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.

Graphical abstractA 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 full-size imageDownload as PowerPoint slide