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Formylbiliverdin and related chlorophyll-derived molecules are possible products of heme catabolism and other biologically important oxidative processes and are likely to be initially formed as metal complexes. To explore the properties of the formylbiliverdin moiety bound to transition metal ions, complexes of octaethylformylbiliverdin (H2OEFB) with Cu(II), Ni(II), and Co(II) have been prepared, since attempts to prepare an iron complex have produced only an unstable material. Transmetalation of MgII(OEFB), made by photooxidation of MgII(octaethyl-porphyrin), with a metal(II) acetate yields the low-spin complexes:  CuII(OEFB), NiII(OEFB), and CoII(OEFB). Single-crystal X-ray diffraction of CuII(OEFB) reveals that it consists of a four-coordinate copper(II) center which is bound to the four nitrogen atoms in distorted planar coordination. The tetrapyrrole ligand has a helical geometry. The structure of the solid is complicated by the existence of three molecules in the asymmetric unit and C−H···O hydrogen bonding between pairs of these in the tab/slot arrangement seen in complexes of octaethylbiliverdin. Both CuII(OEFB) and CoII(OEFB) can be converted to the verdoheme analogues, [CuII(OEOP)]+ and [CoII(OEOP)]+, where OEOP is the anion of octaethyl-5-oxaporphyrin, by the addition of hydrogen peroxide. Additionally, [CuII(OEOP)]+ can be produced by heating a toluene solution CuII(OEFB) in the presence of trifluoroacetic acid under dioxygen. Carbon monoxide is produced when CuII(OEFB) is converted to [CuII(OEOP)]+ by either method. [CuII(OEOP)](PF6) has been characterized by single-crystal X-ray diffraction which shows that the cation has a planar, porphyrin-like structure. The room-temperature EPR spectrum of this complex shows that the copper is four-coordinate with four nitrogen based ligands, but frozen solutions of [CuII(OEOP)]+ show a triplet EPR spectrum indicative of a dimeric species much like that in the X-ray crystal structure. The 1H NMR spectrum of diamagnetic NiII(OEFB) has been shown to be consistent with the helical structure through the use of lanthanide and chiral lanthanide shift reagents. The EPR spectra of CoII(OEFB) show that it forms a low-spin adduct with pyridine and that this adduct acts as a reversible dioxygen carrier. The geometric and electronic structural properties of these complexes of formylbiliverdin are compared to those of analogous compounds of biliverdin and of porphyrins.

Two artificial peptides with pendant pyridine or bipyridine ligands have been synthesized and incorporated into oligomeric strands that are analogous to peptide nucleic acid. Spectrophotometric titrations with Cu(2+) and Fe(2+) show that the oligomers bind stoichiometric quantities of transition metals based on the number of pendant ligands. The identities of the titration products are confirmed by high resolution mass spectrometry. In the case of the bipyridine tripeptides, the titration stoichiometry and mass spectra indicate that the metal ions form interstrand cross-links between two oligopeptides, creating duplex structures linked exclusively by metal ions. Calculated molecular structures of the metalated oligopeptides and duplexes indicate that the peptide backbone acts as a scaffold for the directed assembly of metal ions. Electron paramagnetic resonance spectroscopy of the Cu-containing molecules have varying degrees of electronic interaction based on their charge and supramolecular structure. Cyclic voltammetry of the Fe(2+)- and Cu(2+)-linked bpy oligopeptide duplexes shows that they possess unique electrochemical signatures based on the redox reactivity of the metal complex.

Studies of the reaction of 5-oxaporphyrin iron complexes (verdohemes) with methoxide ion have been undertaken to provide models for the initial step in hydrolysis of verdohemes. Treatment of both low-spin [(py)2FeII(OEOP)]Cl and high-spin {ClFeII(OEOP)} (OEOP is the monoanion of octaethyl-5-oxaporphyrin) with methoxide ion in either pyridine or methanol solution is shown to cause ring opening and addition of methoxide to one end of the tetrapyrrole. Black {FeII(OEBOMe)}2 has been isolated in crystalline form from the reaction of {ClFeII(OEOP)} with methoxide in methanol and studied by single-crystal X-ray diffraction. The molecule has a centrosymmetric dimeric structure composed of two helical Fe(OEBOMe) units with FeN4 coordination which are connected by a pair of Fe−O bonds. Each iron ion has approximately trigonal bipyramidal geometry. Evidence is presented which suggests that {FeII(OEBOMe)}2 dissolves in dichloromethane in the presence of methanol or pyridine to form high-spin {(MeOH) n FeII(OEBOMe)} or {(py) n FeII(OEBOMe)} (n = 1 or 2) through rupture of the Fe−O bonds in the dimer. The electronic absorption spectra of these open chain complexes show characteristic low-energy features at ca. 720 and 810 nm. The 1H NMR spectrum of {FeII(OEBOMe)}2 at 23 °C shows a characteristic upfield methoxy resonance, 16 methylene resonances in the 47 to 5 ppm region, and meso resonances in the 51 to 29 ppm region. The temperature dependence of these spectra shows marked deviations from the Curie law, which are consistent with a dimeric structure.

