Β-pyrrole Positions Research Articles

Optical Anion Sensors Based on Alkyne‐Linked, Functionalized Calix[4]pyrroles.

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Synthesis and simple separation of β-pyrrole sulfonated porphycenes

New water-soluble porphycenes having sulfonic acid groups at the β-pyrrolic positions were synthesized and characterized by UV–vis, NMR, IR, and mass spectroscopy as well as elemental analyses. The series of mono- to tri-sulfonato derivatives were simply separated by the extraction method.

First examples of intramolecular Pd(0) catalysed couplings on ortho-iodinated meso-phenyl porphyrins.

Palladium catalysed intramolecular cyclisations on meso-phenyl porphyrins give access to a range of substituted porphyrins in which one or two of the phenyl ortho-carbons are bonded directly to an adjacent β-pyrrolic position.

Optical Anion Sensors Based on Alkyne-Linked, Functionalized Calix[4]pyrroles

Linking a calix[4]pyrrole anion recognition subunit to chromophores or fluorophores via an alkynyl spacer attached to a β-pyrrolic position of a calix[4]pyrrole core produces a new class of anion sensor that permits the detection of halide and phosphate anions in organic media via direct, so-called "naked-eye" visualization or fluorescence quenching-based spectroscopic means.

Studies on some new meso-aryl substituted octabromo-porphyrins and their Zn(II) derivatives

A series of porphyrins with various tolyl (H 2TTHP) and naphthyl substituents (H 2TNHP) at the meso-positions, their octabromoderivatives (H 2TTBP and H 2TNBP) with Br substituents at β-pyrrole positions and also their Zn(II) derivatives have been synthesised and characterised by 1H NMR, electronic, fluorescence and electrochemical studies. These have the meso-carbons bonded to tolyl moieties at ortho-, meta- and para-positions of the tolyl groups and at the α- or β-position of the naphthyl group. For the octabromoporphyrins, pronounced deshielding of NH protons and a moderate shift of meso-aryl protons to a lower δ value are observed compared to their nonbrominated species. The electronic spectra of ZnTTHP and ZnTNHP have almost the same B and Q bands while the B band of their free-base analogues have H 2TNHP absorbing at a higher wavelength than H 2TTHP. All the octabromoderivatives exhibit a pronounced red shift for both B and Q bands (compared to their nonbrominated forms) and show meso-substituent dependent change in both free-base and metallated forms. The above observations are interpreted in terms of moderate conjugative interaction of the aryl substituent with the π framework and also in terms of energy level reordering which alters the HOMO–LUMO gap. Consistent with the absorption spectral data the emission bands of all the bromoporphyrins were also seen to be red shifted considerably. Significant decrement in quantum yield ( φf) was observed for the bromocompounds. While the φf of nonbrominated porphyrins is seen to be higher than their Zn(II) derivatives the reverse order is observed for the bromoderivatives. The ability of the Zn 2+ ion to make the bromoporphyrins resistant to distortion by bridging the central cavity can be attributed as the cause for this interesting observation. Cyclic voltammetric studies exhibit characteristic quasi-reversible/irreversible oxidation–reduction features for all the free-bases and Zn(II) derivatives. The bromoderivatives on the other hand manifest marginally harder oxidation and very easy reduction features. The data are interpreted in terms of electron withdrawing ability of Br atoms and to reordering of HOMO and LUMO levels due to distortion in the porphyrin ring.

