Structural, Magnetic, and Dynamic Characterization of the (dxz,dyz)4(dxy)1 Ground-State Low-Spin Iron(III) Tetraphenylporphyrinate Complex [(p-TTP)Fe(2,6-XylylNC)2]CF3SO3

Models of the bis-histidine-coordinated ferricytochromes: Mössbauer and EPR spectroscopic studies of low-spin iron(III) tetrapyrroles of various electronic ground states and axial ligand orientations

The EPR and magnetic Mössbauer spectra of a series of axial ligand complexes of tetrakis(2,6-dimethoxyphenyl)porphyrinatoiron(III), [(2,6-(OMe)(2))(4)TPPFeL(2)](+), where L= N-methylimidazole, 2-methylimidazole, or 4-(dimethylamino)pyridine, of one axial ligand complex of tetraphenylporphyrin, the bis(4-cyanopyridine) complex [TPPFe(4-CNPy)(2)](+), and of one axial ligand complex of tetraphenylchlorin, [TPCFe(ImH)(2)](+), where ImH=imidazole, have been investigated and compared to those of low-spin Fe(III) porphyrinates and ferriheme proteins reported in the literature. On the basis of this and previous complementary spectroscopic investigations, three types of complexes have been identified: those having (d(xy))(2)(d(xz),d(yz))(3) electronic ground states with axial ligands aligned in perpendicular planes (Type I), those having (d(xy))(2)(d(xz),d(yz))(3) electronic ground states with axial ligands aligned in parallel planes (Type II), and those having the novel (d(xz),d(yz))(4)(d(xy))(1) electronic ground state (Type III). A subset of the latter type, with planar axial ligands aligned parallel to each other or strong macrocycle asymmetry that yield rhombic EPR spectra, cannot be created using the porphyrinate ligand. Type I centers are characterized by "large g(max)" EPR spectra with g>3.2 and well-resolved, widely spread magnetic Mössbauer spectra having A(zz)/ g(N)mu(N)>680 kG, with A(xx) negative in sign but much smaller in magnitude than A(zz), while Type II centers have well-resolved rhombic EPR spectra with g(zz)=2.4-3.1 and also less-resolved magnetic Mössbauer spectra, and usually have A(zz)/ g(Nmu(N) in the range of 440-660 kG (but in certain cases as small as 180 kG) and A(xx) again negative in sign but only somewhat smaller (but occasionally larger in magnitude) than A(zz), and Type III centers have axial EPR spectra with g( upper left and right quadrants ) approximately 2.6 or smaller and g( vertical line )<1.0-1.95, but often not resolved, and less-resolved magnetic Mössbauer spectra having A(zz)/ g(N)mu(N) in the range of 270-400 kG, and A(xx) again negative in sign but much smaller in magnitude than A(zz). An exception to this rule is [TPPFe(4-CNPy)(2)](+), which has A(xx)/ g(N)mu(N)=-565 kG, A(yy)/ g(N)mu(N)=629 kG, and A(zz)/ g(N)mu(N)=4 kG. A subset of Type II complexes (Type II') have rhombicities ( V/Delta) much greater than 0.67 and A(zz)/ g(N)mu(N) ranging from 320 to 170 kG, with A(xx) also negative but with the magnitude of A(xx) significantly larger than that of A(zz). These classifications are also observed for a variety of ferriheme proteins, and they lead to linear correlations between A(zz) and either A(xx), g(zz), or V/Delta for Types I and II (but not for A(zz) versus V/Delta for Type II'). Not enough data are yet available on Type III complexes to determine what, if any, correlations may be observed.

Pulsed EPR and NMR spectroscopy of paramagnetic iron porphyrinates and related iron macrocycles: how to understand patterns of spin delocalization and recognize macrocycle radicals

