Pyrrole Nitrogen Atoms Research Articles

The structures of (phenylato)( N-2-thiophenecarboxamido- meso-tetraphenylporphyrinato)mercury(II) and bisphenylmercury(II) complex of 21-(4- tert-butyl-benzenesulfonamido)-5,10,15,20-tetraphenylporphyrin

The reaction of PhHgOAc with N-NHCO-2-C 4H 3S-Htpp ( 5) and N- p-HNSO2C6H4 tBu-Htpp ( 4) gave a mercury (II) complex of (phenylato) ( N-2-thiophenecarboxamido- meso-tetra phenylporphyrinato)mercury(II) 1.5 methylene chloride solvate [HgPh( N-NHCO-2-C 4H 3S-tpp) · CH 2Cl 2 · 0.5C 6H 14; 6 · CH 2Cl 2 · 0.5C 6H 14] and a bismercury complex of bisphenylmercury(II) complex of 21-(4- tert-butyl-benzenesulfonamido)-5,10,15,20-tetraphenylporphyrin, [(HgPh)2( N- p-NSO 2C 6H 4 t Bu-tpp); 7], respectively. The crystal structures of 6 · CH 2Cl 2 · 0.5C 6H 14 and 7 were determined. The coordination sphere around Hg(1) in 6 · CH 2Cl 2 · 0.5C 6H 14 and Hg(2) in 7 is a sitting-atop derivative with a seesaw geometry, whereas for the Hg(1) in 7, it is a linear coordination geometry. Both Hg(1) in 6 · CH 2Cl 2 · 0.5C 6H 14 and Hg(2) in 7 acquire 4-coordination with four strong bonds [Hg(1)–N(1) = 2.586(3) Å, Hg(1)–N(2) = 2.118(3) Å, Hg(1)–N(3) = 2.625(3) Å, and Hg(1)–C(50) = 2.049(4) Å for 6 · CH 2Cl 2 · 0.5C 6H 14; Hg(2)–N(1) = 2.566(6) Å, Hg(2)–N(2) = 2.155(6) Å, Hg(2)–N() = 2.583(6) Å, and Hg(2)–C(61) = 2.064(7) Å for 7]. The plane of the three pyrrole nitrogen atoms [i.e., N(1)–N(3)] strongly bonded to Hg(1) in 6 · CH 2Cl 2 · 0.5C 6H 14 and to Hg(2) in 7 is adopted as a reference plane 3N. For the Hg 2+ complex in 6 · CH 2Cl 2 · 0.5C 6H 14, the pyrrole nitrogen bonded to the 2-thiophenecarboxamido ligand lies in a plane with a dihedral angle of 33.4° with respect to the 3N plane, but for the bismercury(II) complex in 7, the corresponding dihedral angle for the pyrrole nitrogen bonded to the NSO 2C 6H 4 t Bu group is found to be 42.9°. In the former complex, Hg(1) 2+ and N(5) are located on different sides at 1.47 and −1.29 Å from its 3N plane, and in the latter one, Hg(2) 2+ and N(5) are also located on different sides at −1.49 and 1.36 Å form its 3N plane. The Hg(1)⋯Hg(2) distance in 7 is 3.622(6) Å. Hence, no metallophilic Hg(II)⋯Hg(II) interaction may be anticipated. NOE difference spectroscopy, HMQC and HMBC were employed to unambiguous assignment for the 1H and 13C NMR resonances of 6 · CH 2Cl 2 · 0.5C 6H 14 in CD 2Cl 2 and 7 in CDCl 3 at 20 °C. The 199Hg chemical shift δ for a 0.05 M solution of 7 in CDCl 3 solution is observed at −1074 ppm for Hg(2) nucleus with a coordination number of four and at −1191 ppm for Hg(1) nucleus with a coordination number of two. The former resonance is consistent with that chemical shift for a 0.01 M solution of 6 in CD 2Cl 2 having observed at −1108 ppm for Hg(1) nucleus with a coordination number of four.

