Electron-deficient Porphyrins Research Articles

Synthesis and Self-Assembly of β-Octa[(4-Diethoxyphosphoryl)phenyl]porphyrins.

The β-substituted porphyrinoids commonly used to form functional assembled systems in nature yet are still scarcely used in material chemistry probably due to the laborious synthesis of these compounds. In this work, β-octa[(4-diethoxyphosphoryl)phenyl]porphyrin (2HOPPP) and its metal (Zn(II), Cd(II), Cu(II), and Ni(II)) complexes were prepared in good yields. These highly soluble chromophores were characterized in solution using spectroscopic (NMR, UV-vis, fluorescence), electrochemical, and spectroelectrochemical methods. Attachment of the electron-deficient residue (ArP(O)(OEt)2) to the porphyrin macrocycle leads to easier reductions and harder oxidations of the macrocycle for all complexes studied as compared to corresponding meso-tetra[4-(diethoxyphosphoryl)phenyl]porphyrin derivatives reported previously. We demonstrated that the strong electron-deficient character of the MOPPP porphyrins results principally from the increase in the number of electron-withdrawing groups at the periphery of the tetrapyrrolic macrocycle. Electron-deficient porphyrins are highly required in supramolecular and material chemistry in part due to their ability to form supramolecular assemblies via the coordination of axial ligands to the central metal atom. According to single-crystal X-ray data, ZnOPPP forms in the crystalline phase dimers in which each of the two tetrapyrrolic macrocycles is connected through an unusual combination of hydrogen bonding of two phosphoryl groups and the water molecules axially coordinated to the zinc atom of the partner molecule. The involvement of water molecules in porphyrin binding allows for an increase of distance between two porphyrin mean N4 planes, up to 4.478 Å. The offset of phosphoryl groups attached to the macrocycle through a 1,4-phenylene spacer withdraws the whole porphyrin macrocycle of one molecule from spatial overlap with the macrocycle of a partner molecule and increases the Zn-Zn distance up to 10.372 Å. This still unknown type of porphyrin dimers allows one to get deeper insights into the organization of naturally occurring tetrapyrrolic macrocycles. ZnOPPP also forms a labile dimeric complex in 5.3 × 10-7-5.8 × 10-5 M chloroform solutions. In contrast, other complexes prepared in this work exist as monomeric species under these experimental conditions. The self-association constant of ZnOPPP has been determined by electronic absorption spectroscopy.

Kinetic and mechanistic aspects of solid state, nanostructured porphyrin diacid photosensitizers in photooxidation of sulfides

The kinetics and mechanism of aerobic photooxidation of sulfides in the presence of a series of electron-rich and electron-deficient porphyrins immobilized on Amberlyst 15 nanoparticles in the form of porphyrin diacids are reported.

One-Pot Approach to Chlorins, Isobacteriochlorins, Bacteriochlorins, and Pyrrocorphins.

A Diels-Alder strategy is reported to synthesize the complete set of hydroporphyrins: chlorins, bacteriochlorins, isobacteriochlorins, and pyrrocorphins. Porphyrins and Ni-porphyrins react with isobenzofuran in very high yields at 70 °C to form the corresponding chlorins. Electron-deficient porphyrins react with a second equivalent of isobenzofuran yielding exclusively bacteriochlorin (82%), and Ni-porphyrin gives only isobacteriochlorin (99%). All cycloadditions are completely regio- and stereoselective. The regiochemistry is correctly predicted using the ACID method.

Insertion of Ni(I) into Porphyrins at Room Temperature: Preparation of Ni(II)porphyrins, and Ni(II)chlorins and Observation of Hydroporphyrin Intermediates.

Reduced Nickel porphyrins play an important role as enzymatic cofactors in the global carbon cycle (cofactor F430), and as powerful catalysts in solar-to-fuel-processes such as the hydrogen evolution reaction, and the reduction of CO and CO2. The preparation of Ni(II)porphyrins requires harsh conditions, and characterization of the reduced species is intricate. We present a very mild, convenient, and high yielding method of inserting Ni into electron rich, and electron deficient porphyrins which at the same time gives access to to Ni(II) phlorins and Ni(II)chlorins and Ni(II)porphyrins.

Synthesis of Functionalized Perfluorinated Porphyrins for Improved Spin Switching.

