Μ-nitrido Diiron Phthalocyanine Research Articles

From mononuclear iron phthalocyanines in catalysis to μ-nitrido diiron complexes and beyond

For a long time biomimetic oxidation catalysis was mainly associated with metal porphyrin complexes. Phthalocyanine complexes, also belonging to large family of macrocyclic complexes have been under-investigated as catalysts until recently. In this review, we present the development of phthalocyanine catalytic chemistry at IRCELYON with a topical focus on contributions from our group during last 20 years and discuss recent research directions from personal perspective. Catalytic chemistry of phthalocyanine complexes is a rapidly growing field including not only oxidations but also many other reactions. Recent discovery of remarkable catalytic properties of μ-nitrido diiron phthalocyanines will give a new impetus and should even accelerate this development. These binuclear N-bridged complexes emerge as promising bio-inspired catalysts, in particular, for challenging reactions such as a low temperature oxidation of methane and oxidative defluorination of heavily fluorinated aromatic compounds. The scope of their catalytic applications is constantly increasing. The bimetallic construction is flexible and has a significant potential for further development including implementation of different ligands, metals and bridging groups. Consequently, the properties of this scaffold can be fine-tuned by platform architecture thus providing novel tools for catalysis.

Alkylthio-tetrasubstituted μ-Nitrido Diiron Phthalocyanines: Spectroelectrochemistry, Electrical Properties, and Heterojunctions for Ammonia Sensing

Alkylthio-tetrasubstituted μ-nitrido diiron phthalocyanine complexes are synthesized with n-butyl, iso-butyl, tert-butyl, and n-hexadecyl alkyl moieties. For the first time, a spectroelectrochemical investigation of μ-nitrido diiron phthalocyanines is achieved at all the redox steps. The complexes are stable in all their redox states, unlike their unsubstituted analogues. The interest of the present complexes is to prepare sensing devices by a solution processing method. Films are characterized by electronic absorption and Raman spectroscopies. Electrical measurements on resistors show the highly resistive behavior of these complexes, whatever the chain length. However, when combined with the lutetium bisphthalocyanine, an intrinsic semiconductor, these complexes form heterojunctions that exhibit a high sensitivity to ammonia, with a very good signal over noise ratio, at room temperature and under atmospheric conditions.

Effect of acidity of oxide support on the activity and stability of μ-nitrido diiron phthalocyanine complex

Various oxides, including Al2O3, Fe2O3, SiO2, and ZrO2, were employed as catalyst supports for the oxidation of cyclohexane by a μ-nitrido-bis[tetra(tert-butyl)phthalocyaninatoiron] complex. These oxide supports were characterized by Brunauer–Emmett–Teller surface area measurements and NH3 temperature-programmed desorption analyses. The catalyst supported on Al2O3, which has the highest acidity among the oxides considered here, showed the best activity with a turnover number (TON) of ˜108.8, without any co-catalyst. Furthermore, recycle tests of the catalysts revealed a TON of 69.3 for the Al2O3-supported catalyst after the third run. The strongly acidic sites in the Al2O3 support could be responsible for the strong adsorption of μ-nitrido diiron phthalocyanine, resulting in enhanced catalytic activity and stability.

Comparison of μ-nitrido diiron phthalocyanine – H2O2 and Fenton systems in transformation of poly- and perfluorinated aromatics does not support the involvement of hydroxyl radicals

μ-Nitrido diiron tetrabutylphthalocyanine, (FePctBu4)2N, in combination with H2O2 can perform an efficient defluorination of heavily fluorinated aromatics via an oxidative pathway. To check whether hydroxyl radicals might be involved in this reaction we have compared catalytic activities of (FePctBu4)2N – H2O2 and FeSO4 – H2O2 systems in the transformation of five differently fluorinated benzenes, perfluoronaphthalene, pentafluoropyridine, and eight functionalized perfluorinated benzenes. In sharp contrast with (FePctBu4)2N – H2O2 system, all fluorinated benzene derivatives were completely stable in the presence of FeSO4 and H2O2 at 60 °C during 15 h. Perfluorinated aromatic compounds functionalized with electron-donating or electron-withdrawing groups as well as pentafluoropyridine exhibited very low conversions. These results unambiguously show that Fenton chemistry is not responsible for the oxidative defluorination of highly fluorinated aromatic compounds performed by the (FePctBu4)2N – H2O2 system.

