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4,4′-biphenylene spaced lipophilic bis-β-diketone ligands of the type 4,4′-bis(RC(O)CH2C(O))C12H8 (R = Pr, Ph, hexyl, octyl, nonyl) have been prepared and used for the liquid–liquid extraction of d-block metal ions. These ligands are expected to interact with divalent metal ions to form charge-neutral trinuclear metallocycles of type [M3(L3)3(solvent)] as has been demonstrated with the previously reported derivative of H2L3 (R = tBu), the X-ray structure of which is reported. Liquid–liquid extraction studies were performed in a two-phase water/chloroform system employing a radiotracer technique for cobalt(II) and zinc(II). These experiments involved the systematic variation of ligand, metal and 4-ethylpyridine concentrations to probe the stoichiometries of the species extracted. Synergistic extraction was observed when 4-ethylpyridine was present with the ligand in the organic phase. Competitive extraction studies demonstrated the ligands are highly selective for copper(II) over cobalt(II), nickel(II), zinc(II) and cadmium(II).

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.