Entrapped complexes

Later reports (58) have questioned whether the earlier report (55) was correct in concluding that the planar cobalt(II) complex of salen was formed in zeolite Y. The characteristics of the supposedly zeolite-entrapped [Con(salen)] are apparently not as similar to the same species in solution as previously reported. For example, planar [Con(salen)] and its adducts with axially disposed bases are generally ESR-detect-able low-spin complexes (59), and cyclic voltammetry of the entrapped complex revealed a Co3+/Co2+ redox transition that is absent in solution (60). These data, and more recent work (58), indicate that, in the zeolite Y environment, [Con(salen)] is probably not a planar system. Further, the role of pyridine in the observed reactivity with dioxygen is unclear, since, once the pyridine ligand is bound to the cobalt center, it is doubtful that the complex could actually even fit in the zeolite Y cage. The lack of planarity may account for the differences in properties between [Con(salen)] entrapped in zeolite Y and its properties in solution. 

In aspect of chip-based technology, electrochemical genosensors based on different materials and transducers have been recently developed in response to clinical demand of giving promising results [18-25]. Different sensor technologies provide a unique platform in order to immobilize molecular receptors by adsorption, crosslinking or entrapment, complexation, covalent attachment, and other related methods on nanomaterials [5,7,26]. 

Schlax, P.J., Xavier, K. A., Gluick, T. C., and Draper, D. E. (2001). Translational repression of the Escherichia coli alpha operon mRNA Importance of an mRNA conformational switch and a ternary entrapment complex. J. Biol. Chem. 276, 38494-38501. 

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). 

This would be the most direct way to monitor electrochemical properties of the entrapped complex, C"+. 

The entrapped complexes are known to catalyze selective oxidation or hydrogenation reactions, depending mainly on the complexed transition metal cation [4, 82, 84]. Recently, two exciting examples have been published describing the synthesis of adipic acid from cyclohexene [93] or even from cyclohexane [94], respectively (cf Figure 6). 

The oxidation of dimethyl sulfide to the corresponding sulfoxide on different zeolites has been reported recently, using zeolite entrapped Cu-ethylenediamine ([Cu(en)2]2") complexes. Spectroscopic comparison between the neat and the NaY, KL, and NaBETA entrapped complexes, shows that the square planar complex undergoes distortion in the zeolite crystal [54-56], Changes in redox properties of the complexes in the zeolites are due to decrease of the HOMO / LUMO levels of the metal complexes upon encapsulation under influence of the electric field existing inside the zeolite [56]. The high activity in ZSM-5, however, points to the existence of extra-pore complexes, probably strongly adsorbed at the external surface. 

In summary, interaction with amylopectin occurs more readily than with amylose but the bonds are less stable in the former and may disintegrate during heating and freezing perhaps only the entrapped complexes are formed. On the other hand, interaction with amylose starch proceeds less readily, but the complexes are enhanced by processing. 

The oxygenation of olefines to form epoxides is also possible by means of Mn (salen) anchored in Y-zeolite [66]. Furthermore selective hydrogenations are known using Pd (salen) encapsulated in X- and Y-zeolite [71]. M. Ichikawa and coworkers used a special electron donor acceptor complex (Na )4(FePc )/NaY for the selective hydrogenation of butadien to 2-butene and found that the entrapped complex results in a higher trans/cis ratio of 2-butenes than the complex on the external surface of NaY [70]. 

Since the non-entrapped complex is water insoluble it is inactive in this solvent under homogeneous conditions. 

Resonance Raman and TR data were able to characterise a zeolite Y-entrapped complex, Ru(bipy)2(pypz), where pypz = 2-(2-pyridyl)pyrazine. They showed that the structure was very similar to that in the free species. Resonance Raman studies have continued to be used to study the electronic excited states of a range of ruthenium bipyridyl and related complexes. 

Hammarsten casein was also used as a high molecular weight substrate. Buffer solutions of 0.1 ionic strength were used and were of the same composition as those described above. A comparison of the enzymatic activities for native and complexed BT served not only to reveal structural changes in the active site after complexation, but also demonstrated the functional property of the complex or calcium alginate gel-entrapped complex as an immobilized enzyme. 

Aluminosilicate molecular sieves with the FAU structure have been crystallized in the presence of several metallophthalocyanines. A percentage of the complexes becomes included into the zeolites. The synthesis of NaX around the metal chelate represents a new method for encapsulating such complexes and modifying zeolite molecular sieves. The entrapped complexes were characterized by XRD, IR and UV-VIS spectroscopy. Preliminary results suggest the metal complexes may function as templates by modifying the gel chemistry. 

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