Nitric Oxide Dioxygenation by O2 Adducts of Manganese Porphyrins.

We describe here nitric oxide dioxygenation (NOD) by the dioxygen manganese porphyrin adducts Mn(Por)(η2-O2) (Por2- = the meso-tetra-phenyl or meso-tetra-p-tolylporphyrinato dianions, TPP2- and TTP2-). The Mn(Por)(η2-O2) was assembled by adding O2 to sublimed layers of MnII(Por). When NO was introduced and the temperature was slowly raised from 80 to 120 K, new IR bands with correlated intensities grew concomitant with depletion of the υ(O2) band. Isotope labeling experiments with 18O2, 15NO, and N18O combined with DFT calculations provide the basis for identifying the initial intermediates as the six-coordinate peroxynitrito complexes (ON)Mn(Por)(η1-OONO). Further warming to room temperature led to formation of the nitrato complexes Mn(Por)(η1-ONO2), thereby demonstrating the ability of these metal centers to promote NOD. However, comparable quantities of the nitrito complexes Mn(Por)(η1-ONO) are also formed. In contrast, when the analogous reactions were initiated with the weak σ-donor ligand tetrahydrofuran or dimethyl sulfide present in the layers, formation of Mn(Por)(η1-ONO2) is strongly favored (∼90%). The latter are formed via a 6-coordinate intermediate (L)Mn(Por)(η1-ONO2) (L = THF or DMS) that loses L upon warming. These reaction patterns are compared to those observed previously with analogous iron and cobalt porphyrin complexes.

Six-Coordinate Nitrato Complexes of Iron Porphyrins with Trans S-Donor Ligands.

The reaction of dimethyl sulfide (DMS) and tetrahydrothiophene (THT) with thin, amorphous layers of the nitrato complexes Fe(Por)(η2-O2NO) (Por = meso-tetraphenylporphyrinato dianion or meso-tetra- p-tolylporphyrinato dianion) at low temperature leads to formation of the corresponding six-coordinate complexes Fe(Por)(L)(η1-ONO2) (L = DMS, THT) as characterized by Fourier transform infrared and optical spectroscopy measurements. Adduct formation was accompanied by bidentate-to-monodentate linkage isomerization of the nitrato ligand, with the FeIII center remaining in a high-spin electronic state. These adducts are thermally unstable; warming to room temperature restores the initial Fe(Por)(η2-O2NO) species.

Six-Coordinate Ferrous Nitrosyl Complex FeII(TTP)(PMe3)(NO) (TTP = meso-Tetra-p-tolylporphyrinato Dianion).

Low-temperature in situ Fourier transform infrared and UV-vis measurements show that trimethylphosphine (PMe3) reacts with microporous layers of FeII(TTP)(NO) (TTP = meso-tetra-p-tolylporphyrinato dianion; NO = nitric oxide) to form the previously unknown six-coordinate complex FeII(TTP)(PMe3)(NO). Upon warming this compound to room temperature in the presence of excess phosphine, the NO ligand is completely replaced by phosphine, resulting in formation of the bis(trimethylphosphine) complex FeII(TTP)(PMe3)2. Simultaneously, the NO released oxidizes free PMe3 to the corresponding phosphine oxide (OPMe3) with concomitant formation of nitrous oxide (N2O).

Nitric oxide interaction with oxy-coboglobin models containing trans-pyridine ligand: two reaction pathways.

