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Oxime-Based Salen-Type Tetradentate Ligands with High Stability against Imine Metathesis Reaction

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posted on 04.03.2005, 00:00 by Shigehisa Akine, Takanori Taniguchi, Wenkui Dong, Sayuri Masubuchi, Tatsuya Nabeshima
Although salen and its analogues are versatile chelate ligands in inorganic and organometallic chemistry, synthesis of unsymmetrical salen derivatives consisting of two different salicylideneimine moieties is difficult because of the CN bond recombination. To develop stable analogues of salen-type ligands, we synthesized a series of new ligands salamo (=1,2-bis(salicylideneaminooxy)ethane) on the basis of O-alkyl oxime instead of the imine moiety. Eight salamo ligands 1ah were prepared in 64−88% yields as colorless crystals from the corresponding salicylaldehydes 2ah. The crystal structure of 1ac suggests that the oxime-OH form is more predominant than the keto-NH form. The reaction of 2ae with excess 1,2-bis(aminooxy)ethane gave monooximes 3ae in 59−86%, which further reacted with a different salicylaldehyde to afford unsymmetrical salamo ligands 48 as stable crystals in 51−70%. No reaction took place when a mixture of salamo derivatives 1a and 1b was treated at 40 °C in H2O/MeCN (5:95). However, the metathesis reaction of salen derivatives 9a and 9b completed in 2 h to give a statistical mixture. Monooxime 3b was much more stable than monoimine 11 which is difficult to be isolated. These results indicate the extremely high stability of the salamo derivatives 1 and precursors 3.