Phthalocyanine complexes, zeolite

The protons released are presumably available to compensate for the loss of the charge balancing cations within the zeolite. In conventional syntheses, the phtha-lonitrile condensation normally requires the nucleophilic attack of a strong base on the phthalonitrile cyano group [176, 177]. This function is presumably accommodated by the Si-O-Al (cation) basic sites within the ion-exchanged faujasite zeolites [178, 179]. The importance of this role is perhaps emphasized by the widespread use of alkali metal exchanged faujasites, particularly the more basic NaX materials of higher aluminium content [180, 181] as hosts for encapsulated phthalocyanine complexes. 

The use of heterogeneous catalysts in the synthesis of urethanes from aliphatic and aromatic amines, C02 and alkyl halides has been explored only recently. Titanosilicate molecular sieves [60a], metal phthalocyanine complexes encapsulated in zeolite-Y [60a], beta-zeolites and mesoporous silica (MCM-41) containing ammonium cations as the templates [60b, c], and adenine-modified Ti-SBA-15 [60d, e] each function as effective catalysts, even without any additional base. 

The involvement of upper excited states following irradiation of the phthalocyanine complexes Rh(Pc)(MeOH)X (where X = Cl, Br, or I) has been investigated. All compounds have the same action spectrum for photoinduced H-abstraction and the emission at 420 nm is attributed to relaxation of an upper tt, tt ) excited state. At high photonic fluxes it has been shown that biphotonic photochemistry involves the n, tt state. A study of the photoaquation of [RhCNH,), ] in fully and partially hydrated zeolite Y has been reported. Reaction within the cavities occurs with a quantum yield that is only 15—20% of that in aqueous solution this is attributed to the decrease in mobility of the water in the zeolite and to the exclusion of water from the ligand-exchange site by the zeolite lattice. ... 

One of the most smdied examples is the mimic of the enzyme cytochrome P-450 in the pores of a faujasite zeolite [196,204,225], The iron-phthalocyanine complex was encapsulated in the FAU supercage and is used as oxidation catalyst for the conversion of cyclohexane and cyclohexanone to adipic acid, an important intermediate in the nylon production. In this case the two step process using homogeneous catalysts could be replaced by a one step process using a heterogeneous catalyst [196]. This allowed better control of the selectivity and inhibited the auto oxidation of the active compound. In order to simulate a catalyst and the reaction conditions which are close to the enzymatic process, the so obtained catalyst was embedded in a polydimethylsiloxane membrane (mimics the phospholipid membrane in the living body) and the membrane was used to limit oxygen availability. With this catalyst alkanes were oxidized at room temperature with rates comparable to those of the enzyme [205]. 

An example for subtle control by substitution of the phtallocyanine structure is reported by Pattons et al. [193] comparing the activity of FePcY with nitro substituted Fe phthalocyanines in zeolite Y. The electron withdrawing effect of the nitro substiment on the benzene ring was expected to enhance the electrophilic character of the active oxygen species and so to increase the activity. For the oxidation of cyclohexane to cyclohexanone and cyclohexanol with tBHP a 10 fold increase in the turnover frequency (TOF)was found for the nitro substituted complex in zeolite Y in comparison to the unsubstituted [204]. However the nitro substimted Fe phthalocyanines were found to be located at the outer surface of the... 

A recent elegant example of the tailoring the chemical properties of encapsulated metal complexes is the work of Balkus etal. who prepared and studied perfluorinated phthalocyanine complexes of Fe, Co, Cu and Ru (Scheme 25)[230] in NaX. Perfluorinating the complexes enhances the stability and catalytic activity of the catalysts in the oxyfiinctionalisation of light alkanes. The rapid deactivation of the catalysts based on Fe, Co and Cu Fj Pc complexes was overcome by using Ru as the metal center. Similar catalysts, i.e.,Co-phthalocyanine (Co-Pc) encapsulated in zeolite Y, are active catalysts for cyclohexene and 1-hexene epoxidation (Scheme 27)[231]. Comparison of the activity of free and encapsulated Co-Pc has shown that the interaction with the zeolite stabilizes the complex. Co-Pc is still active after 24 hrs reaction whereas the free complex in solution is virtually inactive after 15 minutes. 

