Electrochemical reduction of bis(β-diketones): 1,10-di(α-cyclohexanone)decane-1,10-dione and 3,14-diacetylhexadecane-2,15-dione. Characterization and electron paramagnetic resonance properties of their electrogenerated mononuclear copper(II) complexes

Abstract

The preparation and characterization of the bis(β-diketone) 1,10-di(α-cyclohexanone)decane-1,10-dione (H₂L¹1) and 3,14-diacetylhexadecane-2,15-dione (H₂L²2) ligands, are reported. A direct electrochemical generation of Cu^IIL complexes is achieved. The electrochemical reductions of 1 and 2 have been investigated in tetrahydrofuran at a dropping mercury electrode. Voltammetric, coulometric and UV–visible results indicate that their two-electron reduced form ultimately generates the corresponding anions (L¹)^2– and (L²)^2–. The reaction of (L¹)^2– and (L²)^2– with Cu^II trifluoromethanesulfonate gives the corresponding 1 : 1 complexes (Cu^IIL¹1′) and (Cu^IIL²2′). In frozen-chloroform solutions, the latter complexes show well resolved electron paramagnetic resonance (EPR) spectra. The observed hyperfine splittings due to the nuclear spin I= 3/2 of the copper ion allow the determination of the following magnetic parameters: g_∥= 2.304, g_⊥= 2.062, a_∥= 142 × 10^–4 cm^–1, a_⊥= 25.2 × 10^–4 cm^–1 for 1′, and g_∥= 2.277, g_⊥= 2.048, a_∥= 160 × 10^–4 cm^–1, a_⊥= 22.4 × 10^–4 cm^–1 for 2′. The UV–VIS absorption spectra and the results obtained from the experimental and simulated EPR spectra indicate that the complex 1′ has a square-planar geometry whereas 2′ is a square-planar distorted complex.