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

Understanding electronic communication among interacting chromophores provides the foundation for a variety of applications. The ground-state electronic communication in diphenylethyne-linked zinc-porphyrin dyads has been investigated by a novel molecular design strategy that entails introduction of a 13C-atom (*) at specific sites of the porphyrins where there is substantial electron density in the relevant frontier (highest occupied) molecular orbital. The site of 13C substitution is at a meso-position, either the site of attachment of the linker (proximal, "P") or the site trans to the linker (distal, "D"). The substituents (R) at the non-linking meso-positions are mesityl, tridec-7-yl ("swallowtail"), or p-tolyl groups. Altogether five isotopically labeled porphyrin dyads have been prepared. The hole/electron-transfer properties of one-electron oxidized dyads have been examined by electron paramagnetic resonance (EPR) spectroscopy. The introduction of the meso-13C label provides a "clock" (via the hyperfine interactions) that allows investigation of a time scale for hole transfer that is 3-4 times shorter than that provided by the natural abundance 14N nuclei of the pyrrole nitrogen atoms. The EPR studies indicate that the hole transfer, which has been previously shown to be fast on the time scale of the 14N hyperfine clock ( approximately 220 ns), remains fast on the time scale of the 13C hyperfine clock ( approximately 50 ns).

Design and Synthesis of Binucleating Macrocyclic Clefts Derived from Schiff‐Base Calixpyrroles

The syntheses, characterisation and complexation reactions of a series of binucleating Schiff-base calixpyrrole macrocycles are described. The acid-templated [2+2] condensations between meso-disubstituted diformyldipyrromethanes and o-phenylenediamines generate the Schiff-base pyrrolic macrocycles H(4)L(1) to H(4)L(6) upon basic workup. The single-crystal X-ray structures of both H(4)L(3).2 EtOH and H(4)L(6).H2O confirm that [2+2] cyclisation has occurred, with either EtOH or H2O hydrogen-bonded within the macrocyclic cleft. A series of complexation reactions generate the dipalladium [Pd2(L)] (L=L(1) to L(5)), dinickel [Ni2(L(1))] and dicopper [Cu2(L)] (L=L(1) to L(3)) complexes. All of these complexes have been structurally characterised in the solid state and are found to adopt wedged structures that are enforced by the rigidity of the aryl backbone to give a cleft reminiscent of the structures of Pacman porphyrins. The binuclear nickel complexes [Ni2(mu-OMe)2Cl2(HOMe)2(H(4)L(1))] and [Ni2(mu-OH)2Cl2(HOMe)(H(4)L(5))] have also been prepared, although in these cases the solid-state structures show that the macrocyclic ligand remains protonated at the pyrrolic nitrogen atoms, and the Ni(II) cations are therefore co-ordinated by the imine nitrogen atoms only to give an open conformation for the complex. The dicopper complex [Cu2(L(3))] was crystallised in the presence of pyridine to form the adduct [Cu2(py)(L(3))], in which, in the solid state, the pyridine ligand is bound within the binuclear molecular cleft. Reaction between H(4)L(1) and [Mn(thf){N(SiMe(3))2}2] results in clean formation of the dimanganese complex [Mn2(L(1))], which, upon crystallisation, formed the mixed-valent complex [Mn2(mu-OH)(L(1))] in which the hydroxo ligand bridges the metal centres within the molecular cleft.

Cross-Bridging Reaction of 5,20-Diethynyl Substituted Hexaphyrins to Vinylene-Bridged Hexaphyrins

5,20-Diethynyl substituted [26]hexaphyrins(1.1.1.1.1.1) undergo a thermal or spontaneous cross-bridging reaction to provide trans-vinylene-bridged [26]hexaphyrins, probably through strongly folded conformations. Facile aromatic−antiaromatic switching of vinylene-bridged hexaphyrins has been demonstrated upon two-electron redox processes. Curiously, a resonance contributor of the vinylene-bridged hexaphyrins is regarded as [16]diazaannuleno[16]diazaannulene but its contribution should be minor because of the perpendicular geometry of the bridge. Vinylene-bridged hexaphyrin serves as an effective ligand that coordinates two zinc(II) ions using the central vinylene double bond in a η2-fashion along with the three pyrrolic nitrogen atoms.

