Iron TPPS

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

During the course of an attempted recrystallization of this complex from benzene containing chlorinated impurities the solution was exposed to light. The crystalline compound that formed from this solution was identified by X-ray crystallography as Mo(TPP)(Ph)Cl (Fig. 3). The complex contains Irons phenyl and chloro ligands, and the Mo—C and Mo—N4 plane distances are 2.241(1) and 0.125 A, respectively. A systematic synthesis of the complex could not subsequently be developed and consequently other spectroscopic and magnetic data were not collected. 

Iron porphyrins containing vinyl ligands have also been prepared by hydromet-allation of alkynes with Fe(TPP)CI and NaBH4 in toluene/methanol. Reactions with hex-2-yne and hex-3-yne are shown in Scheme 4. with the former giving two isomers. Insertion of an alkyne into an Fe(III) hydride intermediate, Fe(TPP)H, formed from Fe(TPP)Cl with NaBH4, has been proposed for these reactions. " In superficially similar chemistry, Fe(TPP)CI (present in 10 mol%) catalyzes the reduction of alkenes and alkynes with 200 mol% NaBH4 in anaerobic benzene/ethanol. For example, styrene is reduced to 2,3-diphenylbutane and ethylbenzene. Addition of a radical trap decreases the yield of the coupled product, 2,3-diphenylbutane. Both Fe(lll) and Fe(II) alkyls, Fe(TPP)CH(Me)Ph and [Fe(TPP)CH(Me)Ph] , were propo.sed as intermediates, but were not observed directly. ... 

Six-coordinate organoiron porphyrin nitrosyl complexes, Fe(Por)(R)(NO), were prepared from Fe(Por)R (Por = OEP or TPP R = Me, n-Bu, aryl) with NO gas. The NMR chemical shifts were typical of diamagnetic complexes, and the oxidation state of iron was assigned as iron(ll). ... 

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). ... 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Fe—N4 plane displacements of over 0.15 A, consistent with low spin Fe(lll) centers." The Fe—C bond lengths in the iron(lll) complexes are shorter (less than 2 A) than that in the iron(Il) complex (greater than 2 A). The shorter bond length for the phenyl complex Fe(TPP)Ph relative to the methyl complex Fe(TAP)Me is consistent with a stronger aryl Fe—C bond in the former. 

Iron porphyrins (containing TPP, picket fence porphyrin, or a basket handle porphyrin) catalyzed the electrochemical reduction of CO2 to CO at the Fe(I)/Fe(0) wave in DMF, although the catalyst was destroyed after a few cycles. Addition of a Lewis acid, for example Mg , dramatically improved the rate, the production of CO, and the stability of the catalyst. The mechanism was proposed to proceed by reaction of the reduced iron porphyrin Fe(Por)] with COi to form a carbene-type intermediate [Fe(Por)=C(0 )2, in which the presence of the Lewis acid facilitates C—O bond breaking. " The addition of a Bronsted acid (CF3CH2OH, n-PrOH or 2-pyrrolidone) also results in improved catalyst efficiency and lifetime, with turnover numbers up to. 750 per hour observed. ... 

The photochemistry of several of the iron porphyrin halocarbene complexes Fe(TPP)(=CXY) (CXY = CCF, CBr2, CCIF. CCl(CN) and the vinylidene complex Fe(TPP)=C=CAr2 (Ar = p-C6Fl4Cl) has been studied in degassed benzene... 

Decomposition of diazo compounds by iron porphyrins is a convenient method for the synthesis of non-heteroatom carbene-iron porphyrins [22]. Reaction of [Fe(F2o-TPP)] [F20-TPP = meso-tetrakis(pentafluorophenyl)porphyrinato dianion] with diazo compounds N2C(Ph)R (R = Ph, C02Et, C02CH2CH=CH2) under an inert atmosphere afforded complexes [Fe(F2o-TPP)C(Ph)R] in 65-70% yields (Scheme 2). Like the halocarbene complex [Fe(TPP)(CCl2)], [Fe(F2o-TPP)CPh2] reacted with Melm to afford six-coordinate species [(MeIm)Fe(F2o-TPP)CPh2] in 65% isolated yield. 