The oxy-cobolglobin models of the general formula (Py)Co(Por)(O2) (Por = meso-tetraphenyl- and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low-temperature interaction of Py and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Similar to the recently studied (NH3)Co(Por)(O2) system (Kurtikyan et al. J. Am. Chem. Soc., 2012, 134, 13671-13680), this interaction leads to the nitric oxide dioxygenation reaction with the formation of thermally unstable nitrato complexes (Py)Co(Por)(η(1)-ONO2). The reaction proceeds through the formation of the six-coordinate peroxynitrite adducts (Py)Co(Por)(OONO), as was demonstrated by FTIR measurements with the use of isotopically labeled (18)O2, (15)NO, N(18)O, and (15)N(18)O species and DFT calculations. In contrast to the ammonia system, however, the binding of dioxygen in (Py)Co(Por)(O2) is weaker and the second reaction pathway takes place due to autoxidation of NO by rebound O2 that in NO excess gives N2O3 and N2O4 species adsorbed in the layer. This leads eventually to partial formation of (Py)Co(Por)(NO) and (Py)Co(Por)(NO2) as a result of NO and NO2 reactions with five-coordinate Co(Por)(Py) complexes that are present in the layer after the O2 has been released. The former is thermally unstable and at room temperature passes to the five-coordinate nitrosyl complex, while the latter is a stable compound. In these experiments at 210 K, the layer consists mostly of six-coordinate nitrato complexes and some minor quantities of six-coordinate nitro and nitrosyl species. Their relative quantities depend on the experimental conditions, and the yield of nitrato species is proportional to the relative quantity of peroxynitrite intermediate. Using differently labeled nitrogen oxide isotopomers in different stages of the process the formation of the caged radical pair after homolytic disruption of the O-O bond in peroxynitrite moiety is clearly shown. The composition of the layers upon farther warming to room temperature depends on the experimental conditions. In vacuo the six-coordinate nitrato complexes decompose to give nitrate anion and oxidized cationic complex Co(III)(Por)(Py)2. In the presence of NO excess, however, the nitro-pyridine complexes (Py)Co(Por)(NO2) are predominantly formed formally indicating the oxo-transfer reactivity of (Py)Co(Por)(η(1)-ONO2) with regard to NO. Using differently labeled nitrogen in nitric oxide and coordinated nitrate a plausible mechanism of this reaction is suggested based on the isotope distribution in the nitro complexes formed.

Tracking reactive intermediates by FTIR monitoring of reactions in low-temperature sublimed solids: nitric oxide disproportionation mediated by ruthenium(II) carbonyl porphyrin Ru(TPP)(CO).

Interaction of NO ((15)NO) with amorphous layers of Ru(II) carbonyl porphyrin (Ru(TPP)(CO), TPP(2-) = meso-tetraphenylporphyrinato dianion) was monitored by FTIR spectroscopy from 80 K to room temperature. An intermediate spectrally characterized at very low temperatures (110 K) with ν(CO) at 2001 cm(-1) and ν(NO) at 1810 cm(-1) (1777 cm(-1) for (15)NO isotopomer) was readily assigned to the mixed carbonyl-nitrosyl complex Ru(TPP)(CO)(NO), which is the logical precursor to CO labilization. Remarkably, Ru(TPP)-mediated disproportionation of NO is seen even at 110 K, an indication of how facile this reaction is. By varying the quantity of supplied NO, it was also demonstrated that the key intermediate responsible for NO disproportionation is the dinitrosyl complex Ru(TPP)(NO)2, supporting the conclusion previously made from solution experiments.

Nitrosyl isomerism in amorphous Mn(TPP)(NO) solids.

Reaction of NO with amorphous Mn(TPP) layers gives two Mn(TPP)(NO) isomers with linear and bent Mn-N-O geometries that reversibly interconvert with changes in temperature. DFT computations predict that the linear complex is the singlet ground state while the bent structure is a triplet state.

Nitric oxide dioxygenation reaction by oxy-coboglobin models: in-situ low-temperature FTIR characterization of coordinated peroxynitrite.