Cooperative behavior of perfluoro carboxylic acid on cyclohexane oxidation catalyzed by μ-nitrido diiron phthalocyanine complex

μ-Nitrido diiron phthalocyanine complex has shown great performance in cyclohexane oxidation. Catalytic activity of μ-nitrido-bis[tetra(tert-butyl)phthalocyaninatoiron] complex could dramatically be enhanced in the presence of perfluoro carboxylic acid. Turnover number of cyclohexane oxidation reached 504, with 10 mM of trifluoro acetic acid (TFA), which is more than 15-fold increase compared to that without TFA. Interaction between perfluorinated COO− group and metal center is thought to form stable reaction intermediate and accelerate the reaction. Furthermore, relationship between carbon length of perfluoro carboxylic acid and co-catalytic performance of cyclohexane oxidation was also investigated and the shorter perfluoro carboxylic acid provided higher enhancement.

Oxidation of aliphatic and aromatic C[sbnd]H bonds by t-BuOOH catalyzed by μ-nitrido diiron phthalocyanine

Low temperature selective transformation of alkanes to useful products continues to be an important challenge in chemistry and industry. μ-Nitrido diiron phthalocyanines in combination with H2O2 have been recently identified as powerful oxidation catalysts for these challenging reactions due to the formation of ultra-high valent diiron oxo species PcFe(IV)μNFe(IV)O(Pc+). This very strong two-electron oxidizing species is generated from peroxo complex PcFe(IV)μNFe(III)OOR(Pc) (RH in the case of H2O2) via heterolytic OO bond cleavage. Therein we show that the evolution of the peroxo diiron complex depends on the peroxide structure. Using tBuOOH we have demonstrated the formation of an one-electron oxidizing PcFe(IV)μNFe(IV)O(Pc) and tBuO radical via homolytic OO cleavage of the peroxocomplex. The reactivity of the μ-nitrido diiron tetra-t-butylphthalocyanine − tBuOOH catalytic system was investigated in the oxidation of different CH bonds in alkanes, olefins, aromatic and alkylaromatic compounds. The main products of cyclohexane oxidation were cyclohexanone and cyclohexanol whereas bicyclohexyl was formed in minor amounts even in the presence of O2 and tBuOOH. Under optimal conditions, the turnover numbers of almost 5300 have been achieved.

Mild oxidation of ethane to acetic acid by H2O2 catalyzed by supported μ-nitrido diiron phthalocyanines

Low temperature selective transformation of light alkanes of natural gas to useful products continues to be an important challenge in chemistry and industry. μ-Nitrido diiron phthalocyanines were recently identified as powerful oxidation catalysts capable of oxidizing methane, benzene and transforming poly- and perfluorinated aromatic compounds under oxidative conditions. Herein, we have supported two μ-nitrido diiron complexes onto silica, graphite or nafion and their catalytic performance has been evaluated in the heterogeneous oxidation of ethane in water. Acetic acid was obtained with selectivity up to 71%. Turnover number up to 58 and 50–65 % product yields can be achieved. The cleavage of C–C bond also occurred and formic acid was formed as side product. In contrast to methane oxidation, no strong influence of 75 mM acid was observed in ethane oxidation. The reactivities of C–H bonds in methane, ethane and propane were very similar when the reaction was performed in water. In contrast, in diluted acidic solution, the μ-nitrido diiron phthalocyanine – H2O2 system exhibits 5 times higher activity in the oxidation of the methane C–H bond compared to the ethane C–H bond.

