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

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) ų, 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) ų, 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) ų, 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) ų, 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) ų, 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) ų, 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.