A CuN2S2 core with a high potential copper(II/I) couple. Use of a surface modified electrode and generation of five coordinate copper(I)

• A cationic copper(I) complex with N2S2 coordination sphere is described.
• The Cu(II/I) potential is 0.79 versus NHE at a gold electrode modified by SCN−.
• It reacts with PPh3 to yield a five coordinate PPh3 adduct in solution.
• The ΔH0 of the PPh3 ligation to the copper(I) complex is estimated from 1H NMR.

Using the 1:2 condensate of benzil dihydrazone and 2-(methylthio)benzaldehyde as a ligand (L), a cationic copper(I) complex [CuL]PF6·0.5THF (THF ≡ tetrahydrofuran; 1) is synthesized. The X-ray crystal structure of the desolvated species [CuL]PF6 has been determined and is found to contain a CuN2S2 core. The corresponding copper(II) complex could not be obtained. The cyclic voltammograms of 1 at a Pt or a glassy carbon electrode show features of adsorption, but it shows a quasireversible Cu(II/I) couple at a gold electrode modified by thiocyanate. The redox potential is 0.79 V versus NHE. Complex 1 reacts with PPh3 in CH2Cl2 in 1:1 M proportion and subsequent addition of n-hexane yields [CuL(PPh3)]PF6·0.25n-hexane·4H2O (2). Our bp86/6-311g(2d,p) calculations indicate two plausible structures A and B for the cation in 2. In A, copper(I) is 5-coordinate with a trigonal bipyramidal N2S2P coordination sphere (the P atom and an azine N occupy the apices) and in B, the metal has a tetrahedral N2SP coordination sphere with one of the two S atoms of L remaining uncoordinated. Binding of PPh3 to the metal in [CuL]+ is theoretically exothermic; structure B is energetically more stable than A by 4.5 kcal mol−1. In solution there exists an equilibrium between A and B. From variable temperature 1H NMR of 2, an average ΔH0 value of −2.8 kcal mol−1 is obtained for the equilibrium thus showing that enthalpy wise B is more stable than A which matches well with the DFT results. The equilibrium is not favoured entropically. Existence of the equilibrium is also apparent in the 31P NMR of 2.

A copper(I) complex having an N2S2 coordination sphere shows a Cu(II/I) couple with an E0 of 0.79 V versus NHE in dichloromethane at a gold electrode modified by thiocyanate. The complex reacts with PPh3 to generate a five-coordinate copper(I) species in solution.Figure optionsDownload as PowerPoint slide