X-ray absorption and emission spectroscopies of X-bridged diiron phthalocyanine complexes (FePc)2X (X = C, N, O) combined with DFT study of (FePc)2X and their high-valent diiron oxo complexes.

μ-Nitrido diiron phthalocyanine [PcFe(+3.5)NFe(+3.5)Pc](0) is an efficient catalyst, able to catalyze the oxidation of methane under near-ambient conditions. In this work, we compared the properties of structurally similar μ-carbido (1), μ-nitrido (2), and μ-oxo (3) dimers of iron phthalocyanine. The goal was to discern the structural and electronic differences between these complexes and to propose a rationale for the exceptional activity of 2. Extended X-ray fine-structure spectroscopy, high-resolution X-ray emission spectroscopy, and resonant inelastic X-ray scattering were applied to study the geometry and electronic structure of iron species in the series 1-3. The data provided by core hole spectroscopies were compared to the results of DFT calculations and found to coherently describe the structural and electronic properties of 1-3 as having equivalent iron centers with formal iron oxidation degrees of 3, 3.5, and 4 for the μ-oxo, μ-nitrido, and μ-carbido dimers, respectively. However, the bond length to the bringing atom changed in an unexpected sequence Fe-O > Fe-N < Fe-C, indicating redox non-innocence of the brigding μ-carbido ligand in 1. According to the X-ray emission spectroscopy, the μ-nitrido dimer 2 is a low-spin compound, with the highest covalency in the series 1-3. The DFT-calculated geometry and electronic structures as well as core hole spectra of hypothetical high-valent oxo complexes of 1-3 were compared, in order to explain the particular catalytic activity of 2 and to estimate the prospects of spectroscopic observation of such species. It appears that the terminal Fe═O bond is the longest in the oxo complex of 2, due to the strong trans-effect of the nitrido ligand. The corresponding LUMO of the μ-nitrido diiron oxo complex has the lowest energy among the three oxo complexes. Therefore, the oxo complex of 2 is expected to have the highest oxidative power.

Catalytic defluorination of perfluorinated aromatics under oxidative conditions using N-bridged diiron phthalocyanine.

Carbon-fluorine bonds are the strongest single bonds in organic chemistry, making activation and cleavage usually associated with organometallic and reductive approaches particularly difficult. We describe here an efficient defluorination of poly- and perfluorinated aromatics under oxidative conditions catalyzed by the μ-nitrido diiron phthalocyanine complex [(Pc)Fe(III)(μ-N)Fe(IV)(Pc)] under mild conditions (hydrogen peroxide as the oxidant, near-ambient temperatures). The reaction proceeds via the formation of a high-valent diiron phthalocyanine radical cation complex with fluoride axial ligands, [(Pc)(F)Fe(IV)(μ-N)Fe(IV)(F)(Pc(+•))], which was isolated and characterized by UV-vis, EPR, (19)F NMR, Fe K-edge EXAFS, XANES, and Kβ X-ray emission spectroscopy, ESI-MS, and electrochemical techniques. A wide range of per- and polyfluorinated aromatics (21 examples), including C6F6, C6F5CF3, C6F5CN, and C6F5NO2, were defluorinated with high conversions and high turnover numbers. [(Pc)Fe(III)(μ-N)Fe(IV)(Pc)] immobilized on a carbon support showed increased catalytic activity in heterogeneous defluorination in water, providing up to 4825 C-F cleavages per catalyst molecule. The μ-nitrido diiron structure is essential for the oxidative defluorination. Intramolecular competitive reactions using C6F3Cl3 and C6F3H3 probes indicated preferential transformation of C-F bonds with respect to C-Cl and C-H bonds. On the basis of the available data, mechanistic issues of this unusual reactivity are discussed and a tentative mechanism of defluorination under oxidative conditions is proposed.

