Reactions of Nitrogen Oxides with the Five-Coordinate Fe^III(porphyrin) Nitrito Intermediate Fe(Por)(ONO) in Sublimed Solids

Abstract

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.

Chemical and photochemical properties of a ruthenium nitrosyl complex with the N-monosubstituted cyclam 1-(3-Propylammonium)-1,4,8,11-tetraazacyclotetradecane

“…18,25 However, it has been reported that the spectra of reduced froms of nitrosyl complexes show the n(NO) peak at much lower wavenumbers. 65,66 The electronic absorption spectral changes observed for trans-[Ru(NO)Cl(1-pramcyH)](PF 6 ) 3 consist of an absorbance decrease at 263 nm with a concomitant increase at 291 nm. The final electronic absorption spectrum is assigned to the trans-[Ru(NO)Cl(1-pramcyH)] 2+ complex.…”

FTIR and optical spectroscopic studies of the reactions of heme models with nitric oxide and other NOx in porous layered solids

“…DFT computations at the B3LYP/LACVP* level suggest [54] that the nitrito and nitro linkage isomers Fe(P)(ONO) and Fe(P)(NO 2 ) (P = the unsubstituted porphinato dianion) have similar energies with the nitrito species more stable, but only by less than 1 kJ mol −1 . Despite this, the experimental studies showed only the former to be present as the initial product from the reaction of NO 2 with Fe(TPP) or Fe(TTP) in layered solids.…”

“…The room temperature reaction of NO gas with sublimed layers containing pre-formed Fe(Por)(ONO) (Por = TPP or TTP) leads to rapid formation of the familiar nitro-nitrosyl complexes Fe(Por)(NO 2 )(NO) [54]. However, at low temperature (130 K), the reaction nearly quantitatively forms nitrosyl complexes identified as the nitrito isomers Fe(Por)( 1 -ONO)(NO) owing to the characteristic and isotope sensitive nitrosyl and nitrito bands in the vicinity of 1890…”

“…Hence warming Fe(TTP)( 1 -ONO)(NO) results in the nitrito → nitro isomerization of coordinated NO 2 (Scheme 3) as demonstrated by the disappearance of the nitrito bands at 1497 and 935 cm −1 and appearance of new bands at 1456 and 1293 cm −1 characteristic of a nitro complex [54]. This process proceeds in the presence of excess NO as well as after evacuating the NO from the cryostat system at 130 K before warming the Fe(Por)(ONO)(NO) layer on the substrate.…”

“…This linkage isomerization was also probed using optical spectroscopy; however, the differences between the UV-visible spectrum of the nitrito-nitrosyl complex and that of the nitro- nitrosyl analog are too small to differentiate between the two isomers [54]. This spectral similarity may explain why the oftpostulated nitrito-nitrosyl isomer had not been characterized previously by optical spectroscopy.…”