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.     

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.     

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.     

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.     

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.     

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).     

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.     

The binding ability of iron bonded to porphodimethene: structural, magnetic, and electronic relationship to iron porphyrin complexes

The availability of the parent compound, meso-hexaethylporphodimetheneiron(II), [(Et6N4)Fe] (2), of this report results from a novel synthetic methodology that makes [Et6N4Li2] (1) easily available. The major focus is on how the axial positions, which are the key reactive sites in metalloporphyrins, and the electronic configuration of the metal can be affected by the breakdown of the aromaticity of the porphyrin skeleton and by the nonplanar conformation of the ligand. DFT calculations indicate a 3B1(dz2)1(dyz)1 ground state for 2 versus the 3A2(dxz)1(dyz)1 ground state in the porphyrin analogue. The intermediate-spin state (S = 1) of 2 changed drastically upon addition of one or two axial ligands, as hexacoordination is preferred by iron(II). The hexacoordinate complexes [(Et6N4)Fe(L)(L')] (L = L' = THF, 3; L = L' = Py, 4; L = PhNO, L' = Py, 14) have been isolated and structurally characterized. Strong-field ligands lead to a low-spin diamagnetic state for iron(II), namely for complexes 4-7, 9, and 14, whereas 3 is a typical d6 high-spin complex, as is the pentacoordinate [(Et6N4)Fe(CN)]Bu4N (8). The structural analysis showed common features for 6, 7, 9, and 14: i) a small displacement of the metal from the N4 plane, and ii) an N4 cavity, larger than that in the corresponding porphyrins, affecting the Fe-N bond lengths. The 1H NMR spectrum is quite diagnostic of the two-fold symmetry in the diamagnetic hexacoordinate complexes, revealing either a D2h or a C2v symmetry. The CO stretching frequency (1951 cm(-1)) in complex 6 probes the good electron density at the metal. The one-electron oxidation of 2 led to pentacoordinate iron(III) derivatives [(Et6N4)Fe(Cl)] (10), [(Et6N4)2Fe2(mu-O)] (11), and [(Et6N4)2Fe2(mu-p-OC6H4-O)] (12). Complex 10 is a typical high-spin iron(III) (5.85muB at 298 K), while 11 and 12 behave as antiferromagnetic coupled iron(III) (J = -9.4cm(-1), 12, and J = -115cm(-1), 11). In complexes 10, 11, and 12 iron is sitting in a quite distorted square pyramidal geometry, in which the ligand displays a very distorted roof conformation with different degrees of ruffling. Distinctive structural and magnetic features have been found for the nitrosyl derivative [(Et6N4)Fe-NO], which has a low-spin state (S = 1/2) and the following structural parameters: Fe-N-O, 147.3(2) degrees; Fe-N, 1.708(2) A; N-O, 1.172(3) A. A comparative structural, magnetic, and theoretical analysis of the compounds listed above has been made with the analogous porphyrin derivatives. The detailed structural investigation has been mapped through the X-ray analysis of 2, 7, 8, 9, 11, 13, and 14.     

Structural and Spectroscopic Characterization of Iron(III) Dioxoporphodimethene Complexes and Their Autoreduction to an Iron(II) Complex in Pyridine