The oxy-cobolglobin models of the general formula (NH(3))Co(Por)(O(2)) (Por = meso-tetra-phenyl and meso-tetra-p-tolylporphyrinato dianions) were constructed by sequential low temperature interaction of NH(3) and dioxygen with microporous layers of Co-porphyrins. At cryogenic temperatures small increments of NO were introduced into the cryostat and the following reactions were monitored by the FTIR and UV-visible spectroscopy during slow warming. Upon warming the layers from 80 to 120 K a set of new IR bands grows with correlating intensities along with the consumption of the ν(O(2)) band. Isotope labeling experiments with (18)O(2), (15)NO and N(18)O along with DFT calculations provides a basis for assigning them to the six-coordinate peroxynitrite complexes (NH(3))Co(Por)(OONO). Over the course of warming the layers from 140 to 170 K these complexes decompose and there are spectral features suggesting the formation of nitrogen dioxide NO(2). Upon keeping the layers at 180-210 K the bands of NO(2) gradually decrease in intensity and the set of new bands grows in the range of 1480, 1270, and 980 cm(-1). These bands have their isotopic counterparts when (15)NO, (18)O(2) and N(18)O are used in the experiments and certainly belong to the 6-coordinate nitrato complexes (NH(3))Co(Por)(η(1)-ONO(2)) demonstrating the ability of oxy coboglobin models to promote the nitric oxide dioxygenation (NOD) reaction similar to oxy-hemes. As in the case of Hb, Mb and model iron-porphyrins, the six-coordinate nitrato complexes are not stable at room temperature and dissociate to give nitrate anion and oxidized cationic complex Co(III)(Por)(NH(3))(1,2).

Hexacoordinate oxy-globin models Fe(Por)(NH3)(O2) react with NO to form only the nitrato analogs Fe(Por)(NH3)(η1-ONO2), even at ~100 K.

The oxy-globin models Fe(Por)(NH(3))(O(2)), prepared by sequential reactions of O(2) ((18)O(2)) and NH(3) with thin porous layers of Fe(II)(Por), react with NO ((15)NO) at 80-100 K to form only the low-spin nitrato complexes Fe(Por)(NH(3))(η(1)-ONO(2)), thus implying that peroxynitrite intermediates, if formed, must undergo very facile isomerization to the nitrato analog.

Six-coordinate nitrosyl and nitro complexes of meso-tetratolylporphyrinatocobalt with trans sulfur-donor ligands.

By Fourier transform infrared and optical spectroscopy, it has been observed that interactions of dimethyl sulfide and tetrahydrothiophene with nitrosyl and nitro complexes of meso-tetra-p-tolylporphyrinatocobalt [Co(TTP)] lead to the formation of previously unknown six-coordinate species. Nitrosyl complexes of the general formula (S-donor)Co(TTP)(NO) are thermally unstable and can be seen only at low temperatures both in the solid state and in solution. The nitro complexes (S-donor)Co(TTP)(NO(2)) are fairly stable at room temperature in the solid state but partly decompose upon dissolution. The binding constants for these complexation reactions were determined. In contrast to the solid-state iron nitritoporphyrin complexes, oxo-transfer reactions from the coordinated nitro group of Co(TTP)(NO(2)) to the S donors, resulting in oxidation of these sulfides and the formation of Co(TTP)(NO), were not observed.

Six-coordinate nitro complexes of iron(III) porphyrins with trans S-donor ligands. Oxo-transfer reactivity in the solid state.

Spectroscopic studies demonstrate that the 5-coordinate O-nitrito complexes Fe(Por)(eta(1)-ONO) (Por--meso-tetraphenyl- or meso-tetra-p-tolyl-porphyrinato dianions) react with the thioethers (R(2)S) dimethylsulfide and tetrahydrothiophene to give the 6-coordinate N-nitrito complexes Fe(Por)(R(2)S)(NO(2)). These reactions were conducted in low-temperature porous layered solids formed in a cryostat; however, with excess R(2)S in the atmosphere, the same species are moderately stable at room temperature. Six-coordinate O-nitrito isomers were not observed with the R(2)S proximal ligands, even though DFT calculations for the Fe(P)(DMS)(eta(1)-ONO) and Fe(P)(DMS)(NO(2)) models (P = porphinato dianion, DMS = dimethyl sulfide) show the latter to be only modestly lower energy (approximately 8 kJ/mol) than the former. Leaving this system at room temperature in the presence of excess R(2)S leads eventually to the appearance in the FTIR spectra of the nu(NO) band characteristic of the ferrous nitrosyl Fe(Por)(NO). Concomitantly, the mass spectrum of the gas phase demonstrated the molecular peaks of the sulfoxides R(2)SO, indicating oxygen atom transfer reactivity for the ferric porphryinato complexes of nitrite.