Electrostatic forces that determine O,O- trans versus O,S- cis conformers in the aminated porphyrin complexes of Cd(N-NHCO-2-C 4H 3O-tpp)(OAc) and Cd(N-NHCO-2-C 4H 3S-tpp)(OAc)

Two new diamagnetic, mononuclear and aminated porphyrin complexes of O,O- trans-Cd ( 3- trans) and O,S- cis-Cd ( 4- cis) have been synthesized and characterized by 1H, 13C NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido- meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C 4H 3O-tpp)(OAc); 3- trans] and (acetato)(N-2-thiophenecarboxamido- meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C 4H 3S-tpp)(OAc); 4- cis] were determined. The coordination sphere around Cd 2+ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc − group for 3- trans and 4- cis. The plane of three pyrrole nitrogen atoms [i.e., N(1), N(2), N(4) for 3- trans and N(1), N(2), N(3) for 4- cis] strongly bonded to Cd 2+ is adopted as a reference plane 3N. The N(3) and N(4) pyrrole rings bearing the 2-furancarboxamido (Fr) and 2-thiophenecarboxamido groups in 3- trans and 4- cis, respectively, deviate mostly from the 3N plane, thus orienting separately with a dihedral angle of 33.4° and of 31.0°. In 3- trans, Cd 2+ and N(5) are located on different sides at 1.06 and −1.49 Å from its 3N plane, while in 4- cis, Cd 2+ and N(5) are also located on different sides at 1.04 and −1.53 Å from its 3N plane. An attractive electrostatic interaction between the Cd 2+ and O(4) − atoms in furan stabilizes the O,O- trans conformer of 3. A repulsive electrostatic interaction between Cd 2+ and S(1) + destabilizes the O,S- trans conformer of 4. Both of these repulsive and the mutually attractive interactions between S(1) + and O(3) − atoms favor the O,S- cis rotamer of 4 both in the vapor phase and in low polarity solvents. NOE difference spectroscopy, HMQC and HMBC were employed for the unambiguous assignment of the 1H and 13C NMR resonances of 3- trans and 4- cis in CDCl 3 at 20 and −50 °C.

Inverted Sapphyrin: A New Family of Doubly N-Confused Expanded Porphyrins

The synthesis of an inverted, methoxylated sapphyrin derivative is described. This system, wherein inversion of pyrrolic nitrogen atoms is configurationally enforced via the use of a 3,3'-bipyrrolic precursor, displays what is best described as "weak aromaticity" as judged from its spectroscopic features and supporting theoretical calculations.

Thallium complexes of tetraphenylporphyrin: Tl(tpp- N-O)(OAc) and cis-acetato-N- p- tert-butylbenzensulfonylimido- meso-tetraphenylporphyrinatothallium(III) Tl(N- p-NSO 2C 6H 4tBu-tpp)(OAc)

The crystal structures of Tl(tpp- N-O)(OAc) ( 3) and acetato-N- p- tert-butylbenzensulfonylimido- meso-tetraphenylporphyrinatothallium(III) Tl(N- p-NSO 2C 6H 4 t Bu-tpp)(OAc) ( 5) have been determined. The coordination sphere around Tl 3+ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc − group for 3 and 5. The plane of three pyrrole nitrogen atoms (i.e., N(1), N(2) and N(3)), strongly bonded to Tl 3+ in 3 and 5, is adopted as a reference plane, 3 N. The porphyrin ring is severely distorted and the pyrrole ring N(4) bonding to the oxygen and NSO 2C 6H 4 t Bu group makes a dihedral angle of 46.5° and 46.7° with the 3 N plane for 3 and 5, respectively. In 3, Tl 3+ and O(1) are located on the same side at 1.11 and 1.34 Å from its 3 N plane, and in 5, Tl 3+ and N(5) are also located on the same side at 1.15 and 1.30 Å from its 3 N plane. The free energy of activation at the coalescence T c for the intermolecular acetate exchange process of 3 and 5 in CD 2Cl 2 is found to be Δ G 184 ‡ = 39.3 and Δ G 208 ‡ = 44.1 kJ / mol , respectively, through 1H NMR variable temperature measurements. An electronegative substituent, O(1), bonded to Tl in 3 causes a significant negative contribution up to 145 Hz for Δ 3 J(Tl–C, O) and 79 Hz for Δ 2 J(Tl–C, O) for OAc − in 3.