Three iron complexes of the meso-dioxo derivative of octaethylporphryin (trans-H(2)OEPO(2)) were characterized by X-ray diffraction. Green ClFe(III)(trans-OEPO(2)).1.5C(6)H(6) crystallizes in the monoclinic space group P2(1)/c with a = 13.766(3) ?, b = 19.075(3) ?, c = 15.217(3) ?, beta = 99.87(2) degrees at 123 K with Z = 4. Refinement of 2712 reflections with F > 6.0sigma(F) and 223 parameters yielded R = 0.0624, R(w) = 0.0596. The iron complex contains a domed dioxoporphodimethene macrocyclic ligand. The observation of a five-coordinate iron(III) ion with an axial Fe-Cl distance of 2.232(2) ? and in-plane Fe-N distances averaging 2.082 ? is consistent with its high-spin (S = (5)/(2)) character. This monomer is readily converted to the green {Fe(III)(trans-OEPO(2))}(2)O using aqueous hydroxide. {Fe(III)(trans-OEPO(2))}(2)O crystallizes in the monoclinic space group C2/c with a = 23.541(8) ?, b = 15.392(5) ?, c = 18.686(8) ?, and beta = 90.09(3) degrees at 294 K with Z = 8. Refinement of 3472 reflections with F > 6.0sigma(F) and 393 parameters yielded R = 0.0484, R(w) = 0.0527. The complex possesses a crystallographically imposed 2-fold symmetry axis that passes through the oxo ligand. The dioxoporphodimethene ligands within the molecule are roof-shaped and fold away from each other. The axial Fe-O distance is 1.749(1) ? with longer in-plane Fe-N distances (average 2.077 ?). The Fe-O-Fe angle of 165.4(2) degrees deviates significantly from linearity and is more acute than related porphyrin complexes. Pyridine solutions of either the iron(III) monomer or &mgr;-oxo dimer autoreduce over a period of days to give (py)(2)Fe(II)(trans-OEPO(2)). This red compound crystallizes in the space group P2(1) with a = 19.177(4) ?, b = 20.039(4) ?, c = 10.547(2) ?, and beta = 100.36(3) degrees at 130 K with Z = 2. Refinement of 5090 reflections with one restraint and 984 parameters yielded R1 = 0.0684, wR2 = 0.1763. The complex crystallizes with two distinct molecules in the asymmetric unit; each molecule contains a different degree of disorder with respect to the trans meso oxygen atoms (50/50, 71/29). Each independent molecule exhibits severe ruffling of the macrocycle. The six coordinate iron(II) center is diamagnetic. The axial Fe-N(pyridine) distances average 1.98 ?, and the in-plane Fe-N(pyrrole) distances average 1.95 ?. A common trend observed for the dioxoporphodimethene macrocycle in all of these structures is an elongation toward the trans oxidized meso carbons. These complexes were originally prepared as cis and trans isomeric mixtures that can be enriched in the trans isomer by fractional crystallization. This is evident in their distinctive (1)H NMR spectra. In addition, these compounds are characterized by electron impact mass spectrometry and UV-visible, ESR, and infrared spectroscopies.     

Unexpected nitrosyl-group bending in six-coordinate [M(NO)](6) sigma-bonded aryl(iron) and -(ruthenium) porphyrins.

The six-coordinate nitrosyl sigma-bonded aryl(iron) and -(ruthenium) porphyrin complexes (OEP)Fe(NO)(p-C(6)H(4)F) and (OEP)Ru(NO)(p-C(6)H(4)F) (OEP = octaethylporphyrinato dianion) have been synthesized and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting of the MNO group for both [MNO](6) species as well as significant lengthening of trans axial bond distances. In (OEP)Fe(NO)(p-C(6)H(4)F) the Fe-N-O angle is 157.4(2) degrees, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2 degrees, Fe-N(NO) = 1.728(2) A, and Fe-C(aryl) = 2.040(3) A. In (OEP)Ru(NO)(p-C(6)H(4)F) the Ru-N-O angle is 154.9(3) degrees, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8 degrees, Ru-N(NO) = 1.807(3) A, and Ru-C(aryl) = 2.111(3) A. We show that these structural features are intrinsic to the molecules and are imposed by the strongly sigma-donating aryl ligand trans to the nitrosyl. Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe(NO)(p-C(6)H(4)F) and, combined with the results of extended Hückel calculations, show that the observed bending and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO pi-bonding as measured by infrared and (57)Fe M?ssbauer spectroscopies. The solid-state nitrosyl stretching frequencies for the iron complex (1791 cm(-)(1)) and the ruthenium complex (1773 cm(-)(1)) are significantly reduced compared to their respective [MNO](6) counterparts. The M?ssbauer data for (OEP)Fe(NO)(p-C(6)H(4)F) yield the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins.     

