Molecular ship in a bottle

Herron, N. A cobalt oxygen carrier in zeolite Y— a molecular ship in a bottle, Inorg. Chem., 1986, 25, 4714-4717. 

Common to all encapsulation methods is the provision for the passage of reagents and products through or past the walls of the compartment. In zeolites and mesoporous materials, this is enabled by their open porous structure. It is not surprising, then, that porous silica has been used as a material for encapsulation processes, which has already been seen in LbL methods [43], Moreover, ship-in-a-bottle approaches have been well documented, whereby the encapsulation of individual molecules, molecular clusters, and small metal particles is achieved within zeolites [67]. There is a wealth of literature on the immobilization of catalysts on silica or other inorganic materials [68-72], but this is beyond the scope of this chapter. However, these methods potentially provide another method to avoid a situation where one catalyst interferes with another, or to allow the use of a catalyst in a system limited by the reaction conditions. For example, the increased stability of a catalyst may allow a reaction to run at a desired higher temperature, or allow for the use of an otherwise insoluble catalyst [73]. 

In microporous supports or zeolites, catalyst immobilization is possible by steric inclusion or entrapment of the active transition metal complex. As catalyst retention requires the encapsulation of a relatively large complex into cages only accessible through windows of molecular dimensions, the term ship-in-a-bottle has been coined for this methodology. Intrinsically, the size of the window not only determines the retention of the complex, but also limits the substrate size that can be used. The sensitivity to diffusion limitations of zeolite-based catalysis remains unchanged with the ship-in-a-bottle approach. In many cases, complex deformation upon heterogenization may occur. 

Metal phthalocyanines are easily synthesized by vapor-phase condensation of four molecules of dicyanobenzene in the presence of molecular sieves such as faujasites or A1PO-5 (123-126). This results in direct entrapment of the macrocycle inside the molecular sieve s channels and cages. There are also reports of ship-in-a-bottle synthesis of porphyrins in zeolites, but since porphyrin synthesis requires a mixture of pyrrole and an aldehyde instead of a single compound, porphyrin synthesis is a much less clean process than phthalocyanine preparation (127). Alternatively, soluble porphyrins or phthalocyanines can be added to the synthesis gel of, for example, zeolite X. This also results in entrapped complexes (128). 

Metal carbonyl clusters on supports are important to the subject reviewed here because they are the best known precursors of structurally simple supported metal clusters, which are formed by decarbonylation of the precursors. The routes for preparation of molecularly or ionically dispersed metal carbonyl clusters on zeolite and metal oxide supports include syntheses from mononuclear precursors on the support surface [5,9]. Ship-in-a-bottle syntheses of this type take place when the clusters formed in zeolite cages are trapped there because they are too large to fit through the apertures. Syntheses in the nearly neutral NaY zeolite are similar to those occurring on the nearly neutral y-Al203 and in nearly neutral solutions. Examples are the syntheses of [Ir4(CO)i2] [8] and of [Ir6(CO)i6] [10] from [Ir(CO)2(acac)] in the presence of CO. Syntheses in the more basic NaX zeolite are similar to those occurring in basic solutions and on the basic surface of MgO, e g., those of [HIr4(CO)ii]-and [Ir6(CO)i5]2- [11]... 

Up until the late seventies attempts to develop redox molecular sieves were mainly limited to the ion-exchange approach (see later). This situation changed dramatically with the discovery, by Enichem scientists in 1983 [6,7], of the unique activity of titanium silicalite-1 (TS-1) as a catalyst for oxidations with 30% aqueous hydrogen peroxide. Following the success of TS-1, interest in the development, and application in organic synthesis, of redox molecular sieves has increased exponentially and has been the subject of several recent reviews [8-11]. It has even provoked a revival of interest in another approach to producing redox molecular sieves the so-called ship-in-a-bottle method [12-15]. 

The discovery, in the mid-eighties, of the remarkable activity of TS-1 as a catalyst for selective oxidations with aqueous H2O2 fostered the expectation that this is merely the progenitor of a whole family of redox molecular sieve catalysts with unique activities. However, the initial euphoria has slowly been tempered by the realization that framework substitution/attachment of redox metal ions in molecular sieves does not, in many cases, lead to a stable heterogeneous catalyst. Nevertheless, we expect that the considerable research effort in this area, and the related zeolite-encapsulated complexes, will lead to the development of synthetically usefril systems. In this context the development of chiral ship-in-a-bottle type catalysts for intrazeolitic asymmetric oxidation is an important goal. Such an achievement would certainly justify the appellation mineral enzyme . 

