Elucidating the secondary effect in the Lewis acid mediated anodic shift of electrochemical oxidation of a Cu(ii) complex with a N2O2 donor unsymmetrical ligand

Abstract

A metalloligand [CuL] (H₂L = N-α-methylsalicylidene-N′-salicylidene-1,3-propanediamine) was reacted with a series (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Cd²⁺, Pr³⁺, Nd³⁺ and Sm³⁺) of redox-inactive metal ions (in excess) with different Lewis acidities in acetonitrile to form heterometallic complexes in situ. In the cases of K⁺ and Zn²⁺, single crystals were isolated from their respective solutions and solved. The complexes were found to be trinuclear with the phenoxido bridge between the redox-inactive metals and the copper center having closest equilibrium distance. In solution, some metal ions (Na⁺, K⁺, Mg²⁺ and Ca²⁺) formed predominantly 1 : 1 [CuL] : Mⁿ⁺ adducts whereas others did not. Spectroscopic studies revealed that both the energy and intensity of the LMCT band of [CuL] at 363 nm were influenced by the Lewis acidity of the guest metal ions. The blue shift and the hypochromic shift of this band showed a linear dependence on the Lewis acidity of the corresponding redox-inactive metal ions only for 1 : 1 adducts. A combined EPR and d–d transition spectral analysis of these 1 : 1 adducts in acetonitrile at 298 K indicates that there is a change in the coordination geometry around Cu(II) on proximal cation binding of free [CuL]. The correlation of the half wave potential (E_1/2) of the first oxidation of [CuL] with pK_a of the corresponding metal(aqua)ⁿ⁺ ion as a measure of its Lewis acidity revealed that the potentials were linearly dependent for 1 : 1 adducts whereas in the case of the other metal ions an unexpected deviation from linearity was observed. An incremental addition of water to some of these mixtures revealed a decrease in the corresponding oxidation peak potential with a concomitant increase in the molar absorptivity. The molar absorptivities of different mixtures with their corresponding E_1/2 values show a linear dependence with better correlation suggesting that the modulation of electron density around Cu(II) regulates the electrochemical oxidation of the metalloligand.