Interpretation of EXAFS spectra for sitting-atop complexes with the help of computational methods

The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and EXAFS (extended X-ray absorption fine structure) data has been reported for Cu 2+ in acetonitrile, which have been interpreted as the observation of a SAT complex. However, this interpretation has been challenged and other investigations have shown that there are many possible SAT structures. We have recently developed a method to combine quantum mechanical (QM) calculations and EXAFS fits (EXAFS/QM), which in principle is a standard EXAFS fit that employs all multiple-scattering information in an optimum and self-consistent way and uses the QM calculations to ensure that the obtained structures are chemically reasonable. By this approach, we show that out of the 15 putative SAT complexes, structures with the copper ion coordinating to two cis pyrrolenine nitrogen atoms and two or three acetonitrile molecules fit the experimental EXAFS spectrum best. However, an equally good fit can be obtained also by a mixture of the reactant and product complexes.

Residue F4 Plays a Key Role in Modulating Oxygen Affinity and Cooperativity in Scapharca Dimeric Hemoglobin

Residue F4 (Phe 97) undergoes the most dramatic ligand-linked transition in Scapharca dimeric hemoglobin, with its packing in the heme pocket in the unliganded (T) state suggested to be a primary determinant of its low affinity. Mutation of Phe 97 to Leu (previously reported), Val, and Tyr increases oxygen affinity from 8- to 100-fold over that of the wild type. The crystal structures of F97L and F97V show side chain packing in the heme pocket for both R and T state structures. In contrast, in the highest-affinity mutation, F97Y, the tyrosine side chain remains in the interface (high-affinity conformation) even in the unliganded state. Comparison of these mutations reveals a correlation between side chain packing in the heme pocket and oxygen affinity, indicating that greater mass in the heme pocket lowers oxygen affinity due to impaired movement of the heme iron into the heme plane. The results indicate that a key hydrogen bond, previously hypothesized to have a central role in regulation of oxygen affinity, plays at most only a small role in dictating ligand affinity. Equivalent mutations in sperm whale myoglobin alter ligand affinity by only 5-fold. The dramatically different responses to mutations at the F4 position result from subtle, but functionally critical, stereochemical differences. In myoglobin, an eclipsed orientation of the proximal His relative to the A and C pyrrole nitrogen atoms provides a significant barrier for high-affinity ligand binding. In contrast, the staggered orientation of the proximal histidine found in liganded HbI renders its ligand affinity much more susceptible to packing contacts between F4 and the heme group. These results highlight very different strategies used by cooperative hemoglobins in molluscs and mammals to control ligand affinity by modulation of the stereochemistry on the proximal side of the heme.

Cd(II), Zn(II), and Pd(II) Complexes of an Isoindoline Pincer Ligand: Consequences of Steric Crowding

The sterically crowded isoindoline pincer ligand, 6'-MeLH, prepared by condensation of 4-methyl-2-aminopyridine and phthalonitrile, exhibits very different reaction chemistry with Cd2+, Zn2+, and Pd2+. Three different ligand coordination modes are reported, each dependent upon choice of metal ion. This isoindoline binds to Cd2+ as a charge-neutral, zwitterionic, bidentate ligand using imine and pyridine nitrogen atoms to form the eight-coordinate fluxional complex, Cd(6'-MeLH)2(NO3)2. In the presence of Zn2+, however, loss of a pyridine arm occurs through solvolysis and tetrahedrally coordinated complexes are formed with coordination of pyrrole and pyridine nitrogen atoms. Reaction with Pd2+ produces the highly distorted, square planar complex Pd(6'-MeL)Cl in which a deprotonated isoindoline anion coordinates as a tridentate pyridinium NNC pincer ligand.

A New Hydrogen-Bonding Motif with Constituents Bearing Donor and Acceptor Sites 7 Å Apart

Aryl substituted dipyrromethanes [di(pyrrol-2-yl)-phenyl-methanes] with hydrogen acceptor substituents R in para position of the aryl ring [R = CO2Me, CO2H, CONH(iPr) and NH2] located 7 Å apart from the hydrogen donor pyrrole nitrogen atom are shown to self-assemble in the solid state via hydrogen bonds to form rings or chains.

