oxaporphyrin

The iron complexes of 5,10,15,20-tetraphenyl-21-oxaporphyrin (OTPP)H have been investigated. Insertion of iron(II) followed by one-electron oxidation yielded a high-spin, six-coordinate (OTPP)Fe(III)Cl(2) complex. The reduction of (OTPP)Fe(III)Cl(2) has been accomplished by means of moderate reducing reagents producing high-spin five-coordinate (OTPP)Fe(II)Cl. The molecular structure of (OTPP)Fe(III)Cl(2) has been determined by X-ray diffraction. The iron(III) 21-oxaporphyrin skeleton is essentially planar. The furan ring coordinates in the eta(1) fashion through the oxygen atom, which acquires trigonal geometry. The iron(III) apically coordinates two chloride ligands. Addition of potassium cyanide to a solution of (OTPP)Fe(III)Cl(2) in methanol-d(4) results in its conversion to a six-coordinate, low-spin complex [OTPP)Fe(III)(CN)(2)] which is spontaneously reduced to [OTPP)Fe(II)(CN)(2)](-) by excess cyanide. The spectroscopic features of [OTPP)Fe(III)(CN)(2)] correspond to the common low-spin iron(III) porphyrin (d(xy))(2)(d(xz)d(yz))(3) electronic configuration. Titration of (OTPP)Fe(III)Cl(2) or (OTPP)Fe(II)Cl with n-BuLi (toluene-d(8), 205 K) resulted in the formation of (OTPP)Fe(II)(CH(2)CH(2)CH(2)CH(3)). (OTPP)Fe(II)(n-Bu) decomposes via homolytic cleavage of the iron-carbon bond to produce (OTPP)Fe(I). The EPR spectrum (toluene-d(8), 77 K) is consistent with a (d(xy))(2)(d(xz))(2)(d(yz))(2)(d(z)(2)(1)(d[(x)(2)-(y)(2)])(0) ground electronic state of iron(I) oxaporphyrin (g(1) = 2.234, g(2) = 2.032, g(3) = 1.990). The (1)H NMR spectra of (OTPP)Fe(III)Cl(2), (OTPP)Fe(III)(CN)(2), ([(OTPP)Fe(III))](2)O)(2+), and (OTPP)Fe(II)Cl have been analyzed. There are considerable similarities in (1)H NMR properties within each iron(n) oxaporphyrin-iron(n) regular porphyrin or N-methylporphyrin pair (n = 2, 3). Contrary to this observation, the pattern of downfield positions of pyrrole resonances at 156.2, 126.5, 76.3 ppm and furan resonance at 161.4 ppm (273 K) detected for the two-electron reduction product of (OTPP)Fe(III)Cl(2) is unprecedented in the group of iron(I) porphyrins.     

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.     

Nitrosyliron(III) porphyrinates: porphyrin core conformation and FeNO geometry. Any correlation?

