Dalton Transactions Dynamic Article Links Cite this: Dalton Trans., 2012, 41, 2500 www.rsc.org/dalton PAPER Coordination versatility of tridentate pyridyl aroylhydrazones towards iron: tracking down the elusive aroylhydrazono-based ferric spin-crossover molecular materials† Musa S. Shongwe,* a Sumaiya H. Al-Rahbi, a Mariam A. Al-Azani, a Abdulaziz A. Al-Muharbi, a Faizah Al-Mjeni, a Dariusz Matoga, b Abbasher Gismelseed, c Imaddin A. Al-Omari, c Ali Yousif, c Harry Adams, d Michael J. Morris d and Masahiro Mikuriya e Received 25th July 2011, Accepted 10th November 2011 DOI: 10.1039/c1dt11407g The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL 1 ) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL 2 ) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe II (HL) 2 ](ClO 4 ) 2 , [Fe II L 2 ] and [Fe III L 2 ]ClO 4 (HL = HL 1 or HL 2 ) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe III (L 1 ) 2 ]ClO 4 is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe III (L 1 ) 2 ]ClO 4 and [Fe III (L 2 ) 2 ]ClO 4 are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the 6 A 1 ↔ 2 T 2 transition curve is sigmoidal with T 1 2 ~280 K and the SCO is virtually complete. As regards [Fe III (L 1 ) 2 ]ClO 4 ,M¨ ossbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state. Introduction The high efficacy, selectivity and specificity of the coordination of aroylhydrazones towards iron has rendered these Schiff bases prime candidates in the development of iron chelators for the treatment of iron overload in humans arising from b-thalassemia (a genetic disease) and hemochromatosis (chronic iron poisoning). a Department of Chemistry, College of Science, Sultan Qaboos University, PO Box 36, Al-Khod 123, Muscat, Sultanate of Oman. E-mail: musa@ squ.edu.om; Tel: +968 92163719 b Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060, Krak´ ow, Poland c Department of Physics, College of Science, Sultan Qaboos University, PO Box 36, Al-Khod 123, Muscat, Sultanate of Oman d Department of Chemistry, University of Sheffield, Sheffield, UK, S3 7HF e Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2-1 Gakuen, Sanda, 669-1337, Japan †Electronic supplementary information (ESI) available. CCDC reference numbers 836952. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1dt11407g In recent times, there has been a concerted research endeavour to explore a diverse range of aroylhydrazones 1 and other related Schiff bases, such as thiosemicarbazones, 1a,2 as potential iron- chelating drugs prompted by the overriding need for possible replacements for, or alternatives to, desferrioxamine (DFO), 3 the siderophore currently enjoying world-wide clinical use in iron chelation therapy. For decades, DFO has been the sole clinically approved drug for the treatment of iron-overload disorders; how- ever, its major drawback is that it is orally inactive, necessitating frequent and lengthy subcutaneous infusions, thereby causing poor compliance by patients. The iron chelators deferiprone, 3 deferitrin 4 and deferasirox, 5 which have emerged recently, exhibit different degrees of oral effectiveness and are at different devel- opmental stages, but all seem to be credible alternatives to DFO. As for aroylhydrazones, 1 studies have shown that these chelating agents are adequately lipophilic to permeate cell membranes and readily access intracellular iron. The potential biological applications of these ligands extend to prevention of malaria and treatment of several types of aggressive cancer. 1,2 2500 | Dalton Trans., 2012, 41, 2500–2514 This journal is © The Royal Society of Chemistry 2012 Published on 04 January 2012. Downloaded by SULTAN QABOOS UNIVERSITY on 04/03/2015 05:56:09. View Article Online / Journal Homepage / Table of Contents for this issue