Synthetic models for biological trinuclear copper clusters. Trinuclear and binuclear complexes derived from an octadentate tetraamine-tetrabenzimidazole ligand

The new tetraamino-tetratbenzimidazole ligand N,N'-bis{bis[3-(1-methyl-2-benzimidazolyl)-propyl] amino}piperazine (L) has been synthesised together with a series of trinuclear and binuclear complexes. Two terminal binding sites with tridentate linkages (A sites) and one central binding site with the bidentate piperazine residue ( B site) are used by the ligand to bind divalent metal centres in the trinuclear complexes [ (Cu3L)-L-II] (6 +), [ (Cu2ZnL)-Zn-II-L-II] (6 +) and [Cu-II,(CoL)-L-II](6+). In the binuclear complex [(Cu2L)-L-II](4+) each nitrogen donor of the piperazine ring acts as an axial ligand for the two coppers bound to A sites, but these piperazine donors can be easily displaced by protonation to form the [Cu-2(II)(LH2)](6+) species. The structure of this protonated complex has been determined by X-ray analysis. The crystals of composition [ Cu-2(II)(LH2) (CH3CN)(4)] [ClO4](6). 2H(2)O . 3CH(3)CN belong to the monoclinic system, space group P2(1)/n with cell parameters a= 10.661(3), b=23.014(3), c=17.217(4) Angstrom, beta=96.58(2)degrees,Z=2. The cation is arranged on a crystallographic symmetry centre, which is located at the middle of the protonated piperazine ring. The protonation at the piperazine N atoms is supported by the total charge of the cation and by the analysis of the difference Fourier map. The copper ions are five-coordinated, with ligation of the two benzimidazole residues and the tertiary N donor in the basal plane of a distorted square pyramid Two CH,CN molecules, one at the basal, the other at the apical position, complete the coordination polyhedron. The centrosymmetric spacer, between the tertiary N atoms, deviates from the higher C-2h symmetry, so that the two approximately planar N-(CH2)(3) groupings lie in two parallel planes. Complexes containing reduced copper ions, [(Cu2L)-L-I] (2+) and [(Cu2CuL)-Cu-I-L-II](4+), have also been obtained, but these ions do not bind to the piperazine B site which can only be used to coordinate divalent metal ions. The complexes containing Cu(II) centres exhibit EPR signals indicative of mononuclear species with tetragonal symmetry. The different coordination environment of [(Cu2L)-L-II] (4 +) with respect to [(Cu2ZnL)-Zn-II-L-II] (6+), [(Cu2CoL)-Co-II-L-II](6+) or [Cu-2(II)(LH2)](6+) is reflected by a difference in the magnetic parameters of the complexes. The EPR spectrum of [(Cu3L)-L-II](6+) is very similar to those of [(Cu2ZnL)-Zn-II-L-II](6+) and [(Cu2CoL)-Co-II-L-II](6+), typical for Cu(II)-A sites, but the integrated intensity accounts for only about 2.2 paramagnetic centres per molecule. It is likely that a dipolar interaction between one of the Cu(II)-A centres and the Cu(II)-B centre produces severe broadening of the corresponding signals and apparent reduction in the overall EPR intensity. Voltammetric data in acetonitrile solution exhibit quasi-reversible electron transfer for the Cu(II)/Cu(I) couples with estimated reduction potentials in the range 0.39-0.56 V versus NHE. The voltammogram of [(Cu3L)-L-II](6+) run at low concentration and sweep rate shows that the three-electron transfer is split into one-electron and two-electron steps. Binding experiments show that the complexes bind azide molecules in the terminal mode to the copper(II) centres with affinity constants decreasing in the series: [(Cu3L)-L-II](6+) > [(Cu2ZnL)-Zn-II-L-II](6+) > [(Cu2L)-L-II](4+). The complexes derived from L are catalytically active in the air oxidation of di-tert-butylcatechol (DTBC). The oxidations are biphasic, with a fast initial stoichiometric phase corresponding to reduction of a pair of copper(II) centres and oxidation of DTBC to quinone followed by the catalytic reaction. The catalytic reaction follows substrate saturation behaviour, with kinetic constants decreasing in the order: [(Cu3L)-L-II](6+) > [(Cu2CoL)-Co-II-L-II] (6+) > [(Cu2ZnL)-Zn-II-L-II](6+) approximate to [(Cu2L)-L-II]( 4+). Anaerobic experiments show that the two-electron oxidation of DTBC involves reduction of one Cu(II)-A site and the Cu(II)-B site for [(Cu3L)-L-II](6+), both the Cu(II)-A sites of [(Cu2ZnL)-Zn-II-L-II](6+) and [(Cu2L)-L-II](4+), but one Cu(II)-A and co(III) in the case of the mixed [(Cu2CoL)-Co-II-L-II](6+) complex, since the Co(II) ion is rapidly oxidised to Co(III) in the conditions in which the catalytic reaction is carried out. (C) 1998 Elsevier Science S.A. All rights reserved.