Nickel(II) and Zinc(II) meso-Tetracyclohexylporphyrins. Structural and Electronic Effects Induced by meso-Cyclohexyl Substitution in Metalloporphyrins.

The synthesis and X-ray structures of the zinc(II) and nickel(II) complexes of meso-tetracyclohexylporphyrin H(2)(TCHP) are described. The nonplanarity of the meso substituents results in steric crowding at the porphyrin periphery. In the solid state, the nickel(II) complex Ni(TCHP) has a ruffled porphyrin conformation while Zn(TCHP) exhibits a stepped distortion of the macrocycle. In chloroform solution, fast rotation of the cyclohexyl groups on the NMR time scale is observed at room temperature for both complexes. Temperature-dependent (1)H NMR spectra showed that the (-g,g,-g,g) conformer of Zn(TCHP) and Ni(TCHP) is prevalent in solution at low temperatures and gave an estimate for the rotation barrier of the cyclohexyl groups (DeltaG(c)() = 10-12 kcal mol(-)(1)). In both complexes, the porphyrin ring is easier to oxidize and harder to reduce than in their tetraphenylporphyrin M(TPP) congeners, in agreement with the stronger electron-donating effect of the cyclohexyl group. The magnitude of the potential shift is larger for the first oxidation than for the first reduction, reflecting a smaller HOMO-LUMO energy gap and a greater degree of macrocycle distortion than in the M(TPP) derivatives. This information is of importance to understanding the protein regulation of electron-transfer processes by cytochrome c and other redox active proteins. Crystal data: Ni(TCHP).CHCl(3).CH(3)CN, monoclinic, C2/c, a = 27.405(12), b = 10.004(21), c = 32.877(24) Å, beta = 107.71(3) degrees at 127 K, Z = 8. Zn(TCHP), monoclinic, P2(1)/a, a = 11.159(15), b = 11.992(7), c = 13.465(20) Å, beta = 102.85(16) degrees at 127 K, Z = 2.

Synthesis and Electrochemistry of Tetraphenylporphyrins Containing an Antimony-Carbon sigma-Bond.

The following five antimony(V) tetraphenylporphyrins with sigma-bonded antimony-carbon bonds were synthesized: [(TPP)Sb(CH(3))(2)](+)PF(6)(-), [(TPP)Sb(OCH(3))(OH)](+)PF(6)(-), [(TPP)Sb(CH(3))(OH)](+)ClO(4)(-), [(TPP)Sb(CH(3))(OCH(3))](+)ClO(4)(-), and [(TPP)Sb(CH(3))(F)](+)PF(6)(-). Each compound is stable toward air and moisture and has a high melting point (>250 degrees C). The electrochemistry and spectroelectrochemistry of these sigma-bonded porphyrins were examined in benzonitrile or dichloromethane containing 0.1 M tetrabutylammonium perchlorate as supporting electrolyte and the data compared to those for three previously synthesized OEP derivatives containing similar sigma-bonded and/or anionic axial ligands. Each porphyrin shows two reversible reductions and up to a maximun of one oxidation within the potential window of the solvent. Spectroelectrochemical data indicate formation of a porphyrin pi anion radical upon the first reduction as do ESR spectra of the singly reduced species. However, a small amount of the Sb(III) porphyrin products may be generated via a chemical reaction following electron tranfer. An X-ray crystallographic analysis of [(TPP)Sb(CH(3))(F)](+)PF(6)(-) is also presented: monoclinic, space group C2/c, Z = 8, a = 24.068(5) Å, b = 19.456(4) Å, c = 18.745(3) Å, beta = 94.69(2) degrees, R = 0.056.

Electrochemistry and Spectral Characterization of Oxidized and Reduced (TPPBr(x)())FeCl Where TPPBr(x)() Is the Dianion of beta-Brominated-Pyrrole Tetraphenylporphyrin and x Varies from 0 to 8.

The electrochemistry and spectroelectrochemistry of (TPPBr(x)())FeCl (TPPBr(x)() is the dianion of beta-brominated-pyrrole tetraphenylporphyrin and x = 0-8) were examined in PhCN containing tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte. Each compound undergoes two reversible to quasireversible one-electron oxidations and either three or four reductions within the potential limits of the solvent. The two oxidations occur at the conjugated porphyrin pi ring system, and DeltaE(1/2) between these two electrode reactions increases as the molecule becomes more distorted. The overall reduction of each compound involves the stepwise electrogeneration omicronf an iron(II), iron(I), and iron(I) pi anion radical. An equilibrium between chloride-bound and chloride-free iron(II) forms of the porphyrin is observed with association of the anionic ligand being favored for compounds with x > 5. Singly reduced (TPPBr(x)())FeCl (x = 0 to x = 6) forms both mono- and bis-CO adducts in CH(2)Cl(2). Only the mono-CO adduct is observed for (TPPBr(7))FeCl, and there is no binding at all of CO to (TPPBr(8))FeCl. The nu(CO) of both the mono- and bis-adducts increases with increase in the number of Br groups, but in a nonlinear fashion which is explained in terms of two competing effects. One is the electron-withdrawing affinity of the Br substitutents and the other the nonplanarity of the macrocycle.

