NMR spectra of porphyrins. Part 31. Ring currents in hydroporphyrins

Syntheses and Aromaticity Parameters of Hexahydroxypyrrocorphin, Porphotrilactones, and Their Oxidation State Intermediates

Triple OsO₄-mediated dihydroxylation of meso-tetrakis(pentafluorophenyl)porphyrin formed a non-aromatic hexahydroxypyrrocorphin as a single stereo-isomer. A one-step oxidative conversion of all three diol functionalities to lactone moieties generated three out of the four possible porphotrilactone regioisomers that were spectroscopically and structurally characterized. This conversion recovered most of the porphyrinic macrocycle aromatic ring current, as seen in their ¹H NMR spectra and modeled using DFT computations. Stepwise OsO₄-mediated dihydroxylations of porpho-mono- and -di-lactones generated intermediate oxidation state compounds between the pyrrole-three pyrroline macrocycle of the pyrrocorphin and the pyrrole-three oxazolone chromophore of the trilactones. The aromaticity of these chromophores was reduced with increasing number of oxazolone to pyrroline replacements, showing the importance for the presence of three lactone moieties for the retention of the macrocycle aromaticity in the tris-β,β'-modified macrocycles. This work first describes hexahydoxypyrrocorphins, porphotrislactones, and the oxidation state intermediates between them; furthers the understanding of the roles of β-lactone moieties in the expression of porphyrinic macrocycle aromaticity; and generally broadens access to chemically stable pyrrocorphins and pyrrocorphin analogues.

Stepwise Reduction of β-Trioxopyrrocorphins: Collapse of the Oxo-Induced Macrocycle Aromaticity

The diatropic ring current that characterizes the unexpectedly aromatic octaethyltrioxopyrrocorphins gets drastically reduced upon chemical reduction of one and particularly two ketone moieties. With increasing reduction, the chromophores containing one pyrrole, one/two pyrrolinone, and one/two pyrrolines become more similar to regular, nonmacrocycle-aromatic pyrrocorphins (hexahydroporphyrins). Single-crystal diffraction analysis shows the reduction products to be idealized planar. With increasing reduction, their UV-vis spectroscopic signatures are those of conjugated but nonaromatic oligopyrroles. Their diatropic ring currents, as assessed by ¹H NMR spectroscopy, showed them to possess largely nonaromatic π-systems. Dihydroxylation of select β,β'-dioxobacteriochlorin and β,β'-dioxoisobacteriochlorins also resulted in the formation of equivalent mixed pyrrole/two pyrrolinone/pyrroline chromophores. Computations were able to reproduce the experimental trends of the diatropic ring currents and filled in the data for the regioisomers that could not be experimentally accessed. The work further highlights the electronic influence of the β-oxo-substituents and, more specifically, the origin of the aromaticity of the trioxopyrrocorphins. It also presents a series of chemically robust pyrrocorphins, a chromophore class for which many chemically very sensitive members have been reported.

Nickel (II) pyrrocorphin: Enhanced binding ability in a highly reduced porphyrin complex

Pyrrocorphin is an air-sensitive porphyrinoid with a highly reduced hexahydroporphyrin core. In contrast, pyrrolidine-fused pyrrocorphin (Pyr) obtained by successive 1,3-dipolar cycloaddition reactions of azomethine ylide to 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin is less air-sensitive. In order to reveal the effect of highly reduced porphyrin rings on the physicochemical properties of their metal complexes, we have prepared diamagnetic (S=0) Ni(II) complex Ni(II)(Pyr). The addition of excess pyridine (Py) to the toluene solution of Ni(II)(Pyr) yielded five-coordinate Ni(II)(Pyr)(Py), which was then completely converted to six-coordinate paramagnetic (S=1) Ni(II)(Pyr)(Py)₂. The latter was characterized by UV-Vis, ¹H NMR, CV, SQUID, and X-ray crystallography as well as DFT calculations. As compared with analogous complexes of porphyrin (Por), chlorin (Chl), and isobacteriochlorin (Iso) reported by Herges and co-workers (R. Herges et al., Inorg. Chem. 2015), Ni(II)(Pyr)(Py)₂ has longer equatorial NiN and shorter axial NiN bonds. The CV study has shown a large decrease in HOMO-LUMO gap as the reduction of porphyrin ring proceeds, which has further been confirmed by UV-Vis and DFT calculation. Titration studies using ¹H NMR and UV-Vis have shown that the first binding constant of pyridine toward Ni(II)(Pyr) is ca. 4 times as large as that of Ni(II)(Iso) and ca 230 times as large as that of Ni(II)(Por). Thus, we have concluded that the binding constant of pyridine to Ni(II) porphyrinoid increases by the following order: Por<Chl<Iso<Pyr. The high affinity of pyridine binding in Ni(II)(Pyr) was discussed both from the structural and thermodynamic point of view.

