J. Fajer

THE CHEMISTRY OF PORPHYRIN π‐CATIONS*

The redox properties of metalloporphyrins have been studied extensively, and particular attention has been paid to the changes in oxidation states of the metal.’-” In general, the most stable metalloporphyrins are those in which the metal is in the + 2 oxidation state. However, most of the transition metalloporphyrins show a variety of oxidation states. Thus Co(L1) porphyrins can, like the corresponding cobalt-containing vitamin B I Z , be oxidized to the CO(LII),~ or be reduced to the Co(1) complex; and like the Co(1) containing vitamin BI2(BL2 s), the Co(1) porphyrin is nucleophilic and undergoes oxidative addition with alkyl halides to give the corresponding alkyl cobalt porphyrins. Manganese porphyrins show + 2, + 3, and +4 oxidation states,6 and it has recently been shown’ that Pb(I1) porphyrins can be oxidized to Pb(1V) while the more stable Sn(Lv) can be reduced to Sn(LL) ~ y s t e r n s . ~ ~ ~ A particularly striking example of the stability of the divalent complexes is that of the Ag(1L) porphyrins.*O Few examples of divalent silver complexes are known, but the planar tetradentate porphyrin ligand stabilized this unusual oxidation state which can, however, be oxidized to the trivalent Although the redox properties of metalloporphyrins are fascinating and important in their own right, the initial focus and interest in the oxidation states of these systems stems from the redox properties of the cytochromes (which are enzymes containing iron porphyrins, and function catalytically via theFe(I1) Fe(LLL)couple) and the function of hemoglobin (an Fe(I1) porphyrin which, unlike simple Fe(I1) porphyrins, is not oxidized by oxygen to Fe(LI1) but reversibly binds oxygen at the Fe(LL) oxidation level) as well as from the incompletely determined role of valence changes of iron in the peroxidase/catalase class of heme enzymes. The central role of the iron in these naturally occurring systems and the considerable efforts that have been expended on elucidating the roles of metals in these and other metalloporphyrins resulted in the widely held assumption that the macrocyclic porphyrin ligand serves merely to modify the redox potentials of the metals, and to act as a convenient bridge between the metal and the protein-an assumption which is far from true.

Unusual picosecond 1(π, π∗) deactivation of ruffled nonplanar porphyrins

Abstract Time-resolved and steady-state optical data are presented for metal-free meso-tetraalkylporphyrins, H 2 T(alkyl)P. Introduction of bulky tertiary-butyl or adamantyl substituents at the meso positions induces large ruffling distortions of the macrocycle skeleton that give rise to highly novel photophysical properties of the porphyrins. These include ultrashort 1 (π, π ∗ ) lifetimes of 10–50 ps at 296 K that, intriguingly, lenghten to 10–15 ns at 78 K, the typical timescale found at both temperatures for planar derivatives such as tetra( n -pentyl) and tetraphenyl porphyrins. The ability of the nonplanar ruffled porphyrins to undergo additional structural deformations in the excited singlet state likely underlies their dramatically enhanced rates of internal conversion.

Electron transfer reactions in cofacial diporphyrins

Abstract Optical difference spectra consonant with electron transfer from a magnesium porphyrin to a free base porphyrin are obtained following picosecond excitation of cofacial diporphyrins covalently linked approximately 4Aapart. The charge-transfer species decays with a lifetime of 620 ± 20 ps.

Structural effects in chemistry and biology

Conformationally designed, non-planar porphyrins afford new classes of structurally distinct chromophores with significantly altered optical, redox, magnetic, radical and excited state properties. The synthetic, non-planar porphyrins model and illustrate the consequences of the skeletal deformations and plasticity increasingly observed in crystal structures of protein complexes comprising porphyrinic chromophores and prosthetic groups. Conformational variations thus offer attractively simple mechanisms for modulating the physicochemical properties of porphyrins in vivo and in vitro.

Oxidative chemistry of nickel porphyrins

Abstract. The oxidative chemistry of nickel(II) porphyrins is reviewed. Whether electron abstraction occurs from the metal to yield Ni(III) or from the porphyrin to yield Ni(II) π cation radicals is discussed in terms of the relative energy levels of the metal and porphyrin orbitals. The effects of axial ligands in further modulating this ordering as well as the orbital occupancy of Ni(III) are also reviewed. Structural considerations, based on existing stereochemical data for Ni(I), high spin Ni(II) and related Ni(III) tetraaza complexes, are used to predict the metrics of Ni(III) porphyrins for which no structural data are available.

