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

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 °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.