No abstract

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 tetraaza macrocycles 2-oxa-3-oxotetramesitylporphine (|H(2) 1|) and 2-oxa-3-oxotetrakis(2,6-dichlorophenyl)porphine (|H(2) 2|) and the corresponding iron complexes (|Fe(III)(X) 1| and |Fe(III)(X) 2|; X= Cl(-), OH(-), or SO(3)CF(3)(-)) have been synthesized. These macrocycles are derived from porphyrins by transformation of one pyrrole ring to an oxazolone ring. The resulting lactone functionality serves to restrict but not completely block pi-conjugation around the periphery. These complexes thus share properties with both porphyrins and chlorins. The ferric and high-valent iron complexes have been characterized by a variety of spectroscopic techniques. The molecular structure of |Fe(III)(Cl) 2| has been obtained by X-ray crystallography and shows that the structural changes at the macrocycle periphery do not perturb the coordination sphere of iron relative to the corresponding porphyrin complexes. This is illustrated by the observation that Fe-O frequencies in the resonance Raman spectra of the porpholactone analogues of compounds I and II are not substantially different from those of porphyrins and by the axial appearance of the EPR signals of the high-spin ferric complexes. This is consistent with reports that the Fe=O unit of oxidized porphyrins and chlorins is relatively insensitive to alteration of macrocycle symmetry. Nevertheless, probes of properties of the porpholactone macrocycle ((1)H NMR, resonance Raman skeletal modes) show effects of the asymmetry induced by the oxazolone ring. On the basis of (1)H NMR, EPR, Mössbauer, and resonance Raman data, the singly occupied molecular orbital of oxoferryl porpholactone pi-cation radicals correlates with the a(1u) molecular orbital of porphyrins under D(4)(h)() symmetry. Moreover, the paramagnetic properties and the intramolecular exchange interaction of ferryl iron and the porpholactone pi-radical have been characterized by EPR and magnetic Mössbauer measurements and spin-Hamiltonian analyses. The values J(0) = 17 cm(-)(1) and J(0) = 11 cm(-)(1) obtained for the exchange coupling constants of the oxoferryl porpholactone pi-cation radical complexes |Fe(IV)=O 1|(+) and |Fe(IV)=O 2|(+), respectively, are among the lowest found for synthetic compound I analogues.

experiments we report here, we have measured spectral changes associated with both the disulfide radical anion and the a subunit heme group. Moreover, since we have found that C02*~r educes the heme group in Fe(III)-a-Hb with no attached disulfide and predominantly the disulfide of Fe(III)-a-Hb-SSR, unwanted side reactions due to "contaminating" radical reduction at other protein sites most presumably do not occur in this system.