Solvent effects on some newmeso-aryl substituted octabromoporphyrins

A series of porphyrins with tolyl and naphthyl substituents at themeso positions, their octabromoderivatives (OBP) with Br substituents at β-pyrrole positions are synthesised and characterised by chemical analysis,1H NMR and electronic spectral studies. It is seen that all the OBPs exhibit pronounced red shifts in both the Soret andQ bands of their electronic spectra compared to their non-brominated form in various polar and nonpolar solvents, the energy difference Δv being in the range 2300–2700 cm−1. The high energyB band of naphthyl porphyrins (both brominated and nonbrominated) are found to be more red-shifted than that of tolyl porphyrins, owing to the noticeable mesomeric effect of the naphthyl groups. Detailed spectral studies reveal that while none of the nonbrominated porphyrin show solvent-dependent change in theirB andQ bands, all the OBPs manifest significant shifts depending on the nature of solvents. Solvent-solute interaction can be considered to be of strong dipole-dipole nature for OBPs with polar solvents and of π-π type with aromatic non-polar solvents. In the brominated form we find two categories of porphyrins exhibiting distinctly different absorption phenomena in aromatic solvents. The OBPs havingmeso-groups not shielding the porphyrin π-framework exhibit additional absorption peaks (split Soret peaks and broadened Q bands) in some aromatic solvents. This could be explained in terms of π-π type donor-acceptor (DA) complex formation between such bromoporphyrins (acceptor) and the aromatic solvent molecules (donor) that is not possible for OBPs that have bulkymeso groups that block the approach of aromatic solvent molecules close to the porphyrin framework

Studies on iron(III) and manganese(III) derivatives of new meso-aryl substituted and brominated porphyrins

A series of FeIII and MnIII porphyrins with various tolyl and naphthyl substituents at the meso positions, and their perbromoderivatives with Br substituents at the β-pyrrole positions, have been synthesised and investigated. As seen in the case of the free-base porphyrins, both FeIII and MnIII derivatives of the Br-substituted porphyrins also exhibit pronounced red-shifts in both B and Q bands compared to their nonbrominated analogues. This is attributed to the electron-withdrawing ability of eight Br substituents at β-pyrrole positions and is also due to distortion brought about in the π-framework by the bulky substituents including those at the meso positions. The naphthyl groups seem to be making mesomeric contributions for both nonbrominated and brominated porphyrins of these metal ions as is evident from the higher wavelength absorption of the B band as compared to the tolyl derivatives. While the meso-substituent do not exhibit any isomer dependent change on the electronic properties of FeIII porphyrins, they show a noticeable effect in the MnIII derivatives. During the metallation of meso-tetratolylporphyrins by FeIII ions μ-oxo dimeric compounds are formed, while the naphthyl porphyrins and the bromoderivatives do not form such dimeric species. The presence of bulky groups at the meso positions and heavy bromines on the β-pyrrole positions can be considered to prevent the formation of catalytically inert μ-oxo dimers.

β-Substituted copper porphyrin cations: A 2u or A 1u radicals?

DFT calculations have been performed on a series of β-substituted copper porphyrins, CuP–X, with particular regard to substituent effects upon the relative energies of two frontier occupied, nearly degenerate a 2u and a 1u orbitals. Substitution by electron-withdrawing groups only slightly enlarges the a 2u–a 1u separation of CuP. Electron-donating groups CH 2OH and OCH 3 raise the energies of a 2u and a 1u uniformly. In contrast, SH and NH 2 reverse the normal ordering; these [CuP–X] + cation radicals are mainly a 1u in character. Electronic structures of CuP derivatives substituted at all eight β-pyrrole positions are also examined.

Perhalogenated porphyrins as a sink of excitation energy in porphyrin heterodimers

The photophysics of a new family of free-base and zinc derivatives of meso- tetraphenylporphyrin heterodimers have been studied by UV-vis absorption, fluorescence and nanosecond flash photolysis techniques. An almost complete (≈99%) and directionally controlled transfer of excitation energy from a donor porphyrin moiety was obtained by multiple bromination (four and eight Br substituents in the two series of compounds investigated) on the β-pyrrole positions of the acceptor porphyrin molecule. The covalently linked porphyrin dimers populate almost exclusively low energy triplet states because of the extremely efficient intramolecular singlet-to-triplet inter system crossing (ISC) process which is enhanced by the multiple heavy atoms substitutions. The nature of the electronic interactions determining the actual relaxation pathway followed by the porphyrin donor-acceptor pair is discussed.

