Cyclic and open-chain aza–oxa ferrocene-functionalised derivatives as receptors for the selective electrochemical sensing of toxic heavy metal ions in aqueous environments

Abstract

A new family of aza–oxa open-chain and macrocyclic molecules functionalised with ferrocenyl groups have been synthesized and characterised. The crystal structures of the [HL¹][PF₆], [H₂L³][PF₆]₂, [H₂L⁴][PF₆]₂ and [H₂L⁵][PF₆]₂ salts have been determined by single crystal X-ray procedures {L¹ = 10-ferrocenylmethyl-1,4,7-trioxa-10-azacyclododecane, L³ = 7,13-bis(ferrocenyl)-1,4,10-trioxa-7,13-diazacyclopentadecane, L⁴ = 1,8-bis[bis(ferrocenylmethylamino)]-3,6-dioxaoctane, L⁵ = 1,8-bis(ferrocenylmethylamino)-3,6-dioxaoctane}. They consist of cationic protonated amines linked via ionic interactions with hexafluorophosphate anions. Additionally hydrogen-bonding interactions have also been found. The receptors have been designed to promote discrimination, using electrochemical techniques, between toxic heavy metal ions such as Hg²⁺ over other commonly water present cations in aqueous environments. The presence in the receptors of oxygen and nitrogen donor atoms has been used to control the selectivity of large metal ions over small ones. Potentiometric and electrochemical studies have been mainly carried out to find pH ranges of selective electrochemical recognition. Potentiometric titrations were carried out in water (25 °C, 0.1 mol dm^–3 potassium perchlorate) for L¹ and L² [1,1′-(5,8-dioxa-2,11-diazadodecane-1,12-diyl)ferrocene] and in 1,4-dioxane–water (25 °C, 0.1 mol dm^–3 potassium nitrate) for L³ and L⁵ with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Hg²⁺. All receptors show larger stability constants with Hg²⁺ than with the remaining metal ions studied. This is especially so for L¹ and L². The receptors L¹, L² and L⁵ are able electrochemically and selectively to sense the presence of Hg²⁺, whereas maximum electrochemical shifts are produced in L³ upon addition of Pb²⁺. Of importance is the large and selective electrochemical shift monitored in water for L² and Hg²⁺ with an anodic displacement of the oxidation potential of ca. 130 mV which is one of the largest shifts ever reported in electrochemical cation sensing in water using related receptors. A good agreement has been found between potentiometric and electrochemical results. Selective electrochemical response against Hg²⁺ appears to be associated with (i) pH ranges of selective complexation or (ii) the existence of strong predominant receptor–metal complexes in a wide pH range. Additionally the electrochemical behaviour of receptors L¹ and L² in the presence of metal ions can be roughly predicted from potentiometric data. The stability constants of the complexes between L¹ and Cu²⁺, Cd²⁺, Pb²⁺ and Hg²⁺ were also determined in the presence of Cl^–. Whereas there is no important change in the stability constants of the L–H⁺–M²⁺ systems when M²⁺ = Cu²⁺, Cd²⁺ or Pb²⁺, there is a decrease of the co-ordination ability of L¹ towards Hg²⁺. This is also reflected in electrochemical studies which demonstrate that [Hg(L¹)]²⁺ electrochemically sense Cl^– at pH 7. To the best of our knowledge this is the first time it has been shown that metal complexes functionalised with ferrocenyl groups can electrochemically sense anions.