Pyrrole, protonated

Apart from the A-methyl group, three double-bond equivalents and three multiplets remain in the chemical shift range appropriate for electron rich heteroaromatics, Sh = 6.2 to 6.9. A-Methyl-pyrrole is such a compound. Since in the multiplets at Sh = 6.25 and 6.80 the Jhh coupling of 4.0 Hz is appropriate for pyrrole protons in the 3- and 4-positions, the pyrrole ring is deduced to be substituted in the 2-position. 

H NMR spectroscopy studies of iron(IIl) a-alkyl and o-aryl porphyrins have been very important in elucidating spin states. Alkyl and most aryl complexes with simple porphyrin ligands (OEP, TPP, or TTP) are low spin, S — I /2 species. NMR spectra for the tetraarylporphyrin derivatives show upheld resonances for the porphyrin pyrrole protons (ca. — 18 to —35 ppm), and alternating upfield and downfield hyperfine shifts for the axial alkyl or aryl resonances. For -alkyl complexes, the a-protons show dramatic downfield shifts (to ca. 600 ppm), upfield shifts for the /3-protons (—25 to — 160 ppm) and downfield shifts for the y-protons (12 ppm). The cr-protons of alkyliron porphyrins are not usually detected as a result of their large downfield shift and broad resonance. These protons were first detected by deuterium NMR in the dcuterated complexes Fe(TPP)CD3 (532 ppm) and Fe(TPP)CD2CDi (562, -117 ppm). ... 

However, it was pointed out that two other observations are out of line with the iron(I) formulation and more consistent with an iron(II)-porphyrin radical anion [290] (1) the low-intensity red-shifted Soret band in the UV-VIS spectrum with broad maxima in the a,(3-region compared to, for instance, Fe(TPP) in THF, is typical of a porphyrin radical, and (2) the bond lengths of the porphyrin core indicate population of the (antibonding) LUMO of the ligand (i.e., the presence of an extra electron in the re-system). The presence of porphyrin radical character in the electronic ground state was also inferred from the paramagnetic NMR-shifts of the pyrrole protons at the meso and p-carbon atoms [291]. 

Optical Properties. The peripheral positive charges on the pz greatly lower the pvalues of the pyrrole protons of 42 in aqueous solution. Titrations... 

An extensive EPR and ENDOR study of Cu(TPP) and Ag(TPP) (Fig. 32) doped into (H20)Zn(TPP) single crystals has been published by Brown and Hoffman66,171. The complete set of hfs and quadrupole tensors for the 14N nuclei and the hfs tensors of the Ag(II) and Cu(II) ions and of the pyrrole protons are reported. A detailed analysis of the magnetic parameters (Table 6.1), which are measured with high accuracy, has been presented using the standard MO treatment66. ... 

Proton ENDOR. In both metal complexes four pairs of magnetically equivalent pyrrole protons along with some weakly coupled phenyl protons have been observed. 

This indicates that the paramagnetic guest compounds exhibit the center of symmetry required by the Zn(TPP) host crystal structure. Two types of pyrrole protons (1, 3 and 2, 4, Fig. 32) with slightly different hf values are found in Ag(TPP), indicating that the site symmetry of Ag(TPP) cannot be higher than S4 or C4. 

Considering a five-point spin density distribution (central ion and four nitrogens) for the determination of the dipolar proton hfs tensors in Ag(TPP) (5.5), the computed ADD principal values are found to be close to the experimental results. It should be noted that in Ag(TPP) the Mulliken population, UN, on the nitrogen nearest to the pyrrole proton provides a larger contribution to ADD along the Ag-H direction than the population UAg. 

As a consequence of the smaller covalence of Cu(TPP), the pyrrole proton tensors are nearly axially symmetric and the Cu-H distances calculated with the entire unpaired electron at the Cu(II) ion are in excellent agreement with X-ray data. The difference in covalency of Ag(TPP) and Cu(TPP) is also reflected by the s-spin densities on the pyrrole protons which amount to PH(Ag) = 0.15% and pH(Cu) = 0.093%, respectively. A comparison with the corresponding data of an Xa calculation on Cu(II)-porphine1725 (oh(Cu) = 0.071%) indicates that the state-of-the-art electronic structure calculation underestimates the amount of unpaired spin density on the porphyrin ring. 

