Nitrosoarene Complexes of Manganese Porphyrins

Insights into Nitrosoalkane Binding to Myoglobin Provided by Crystallography of Wild-Type and Distal Pocket Mutant Derivatives

Nitrosoalkanes (R-N═O; R = alkyl) are biological intermediates that form from the oxidative metabolism of various amine (RNH2) drugs or from the reduction of nitroorganics (RNO2). RNO compounds bind to and inhibit various heme proteins. However, structural information on the resulting Fe-RNO moieties remains limited. We report the preparation of ferrous wild-type and H64A sw MbII-RNO derivatives (λmax 424 nm; R = Me, Et, Pr, iPr) from the reactions of MbIII-H2O with dithionite and nitroalkanes. The apparent extent of formation of the wt Mb derivatives followed the order MeNO > EtNO > PrNO > iPrNO, whereas the order was the opposite for the H64A derivatives. Ferricyanide oxidation of the MbII-RNO derivatives resulted in the formation of the ferric MbIII-H2O precursors with loss of the RNO ligands. X-ray crystal structures of the wt MbII-RNO derivatives at 1.76-2.0 Å resoln. revealed N-binding of RNO to Fe and the presence of H-bonding interactions between the nitroso O-atoms and distal pocket His64. The nitroso O-atoms pointed in the general direction of the protein exterior, and the hydrophobic R groups pointed toward the protein interior. X-ray crystal structures for the H64A mutant derivatives were determined at 1.74-1.80 Å resoln. An analysis of the distal pocket amino acid surface landscape provided an explanation for the differences in ligand orientations adopted by the EtNO and PrNO ligands in their wt and H64A structures. Our results provide a good baseline for the structural analysis of RNO binding to heme proteins possessing small distal pockets.

Does the Red Shift in UV-Vis Spectra Really Provide a Sensing Option for Detection of N-Nitrosamines Using Metalloporphyrins?

N-nitrosamines are widespread cancerogenic compounds in human environment, including water, tobacco products, food, and medicinal products. Their presence in pharmaceuticals has recently led to several recalls of important medicines from the market, and strict controls and tight limits of N-nitrosamines are now required. Analytical determination of N-nitrosamines is expensive, laborious, and time-inefficient making development of simpler and faster techniques for their detection crucial. Several reports published in the previous decade have demonstrated that cobalt porphyrin-based chemosensors selectively bind N-nitrosamines, which produces a red shift of characteristic Soret band in UV-Vis spectra. In this study, a thorough re-evaluation of metalloporphyrin/N-nitrosamine adducts was performed using various characterization methods. Herein, we demonstrate that while N-nitrosamines can interact directly with cobalt-based porphyrin complexes, the red shift in UV-Vis spectra is not selectively assured and might also result from the interaction between impurities in N-nitrosamines and porphyrin skeleton or interaction of other functional groups within the N-nitrosamine structure and the metal ion within the porphyrin. We show that pyridine nitrogen is the interacting atom in tobacco-specific N-nitrosamines (TSNAs), as pyridine itself is an active ligand and not the N-nitrosamine moiety. When using Co(II) porphyrins as chemosensors, acidic and basic impurities in dialkyl N-nitrosamines (e.g., formic acid, dimethylamine) are also UV-Vis spectra red shift-producing species. Treatment of these N-nitrosamines with K2CO3 prevents the observed UV-Vis phenomena. These results imply that cobalt-based metalloporphyrins cannot be considered as selective chemosensors for UV-Vis detection of N-nitrosamine moiety-containing species. Therefore, special caution in interpretation of UV-Vis red shift for chemical sensors is suggested.

Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O2

The oxidation of transition metals such as manganese and copper by dioxygen (O₂) is of great interest to chemists and biochemists for fundamental and practical reasons. In this report, the O₂ reactivities of 1:1 and 1:2 mixtures of [(TPP)Mn^II] (1; TPP: Tetraphenylporphyrin) and [(tmpa)Cu^I(MeCN)]⁺ (2; TMPA: Tris(2-pyridylmethyl)amine) in 2-methyltetrahydrofuran (MeTHF) are described. Variable-temperature (-110 °C to room temperature) absorption spectroscopic measurements support that, at low temperature, oxygenation of the (TPP)Mn/Cu mixtures leads to rapid formation of a cupric superoxo intermediate, [(tmpa)Cu^II(O₂^•-)]⁺ (3), independent of the presence of the manganese porphyrin complex (1). Complex 3 subsequently reacts with 1 to form a heterobinuclear μ-peroxo species, [(tmpa)Cu^II-(O₂^2-)-Mn^III(TPP)]⁺ (4; λ_max = 443 nm), which thermally converts to a μ-oxo complex, [(tmpa)Cu^II-O-Mn^III(TPP)]⁺ (5; λ_max = 434 and 466 nm), confirmed by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy. In the 1:2 (TPP)Mn/Cu mixture, 4 is subsequently attacked by a second equivalent of 3, giving a bis-μ-peroxo species, i.e., [(tmpa)Cu^II-(O₂^2-)-Mn^IV(TPP)-(O₂^2-)-Cu^II(tmpa)]²⁺ (7; λ_max = 420 nm and δ_pyrrolic = -44.90 ppm). The final decomposition product of the (TPP)Mn/Cu/O₂ chemistry in MeTHF is [(TPP)Mn^III(MeTHF)₂]⁺ (6), whose X-ray structure is also presented and compared to literature analogs.

Insight into the preferential N-binding versus O-binding of nitrosoarenes to ferrous and ferric heme centers

Nitrosoarenes (ArNOs) are toxic metabolic intermediates that bind to heme proteins to inhibit their functions. Although much of their biological functions involve coordination to the Fe centers of hemes, the factors that determine N-binding or O-binding of these ArNOs have not been determined. We utilize X-ray crystallography and density functional theory (DFT) analyses of new representative ferrous and ferric ArNO compounds to provide the first theoretical insight into preferential N-binding versus O-binding of ArNOs to hemes. Our X-ray structural results favored N-binding of ArNO to ferrous heme centers, and O-binding to ferric hemes. Results of the DFT calculations rationalize this preferential binding on the basis of the energies of associated spin-states, and reveal that the dominant stabilization forces in the observed ferrous N-coordination and ferric O-coordination are dπ-pπ* and dσ-pπ*, respectively. Our results provide, for the first time, an explanation why in situ oxidation of the ferrous-ArNO compound to its ferric state results in the observed subsequent dissociation of the ligand.

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

A series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O₂ activation by copper(I). The reaction of the copper(I) complex of N,N,N',N'-tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet-visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η²-NO and dinuclear μ-η²:η¹ for ArNO^•-, and dinuclear μ-η²:η² for ArNO^2-. ¹⁵N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm^-1 for κN-ArNO, 1226 cm^-1 for η²-ArNO^•-, 1133 cm^-1 for dinuclear μ-η²:η¹-ArNO^•-, and 875 cm^-1 for dinuclear μ-η²:η² for ArNO^2-). The ¹⁵N NMR signal disappears for the ArNO^•- species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O₂ analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion.

Synthesis and molecular structures of N,N-dialkyl-4-nitrosoaniline adducts of formally d6 metalloporphyrins of ruthenium and cobalt

Para-aminosubstituted nitrosoarenes react with Ru ( CO )( OEP ) or [ Co ( TPP )( THF )2] SbF 6 (OEP2- = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion, TPP2- = 5,10,15,20-tetraphenylporphyrinato dianion) to generate Ru ( OEP )( ONC 6 H 4 NMe 2)2 and [ Co ( TPP )( ONC 6 H 4 NR 2)2] SbF 6 ( R = Me , Et ), respectively, in fair to high yields. These N -bound nitrosoarene complexes have been characterized by spectroscopic methods. The complexes Ru ( OEP )( ONC 6 H 4 NMe 2)2 and [ Co ( TPP )( ONC 6 H 4 NMe 2)2] ClO 4 have also been characterized by single-crystal X-ray crystallography. Their structures represent the first reported solid-state structures of Ru and Co porphyrins containing C-nitroso ligands.

