American Chemical Society
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A Route to Bis(benzimidazole) Ligands with Built-In Asymmetry:  Potential Models of Protein Binding Sites Having Histidines of Different Basicity

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journal contribution
posted on 1996-12-18, 00:00 authored by Craig J. Matthews, Troy A. Leese, William Clegg, Mark R. J. Elsegood, Lynne Horsburgh, Joyce C. Lockhart
A synthetic strategy has been developed for bis(benzimidazole) ligands in which the two halves are different (411), 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.