A series of dichloroferrous complexes with ligands derived from the tris(2-pyridylmethyl)amine tripod has been prepared and characterized. The X-ray crystal structures of the complexes [bis(2-bromo-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe(II)Cl(2) ((Br(2)TPA)Fe(II)Cl(2)) and [bis(2-phenyl-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe(II)Cl(2), ((Ph(2)TPA)Fe(II)Cl(2)) are reported. In these complexes, the tripod coordinates in the tridentate mode, with a substituted pyridyl arm dangling away from the metal. Both complexes have a trigonal bipyramidal iron center with two equatorial chloride ions. Their crystal structures are compared with those of the [tris(2-pyridylmethyl)amine]Fe(II)Cl(2) and [(2-bromo-6-pyridylmethyl)bis(2-pyridylmethyl)amine]Fe(II)Cl(2) complexes ((TPA)Fe(II)Cl(2) and (BrTPA)Fe(II)Cl(2), respectively) in which the ligand coordinates in the tetradentate mode. For all complexes, the metal to ligand distances are systematically above the value of 2.0 A, and (1)H NMR displays paramagnetically shifted resonances with short relaxation times. This indicates that the iron is in a high-spin state. Electric conductivity measurements show that, for all complexes, the measured values lie within the same range, significantly below those expected for ionic complexes. Together with the analysis of the UV-visible and NMR data, this strongly suggests that the coordination mode of the tripod is retained in solution.

We have synthesized the mono, di-, and tri-alpha-fluoro ligands in the tris(2-pyridylmethyl)amine (TPA) series, namely, FTPA, F(2)TPA and F(3)TPA, respectively. Fluorination at the alpha-position of these nitrogen-containing tripods shifts the oxidation potential of the ligand by 45-70 mV per added fluorine atom. The crystal structures of the dichloroiron(II) complexes with FTPA and F(2)TPA reveal that the iron center lies in a distorted octahedral geometry comparable to that already found in TPAFeCl(2). All spectroscopic data indicate that the geometry is retained in solution. These three isostructural complexes all react with molecular dioxygen to yield stable mu-oxodiiron(III) complexes. Crystal structure analyses are reported for each of these three mu-oxo compounds. With TPA, a symmetrical structure is obtained for a dicationic compound with the tripod coordinated in the kappa(4)N coordination mode. With FTPA, the compound is a neutral mu-oxodiiron(III) complex with a kappa(3)N coordination mode of the ligand. Oxygenation of the F(2)TPA complex gave a neutral unsymmetrical compound, the structure of which is reminiscent of that already found with the trifluorinated ligand. On reduction, all mu-oxodiiron(III) complexes revert to the starting iron(II) species. The oxygenation reaction parallels the well-known formation of mu-oxo derivatives from dioxygen in the chemistry of porphyrins reported almost three decades ago. The striking feature of the series of iron(II) precursors is the effect of the ligand on the kinetics of oxygenation of the complexes. Whereas the parent complex undergoes 90 % conversion over 40 h, the monofluorinated ligand provides a complex that has fully reacted after 30 h, whereas the reaction time for the complex with the difluorinated ligand is only 10 h. Analysis of the spectroscopic data reveals that formation of the mu-oxo complexes proceeds in two distinct reversible kinetic steps with k(1) approximately 10 k(2). For TPAFeCl(2) and FTPAFeCl(2) only small variations in the k(1) and k(2) values are observed. By contrast, F(2)TPAFeCl(2) exhibits k(1) and k(2) values that are ten times higher. These differences in kinetics are interpreted in the light of structural and electronic effects, especially the Lewis acidity at the metal center. Our results suggest coordination of dioxygen as an initial step in the process leading to formation of mu-oxodiiron(III) compounds, by contrast with an unlikely outer-sphere reduction of dioxygen, which generally occurs at negative potentials.

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Despite four additional electron‐withdrawing substituents, the octahalogenated porphyrins 2 are more easily oxidized than their tetra‐halogenated counterparts 1. This apparent paradox can be explained by the structures of the molecules: porphyrins 2 are almost planar, whereas in 1 steric interactions are minimized by a saddle conformation, which makes these porphyrins easier to oxidize.

The X-ray structures of the 8-substituted-pyrrole tetraarylporphyrin 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetramesitylporphyrin (H2TMOBP), its nickel(I1) complex (NiTMOBP), and the nickel(I1) derivative of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20tetrakis(pentafluoropheny1)porphyrin (NiTPFPOBP.1/2CH2C1z) are reported. The synthesis of the free base H,TPFPOBP is also described. All these molecules are nonplanar, displaying saddle-shaped conformations. The saddle distortions minimize the intramolecular steric interactions between the bromine substituents and the ortho (ortho') carbon atoms or ortho (ortho') substituents of the phenyl rings, and consequently, the corresponding contact distances have similar values in all compounds. Because of the distortion of the porphyrin cores, the cavity defined by the ortho (ortho') substituents gives slightly less steric protection than in the nonbrominated nearly planar ortho-and ortho'-substituted tetraarylporphyrin complexes. Crystallographic data: C56H4sN4Brs (H2TMOBP), tetragonal, space group P42,c, a = b = 15.085 (4) A, c = 14.056 (4) A, V = 3198.8 A', Z = 2, RF = 0.054, RwF = 0.069 based on 638 reflections with I > 3a(10), T = 298 K; NiC56H44N4Brs,1s (NiTMOBP), tetragonal, space group P421c, a = b = 15.024 (3) A, c = 14.068 (6) 1, V = 3175.2 A', Z = 2, RF = 0.049, R w F = 0.057 based on 877 reflections with lo > 2.5u(10), T = 298 K; NiCMN4F2 BrsJ/2CH2C12 (NiTPFPOBP-1/2CH2C1z), monoclinic, space group C2/c, a = 18.105 (3) A, b = 22.141 (5) A, c = 24.301 ( 5 ) 1, B = 93.32 (2)O, V = 9725.5 A', Z = 8, RF = 0.047, RwF = 0.064 based on 2896 reflections with Io > 3a(10), T = 298 K.