Syntheses of regioselectively functionalized, asymmetric porphyrins via “2+2” condensation

Attachment of functional groups at the designated β-pyrrolic positions in asymmetrically substituted porphyrins is important in building customized model systems. We have developed a synthetic method of mono-functionalized dipyrromethanes at one of the β-pyrrolic positions. Synthesis of β-mono-brominated AB 2 C and ABCD type porphyrins was achieved by “2+2” condensation therefrom.

5,15-Diaryl-β-substituted-porphyrinato-manganese(III) chlorides as probes for structure–activity relationships in porphyrin-based epoxidation catalysts

Manganese complexes of 5,15-diaryl-β-substituted-porphyrins were prepared and their behaviour as oxidation catalysts was studied. The role of the pyrrolic and meso-substituents on the activity and selectivity of these catalysts was studied to reveal new structure–activity relationships in these porphyrin-based epoxidation catalysts. The beneficial effect of the halogen atoms at the meso-phenyls is still observed with these catalysts but, for the first time, a strong dependence on the selectivity of the epoxide production was found to be dependent on the nature of the non-halogen substituents at the β-pyrrolic positions of the porphyrin.

Electrochemical and in situ spectroelectrochemical behavior of 2,3,7,8,12,13,17,18-octakis(hexyl-thio) tetraazaporphyrin (H2OHTTAP) and [Zn(II)OHTTAP

The characterization of free base porphyrin 2,3,7,8,12,13,17,18-octakis(hexyl-thio) tetraazaporphyrin (H2OHTTAP) and its zinc(II) complexes [ Zn (II)OHTTAP] containing eight thioether groups at the β-pyrrole positions of the macrocycle was reported. Results obtained by cyclic voltammetry and differential pulse voltammetry indicated a five-electron reduction in five steps for each complex. They were oxidized in two single-electron-transfer steps to yield π-cation radicals and dications and reduced in three single-electron-transfer steps to yield π-anion radicals, dianions and trianions, respectively. The redox property of H2OHTTAP was unusual as compared to porphyrins (PPs) and phthalocyanines (Pcs). Each process was monitored by in situ thin-layer spectroelectrochemistry, which indicated that only the ligand was electroactive. The existence of the eight hexylthio groups was responsible for the intrastack interactions and enhanced intracolumnar and intercolumnar electron motions, resulting in improved conductivity.

Differentiation of positional isomers of nitro meso-tetraphenylporphyrins by tandem mass spectrometry

We studied by tandem mass spectrometry two isomers of nitro meso-tetraphenylporphyrin, one with a nitro group in the para position of a phenyl ring and the other with the same group in a β-pyrrolic position, and their copper complexes. Collisional activation of the molecular ions of both free-base porphyrins and of their copper complexes produces an array of product ions that permit ready differentiation of the two positional isomers. The diagnostic ions, when the nitro group is in a β-pyrrolic position, may be produced through intramolecular and double cyclization processes, triggered by the interaction of the nitro substituent with the neighboring meso-phenyl ring. These diagnostic ions do not form when the nitro group is in the para position. The gas-phase processes have precedents in solution chemistry.

Perbrominated 2-nitrotetraphenylporphyrins: electrochemical and axial ligation properties

A new series of perbrominated 2-nitro-5,10,15,20-tetraphenylporphyrins, H2TPPBrn(NO2) (n = 6 and 7) and their metal (CuII and ZnII) complexes have been synthesised and characterised. The presence of mixed electron withdrawing (bromo- and nitro-) substituents at the β-pyrrole positions induces interesting electrochemical and axial ligation properties. Perbrominated nitroporphyrins exhibit two successive one electron redox potentials with a positive shift of >100 mV relative to their corresponding octabromotetraphenylporphyrin (MTPPBr8) complexes. Axial ligation of various nitrogenous bases to ZnTPPBrn(NO2) complexes showed facile ligand binding with >50% enhancement in the equilibrium constants, Keq, relative to ZnTPPBr8. Surprisingly, MTPPBrn(NO2) complexes show similar axial ligation and electrochemical redox behaviour. The anodic shift in electrochemical redox potentials and enhanced Keq of MTPPBrn(NO2) complexes have been attributed to the increased electron deficiency of the porphyrin π-system.