Pulsed EPR spectroscopic techniques, including ESEEM (electron spin echo envelope modulation) and pulsed ENDOR (electron-nuclear double resonance), are extremely useful for determining the magnitudes of the hyperfine couplings of macrocycle and axial ligand nuclei to the unpaired electron(s) on the metal as a function of magnetic field orientation relative to the complex. These data can frequently be used to determine the orientation of the g-tensor and the distribution of spin density over the macrocycle, and to determine the metal orbital(s) containing unpaired electrons and the macrocycle orbital(s) involved in spin delocalization. However, these studies cannot be carried out on metal complexes that do not have resolved EPR signals, as in the case of paramagnetic even-electron metal complexes. In addition, the signs of the hyperfine couplings, which are not determined directly in either ESEEM or pulsed ENDOR experiments, are often needed in order to translate hyperfine couplings into spin densities. In these cases, NMR isotropic (hyperfine) shifts are extremely useful in determining the amount and sign of the spin density at each nucleus probed. For metal complexes of aromatic macrocycles such as porphyrins, chlorins, or corroles, simple rules allow prediction of whether spin delocalization occurs through sigma or pi bonds, and whether spin density on the ligands is of the same or opposite sign as that on the metal. In cases where the amount of spin density on the macrocycle and axial ligands is found to be too large for simple metal-ligand spin delocalization, a macrocycle radical may be suspected. Large spin density on the macrocycle that is of the same sign as that on the metal provides clear evidence of either no coupling or weak ferromagnetic coupling of a macrocycle radical to the unpaired electron(s) on the metal, while large spin density on the macrocycle that is of opposite sign to that on the metal provides clear evidence of antiferromagnetic coupling. The latter is found in a few iron porphyrinates and in most iron corrolates that have been reported thus far. It is now clear that iron corrolates are remarkably noninnocent complexes, with both negative and positive spin density on the macrocycle: for all chloroiron corrolates reported thus far, the balance of positive and negative spin density yields -0.65 to -0.79 spin on the macrocycle. On the other hand, for phenyliron corrolates, the balance of spin density on the macrocycle is zero, to within the accuracy of the calculations (Zakharieva, O.; Schünemann, V.; Gerdan, M.; Licoccia, S.; Cai, S.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636-6648), although both negative and positive spin densities are found on the individual atoms. DFT calculations are invaluable in providing calculated spin densities at positions that can be probed by (1)H NMR spectroscopy, and the good agreement between calculated spin densities and measured hyperfine shifts at these positions leads to increased confidence in the calculated spin densities at positions that cannot be directly probed by (1)H NMR spectroscopy. (13)C NMR spectroscopic investigations of these complexes should be carried out to probe experimentally the nonprotonated carbon spin densities.

Importance of the C-H...N weak hydrogen bonding on the coordination structures of manganese(III) porphyrin complexes

The reactions between Mn(Por)Cl and Bu(4)N(+)CN(-) have been examined in various solvents by UV-vis and (1)H NMR spectroscopy, where Por's are dianions of meso-tetraisopropylporphyrin (T(i)PrP), meso-tetraphenylporphyrin (TPP), meso-tetrakis(p-(trifluoromethyl)phenyl)porphyrin (p-CF(3)-TPP), meso-tetramesitylporphyrin (TMP), and meso-tetrakis(2,6-dichlorophenyl)porphyrin (2,6-Cl(2)-TPP). Population ratios of the reaction products, Mn(Por)(CN) and [Mn(Por)(CN)(2)](-), have been sensitively affected by the solvents used. In the case of Mn(T(i)PrP)Cl, the following results are obtained: (i) The bis-adduct is preferentially formed in dipolar aprotic solvents such as DMSO, DMF, and acetonitrile. (ii) Both the mono- and bis-adduct are formed in the less polar solvents such as CH(2)Cl(2) and benzene though the complete conversion to the bis-adduct is achieved with much smaller amount of the ligand in benzene solution. (iii) Only the mono-adduct is formed in CHCl(3) solution even in the presence of a large excess of cyanide. (iv) Neither the mono- nor the bis-adduct is obtained in methanol solution. The results mentioned above have been explained in terms of the C-H.N and O-H.N hydrogen bonding in chloroform and methanol solutions, respectively, between the solvent molecules and cyanide ligand; hydrogen bonding weakens the coordination ability of cyanide and reduces the population of the bis-adduct. The importance of the C-H.N weak hydrogen bonding is most explicitly shown in the following fact: while the starting complex is completely converted to the bis-adduct in CH(2)Cl(2) solution, the conversion from the mono- to the bis-adduct is not observed even in the presence of 7000 equiv of Bu(4)N(+)CN(-) in CHCl(3) solution. The effective magnetic moments of the bis-adduct has been determined by the Evans method to be 3.2 micro(B) at 25 degrees C, suggesting that the complex adopts the usual (d(xy))(2)(d(xz), d(yz))(2) electron configuration despite the highly ruffled porphyrin core expected for [Mn(T(i)PrP)(CN)(2)](-). The spin densities of [Mn(T(i)PrP)(CN)(2)](-) centered on the pi MO have been determined on the basis of the (1)H and (13)C NMR chemical shifts. Estimated spin densities are as follows: meso-carbon, -0.0014; alpha-pyrrole carbon, -0.0011; beta-pyrrole carbon, +0.0066; pyrrole nitrogen, -0.022. The spin densities at the pyrrole carbon and meso nitrogen atoms are much smaller than those of the corresponding [Mn(TPP)(CN)(2)](-), which is ascribed to the nonplanar porphyrin ring of [Mn(T(i)PrP)(CN)(2)](-). This study has revealed that the C-H.N weak hydrogen bonding is playing an important role in determining the stability of the manganese(III) complexes.

