Coordination of Zn(II), Cd(II), Hg(II), and Ag(I) by Bis(benzimidazole) Ligands

Abstract

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 pKa 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.