Conformational landscapes of nonplanar porphyrins: superstructure, ligation, binding pockets and oxidation effects in Cu(II) porphyrins

Abstract The crystal structure of the porphyrin π cation radical Cu(II)OETPP + pyl 3 − ·py is reported (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin; py = pyridine). The compound is readily prepared by oxidation of Cu(II)OETPP in CH 2 Cl 2 with excess I 2 in the presence of py. The structure exhibits the following features: (1) The porphyrin is oxidized, not the Cu(II). (2) The Cu(II) is pentacoordinated with one py as axial ligand. (3) The sterically crowded macrocycle is saddle shaped. This distortion and the peripheral substituents combine to form superstructural, orthogonal ‘trenches’ above and below the porphyrin plane. (4) The trenches impose a specific orientation on the ligated axial py along the NCuN axis of the porphyrin and also form a ‘distal’ binding pocket that traps a second molecule of py. (5) The Cu(II)OETPP + py radical is diamagnetic because of antiferromagnetic coupling between the unpaired electrons of the Cu d x 2 -v 2 and the porphyrin π orbitals which is favored by the nonplanarity of the macrocycle. (6) The radical is more distorted than its nonplanar parent and provides additional support for the thesis that nonplanar porphyrins can easily undergo additional structural excursions. Conformational changes of the type reported here may also influence the reactivities of the π cation radicals formed in the (photo)catalytic cycles of photosynthetic and hemoproteins also comprised of nonplanar porphyrins. Crystallographic data: Cc ( No. 9), a = 21.706(2) A , b = 16.077(2) A , c = 19.993(2) A , β = 103.34(1)°, V = 6789(1) A 3 , Z = 4 .

Molecular structure of (5,10,15,20-tetrabutyl-2,3,7,8,12,13,17,18- octaethylporphyrinato)nickel(II)—correlation of nonplanarity with frontier orbital shifts †

Highly substituted porphyrins exhibit significantly distorted, nonplanar conformations in the solid state. The crystallographically determined degree of nonplanarity correlates with a bathochromic shift of the absorption maxima in solution. In addition, nonplanar porphyrins with meso aryl groups show increasing in-plane rotation of the meso aryl groups, which potentially could account for the observed changes in spectroscopic and physicochemical properties of nonplanar porphyrins. A crystal structure analysis of the title compound NiTBuOEP reveals a highly nonplanar conformation with an average deviation from planarity for the 24 macrocycle atoms of 0.462 A and displacements of the meso carbon atoms from the 4N-plane of 1.044 A. The average Ni–N bond distance in the crystal (1.873(3) A) is in good agreement with the Ni–N bond distance in solution (1.87 A) that was determined by EXAFS. Compared to more planar reference compounds, NiTBuOEP exhibits significantly red-shifted absorption spectra in solution, correctly predicted by INDO/s calculations. As the shortness of the Ni–N bonds has been shown to be an excellent indicator for the degree of conformational distortion in porphyrins, this proves that the highly nonplanar conformation of sterically strained porphyrins is maintained in solution. Thus, the physical and chemical properties measured in solution do indeed reflect the stereochemistry of the single crystals. In addition, the use of only alkyl substituents in NiTBuOEP to cause nonplanarity obviously circumvents potential electronic effects due to aryl ring interactions.

Chlorophyll chemistry before and after crystals of photosynthetic reaction centers

The experimental, theoretical and structural research leading to the identification and characterization of the (bacterio) chlorophyll species that mediate the primary events of solar energy transduction and dynamics is reviewed and examined from the author’s perspective.

Correlation of macrocycle distortion with oxidation potentials of iron(III) porphyrins: molecule structure of the sterically crowded chloro-iron(III) 7,8,17,18-tetrabromo-5,10,15,20-tetraphenylporphyrin

Abstract The chloro-iron(III) complex of the tetraphenylporphyrin tetrabrominated at the antipodal β-pyrrole positions [(7,8,17,18-tetrabromo-5,10,15,20-tetraphenyl)porphyrin] has been synthesized and characterized by spectroscopy and X-ray crystallography. The iron atom is bonded to the chloride ion and the four pyrrole nitrogens. The Fe-Cl bond distance is 2.209(4) Å, and the mean value of the two opposite Fe-Np lengths at the brominated pyrrole rings is 2.079(8) Å, whereas the mean value of the two opposite Fe-Np′ bond distances at the non-brominated pyrrole rings is 2.041(8) Å. The X-ray structure determination and the analysis of the UV-Vis spectra obtained in solution and on thin films indicate that |FeCl(tpp-Br4)| (1) is principally saddle-shaped in the solid state and in solution. Variable-temperature (195–325 K) 1H NMR spectroscopy confirms the high-spin state (S=5/2) of the iron(III) center and indicates that the saddle-shaped conformation of 1 is maintained in solution. EPR spectra obtained in frozen CH2Cl2 solution and in the solid state show a rhombic symmetry with g values of 6.25, 5.70 and 1.99. Kadish et al. have shown that the one-electron oxidation potential of 1 increases only by 0.06 V relative to that of the non-brominated complex |FeCl(tpp)|. The present study indicates that the increase of the first oxidation potential of 1 is related to the non-planar distortion of the porphyrin. Relative to the unbrominated derivative |FeCl(tpp)|, this distortion destabilizes the π system of the macrocycle and thus compensates for the effects of the four electron-withdrawing bromine substituents.

A cell for extended x-ray absorption fine structure studies of oxygen sensitive products of redox reactions

We describe a cell suitable for extended x‐ray absorption fine structure (EXAFS) studies of oxygen and/or water sensitive products of redox reactions. The cell utilizes aluminized Mylar windows that are transparent to x rays, provide low gas permeability, and allow vacuum to be maintained in the cell. The windows are attached to the glassware with an epoxy that resists attack by common organic solvents. Additional side arms allow multiple spectroscopic probes of the same sample under anaerobic and anhydrous conditions.