Towards synthetic models for trinuclear copper active sites of ascorbate oxidase and laccase: self-assembly, crystal structure and magnetic properties of the copper(II) complexes of 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene†

Abstract

Reaction of the hydrobromide salt of 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene, L·9HBr, with copper(II) nitrate followed by cation-exchange chromatographic purification afforded [Cu₃L(H₂O)₆][ClO₄]₆·6H₂O 1. The ESR and magnetic susceptibility data indicated that the complex consists of three identical non-interacting copper(II) centres. Reaction of 1 with a phosphate source produces {[Cu₃L(µ-OH)(µ₃-HPO₄)(H₂O)][PF₆]₃·3H₂O}_n2 the polymeric lattice of which contains trinuclear copper(II) sites with structural similarities to laccase (Lc) and ascorbate oxidase (AO). These trinuclear sites consist of two type 3 copper(II) centres, at a separation of 3.557(4) Å, linked by an hydroxo bridge and two phosphate oxygens while another phosphate oxygen links these two centres to the further removed type 2 copper(II) centre, establishing separations of 4.561(4) and 5.474(4) Å. The magnetic properties of 2 were investigated in the temperature range 4.2–300 K and they revealed an S = ½ molecular ground state arising from antiferromagnetic coupling. A number of models have been employed in order quantitatively to fit the µ_effversus temperature data including those applicable to the resting oxidised state of laccase and ascorbate oxidase, vis-à-vis dimer plus uncoupled monomer, but the best fits were obtained using a symmetrical trinuclear approximation with J₁₂ for the doubly bridged [CuL(µ-OH)(µ-HPO₄)Cu] moiety of –53 or ca. –80 cm^–1 combined respectively with J for the two equal three-atom phosphato bridges of –90 or –77 to –90 cm^–1. The 77 K solid-state ESR spectrum of 2 is unusual for S = ½ ground-state systems and shows six components of a probable seven-line copper hyperfine multiplet between 2500 and 3100 G and a strong x, y resonance at ca. 3200 G, most probably due to weak copper(II) pair interactions or the superimposition of S = ½ signals from both molecular doublets. The 77 K solution ESR spectrum (dimethylformamide, water–glycol 1∶1) for 2 is typical of a monomeric copper(II) centre and closely resembles the ESR spectrum of the type 2 site in Lc and AO. The ESR, electrospray mass spectrometric and NMR data indicate that 2 dissociates in solution to give a trinuclear unit consisting of two type 3 copper(II) centres (ESR silent), which are linked by the hydroxo and phosphate group (phosphate is released from the complex only on addition of acid), and an isolated type 2 copper(II) centre which is probably responsible for the ESR features.