A series of dinuclear copper(II) complexes has been synthesized with the aim to investigate their applicability as potential structure and function models for the active site of catechol oxidase enzyme. They have been characterized by routine physicochemical techniques as well as by X-ray single-crystal structure analysis: [Cu2(H2L2 2)(OH)(H2O)(NO3)](NO3) 3·2H2O (1), [Cu(HL14)(H 2O)(NO3)]2(NO3)2· 2H2O (2), [Cu(L11)(H2O)(NO3)] 2 (3), [Cu2(L23)(OH)(H2O) 2](NO3)2, (4) and [Cu2(L2 1)(N3)3] (5) [L1 = 2-formyl4-methyl-6R- iminomethyl-phenolato and L2 = 2,6-bis(R-iminomethyl)-4-methyl-phenolato; for L11 and L21, R = N-propylmorpholine; for L22, R = N-ethylpiperazine; for L23, R = N-ethylpyrrolidine, and for L14, R = N-ethylmorpholine]. Dinuclear 1 and 4 possess two "end-off" compartmental ligands with exogenous μ-hydroxido and endogenous μ-phenoxido groups leading to intermetallic distances of 2.9794(15) and 2.9435(9) Å, respectively; 2 and 3 are formed by two tridentate compartmental ligands where the copper centers are connected by endogenous phenoxido bridges with Cu-Cu separations of 3.0213(13) and 3.0152(15) Å, respectively; 5 is built by an end-off compartmental ligand having exogenous μ-azido and endogenous μ-phenoxido groups with a Cu-Cu distance of 3.133(2) Å (mean of two independent molecules). The catecholase activity of all of the complexes has been investigated in acetonitrile and methanol medium by UV-vis spectrophotometric study using 3,5-di-tert- butylcatechol (3,5-DTBC) and tetrachlorocatechol (TCC) as substrates. In acetonitrile medium, the conversion of 3,5-DTBC to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) catalyzed by 1-5 is observed to proceed via the formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically for the first time. In methanol medium no such enzyme-substrate adduct has been detected, and the 3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by 1-5 very efficiently. The substrate TCC forms an adduct with 2-5 without performing further oxidation to TCQ due to the high reduction potential of TCC (in comparison with 3,5-DTBC). But most interestingly, 1 is observed to be effective even in TCC oxidation, a process never reported earlier. Kinetic experiments have been performed to determine initial rate of reactions (3,5-DTBC as substrate, in methanol medium) and the activity sequence is 1 > 5 > 2 = 4 > 3. A treatment on the basis of Michaelis-Menten model has been applied for kinetic study, suggesting that all five complexes exhibit very high turnover number, especially 1, which exhibits turnover number or Kcat of 3.24 × 104 (h-1), which is ∼3.5 times higher than the most efficient catalyst reported to date for catecholase activity in methanol medium. © 2008 American Chemical Society.