Catecholate adducts of binuclear copper complexes modelling the type 3 copper active site - Spectroscopic characterization and relevance to the tyrosinase reaction

The p-nitrochatecholate (NCat) adducts 2 and 4 of the two copper(H) complexes [Cu-2(L66)](ClO4)(4) (1; L66 = alpha,alpha'-bis{bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino}-m-xylene) and [Cu(L6)](ClO4)(2) (3; L6 = N,N-bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amine), respectively, have been prepared and investigated by infrared, Raman and electronic absorption spectroscopies. The combined spectroscopic information suggests that the binding mode of p-nitrocatecholate in 2 and 4 is similar for both adducts although for catecholate complexes of mononuclear Cu complexes like 4 an eta(2) coordination applies and for binuclear copper sites like 2 an eta(1):eta(1) bridging modes has been postulated. In addition a double catecholate adduct of 1, [Cu-2(L66)(NCat)(2)] (5), has been prepared which again shows spectral features similar to 1 and 3. Based on these results it is suggested in all cases catecholate is binding in an eta(2) mode to one copper atom, eventually exhibiting an additional eta(1) bridging coordination to a second copper atom. This arrangement also plays an important role in inhibitor complexes of type 3 copper proteins like tyrosinase and catechol oxidase. The relevance of the eta(1):eta(2) binding mode of catecholate to the mechanism of the tyrosinase reaction is discussed.