The Effects of Axial Ligands on Electron Distribution and Spin States in Iron Complexes of Octaethyloxophlorin, Intermediates in Heme Degradation

The results presented here show that the nature of the axial ligand can alter the distribution of electrons between the metal and the porphyrin in complexes where there is an oxygen atom replacing one of the meso protons. The complexes (1-MeIm)(2)Fe(III)(OEPO) and (2,6-xylylNC)(2)Fe(II)(OEPO(*)) (where OEPO is the trianionic octaethyloxophlorin ligand and OEPO(*) is the dianionic octaethyloxophlorin radical) were prepared by addition of an excess of the appropriate axial ligand to a slurry of [Fe(III)(OEPO)](2) in chloroform under anaerobic conditions. The magnetic moment of (2,6-xylylNC)(2)Fe(II)(OEPO(*)) is temperature invariant and consistent with a simple S = (1)/(2) ground state. This complex with an EPR resonance at g = 2.004 may be considered as a model for the free-radical like EPR signal seen when the meso-hydroxylated heme/heme oxygenase complex is treated with carbon monoxide. In contrast, the magnetic moment of (1-MeIm)(2)Fe(III)(OEPO) drops with temperature and indicates a spin-state change from an S = (5)/(2) or an admixed S = (3)/(2),(5)/(2) state at high temperatures (near room temperature) to an S = (1)/(2) state at temperatures below 100 K. X-ray diffraction studies show that each complex crystallizes in centrosymmetric form with the expected six-coordinate geometry. The structure of (1-MeIm)(2)Fe(III)(OEPO) has been determined at 90, 129, and 296 K and shows a gradual and selective lengthening of the Fe-N(axial bond). This behavior is consistent with population of a higher spin state at elevated temperatures.

Metallobiliverdin radicals--DFT studies

Several aspects of the molecular and electronic structure of biliverdin derivatives have been studied using density functional theory (DFT). The calculations have been performed for complexes of trianion (BvO2)3- and dianion [BvO(OH)]2-, derived from two tautomeric forms of biliverdin, BvO2H3 and [BvO(OH)]H2, with redox innocent metal ions: lithium(I), zinc(II), and gallium(III). One-electron-oxidized and reduced forms of each complex (cation and anion radicals) have been also considered. The molecular structures of all species investigated are characterized by a helical arrangement of tetrapyrrolic ligands with the metal ion lying in the plane formed by the two central pyrrole rings. The spin density distribution in four types of metallobiliverdin radicals--[(BvO2.)Mn+]n-2,[[BvO(OH).]Mn+]n-1 (cation radicals),[(BvO2.)Mn+]n-4,[[BvO(OH).]Mn+]n-3 (anion radicals)--has been investigated. In general, the absolute values of spin density on meso carbon atoms were larger than for the beta-carbon atoms. Sign alteration of spin density has been found for meso positions, and also for the beta-carbon atoms of at least two pyrrole rings. The calculated spin density maps accounted for the essential NMR spectroscopic features of iron biliverdin derivatives, including the considerable isotropic shifts detected for the meso resonances and shift alteration at the meso and beta-positions.

A remarkable skeletal rearrangement of a coordinated tetrapyrrole: chemical consequences of palladium pi-coordination to a bilindione

Pd4(OEB)2, in which a [Pd2]2+ unit is bound in pi-fashion to olefinic sites that are exocyclic to pyrrole rings of the octaethylbilindione ligand, undergoes an unprecedented sequence of reactions that results in the rearrangement of the framework of the bilindione ligand and the formation of trans-Pd(py)2I2. This process of bilindione rearrangement and oxidation occurs as a direct consequence of the pi-coordination of the palladium. The reaction results in the migration of a nitrogen atom from a pyrrole carbon atom to what was formerly a meso carbon atom to transform a former pyrrole ring into a six-membered ring. This process also involves cleavage of the Pd-Pd and Pd-C bonds, oxidation of palladium, and introduction of an oxygen atom (from water) not necessarily in this particular sequence.

