Tuning the Activity of Catechol Oxidase Model Complexes by Geometric Changes of the Dicopper Core

摘要

Dicopper(II) complexes of a series of different pyrazolate-based dinucleating ligands [L~1]~--[L~4]~- have been synthesized and characterized structurally and spectroscopically. A major difference between the four complexes is the individual metal-metal separation that is enforced by the chelating side arms of the pyrazolate ligand scaffold: it varies from 3.45A in 2(centre dot)(BF_4)_4 to 4.53A in 4(centre dot)(ClO_4)_2. All complexes have been evaluated as model systems for the catechol oxidase enzyme by using 3,5-di-tert-butylcatechole (DTBC) as the test substrate. They were shown to exhibit very different catecholase activities ranging form very efficient to poor catalysts (k_(obs) between 2430 +- 202 and 22.8 +- 1.2 h~(-1)), with an order of decreasing activity 2(centre dot)(ClO_4)_4 > 1(centre dot)(ClO_4)_2 > 3(centre dot)(ClO_4)_2 4(centre dot)(ClO_4)_2. A correlation of the catecholase activities with the variation in Cu…Cu distances, as well as other effects resulting from the distinct redox potentials, neighboring groups, and the individual coordination spheres are discussed. Saturation behavior for the rate dependence on substrate concentration was observed in only two cases, that is, for the most active 2(centre dot)(ClO_4)_4 and for the least active 4(centre dot)(ClO_4)_2, whereas a catalytic rate that is almost independent of substrate concentration (within the range studied) was observed for 1(centre dot)(ClO_4)_2 and 3(centre dot)(ClO_4)_2. H_2O_2 was detected as the product of O_2 reduction in the catecholase reaction of the three most active systems. The structures of the adducts of "L~3Cu_2" and "L~4Cu_2" with a substrate analogue (tetrachlorocatecholate, TCC) suggest a bidentate substrate coordination to only one of the copper ions for those catalysts that feature short ligand side arms and correspondingly exhibit larger metal-metal separations; this possibly contributes to the lower activity of these systems. TCC binding is supported by several H-bonding interactions to water molecules at the adjacent copper or to ligand-side-arm N-donors; this emphasizes the importance of functional groups in proximity to the bimetallic active site.