Five- and six-coordinate ruthenium(II) porphyrin tetiaryphosphine complexes,and their reactions with dioxygen via inner- and outer-sphere mechanisms

The 5-coordinate ruthenium(II) octaethylporphyrin complex Ru(OEP)(PPh₃) has been prepared by reduction of Ru(OEP)(PPh₃)Br using zinc amalgam. Both the Ru(OEP)(PPh₃)₃ complexes (n = 1,2) undergo reaction in toluene with O₂ to generate OPPh₃, RuO₂, and the parent porphyrin H₂(OEP); trace water and the μ-oxo dimer [Ru(OEP)(OH)]₂O are implicated in the oxidation reaction, which is considered to be initiated by coordination of O₂ to Ru(OEP)(PPh₃). In contrast, a catalytic O₂-oxidation of excess PPh₃ to the oxide probably goes via an initial outer-sphere reaction with Ru(OEP)(PPh₃)₂, that generates superoxides and Ru(III), both detectable by ESR; the superoxide is believed to be stabilized via portion addition as HO₂· that subsequently disproportionates to O₂ and H₂O₂. PPh₃ is oxidized by the peroxide, and during a reduction step that regenerates the Ru(II) catalyst from Ru(III).     

Carbon-13 nuclear magnetic resonance spectroscopy of high-spin iron(III) prophyrin compounds

¹³C nuclear magnetic resonance spectra have been obtained for variety of high-spin iron(III) porphyrin compounds and corresponding μ-oxo-bridged dimeric species. Large hyperfine shifts and significant line broadening are observed. The monomeric exhibit hyperfine shifts which are downfield with te exception of an upfield shift for the meso-carbon atom. Possible unpaired spin delocalization mechanisms and prospects for observing ¹³C NMR porphyrin resonances in high-spin ferrihemoproteins are discussed. Spectra reported here provide strategy for incorporation of ¹³C labels in hemoproteins either by biosynthetic or chemical means. The vinyl-CH₂ resonances of iron(III) protoporphyrin IX located 260 parts per million downfield from tetramethylsilane are especially attractive from the standpoint of chemical labeling.     

Mixed-valence porphyrin π-cation radical derivatives: Electrochemical investigations

The electrochemistry of [Cu(OEP)] and [Ni(OEP)] are compared with the mixed-valence π-cations and . These electrochemical studies, carried out with cyclic voltammetry and hydrodynamic voltammetry, show that the mixed valence π-cations have distinct electrochemical properties, although the differences between the [M(OEP)]^+/0 and processes are subtle.     

Axial pertubations on the electronic and magnetic properties of ferric porphyrins: II. Solvent effects on the proton NMR spectra of low-spin cyano complexes

The proton NMR spectra of a series of low-spin bis-cyano ferric complexes of tetraarylporphyrins and octaethylporphyrin in a variety of solvents have been recorded and analyzed. The hyperfine shifts are shown to be very sensitive to the solvent, experiencing an overall downfield bias as the solvent hydroge-bonding donor strength increased. The characteristic pattern of the contact and dipolar shifts for the meso-aryl group in tetraarylporphyrin complexes are shown to permit a quantitative separation of the dipolar and contact contributions to the hyperfine shift. The separated components indicate that increased solvent hydrogen bonding strength significantly decreases the magnetic anisotropy of the iron and diminishes porphyrin → iron π bonding. The changes in anisotropy with solvent are shown to be consistent with the coordinated cyanide acting as a proton acceptor. Although similar effects are found to be absent in bis-imidazole complexes, a downfield bias of half the magnitude of the bis-cyano complexes is observed in mixed cyano/imidazole complexes. Hence, the heme hyperfine shifts in cyano-metmyoglobins and -hemoglobins may serve as probes for the protonation of the distal histidyl imidazole.     

Axial perturbations on the electronic and magnetic properties of ferric porphyrins I. Effect of axial ligand basicity in bis-substituted pyridine complexes of synthetic porphyrins

The proton NMR spectra of the bis-4-substituted pyridinates of ferric tetrapheylporphyrin and octaethylporphyrin complexes have been recorded and analyzed fort he purpose of ascertaining the influence of variable axial lignad basicity on the bonding and magnetic properties of the iron. Under the conditions of slow ligand exhange where the bis stoichiometry can be established, all complexes exist exlusively in the low-spin, $\text{S}\text{=}\text{1}\text{2}$, state. The hyperfine shifts at ?60° C for both the porphyrins and axial ligands are shown to be very sensitive to the basicity of the substituted pyridine, as measured by its pK_a. For the tetraphenylporphyrin complexes, we illustrate that the pattern of the meso-phenyl hyperfine shifts permits a quantitative separation of the contact and dipolar contributions to these shifts. This separation reveals that the shift variations with pyridine pK_a are dominated by changes in the magnetic susceptibility anisotropy (dipolar shift), which decreases markedly upon lowering the pyridine basicity; ESR data support this conclusion in the few samples investigated. However, this trend in magnetic anisotropy with ligand basicity is not valid when comparing pyridines with other ligands such as imidazoles. The important change in the contact shift reflects a decrease in porphyrin → iron π change transfer as the ligand basicity is lowered. A correlation between increase in proton NMR linewidth and magnetic anisotrophy of the iron suggests that electron spin relaxation occurs via a process which couples the same levels that control the magnetic anisotropy.     

Proton NMR study of coordinated imidazoles in low-spin ferric heme complexes. Assignment of single proton histidine resonance in hemoproteins

The proton signals for the coordinated axial imidazoles in a series of low-spin ferric bis-imidazole complexes with natural porphyrin derivatives have been located and assigned. The methyl signals of several methyl-substituted imidazoles have also been resolved for the mixed ligand complexes of imidazole and cyanide ion. The imidazole spectra for the bis complexes are essentially the same as those reported earlier for synthetic porphyrins, with the hyperfine shifts exhibiting comparable contributions from the dipolar and contract interactions. The contact contribution reflects spin transfer into a vacant imidazole π orbital. The spectra of both the mono- and bis-imidazole complex concur in predicting that only the 2-H and 5CH₂ signals of an axial histidine are likely to resonate clearly outside the diamagnetic 0 to ?10 ppm from TMS region in hemoproteins. However, both the 2-H and 4-H imidazole peaks are found to be too broad to detect in a hemoprotein. Hence, it is suggested that the pair of non-heme, single proton resonances in low-spin met-myoglobin cyanides arise from the non-equivalent methylene protons at the 5-position of the histidyl imidazole. Both the resonance positions and relative linewidths in the model compounds are consistent with the data for this pair of protons in myoglobins. The possible interpretations of the average downfield bias of these signals as well as the magnitude of their spacing, are discussed in terms of the conformation of the proximal histidine relative to the heme group.