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Interaction of copper(II) salts with 2,2′-dipyridylamine (1), N-cyclohexylmethyl-2,2′-dipyridylamine (2), di-2-pyridylaminomethylbenzene (3), 1,2-bis(di-2-pyridylaminomethyl)-benzene (4), 1,3-bis(di-2-pyridylaminomethyl)benzene (5), 1,4-bis(di-2-pyridylaminomethyl)benzene (6), 1,3,5-tris(di-2-pyridylaminomethyl)benzene (7) and 1,2,4,5-tetrakis(di-2-pyridylaminomethyl)benzene (8) has yielded the following complexes: [Cu(2)(μ-Cl)Cl]2, [Cu(3)(μ-Cl)Cl]2 · H2O, [Cu2(4)(NO3)4], [Cu2(5)(NO3)4] · 2CH3OH, [Cu2(6)(CH3OH)2(NO3)4], [Cu4(8)](NO3)4] · 4H2O while complexation of palladium(II) with 1, 4, 5 and 6 gave [Pd(1)2](PF6)2 · 2CH3OH, [Pd2(4)Cl4], [Pd2(4)(OAc)4], [Pd2(5)Cl4], [Pd2(6)Cl4] and [Pd2(6)(OAc)4] · CH2Cl2, respectively. X-ray structures of [Cu(2)(μ-Cl)Cl]2, [Cu(3)(μ-Cl)Cl]2 · 2C2H5OH, [Cu2(6)(CH3OH)2(NO3)4], [Pd(1)2](PF6)2 · 2CH3OH, [Pd2(4)(OAc)4] · 4H2O and [Pd2(6)(OAc)4] · 2CH2Cl2 are reported. In part, the inherent flexibility of the respective ligands has resulted in the adoption of a diverse range of coordination geometries and lattice arrangements, with the structures of [Pd2(4)(OAc)4] · 4H2O and [Pd2(6)(OAc)4] · 2CH2Cl2, incorporating the isomeric ligands 4 and 6, showing some common features. Liquid–liquid (H2O/CHCl3) extraction experiments involving copper(II) and 1–3, 5, 7and 8 show that the degree of extraction depends markedly on the number of dpa-subunits (and concomitant lipophilicity) of the ligand employed with the tetrakis-dpa derivative 8 acting as the most efficient extractant of the six ligand systems investigated.

Three copper(II) complexes derived from 2-(hydroxymethyl)pyridine (LH) have been synthesised and a comparative X-ray investigation of their respective crystal structures undertaken. In the absence of added base, LH reacts with copper(II) chloride or nitrate to yield the five-coordinate [Cu(LH)2Cl]Cl and six-coordinate [Cu(LH)2(NO3)2] species. In the chloro complex the coordination geometry is distorted trigonal bipyramidal, with the pyridyl nitrogens occupying the axial position and two hydroxyl oxygens from the bidentate ligands together with a chloro group occupying the equatorial sites. The structure of the nitrate species, published previously, has been reinvestigated at low temperature in order to specify the weak interactions in the crystal; it contains two molecules of bidentate LH bound trans in the equatorial plane while monodentate nitrato ligands occupy the axial sites. In each of these complexes the hydroxyl protons act as hydrogen bond donors, interacting with the non-coordinated chloride anion in [Cu(LH)2Cl]Cl and the coordinated nitrato groups in [Cu(LH)2(NO3)2] to form bridged, hydrogen-bonded, copper(II)-organic arrays in each case; offset face-to-face π-stacking in the latter produces a two dimensional structure. Further weak CH⋯Cl, CH⋯O interactions stabilise both arrangements. The X-ray structure of the complex [Cu(L)2] · 4H2O containing the deprotonated ligand is also described. The presence of the latter results in the above hydrogen bonding arrangements being ‘switched off’ and instead a new two-dimensional network involving bridging tetrameric water clusters hydrogen bound to adjacent ligand hydroxo groups to give extended sheets is generated; offset face-to-face π-stacking occurs between sheets to yield a three dimensional array.

In the discussion that follows some of the more recent progress in the area of anion binding by synthetic polyamine receptors is presented, with emphasis given to work undertaken by the authors’ groups. A continuing theme in these studies has been the relationship between receptor structure and its anion extraction properties. Systematic solvent extraction and structural studies for halide and perrhenate complexes with polyamines of tripodal, macrocyclic and macrobicyclic architecture that contain both aromatic moieties and four to eight amine functions have been performed in order to derive relevant structure-binding/extractability relationships. The results demonstrate that the binding and extraction behaviour of the polyamines towards halides and perrhenate is a complex function of their structural features, degree of protonation and lipophilic properties. The extraction is characterized by the preferred formation of mono- and diprotonated amine species in the organic phase. X-ray structure studies of iodide and perrhenate complexes with open-chain tetraamino derivatives and octaamino cryptands in different protonation states lead to the conclusion that in the first case only limited chelation of the anion occurs and in the second only highly protonated species are able to encapsulate the anion. The structural patterns observed are strongly influenced by the presence of water molecules in the crystals.

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) A] 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.