Electrochemical and Spectral Characterization of Iron Corroles in High and Low Oxidation States: First Structural Characterization of an Iron(IV) Tetrapyrrole pi Cation Radical.

The electrochemistry and spectroscopic properties of three iron corroles were examined in benzonitrile, dichloromethane, and pyridine containing 0.1 M tetra-n-butylammonium perchlorate or tetra-n-ethylammonium hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)Fe(IV)(C(6)H(5)), (OEC)Fe(IV)Cl, and (OEC)Fe(III)(py), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. Each iron(IV) corrole undergoes two one-electron reductions and two or three one-electron oxidations depending upon the solvent. Under the same solution conditions, the iron(III) corrole undergoes a single one-electron reduction and one or two one-electron oxidations. Each singly oxidized and singly reduced product was characterized by UV-vis and/or EPR spectroscopy. The data indicate a conversion of (OEC)Fe(IV)(C(6)H(5)) and (OEC)Fe(IV)Cl to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole pi cation radicals upon a one-electron oxidation. The metal center in [(OEC)Fe(III)(C(6)H(5))](-) is low spin (S = (1)/(2)) as compared to electrogenerated [(OEC)Fe(III)Cl](-), which contains an intermediate-spin (S = (3)/(2)) iron(III). (OEC)Fe(III)(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)Fe(III)(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) pi cation radical. Singly oxidized [(OEC)Fe(IV)(C(6)H(5))](*)(+) is the first iron(IV) tetrapyrrole pi cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P&onemacr; with a = 10.783(3) Å, b = 13.826(3) Å, c = 14.151(3) Å, alpha = 78.95(2) degrees, beta = 89.59(2) degrees, and gamma = 72.98(2) degrees at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F(o)(2) yields R1 = 0.0864 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) Å. The formulation of the electron distribution in this compound was confirmed by Mössbauer, X-ray crystallographic, and magnetic susceptibility data as well as by EPR spectroscopy, which gives evidence for strong antiferromagnetic coupling between the iron(IV) center and the singly oxidized corrole macrocycle.

Electrochemical and Spectral Characterization of Iron Corroles in High and Low Oxidation States: First Structural Characterization of an Iron(IV) Tetrapyrrole π Cation Radical.

The electrochemistry and spectroscopic properties of three iron corroles were examined in benzonitrile, dichloromethane, and pyridine containing 0.1 M tetra-n-butylammonium perchlorate or tetra-n-ethylammonium hexafluorophosphate as supporting electrolyte. The investigated compounds are represented as (OEC)FeIV(C6H5), (OEC)FeIVCl, and (OEC)FeIII(py), where OEC is the trianion of 2,3,7,8,12,13,17,18-octaethylcorrole. Each iron(IV) corrole undergoes two one-electron reductions and two or three one-electron oxidations depending upon the solvent. Under the same solution conditions, the iron(III) corrole undergoes a single one-electron reduction and one or two one-electron oxidations. Each singly oxidized and singly reduced product was characterized by UV-vis and/or EPR spectroscopy. The data indicate a conversion of (OEC)FeIV(C6H5) and (OEC)FeIVCl to their iron(III) forms upon a one-electron reduction and to iron(IV) corrole π cation radicals upon a one-electron oxidation. The metal center in [(OEC)FeIII(C6H5)]- is low spin (S = 1/2) as compared to electrogenerated [(OEC)FeIIICl]-, which contains an intermediate-spin (S = 3/2) iron(III). (OEC)FeIII(py) also contains an intermediate-spin-state iron(III) and, unlike previously characterized (OEC)FeIII(NO), is converted to an iron(IV) corrole upon oxidation rather than to an iron(III) π cation radical. Singly oxidized [(OEC)FeIV(C6H5)]•+ is the first iron(IV) tetrapyrrole π cation radical to be isolated and was structurally characterized as a perchlorate salt. It crystallizes in the triclinic space group P1̄ with a = 10.783(3) Å, b = 13.826(3) Å, c = 14.151(3) Å, α = 78.95(2)°, ß = 89.59(2)°, and γ = 72.98(2)° at 293 K with Z = 2. Refinement of 8400 reflections and 670 parameters against F o2 yields R1 = 0.0864 and wR2 = 0.2293. The complex contains a five-coordinated iron with average Fe-N bond lengths of 1.871(3) Å. The formulation of the electron distribution in this compound was confirmed by Mössbauer, X-ray crystallographic, and magnetic susceptibility data as well as by EPR spectroscopy, which gives evidence for strong antiferromagnetic coupling between the iron(IV) center and the singly oxidized corrole macrocycle.