The iron atom in [Fe(F2o-TPP)CPh2] is 0.294 A out of the mean porphyrin plane toward the carbene group, whereas [(MeIm)Fe(F2o-TPP)CPh2] exhibits a considerably smaller displacement of the iron atom from the mean plane of the porphyrin ring (0.122 A toward the CPh2 group). In contrast, the iron atom of... 

Interestingly, [Ee(F20-TPP)C(Ph)CO2Et] and [Fe(p2o-TPP)CPh2] can react with cyclohexene, THF, and cumene, leading to C-H insertion products (Table 3) [22]. The carbenoid insertion reactions were found to occur at allylic C-H bond of cyclohexene, benzylic C-H bond of cumene, and ot C-H bond of THF. This is the first example of isolated iron carbene complex to undergo intermolecular carbenoid insertion to saturated C-H bonds. 

In 1982, Breslow and coworkers reported the first example of iron-catalyzed nitrene C-H bond insertion [29]. They used [Fe(TTP)] as catalyst and PhINTs as nitrene precursor to achieve C-H bond amination of cyclohexane. However, the product yield was low (around 10%). Subsequently, the same authors found that iminoio-dane 7 derived from 2,5-diisopropylbenzenesuIfonamide underwent intramolecular C-H amination efficiently with [Fe(TPP)Cl] as catalyst at room temperature, giving the insertion product in 77% yield (Scheme 29) [85]. 

More recently 233) it has been reported that cross-linked polystyrene containing imidazole ligands did not provide a support rigid enough to prevent dimerization, and that the p-oxo dimer was benzene extracted from oxygenated tetraphenyl porphyrin iron(ll), Fe(TPP), which had been attached to the modified polystyrene. A discussion of model synthetic porphyrins, from which definitive structural and other physical data are obtained, is given in section V.C. 

A silica gel containing 3-imidazole propyl groups bonded to surface atoms of silicon has been prepared (253), and was treated 254) with a solution of Fe(TPP)L2, where L = py or pip, where upon the iron(II) porphyrin became attached to the imidazole groups on the surface of the gel,... 

Fig. 5.16 Calculated structure of ferrous nitrosyl tetraphenylporphyrin Fe(TPP)(NO), resulting from geometric optimization with the B3LYP functional and 6-3IG -I- VTZ basis set [101]. Color scheme cyan = iron, green = carbon, blue = nitrogen, red = oxygen (taken from [101])...

THF solution [290]. There had been much confusion in the past about the magnetic moment of the iron(I) species and the correct Mossbauer and NMR properties caused by impurities of the samples and possible unknown axial ligation. However, with clean crystallized samples with known molecular structure [291], the first reduction product of iron(II)(tetraphenylporyphrin), [Fe(TPP)], can be clearly characterized as a quasiplanar iron(I) complex with spin S = 1/2 (EPR = 2.28,... 

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]. 

The second reduction step of Fe(II)(TPP) yields an extremely air-sensitive green product which can be assigned the formula [Fe(I)(TPP) ] because of the red shift of the Soret band in the UV-VIS spectrum. The pure material is diamagnetic (S = 0) but this does not allow one to distinguish between the three possible descriptions as an iron(O) / -porpyhrin, a spin-coupled S = 1/2 iron(I)-porphyrin... 

The Mossbauer spectrum of the dianion was a surprise because the isomer shift (5 = 0.48 mm s ) was substantially more negative than that of the monoanion, and resembles more that of Fe(II)(TPP) than that of [Fe(I)TPP] . Reed and Scheldt therefore suggested a resonance hybrid of iron(I) and iron(II) for the electronic structure of the dianion molecule instead of the formulation [Fe(0)(TPP)] [290, 291]. 

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