Electronic structure of iron chlorins: characterization of bis(l-valine methyl ester)(meso-tetraphenylchlorin)iron(III)triflate and bis(l-valine methyl ester)(meso-tetraphenylchlorin)iron(II)

The synthesis and characterization of the two iron chlorin complexes [Fe(III)(TPC)(NH(2)CH(CO(2)CH(3))(CH(CH(3))(2)))(2)]CF(3)SO(3) (1) and Fe(II)(TPC)[(NH(2)CH(CO(2)CH(3))(CH(CH(3))(2))](2) (2) are reported. The crystal structure of complex 1 has been determined. The X-ray structure shows that the porphyrinate rings are weakly distorted. The metal-nitrogen distances to the reduced pyrrole N(4), 2.034(4) A, and to the pyrrole trans to it N(2), 2.012(4) A, are longer than the distances to the two remaining nitrogens [N(1), 1.996(4) A, and N(3), 1.984(4) A], leading to a core-hole expansion of the macrocycle due to the reduced pyrrole. The (1)H NMR isotropic shifts at 20 degrees C of the different pyrrole protons of 1 varied from -0.8 to -48.3 ppm according to bis-ligated complexes of low-spin ferric chlorins. The EPR spectrum of [Fe(TPC)(NH(2)CH(CO(2)CH(3))(CH(CH(3))(2)))(2)]CF(3)SO(3) (1) in solution is rhombic and gives the principal g values g(1) = 2.70, g(2) = 2.33, and g(3) = 1.61 (Sigmag(2) = 15.3). These spectroscopic observations are indicative of a metal-based electron in the d(pi) orbital for the [Fe(TPC)(NH(2)CH(CO(2)CH(3))(CH(CH(3))(2)))(2)]CF(3)SO(3) (1) complex with a (d(xy))(2)(d(xz)d(yz))(3) ground state at any temperature. The X-ray structure of the ferrous complex 2 also shows that the porphyrinate rings are weakly distorted. The metal-nitrogen distances to the reduced pyrrole N(4), 1.991(5) A, and to the pyrrole trans to it N(2), 2.005(6) A, are slightly different from the distances to the two remaining nitrogens [N(1), 1.988(5) A, and N(3), 2.015(5) A], leading to a core-hole expansion of the macrocycle due to the reduced pyrrole.

Models of the low-spin iron(III) hydroperoxide intermediate of heme oxygenase: magnetic resonance evidence for thermodynamic stabilization of the d(xy) electronic state at ambient temperatures

The (13)C pulsed ENDOR and NMR study of [meso-(13)C-TPPFe(OCH(3))(OO(t)Bu)](-) performed in this work shows that although the unpaired electron in low-spin ferrihemes containing a ROO(-) ligand resides in a d(pi) orbital at 8 K, the d(xy) electron configuration is favored at physiological temperatures. The variable temperature NMR spectra indicate a dynamic situation in which a heme with a d(pi) electron configuration and planar porphyrinate ring is in equilibrium with a d(xy) electron configuration that has a ruffled porphyrin ring. Because of the similarity in the EPR spectra of the hydroperoxide complexes of heme oxygenase, cytochrome P450, and the model heme complex reported herein, it is possible that these two electron configurations and ring conformations may also exist in equilibrium in the enzymatic systems. The ruffled porphyrinate ring would aid the attack of the terminal oxygen of the hydroperoxide intermediate of heme oxygenase (HO) on the meso-carbon, and the large spin density at the meso-carbons of a d(xy) electron configuration heme suggests the possibility of a radical mechanism for HO. The dynamic equilibrium between the ruffled (d(xy)) and planar (d(pi)) conformers observed in the model complexes also suggests that a flexible heme binding cavity may be an important structural motif for heme oxygenase activity.