Other transition metals may be incorporated as carbonyl complexes. Catalytically active complexes of Mn and Fe were prq>ared by synthesizing the complex inside the pores of NaX and NaY zeolites [68,69]. The occluded Mn-bipyridyl and Fe-phthalocyanine complexes catalyze the oxidation of cyciohexene to adipic acid. 

Parton et al. [126] reported on the development of a synthetic system that mimics the cytochrome P-450 enzyme. They embedded zeolite Y crystallites containing encapsulated iron phthalocyanine complexes in a polymer membrane. Using tertiary-butylhydroperoxide as oxidant, this catalytic system oxidizes alkanes at room temperature with rates comparable to those of the real enzyme. 

The macrocycle phthalocyanine contains 8 N atoms, but usually only the four N-atoms on the inner side of the cycle are able to coordinate. In fact, in most cases the synthesis of phthalocyanine is realized in the presence of a metal ion as the template. It is also possible to attach various substituents on the phthalocyanine macrocycle. As for porphyrin, when coordinating to a metal ion, the H-atoms of the two NH groups on the inner side of the phthalocyanine cycle are replaced. The incorporation of metal porphyrin and phthalocyanine complexes into porous crystals has been gaining increasing interest. The properties of the complexes located in zeolite channels or cages are usually different from those of the compounds in solution, and they may find applications in areas such as catalysis, photochemistry, electrochemistry, and biomimetics. 

Transition metal complexes of phthalocyanine encaged in faujasite type zeolites have been reported as efficient catalysts in the oxidation of alkanes at room temperature and atmospheric pressure [6-13]. These catalysts constitute potential inorganic mimics of remarkable enzymes such as monooxygenase cytochrome P-450 which displays the ultimate in substrate selectivity. In these enzymes the active site is the metal ion and the protein orientates the incoming substrate relative to the active metal center. Zeolites can be used as host lattices of metal complexes [14, 15]. The cavities of the aluminosilicate framework can replace the protein terciary structure of natural enzymes, thus sieving and orientating the substrate in its approach to the active site. Such catalysts are constructed by the so-called ship in a bottle synthesis the metal phthalocyanine complexes are synthesized in situ within the supercages of the zeolite... 

UV spectra used for a semi-quantitive determination of the amount of intracrystalline phthalocyanine complexes were taken on a Perkin Elmer UV-visible spectrophotometer. A calibration curve was obtained by dissolving known amounts of metal complex in concentrated sulfuric acid. Zeolite was added to take into account matrix effects. Surface area and pore volume measurements were performed on a Micromeritics ASAP 2000 by absorption of nitrogen gas at liquid nitrogen temperature. X-ray powder diffraction of the zeolites was used to ensure good crystallinity after the exchange and encapsulation procedures... 

The crystallization of zeolites and molecular sieves with metal complexes represents a fresh strategy for the synthesis of these materials as well as a novel method for encapsulation of metal chelate complexes. We have shown that several first row tranistion metal phthalocyanines complexes can be encapsulated in X and A type zeolites by synthesizing the zeolite around the metal complex. Preliminary results indicate the concentration and type of phthalocyanine complex modify the crystallization of X type zeolites. The extension of this method to other metal chelate complexes and molecular sieves is currently under investigation. 

The zeolite cavities can be considered as peculiar reaction nanovessels where the chemical processes carried out inside them and their products are affected by the confines in which they are being performed. This main principle was proven in mid-70 s when the first synthesis of neutral phthalocyanine complexes encapsulated in Y zeolites via intracrystalline assembling was performed at Moscow State University [1,2]. Once formed within the bottle-shaped supercages of Y zeolite, the resulting electroneutral complexes cannon leave them because of spacial restrictions. Later, this new type of inclusion compounds was termed as "ship-in-a-bottle" systems [3]. 

P-12 - Effects of molecular confinement on structure and catalytic behaviour of metal phthalocyanine complexes encapsulated in zeolite-Y... 