Catalytic dioxygen activation by (nitro)(meso-tetrakis(2-n-methylpyridyl)porphyrinato)cobalt(III) cation derivatives electrostatically immobilized in nafion films: an experimental and DFT investigation.

Complexes of the (nitro)( meso-tetrakis(2- N-methylpyridyl)porphyinato)cobalt(III) cation, [LCoTMpyP(2)(NO 2)] (4+), in which L = water or ethanol have been immobilized through ionic attraction within Nafion films (Naf). These immobilized six-coordinate species, [LCoTMPyP(2)(NO 2)/Naf], have been found to catalyze the oxidation of triphenylphosphine in ethanol solution by dioxygen, therefore retaining the capacity to activate dioxygen catalytically without an additional reducing agent as was previously observed in nonaqueous solution for the non-ionic (nitro)cobalt porphyrin analogs. Heating these immobilized six-coordinate species under vacuum conditions results in the formation of the five-coordinate nitro derivatives, [CoTMPyP(2)(NO 2)/Naf] at 85 degrees C and [CoTMPyP(2)/Naf] at 110 degrees C. The catalytic oxidation of gas-phase cyclohexene with O 2 is supported only by the resulting immobilized five-coordinate nitro complex as was previously seen with the corresponding solution-phase catalyst in dichloromethane solution. The simultaneous catalytic oxidation of triphenylphosphine and cyclohexene with O 2 in the presence of the Nafion-bound six-coordinate ethanol nitro complex is also observed; however, this process is not seen for the CoTPP derivative in dichloromethane solution. The oxidation reactions do not occur with unmodified Nafion film or with Nafion-supported [BrCo(III)TmpyP]/Naf or [Co(II)TmpyP]/Naf, indicating the necessity for the nitro/nitrosyl ligand in the oxidation mechanism. The existence of a second reactive intermediate is indicated because the two simultaneous oxidation reactions depend on two distinct oxygen atom-transfer steps having different reactivity. The absence of homogeneous cyclohexene oxidation by the six-coordinate (H 2O)CoTPP(NO 2) derivatives in the presence of Ph 3P and O 2 in dichloromethane solution indicates that the second reactive intermediate is lost by an unidentified route only in solution, implying that the immobilization of it in Nafion allows it to react with cyclohexene. Although direct observation of this species has not been achieved, a comparitive DFT study of likely intermediates in several catalytic oxidation mechanisms at the BP 6-31G* level supports the possibility that this intermediate is a peroxynitro species on the basis of relative thermodynamic accessibility. The alternate intermediates evaluated include the reduced cobalt(II) porphyrin, the dioxygen adduct cobalt(III)-O 2 (-), the oxidized cobalt(II) pi-cation radical, and the nitrito complex, cobalt(III)-ONO.

Six-coordinate nitrato complexes of iron(III) porphyrins.

The interaction of tetrahydrofuran (THF) with thin films of the nitrato complexes Fe(III)(Por)(eta(2)-O(2)NO) [Por = meso-tetraphenylporphyrinato (TPP) and meso-tetratolylporphyrinato (TTP) dianion] at low temperature leads to the formation of the six-coordinate nitrato complex Fe(Por)(THF)(NO(3)), which was characterized by IR and UV-visible spectroscopies. Formation of the THF adduct was accompanied by nitrate linkage isomerization from bidentate to monodentate coordination. The iron(III) center remains in a high spin state in contrast with the previously observed low-spin nitratonitrosyl complex Fe(TPP)(NO)(eta(10-ONO(2)). Upon warming, THF dissociates to restore the initial five-coordinate bidentate nitrato complex.

Interaction of nitrogen bases with iron-porphyrin nitrito complexes Fe(Por)(ONO) in sublimed solids.