Reversible NO motion in crystalline [Fe(Porph)(1-MeIm)(NO)] derivatives

The synthesis, characterization, and X-ray structures of three low-spin (nitrosyl)iron(II) tetraarylporphyrinates, [Fe(TpXPP)(NO)(1-MeIm)], where X = F (in a triclinic and a monoclinic form) and 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.     

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.     

Electron distribution in iron octaethyloxophlorin complexes. Importance of the Fe(III) oxophlorin trianion form in the bis-pyridine and bis-imidazole complexes

The apportionment of electrons between iron and the porphyrinic macrocycle in complexes of octaethyloxophlorin (H3OEPO) has been a vexing problem. In particular, for (Py)2Fe(OEPO), which is an important intermediate in heme degradation, three resonance structures involving Fe(III), Fe(II), or Fe(I), respectively, have been considered. To clarify this matter, the electronic and geometric structures of (Py)2Fe(III)(OEPO), (Im)2Fe(III)(OEPO).2THF, and (Im)2Fe(III)(OEPO).1.6CHCl3 have been examined by single-crystal X-ray diffraction, measurement of magnetic moments as a function of temperature, and EPR and NMR spectral studies. The results clearly show that both complexes exist in the Fe(III)/oxophlorin trianion form rather than the Fe(II)/oxophlorin radical form previously established for (2,6-xylylNC)(2)Fe(II)(OEPO.). In the solid state from 10 to 300 K, (Py)2Fe(III)(OEPO) exists in the high-spin (S = 5/2) state with the axial ligands in parallel planes, a planar porphyrin, and long axial Fe-N distances. However, in solution it exists predominantly in a low-spin (S = 1/2) form. In contrast, the structures of (Im)2Fe(III)(OEPO).2THF and (Im)2Fe(III)(OEPO).1.6CHCl3 consist of porphyrins with a severe ruffled distortion, axial ligands in nearly perpendicular planes, and relatively short axial Fe-N distances. The crystallographic, magnetic, EPR, and NMR results all indicate that (Im)2Fe(III)(OEPO) exists in the low-spin Fe(III) form in both the solid state and in solution.     

Structure-function correlations in Iron(II) tris(pyrazolyl)borate spin-state crossover complexes

Iron(II) poly(pyrazolyl)borate complexes have been investigated to determine the impact of substituent effects, intramolecular ligand distortions, and intermolecular supramolecular structures on the spin-state crossover (SCO) behavior. The molecular structure of Fe[HB(3,4,5-Me3pz)3]2 (pz = pyrazolyl ring), a complex known to remain high spin when the temperature is lowered, reveals that this complex has an intramolecular ring-twist distortion that is not observed in analogous complexes that do exhibit a SCO at low temperatures, thus indicating that this distortion greatly influences the properties of these complexes. The structure of Fe[B(3-(cy)Prpz)4]2.(CH3OH) ((cy)Pr = cyclopropyl ring) at 294 K has two independent molecules in the unit cell, both of which are high spin; only one of these high-spin iron(II) sites, the site with the lesser ring-twist distortion, is observed to be low-spin iron(II) in the 90 K structure. A careful evaluation of the supramolecular structures of these complexes and several similar complexes reported previously revealed no strong correlation between the supramolecular packing forces and their SCO behavior. Magnetic and M?ssbauer spectral measurements on Fe[B(3-(cy)Prpz)4]2 and Fe[HB(3-(cy)Prpz)3]2 indicate that both complexes exhibit a partial SCO from fully high-spin iron(II) at higher temperatures, respectively, to a 50:50 high-spin/low-spin mixture of iron(II) below 100 K. These results may be understood, in the former case, by the differences in ring-twisting and, in the latter case, by a phase transition; in all complexes in which a phase transition is observed, this change dominates the SCO behavior. A comparison of the M?ssbauer spectral properties of these two complexes and of Fe[HB(3-Mepz)3]2 with that of other complexes reveals correlations between the M?ssbauer-effect isomer shift and the average Fe-N bond distance and between the quadrupole splitting and the average FeN-NB intraligand dihedral torsion angles and the distortion of the average N-Fe-N intraligand bond angles.     

