title: Oxime-based salen-type tetradentate ligands with high stability against imine metathesis reaction.
abstract: 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 1a-h were prepared in 64-88% yields as colorless crystals from the corresponding salicylaldehydes 2a-h. The crystal structure of 1a-c suggests that the oxime-OH form is more predominant than the keto-NH form. The reaction of 2a-e with excess 1,2-bis(aminooxy)ethane gave monooximes 3a-e in 59-86%, which further reacted with a different salicylaldehyde to afford unsymmetrical salamo ligands 4-8 as stable crystals in 51-70%. No reaction took place when a mixture of salamo derivatives 1a and 1b was treated at 40 degrees 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.
authors.affiliation: Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan.
authors.forename: Shigehisa
authors.initials: S
authors.lastname: Akine
authors.forename: Takanori
authors.initials: T
authors.lastname: Taniguchi
authors.forename: Wenkui
authors.initials: W
authors.lastname: Dong
authors.forename: Sayuri
authors.initials: S
authors.lastname: Masubuchi
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Nabeshima
mesh_terms: D002614:Chelating Agents / Q000138:chemical synthesis* / Q000737:chemistry; D004980:Ethane / Q000031:analogs & derivatives* / Q000138:chemical synthesis* / Q000737:chemistry; D005029:Ethylenediamines / Q000737:chemistry*; D007097:Imines / Q000737:chemistry*; D008024:Ligands; D008956:Models, Chemical; D015394:Molecular Structure; D010091:Oximes / Q000138:chemical synthesis* / Q000737:chemistry*
chemical_list: C499745:1,2-bis(salicylideneaminooxy)ethane; D002614:Chelating Agents; D005029:Ethylenediamines; D007097:Imines; D008024:Ligands; D010091:Oximes; C011452:disalicylaldehyde ethylenediamine; D004980:Ethane