The synthesis, structural, and spectroscopic characterization of (nitrosyl)iron(III) porphyrinate complexes designed to have strongly nonplanar porphyrin core conformations is reported. The species have a nitrogen-donor axial ligand trans to the nitrosyl ligand and display planar as well as highly nonplanar porphyrin core conformations. The systems were designed to test the idea, expressly discussed for the heme protein nitrophorin (Roberts, et al. Biochemistry 2001, 40, 11327), that porphyrin core distortions could lead to an unexpected, bent geometry for the FeNO group. For [Fe(OETPP)(1-MeIm)(NO)]ClO(4).C(6)H(5)Cl (H(2)OETPP = octaethyltetraphenylporphyrin), the porphyrin core is found to be severely saddled. However, this distortion has little or no effect on the geometric parameters of the coordination group: Fe-N(p) = 1.990(9) A, Fe-N(NO) = 1.650(2) A, Fe-N(L) = 1.983(2) A, and Fe-N-O = 177.0(3) degrees. For the complex [Fe(OEP)(2-MeHIm)(NO)]ClO(4).0.5CH(2)Cl(2) (H(2)OEP = octaethylporphyrin), there are two independent molecules in the asymmetric unit. The cation denoted [Fe(OEP)(2-MeHIm)(NO)](+)(pla) has a close-to-planar porphyrin core. For this cation, Fe-N(p) = 2.014(8) A, Fe-N(NO) = 1.649(2) A, Fe-N(L) = 2.053(2) A, and Fe-N-O = 175.6(2) degrees. The second cation, [Fe(OEP)(2-MeHIm)(NO)](+)(ruf), has a ruffled core: Fe-N(p) = 2.003(7) A, Fe-N(NO) = 1.648(2) A, Fe-N(L) = 2.032(2) A, and Fe-N-O = 177.4(2) degrees. Thus, there is no effect on the coordination group geometry caused by either type of nonplanar core deformation; it is unlikely that a protein engendered core deformation would cause FeNO bending either. The solid-state nitrosyl stretching frequencies of 1917 cm(-)(1) for [Fe(OEP)(2-MeHIm)(NO)]ClO(4) and 1871 cm(-)(1) for [Fe(OETPP)(1-MeIm)(NO)]ClO(4) are well within the range seen for linear Fe-N-O groups. M?ssbauer data for [Fe(OEP)(2-MeHIm)(NO)]ClO(4) confirm that the ground state is diamagnetic. In addition, the quadrupole splitting value of 1.88 mm/s and isomer shift (0.05 mm/s) at 4.2 K are similar to other (nitrosyl)iron(III) porphyrin complexes with linear Fe-N-O groups. Crystal data: [Fe(OETPP)(1-MeIm)(NO)]ClO(4).C(6)H(5)Cl, monoclinic, space group P2(1)/c, Z = 4, with a = 12.9829(6) A, b = 36.305(2) A, c = 14.0126(6) A, beta = 108.087(1) degrees; [Fe(OEP)(2-MeHIm)(NO)]ClO(4).0.5CH(2)Cl(2), triclinic, space group Ponemacr;, Z = 4, with a = 14.062(2) A, b = 16.175(3) A, c = 19.948(3) A, alpha = 69.427(3) degrees, beta = 71.504(3) degrees, gamma = 89.054(3) degrees.     

Octahedral (cis-cyclam)iron(III) complexes with O,N-coordinated o-iminosemiquinonate(1-) pi radicals and o-imidophenolate(2-) anions

Three octahedral complexes containing a (cis-cyclam)iron(III) moiety and an O,N-coordinated o-iminobenzosemiquinonate pi radical anion have been synthesized and characterized by X-ray crystallography at 100 K: [Fe(cis-cyclam)(L(1-3)(ISQ))](PF(6))(2) (1-3), where (L(1-3)(ISQ)) represents the monoanionic pi radicals derived from one-electron oxidations of the respective dianion of o-imidophenolate(2-), L(1), 2-imido-4,6-di-tert-butylphenolate(2-), L(2), and N-phenyl-2-imido-4,6-di-tert-butylphenolate(2-), L(3). Compounds 1-3 possess an S(t) = 0 ground state, which is attained via strong intramolecular antiferromagnetic exchange coupling between a low-spin central ferric ion (S(Fe) = 1/2) and an o-imino-benzosemiquinonate(1-) pi radical (S(rad) = 1/2). Zero-field M?ssbauer spectra of 1-3 at 80 K confirm the low-spin ferric electron configuration: isomer shift delta = 0.26 mm s(-1) and quadrupole splitting DeltaE(Q) = 1.96 mm s(-1) for 1, 0.28 and 1.93 for 2, and 0.33 and 1.88 for 3. All three complexes undergo a reversible, one-electron reduction of the coordinated o-imino-benzosemiquinonate ligand, yielding an [Fe(III)(cis-cyclam)(L(1-3)(IP))](+) monocation. The monocations of 1 and 2 display very similar rhombic signals in the X-band EPR spectra (g = 2.15, 2.12, and 1.97), indicative of low-spin ferric species. In contast, the monocation of 3 contains a high-spin ferric center (S(Fe) = 5/2) as is deduced from its M?ssbauer and EPR spectra.     

Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles

Starting from their six-coordinate iron(II) precursor complexes [L^8RFe(MeCN)]²⁺, a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L^8H and its new octamethylated derivative L^8Me, both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl⁻, OTf⁻, MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and ⁵⁷Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ-oxo diiron(III) complexes were also studied.     