Effects of solvents on the electron configurations of the low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]iron(III) complex: importance of the C-H...N weak hydrogen bonding

There are two types of electron configurations, (d(xy))(2)(d(xz), d(yz))(3) and (d(xz), d(yz))(4)(d(xy))(1), in low-spin iron(III) porphyrin complexes. To reveal the solvent effects on the ground-state electron configurations, we have examined the (13)C- and (1)H-NMR spectra of low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]ferrate(III) in a variety of solvents, including protic, dipolar aprotic, and nonpolar solvents. On the basis of the NMR study, we have reached the following conclusions: (i) the complex adopts the ground state with the (d(xz), d(yz))(4)(d(xy))(1) electron configuration, the (d(xz), d(yz))(4)(d(xy)())(1) ground state, in methanol, because the d(pi) orbitals are stabilized due to the O-H...N hydrogen bonding between the coordinated cyanide and methanol; (ii) the complex also exhibits the (d(xz), d(yz))(4)(d(xy))(1) ground state in nonpolar solvents, such as chloroform and dichloromethane, which is ascribed to the stabilization of the d(pi) orbitals due to the C-H...N weak hydrogen bonding between the coordinated cyanide and the solvent molecules; (iii) the complex favors the (d(xz), d(yz))(4)(d(xy))(1) ground state in dipolar aprotic solvents, such as DMF, DMSO, and acetone, though the (d(xz), d(yz))(4)(d(xy))(1) character is less than that in chloroform and dichloromethane; (iv) the complex adopts the (d(xy))(2)(d(xz), d(yz))(3) ground state in nonpolar solvents, such as toluene, benzene, and tetrachloromethane, because of the lack of hydrogen bonding in these solvents; (v) acetonitrile behaves like nonpolar solvents, such as toluene, benzene, and tetrachloromethane, though it is classified as a dipolar aprotic solvent. Although the NMR results have been interpreted in terms of the solvent effects on the ordering of the d(xy) and d(pi) orbitals, they could also be interpreted in terms of the solvent effects on the population ratios of two isomers with different electron configurations. In fact, we have observed the unprecedented EPR spectra at 4.2 K which contain both the axial- and large g(max)-type signals in some solvents such as benzene, toluene, and acetonitrile. The observation of the two types of signals has been ascribed to the slow interconversion on the EPR time scale at 4.2 K between the ruffled complex with the (d(xz), d(yz))(4)(d(xy))(1) ground state and, possibly, the planar (or nearly planar) complex with the (d(xy))(2)(d(xz), d(yz))(3) ground state.

meso Substituent effects on the geometric and electronic structures of high-spin and low-spin iron(III) complexes of mono-meso-substituted octaethylporphyrins

Introduction of a single meso substituent into ClFe(III)(OEP) or K[(NC)(2)Fe(OEP)] results in significant changes in the geometric and/or spectroscopic properties of these complexes. The mono-meso-substituted iron(III) complexes ClFe(III)(meso-Ph-OEP), ClFe(III)(meso-n-Bu-OEP), ClFe(III)(meso-MeO-OEP), ClFe(III)(meso-Cl-OEP), ClFe(III)(meso-NC-OEP), ClFe(III)(meso-HC(O)-OEP), and ClFe(III)(meso-O(2)N-OEP) have been isolated and characterized by their UV/vis and paramagnetically shifted (1)H NMR spectra. The structures of both ClFe(III)(meso-Ph-OEP) and ClFe(III)(meso-NC-OEP) have been determined by X-ray crystallography. Both molecules have five-coordinate structures typical for high-spin (S = 5/2) iron(III) complexes. However, the porphyrins themselves no longer have the domed shape seen in ClFe(III)(OEP), and the N(4) coordination environment possesses a slight rectangular distortion. These high-spin, mono-meso-substituted iron(III) complexes display (1)H NMR spectra in chloroform-d solution which indicate that the conformational changes seen in the solid-state structures are altered by normal molecular motion to produce spectra consistent with C(s) molecular symmetry. In pyridine solution the high-spin six-coordinate complexes [(py)ClFe(III)(meso-R-OEP)] form. In methanol solution in the presence of excess potassium cyanide, the low-spin six-coordinate complexes K[(NC)(2)Fe(III)(meso-R-OEP)] form. The (1)H NMR spectra of these show that electron-donating substituents produce an upfield relocation of the meso-proton chemical shifts. This relocation is interpreted in terms of increased contribution from the less common (d(xz),d(yz))(4)(d(xy))(1) ground electronic state as the meso substituent becomes more electron donating.