We have established a method to synthesize perfluorinated meso-phenylporphyrins with one phenyl group bearing a substituent in the ortho position. These novel electron-deficient porphyrins are interesting for model enzymes, catalysis, photodynamic therapy, and electron transfer. The key step is the synthesis of an iodine-substituted porphyrin and its Suzuki cross coupling with boronic acid derivatives. We applied the novel strategy to synthesize a highly electron-deficient, azopyridine-substituted Ni-porphyrin that undergoes an improved ligand-driven coordination-induced spin-state switch.

ChemInform Abstract: Towards meso‐meso‐Linked Porphyrin Arrays and meso‐Aryl Expanded Porphyrins

AbstractReview: 117 refs.

Towards meso-meso-linked porphyrin arrays and meso-aryl expanded porphyrins.

meso-meso-Linked porphyrin arrays and meso-aryl-substituted expanded porphyrins have continuously fueled my imagination for many years. In this account, my expertise in chemical research is retrospectively summarized with a particular focus on how these two novel categories of porphyrinoids were found by our group. As part of our photosynthetic model studies in collaboration with Prof. N. Mataga, the energy-gap dependence of intramolecular charge separation was examined by exploring the photoexcited dynamics of 1,4-phenylene-bridged hybrid porphyrin dimers. This study required electron-deficient porphyrins in the dimers that could serve as an electron-accepting unit towards an octaalkyl-substituted Zn(II) porphyrin donor. To this end, we employed meso-nitrated porphyrins and meso-pentafluorophenyl porphyrins. Efforts to prepare these electron-deficient porphyrins allowed us to serendipitously find both a meso-meso-linked porphyrin dimer and a series of meso-pentafluorophenyl-substituted expanded porphyrins. The meso-meso-linked Zn(II) porphyrin dimer was found as a byproduct in the nitration of 5,10-diaryl Zn(II) porphyrin with AgNO2 but became a major product in the reaction with AgPF6. This finding opened up a new path to directly linked porphyrin oligomers. The series of meso-pentafluorophenyl-substituted expanded porphyrins were prepared via BF3·OEt2-catalyzed condensation of pyrrole and pentafluorobenzaldehyde when the reaction was run at tenfold-higher substrate concentrations, as compared to the optimal conditions for the synthesis of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin. These expanded porphyrins have been shown to have attractive attributes such as flexible structures, versatile electronic states, multi-metal coordination, anion sensing, and large nonlinear optical properties. While these studies were mostly curiosity-driven, some of our work covers rather more general interests: how linearly connected molecules can be rationally synthesized and isolated in a pure and discrete form, how large π-conjugation can be realized to allow for very low energy electronic transitions, and how easily Möbius aromatic and antiaromatic molecules can be prepared.

Substitution effects on the UV–vis and 1H NMR spectra of the dications of meso and/or β substituted porphyrins with trifluoroacetic acid: Electron-deficient porphyrins compared to the electron-rich ones

Abstract Spectral shifts upon core protonation of a series of electron-rich and electron-deficient porphyrins with five and six membered rings at the meso-positions have been studied by UV–vis and 1H NMR spectroscopy. The direction of the shift of the Q(0,0) band of different porphyrins depends on the electron-donating or electron-withdrawing character of the meso-substituent and therefore it may be used to evaluate the relative electron donor ability of the substituents. Also, the shift of the Soret bands was found to be influenced to a lesser extent by the electron donor ability of substituted groups at the porphyrin periphery. Although, large red shifts of the Soret band of H4T(o-NO2)P2+, H4T(p-SCH3P)P2+ and H4T(p-NO2P)P2+ indicate a significant out-of-plane deformation of porphyrin core, negligible upfield shifts of the β protons in their 1H NMR spectra show that the practice of using the change in the chemical shift of the β protons as an indicator of structurally induced changes in the porphyrin ring current should be approached with more caution. Interestingly, high level ab initio and DFT calculations demonstrate that in spite of the better electron donating ability of –OCH3 to benzene ring compared to that of –SCH3, meso-(p-SCH3)phenyl group is a clearly better electron donor to the porphyrin core than meso-(p-OCH3)phenyl one.