Degradation of chlorinated phenols in water in the presence of H2O2 and water-soluble μ-nitrido diiron phthalocyanine

Efficient disposal of pollutants is a key problem in the environmental context. In particular, chlorinated aromatic compounds are recalcitrant to biodegradation and conventional treatment methods. Iron phthalocyanines were previously shown to be efficient catalysts for the oxidative degradation of chlorinated phenols considered as priority pollutants. We have recently discovered μ-nitrido diiron phthalocyanines as powerful oxidation catalysts. Herein, we evaluate these emerging catalysts in the oxidation of chlorinated phenols in comparison with conventional mononuclear complex. Catalytic performance of iron tetrasulfophthalocyanine (FePcS) and corresponding μ-nitrido dimer [(FePcS)2N] have been compared in the oxidation of chlorinated phenols by hydrogen peroxide in water. The oxidative degradation of 2,6-dichlorophenol (DCP) and 2,4,6-trichlorophenol (TCP) has been studied. The (FePcS)2N exhibited better catalytic properties than mononuclear FePcS in terms of conversion and mineralization (transformation of organic chlorine to Cl− and decrease of total organic carbon due to the formation of CO2). Kinetics of the DCP oxidation indicated that different reaction mechanisms are involved in the presence of FePcS and (FePcS)2N. The high catalytic activity of (FePcS)2N in the degradation and mineralization of chlorinated phenols make μ-nitrido diiron phthalocyanines promising catalyst to apply also in environmental remediation.

The first push-pull μ-nitrido iron phthalocyanine dimer

The first push-pull μ-nitrido diiron phthalocyanine complex was prepared and characterized. The EPR spectrum confirms the Fe +3.5– N – Fe +3.5 formalism. The stability of this dimer in oxidant media (up to 1000 equivalent of H 2 O 2) was studied by UV-vis spectroscopy. The push-pull μ-nitrido diiron phthalocyanine exhibited excellent stability because of the delocalization of an unpaired electron.

Diiron complexes on macrocyclic porphyrin-like platform as oxidation catalysts: reactivity and mechanistic considerations

Diiron macrocyclic complexes have been often considered as catalytically inert. However, numerous published examples of the catalytic oxidation reactions mediated by these complexes indicate their potential in catalysis. Mechanistic background for their applications as oxidation catalysts and their relationship with enzymatic and biomimetic oxidation involving cytochrome P-450 and soluble methane monooxygenase are discussed. A special emphasis was put on the N-bridged diiron phthalocyanine complexes as an emerging class of oxidation catalysts. Their unusual Fe(μN)Fe structure has interesting catalytic properties and reactivity. In addition to oxidation of strong C–H bonds in alkanes including methane, aromatic, and alkylaromatic compounds, μ-nitrido diiron phthalocyanines catalyze the oxidative dehalogenation and the formation of C–C bonds. A clean and practical character of the catalytic systems coupled with real availability of the phthalocyanines suggests a possibility of the application of this approach in industry. Great potential for further developments by modification of the catalysts structure can be envisioned.

Study of N-bridged diiron phthalocyanine relevant to methane oxidation: Insight into oxidation and spin states from high resolution 1s core hole X-ray spectroscopy

μ-Nitrido diiron phthalocyanine [PcFe+3.5NFe+3.5Pc]0 is a highly efficient catalyst, able to oxidize methane under near-ambient conditions. In this work, high resolution X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) were applied to study iron species in the series of μ-nitrido diiron phthalocyanines including initial [PcFe+3.5NFe+3.5Pc]0 and oxidized complexes [PcFeIVNFeIVPc]+PF6 and [PcFeIVNFeIV(Pc+)]2+Br2 as model compounds for the intermediates in the catalytic cycle. These systems contain 3d4 configuration of iron in high oxidation state Fe(IV). XES spectra of Kβ line are sensible to the local iron spin density and show unexpected difference in the spin state between the initial [PcFe+3.5NFe+3.5Pc]0 (LS), one-electron oxidized [PcFeIVNFeIVPc]+PF6 (HS) and two electron oxidized cation radical species [PcFeIVNFeIV(Pc+)]2+Br2 (LS). Fe K-edge XANES spectra were recorded at the fluorescence energy of the main Kβ1,3 line or Kβ′ line in the Kβ emission spectrum. The conclusions of RIXS study corroborated XES data. Our study suggests that the main catalytic pathway of oxidation includes two-electron transformation from the LS initial complex to the LS two–electron oxidized [PcFeIVNFeIV(Pc+)(O)]0 to generate powerful oxidant able to oxidize methane via two-electron process.

