Structural deformations and bond length alternation in porphyrin pi-cation radicals.

This review discusses the structural changes that occur when the porphyrin ring of metalloporphyrin complexes is oxidized to form a pi-cation radical species. Although various differences in core conformation between the pi-cation derivative and the unoxidized homologue have been observed, there does not appear to be a general pattern of change. A frequently observed feature in pi-cation derivatives is the appearance of an alternating bond distance pattern in the inner ring of the porphyrin consistent with a localized structure rather the delocalized structure usually seen. The pattern, first seen in cofacial dimeric structures, has now been seen in monomeric systems as well. The nature and frequency of the observation and possible explanation are given.

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 Mössbauer 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 Mössbauer 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.

mu-Oxo-bis[(2,3,12,13-tetrabromo-5,10,15, 20-tetraphenylporphyrinato)iron(III)] bis(dichloromethane) solvate.

The precise structure of the title compound, [Fe(2)O(C(44)H(24)Br(4)N(4))(2)].2CH(2)Cl(2), is reported. The Fe-N distances are non-equivalent in pairs because of the asymmetric peripheral substitution; the values are 2.098 A to the brominated rings and 2.041 A to the other two rings. The Fe-O bond distance is 1.7583 (4) A. The molecule has required twofold symmetry so that there is one unique porphyrin macrocycle and one Fe-O bond length in contrast to a previous report on the same species.

Definitive assignment of the g tensor of [Fe(OEP)(NO)] by single-crystal EPR.

Single-crystal EPR measurements have been performed on the triclinic form of [Fe(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 7404) and on the isomorphous cobalt derivative [Co(OEP)(NO)] (Ellison, M. K.; Scheidt, W. R. Inorg. Chem. 1998, 37, 382) which has been doped with [Fe(OEP)(NO)]. Principal values of the g tensor determined at room temperature are gmax = 2.106, gmid = 2.057, and gmin = 2.015. The principal direction associated with the minimum g value lies 8 degrees from the Fe-N(NO) direction, 2 degrees from the normal to the heme plane, and 42 degrees from the N-O direction. The direction associated with the maximum g value lies 9 degrees from the normal to the Fe-N-O plane. The fact that the direction of gmin is near the Fe-N(NO) direction is consistent with the dominant role of spin-orbit coupling at the iron atom in determining the g tensor and with the picture of the electronic structure of the compound from restricted calculations, which makes the half-filled orbital mostly dz2 on the iron atom. The hyperfine tensor is nearly isotropic and was only resolved in the doped samples. Principal values of the A tensor determined at room temperature are 40.9, 49.7, and 42.7 MHz. Principal values of the g tensor determined from the doped samples at 77 K are gmax = 2.110, gmid = 2.040, and gmin = 2.012. Principal values of the A tensor are 42.5, 52.8, and 44.1 MHz at 77 K. The small change in g values with temperature is in contrast to the large temperature dependence on g values observed in samples of MbNO (Hori et al. J. Biol. Chem. 1981, 256, 7849).

An integrated approach to the mid-spin state (S = 3/2) in six-coordinate iron(III) chiroporphyrins.

An intermediate-spin state very close to the mid-spin state (S = 3/2) can be stabilized in a ferric porphyrin by an integrated approach which combines the favorable effects of a weak axial field strength and of a small macrocycle hole. Axial ligand exchange by reaction of chloroiron(III)tetramethylchiroporphyrin [(TMCP)FeCl] with silver perchlorate in ethanol-chloroform leads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline products containing a bisethanol complex [[(TMCP)FeIII(EtOH)2]ClO4] and three variants of a mixed ethanol-water complex [[(TMCP)FeIII(EtOH)(H2O)]ClO4] have been structurally characterized in the solid state. The small hole of the ruffled chiroporphyrin and the weak axial oxygen ligation result in strongly tetragonally distorted complexes. The six-coordinate species exhibit long axial Fe-O bond distances (2.173(5)-2.272(4) A) and the shortest equatorial Fe-N(av) distances (1.950(5)-1.978(7) A) found as yet in a ferric porphyrin, reflecting a singly occupied dz2 orbital and a largely depopulated dx2-y2 orbital. An intriguing case of bond-stretch isomerism is seen for the axial Fe-O bonds in two crystallographically independent mixed ethanol-water species, and it is accounted for by their distinct intra- and intermolecular hydrogen-bond arrays. The Mössbauer spectrum (delta = 0.35(1) mm s-1 and delta EQ = 3.79(1) mm s-1 at 77 K) indicates a strong tetragonal distortion around the ferric ion, in agreement with the structural data. The value of the magnetic moment (mu eff = 3.8 mu B in the range 50-300 K) strongly supports a mid-spin state (S = 3/2). The EPR spectrum at 80 K (g perpendicular approximately 4.0, g parallel approximately 2.00) is consistent with a nearly pure mid-spin state (4A2) with little rhombic distortion. The 1H NMR spectra in CDCl3-EtOH exhibit upfield-shifted resonances for the pyrrole protons (delta approximately -30 ppm) which are consistent with the depopulated iron dx2-y2 orbital. Solution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussed on the basis of the NMR data. The bisethanol and ethanol-water species are potential models of unknown hemoprotein ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obtained by site-directed mutagenesis.

