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 t ogether with a series of trinuclear and binuclear complexes. Two termi nal binding sites with tridentate linkages (A sites) and one central b inding site with the bidentate piperazine residue ( B site) are used b y the ligand to bind divalent metal centres in the trinuclear complexe s [ (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 do nor 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 symm etry centre, which is located at the middle of the protonated piperazi ne ring. The protonation at the piperazine N atoms is supported by the total charge of the cation and by the analysis of the difference Four ier map. The copper ions are five-coordinated, with ligation of the tw o benzimidazole residues and the tertiary N donor in the basal plane o f a distorted square pyramid Two CH,CN molecules, one at the basal, th e other at the apical position, complete the coordination polyhedron. The centrosymmetric spacer, between the tertiary N atoms, deviates fro m the higher C-2h symmetry, so that the two approximately planar N-(CH 2)(3) groupings lie in two parallel planes. Complexes containing reduc ed copper ions, [(Cu2L)-L-I] (2+) and [(Cu2CuL)-Cu-I-L-II](4+), have a lso been obtained, but these ions do not bind to the piperazine B site which can only be used to coordinate divalent metal ions. The complex es containing Cu(II) centres exhibit EPR signals indicative of mononuc lear species with tetragonal symmetry. The different coordination envi ronment 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 spec trum 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, b ut the integrated intensity accounts for only about 2.2 paramagnetic c entres per molecule. It is likely that a dipolar interaction between o ne of the Cu(II)-A centres and the Cu(II)-B centre produces severe bro adening of the corresponding signals and apparent reduction in the ove rall 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. Th e voltammogram of [(Cu3L)-L-II](6+) run at low concentration and sweep rate shows that the three-electron transfer is split into one-electro n and two-electron steps. Binding experiments show that the complexes bind azide molecules in the terminal mode to the copper(II) centres wi th affinity constants decreasing in the series: [(Cu3L)-L-II](6+) > [( Cu2ZnL)-Zn-II-L-II](6+) > [(Cu2L)-L-II](4+). The complexes derived fro m L are catalytically active in the air oxidation of di-tert-butylcate chol (DTBC). The oxidations are biphasic, with a fast initial stoichio metric phase corresponding to reduction of a pair of copper(II) centre s and oxidation of DTBC to quinone followed by the catalytic reaction. The catalytic reaction follows substrate saturation behaviour, with k inetic 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-I I](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 catalyti c reaction is carried out. (C) 1998 Elsevier Science S.A. All rights r eserved.