The reactions of the nitrogen Lewis bases (B) 1-methylimidazole (1-MeIm), pyridine (Py), and NH3 as gases with sublimed layers containing the 5-coordinate nitrito iron(III)-porphyrinato complexes Fe(Por)(eta1-ONO) (1) are described (Por = meso-tetraphenyl-porphyrinato or meso-tetra-p-tolyl-porphyrinato dianions). In situ FTIR and optical spectra are used to characterize the formation of the 6-coordinate nitro complexes formed by the reaction of 1 with B = 1-MeIm, Py, or NH3. These represent the first examples of 6-coordinate amino-nitro complexes with sterically unprotected iron-porphyrins. The interaction of ammonia with Fe(Por)(ONO) at 140 K initially led to the nitrito species Fe(Por)(NH3)(eta1-ONO), and this species isomerized to the nitro complexes Fe(Por)(NH3)(eta1-NO2) upon warming to 180 K. When the latter were warmed to room temperature under intense pumping, the initial nitrito complexes Fe(Por)(eta1-ONO) were restored. Assignments of vibrational frequencies for the coordinated nitro group in 6-coordinate iron-porphyrin complexes are given and confirmed using 15N-labeled nitrogen dioxide to identify characteristic infrared bands. For M(Por)(B)(NO2) complexes (M = Fe or Co), an inverse correlation between the net charge transfer from the axial ligand B to the nitro group and the value of Deltanu = nua(NO2) - nus(NO2) is proposed. These observations are discussed in the context of growing interest in potential physiological roles of nitrite ion reactions with ferro- and ferri-heme proteins.

Reactions of nitrogen oxides with the five-coordinate Fe(III)(porphyrin) nitrito intermediate Fe(Por)(ONO) in sublimed solids.

Detailed experimental studies are described for reactions of several nitrogen oxides with iron porphyrin models for heme/NxOy systems. It is shown by FTIR and optical spectroscopy and by isotope labeling experiments that reaction of small increments of NO2 with sublimed thin layers of the iron(II) complex Fe(Por) (Por = meso-tetraphenylporphyrinato dianion, TPP, or meso-tetra-p-tolylporphyrinato dianion, TTP) leads to formation of the 5-coordinate nitrito complexes Fe(Por)(eta1-ONO) (1), which are fairly stable but very slowly decompose under vacuum giving mostly the corresponding nitrosyl complexes Fe(Por)(NO). Further reaction of 1 with new NO2 increments leads to formation of the nitrato complex Fe(Por)(eta2-O2NO) (2). The interaction of NO with 1 at low temperature involves ligand addition to give the nitrito-nitrosyl complexes Fe(Por)(eta1-ONO)(NO) (3); however, these isomerize to the nitro-nitrosyl analogs Fe(Por)(eta1-NO2)(NO) (4) upon warming. Experiments with labeled nitrogen oxides argue for an intramolecular isomerization ("flipping") mechanism rather than one involving dissociation and rebinding of NO2. The Fe(III) centers in the 6-coordinate species 3 and 4 are low spin in contrast to 1, which appears to be high-spin, although DFT computations of the porphinato models Fe(P)(nitrite) suggest that the doublet nitro species and the quartet and sextet nitrito complexes are all relatively close in energy. The nitro-nitrosyl complex 4 is stable under an NO atmosphere but decomposes under intense pumping to give a mixture of the ferrous nitrosyl complex Fe(Por)(NO) and the ferric nitrito complex Fe(Por)(eta1-ONO) indicating the competitive dissociation of NO and NO2. Hence, loss of NO from 4 is accompanied with nitro --> nitrito isomerization consistent with 1 being the more stable of the 5-coordinate NO2 complexes of iron porphyrins.

Low-temperature spectral observation of the first six-coordinate nitrosyl complexes of cobalt(II) meso-tetratolylporphyrin with trans nitrogen base ligands.

Low-temperature interaction of nitrogen base ligands with layered Co(TTP)(NO) (TTP = meso-tetratolylporphyrinato dianion) as well as its toluene solution leads to the formation of the first six-coordinate species of the general formula (B)Co(TTP)(NO) (where B = piperidine and pyridine). The nu(NO) stretching bands of these species appear at lower frequencies compared with the five-coordinate nitrosyl derivative and depend on the nature of the trans axial ligand. The equilibrium constants and enthalpies of formation of these new species are determined. Fairly stable at low-temperature conditions in the solid state, they slowly dissociate the nitrogen base ligands upon warming to restore the five-coordinate nitrosyl complex Co(TTP)(NO).

