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

Reaction of the hydrobromide salt of ,3,5-tris(1,4,7-triazacyclonon-1- ylmethyl)benzene, L.9HBr, with copper(II) nitrate followed by cation-e xchange chromatographic purification afforded [Cu3L(H2O)(6)][ClO4](6) . 6H(2)O 1. The ESR and magnetic susceptibility data indicated that th e complex consists of three identical non-interacting copper(II) centr es. Reaction of 1 with a phosphate source produces {[Cu3L(mu-OH)(mu(3) -HPO4)(H2O)][PF6](3) . 3H(2)O}(n) 2 the polymeric lattice of which con tains trinuclear copper(II) sites with structural similarities to lacc ase (Lc) and ascorbate oxidase (AO). These trinuclear sites consist of two type 3 copper(II) centres, at a separation of 3.557(4) A, linked by an hydroxo bridge and two phosphate oxygens while another phosphate oxygen links these two centres to the further removed type 2 copper(I I) centre, establishing separations of 4.561(4) and 5.474(4) Angstrom. The magnetic properties of 2 were investigated in the temperature ran ge 4.2-300 K and they revealed an S = 1/2 molecular ground state arisi ng from antiferromagnetic coupling. A number of models have been emplo yed in order quantitatively to fit the mu(eff) versus temperature data including those applicable to the resting oxidised state of laccase a nd ascorbate oxidase, vis-a-vis dimer plus uncoupled monomer, but the best fits were obtained using a symmetrical trinuclear approximation w ith J(12) for the doubly bridged [CuL(mu-OH)(mu-HPO4)Cu] moiety of -53 or ca. -80 cm(-1) combined respectively with J for the two equal thre e-atom phosphato bridges of -90 or -77 to -90 cm(-1). The 77 K solid-s tate ESR spectrum of 2 is unusual for S = 1/2 ground-state systems and shows six components of a probable seven-line copper hyperfine multip let 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 superim position of S = 1/2 signals from both molecular doublets. The 77 K sol ution ESR spectrum (dimethylformamide, water-glycol 1:1) for 2 is typi cal of a monomeric copper(II) centre and closely resembles the ESR spe ctrum of the type 2 site in Lc and AO. The ESR, electrospray mass spec trometric and NMR data indicate that 2 dissociates in solution to give a trinuclear unit consisting of two type 3 copper(II) centres (ESR si lent), which are linked by the hydroxo and phosphate group (phosphate is released from the complex only on addition of acid), and an isolate d type 2 copper(II) centre which is probably responsible for the ESR f eatures.