NMR spectral properties of 16 synthetic bacteriochlorins with site-specific 13C or 15N substitution

The 1 H , 13 C , and 15 N nuclear magnetic resonance (NMR) spectral properties have been examined of a family of synthetic bacteriochlorins wherein each member incorporates a pair of 13 C or 15 N atoms. The atom locations span the inner core of the macrocycle: (1) 15 N at the 21,23- or 22,24-positions; (2) 13 C at the meso- (5,15- or 10,20-) positions; (3) 13 C at the pyrrole α-positions (1,11- or 4,14-positions); and (4) 13 C at the pyrroline α-positions (6,16- or 9,19-positions). Each bacteriochlorin lacks peripheral substituents other than a geminal dimethyl group at the 8- and 18-positions to preclude adventitious dehydrogenation. In total, eight free base and eight zinc bacteriochlorin isotopologs were examined to directly assign 1 H , 13 C and 15 N resonances of the macrocycle skeleton. Complete and unambiguous assignments, including those for all tertiary and quaternary carbons, were accomplished chiefly by direct inspection of 1D NMR spectra of each isotopolog. Coupling constants (1 H –1 H , 13 C –1 H , 15 N –1 H , 13 C –13 C and 15 N –13 C ), which are rarely reported for tetrapyrroles, also were extracted. The 1 H and 13 C chemical shifts were then compared to those of unsaturated analogs (chlorin, porphyrin) and natural bacteriochlorophylls. The comprehensive set of NMR spectroscopic properties of sparsely substituted bacteriochlorins provides valuable information for understanding substitution effects and aromaticity in structurally more elaborate counterparts.

Specific coupling between the 13-keto carbonyl and chlorin skeletal vibrational modes of synthetic 13(1)-(18)O-(un)labelled metallochlorophyll derivatives

Metal complexes of methyl 13(1)-(18)O-labelled pyropheophorbide-a1-M-(18)O (M=Zn, Cu and Ni) were prepared by metallation of the (18)O-labelled free base (1-(18)O) and (18)O-labelling of unlabelled nickel complex (1-Ni). The FT-IR spectra of 1-Zn and 1-Zn-(18)O in CH(2)Cl(2) showed that the 13-keto carbonyl stretching vibration mode moved to about a 30-cm(-1) lower wavenumber by (18)O-labelling of the 13(1)-oxo moiety. In 1-Cu-(18)O and 1-Ni-(18)O, the 13-C==(18)O stretching modes were close to the highest-energy wavenumber mode of chlorin skeletal C-C/C-N vibrations at around 1650cm(-1) and they were coupled in CH(2)Cl(2) to give two split IR bands (Fermi resonance). A similar coupling was observed in the resonance Raman scattering of 1-Ni-(18)O in the solid state. The hydrogen-bonded 13-C==(16)O vibration mode of 1-Ni similarly coupled with the skeletal C-C/C-N mode in CCl(4) containing 1% (v/v) 1,1,1,3,3,3-hexafluoro-2-propanol, while such a coupling was not observed in a neat CCl(4) solution of 1-Ni possessing the 13-C==(16)O free from any interaction. The skeletal C-C/C-N band selectively coupled with the 13-C==O, not with the 3-C==O, when the difference in their peak maxima was less than 20 cm(-1).

Zinc−Porphyrin Phosphonate Coordination: Structural Control through a Zinc Phosphoryl−Oxygen Interaction

Inter- and intramolecular zinc-porphyrin phosphonate coordination in the cis and trans isomers of zinc tetraphenylporphyrinstyryldiphosphonate is reported, demonstrating the potential of the zinc phosphoryl-oxygen interaction for structural control.

Diastereopure Fe(II) and Zn(II) Complexes Derived from a Tridentate N,N‘,N-Bis(methyl-l-prolinate)-Substituted Pyridine Ligand

A series of mononuclear iron(II) and zinc(II) complexes of the new chiral Py(ProMe)2 ligand (Py(ProMe)2 = 2,6-bis[[(S)-2-(methyloxycarbonyl)-1-pyrrolidinyl]methyl]pyridine) have been prepared. The molecular geometry in the solid state (X-ray crystal structures) of the complexes [FeCl2(Py(ProMe)2)] (1), [ZnCl2(Py(ProMe)2)] (2), [Fe(OTf)2(Py(ProMe)2)] (3), [Fe(Py(ProMe)2)(OH2)2](OTf)2 (4), and [Zn(OTf)(Py(ProMe)2)](OTf) (5) are reported. They all show a meridional NN'N coordination of the Py(ProMe)2 ligand. The bis-chloride derivatives 1 and 2 represent neutral isostructural five-coordinated complexes with a distorted geometry around the metal center. Unusual seven-coordinate iron(II) complexes 3 and 4 having a pentagonal bipyramidal geometry were obtained using weakly coordinating triflate anions. The reaction of Zn(OTf)2 with the Py(ProMe)2 ligand afforded complex 5 with a distorted octahedral geometry around the zinc center. All complexes were formed as single diastereoisomers. In the case of complexes 3-5, the oxygen atoms of both carbonyl groups of the ligand are also coordinated to the metal. The stereochemistry of the coordinated tertiary amine donors in complexes 3-5 is of opposite configuration as in complexes 1 and 2 as a result of the planar penta-coordination of the ligand Py(ProMe)2. Complexes 1, 2, and 5 have an overall -configuration at their metal center, while the Fe(II) ion in complexes 3 and 4 has the opposite delta-configuration (crystal structures and CD measurements). The magnetic moments of iron complexes 1, 3, and 4 correspond to that of high-spin d6 Fe(II) complexes. The solution structures of complexes 1-5 were characterized by means of UV-vis, IR, conductivity, and CD measurements and their electrochemical behavior. These studies showed that the coordination environment of 1 and 2 observed in the solid state is maintained in solution. In coordinating solvents, the triflate anion (3, 5) or water (4) co-ligands of complexes 3-5 are replaced by solvent molecules with retention of the original pentagonal bipyramidal and octahedral geometry, respectively.