Modulation of metal displacements in a saddle distorted macrocycle: synthesis, structure, and properties of high-spin Fe(III) porphyrins and implications for the hemoproteins

A rare family of five and six-coordinated high-spin Fe(III) porphyrins incorporating weak axial ligands are synthesized and structurally characterized which demonstrate, for the first time, stepwise metal displacements in a single distorted macrocyclic environment that has generally been seen in many biological systems. The introduction of four nitro groups into the meso-positions of octaethyl porphyrin severely distorts the porphyrin geometry and provides an interesting modulation of the macrocycle properties which enables the facile isolation of "pure" high-spin Fe(III)(tn-OEP)Cl, Fe(III)(tn-OEP)(MeOH)Cl, and Fe(III)(tn-OEP)(H2O)2(+) in excellent yields in a saddle distorted macrocyclic environment that are known to stabilize intermediate spin states. The stepwise out-of-plane displacements of iron are as follows: 0.47 A for Fe(III)(tn-OEP)Cl; 0.09 A for Fe(III)(tn-OEP)(MeOH)Cl, and 0.01 A for Fe(III)(tn-OEP)(H2O)2(+) from the mean plane of the porphyrins. However, in both five and six-coordinated Fe(III) porphyrins, the Fe-Np distances are quite comparable while the porphyrin cores have expanded significantly, virtually to the same extent for the six-coordinate complexes reported here. The large size of the high-spin iron(III) atom in Fe(III)(tn-OEP)(H2O)2(+) is accommodated perfectly with no displacement of the metal. This expansion is accompanied by a significant decrease of the saddle distortion with a clear increase of the ruffling. Furthermore, the Fe atom in Fe(III)(tn-OEP)(MeOH)Cl is not out of plane because of the larger atom size; however, the displacement of the iron depends on both the relative strength of the axial ligands, as well as the nature and extent of the ring deformation. Our characterization demonstrates that increase in ruffling and/or decrease in macrocycle deformation brings the iron atom more into the plane in a distorted macrocyclic environment. Our observations thus suggest that the displacements of iron in proteins are the consequences of nonequivalent axial coordination, as well as protein induced deformations at the heme. The high-spin nature of the complexes reported here is believed to be due to the larger Fe-Np distances which then reduce substantially the interaction between iron d(x2)-y2 and porphyrin a(2u) orbital. The Fe(III)/Fe(II) reduction potential of Fe(III)(tn-OEP)Cl shows a reversible peak at large positive value (0.20 V), and no ring-centered oxidation was observed within the solvent limit (approximately 1.80 V). It is thus easier to reduce Fe(III)(tn-OEP)Cl by almost 700 mV compared to Fe(III)(OEP)Cl while oxidations are very difficult. Furthermore, the addition of 3-Cl-pyridine to Fe(III)(tn-OEP)Cl in air undergoes spontaneous auto reduction to produce the rare air-stable Fe(II)(tn-OEP)(3-Cl-py)2 that shows Fe(II)/Fe(III) oxidation peaks at high positive potential (0.79 V), which is approximately 600 mV more anodic compared to [Fe(II)(tn-OEP)Cl](-). This large anodic shift illustrates the effective removal of metal-centered electron density by the macrocycle when the metal is constrained to reside in the porphyrin plane.     

Relative axial ligand orientation in bis(imidazole)iron(II) porphyrinates: are "picket fence" derivatives different?

The synthesis of three new bis(imidazole)-ligated iron(II) picket fence porphyrin derivatives, [Fe(TpivPP)(1-RIm) 2] 1-RIm = 1-methyl-, 1-ethyl-, or 1-vinylimidazole) are reported. X-ray structure determinations reveal that the steric requirements of the four alpha,alpha,alpha,alpha-o-pivalamidophenyl groups lead to very restricted rotation of the imidazole ligand on the picket side of the porphyrin plane; the crowding leads to an imidazole plane orientation eclipsing an iron-porphyrin nitrogen bond. An unusual feature for these diamagnetic iron(II) species is that all three derivatives have the two axial ligands with a relative perpendicular orientation; the dihedral angles between the two imidazole planes are 77.2 degrees , 62.4 degrees , and 78.5 degrees . All three derivatives have nearly planar porphyrin cores. M?ssbauer spectroscopic characterization shows that all three derivatives have quadrupole splitting constants around 1.00 mm/s at 100K.     

