A wine red copper(II) complex of a tetradentate nitrogen donor showing two-electron oxidation. Generation of a copper(II)-phosphine bond

Abstract

<p>Using the 1 : 2 condensate of benzil and 2-hydrazinopyridine as the ligand LH₂ (H: dissociable NH proton), the red complex Cu(LH₂)(ClO₄)₂ (<b>1</b>) was synthesized. The ligand also afforded the orange [Zn(LH₂)(OH₂)₂](ClO₄)₂ (<b>2</b>). The X-ray crystal structures of the ligand, <b>1</b> and <b>2</b> have been determined. The metals in <b>1</b> and <b>2</b> have octahedral N₄O₂ environments. <b>1</b> is paramagnetic with <i>μ</i>_eff of one unpaired electron (1.63 <i>μ</i>_B and displays an axial EPR spectrum in the solid state with <<i>g</i>> = 2.07, characteristic of a (d_x2_−y2)¹ ground state (<i>g</i>_|| > <i>g</i>_⊥; <i>A</i>_|| = 16 mT). In cyclic voltammetry, <b>1</b> displays a two-electron oxidation around 0.9 V <i>versus</i> NHE. The two-electron oxidized (coulometrically) solution of <b>1</b> (golden yellow) gives an EPR spectrum with <<i>g</i>> = 2.17 and <i>g</i>_|| < <i>g</i>_⊥. The reaction of PPh₃ with <b>1</b> yields the orange complex [Cu(LH₂)(PPh₃)](ClO₄)₂ (<b>4</b>). With the assumed chemical formula, the effective magnetization of <b>4</b> corresponds to one electron. Its EPR spectrum in the solid state is isotropic with <i>g</i> = 2.07. This <i>g</i> value yields a theoretical <i>μ</i>_eff of 1.80 <i>μ</i>_B at 298 K from Curie’s law, which matches very well with the experimental value of 1.89 <i>μ</i>_B at room temperature. Since single crystals of <b>4</b> could not be obtained, DFT calculations at the UBP86/6–311G(2d,p) level have been carried out and indicate that the cation in <b>4</b> is square pyramidal with the phosphine at the apex. The ease of the oxidation of the metal in <b>1</b> leads to the stabilization of the rare Cu(II)-P bond in <b>4</b>.</p