Ring-opening and meso substitution from the reaction of cyanide ion with zinc verdohemes

The reactivity of zinc verdoheme, [Zn(II)(OEOP)](O(2)CCH(3)) where OEOP is the monoanion of octaethyl-5-oxaporphyrin, with cyanide ion has been shown to be a complex process that involves not only the expected ring-opening of the macrocycle, as occurs with other nucleophiles (methoxide, methanethiolate, dimethylamide), but also substitution at one or two of the meso positions. The ring-opened products have been subjected to crystallographic study. The structures of mu-H(2)O-[Zn(II)(OEB-10,19-(CN)(2))](2) and mu-H(2)O-[(Zn(II)(OEB-10,15,19-(CN)(3))](2) both consist of two helical tetrapyrrole subunits that are coordinated to a zinc ion through four Zn-N bonds. The two zinc ions are coordinated to a bridging water molecule that is also hydrogen bonded to a lactam oxygen atom at one end of each tetrapyrrole subunit. Thus the chiral sense of one helical Zn(II)(OEB-10,19-(CN)(2)) portion is transmitted to the other Zn(II)(OEB-10,19-(CN)(2)) unit and the resulting binuclear unit is chiral. In contrast Co(II)(OEB-15,19-(CN)(2)), which was obtained by the insertion of Co(II) into the free ligand, is monomeric with a four-coordinate cobalt ion. A series of DFT geometry optimization calculations were performed on zinc complexes of 5-oxaporphyrins (verdoheme), verdins (bilindione), 4-cyano-5-oxaporphyrins, and 19-cyanoverdins in an effort to gain insights to the features of these complexes and the reactions that lead to meso-cyano-substituted cyanoverdins.

Verdoheme reactivity. Remarkable paramagnetically shifted (1)H NMR spectra of intermediates from the addition of hydroxide or methoxide with Fe(II) and Fe(III) verdohemes

Studies of the reaction of 5-oxaporphyrin iron complexes (verdohemes) with methoxide ion or hydroxide ion have been undertaken to understand the initial step of ring opening of verdohemes. High-spin [ClFe(III)(OEOP)] undergoes a complex series of reactions upon treatment with hydroxide ion in chloroform, and similar species are also detected in dichloromethane, acetonitrile, and dimethyl sulfoxide. Three distinct paramagnetic intermediates have been identified by (1)H NMR spectroscopy. These reactive species are formed by addition of hydroxide to the macrocycle and to the iron as an axial ligand. Treatment of low-spin [(py)(2)Fe(II)(OEOP)]Cl (OEOP is the monoanion of octaethyl-5-oxaporphyrin) with excess methoxide ion in pyridine solution produces [(py)(n)()Fe(II)(OEBOMe)] (n = 1 or 2) ((OEBOMe), dianion of octaethylmethoxybiliverdin), whose (1)H NMR spectrum undergoes marked alteration upon addition of further amounts of methoxide ion. An identical (1)H NMR spectrum, which is characterized by methylene resonances with both upfield and downfield paramagnetic shifts, is formed upon treatment of [Fe(II)(OEBOMe)](2) with methoxide in pyridine solution and results from the formation of [(MeO)Fe(II)(OEBOMe)](-).

Manganese(III) biliverdin IX dimethyl ester: a powerful catalytic scavenger of superoxide employing the Mn(III)/Mn(IV) redox couple

