An unprecedented one-step synthesis of octahedral Cu(ii)-bis(iminoquinone) complexes and their reactivity with NaBH4

Abstract

A new non-innocent ligand, H₂L^{AP(o-NO₂-OPh)}, was synthesized. The ligand H₂L^{AP(o-NO₂-OPh)} reacted with 0.5 equivalents of CuCl₂·2H₂O and provided the corresponding mononuclear four-coordinate [Cu(II)-bis(iminosemiquinone)] complex (1). Interestingly, the ligand upon reacting with 2 equivalents of CuCl₂·2H₂O in the presence of Et₃N and air provided the corresponding trans-dichloride-bound six-coordinate mononuclear Cu(II)-complex (2) in one step. To extend the validity of the newly developed one-step synthesis of the trans-dichloride-bound Cu(II)-bis(iminoquinone) complex using a non-innocent ligand and 2 equivalents of CuCl₂·2H₂O, the previously reported non-innocent ligand H₂L^AP was further examined. Thus the formed trans-dichloride-bound Cu(II)-bis(iminoquinone) complex was designated as complex 3. The complexes were characterized by IR, mass, UV-Vis/NIR, X-band EPR, and X-ray single crystal diffraction techniques. Molecular structure analysis confirmed that in 1 the oxidation state of the coordinating ligands was [L^{ISQ(o-NO₂-OPh)}]¹⁻, i.e., one-electron oxidized iminosemiquinone. In 2 and 3·CH₂Cl₂, both the coordinated-ligands were present in their two-electron oxidized iminoquinone form. The iminoquinone-complex formation was found to proceed with the generation of CuCl salt. X-band EPR spectrum measurement confirmed that both the iminoquinone-complexes were paramagnetic and the unpaired electron was located at the 3d_x²−y² orbital of Cu(II) ions. When the Cu(II)-bis(iminoquinone) complex 3 was subjected to react with NaBH₄ in dry CH₃CN, H₂ gas was formed along with the generation of the corresponding Cu(II)-bis(iminosemiquinone) complex. GC analyses were performed for the identification of H₂ gas.