pH-controlled change of the metal coordination in a dicopper(II) complex of the ligand H-BPMP: Crystal structures, magnetic properties, and catecholase activity

The dinucleating ligand 2,6-bis[(bis(2-pyridylmethyl)amino)methy]-4-methylp henol (H-BPMP) has been used to synthesize the three dinuclear Cu(II) compl exes [Cu-2(BPMP)(OH)][ClO4](2). 0.5C(4)H(8)O (1), [Cu-2(BPMP)(H2O)2](ClO4)( 3). 4H(2)O (2), and [Cu-2(H-BPMP)][(ClO4)(4)]. 2CH(3)CN (3). X-ray diffract ion studies reveal that 1 is a mu-hydroxo, mu-phenoxo complex, 2 a diaqua, mu-phenoxo complex, and 3 a binuclear complex with Cu-Cu distances of 2.96, 4.32, and 6.92 Angstrom, respectively. Magnetization measurements reveal t hat 1 is moderately antiferromagnetically coupled while 2 and 3 are essenti ally uncoupled. The electronic spectra in acetonitrile or in water solution s give results in accordance with the solid-state structures. 1 is EPR-sile nt, in agreement with the antiferromagnetic coupling between the two copper atoms. The X-band spectrum of powdered 2 is consistent with a tetragonally elongated square pyramid geometry around the Cu(II) ions, in accordance wi th the solid-state structure, while the spectrum in frozen solution suggest s a change in the coordination geometry. The EPR spectra of 3 corroborate t he solid-state and UV-visible studies. The H-1 NMR spectra also lead to obs ervations in accordance with the conclusions from other spectroscopies. The electrochemical behavior of 1 and 2 in acetonitrile or in water solutions shows that the first reduction (Cu(II)Cu(II)-Cu(II)Cu(I) redox couple) is r eversible and the second (formation of Cu(I)Cu(I)) irreversible. In water, 1 and 2 are reversibly interconverted upon acid/base titration (pK 4.95). I n basic medium a new species, 4, is reversibly formed (pK 12.0), identified as the bishydroxo complex. Only 1 exhibits catecholase activity (oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone, v(max) = 1.1 x 10(-6) M-1 s(-1) and K-M = 1.49 mM). The results indicate that the pH depen dence of the catalytic abilities of the complexes is related to changes in the coordination sphere of the metal centers.