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 Epa=+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 Ipc/Ipa= 0.80. This CuII–CuIII couple shows greater reversibility at 254 K with Epa=+0.91 and Epc=+0.81 V vs.[Fe(η-C5H5)2]-[Fe(η-C5H5)2]+ and Ipc/Ipa= 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+.