FULL PAPER DOI: 10.1002/ejic.200600022 Synthesis, Structures and Catecholase Activity of a New Series of Dicopper(II) Complexes of Reduced Schiff Base Ligands Bellam Sreenivasulu, [a] Fei Zhao, [b] Song Gao, [b] and Jagadese J. Vittal* [a] Keywords: Biomimetic studies / Copper / Reduced Schiff-base complexes / Tridentate ligands / Catecholase activity / Magnetic properties A series of dinuclear Cu II complexes of reduced Schiff bases from substituted salicylaldehydes and amino acids have been synthesized and characterized. They are: [Cu 2 (RScp11) 2 - (H 2 O) 2 ] {H 2 RScp11 = 1-[(2-hydroxy-5-R-benzyl)amino]cyclo- pentane-1-carboxylic acid; R = H (1), Cl (2), CH 3 (3), OH (4)}, [Cu 2 (RSch11) 2 (H 2 O) x ] {H 2 RSch11 = 1-[(2-hydroxy-5-R-ben- zyl)amino]cyclohexane-1-carboxylic acid; R = H and x =1(5), R = Cl and x =2(6), R = CH 3 and x =2(7)}, [Cu 2 (RSch12) 2 - (H 2 O) 2 ] {H 2 RSch12 = 2-[(2-hydroxy-5-R-benzyl)amino]cyclo- hexane-1-carboxylic acid; R = H (8), CH 3 (10) and [Cu 2 (ClSch12) 2 ]·2H 2 O (9)}, [Cu 2 (Diala5) 2 (H 2 O) 2 ]·H 2 O [H 3 - Diala5 = N-(2,5-dihydroxybenzyl)-L-alanine] (11), [Cu 2 - (Diala4) 2 (H 2 O) 2 ]·H 2 O [H 3 Diala4 = N-(2,4-dihydroxybenzyl)- L-alanine] (12), and [Cu 2 (Diala3) 2 (H 2 O) 2 ]·H 2 O [H 3 Diala3 = N- (2,3-dihydroxybenzyl)-L-alanine] (13). They were isolated and characterized by chemical and spectroscopic methods. Single crystal X-ray crystallographic studies have revealed Introduction Among the well-known representatives of Type III cop- per proteins, catechol oxidase with active dicopper(II) sites is a ubiquitous enzyme in living systems for catalyzing the oxidation of a wide range of ortho-diphenols to ortho-diqui- nones. The subsequent auto polymerization of the highly active quinones into polyphenolic catechol melanins is con- sidered to be responsible for the defense mechanism ob- served in plants against pathogens or pests. [1] In fact, the two copper atoms of dicopper(II) bio-active centers present in different metalloenzymes are found to act cooperatively within the proximity of ca. 3.5 Å with each Cu II center co- ordinated by three histidine donors. [2,3] As confirmed by the recent X-ray crystal structure analysis, catechol oxidase in the met oxidized form contains dicopper centers with a Cu···Cu distance of 2.9 Å. [3] Modeling the features of bio- [a] Department of Chemistry, National University of Singapore, Science Drive 3, Singapore 117543 Fax: +65-6779-1691 E-mail: chmjjv@nus.edu.sg [b] College of Chemistry and Molecular Engineering, Peking Uni- versity, Beijing 100871, P. R. China Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2006, 2656–2670 2656 that [Cu 2 (Scp11) 2 (MeOH) 2 ] (1a), [Cu 2 (ClScp11) 2 (DMF)- (H 2 O)]·MeCN (2a), [Cu 2 (MeScp11) 2 (MeOH) 2 ]·2 MeOH (3a), [Cu 2 (ClSch11) 2 (MeOH) 2 ]·2 MeOH (6a), [Cu 2 (ClSch12) 2 ]· 2 MeOH (9a), and [Cu 2 (Diala4) 2 (DMSO) 2 ]·2 DMSO·2 acetone (12a) have 1D hydrogen-bonded polymeric structures while 4 has a 3D hydrogen-bonded network structure. Complex 8 displays a 2D coordination polymeric network structure. The complexes 1–13 have been investigated as functional models for the catechol oxidase by employing 3,5-di-tert-butylcate- chol as a model substrate. Electron-withdrawing substituents reduced the activity while electron-donating substituents en- hanced the activity. Variable-temperature magnetic studies conducted on compound 8 suggest the presence of strong inter-dimer antiferromagnetic coupling. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) relevant dinuclear active sites through synthetic analogues basically involves the judicious design of binucleating li- gands to meet the conditions such as bridging mode con- trolling the metal–metal distance, steric-electronic proper- ties, and geometry around the metal centers. [4] Conse- quently, dinuclear copper(II) complexes of the binucleating ligands have been extensively documented as bimetallic cat- alysts due to their potential ability to mimic the functions of the so-called catechol oxidase. [2,4–5] Our research group has been interested in the coordina- tion chemistry of reduced Schiff-base ligands, N-(2-hy- droxybenzyl)-amino acids, which have the potential to form dinuclear complexes through a bridging phenolate group. In addition, these ligands can also afford the choice of coor- dination environments that determine the nature of metal ions that can be bound within the closest proximity. Apart from the studies illustrating interesting solid-state supramo- lecular transformation of Cu II and Zn II complexes, [6] and the novel helical stair-case structure in the Ni II complex, [7] our research group has reported several dicopper(II) com- plexes derived from reduced Schiff bases as functional mod- els for catechol oxidase. [8] Very recently, we have also dem- onstrated that the dicopper(II) complexes containing the weakly coordinating sulfonate donor group are more active