Substitution effects on the catalytic activity of Mn(III)-porphyrins in epoxidation of alkenes with iodosylbenzene: A comparison between the electron-rich and electron-deficient porphyrins

Oxidation of different olefins with iodosylbenzene in the presence of Mn(III) complexes of meso-tetra( para-tolyl)porphyrin, meso-tetra( ortho-tolyl)porphyrin, meso-tetra(thien-2-yl)porphyrin and β-hexaboromo- meso-tetra(thien-2-yl)porphyrin as catalyst has been studied. Oxidation of cis- and trans-stilbene in a competitive reaction strongly suggests the involvement of a high valent (porphyrin)Mn O as the active oxidant intermediate, in the case of each catalyst. Clear observation of the band relevant to a (porphyrin)Mn(IV) O species in the presence of excess amounts of styrene shows the stability of this moiety towards reaction with olefins. Although, the stability of metalloporphyrins towards oxidative degradation decreases in the order MnT( o-tolyl)P(OAc) > MnT(thien-2-yl)PBr 6(OAc) > MnT( p-tolyl)P(OAc) ⩾ MnT(thien-2-yl)P(OAc), a complex pattern of catalytic activity and product (epoxide) selectivity has been found for the Mn-porphyrins in oxidation of various alkenes.

Regiospecifically α-13C-Labeled Porphyrins for Studies of Ground-State Hole Transfer in Multiporphyrin Arrays

Insight into the electronic communication between the individual constituents of multicomponent molecular architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-linked zinc porphyrin dyad was prepared wherein one porphyrin bears two (13)C atoms and the other porphyrin is unlabeled. The (13)C atoms are located at the 1- and 9-positions (alpha-carbons symmetrically disposed to the position of linker attachment), which are sites of electron/spin density in the a(1u) HOMO of the porphyrin. The (13)C labels were introduced by reaction of KS(13)CN with allyl bromide to give the allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methylthio)pyrrole-2-(13)C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave dipyrromethane-1,9-(13)C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing (13)C labels at the 1,9-positions and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospecifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin constituents of the dyad is slow (<10(6) s(-1)) on the EPR time scale at room temperature. The slow rate stems from the a(1u) HOMO of the electron-deficient porphyrins, which has a node at the site of linker connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a(2u) and has a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5 x 10(7) s(-1)) on the EPR time scale at room temperature.

Regioselective Synthesis of Antipodal β-Tetrabrominated meso-Tetraarylporphyrins

Abstract A series of meso-tetraarylporphyrins bearing electron-donor or electron-withdrawing substituents at the meso-phenyl groups were examined for antipodal β-pyrrole tetrabromination using different brominating agents. Porphyrins bearing electron-donor substituents undergo bromination at moderate temperatures with N-bromosuccinimide as the brominating agent while the electron-deficient porphyrins require liq. Br2 at room temperature. The brominated porphyrins were isolated in moderate to very good yields. Synthesized compounds were characterized by electronic absorption, 1H NMR, and mass spectroscopic methods. Regioselectivity of an electron-deficient porphyrin, 2,3,12,13-tetrabromo-5,10,15,20-tetrakis(4-methoxycarbonylphenyl)porphyrin was examined by single-crystal X-ray structure analysis and showed antipodal β-pyrrole bromo substitution.

Two coordinatively linked supramolecular assemblies constructed from highly electron deficient porphyrins

The synthesis of a pair of electron-deficient porphyrin building blocks and their resulting coordinatively linked supramolecular assemblies are described. The perfluorophenyl substituted porphyrins feature pendant pyridines which, upon reaction with Re(CO)5Cl assemble into discrete dimers and tetramers, as dictated by the geometry of the porphyrin monomer. The resulting supramolecular complexes as well as their constituent porphyrins display several interesting and potentially useful properties owing to the electron-withdrawing nature of the perfluorophenyl funtionalities. Structural, spectroscopic, and electrochemical data indicate that the electron-deficient porphyrins remain planar, allowing for modulation of spectral and redox properties, as well as for enhancement of the affinity of the porphyrins for Lewis-basic ligands.

Preparation, electrochemical and spectral properties of N-methylated pyridylethynyl porphyrins.

A new series of electron-deficient porphyrins were prepared by attaching one or two N-methylated 2-, 3- or 4-pyridylethynyl groups to the 10,20-meso positions of (5,15-biphenylporphinato)zinc(II). Electrochemical studies showed significant changes in the reduction potentials of these porphyrins, and N-methyl-2-pyridylethyne is the strongest electron-withdrawing substituent in the series. UV-visible spectra demonstrated largely red-shifted absorptions, and N-methyl-4-pyridylethyne has the greatest impact to the porphyrin absorptions. Electrochemical, UV-visible and EPR results concluded that porphyrins Zn2 and Zn6 reversibly undergo two one-electron porphyrin-ring reductions to their anion radicals then dianions. The first reductions of porphyrins Zn1, Zn3, Zn4 and Zn5 were irreversible one-electron transfer processes. The instability of these reduction products was suggested to result from the eletrophilic attacks at the substituents.