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.     

New class of verdoheme analogues with weakly coordinating anions: the structure of (mu-oxo)bis[(octaethyloxoporphinato)iron(III)] hexafluorophosphate

Three new verdoheme analogues with weakly coordinating anions, [OEOPFe(II)X], where OEOP is the monoanion of octaethyloxoporphyrin and X = PF(6), ClO(4), and BF(4), have been synthesized and characterized by spectroscopic methods. (1)H NMR spectroscopy reveals that the [OEOPFe(II)X] species are paramagnetic, and the iron is five-coordinate (S = 2). The oxidation of [OEOPFe(II)PF(6)] with dioxygen yields [(OEOPFe)(2)O](PF(6))(2). The structure of (mu-oxo)bis[(octaethyloxoporphinato)iron(III)] has been determined by X-ray diffraction analysis. The eight Fe-N bond distances have an average value of 2.077(3) Angstroms. The oxygen atom sits on the inversion center, and the average axial Fe-O bond length is 1.756(3) Angstroms. The average displacement of the iron(III) atom from the mean porphinato core is 0.60 Angstroms. Crystal data: crystal system, monoclinic; a = 8.7114(10) Angstroms; b = 26.102(4) Angstroms; c = 15.8323(14) Angstroms; beta = 104.134(6) degrees ; space group P2(1)/c; V = 3491.1(7) Angstroms (3); Z = 2; R1 = 0.0546, wR2 =0.1145 for data with I > 2sigma(I).     

A two-dimensional iron(II) carboxylate linear chain polymer that exhibits a metamagnetic spin-canted antiferromagnetic to single-chain magnetic transition

A two-dimensional iron(II) carboxylate coordination polymer, [Fe(pyoa)2]infinity, where pyoa is 2-(pyridin-3-yloxy)acetate, has been prepared by hydrothermal synthesis. Its crystal structure reveals a single iron(II) site with an elongated octahedral coordination environment containing four equatorial carboxylate oxygens and two axial pyridyl nitrogens; the iron(II) sites are linked by syn-anti micro-carboxylates to form chains along the b axis that have an Fe...Fe separation of 4.910 A. The shortest interchain and interlayer Fe...Fe distances are 6.453 and 11.125 A, respectively. The 4.2-295 K M?ssbauer spectra of [Fe(pyoa) 2] infinity consist of a single paramagnetic high-spin iron(II) quadrupole doublet. The axial Fe-N bond direction defines the Jahn-Teller axis at an iron(II) site and, consequently, the orientation of the single-ion magnetic anisotropy. Thus, along the b axis in a given chain, the spins are collinear and parallel to the Jahn-Teller axis. The Jahn-Teller axes of adjacent intralayer chains have different orientations with an angle of 79.2 degrees between the axes in adjacent chains in a bc layer. [Fe(pyoa)2]infinity exhibits field-induced metamagnetic behavior such that, in an applied field smaller than the critical field, the iron(II) spin-canted moments experience intrachain ferromagnetic interactions and weak interchain antiferromagnetic interactions; the spin canting yields weak ferromagnetism. In an applied field larger than the critical field, the weak antiferromagnetic interchain interactions are overwhelmed to yield superparamagnetic-like slow-magnetic relaxation with an energy barrier of 23(3) K. Single-crystal magnetic studies reveal a quasi-uniaxial magnetic anisotropy with the a axis as the easy-magnetic axis and the b axis as the hard-magnetic axis; the susceptibility measured along the easy a axis may be fit with the Glauber model to yield an effective intrachain exchange coupling constant of 2.06(8) K. A dynamic analysis of the susceptibility yields a 6.3(1) K energy barrier for intrachain domain wall creation. The observed field-assisted superparamagnet-like behavior is consistent with the dynamics of a single-chain magnet. Thus, [Fe(pyoa)2]infinity is best considered as a "metamagnetic-like" single-chain magnet.