Coordination of Zn(II), Cd(II), Hg(II), and Ag(I) by bis(benzimidazole) ligands

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 P2(1)/c, a = 10.3247(11) Angstrom, b = 11.0320(12) Angstrom, c = 20.761(2) Angstrom, beta = 96.465(3)degrees, V = 2349.7(4) Angstrom(3), Z = 4), [Zn(1)Cl-2]. CH3OH ([C18H18Cl2N4SZn]. CH4O, triclinic, space group <P(1)over bar>, a = 9.0917(9) Angstrom, b = 9.9186(10) Angstrom, c = 12.2079(12) Angstrom, alpha = 99.710(3)degrees, beta = 94.252(2), gamma = 95.598(3)degrees, V = 1075.3(2) Angstrom(3), Z = 2), [Hg(1)Br-2]. CH3OH ([C18H18Br2HgN4S]. CH4O, triclinic, space group <P(1)over bar>, a = 9.1956(12) Angstrom, b = 9.9775(13) Angstrom, c = 12.462(2) Angstrom, alpha = 99.239(3), beta = 97.676(3)degrees, gamma = 94.036(3)degrees, V = 1113.4(3) Angstrom(3), Z = 2), [Cd(1)Cl-2] ([C18H18CdCl2N4S], monoclinic, space group P2(1)/n, a = 14.5219(11) Angstrom, b = 8.5315(7) Angstrom, c = 15.6182(12) Angstrom, beta = 92.063(2)degrees, V = 1933.7(3) Angstrom(3), Z = 4), and [Ag(4)](NO3) ([C19H20AgN4S](NO3), monoclinic, space group C2/c, a = 13.7339(3) Angstrom, b = 17.2710(1) Angstrom, c = 9.8866(2) Angstrom, beta = 115.399(1)degrees, V = 2118.4(1) Angstrom(3), Z = 4) have been determined. Ligand 3 provided the expected N2O donor set to Zn(II) forming part of a pseudotrigonal bipyramidal coordinating environment, tentatively extrapolated to the Zn(II), Cd(II), and Hg(lI) complexes of ligand 2 from H-1 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(LI) 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 Hg-199 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 H-1 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 Hg-199, C-13, and H-1 NMR. It is considered that there is little or no exchange occurring in a region of excess mercury on the Hg-199 NMR time scale, and in a region of excess ligand 1 the 1/1 complex predominates.