Group 10 Metal Complexes of meso-Aryl-Substituted [26]Hexaphyrins with a Metal−Carbon Bond

Nickel(II), palladium(II), and platinum(II) complexes of meso-aryl-substituted [26]hexaphyrin have been prepared and structurally characterized. In mono-metalated complexes, the metal ion is commonly bound with three pyrrolic nitrogen atoms and one pyrrolic beta-carbon, whereas a bis-Pd(II) complex exhibits a unique structure involving a Pd-C(alpha) bond. These results reveal novel coordination abilities of the hexaphyrin that lead to a metal-carbon bond at either pyrrolic C(beta) or C(alpha) position.

Calculation of Vibrational Spectra of Linear Tetrapyrroles. 3. Hydrogen-Bonded Hexamethylpyrromethene Dimers

The structure and vibrational spectra of hexamethylpyrromethene (HMPM) have been investigated by X-ray crystallography, IR and Raman spectroscopies, and density functional theory calculations. HMPM crystallizes in the form of dimers, which are held together by bifurcated N-H(...N)(2) hydrogen bonds, involving one intramolecular and one intermolecular N-H...N interaction. The monomers are essentially planar, and the mean planes of the monomers lie approximately perpendicular to one another, so that the four N atoms in the dimer form a distorted tetrahedron. The structure of the HMPM dimer is well-reproduced by B3LYP/6-31G calculations. A comparison of the calculated geometry of the dimer with that of the monomer reveals only small changes in the N-H...N entity and the methine bridge angles upon dimerization. These are a result of weakening of the intramolecular N-H...N hydrogen bond and the formation of a more linear N-H...N intermolecular hydrogen bond. Using an empirical relation between the shift of the N-H stretching frequency of pyrrole and the enthalpy of adduct formation with bases [Nozari, M. S.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 7086-7090], estimates of the strength of the intra- and intermolecular hydrogen bonds are obtained. IR and Raman spectroscopies of HMPM and its isotopomers deuterated at the pyrrolic nitrogen atom and at the methine bridge reveal that the molecule is monomeric in nonpolar organic solvents but dimeric in a solid Ar matrix and in KBr pellets. The matrix IR spectra show a splitting of vibrational modes for the dimer, particularly those involving the N-H coordinates. Due to intrinsic deficiencies of the B3LYP/6-31G approximation, a satisfactory reproduction of these modes of the monomeric and dimeric HMPM requires specific adjustments of the NH scaling factors for the calculated force constants and, in the case of the NH out-of-plane modes of HMPM dimers, also of intra- and intermolecular coupling constants. This parametrization does not significantly affect the other calculated modes, which in general reveal a very good agreement with the experimental data.

Phosphorylation of N‐iso‐propyl‐ and N‐tert‐butylpyrroles with phosphorus(III) halides

AbstractThe investigation concerns the effect of a bulky substituent at the pyrrole nitrogen atom on the orientation and regioselectivity of pyrrole phosphorylation with phosphorus(III) halides. As shown, phosphorylation of N‐iso‐propylpyrrole with phosphorus tribromide or trichloride proceeds nonregioselectively at positions 2 and 3 but it is followed by the 2 → 3 migration of the dihalogenophosphine group which quantitatively yields the 3‐isomer. N‐tert‐butylpyrrole is regioselectively phosphorylated with halogenophosphines at position 3. The tert‐butyl substituent at the nitrogen atom does not preclude the binding of even two or three pyrrolyl residues to the phosphorus atom. The key compounds, 3‐pyrrolyldihalogenophosphines, were isolated in a pure state, characterized and used to obtain a number of stable phosphorus(V) derivatives. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:599–604, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20158

Heme Distortion Modulated by Ligand-Protein Interactions in Inducible Nitric-oxide Synthase

