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.

Crystal structure of (E)-1-(4-(((E)-3,5-dibromo-2-hydroxybenzylidene)amino)phenyl)ethan-1-one O-methyl oxime, C17H16Br2N2O2

“…These compounds are an important class of organic compounds because of their applications in many elds of organic, biological, analytical chemistry [5,6] and material science [7]. Moreover, oximes frequently exhibit satisfactory insecticidal, fungicidal or herbicidal activity.…”

Crystal structure of bis[(3,3'-dimethoxy-2,2'-1,3-propylene)dioxybis( nitrilomethylidyne)diphenolato]-bis(acetato-O1,O2)- bis(ethanol-O)trinickel(II) — ethanol — acetone (1:2:1.06), Ni3(C19H20N2O6)2(C2H5OH)2(C2H3O2)2 · 2C2H5OH · 1.06C3H6O

“…In particular, replacement of some atoms of the ligand with other elements often changes its properties drastically [4,5]. If an O-alkyloxime moiety (-CH=N-O-(CH 2)n-O-N=CH-)i su sed instead of the imine moiety, the larger electronegativity of oxygen atoms is expected to strongly affect the electronic properties of N 2 O 2 coordination sphere, which can lead to different and novel properties and structures of the resulted complexes [6,7]. Recently, we studied trinuclear complexes of cobalt [8,9].…”

1,3-Bis[(4-methylbenzylidene)aminooxy]propane

“…Much interest has been focused on bisoxime compounds, in which the large electronegativity of O atoms is expected to affect strongly the electronic properties of the nitrogen atoms, and exhibit high stability against imine metathesis reactions ( Akine et al, 2005). Some of them or their metal complexes are used in wide field due to their variety of applications, especially for catalysis and biological processes, magnetism, and supramolecular architectures (Atwood et al, 2001;Yeap et al, 2008;Dong et al, 2008).…”

“…For bisoximes and their applications, see: Akine et al (2005); Atwood & Harvey (2001); Dong et al (2008Dong et al ( , 2009; He et al (2008); Yeap et al (2008).…”