The coordinating environments provided by the 1-(5,6-dimethylbenzimidazolyl)-3-benzimidazolyl-2-thiapropane (1), 1-(5,6-dimethylbenzimidazolyl)-3-benzimidazolyl-2-oxapropane (2), 1-(N-methyl)benzimidazolyl-3-benzimidazolyl-2-oxapropane (3), and 1,7-bis(benzimidazol-2-yl)-2,6-dithiaheptane (4) ligands for a series of soft-to-borderline metal ions have been demonstrated by crystallographic structure determination in the solid state, supplemented by multinuclear NMR studies in solution. Structures of [Zn(3)(H2O)(MeCN)](ClO4)2 ([C19H21N5O2Zn](ClO4)2, monoclinic, space group P21/c, a = 10.3247(11) Å, b = 11.0320(12) Å, c = 20.761(2) Å, β = 96.465(3)°, V = 2349.7(4) Å3, Z = 4), [Zn(1)Cl2]·CH3OH ([C18H18Cl2N4SZn]·CH4O, triclinic, space group P1̄, a = 9.0917(9) Å, b = 9.9186(10) Å, c = 12.2079(12) Å, α = 99.710(3)°, β = 94.252(2), γ = 95.598(3)°, V = 1075.3(2) Å3, Z = 2), [Hg(1)Br2]·CH3OH ([C18H18Br2HgN4S]·CH4O, triclinic, space group P1̄, a = 9.1956(12) Å, b = 9.9775(13) Å, c = 12.462(2) Å, α = 99.239(3), β = 97.676(3)°, γ = 94.036(3)°, V = 1113.4(3) Å3, Z = 2), [Cd(1)Cl2] ([C18H18CdCl2N4S], monoclinic, space group P21/n, a = 14.5219(11) Å, b = 8.5315(7) Å, c = 15.6182(12) Å, β = 92.063(2)°, V = 1933.7(3) Å3, Z = 4), and [Ag(4)](NO3) ([C19H20AgN4S](NO3), monoclinic, space group C2/c, a = 13.7339(3) Å, b = 17.2710(1) Å, c = 9.8866(2) Å, β = 115.399(1)°, V = 2118.4(1) Å3, Z = 4) have been determined. Ligand 3 provided the expected N2O donor set to Zn(II) forming part of a pseudo-trigonal bipyramidal coordinating environment, tentatively extrapolated to the Zn(II), Cd(II), and Hg(II) complexes of ligand 2 from 1H NMR studies. The thioether-bridged ligands (1 and 4) displayed a strikingly different coordination mode; in none of these structures was the thioether found to be coordinated to the central metal ion, resulting in pseudotetrahedral structures for the Zn(II) and Hg(II) complexes of ligand 1, a bis(chloro)-bridged dimeric five-coordinate complex of Cd(II) with ligand 1, and a near-linear coordination mode for the Ag(I) complex of ligand 4. No significant distinction in geometric parameters around the metal ions could be observed resulting from the introduction of asymmetry and a change of pK a within the bis(benzimidazole) ligands. Using 199Hg NMR spectroscopy, it was demonstrated that the thioether of ligand 1 did not coordinate in solutions of the 1:1 HgBr2 complex; negligible shifts of the signals corresponding to the aliphatic bridging protons in the 1H NMR spectra of ligand 1 when coordinated to Zn(II), Cd(II), and Hg(II) also suggested a noncoordinating behavior. Exchanges occurring in mixtures of HgBr2 and ligand 1 were examined using 199Hg, 13C, and 1H NMR. It is considered that there is little or no exchange occurring in a region of excess mercury on the 199Hg NMR time scale, and in a region of excess ligand 1 the 1/1 complex predominates.

