New copper(II) salicylaldimine derivatives for mild oxidation of cyclohexane

Abstract

Two new salicylaldiminato-copper(II) complexes, [ $$\hbox {Cu}(\mathbf{L}^{\mathbf{1}})_{2}$$ ] (1) and [ $$\hbox {Cu}(\mathbf{L}^{\mathbf{2}})_{2}$$ ] (2) (where $$\mathbf{HL}^{\mathbf{1}}=4$$ -tert-Butyl-2-[(thiophen-2-ylmethylimino)-methyl]-phenol and $$\mathbf{HL}^{2} = 2,4$$ -Di-tert-butyl-6-[(thiophen-2-ylmethylimino)-methyl]-phenol), endowed with a pendant thiophenyl moiety, were synthesized and characterized using standard spectroscopic techniques (FT-IR, UV-Vis, MS) and elemental analysis. Complexes 1 and 2 were unequivocally characterized by single crystal X-ray crystallography, which confirmed bidentate bis-chelation of the deprotonated - $$\mathbf{L}^{\mathbf{1}}$$ and - $$\mathbf{L}^{\mathbf{2}}$$ ligands to the copper (II) centres via the phenoxo and imine atoms forming square planar complexes. The copper(II)-hydroperoxo derivatives of 1 and 2 ([ $$(\mathbf{L}^{\mathbf{1}})_{2}\hbox {Cu}^\mathrm{II}\hbox {-OOH}$$ ] (3) and [ $$(\mathbf{L}^{\mathbf{2}})_{2}\hbox {Cu}^\mathrm{II}\hbox {-OOH}$$ ] (4)) were also synthesized and the formation of the active intermediate in solution studied. Complexes 1 and 2 were tested as catalyst precursors in cyclohexane oxidation under mild reaction conditions using hydrogen peroxide ( $$\hbox {H}_{2} \hbox {O}_{2}$$ ) as a terminal oxidant, and were found to catalyse oxidation of the substrate with yields comparable to similar mononuclear and even multinuclear copper complexes. Synopsis Synthesis, characterisation, and molecular structure of salicylaldiminato-copper(II) complexes, and their catalytic evaluation in the oxidation of cyclohexane employing hydrogen peroxide as a terminal oxidant have been studied. The complexes catalysed conversion of cyclohexane with appreciable yields.