The catalytic center of nitric-oxide synthase (NOS) consists of a thiolate-coordinated heme macrocycle, a tetrahydrobiopterin (H4B) cofactor, and an l-arginine (l-Arg)/N-hydroxyarginine substrate binding site. To determine how the interplay between the cofactor, the substrates, and the protein matrix housing the heme regulates the enzymatic activity of NOS, the CO-, NO-, and CN(-)-bound adducts of the oxygenase domain of the inducible isoform of NOS (iNOS(oxy)) were examined with resonance Raman spectroscopy. The Raman data of the CO-bound ferrous protein demonstrated that the presence of l-Arg causes the Fe-C-O moiety to adopt a bent structure because of an H-bonding interaction whereas H4B binding exerts no effect. Similar behavior was found in the CN(-)-bound ferric protein and in the nitric oxide (NO)-bound ferrous protein. In contrast, in the NO-bound ferric complexes, the addition of l-Arg alone does not affect the structural properties of the Fe-N-O moiety, but H4B binding forces it to adopt a bent structure, which is further enhanced by the subsequent addition of l-Arg. The differential interactions between the various heme ligands and the protein matrix in response to l-Arg and/or H4B binding is coupled to heme distortions, as reflected by the development of a variety of out-of-plane heme modes in the low frequency Raman spectra. The extent and symmetry of heme deformation modulated by ligand, substrate, and cofactor binding may provide important control over the catalytic and autoinhibitory properties of the enzyme.

Dicopper(II) and dinickel(II) complexes of Schiff-base macrocycles derived from 5,5-dimethyl-1,9-diformyldipyrromethane

The synthesis and characterisation of two dicopper(II) and two dinickel(II) macrocyclic complexes, [CuII2LPr] (10), [CuII2LBu] (11), [NiII2LPr] (12) and [NiII2LBu] (13), are reported. The two new Schiff-base macrocycles (LPr)4− and (LBu)4− are isolated as dimetallic complexes 10–13 by the [2+2] condensation of 5,5-dimethyl-1,9-diformyldipyrromethane (9) and 1,3-diaminopropane or 1,4-diaminobutane, respectively, using Cu2+ or Ni2+ template ions. Single crystal X-ray structure determinations carried out on 10–13 show that each metal atom is in a square planar N4 geometry, being bound to two deprotonated pyrrole nitrogen atoms of one dipyrromethane unit and to the two adjacent imine nitrogen atoms. NMR spectra obtained for the two dinickel(II) complexes 12 and 13 show that in CDCl3 solution they are highly symmetrical and diamagnetic.

Molecular and electronic structures of the trithiapenthalene antrone and its oxygen and nitrogen analogues by XPS

Trithiapentalene antrone (I antr), its newly synthesized oxy-(II antr) and aza-(III antr) analogues, and their models were investigated by XPS method for the first time. Quantum-chemical calculations in all-valence (σ–π) approach (INDO/S) were performed. The distribution of the electron density on sulfur, nitrogen, and oxygen atoms was determined. The correlation of calculated charges with binding energies and chemical shifts was found. A new chromophore system was revealed. The electron-donor influence of a pyrrole nitrogen atom on the aromatic π-system was shown.

Oxidation and oxygenation of iron complexes of 2-aza-21-carbaporphyrin.