A manganese(III) complex of biliverdin IX dimethyl ester, (MnIIIBVDME)2, was prepared and characterized by elemental analysis, UV/vis spectroscopy, cyclic voltammetry, chronocoulometry, electrospray mass spectrometry, freezing-point depression, magnetic susceptibility, and catalytic dismuting of superoxide anion (O2.-). In a dimeric conformation each trivalent manganese is bound to four pyrrolic nitrogens of one biliverdin dimethyl ester molecule and to the enolic oxygen of another molecule. This type of coordination stabilizes the +4 metal oxidation state, whereby the +3/+4 redox cycling of the manganese in aqueous medium was found to be at E1/2 = +0.45 V vs NHE. This potential allows the Mn(III)/Mn(IV) couple to efficiently catalyze the dismutation of O2.- with the catalytic rate constant of kcat = 5.0 x 10(7) M-1 s-1 (concentration calculated per manganese) obtained by cytochrome c assay at pH 7.8 and 25 degrees C. The fifth coordination site of the manganese is occupied by an enolic oxygen, which precludes binding of NO., thus enhancing the specificity of the metal center toward O2.-. For the same reason the (MnIIIBVDME)2 is resistant to attack by H2O2. The compound also proved to be an efficient SOD mimic in vivo, facilitating the aerobic growth of SOD-deficient Escherichia coli.

Redox characteristics of nickel and palladium complexes of the open-chain tetrapyrrole octaethylbilindione: a biliverdin model

Octaethylbiliverdin, C35H46N4O2, is a linear tetrapyrrole that can exist in coordinated form as the fully reduced trianion (OEB)3-, as the two electron oxidized monoanion, (OEBOx)-, or as the one electron oxidized radical, (OEB.)2-. The three-membered electron-transfer series involving [M(OEB)]n with n = +1, 0, and -1, and M = Ni or Pd has been characterized electrochemically. The most highly oxidized members of these series have been isolated in the form of their diamagnetic triiodide salts, [NiII(OEBOx)]I3 and [PdII(OEBOx)]I3, and characterized spectroscopically and by X-ray diffraction. [NiII(OEBOx)]I3 crystallizes in the orthorhombic space group Pbcn with a = 15.121(3) A, b = 16.777(3) A, and c = 14.628(3) A at 130(2) K with Z = 4. Refinement of 3311 reflections with 209 parameters and no restraints yielded wR2 = 0.136 and R = 0.054 for 2643 reflections with I > 2 sigma(I). The structure involves helical coordination of the linear tetrapyrrole ligand about the nickel with all four nitrogen atoms coordinated to the metal and Ni-N distances of 1.867(5) and 1.879(5) A. The triiodide ion is not coordinated to the nickel but sits over one of the meso carbon atoms of the tetrapyrrole. In the solid state, pairs of [NiII(OEBOx)]+ crystallize about a center of symmetry so that two identical tab/slot hydrogen-bonded arrangements involving the lactam oxygen of one complex and methine and two methylene protons of the adjacent cation. Similar hydrogen-bonded motifs are found in other complexes derived from octaethylbilindione. [PdII(OEBOx)]I3 is isomorphic with the nickel analogue and crystallizes in the orthorhombic space group Pbcn with a = 15.2236(6) A, b = 16.7638(7) A, and c = 14.6289(6) A at 90(2) K with Z = 4. Refinement of 6123 reflections with 209 parameters and no restraints yielded wR2 = 0.094 and R = 0.036 for 4042 reflections with I > 2 sigma(I). The odd electron compound PdII(OEB.) has also been isolated and characterized by single-crystal X-ray diffraction. Black PdII(OEB.) crystallizes in the monoclinic space group I2/a with a = 13.274(3) A, b = 18.655(4) A, c = 14.114(3) A, and beta = 116.00(3) degrees at 140(2) K with Z = 4. Refinement of 2030 reflections with 195 parameters and no restraints yielded wR2 = 0.090 and R = 0.042 for 1715 reflections with I > 2 sigma(I). The structure again involves helical coordination of the linear tetrapyrrole to the palladium through the four pyrrole nitrogen atoms. Reduction of the complex causes a slight elongation of the Pd-N bonds from 1.983(2) and 1.986(2) A in [PdII(OEBOx)]I3 to 2.011(4) and 2.012(4) A in PdII(OEB.).