Molecular structure of an Fe(IV) species:.

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

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. Mössbauer 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 Mössbauer 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 Mössbauer 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.

Magnetic interactions in the high-spin iron(III) oxooctaethylchlorinato derivative [Fe(oxoOEC)(Cl)] and its pi-cation radical [Fe(oxoOEC.)(Cl)]SbCl6.

The preparation and characterization of the beta-oxochlorin derivative [3,3,7,8,12,13,17,18-octaethyl-(3H)-porphin-2-onato(2-)]iron(III) chloride, [Fe(oxoOEC)(Cl)], and its pi-cation radical derivative [Fe(oxoOEC.)(Cl)]SbCl6 is described. Both compounds have been characterized by single-crystal X-ray structure determinations, IR, UV/vis/near-IR, and Mössbauer spectroscopies, and temperature-dependent magnetic susceptibility measurements. The macrocycles of [Fe(oxoOEC)(Cl)] and [Fe(oxoOEC.)(Cl)]SbCl6 are both saddled, and [Fe(oxoOEC.)(Cl)]-SbCl6 is slightly ruffled as well. [Fe(oxoOEC)(Cl)] shows a laterally shifted dimeric unit in the solid state, with a mean plane separation of 3.39 A and a lateral shift of 7.39 A. Crystal data for [Fe(oxoOEC)(Cl)]: triclinic, space group P1, Z = 2, a = 9.174(2) A, b = 13.522(3) A, c = 14.838(3) A, alpha = 95.79(3) degrees, beta = 101.46(2) degrees, gamma = 104.84(3) degrees. Upon oxidation, the inter-ring geometric parameters increase; the mean plane separation and the lateral shift of the dimeric unit of [Fe(oxoOEC.)(Cl)]SbCl6 are 4.82 and 8.79 A, respectively. Crystal data for [Fe(oxoOEC.)(Cl)]SbCl6: monoclinic, space group Cc, Z = 4, a = 19.8419(13) A, b = 10.027(2) A, c = 22.417(4) A, beta = 96.13(2) degrees. A broad near-IR absorption band appears at 1415 nm for the pi-cation radical, [Fe(oxoOEC.)(Cl)]SbCl6. Zero-field Mössbauer measurements at 4.2 K for both [Fe(oxoOEC)(Cl)] and [Fe(oxoOEC.)(Cl)]SbCl6 confirmed that the oxidation state of the iron atom did not change upon chemical oxidation. Solid-state magnetic susceptibility measurements for [Fe(oxoOEC.)(Cl)]SbCl6 resulted in a large temperature dependence of the magnetic moment that can best be fit with a model that includes a zero-field splitting parameter of D = 6 cm-1, antiferromagnetic intermolecular iron-iron coupling (2JFe-Fe = -0.14 cm-1), antiferromagnetic intramolecular iron-radical coupling (2JFe-r = -76 cm-1), and antiferromagnetic radical-radical coupling (2Jr-r = -13 cm-1).

Molybdenum(V) on an oxide string. Formation and solution stability of the linear complex (mu-trans-dioxo(tetraphenylporphyrinato)molybdenato(V))- bis(oxo(tetraphenylporphyrinato)molybdenum(V)) perchlorate.

The unusual linear trinuclear complex [Mo3O4(TPP)3]+ is formed in solution upon the reaction of [MoO(TPP)-(OClO3)] with [[MoO(TPP)]2O], and an equilibrium between [Mo3O4(TPP)3]+ and its constituent species is rapidly established. Spectrophotometric experiments suggest that [Mo3O4(TPP)3]+ is the predominant species found in solutions resulting from the mixture of [MoO(TPP)(OClO3)] and [[MoO(TPP)]2O], and its formation is strongly favored (log K = 5.5 +/- 0.5 M-1). No evidence of higher oligomers has been observed. A mechanism for the formation of [Mo3O4(TPP)3]+ by the controlled hydrolysis of [MoO(TPP)(OClO3)] is proposed.

Nitrato(2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III).

The crystal structure of [Fe(C36H44N4)(NO3)] has been determined in the space group P1. The unit cell contains two molecules. The Fe atom is displaced out of the porphyrin plane by 0.50 A, the average Fe-Np distance is 2.056 (1) A (where Np is a porphyrin N atom) and the Fe-O(NO3) bond length is 2.016 (3) A.

Chloro(2,3,7,8,12,13,17,18-octaethylporphinato)gallium(III).

The crystal structure of [Ga(C36H44N4)Cl].1.45CH2C(l)2 has been determined. Examination of the crystal established a two-molecule triclinic unit cell with space group P1. The asymmetric unit contains one porphyrin molecule and two solvate molecules, one as an approximate half molecule with required inversion symmetry. All measurements were made at 127(2) K. The average Ga-N distance is 2.035(4) A and the axial Ga-Cl distance 2.240(1) A. The displacement of the Ga atom from the N4 porphyrin plane is 0.40 A.

mu-Oxo-bis[(5,10,15,20-tetraphenylporphyrinato)oxomolybdenum(V)].

