Flexible ligand method

Three methods can be followed for the synthesis of a SIB catalyst (i) zeolite synthesis around the metal complex (ii) template synthesis and (iii) the flexible ligand method. 

The flexible ligand method, consisting of the tetramerization of pyrrole and an aldehyde, can be used for the entrapment of metallo-porphyrins (MePOR) in situ in the supercages of faujasite-type zeolites (Scheme 10.7).[74]... 

Flexible ligand method. 0.9 g of 1,2-phenylenediamine were slowly added with stirring to 2 g of salicylaldehyde. After 10 minutes stirring, the mixture was allowed to cool down and the product was transferred to 40 mL of ethanol. The solids were filtered off, recrystallized from ethanol and dried in vacuo overnight. 1 g of CoNaY was mixed with 2 g salophen and heated in an open tube to 450 K with continuous stirring for 12 hours. The molten slurry was allowed to cool, and the zeolite was soxhiet extracted with methylene chloride. The solid material was dried in vacuo overnight. 

The basic approach to prepare Co(II)-complexes of salen (N,lSr-bis(salicylidene)ethylene-diamine)-type molecules is the flexible ligand method [9]. In this process the Schiffbase ligand can diffuse by twisting into the zeolite where it becomes too large to exit by complexation with the cobalt ion. The flexible ligand method, however, was not usefiil for the preparation of Co-salophen/ zeolite catalyst, because the product was inactive in the oxidation reactions. The salophen molecule does not seem to be flexible enough and can not get into the zeolite to produce the suitable complex in the supercage. 

We have also prepared the Co-salophen/zeolite catalyst, using the template synthesis and the flexible ligand method. The Co-salophen/zeolite catalyst prepared by the template synthesis method proved to be active in the oxidation of hydroquinone and in the aerobic oxidation of 1-octene and the acetoxylation of cyclohexene. The zeolite-encapsulated catalyst was active and produced the same selectivity and yield as the free complex. It was also possible to remove the catalyst and to reuse it in subsequent experiments. 

The Co(salophen) complex encapsulated in zeolite was prepared by two different methods. The flexible ligand method involves the diffusion of the Schiff base ligand into the zeolite, where upon complexation with the Co ion becomes too large to exit. However, this method was not useful for the preparation of Co(salophen)/zeolite, because the catalyst was not active in the oxidation reaction. For this reason the synthesis, originally used for the preparation of the free Co(salophen) complexes was modified and the principles of template synthesis method was used to prepare the Co(salophen)/zeolite catalyst. 

The encapsulated catalyst can be obtained by assembling the catalytic complex within the pores or by building the support around the isolated complex (vide infra). Both methods today are known in literature as the flexible ligand and zeolite synthesis method, respectively. In the flexible ligand method the ligand or its precursors should be able to diffuse freely through the zeolite pores exchanged with the preferred transition metal ion,... 

Mills JEJ, De Esch IJP, Perkins TDJ, Dean PM. SLATE a method for the superposition of flexible ligands. J Corn-put Aided Mol Design 2001 15 81-96. 

Lemmen, C., Lengauer, T., and Klebe, G. ELEXS a method for fast flexible ligand superposition./. Med. Chem. 1998, 41, 4502-4520. 

Automated flexible ligand docking method and its application for database search./. Comput. Chem. 1997, 78, 1812-1825. 

An even more common method for carbacyclic ring construction is the Diels-Alder reaction. Mukund Sibi of North Dakota State University reports (J. Am. Chem. Soc. 125 9306, 2003) that the flexible ligand 6 works particularly well in mediating the enantioselective addition of 4 to 5, to give 7. 

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