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

1996, Inorganic Chemistry

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)(NCCH 3 )(OH 2 )](ClO 4 ) 2 (complex A, [C 20 H 23 -CuN 5 O 2 ](ClO 4 ) 2 , monoclinic, space group P2 1 /c, a ) 10.2168 Br 2 ]‚MeOH (complex D, [C 16 H 14 Br 2 CuN 4 O]‚CH 4 O, orthorhombic, space group Pbca, a ) 14.325(2) Å, b ) 13.919(2) Å, c ) 18.837(2) Å, V ) 3756.0(9) Å 3 , Z ) 8), [Cu(4)Br 2 ]‚MeOH (complex E, [C 18 H 18 Br 2 CuN 4 O]‚CH 4 O, triclinic, space group P1 h, a ) 7.3120(11) Å, b ) 9.9460(15) Å, c ) 15.189(2) Å, R ) 87.476(4)°, ) 89.093(4)°, γ ) 68.673(3)°, V ) 1028.0(3) Å 3 , Z ) 2), and Cu(10)Br 2 (complex F, C 16 H 13 Br 2 CuN 3 OS, monoclinic, space group P2 1 /c, a ) 7.3130(9) Å, b ) 15.861(2) Å, c ) 14.846(2) Å, ) 98.318 °, V ) 1704.0(4) Å 3 , 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.