EFFECT OF LIGAND DONORS ON THE CATALYTIC PROPERTIES OF METAL-COMPLEXES - COPPER(II) COMPLEXES AS CATALYSTS FOR THE OXIDATION OF 3,5-DI-TERT-BUTYLCATECHOL

Copper(II) complexes 15 with the tetradentate ligand L(1) = imethylpyr azol-1-ylmethyl)-1-hydroxy-2-aminoethane (pzmhe) and 6-10 with the ana logous N3O benzimidazole ligand L(2) = is(2-benzimidazolylmethyl)-1-hy droxy-2-aminoethane (bbmhe) were prepared. The five-coordinate complex es [Cu(L)X] Y-1,Y-2 (with 1, 6: X = H2O, Y = BF4; 2, 7: X = NO3, Y = N O3; 3, 8: X = Br, Y = Br; 4, 9: X = Cl, Y = Cl; 5, 10: X = N-3, Y = BF 4) were characterized by chemical analysis, absorption and EPR spectro scopies and cyclic voltammetry. The data show the complexes have squar e pyramidal geometry. All display chemically reversible one-electron r edox behavior in acetonitrile. The E(o)' values are more positive with L(1) than with L(2) by 144-234 mV. Oxidation of 3,5-di-tert-butylcate chol to 3,5-di-tert-butylquinone by these complexes was studied. The r eaction rate depends on the nature of the heterocyclic donor, its basi city, steric effects, reduction potentials, and the type of exogenous donor present. Large rate variations were observed with the most effec tive catalysts those with pzmhe; the [Cu(pzmhe)N-3]BF4 complex was mos t active with [Cu(bbmhe)Br] Br and [Cu(bbmhe)Cl]Cl complexes relativel y inactive. Electrochemical data showed a non-linear relationship betw een their ability to oxidize catechols and their reduction potentials. The results point to the bulky ligand environment provided by the ben zimidazoles as being responsible for the diminution of their rates of catalysis compared to the pyrazole complexes.