ChemInform Abstract: Phthalocyanine Metal Complexes: Versatile Catalysts for Selective Oxidation and Bleaching

AbstractReview: 69 refs.

µ-Nitrido Bridged Diiron Phthalocyanines: Old Complexes for New Catalytic Applications

Binuclear N-bridged M-N-M (M=transition metal) complexes on phthalocyanine platform constitute an emerging class of oxidation catalysts. Their unusual structure provides unprecedented catalytic properties and reactivity. The most significant so far example is the mild oxidation of methane. In addition to oxidation of strong C-H bonds in alkanes, aromatic and alkylaromatic compounds μ-nitrido diiron phthalocyanines catalyze the formation of C-C bonds exemplified by hydroacylation of olefins with aldehydes. A clean and practical character of this system coupled with real availability of the phthalocyanines suggests a high application potential of this approach. This system has a great potential for further development by modification of the structure.

Phthalocyanine metal complexes: Versatile catalysts for selective oxidation and bleaching

Iron and other metal phthalocyanine complexes can mediate both selective oxidation and bleaching reactions depending on the choice of oxidant. Olefins, phenols, condensed aromatics can efficiently be oxidized to valuable products by tBuOOH. The use of H 2O 2 leads to a different kind of reactivity providing efficient systems for catalytic degradation of recalcitrant pollutants and bleaching. The discovery of remarkable catalytic properties of μ-nitrido diiron phthalocyanine complexes exemplified by mild catalytic oxidation of methane in water at near-ambient conditions is of especial significance. This emerging type of oxidation catalysts has a great potential for the development. In this context, a combinatorial approach might be particularly fruitful. First, different macrocyclic ligands can be used, e.g. phthalocyanines, porphyrins, porphyrazines, corroles, corrolazines, etc. to produce either symmetrical dimers with the same ligand or unsymmetrical dimers having two different macrocyclic ligands. Secondly, a number of transition metals (Fe, Mn, Ru, Cr, V, Ti, etc.) can be used for the preparation of homoleptic or heteroleptic dimers having the same or different metals. Finally, several single bridging atoms can be used including O, N, C. The great variability of possible metal, ligand, bridging atom combinations might provide a large variety of complexes with different and tunable catalytic properties.

Preparation of N-bridged diiron phthalocyanines bearing bulky or small electron-withdrawing substituents

The synthesis of novel μ-nitrido diiron phthalocyanine with electron-withdrawing substituents is reported: μ-nitrido tetra-methylsulfonylphthalocyanine (13), μ-nitrido tetra-ethylsulfonylphthalocyanine (14), μ-nitrido tetra-adamantylsulfonylphthalocyanine (15), μ-nitrido tetra-cyclohexylsulfonylphthalocyanine (16). These complexes were characterized by ESI-MS, UV-vis, FT-IR and EPR techniques. The state of these complexes depends on the size of the substituents. Complexes 13 and 14 bearing small methylsulfonyl and ethylsulfonyl substituents are cationic ( PcFe IV NFe IV Pc )+ N 3- complexes while complexes 15 and 16 with bulkier adamantylsulfonyl and cyclohexylsulfonyl substituents are formally neutral PcFe III NFe IV Pc complexes which can be represented as PcFe (+3.5) NFe (+3.5) Pc .