Enclosed cavities in self-assembling capsules can even stabilize labile molecules formed in situ by the reaction of smaller molecular components. Thus, the condensation reaction of trimethoxysilanes in the cavity of 8 led exclusively to cyclic, trirneric silanol 14, which was never isolated before. In a typical reaction, phenyltrimethoxysilane was suspended in D2O solution of 4 at I00°C (Fig. 6). After I h. the H-NMR spectrum showed the exclusive presence of only one complex 8-14. The formation 844 was also evidenced by ESI-MS and single-crystal x-ray crystallography. Not only the cyclic trimers 14 are formed in a ship-in-a-bottle fashion, but they were protected... 

Shape-selective catalysis is not limited to the use of the molecular sieve. There are many applications that involve transition metal species that are supported inside or synthesized inside the cages or pores channels of molecular sieves ( ship-in-a-bottle") that can be used for catalyzing many selective reactions, such as hydrogenation and oxidation. 

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. 

For example, MPc complexes are synthesized in the zeolite framework by subjecting the zeolite to metal ion exchange and then treating it with molten dicyanobenzene. These "ship-in-a-bottle complexes cannot leave the zeolite without destroying the framework. Such zeolite catalysts, whose super-cages serve as a sort of reaction flask with molecular dimensions, continue to possess shape selectivity, reactant selectivity, regioselectivity and stereoselectivity. 

The application of zeolite encapsulated metal chelate complexes in catalysis is a promising area of research. In particular shape selective oxidations catalyzed by metallophthalocyanines (MPc), shown in Figure 1, included in synthetic faujasite (FAU) type zeolites (2-10) appear to be competitive with other molecular sieve based catalysts that may have commercial potential. The restricted apertures ( 7.4 A) to the supercages (12A) in FAU type zeolites precludes removal of the large MPc complex unless the zeolite lattice is destroyed. Such physically trapped complexes have been termed ship-in-a-bottle complexes as well as zeozymes (to reflect the biomimetic reactivity that is often associated with these catalysts). 

In principle, the formation of the porous matrix around a preformed molecular catalyst (bottle-around-ship) and the construction of the molecular catalyst in a preformed porous material (ship-in-a-bottle) have become two popular strategies for the encapsulation of molecular catalysts since the 1980s. Complexes such as [(BINAP)Ru(p-cymene)Cl)Clj, [(MeDuphos)Rh(cod)]OTf, (Salen)Mn, PrPybox-RuClj, and so on, have been entrapped in a silicon membrane, poly (vinyl alcohol) film, microcapsules, or silica matrix via the in situ formation of the network around the complexes (polymerization and sol-gel process were involved in the network formation) [53-55], However, the catalysts prepared are generally poor in activity, selectivity, and stability. The swelling of the polymer host material and the inhomogeneous cavity formed around the metal complexes may be the main reasons for the low activity and stabihty. In addition, the in situ formation of the... 

Catalysis by zeolites is a rapidly expanding field. Beside their use in acid catalyzed conversions, several additional areas can be identified today which give rise to new catalytic applications of zeolites. Pertinent examples are oxidation and base catalysis on zeolites and related molecular sieves, the use of zeolites for the immobilization of catalytically active guests (i.e., ship-in-the-bottle complexes, chiral guests, enzymes), applications in environmental protection and the development of catalytic zeolite membranes. Selected examples to illustrate these interesting developments are presented and discussed in the paper. 

The major activities in the science and application of zeolite catalysts are still observed in the field of (shape selective) acid catalysis. However, additional thrust areas can be clearly identified today, viz. zeolites in oxidation or base catalysis, applications in environmental protection, catalysis by ship-in-the-bottle complexes, to enumerate just a few. Many aspects of zeolite catalysis have been covered in a number of recent review articles [e g., 1-6] including the potential catalytic applications of ultra-large pore molecular sieves [7]. Hence there is no real need, nor would it be feasible on the limited number of pages allotted to this review, to cover every aspect fi om the huge amount of work done recently in the field. Rather, the authors restricted themselves to selected topics in catalysis by zeolites which, in their own view, deserve particular attention in the years to come. 

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