Macrocyclization of cyclic thiosemicarbazones with mercury s M. Ara Ânzazu Blanco, a Elena Lo Âpez-Torres, a M. Antonia Mendiola, a,p Ernesto Brunet b and M. Teresa Sevilla c a Departamento de Quõ Âmica Inorga Ânica, Universidad Auto Ânoma de Madrid, Cantoblanco, 28049Madrid, Spain b Departamento de Quõ Âmica Orga Ânica, Universidad Auto Ânoma de Madrid, Cantoblanco, 28049Madrid, Spain c Departamento de Quõ Âmica Analõ Âtica y Ana Âlisis Instrumental, Universidad Auto Ânoma de Madrid, Cantoblanco, 28049Madrid, Spain Received 25 May 2001; revised 22 October 2001; accepted 17 December 2001 AbstractÐA new cyclic Schiff base L 1 H 3 derived from benzil and thiosemicarbazide has been prepared in the presence of NaBH 4 . Reactions of the new molecule and the cyclic 5-methoxy-5,6-diphenyl-4,5-dihydro-2H-[1,2,4]triazine-3-thione L 2 H 2 with mercury chloride and nitrate are reported. Complexes of 1:2 stoichiometry are obtained from L 1 H 3 , but the reactions from L 2 H 2 yield complexes of the macrocyclic Schiff base 5,6,11,12-tetraphenyl-1,2,4,7,8,10-hexaaza-cyclododeca-4,6,10,12-tetraene-3,9-dithione L 3 H 2 , which indicates the macrocyclization of L 2 H 2 . A mechanism for the cyclization reaction based on the rupture of the C±N single bond in L 2 H 2 is proposed. The electrochemical results of complex L 3 Hg makes it and its precursors, L 2 H 2 and L 3 H 2 , outstanding candidates for mercury determination by appropriate elect modi®cation. q 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Schiff base macroligands derived from thiosemicarbazide and theircomplexes are of signi®cant interest for their pharmacological properties as antibacterial and anticancer agents. 1±8 Macrocyclic ligands allow selective complex- ation and extraction of metalliccationsand anionsof environmental importance. 9±12 In particular, the accidental liberation of mercury in the environment poses dreadful toxicity problems, but only recent studies have considered the reactivity of macroligands containing sulfur to entrap this metal. 13±18 In the last few years, a great deal of research has been aimed to design compounds highly selective and sensitive to heavy metal ions, in order to detect, and if possible, remove them. Among the many different strategies proposed, the prepa- ration of electrochemical and ¯uorescent sensors should be highlighted. 19±21 The number and relative position of the donor atoms and the cavity size in macrocyclic compounds conferthese molecules with special reactivity. From speci®c dicarbonyl and diamine precursors, the structure of the condensation productcan be conditioned by controlling the reaction conditions. Thus,[111], [112] and [212]condensation products, open chain and cyclic structures can be obtained by selecting the appropriate solvent, pH, temperature and the presence of metal ions.Metalsalts have been used as templates to yield macrocyclic Schiff bases from diamine and dicarbonyl precursors. 22,23 In these cases the size of the metal, its charge, acidic character and favorable geometri have to be considered. In previous papers, we have reported the preparation of several macroligands derived from thiosemicarbazide and benzil. 24,25 The obtained results showed the importance of the reactionconditionsto get differentcondensation products: open chain, cyclic L 2 H 2 and macrocyclic L 3 H 2 compounds. We havealso studiedtheir complexation reactions with transition metals ions, in order to know the structural features of the formed complexes and their pote tial use in determination of metals. 26±29 Some of the studied coppercomplexes have been used for the preparation of sensitive potentiometric sensors for this metal. 30 The aim of this work is to get hydrogenated molecules from the sameprecursors, which would permitto obtainnew ligands with functionalized pendant arms by subsequent reactions, and to study the interaction of the obtained new molecules and the previously published [111] and [212] condensation products, 25,27 (see Scheme 1) with mercury chloride and nitrate to determine their conduct towardsthis toxicmetal.The compoundshavebeen characterized by elemental analysis, mass spectrometry, IR and 1 H and 13 C NMR spectroscopy. We also explored the electrochemical behavior of the complexes by cyclic voltammetry in order to check their potentiality as sensors for mercury. Tetrahedron 58 (2002) 1525±1531 Pergamon TETRAHEDRON 0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0040-402 0(02)00016-9 Keywords: thiosemicarbazones; mercury complexes; nitrogen macrocycles; templates. p Corresponding author. Tel.:134-91-397-4844; fax:134-91-397-4833; e-mail: antonia.mendiola@uam.es