Studies on transition-metal macrocyclic complexes. Single-crystal X-ray structure and electrochemistry of the bis-macrocyclic complex [Cu(L)2][ClO4]2·2H2O (L = 1,4,7-triazacyclononane)

Abstract

A structural and electrochemical study of the bis-sandwich complex [Cu(L)₂]²⁺(L = 1,4,7-triazacyclononane) is reported. [Cu(L)₂][ClO₄]₂·2H₂O crystallises in the monoclinic space group Ca(alternative setting for P2₁/a with c axis unique), with a= 9.533(3), b= 16.909(8), c= 13.653(9)Å, γ= 89.22(3)°, and Z= 4. The structure of the complex shown a centrosymmetric CuN₆²⁺ cation with a distorted octahedral copper(II) ion bound to each nitrogen donor atom of the macrocyclic ligands [Cu–N = 2.233(7), 2.176(6), and 2.087(6)Å]. The stereochemistry of [Cu(L)₂]²⁺ is compared to the structures of related complexes of tridentate ligands. Cyclic voltammetry of [Cu(L)₂][ClO₄]₂ in acetonitrile (0.1 mol dm^–3 NBuⁿ₄ClO₄) at platinum electrodes shows an irreversible oxidation at E_pa=+0.95 V vs.[Fe(η-C₅H₅)₂]-[Fe(η-C₅H₅)₂]⁺ at a scan rate of 130 mV s^–1 at 292 K; at an increased scan rate of 1.3 V s^–1, the redox couple becomes more reversible and I_pc/I_pa= 0.80. This Cu^II–Cu^III couple shows greater reversibility at 254 K with E_pa=+0.91 and E_pc=+0.81 V vs.[Fe(η-C₅H₅)₂]-[Fe(η-C₅H₅)₂]⁺ and I_pc/I_pa= 1 at a scan rate of 260 mV s^–1. The electrochemical behaviour of [Cu(L)₂]²⁺ is compared to that of the mono-macrocyclic species [Cu(L)]²⁺.