Factors affecting the electronic ground state of low-spin iron(III) porphyrin complexes

To determine the factors affecting the ground-state electron configuration of low-spin Fe(III) porphyrin complexes, we have examined the (1)H NMR, (13)C NMR, and EPR spectra of a series of low-spin bis-ligated Fe(III) porphyrin complexes [Fe(Por)L(2)](+/-), in which the positions of porphyrin substituents and the coordination ability of axial ligands are different. The seven porphyrins used in this study are meso-tetraalkylporphyrins (TRP: R is propyl, cyclopropyl, or isopropyl), meso-tetraphenylporphyrin (TPP), meso-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin, and 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octaalkylporphyrins (ORTPP: R is methyl or ethyl). The porphyrin cores of TRP are more or less S(4)-ruffled depending on the bulkiness of the alkyl substituents, while those of ORTPP are highly S(4)-saddled. Three types of axial ligands are examined which have the following characteristics in ligand field theory: they are (i) strong sigma-donating imidazole (HIm), (ii) strong sigma-donating and weak pi-accepting cyanide (CN(-)), and (iii) weak sigma-donating and strong pi-accepting tert-butyl isocyanide ((t)BuNC). In the case of the bis(HIm) complexes, only the isopropyl complex, [Fe(T(i)PrP)(HIm)(2)](+), has shown the less common (d(xz), d(yz))(4)(d(xy))(1) ground state; the other six complexes have exhibited the common (d(xy))(2)(d(xz), d(yz))(3) ground state. When the axial imidazole is replaced by cyanide, even the propyl and cyclopropyl complexes have shown the (d(xz), d(yz))(4)(d(xy))(1) ground state; the TPP and ORTPP complexes have still maintained the common (d(xy))(2)(d(xz), d(yz))(3) ground state. In the case of the bis((t)()BuNC) complexes, all the complexes have shown the (d(xz), d(yz))(4)(d(xy))(1) ground state. However, the contribution of the (d(xz), d(yz))(4)(d(xy))(1) state to the electronic ground state differs from complex to complex; the (d(xz), d(yz))(4)(d(xy))(1) contribution is the largest in [Fe(T(i)PrP)((t)()BuNC)(2)](+) and the smallest in [Fe(OETPPP)((t)BuNC)(2)](+). We have then examined the electronic ground state of low-spin [Fe(OEP)((t)BuNC)(2)](+) and [Fe(ProtoIXMe(2))((t)BuNC)(2)](+); OEP and ProtoIXMe(2) represent 2,3,7,8,12,13,17,18-octaethylporphyrin and protoporphyrin-IX dimethyl ester, respectively. These porphyrins have a(1u) HOMO in contrast to the other seven porphyrins that have a(2u) HOMO. The (13)C NMR and EPR studies have revealed that the contribution of the (d(xz), d(yz))(4)(d(xy))(1) state in these complexes is as small as that in [Fe(OETPP)((t)BuNC)(2)](+). On the basis of these results, we have concluded that the low-spin iron(III) porphyrins that have (i) strong axial ligands, (ii) highly saddle shaped porphyrin rings, (iii) porphyrins with a(1u) HOMO, and (iv) electron withdrawing substituents at the meso positions tend to maintain the common (d(xy))(2)(d(xz), d(yz))(3) ground state.

1H pulsed ENDOR and ESEEM evidence that the bis-imidazole complexes of iron(III) tetraphenylchlorin and tetraphenylporphyrin have the same order of g values, and the same electronic ground state

The electronic structures of the bis-imidazole complexes of iron(III) tetraphenylporphyrin ([(TPP)Fe(ImH)(2)](+)) and iron(III) tetraphenylchlorin ([(TPC)Fe(ImH)(2)](+)) in frozen glassy solutions have been studied by the pulsed electron nuclear double resonance (ENDOR) technique of Mims and by electron spin-echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra have been used to determine the orientation of the imidazole ligand planes with respect to the g tensor axes. In the ENDOR spectra, the manifestations of the implicit TRIPLE effect described and explained earlier by Doan et al. (J. Am. Chem. Soc. 1996, 118, 7014) were seen. In this work, the explicit expressions describing this effect were derived for the first time and used to successfully simulate the proton ENDOR spectra at the low- (LF) and high-field (HF) edges of the EPR spectrum. Using pulsed ENDOR, we have been able to determine the spin density distributions in the pi-systems of both tetrapyrroles and show that [(TPC)Fe(ImH)(2)](+) has the electronic orbital ground state (d(xy)())(2)(d(xz)(),d(yz)())(3), the same as that known for [(TPP)Fe(ImH)(2)](+), and the largest principal g value corresponds to the g tensor axis 3, which is normal to the heme plane. For the TPP complex, the g tensor axis 1, corresponding to the smallest principal g value, was found to be at an angle phi(1) of 30-35 degrees from the N-Fe-N axis, with the ligand planes rotated by the angle of 20-25 degrees in the opposite direction. For the TPC complex, phi(1) was found to be about 25 degrees from the direction N(I)-Fe-N(III), where N(I) corresponds to the nitrogen of the saturated pyrrole ring. The ligand planes in this complex were found to be oriented at an angle of about 10 degrees in the opposite direction.