Copper porphyrin is one of the best-characterized of the metalloporphyrins, and its electron spin resonance (ESR) spectrum has been known for a quarter of a century.(17) More recently, electron nuclear double resonance (ENDOR) investigations have provided the complete hyperfine tensors for the metal, the nitrogens and the pyrrole protons.(18) We have used this detailed knowledge earlier(, ) to assess the quality of scattered-wave calculations. 

All the Ru complexes are diamagnetic and display well-revolved NMR spectra. The pyrrolic proton signals are shifted about 0.4 ppm downfield from those of the carbonylruthen-ium(II) precursors. 

A chiral dichlororuthenium(IV) complex of a Z)4-symmetric porphyrin, [Ru (Z)4-por )(Cl)2], has been prepared by heating [Ru (Z>4-por )(CO)(MeOH)] in CCI4. The complex is characterized by NMR (paramagnetically shifted pyrrolic protons at = 52.3 ppm), FAB-MS, and magnetic susceptibility measurement (/.teff= 3.1/.tB). It is a very active catalyst for enantioselective alkene epoxidations using 2,6-dichloropyridine A-oxide as the terminal oxidant, with a turnover number of up to 2000 the ee of the epoxides is 50-80%. The complex can be incorporated into sol-gel and turnovers of over 10" can be achieved." ... 

The PMR spectra of substituted derivatives, however, provide additional evidence for the structure of bridged [12]annulenes. A comparison of 92a and 92b reveals a normal control of the cyano group over the chemical shift of the neighboring protons (A = 1.0 ppm), the influence on the remote pyrrolic protons being small (A = 0.11-0.39 ppm). Hence diazapyracylenes should be composed of loosely connected closed-shell moieties (formula 97a) the influence of the substituents on the chemical shift being limited to the substituted part of the molecule.112... 

This provides a simple route to difficult-to-obtain compounds of the 4,5-dihydrobenzo[g]indole series, which are rather promising in the search for new bioactive substances and dyes. 4,5-Dihydrobenzo[g]indole (see Table XIX) was isolated by column chromatography (A1203, Et20/hexane 1 2) as colorless crystals that turn blue in air. The H-NMR spectrum of pyrrole 36 (CDCI3, ppm) shows 7.1 m (four benzene protons) 6.8 t and 6.0 t (two pyrrole protons) and 2.8 m (four protons in positions 4 and 5). The IR spectrum shows the characteristic bands of the pyrrole and benzene moieties (cm-1) 695, 730, 765, 1510, 1560, 1580, 1610, 3040, 3050, 3100, 3390, and 3430. 

Fig. 5.8. H NMR spectra of high spin Fe(lII) porphyrins. (A) Fe TPP-C1 (no substituent at the pyrrole positions). Signal a refers to pyrrole protons signals b, c and d refer to the meta, ortho and para phenyl protons respectively [19]. (B) Fe (protoporphyrin IX)-C1 (see Fig. 5.7A for the ligand). Signals a belong to the methyl groups, signals b and g to the 13,17 a-CH2 and the 3,8 a-CH signals d to the COOH signals e to the 13,17 P-CH2 signals h and i to the 3,8 P-CH cis and p-CH trans respectively [20].

For 5 = 2 systems, a predominance of cr delocalization is expected as well as moderate rc delocalization at the meso positions [58]. A spectrum is reported in Fig. 5.24 [58], where the pyrrole protons are downfield. 

Reedijk and co-workers have prepared and studied a variety of Ru(ll) complexes of pyridyl- and pyrazinyl-substituted 1,2,4-triazoles of which 26 is an example. In ail cases the heterocyclic ligands were chelated to the metal center via one azole and one azine nitrogen atom and, in addition to various electrochemical and spectroscopic studies, pKa values for the pyrrole proton of the triazole ring were obtained. The Ru(bpy)2 + com-... 

Experimental Antibiotic Pyrrole Proton Complexation Shifts (A6, ppm) In the Netropsln Poly(dA-dT) Complex3... 

Attempts to correlate stability constant data for 1 and 2 in acetonitrile with the chemical shift changes, A<5, of the pyrrole proton led to straight lines (Fig. 5). These findings clearly demonstrate the selective behaviour of these receptors for the fluoride anion. 

Fig. 5. Linear relationship between log Ks of 1 and 2 and A<5 values for the pyrrole proton in acetonitrile at 298 K.

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