Synthesis and solid-state molecular structures of bis- and mono-nitrosobenzene complexes of ruthenium porphyrins

Bis-nitrosobenzene complexes of the form (por)Ru(PhNO)2 (por = TPP, TTP; TPP = tetraphenylporphyrinato dianion, TTP = tetratolylporphyrinato dianion) have been prepared in good yields from the reaction of the (por)Ru(CO) precursor with excess PhNO in dichloromethane. The IR spectra of the complexes (as KBr pellets) displayed new bands at ~1348 cm–1, due to υNO. The solid-state molecular structure of (TPP)Ru(PhNO)2 was determined by single-crystal X-ray diffraction, and revealed that the PhNO ligands are bound to the Ru center via the N-binding mode. Reactions of the (por)Ru(PhNO)2 complexes with excess 1-methylimidazole gave the mono-nitrosobenzene complexes (por)Ru(PhNO)(1-MeIm). The IR spectra revealed a lowering of υNO in these mononitrosobenzene derivatives by ~27 cm–1, a feature consistent with the replacement of one π-acid PhNO ligand with the more basic 1-MeIm ligand. The solid-state molecular structure of (TPP)Ru(PhNO)(1-MeIm) reveals, in addition to the N-binding of the PhNO ligand, an essentially parallel arrangement of the C-N-O (of PhNO) and imidazole planes; this is in contrast with the (TPP)Ru(PhNO)2 complex, in which the C-N-O planes (of PhNO) are essentially perpendicular.Key words: nitroso, X-ray, ruthenium, porphyrin, imidazole.

Interaction between dioxoruthenium(VI) porphyrins and hydroxylamines: coordination of N-substituted hydroxylamine to ruthenium and X-ray crystal structures of ruthenium complexes with a unidentate nitrosoarene ligand

The interactions between dioxoruthenium(VI) porphyrins 1 with N-phenylhydroxylamine or unsubstituted hydroxylamine are described. Reaction of complexes 1 with excess PhNHOH leads to isolation of bis(nitrosobenzene)ruthenium(II) porphyrins 3 and mono(nitrosobenzene)ruthenium(II) porphyrins 4. Both the types of ruthenium complexes are characterized by 1H NMR, IR, and UV/Vis spectroscopy, and mass spectrometry. The X-ray structure determinations on [Ru(II)(TPP)(PhNO)2] (3a), [Ru(II)(2,6-Cl-TPP)(PhNO)2] (3e), and [Ru(II)(4-MeO-TPP)(PhNO)(PhNH2)] (4d) (TPP tetraarylporphyrin) disclose a unidentate nitrosoarene coordination in all these complexes, with Ru-N(PhNO) bond lengths of 2.003(3) (3a, average), 1.991(3) (3e, average), and 2.042(2) A (4d). In the case of 4d, the Ru-N(PhNH2) bond length is found to be 2.075(3) A. Mechanistic investigations reveal the formation of intermediates [Ru(II)(Por)(PhNO)(PhNHOH)] (5; Por=porphyrin), a ruthenium complex with N-substituted hydroxylamine ligand, in the "1 + PhNHOH" system. The Ru-NH(OH)Ph moiety in 5 undergoes no rapid exchange with free PhNHOH in solution at room temperature, as revealed by 1H NMR spectroscopy. Unlike the interaction between complexes 1 and PhNHOH, reaction of such complexes with NH2OH affords nitrosylruthenium(II) porphyrins [Ru(II)(Por)(NO)(OH)] (6).

Nitrosylation of Octaethylporphyrin Osmium Complexes with Alkyl Nitrites and Thionitrites: Molecular Structures of Three Osmium Porphyrin Derivatives

(OEP)Os(CO) reacts with n-butyl nitrite to give, after workup, the (OEP)Os(NO)(O-n-Bu) trans addition product (OEP = octaethylporphyrinato dianion). Similarly, the reaction of (OEP)Os(CO) or [(OEP)Os](2) with isoamyl nitrite gives the corresponding nitrosyl alkoxide, (OEP)Os(NO)(O-i-C(5)H(11)). The related reactions of (OEP)Os(CO) or [(OEP)Os](2) with isoamyl thionitrite gives the (OEP)Os(NO)(S-i-C(5)H(11)) nitrosyl thiolate. The reaction of the [(OEP)Os](2)(PF(6))(2) reagent with isoamyl thionitrite gives the nitrosylation product, [(OEP)Os(NO)]PF(6), which undergoes anion hydrolysis to give the isolable difluorophosphate (OEP)Os(NO)(O(2)PF(2)) derivative. Interestingly, the reaction of O(2)NC(6)H(4)N=NSPh with [(OEP)Os](2) gives the (OEP)Os(SPh)(2) product with loss of the arylazo fragments. The solid-state structures of (OEP)Os(NO)(O-n-Bu), (OEP)Os(NO)(O(2)PF(2)), and (OEP)Os(SPh)(2) have been determined by X-ray crystallography.