Abstract:
Iron complexes of tetra-amido macrocyclic ligand (TAML) systems have previously been developed to act as catalysts for oxidations with hydrogen peroxide. In an extension of this work it is now reported that metallation of the TAML ligand 11,12-benzo-2,5,6,9-tetraoxo3,3,8,8-tetramethyl-1,4,7,10-tetraazacyclododecane (H4B J ) with Cu(OAc)2 using potassium tbutoxide as a base produces [NEt4][CuBJ]. Full characterization of this product, including by an X-ray crystal structure determination, revealed a square planar geometry for the copper centre, with the copper atom in a +3 oxidation state. The complex showed minimal ability to catalyse the oxidation of orange II dye by hydrogen peroxide, unlike iron or cobalt derivatives of this ligand. Iron complexes of both the H4BJ and related H4B* ligand (3,4,8,9-tetrahydro-3,3,6,6,9-hexamethyl-1H-1,4,8,11-benzotetraazocyclotridecane-2,5,7,10-(6H, 11H)-tetrone) have previously been reported to be susceptible to degradation, especially in the presence of hydrogen peroxide but when a substrate is absent, or to a lesser extent when stored in buffer solutions for extended periods. A potential method for preventing degradation under these conditions is by adsorption onto an activated carbon solid support. Preliminary tests by collaborators at Carnegie Mellon University suggested that the iron TAML catalyst FeB*remained active for weeks in the presence of hydrogen peroxide when adsorbed onto an active carbon that was derived from coconut husks. However, subsequent careful investigation of this work revealed that the high bleach rates were due to activated carbon alone catalysing the large majority of this bleaching combined with hydrogen peroxide accumulation in the buffer solution under the conditions used. Modification of the original method to prevent this showed some initial acceleration of bleaching, however after 7 hours in buffer solution containing hydrogen peroxide there was no significant difference between the catalyst and blank activated carbon samples for both the Na2[FeB*(Cl)] and K2[FeBJ(Cl)] catalysts. The further characteristic of the TAML complexes studied was their ability to activate oxidation reactions with hypochlorite. All catalysts studied showed a maximum rate of reactivity at pH 7, with initial rates of 2.637 x 10-8 mol L-1 s -1, 2.781 x 10-7 mol L-1 s -1, 2.140 x 10-6 mol L-1 s -1, and 1.634 x 10-7 mol L-1 s -1, for the K2[FeBJ(Cl)], Na2[FeB*(Cl)],[NEt4][CoBJ], and [NEt4][CuBJ] complexes respectively. On the basis of the kinetic data obtained, a possible mechanism for oxidations with hypochlorite was postulated for the cobalt and copper catalysts. As the reaction rates for both were dependent on the dye and hypochlorite concentrations, a “two-substrate” mechanism was proposed. The data indicated this most likely involved a sequential binding mechanism involving a ternary intermediate complex.