Heterotrinuclear Oxo-Bridged Fe(III)ORu(IV)OFe(III) Complexes of Ruthenium Porphyrin and Borylated Low-Spin Iron Dioximes(1)

The synthesis, structure, spectra, and reactivity of diamagnetic linear chain heterotrinuclear oxo-bridged complexes L-Fe(III) N(4)-O-Ru(IV)(TPP')-O-Fe(III)N(4)-L are described where TPP' is tetrakis(4-methoxyphenylporphyrinate), N(4) is one of the borylated bis(dioximate) macrocycles, N(4) = ((DMG)BF(2))(2) in 1, ((DMG)BPh(2))(2) in 2, and ((DPG)BF(2))(2) in 3, and L is a monodentate terminal ligand, L = BuNH(2), CH(3)CN, Py, 1-MeIm, (BuO)(3)P, etc. Crystal data for 3-(BuNH(2))(2) = [(BuNH(2))Fe((DPG)BF(2))(2)-O](2)Ru(TPP') monoclinic space group P2(1)/n, a = 16.845 Å, b = 27.184 Å, c = 24.593 Å, beta = 90.41 degrees, Fe-O = 1.79(1) Å, Ru-O = 1.80(1) Å, Fe-O-Ru = 175 degrees. Large porphyrin and phenyl ring current shifts permit ready characterization of the materials by (1)H NMR. A red-shifted Soret band at 433 nm and two charge transfer transitions in the 650-850 nm region are observed in the visible spectra. Clean reduction of 1, 2, and 3-(CH(3)CN)(2) with 4-tert-butylcatechol is observed giving Fe(II) and Ru(II) products. The kinetics of reduction parallel those of low-spin (&mgr;-oxo)diiron FeN(4) systems showing a 1/[CH(3)CN] dependence, but the rates are 10(3)-10(5) times slower. The Ru(IV) is proposed to stabilize the &mgr;-oxo-Fe bonds toward reductive cleavage via delocalization of the oxo pi electrons onto the ruthenium. Electrochemical data for the heterotrinuclear Fe-O-Ru-O-Fe systems are compared with those for the binuclear Fe-O-Fe systems. The HOMO is stabilized by 120 mV over the corresponding Fe-O-Fe system.

Proton NMR of Escherichia coli sulfite reductase: studies of the heme protein subunit with added ligands

The heme protein subunit of sulfite reductase (SiR-HP; M(r) 64,000) from Escherichia coli as isolated contains the isobacteriochlorin siroheme exchange-coupled to a [4Fe-4S] cluster in the 2+ oxidation state. SiR-HP in the presence of a suitable electron donor can catalyze the six-electron reductions of sulfite to sulfide and nitrite to ammonia. Paramagnetic 1H NMR was used to study the low-spin complexes of SiR-HP formed by binding the exogenous inhibitor cyanide or the substrates sulfite and nitrite. As a model, the cyanide complex of purified siroheme was also prepared. The NMR spectrum of isolated ferric low-spin siroheme-CN is consistent with spin density being transferred into the a2u molecular orbital, an interaction which is symmetry-forbidden in porphyrins. The pattern of proton NMR shifts observed for isolated ferric low-spin siroheme-CN is very similar to those obtained for the protein-cyanide complex. NMR spectra of the cyanide complex of SiR-HP were obtained in all three accessible redox states. The pattern of hyperfine shifts observed for the one-electron and two-electron reduced cyanide complexes is typical of those seen for [4Fe-4S] clusters in the 2+ and 1+ oxidation states, respectively. Resonances arising from the beta-CH2 protons of cluster cysteines have been assigned for all complexes studied utilizing deuterium substitution. The cyanide-, sulfite-, and nitrite-ligated states possessed an almost identically shifted upfield cluster cysteine resonance whose presence indicates that covalent coupling exists between siroheme and cluster in solution. Data are also presented for the existence of a secondary anion binding site, the occupancy of which perturbs the oxidized SiR-HP NMR spectrum, where binding occurs at a rate much faster than that of ligand binding to heme.