Synthesis, structure and metal redox of a family of copper complexes derived from hexadentate ligands incorporating thioether and triazene 1-oxide functions

Abstract

Hexadentate ligands of type RN(O)NN(H)C₆H₄S(CH₂)_xSC₆H₄(H)NN(O)NR (H₂L¹: R = Me, x = 2; H₂L²: R = Ph, x = 2 ; H₂L³: R = Me, x = 3 ; general abbreviation H₂L) have afforded copper(II) complexes of type [CuL]. The X-ray structure of [CuL²] ·1/2CH₂Cl₂ reveals the presence of a distorted CUS₂N₂O₂ coordination sphere which is strongly elongated along an OCuS axis. The axial and equatorial bond lengths are: Cu---S, 2.636(5) and 2.473(6) A Cu---0, 2.222(10) and 2.039(11) Å respectively. Both the Cu---N bonds, 1.939(13) and 1.904(12) Å lie on the equatorial plane. The structure is compared with that of [ZnL²] · 1/2CH₂Cl₂, which does not display the above-noted axial elongation. The findings are rationalized in terms of simple angular overlap considerations. The EPR spectrum of [CuL²] · 1/2CH₂Cl₂ has g_|; > g_⊥, showing that the hole lies in the d_x2_-y2 orbital. The [CuL] complexes are electroactive in dichloromethane solutions and successive quasireversible responses due to the [Cu^IIL]/[Cu^IL]^- and [Cu^IIIL]⁺/[Cu^IIL] couples are observed, the E_1/2 values being ~-0.9 V and ~0.8 V vs SCE, respectively. The E_1/2 of the copper(III---copper(II) copple lies close to that of the corresponding nickel (III)---nickel(II) couple of [NiL]. A thermodynamic rationale is provided.