In situ FT-IR and UV-vis spectroscopy of the low-temperature NO disproportionation mediated by solid state manganese(II) porphyrinates.

The heterogeneous reaction between NO gas and sublimed layers of manganese(II) porphyrinato complexes Mn(Por) (Por = TPP (tetraphenylporphyrinato dianion), TMP (tetramesitylporphyrinato dianion), or TPP(d20) (perdeuterated tetraphenylporphyrinato dianion)) has been monitored by IR and optical spectroscopy over the temperature range of 77 K to room temperature. These manganese porphyrins promote NO disproportionation to NO2 species and N2O, and the reaction proceeds via several distinct stages. At 90 K, the principal species observed spectrally are the nitric oxide dimer, cis-ONNO, two manganese nitrosyls, the simple NO adduct Mn(Por)(NO), and another intermediate (1) that is apparently critical to the disproportionation mechanism. This key intermediate is formed prior to N2O evolution, and proposals regarding its likely structure are offered. When the system is warmed to 130 K, the disproportionation products, N2O and the O-coordinated nitrito complex Mn(Por)(NO)(ONO) (2), are formed. IR spectral changes show that, upon further warming to 200 K, 2 isomerizes into the N-bonded nitro linkage isomer Mn(Por)(NO)(NO2) (3). After it is warmed to room temperature, the latter species loses NO and converts to the known 5-coordinate nitrito complex Mn(Por)(ONO) (4).

Reactions of nitrogen oxides with heme models. Spectral and kinetic study of nitric oxide reactions with solid and solute Fe(III)(TPP)(NO3).

The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato iron(III) complex Fe(III)(TPP)(eta(2)-O(2)NO) (1, TPP = meso-tetraphenyl porphyrinate(2)(-)) leads to formation of several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe(TPP)(NO(2))(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not simply unreacted 1 but is the result of a series of transformations taking place in the layered material. Thus, the nitrato complex formed from solid Fe(TPP)(eta(2)-O(2)NO) maintained under a (15)NO atmosphere was found to be the labeled analogue Fe(TPP)(eta(2)-O(2)(15)NO). The reactivities of the layered solids are compared to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the labeled nitrogen oxides, the potential exchange reactions between N(2)O(3) and (15)NO were examined, and complete isotope scrambling was observed between these species under the reaction conditions (T = 140 K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(eta(1)-ONO(2))(NO) (2) as an intermediate. This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate radical NO(3)(*)( )()to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack on the coordinated nitrate of 2 resulting in the release of N(2)O(4) concerted with electron transfer to the metal center is proposed.

Low temperature NO disproportionation by Mn porphyrin. Spectroscopic characterization of the unstable nitrosyl nitrito complex MnIII(TPP)(NO)(ONO).

Reaction of NO gas with sublimed layers of the Mn(II)TPP (TPP =meso-tetraphenylporphyrinato2-) at low temperature leads to nitric oxide disproportionation. UV-Vis and FTIR spectroscopy with isotopically substituted nitrogen oxides revealed formation of the unstable species identified as trans-Mn(III)(TPP)(NO)(ONO).

Interaction of nitrogen oxides with sublimed layers of (meso-tetraphenylporphyrinato)cobalt(II); IR evidence of oxo-transfer from (nitro)porphyrinatocobalt(III) to free nitric oxide.

Interaction of a low-pressure NO2 with sublimed layers of (meso-tetraphenylporphyrinato)cobalt(II) (Co(TPP)) leads to formation of 5-coordinate nitro complex Co(III)(TPP)(NO2). Upon exposure of these layers to pyridine vapors, the fast reaction with formation of 6-coordinate nitro-pyridine porphyrins (Py)Co(III)(TPP)(NO2) occurs. By means of IR spectroscopy and use of nitrogen oxide isotopomers, it is shown that an oxo-transfer reaction occurs from 5-coordinate species to free nitric oxide (NO) while the 6-coordinate complex is rather inert. It is also demonstrated that the stepwise addition of low-pressure NO2 to nitrosyl complex Co(TPP)(NO) leads to formation of the nitro complex most likely by an exchange reaction.