Synthesis, characterization and biomimetic oxygenations of manganese(II), iron(III) and copper(II) pyridyl hydrazone com- plexes Abd El-Mottaleb M. Ramadan* and Ibrahim M. El-Mehasseb Chemistry Department, Faculty of Education, Tanta University, Kafr El-Sheikh, Egypt Summary The preparation and characterization of Mn II , Fe III and Cu II complexes of three tridentate pyridyl hydrazones are reported. The ligands were prepared via Schi base condensation of 6-chloro-2-hydrazopyridine with a- formyl-(L 1 ), a-acetyl-(L 2 ), or a-benzoyl-(L 3 ) pyridine. The structural characterization of the compounds prepared was based on elemental analyses, electrical conductance and magnetic moment measurements, 1 H- n.m.r., i.r., u.v.±vis. and e.s.r spectroscopic methods. The overall structure and reactivity of the metal chelates critically depend on the ligand substituents within the carbonyl moiety. Octahedral and tetrahedral mono- meric species were proposed for Mn II complexes, and an octahedral environment for the Fe III complexes. Re- garding the copper(II) complexes, a monomeric square- planar and a dimeric structure with a chloride bridge in square-pyramidal geometry were suggested. In the presence of molecular oxygen, Mn II and Cu II complexes catalyse the oxidative transformation of catechol (ben- zene-1,2-diol) to the corresponding o-benzoquinone. Iron(III) complexes catalyse the aerobic oxidation of catechol to the intradiol cleavage product. The catalytic activity has been correlated with the Lewis acidity of the metal centres created according to the nature of the ligand substituents. The probable mechanistic implica- tions of the catalysed oxidation reactions are discussed. Introduction Oxygenase enzymes catalyse reactions of molecular oxy- gen with organic substrates in which oxygen atoms from dioxygen are incorporated into the ®nal oxidized prod- uct (1) . Dioxygenase enzymes, which direct both atoms of O 2 into the product, are known that contain haeme iron, nonhaeme iron, copper or manganese (2,3) . Intradiol catechol dioxygenases catalyse the degradation of cat- echol, and its derivatives, to give cis, cis-muconic acids via a mechanism involving a high-spin iron(III) centre (4) . The two best-characterized members of this class are the nonhaeme iron, catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD). The e.p.r. spectra of (CTD) and (PCD) show a resonance at g  4:3, characteristic of high-spin iron(III) in a so- called rhombic (low symmetry) environment (4) and the MoÈssbauer parameters are also characteristic of high- spin iron(III). X-ray crystallography and spectroscopic studies demonstrated that the iron(III) centre is bound to two histidine and two tyrosine ligands, the ®fth ligand being H 2 O (4) . Several iron(III) complexes have been designed as models of catechol 1,2-dioxygenase and extensively studied (5) . Based on the spectroscopic in- vestigations during the catalytic oxidative cleavage of catechol to cis, cis-muconic acid, a substrate rather than a dioxygen activation mechanism was proposed by Que and Cox (6) . Such studies have led also to the conclusion that the product yields and reactivity of the synthetic models depend on the Lewis-acidity of the central iron(III). The copper-containing monooxygenase tyrosinase catalyses the ortho hydroxylation of phenols to give catechols with possible subsequent oxidation to o-quin- ones (7) . The oxidative transformation of catechol or its derivatives to light-absorbing o-quinones catalysed by copper enzymes is clearly visible after their polymer- ization to melanins, which range from red to dark brown (8) . Model studies of the oxidation of phenolic substrates with copper complexes have generally re- vealed that the rate of oxidation of catechol to o-ben- zoquinone is dependent on the basicity of donating, nitrogen-containing ligands (9) . Therefore, structural and electronic properties of copper(II) complexes that mimic metalloproteins are of interest. Studies on mangan- ese(II) complexes that catalyse oxidative transformation of catechols to o-quinones are scare (10) . To obtain functional oxygenase models it is necessary to have free or exchangeable cis-coordination sites on the metal ion for the intermediate adduct formation during the catalytic oxidation cycle. Therefore, metal chelates that incorporate good leaving groups, e.g.H 2 O, Cl, etc., and ligands with a low number of donor atoms seem to be of great promise. Owing to the above reasons a new type of tridentate ligand has been synthesized, which is shown in Figure 1. The aim of this work was to synthesize and charac- terize manganese(II), iron(III) and copper(II) pyridyl hydrazone complexes as structural and functional models for catechol 1,2-dioxygenase and catechol oxid- ase. Furthermore, investigating the eect of electron donating substituents on the Schi base linkage would provide further information pertinent to an improved understanding of structure versus spectra correlation and reactivity of the reported synthetic models. 0340±4285 Ó 1998 Chapman & Hall Figure 1. Pyridyl hydrazones. Transition Met. Chem., 23, 183±189 (1998) Biomimetic oxygenations of Mn II , Fe III and Cu II complexes 183 * Author to whom all correspondence should be directed.