The tris(2-fluoro-6-pyridylmethyl)amine ligand, F3TPA, can easily be prepared by reaction of 2-fluoro-6-bromomethylpyridine with NH4Cl in the presence of NaOH. Complexation to FeCl2 affords the high-spin F3TPAFe(II)Cl2 complex, the X-ray structure of which is reported. The three fluorine substituents provide enough steric hindrance to force the tripod to coordinate in the tridentate mode, affording a trigonal bipyramidal iron center. This complex is thermally stable, and it reacts instantaneously with molecular dioxygen to afford the unsymmetrical micro-oxo dimer F3TPAFe(III)ClOFe(III)Cl3 as the major product, together with small amounts of the mixed salt [F3TPAFe(II)Cl]2, [Fe(III)2OCl6]. These two complexes have been isolated and characterized by X-ray diffraction analysis. A mechanism by which they are obtained is suggested and seems to parallel the well-known process of autoxidation of ferrous porphyrins.

The iron(II1) "picket-fence" porphyrin complex [ Fe1ii(TPpivP)(OS0,CF3)(H20)] (1) was synthesized and characterized by its UV-visible, 'H NMR, EPR, magnetic, and Mossbauer properti'es. The X-ray structure of 1 was determined at -100 OC. Crystal data: [Fe(TPpivP)(OS0,CF3)(HzO)] (C65H6sN80sF3sFe); monoclinic; a = 13.161 (3), b = 19.196 (6), c = 26.212 (6) A; 0 = 103.34 (2)"; Z = 4; d,,, = 1.270 g cm-); space group P2i/c. The six-coordinate iron atom is bonded to the four porphyrinato nitrogens (Fe-N, = 2.021 (16) A), to an oxygen atom of the triflate ion (Fe-O(triflate) = 2.188 (5) A), placed inside the molecular cavity of the picket-fence porphyrin, and to a water molecule (Fe-O(water) = 2.133 (5) A). The susceptibility measurementsin an external field of 1.5 T show that the effective magnetic moment varies from 4.0 to 5.7 pB in the temperature range 2-300 K. The EPR data yield g, = 5.7, which corresponds to a mixture of 85% spin sextet $Ai and 15% spin quartet 'A2 in the lowest Kramers doublet. This mixture is somewhat stronger than in typical high-spin iron porphyrins. Mossbauer spectra were recorded at temperatures varying from 4.2 to 300 K in fields of 0-6.21 T. They exhibit temperature-independent quadrupole splitting, AEQ = 2.2 mm s-l, which lies between the AEQ values characteristic for high-spin (S = 5/2) and intermediate-spin (S = 3/z) porphyrins. The magnetic hyperfine patterns of the measured Mossbauer spectra, in the slow and fast relaxation limit, are successfully simulated within the IO-state model for the spin mixture between 6A, and 4A2 by using parameters that have been derived from susceptibility and EPR data within the same model. The applicability of the usual spin (S = 5/2) Hamiltonian analysis and its relation to the IGstate model are discussed. Spinspin and spin-lattice relaxation effects are explicitly accounted for in the intermediate relaxation regime within the framework of the (S = 5/z) spin Hamiltonian. The degree of spin mixture together with the axial and porphyrinato-nitrogen coordination of iron in 1 and related complexes is discussed on the basis of a putative spin-state/stereochemical relationship.

Freeze-quenched intermediates of substrate-free cytochrome 57 Fe-P450 cam in reaction with peroxy acetic acid as oxidizing agent have been characterized by EPR and Mo «ssbauer spectroscopy. After 8 ms of reaction time the reaction mixture consists of V V90% of ferric low-spin iron with g-factors and hyperfine parameters of the starting material ; the remaining V V10% are identified as a free radical (SP P = 1/2) by its EPR and as an iron(IV) (S = 1) species by its Mo «ssbauer signature. After 5 min of reaction time the intermediates have disappeared and the Mo «ssbauer and EPR-spectra exhibit 100% of the starting material. We note that the spin-Hamiltonian analysis of the spectra of the 8 ms reactant clearly reveals that the two paramagnetic species, e.g. the ferryl (iron(IV)) species and the radical, are not exchanged coupled. This led to the conclusion that under the conditions used, peroxy acetic acid oxidized a tyrosine residue (probably Tyr-96) into a tyrosine radical , and the iron(III) center of substrate-free P450 cam to iron(IV). ß