Metal phthalocyanine complexes (MPc M = V, Co and Cu) encapsulated in zeolite-Y were prepared by in-situ ligand synthesis and characterized by chemical and thermal analyses and FT-IR, diffuse reflectance UV-vis and EPR spectroscopic techniques. The studies provided evidence for the encapsulation of MPc inside the supercages of zeolite-Y. The Pc moiety distorts from square planarity as a consequence of encapsulation. The encapsulated complexes exhibited enhanced styrene epoxidation activity with /e/-/-butylhydroperoxide compared to the neat complexes in homogeneous medium. The activity and product selectivity of the encapsulated complexes varies with the central metal atom. 

The most important class of solid-state enzyme mimics is based on zeolites. Zeolites are solid materials composed of Si04 or AIO4 tetrahedra linked at their corners, affording a three-dimensional network with small pores of molecular dimensions. They possess a unique feature of a strictly uniform pore diameter. In particular, zeolites with encapsulated metal complexes are used as inimics of cytochrome P-450.An efficient enzyme mimic was obtained by encapsulating an iron phthalocyanine complex into crystals of zeolite Y, which were, in turn, embedded into a polydimethylsiloxane membrane acting as a mimic of the phospholipid membrane.With t-butylhydroperoxide as the oxidant, the system hydroxyl-ates alkanes at room temperature with rates comparable to those for the enzyme. It shows similar selectivity (preference oxidation of tertiary C-H bonds) and a large kinetic isotope effect of nine. 

Phthalocyanine complexes within zeolites have also been prepared by the ship-in-a-bottle method (see Section 6.6), and have subsequently been investigated as selective oxidation catalysts, where their planar metal-N4 centres mimic the active sites of enzymes such as cytochrome P450, which is able to oxidize alkanes with molecular oxygen. Cobalt, iron and ruthenium phthalocyanines encapsulated within faujasitic zeolites are active for the oxidation of alkanes with oxygen sources such as iodosobenzene and hydroperoxides. Following a similar route, Balkus prepared Ru(II)-perchloro- and perfluorophthalocyanines inside zeolite X and used these composites for the selective catalytic oxidation of alkanes (tert-butylhydroperoxide). The introduction of fluorinated in place of non-fluorinated ligands increases the resistance of the complex to deactivation. 

The in-s rtu synthesis of metalio-phthalocyanines in zeolites can be done via three distinct literature procedures (Scheme 1). The required amount of a given transition metal is brought into the zeolite via a simple ion exchange (procedure A), through adsorption of a TM-carbonyl (B) or a metallocene (C). After removal of the respective TM ligands (water, CO and cyclopentadiene, respectively), 1,2-dicyanobenzene (DCB) is adsorbed onto the TM-zeolite and the mixture heated to form the MePc compiex. Rnally, the sample has to be purified in such a way that oniv occiuded MePc complex remains in the zeolite. The association of DCB with the supercage TM ions in Y zeolite, are schematically shown in Fig. 3. 

Metal carbonyls are also suitable for assembling phthalocyanine systems [455, 474]. The use of volatile metal-containing compounds, including carbonyls, as matrix sources is of practical interest because of the possibility of obtaining catalysts with molecularly dispersed heterogeneous phthalocyanine complexes supported on zeolites [475-477]. 

The various strategies for preparation of zeolite encapsulated phthalocyanine complexes have largely involved the condensation of dicyanobenzene (DCB) around an intrazeolite metal ion to form the MPc complex. The efficiency of this template synthesis depends on the nature and location of the intrazeolite metal ion to be complexed. For example, metals have been introduced to the zeolite by ion exchange (7-13), metal carbonyls (14-19) and metallocene complexes (2-5,19-21) prior to reaction with DCB. Some of the advantages and disadvantages of these methods have been detailed by Jacobs (2). However, there are several problems that are inherent to the template synthesis in general. Often there is incomplete... 

Even larger virtual pressure effects (up to about 30 kbar) are observed when bulkier molecules, such as metal phthalocyanines, are synthesized in-situ within zeolite Y cages [3,16]. Fig. 4 represents an encaged Co-phthalocyanine complex within a faujasite framework supercage, experiencing distortions which are reflected in the electronic vibration transitions of the macrocycle. 

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