Synthesis and characterization of beta-trifluoromethyl-meso-tetraphenylporphyrins.

Beta-trifluoromethyl-meso-tetraphenylporphyrins were synthesized to investigate the electronic and steric effects of the trifluoromethyl groups on the macrocycle. Preparation of these novel porphyrins was carried out by copper-assisted trifluoromethylation of beta-tetrabromo-meso-tetraphenylporphyrin metal complexes and in situ generated CF3Cu. For comparison, the beta-methyl analogues were also prepared. Analysis of beta-trifluoromethylporphyrins by UV-vis, NMR, and cyclic voltammetry (CV) showed that the electron-withdrawing effects of the trifluoromethyl groups on the antipodal pyrroles required the macrocycle to take a fixed 18pi-electron pathway. UV-vis, CV, and molecular modeling studies suggest that the novel porphyrins are distorted following introduction of trifluoromethyl groups onto the pyrrolic beta-position of meso-tetraphenylporphyrin. The pK(a) difference of beta-tetrakis(trifluoromethyl)-meso-tetraphenylporphyrin from that of DBU in CH2Cl2, obtained by spectrophotometric titration, affirms that it is one of the most electron-deficient porphyrins so far prepared.

ChemInform Abstract: Amidation of Saturated C-H Bonds Catalyzed by Electron-Deficient Ruthenium and Manganese Porphyrins. A Highly Catalytic Nitrogen Atom Transfer Process.

Amidation of a variety of hydrocarbons with PhI=NTs catalyzed by ruthenium and manganese meso-tetrakis(pentafluorophenyl)porphyrins 1 and 2 afforded N-substituted amides in up to 92% yields with good to excellent substrate conversions. By employing catalyst 2, exceptionally high turnovers (up to 2600) were achieved, and the amidations can be effected by directly using PhI(OAc)(2)/NH(2)R as amidating reagents; in the case of R = COCF(3) a direct amination was realized in up to 90% yield.

2,3,7,8,12,13,17,18-Octafluoro-5,10,15,20-tetraarylporphyrins and Their Zinc Complexes: First Spectroscopic, Electrochemical, and Structural Characterization of a Perfluorinated Tetraarylmetalloporphyrin

We report a convenient and general synthesis of β-octafluoroporphyrins bearing meso-tetraaryl substituents, including the first synthesis and full characterization of a perfluorinated tetraarylporphyrin, perfluoro-5,10,15,20-tetraphenylporphyrin, 2. The structural, spectroscopic, and electrochemical data indicate that β-octafluoro-meso-tetraarylporphyrins are a new class of planar, electron-deficient ligands. Particularly impressive is the 0.5 V window over which the formal oxidation potential can be tuned using only aryl substituents. The invariance of the ligand structure with increasingly positive formal oxidation potential is a key advance; electronic effects have been severed from the nonplanar conformations exhibited by all other highly electron-deficient porphyrins.

5-Perfluoroalkyldipyrromethanes and porphyrins derived therefrom

Acid-catalyzed condensation of pyrrole and perfluoroalkyl aldehydes produced, in a single step, α,β-unsubstituted 5-perfluoroalkyldipyrromethanes, which were isolated and characterized for the first time. These dipyrromethanes were used as the key intermediates in the synthesis of trans-perfluroalkylporphyrins as well as meso-tetrakis(perfluoroalkyl)porphyrins. The two-step procedure described here provides access to a wide variety of porphyrins with trans perfluoroalkyl groups. Key words: 5-perfluoroalkyldipyrromethanes, perfluoroalkylporphyrins, electron-deficient porphyrins.

Nitrosyl iron(II) complexes of porphyrins substituted with highly electron-withdrawing CF 3 groups: Electronic absorption, MCD and EPR spectral study

Five- and six-coordinated nitrosyl iron(II) complexes of etioporphyrin I and electron-deficient porphyrins with one, two or four CF 3 groups at pyrrole β-positions of a porphyrin have been prepared and the electronic absorption, MCD, IR and EPR spectra of their complexes have been measured at room temperature and 77 K. Both the line shape and parameters varied with the number of CF 3 substituents with a highly electron-withdrawing ability. Their data were evaluated by using the first redox potentials of free base porphyrin as a measure of the electron-withdrawing or -donating ability of the porphyrin peripheral substituents.