Meso and β-pyrrole sulfonated porphyrins obtained by sulfonation of 5,15-bis(phenyl)porphyrin

The sulfonation of 5,15-bis(phenyl)porphyrin (DPP) in concentrated sulfuric acid results in substitution on the β-pyrrolic positions and the expected sulfonation on the para positions of the phenyl substituents. The sodium salt of 5,15-bis(4-sulfonatophenyl)porphyrin-2-sulfonic acid ( S 3 DPP ) was prepared in good yield, the sodium salts of 5,15-bis(4-sulfonatophenyl)porphyrin ( S 2 A DPP ), 5-phenyl-15-(4-sulfonatophenyl)porphyrin-2-sulfonic acid ( S 2 B DPP ) and 15-phenyl-5-(4-sulfonatophenyl)porphyrin ( S 1 A DPP ) were isolated, and 5,15-bis(phenyl)porphyrin-2-sulfonic acid ( S 1 B DPP ) was detected in a mixture with S 1 A DPP . The β-pyrrole sulfonato substituents are stable and their potential synthetic use is shown by the preparation of the trisodium salt of 10-bromo-5,15-bis(4-sulfonatophenyl)porphyrin-2-sulfonic acid ( BrS 3 DPP ) by NBS bromination of S 3 DPP . In a methanol/sulfuric acid mixture the sulfonation occurs on the meso positions, and the tetrasodium salt of 10,20-bis (4-sulfonatophenyl)porphyrin-5,15-disulfonic acid ( S 4 DPP ) was obtained.

Pernitrated metalloporphyrins as catalysts in oxidation with magnesium monoperoxophthalate: Part II. Epoxidation of linear olefins

The system composed of manganese (2,6)-dichlorotetraphenyl- or pentafluorotetraphenyl-porphyrins, bearing nitro groups at the β-pyrrolic positions as catalysts and monoperoxyphthalic acid magnesium salt (MMPP) as oxidant is an efficient model system for epoxidation of dodecene-1 and propene. A comparison with the corresponding system based on iron and cobalt complexes as well as the influence of the addition of a nitrogenous base are also described.

The oxidation of ethylbenzene and other alkylaromatics by dioxygen catalysed by iron(III) tetrakis(pentafluorophenyl)porphyrin and related iron porphyrins

The oxidation of ethylbenzene with dioxygen catalysed by iron(III) porphyrins in a solvent free system has been studied over the temperature range 30–110 °C. The time dependence of the formation of the three main products, 1-phenylethanol, acetophenone and 1-phenylethyl hydroperoxide, and the fate of the iron porphyrin are interpreted in terms of a free radical autoxidation mechanism. The yields of the oxidation products are determined by the rate of reaction and by the lifetime of the catalyst. Catalyst degradation is shown to involve reaction of the porphyrin ligand with 1-phenylethoxyl and 1-phenylethylperoxyl radicals. The disadvantages of increased induction periods and longer reaction times of the oxidations observed at lower reaction temperatures are counter balanced by increased catalyst turnovers. Less extensive studies on the oxidations of toluene, cumene, (2-methylpropyl)benzene and tert-butylbenzene support the overall mechanism proposed for ethylbenzene. A comparative study using the catalysts iron(III) 2,3,7,8,12,13,17,18-octachloro-5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin and iron(III) tetrakis(pentafluorophenyl)porphyrin and five of its derivatives reveals that halogenation of the β-pyrrole positions markedly increases the activity of the catalysts but not the stability of the porphyrin towards degradation. The highest yields were obtained with the μ-oxodimer of iron(III) tetrakis(pentafluorophenyl)porphyrin and iron(III) tetrakis(4-dimethylamino-2,3,5,6-tetrafluorophenyl)porphyrin.

