PREPARATION, STRUCTURE AND PROPERTIES OF A COPPER(II) COMPLEX WITH A NEW TRIPODAL TETRADENTATE LIGAND, -PYRIDYL)METHYL)((5-CARBOXY-2-PYRIDYL)METHYL)AMINE (BPCA), AND REACTION OF ITS CU(I) COMPLEX WITH DIOXYGEN

A copper complex with a new tripodal tetradentate ligand, pyridyl)meth yl}{(5-carboxyl-2-pyridyl)methyl}amine (BPCA), has been prepared as a model metal centre of copper enzymes that react with dioxygen, the beh aviour of which in solution has been examined on the basis of absorpti on and ESR spectra. Addition of NaN3 to an H2O/CH3OH solution containi ng Cu(ClO4)(2). 6H(2)O-BPCA with 1:1 molar ratio has afforded the comp lex [Cu(bpca)(N-3)]ClO4. It crystallizes in the monoclinic space group P2/a, a = 17.478(3), b = 9.773(1), c = 22.388(5) Angstrom, beta = 102 .92(2)degrees, Z = 4. The crystal structure reveals that the coordinat ion around the central metal ion forms a trigonal bipyramid with three pyridine nitrogen atoms of BPCA (Cu-N(2) = 2.01(2), Cu-N(3) = 2.09(1) and Cu-N(4) = 2.22(1) Angstrom) in the trigonal plane and with a tert iary amino nitrogen atom of BPCA (Cu-N(1) = 1.99(1) Angstrom) and an a zide anion (Cu-N(7) = 1.96(2) Angstrom) in axial positions. The azide ion is coordinated in an end-on configuration, the coordinating nitrog en of which forms hydrogen bonds with two amide NH groups of BPCA; the terminal nitrogen is weakly bound with the carboxyl group of BPCA by hydrogen bonds through a water molecule. Cyclic voltammetry of an acet onitrile solution containing Cu(ClO4)(2). 6H(2)O-BPCA-KCl with 1:1:1 m olar ratio exhibits reversible one-electron redox behaviour with E-1/2 = +0.24 V vs NHE, indicating that the BPCA ligand can stabilize the l ower oxidation state of copper. Reaction of [Cu-I(bpca)]ClO4 with diox ygen in methanol or propionitrile at -78 degrees C gives marked absorp tion changes which suggest the formation of a Cu-O-2 complex.