A synthetic strategy has been developed for bis(benzimidazole) ligands in which the two halves are different (4−11), and consequently of different basicity, which could be important for biomimicry and metal ion transport. Their coordination chemistry toward copper(II) has been studied in the solid state via X-ray crystallography. The ligands were complexed with copper(II) bromide and perchlorate salts to yield complexes of a 1:1 stoichiometry. Crystal structures have been determined and compared for [Cu(4)(NCCH3)(OH2)](ClO4)2 (complex A, [C20H23CuN5O2](ClO4)2, monoclinic, space group P21/c, a = 10.2168(7) Å, b = 30.740(2) Å, c = 8.3403(6) Å, β = 105.960(2)°, V = 2518.4(3) Å3, Z = 4), [Cu(6)Br2]·DMF (complex B, [C17H16Br2CuN4O]·C3H7NO, monoclinic, space group P21/c, a = 8.3348(11) Å, b = 18.165(2) Å, c = 14.140(2) Å, β = 91.646(3)°, V = 2140.0(5) Å3, Z = 4), [Cu(8)Br2]·DMF·H2O (complex C, [C16H13Br2CuN5O3]·C3H7NO·H2O, monoclinic, space group P21/c, a = 8.7241(10) Å, b = 18.172(2) Å, c = 14.506(2) Å, β = 97.376(2)°, V = 2280.7(4) Å3, Z = 4), [Cu(3)Br2]·MeOH (complex D, [C16H14Br2CuN4O]·CH4O, orthorhombic, space group Pbca, a = 14.325(2) Å, b = 13.919(2) Å, c = 18.837(2) Å, V = 3756.0(9) Å3, Z = 8), [Cu(4)Br2]·MeOH (complex E, [C18H18Br2CuN4O]·CH4O, triclinic, space group P1̄, a = 7.3120(11) Å, b = 9.9460(15) Å, c = 15.189(2) Å, α = 87.476(4)°, β = 89.093(4)°, γ = 68.673(3)°, V = 1028.0(3) Å3, Z = 2), and Cu(10)Br2 (complex F, C16H13Br2CuN3OS, monoclinic, space group P21/c, a = 7.3130(9) Å, b = 15.861(2) Å, c = 14.846(2) Å, β = 98.318(2)°, V = 1704.0(4) Å3, Z = 4). The Cu(II) complexes were found to be five coordinate, lying between perfect square-based-pyramidal (SBP) and trigonal-bipyramidal (TBP) extremes; in each case the ligands act as tridentate donors coordinating through the pyridine-like nitrogens of the benzimidazole moieties and the ether donor atom of the linking bridge. Use of the structural index parameter (τ) for five-coordinate metal complexes indicated that all the copper(II) complexes exhibit a greater tendency toward square-based-pyramidal geometry (i.e. τ < 0.5). Comparison of the symmetrical bis(benzimidazole) complex with the other, asymmetric, complexes revealed no significant change in the geometrical parameters around the copper(II) ion consequent on introduction of asymmetry and a change of pKa within the bis(benzimidazole) fragment. The degree of hydrogen bonding, solvent of crystallization, and the nature of the anion have a greater impact on the geometrical parameters and coordination environment of the copper(II) ion. The import for biological metal coordination is considered.

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lonophores of the bis(crown ether) type, with polyamine linkers, examples of which are known to bind simultaneously to a cation and its counter anion, have been prepared. One-pot reductive amination, used to couple a benzo-crown aldehyde with aliphatic (both linear and cyclic) and aromatic polyamines, proved to be the most efficient route. A strong inhibition of one synthetic route (amine benzylation) was observed.The competitive interaction between chloride and other anions for binding with the protonated ionophores was examined using 35CI N M R spectroscopy. Sandwich complexing of potassium contrasting with single-face complexation of sodium was demonstrated by 13C N MR spectroscopy.Biological systems frequently organise a molecule with two distinct subunits by a strong ionic or co-ordinative type interaction with a metal ion, which holds the units in position for some other biological strategy. For example the two halves of the iron protein of the nitrogenase complex are organised by an Fe,S, cluster linked to cysteines coming from each half, forming a receptor site into which fits the ATP/ADP unit involved in energy transfer for the hydrogenation of dinitrogen. Such examples have prompted chemical mimicry. In this work a promising bis(crown ether) structure was designed to co-ordinate anions within a cavity produced by organisation of the two ends of the molecule with a strong interaction. In the examples tried ' this 'organisation' is a sandwiching of an alkalimetal cation between two crown ethers. Molecules containing two crown ether moieties, separated by a linking group, are known to sandwich potassium but crystallographic evidence of the structure has been sparse. A recent crystal structure determination of a rubidium complex of a Schiffbase-linked bis(crown ether) shows a double sandwich structure, not previously demonstrated.' This has a potential binding site between the two aromatic fragments which link the two crowns of each individual bis(crown ether), which may be represented as shown in Scheme 1, which we have used previously to describe our designThe 1 : 1 sandwich structure (see Scheme 1) has also been demonstrated crystallographically.6 This paper gives details of synthetic routes to, and the separation and characterisation of, multireceptors which should exhibit multiple binding as outlined by us previously in a preliminary report.' 1980,47,81.

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