Neocuproine-extended porphyrin coordination complexes. 2. Spectroscopic properties of the metalloporphyrin derivatives and investigations into the HOMO ordering.

The synthesis of a porphyrin compound, 1, containing a 2,9-dimethyl-1,10-phenanthroline moiety that is fused at the beta-pyrrole positions is reported. The absorption spectra of the free-base, copper(II), and zinc(II) derivatives have been studied. On the basis of absorption band intensities, the HOMO of the free base (H21) and its copper and zinc complexes (Cu1 and Zn1) was determined to be of a1u symmetry. Relative to H21, compounds Cul and Znl show enhanced spectral changes upon external metal ion binding. Although the HOMO is the same in all three compounds, the energy gap between the two highest occupied orbitals is greater for Cu1 and Zn1 than it is for the free-base compound. Several metal ions (Ni2+, Cu+, Cu2+, Zn2+, Li+) were examined in their binding to the phenanthrolinic group by measuring the resulting changes in the absorption spectra. It is shown that the observed changes in the absorption spectra are insensitive to the nature of the metal ion coordinated by the phenanthroline moiety. Significant differences in the absorption and emission spectra between Zn1 and [Zn(Zn1)2]2+ clearly demonstrate that the porphyrin pi-system is strongly affected by the binding of metal ions at the fused phenanthrolinic moiety.

Hydroxylation of simple alkanes by iodosylbenzene is catalyzed more efficiently by second than by third generation iron(III) porphyrins

The catalytic activities of aryl-chlorinated iron tetraarylporphyrins with and without chloro substituents at the β-pyrrole positions—third and second generation catalysts, respectively—were compared for the hydroxylation of ethylbenzene and cyclohexane by iodosylbenzene. The results reveal that despite the somewhat larger stability of the former complexes to the oxidative reaction conditions, they are less efficient catalysts than the corresponding unsubstituted complexes, which catalyze the transformation of the alkanes into their oxygenated products with almost 80% yield at more than 10% conversion. It is proposed that for the third generation catalyst the extremely short life time of the most potent intermediate is responsible for the relatively low efficiency in catalysis.

Electrochemical and Spectroelectrochemical Characterization of Water-soluble, β-Pyrrolebrominated Cobalt Porphyrins

The electrochemical and spectroelectrochemical characterization of two water-soluble porphyrins, namely the tetrachloro salt of cobalt(II) tetrakis-(N-methyl pyridyl)-β-octabromoporphyrin, [( Br 8 TMPyP ) Co II] Cl 4, and the tetrasodium salt of cobalt(II) tetrakis-(4-sulfonatophenyl)-β-octabromoporphyrin, Na 4[( Br 8 TPPS ) Co II], under different pH conditions is reported. The investigated porphyrins are highly non-planar and electron-deficient owing to the presence of eight bromides at the β-pyrrole positions and the four meso-aryl substituents. The redox potentials corresponding to the first oxidation and first reduction are shifted positively compared with the redox potentials of the respective unbrominated porphyrin derivatives. Spectroelectrochemical studies reveal the formation of a cobalt(III) complex during the first oxidation of both cobalt porphyrins. The first reduction of Br 8 TPPS ) Co II]4− results in a cobalt(I) complex, while involvement of the peripheral N-methyl pyridyl groups in the case of Br 8 TMPyP ) Co II]4+ is suggested. The peak potentials of the first oxidation corresponding to a Co II/ Co III redox couple of both investigated compounds are found to be pH-dependent. Debromination of the β-pyrrole bromo substituents of Br 8 TMPyP ) Co II]4+ at more negative potentials is observed. The UV-vis spectrum obtained after bulk electrolysis of Br 8 TMPyP ) Co II]4+ at −1.0 V vs Ag / AgCl followed by reoxidation at 0.2 V indicates complete elimination of the bromo substituents of the porphyrin periphery.