Ruthenium and iron complexes with benzotriazole and benzimidazole derivatives as simple models for proton-coupled electron transfer systems

Journal of the Brazilian Chemical Society

Rocha, Reginaldo C.¹  Rein, Francisca N.¹  Toma, Henrique E.¹  Universidade de São Paulo, São Paulo, Brazil¹ 

关键词: ruthenium complexes;    iron complexes;    monoacid N-heterocycles;    redox potentials;    pH-dependent electrochemistry;   

DOI  :  10.1590/S0103-50532001000200018

学科分类：化学（综合）

来源: SciELO

【 摘 要 】

Proton-coupled electron transfer (PCET) is known to play an important pathway of charge transport in a variety of aqueous electrochemical processes1,2 and biochemical3,4, inorganic5 and organic reactions4,5. Recent developments in supramolecular solid-state chemistry are directed toward rational design of PCET systems with novel electronic or photonic material properties6. The overall redox mechanisms, however, are often quite complex, involving multiple electron and proton transfers. But, by limiting the inner coordination sphere to one ionizable proton, it is possible to obtain complexes which exhibit one-electron, one-proton redox reactions over relatively broad pH and potential regions5. In order to develop simple models for PCET, we have synthesized coordinatively saturated, substitutionally inert transition-metal complexes such that oxidation of the metal is coupled to proton transfer at a remote site on the coordinated acid N-heterocyclic ligand.In this direction, ruthenium7 and iron8,9 complexes with d5/d6 configurations are very useful due to their rich and versatile redox chemistry, in addition to their usually favorable kinetic properties and stability. Heterocyclic nitrogens play in turn an important role in coordination chemistry10. Imidazole, for instance, is an ubiquitous ligand11 in chemical and biological systems as it appears as such in proteins, and, together with its derivatives, have been extensively employed as models in a wide range of inorganic subject areas, from biological applications to electronic devices and materials12,13. The triazole derivatives have also been largely studied in ruthenium and osmium chemistry of discrete or supermolecules14. In particular, benzotriazole is largely employed as an efficient corrosion inhibitor for copper and its alloys15 and provides a model ligand for bioinorganic studies involving the interaction of nucleic bases or DNA fragments and transition metal complexes. Nevertheless, its coordination chemistry with iron and second-row transition metals has been little explored.Although the reactivity and the properties of transition metal complexes with imidazole and triazole derivatives have been studied over the last decades, a few cases involving the interaction of the benzimidazole (bimH) and benzotriazole (btaH) ligands (Scheme 1) with iron(II,III) and ruthenium(II,III) complexes are known16-22. Actually, to the best of our knowledge, we have investigated the coordination chemistry of benzotriazole with pentacyano-ferrate(II)/(III)17,21, pentaammineruthenium(II)/(III)18,21,22 and ethylenediaminetetraacetatoruthenium (II)/(III)19,20,22 for the first time. In this present work, a larger series involving these metal units was studied, by including a structural analogue of the benzotriazole ligand (namely, benzimidazole). We explored in detail the relationship between the acid-base equilibria and electrochemistry, focusing on their proton-coupled electron transfer (PCET) reactions.   ExperimentalSyntheses and preparation of compoundsChemicals. RuCl3.nH2O (Inco Europe Ltd.), [Ru(NH3)6]Cl3 (Johnson Matthey Chem.; recrystallized), Na2[Fe(CN)5NO] (J.T.Baker), benzimidazole and benzotriazole (Aldrich Chem.). All organic solvents employed in the syntheses were analytical reagent grade and were used without further purification. Deionized water (NanopureTM –Barnstead) was used through out the experiments. Other chemicals used here were also purchased from Sigma-Aldrich Co. and used as supplied. Argon gas (White-Martins) was employed to deaerate the solutions during the experiments.Starting materials. Na3[Fe(CN)5(NH3)] 23, [Ru(NH3)5 Cl]Cl224 and Ru(Hedta)H2O25 were synthesized according to the procedures previously reported in the literature. Elemental analyses, spectroscopic and electrochemical methods were used to assess the purity of the compounds.[Run(NH3)5(LH)](PF 6)n×xH2O (where LH = bimH or btaH). In the preparation of the ammineruthenium derivatives, 58.6 mg of [Ru(NH3)5Cl]Cl2 (0.2 mmol) were dissolved in 5 cm3 of degassed water, after which some amalgamated zinc pieces were added. After reacting for 30 min, the resulting yellow solution was anaerobically transferred to a recipient containing 238.2 mg of benzotriazole or 236.2 mg of benzimidazole (2.0 mmol) under an argon atmosphere (in the benzimidazole case, 5% in volume of ethanol was previously added to make the dissolution of the ligand easier), and left for 30 min. Then, 5 cm3 of a concentrated ammonium hexafluoro-phosphate was dripped onto the orange color reactional mixture. A yellow precipitate was immediately formed. The solid product was collected on a filter, washed with a small volume of ethanol, and dried in vacuum in the presence of anhydrous calcium chloride. Yield: 80–90%.In the

【 授权许可】

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