Electrochemical recognition of anions by 1,1′-N,N′-ferrocenoylbisamino acid esters

A series of novel ferrocene-based receptors, 1,1′-N,N′-ferrocenoylbisamino acid methyl esters 2–5 have been prepared and their electrochemical properties determined. The amino acids employed were glycine (2), β-alanine (3), γ-aminobutyric acid (4) and l-norleucine (5). These receptors are composed of an electroactive core and two parallel strands of amino acids that can interact with anions via electrostatic interactions in the oxidized state as well as secondary interactions, such as hydrogen bonding and hydrophobic interactions. Furthermore, the semi-rigid molecular clefts between the two strands of amino acids in these receptors are capable of discerning anions of different geometries and sizes. The anion sensing capabilities of receptors 2–5 were studied using cyclic voltammetry (CV). The anions studied were chloride (Cl−), nitrate (NO3−), dihydrogen phosphate (H2PO4−), hydrogen sulfate (HSO4−), acetate (CH3COO−) and neurologically important anions such as lactate (CH3CH(OH)COO−), pyruvate (CH3COCOO−) and glutamate (HOOC–CH(NH2)CH2CH2COO−). The receptors 2–5 exhibit selectivity towards chloride, dihydrogen phosphate and acetate, over hydrogen sulfate and nitrate ions and generate a redox response in organic media. Also, binding studies of receptor 3 with neurologically important anions show it displays selectivity towards lactate and pyruvate, over glutamate ions and generates a redox response. However the response is ill defined in all cases, with poorly separated “free” and “ion-pair” peaks, which preclude accurate measurement of the response by CV. These results emphasize the considerations required for the design of ferrocene-based receptors and the necessary parameters for efficient electrochemical recognition of small anions by CV.