Oxidation of methane and ethylene in water at ambient conditions

Available spectroscopic, labelling and reactivity data show that stable μ-nitrido diiron phthalocyanine activates H 2O 2 to form a high-valent diiron oxo species. This species is a very powerful oxidant which oxidizes methane in pure water at 25–60 °C to methanol, formaldehyde and formic acid. The catalytic activity can significantly be increased in the presence of a diluted acid solution. Thus, a high turnover number of 209 was attained in 0.075 M H 2SO 4. Oxidation of ethylene resulted in the formation of formic acid as a major product and formaldehyde with high turnover numbers. Under optimal conditions, 426 mol HCOOH and 37 mol CH 2O per mole of catalyst were obtained in pure water. The practical and green features of this novel approach (H 2O 2 as the clean oxidant, water as the clean reaction medium, easily accessible solid catalyst) as well as the relevance to biological oxidation (binuclear structure of bio-inspired complex) are of great importance both from practical and fundamental points of view.

Preparation and characterization of μ-nitrido diiron phthalocyanines with electron-withdrawing substituents: application for catalytic aromatic oxidation

Mu-nitrido-bis [tetra-(hexyl-sulfonyl)phthalocyaninatoiron] (3a) and mu-nitrido-bis [tetra-(tert-butylsulfonyl) phthalocyaninatoiron] (3b) complexes have been prepared and fully characterized by electrospray ionization mass spectrometry, UV-Vis, FTIR, EPR, Mössbauer techniques as well as by X-ray photoelectron and Fe K-edge X-ray absorption spectroscopies. Small changes at the periphery of the phthalocyanine ligand introduce a difference in the iron oxidation state. While 3b with tert-butyl substituents is a neutral complex with a mixed-valence Fe(3.5)-N-Fe(3.5) structural unit, 3a having n-hexyl substituents is an oxidized cationic Fe(IV)-N-Fe(IV) complex. The structural parameters of N-bridged diiron phthalocyanine with a Fe(3.5)-N-Fe(3.5) unit were determined for the first time. Iron atoms in 3b are displaced out of plane by 0.24 A and the Fe-N bond distance of the linear Fe-N-Fe fragment is equal to 1.67 A. Both complexes selectively catalyze benzylic oxidation of alkyl aromatic compounds by tBuOOH. Toluene was oxidized to benzoic acid with 80% selectivity, and the total turnover number was as high as 197. p-Toluic acid was the principal product of p-xylene oxidation. In this case the turnover number achieved 587 substrate molecules per molecule of catalyst. The described catalytic system is complementary to the recently reported system based on mu-nitrido diiron tetrabutylphthalocyanine-H2O2 which effectively oxidizes the benzene ring.

Diiron N-bridged species bearing phthalocyanine ligand catalyzes oxidation of methane, propane and benzene under mild conditions

Transformation of methane, the most abundant and the least reactive compound of natural gas to valuable products is one of the most difficult chemical problems of great practical importance. In Nature, methane monooxygenase enzymes transform methane to methanol in water under physiological conditions. However, chemical analogs for such a transformation are unknown. Here, we show the mild and efficient aqueous oxidation of methane by hydrogen peroxide, an ecologically and biologically relevant oxidant catalyzed by supported μ-nitrido diiron phthalocyanine dimer, (FePctBu4)2N . This bio-inspired complex containing a stable Fe – N – Fe motif catalyzes the oxidation of methane to methanol which is further transformed to formaldehyde and formic acid as is demonstrated using13CH4and18O labelling. (FePctBu4)2N - H2O2system shows a high activity in the oxidation of benzene to phenol which occurs via formation of benzene oxide and exhibits NIH shift typically accociated with biological oxidation. Mechanistic features of oxidation of methane and benzene as well as detected intermediate hydroperoxo- and high valent oxo diiron complexes support an O-atom transfer reaction mechanism relevant to bio-oxidation.