Design and synthesis of heteroditopic aza-thioether macrocycles for metal extraction

Abstract

A series of pendant arm aza-thioether macrocycles containing hydrogen-bonding amide functionalities have been synthesised and the single crystal X-ray structures of [12]aneS₃N–CONHCOPh, [12]aneS₃N–CSNHCOPh, [12]aneS₂ON–CONHCOPh, [12]aneS₂ON–CH₂CONHCONH–^tBu, [15]aneS₂ON₂–(CH₂CONHCONH–^tBu)₂ and [15]aneS₃N₂–(CH₂CONHPh)₂ are presented. The structural results confirm that the majority of the macrocyclic C–S linkages adopt gauche conformations with all sulfur atoms being exo-orientated with respect to the macrocyclic ring, while the oxygen atoms, when present, are endo-orientated. The pendant arms, formed by reaction of the parent macrocycle with acyl isocyanates, are twisted from co-planarity and typically form a distorted U-shape with angles between planes formed around the carbonyl units varying from 50.81° to 68.05°. The dominant motif in these structures involves dimeric units linked through hydrogen-bonds: however, such a network is disrupted in [12]aneS₃N–CSNHCOPh by the substitution of a carbonyl unit for a thiocarbonyl unit. When more than one amide unit is present in the pendant arm, intramolecular hydrogen-bond networks are observed resulting in coplanar pendant arms and highly directional hydrogen-bonding. Inclusion of a methylene linker between the macrocycle and carbonyl function in the pendant arm inhibits intermolecular hydrogen-bond formation resulting in interaction of the amide proton with the macrocyclic nitrogen centres. A single crystal X-ray structure of [Ag([12]aneS₃N–CH₂CONH–^tBu)(NO₃)] confirms binding of Ag(I) to the macrocyclic moiety with additional hydrogen-bonding [N–H⋯O 2.901(3) Å] between the macrocyclic pendant arm amide unit and the nitrate anion demonstrating that in the solid-state these ligands can act as heteroditopic receptors for metal salts. Overall, the solid-state structure of this complex is a one-dimensional polymer in which the Ag(I) cation is in a distorted square-pyramidal geometry with the apical sulfur donor emanating from an adjacent macrocyclic molecule. The efficacy of these ligands in competitive membrane transport and liquid–liquid metal ion extraction of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ag(I) and Pb(II) has been assessed using selected ligands. In preliminary experiments, clear extraction and transport selectivity for Ag(I) was observed in all systems investigated, while only modest extraction of TcO₄⁻ was identified.