Coordination geometries salen ligands

Hypercoordinate silicon complexes with tetradentate (O, N, N, 0)-chelating ligands of the salen type are expected to exhibit unusual chemical and physical properties because of the higher coordination number of the silicon atom [1,2]. Therefore, several attempts were made to synthesize such compounds [2, 3]. Starting from easily available silicon compounds such as SiCU or other chlorosilanes, conversion with salen type ligands mostly yielded complexes with a hexacoordinate [2, 3] and, in some cases, pentacoordinate silicon atom [4]. Unfortunately, there are only a few examples where the coordination geometry has been confirmed by X-ray structure analysis [2, 4]. 

The ship-in-a-bottle technique is perhaps the most common method for encapsulation of transition metal complexes. In this way the tetradentate Schiff base ligand SALEN (bis-salicylidene) ethylenediamine can diffuse through the 12 MR windows of faujasite. Then, when complexed with a previously exchanged metal ion, nearly square planar coordination geometry is formed inside the a-cages [97-100], Mn complexes with a chiral ligand, prepared by the ship-in-a-bottle technique inside Y and EMT zeolites, have enantioselectively carried at the epoxidation of olefins [101,102]. 

Parallel to the well-represented salens and porphyrins some other ligand systems find successful utilisation with aluminium in the formation of cyclic carbonates. To stay with a planar N2O2 coordination geometry the group of Zevaco and Dinjus used a ligand system showing some parallels with the acen geometry as shown in Scheme 18.45. 

A series of five- and six-coordinate chiral [Mn (salen)] complexes known to be effective AE catalysts has been characterized by X-ray crystallography [56]. Comparison of some of these structures revealed that the ligand geometry around the metal center and the chiral diimine backbone remained remarkably constant, while the salicylidene regions of the complexes adopted a wide range of conformations. The conformational variations observed in the solid state likely reflect accessible solution conformations of the [Mn (salen)] complexes and possibly their oxo counterparts. 

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