The crystal structure of a new solvated crystal form of the title complex, [¿Mo(O)(TPP)¿2O].1.5H2O.0.5CH2Cl2 (TPP = C44H28N4), has been determined. The dimeric molecule is in a general position, and the overall structural features are extremely similar to those of the previously reported non-solvated form.

(Ethanol)(2,3,7,8,12,13,17,18-octaethylporphinato)manganese(III) perchlorate.

The crystal structure of [Mn(C36H44N4)(C2H6O)]ClO4 has been determined. The average Mn-Np bond distance is 1.997 (5) A and the axial Mn-O bond length is 2.145 (2) A. The manganese(III) ion has a displacement of 0.17 A from the mean plane of the 24-atom porphinato core.

Chloro(5,10,15,20-tetraphenylporphyrinato)manganese(III) with 4/m symmetry.

The crystal structure of [Mn(TPP)Cl] in space group I4/m (where TPP is C44H28N4) has been determined. The unit cell contains two full molecules, with one eighth of a molecule unique. An out-of-plane model for the Mn atom was applied and all non-H atoms were refined anisotropically. The Mn--N distance is 2.002 (3) A, the axial Mn--Cl distance is 2.297 (15) A and the out-of-plane displacement of the Mn atom is 0.16 A. The possibility of a reverse-doming porphyrin core conformation is mentioned briefly.

Diaqua(5,10,15,20-tetraphenylporphinato)iron(III) Perchlorate, [Fe(C44H28N4)(H2O)2]ClO4.

The crystal structure of a new crystal form of [Fe(TPP)(H2O)2]ClO4 has been determined. The asymmetric unit contains one molecule in a general position and a half molecule with required inversion symmetry. The two independent molecules have almost identical average values for the equatorial Fe-Np bond lengths [2.029 (4) and 2.028 (6) A], and the axial Fe-O bond lengths are 2.140 (2) and 2.121 (3) A for molecule 1 (in a general position) and 2.126 (2) A for molecule 2 (in a special position). A brief comparison of the core structure and hydrogen-bonding environment of this molecule with two other crystal forms is described.

(2,3,7,8,12,13,17,18-Octaethylporphinato)perchloratomanganese(III).

The crystal structure of [Mn(OEP)(OClO3)] (where OEP = C36H44N4) has been determined. The axial Mn-O bond length is 2.183 (2) A and the equatorial Mn-Np bond lengths have an average value of 2.000 (5) A. The molecules form weak dimers in the solid state with interring distances of 3.49 A and a lateral shift of 3.27 A. The compound is found to be isomorphous with the iron derivative [Fe(OEP)(OClO3)]. A brief comparison of the structural parameters for the two molecules is given.

Structure of (cyano)(2,3,7,8,12,13,17,18-octaethylporphinato)(pyridine)iron(III) chloroform solvate.

[Fe(C36H44N4)(CN)(C5H5N)].CHCl3, Mr = 813.12, monoclinic, P2(1)/n, a = 10.345 (2), b = 14.748 (2), c = 27.302 (3) A, beta = 92.12 (3) degrees, V = 4162.6 A3, Z = 4, D chi = 1.30, Dm = 1.29 g cm-3, Mo K alpha, lambda = 0.71073 A, mu = 5.92 cm-1, F(000) = 1708, T = 293 K, R = 0.046 for 5193 unique observed [F greater than 3 sigma(F)] reflections. The iron(III) atom is six-coordinate with an average Fe-N(p) distance of 1.980 (4) A and axial Fe-C and Fe-N distances of 1.934 (4) and 2.087 (3) A, respectively.

Structure of (2,3,7,8,12,13,17,18-octaethylporphinato)copper(II).

[Cu(C36H44N4)], Mr = 596.3, triclinic, P1, a = 13.314(5), b = 13.392(5), c = 4.805(3) A, alpha = 92.42(4), beta = 93.38(4), gamma = 113.08(1) degrees, V = 784.8 A3, Z = 1, Dx = 1.26 g cm-3, Dm = 1.25 g cm-3, lambda (Mo K alpha) = 0.71073 A, mu = 7.25 cm-1, F(000) = 317, T = 293 K, R = 0.055 for 2166 unique observed reflections.

Structure of (2,3,7,8,12,13,17,18-octaethylporphinato)dioxoosmium(VI).

[Os(C36H44N4)O2].C6H5Cl, Mr = 867.53, triclinic, P1, a = 8.177 (1), b = 10.668 (1), c = 11.791 (1) A, a = 73.91 (1), beta = 83.18 (1), gamma = 75.07 (1) degree, V = 953.6 A3, Z = 1, Dx = 1.51 g cm-3, Dm = 1.50 g cm-3, Mo K alpha, lambda = 0.71073 A, mu = 34.5 cm-1, F(000) = 438, T = 293 K, R = 0.048 for 5721 unique observed reflections. The Os atom is centered in the porphinato plane with an average Os-N = 2.052 (6) A and Os-O = 1.745 (5) A. The 24-atom porphyrin core is effectively planar with the largest deviation from the mean plane less than 0.02 A.