Oxidation and oxygenation of (HCTPPH)Fe(II)Br an iron(II) complex of 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH)H2 have been followed by 1H and 2H NMR spectroscopy. Addition of I2 or Br2 to the solution of (HCTPPH)Fe(II)Br in the absence of dioxygen results in one-electron oxidation yielding [(HCTPPH)Fe(III)Br]+. One electron oxidation with dioxygen, accompanied by deprotonation of a C(21)H fragment and formation of an Fe-C(21) bond, produces an intermediate-spin, five-coordinate iron(III) complex (HCTPP)Fe(III)Br. In the subsequent step an insertion of the oxygen atom into the preformed Fe(III)-C(21) bond has been detected to produce [(CTPPO)Fe(III)Br]-. Protonation at the N2 atom affords (HCTPPO)Fe(III)Br. The considered mechanism of (HCTPPH)Fe(II)Br oxygenation involves the insertion of dioxygen into the Fe-C bond. The 1H NMR and 2H NMR spectra of paramagnetic iron(III) complexes were examined. Functional group assignments have been made with use of selective deuteration. The characteristic patterns of pyrrole and 2-NH resonances have been found diagnostic of the ground electronic state of iron and the donor nature localized at C(21) center as exemplified by the 1H NMR spectrum of intermediate-spin (HCTPP)Fe(III)Br: beta-H 7.2, -10.6, -19.2, -20.6, -23.2, -24.9, -43.2; 2-NH -76.6 (ppm, 298 K). The structures of two compounds (HCTPP)Fe(III)Br and (HCTPPO)Fe(III)Br, were determined by X-ray diffraction studies. In the first case, the iron(III) is five-coordinate with bonds to three pyrrole nitrogen atoms (Fe-N distances: 1.985(8), 2.045(7), 2.023(8) A), and the pyrrolic trigonal carbon (Fe-C: 1.981(8) A). The iron(III) of (HCTPPO)Fe(III)Br forms bonds to three pyrrole nitrogen atoms (Fe-N distances 2.104(5), 2.046(5), 2.102(5) A). The Fe-O 2.041(5) A and Fe-C(21) 2.192(5) A distances suggests a direct interaction between the iron center and the pi electron density on the carbonyl group in a eta2 fashion.

The structure of sitting-atop complexes of metalloporphyrins studied by theoretical methods

The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are still protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and extended X-ray absorption fine structure (EXAFS) data has been reported for Cu2+ in acetonitrile. We have used density functional calculations to obtain reasonable models for SAT complexes of porphyrins with Mg2+, Fe2+, and Cu2+. The results show that there are many possible SAT complexes with 1–5 solvent molecules, one or two metal ions, and cis or trans protonation of the porphyrin ring. Many of these have similar energies and their relative stabilities vary with the metal ion. A complex with two cis pyrrolenine nitrogens atoms and 2–4 solvent molecules coordinated to Cu2+ fits the NMR and EXAFS data best. However, we cannot fully exclude the possibility that what is observed is rather a mixture of a doubly protonated porphyrin and the copper porphyrin. Mg2+ has a lower affinity for porphyrin and stronger affinity for water, so a complex with five water molecules and only one bond to porphyrin seems to be most stable. For Fe2+, a cis structure with two first-sphere water molecules and four interactions to the porphyrin seems to be most likely.

Facile ring opening of iron(III) and iron(II) complexes of meso-amino-octaethylporphyrin by dioxygen.

Pyridine solutions of ClFe(III)(meso-NH(2)-OEP) undergo oxidative ring opening when exposed to dioxygen. The high-spin iron(III) complex, ClFe(III)(meso-NH(2)-OEP), has been isolated and characterized by X-ray crystallography. In the solid state, it has a five-coordinate structure typical for high-spin (S = 5/2) iron(III) complex. In chloroform-d solution, ClFe(III)(meso-NH(2)-OEP) displays an (1)H NMR spectrum characteristic of a high-spin, five-coordinate complex and is unreactive toward dioxygen. However, in pyridine-d(5) solution a temperature-dependent equilibrium exists between the high-spin (S = 5/2), six-coordinate complex, [(py)ClFe(III)(meso-NH(2)-OEP)], and the six-coordinate, low spin (S = 1/2 with the less common (d(xz)d(yz))(4)(d(xy))(1) ground state)) complex, [(py)(2)Fe(III)(meso-NH(2)-OEP)](+). Such pyridine solutions are air-sensitive, and the remarkable degradation has been monitored by (1)H NMR spectroscopy. These studies reveal a stepwise conversion of ClFe(III)(meso-NH(2)-OEP) into an open-chain tetrapyrrole complex in which the original amino group and the attached meso carbon atom have been converted into a nitrile group. Additional oxidation at an adjacent meso carbon occurs to produce a ligand that binds iron by three pyrrole nitrogen atoms and the oxygen atom introduced at a meso carbon. This open-chain tetrapyrrole complex itself is sensitive to attack by dioxygen and is converted into a tripyrrole complex that is stable to further oxidation and has been isolated. The process of oxidation of the Fe(III) complex, ClFe(III)(meso-NH(2)-OEP), is compared with that of the iron(II) complex, (py)(2)Fe(II)(meso-NH(2)-OEP); both converge to form identical products.