Oxidative degradation of iron porphyrins under coupled oxidation conditions, which generally has involved pyridine as solvent and source of axial ligands, results in the production of verdoheme, the iron complex of 5-oxaporphyrin in which an oxygen atom replaces a methine group. Under pyridine-free conditions, ClFeIII(OEP) (OEP is the dianion of octaethylporphyrin) and {FeIII(OEPO)}2 (OEPO is the trianion of octaethyloxophlorin) have been converted into the 5-oxaporphyrin complex (verdoheme) {(NC)2FeIII(OEOP)} (OEOP is the anion of octaethyl-5-oxaporphyrin). Additionally, through control of the cyanide ion concentration, the oxidation of ClFeIII(OEP) can be stopped to produce {FeIII(OEPO)}2 or allowed to proceed to give {(NC)2FeIII(OEOP)}. A convenient method for the preparation of {FeIII(OEPO)}2, and from it the free base H2OEPOH, is reported. Anionic complexes, [(NC)(py)Fe(OEPO)]- and [(NC)2Fe(OEPO)]2-, related to these oxidative degradation processes have been obtained through the addition of bis(triphenylphosphine)iminium cyanide to (py)2Fe(OEPO) in pyridine solution or through cleavage of {FeIII(OEPO)}2 with potassium cyanide in methanol or with tetra(n-butyl)ammonium cyanide in chloroform solution. The products have been characterized principally through their characteristic 1H NMR spectra, which show sizable hyperfine shifts with meso resonances upfield and methylene resonances with both up- and downfield shifts. In the presence of air, [(NC)2Fe(OEPO)]2- undergoes oxidation to form the 5-oxaporphyrin complex (verdoheme) {(NC)2FeIII(OEOP)}. Treatment of BrFeIII(OEPO•) with CN- produces [(NC)2Fe(OEPO)]-, which can also be obtained through oxidation of [(NC)2Fe(OEPO)]2- with diiodine.

The green product of coupled oxidation of octaethylporphyriniron(i1) in pyridine has been determined by X-ray crystallography to be the low-spin (diamagnetic) bis(pyridine)iron(ii) complex of the octaethyloxaporphyrin anion.

To obtain a better understanding of the effects of ortho-halogen and related substituents on the phenyl rings of TPPH2 and (TPP)Fe(III) complexes, a series of unsymmetrically substituted tetraphenylporphyrins have been synthesized. In each of these complexes one phenyl ring bears halogen(s) on one (or both) ortho position(s), while the other three phenyl rings carry para-methoxy substituents. The free-base porphyrins were characterized by UV−visible and 1H NMR spectroscopy. The resonance of the pyrrole protons closest to the phenyl ring bearing the ortho substituent, Ha, of the free-base porphyrins shows a progressive shift to higher shielding as the atomic radius and Hammett substituent constant σ p of the substituent increases. However, the fact that 2,6-substitution has a similar effect as 2-substitution suggests that size effects are more important than through-bond electronic effects. Equilibrium constants, β2 III, for addition of N-methylimidazole to the series of complexes (o-X)(p-OCH3)3(TPP)FeCl and (2,6-X2)(p-OCH3)3(TPP)FeCl were measured in chloroform at 25 °C. log(β2 III) increases in the order F < Cl < Br < I < F2 < CF3 < Cl2 < Br2, and all β2 III values for mixed substituent porphyrins except the monofluoro-containing complex are larger than the β2 III for the reaction between the symmetrical non-ortho-substituted parent compound, (p-OCH3)4(TPP)FeCl, and NMeIm. Hence, ortho-halogen and -CF3 substituents increase the values of β2 III in order of increasing size, with the 2,6-disubstituted phenyls causing an increase of β2 III by more than a factor of 2 over that for 2-substituted phenyls. In the strongly solvating solvent dimethylformamide, where dissociation of the anion has already taken place, the opposite order of log(β2 III) is observed (F > CF3 > Cl2 ≫ p-OCH3). Both sets of equilibrium constant data suggest that ortho-halogens and -CF3 groups are electron donating, which we believe is due to direct overlap between the electron clouds of the halogens and the π system of the porphyrin. This direct overlap of electron clouds decreases the Lewis acidity of Fe(III), both making it easier for Cl- to dissociate from the Fe+Cl- starting material in CHCl3 and making the complex less stable; the former contribution is more important in CHCl3, while the latter becomes evident in DMF. The 1H NMR spectra of the same series of low-spin Fe(III) complexes show a decrease in the spread of the pyrrole-H resonances in the order of Cl2 > F2 > Br2 ≫ F > Cl > Br ∼ CF3 ≫ I. This order suggests that the apparent electron-donating characteristics of the substituents decrease in the listed order, which is quite different from that derived from the values of log(β2 III) measured in chloroform solution. The differences probably result from the fact that these two physical properties are sensing different effects of the ortho substituents:  log(β2 III) values probe changes in the σ basicity of the pyrrole nitrogens and hence the Lewis acidity of the metal, while the spread of pyrrole-H resonances probes changes in the π el...