Six-coordinate and five-coordinate Fe(II)(CN)(2)(CO)(x) thiolate complexes (x = 1, 2): synthetic advances for iron sites of [NiFe] hydrogenases

The dicyanodicarbonyliron(II) thiolate complexes trans,cis-[(CN)(2)(CO)(2)Fe(S,S-C-R)](-) (R = OEt (2), N(Et)(2) (3)) were prepared by the reaction of [Na][S-C(S)-R] and [Fe(CN)(2)(CO)(3)(Br)](-) (1). Complex 1 was obtained from oxidative addition of cyanogen bromide to [Fe(CN)(CO)(4)](-). In a similar fashion, reaction of complex 1 with [Na][S,O-C(5)H(4)N], and [Na][S,N-C(5)H(4)] produced the six-coordinate trans,cis-[(CN)(2)(CO)(2)Fe(S,O-C(5)H(4)N)](-) (6) and trans,cis-[(CN)(2)(CO)(2)Fe(S,N-C(5)H(4))](-) (7) individually. Photolysis of tetrahydrofuran (THF) solution of complexes 2, 3, and 7 under CO led to formation of the coordinatively unsaturated iron(II) dicyanocarbonyl thiolate compounds [(CN)(2)(CO)Fe(S,S-C-R)](-) (R = OEt (4), N(Et)(2) (5)) and [(CN)(2)(CO)Fe(S,N-C(5)H(4))](-) (8), respectively. The IR v(CN) stretching frequencies and patterns of complexes 4, 5, and 8 have unambiguously identified two CN(-) ligands occupying cis positions. In addition, density functional theory calculations suggest that the architecture of five-coordinate complexes 4, 5, and 8 with a vacant site trans to the CO ligand and two CN(-) ligands occupying cis positions serves as a conformational preference. Complexes 2, 3, and 7 were reobtained when the THF solution of complexes 4, 5, and 8 were exposed to CO atmosphere at 25 degrees C individually. Obviously, CO ligand can be reversibly bound to the Fe(II) site in these model compounds. Isotopic shift experiments demonstrated the lability of carbonyl ligands of complexes 2, 3, 4, 5, 7, and 8. Complexes [(CN)(2)(CO)Fe(S,S-C-R)](-) and NiA/NiC states [NiFe] hydrogenases from D. gigas exhibit a similar one-band pattern in the v(CO) region and two-band pattern in the v(CN) region individually, but in different positions, which may be accounted for by the distinct electronic effects between [S,S-C-R](-) and cysteine ligands. Also, the facile formations of five-coordinate complexes 4, 5, and 8 imply that the strong sigma-donor, weak pi-acceptor CN(-) ligands play a key role in creating/stabilizing five-coordinate iron(II) [(CN)(2)(CO)Fe(S,S-C-R)](-) complexes with a vacant coordination site trans to the CO ligand.     

Six-coordinate nitrato complexes of iron(III) porphyrins

The interaction of tetrahydrofuran (THF) with thin films of the nitrato complexes Fe(III)(Por)(eta(2)-O(2)NO) [Por = meso-tetraphenylporphyrinato (TPP) and meso-tetratolylporphyrinato (TTP) dianion] at low temperature leads to the formation of the six-coordinate nitrato complex Fe(Por)(THF)(NO(3)), which was characterized by IR and UV-visible spectroscopies. Formation of the THF adduct was accompanied by nitrate linkage isomerization from bidentate to monodentate coordination. The iron(III) center remains in a high spin state in contrast with the previously observed low-spin nitratonitrosyl complex Fe(TPP)(NO)(eta(10-ONO(2)). Upon warming, THF dissociates to restore the initial five-coordinate bidentate nitrato complex.     

mer-[Fe III(bpca)(CN)3]-: a new low-spin iron(III) complex to build heterometallic ladder-like chains

The novel mononuclear complex PPh(4)-mer-[Fe(III)(bpca)(3)(CN)(3)].H(2)O (1) [PPh(4)(+) = tetraphenylphosphonium cation and bpca = bis(2-pyridylcarbonyl)amidate anion] and ladder-like chain compound [[Fe(III)(bpca)(micro-CN)(3)Mn(II)(H(2)O)(3)] [Fe(III)(bpca)(CN)(3)]].3H(2)O (2) have been prepared and characterized by X-ray diffraction analysis. Compound 1 is a low-spin iron(III) compound with three cyanide ligands in mer arrangement and a tridentate N-donor ligand building a distorted octahedral environment around the iron atom. Compound 2 is an ionic salt made up of cationic ladder-like chains [[Fe(III)(bpca)(micro-CN)(3)Mn(II)(H(2)O)(3)]](+) and uncoordinated anions [Fe(III)(bpca)(3)(CN)(3)](-). The magnetic properties of 2 correspond to those of a ferrimagnetic chain with significant intrachain antiferromagnetic coupling between the low-spin iron(III) centers and the high-spin manganese(II) cations. This compound exhibits ferrimagnetic ordering below 2.0 K.